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Gestational trophoblastic neoplasia—pathogenesis and potential therapeutic targets
Review
Gestational trophoblastic neoplasia—pathogenesis and potential therapeutic targets Ie-Ming Shih Lancet Oncol 2007; 8: 642–50 Departments of Pathology, Oncology, and Gynecology and Obstetrics, Johns Hopkins Medical Institutions, Baltimore, MD, USA (I-M Shih MD) Correspondence to: Dr Ie-Ming Shih [email protected]
Gestational trophoblastic neoplasia comprises a unique group of human neoplastic diseases that derive from fetal trophoblastic tissues and represent semiallografts in patients. This group is composed of choriocarcinoma, placentalsite trophoblastic tumour, and epithelioid trophoblastic tumour, and many forms are derived from the precursor lesions, hydatidiform moles. Although most patients with gestational trophoblastic neoplasia are cured by chemotherapy and tumour resection, some patients suffer from metastatic diseases that are refractory to conventional chemotherapy. Therefore, new therapeutic regimens are needed to reduce the toxic effects associated with current chemotherapy and to salvage the occasional non-operable patients with recurrent and chemoresistant disease. Until the fundamental biology of gestational trophoblastic neoplasia becomes more clearly understood, development of a new treatment will remain empirical. This review will briefly summarise the recent advances in understanding the molecular aetiology of this group of diseases and highlight the molecules that can be potentially used for therapeutic targets to treat metastatic gestational trophoblastic neoplasia.
Introduction Gestational trophoblastic diseases represent a spectrum of related disorders including benign trophoblastic lesions, premalignant hydatidiform moles, clinically malignant invasive hydatidiform moles, and neoplastic diseases. Hydatidiform moles, especially the invasive moles, are sometimes clinically considered as gestational trophoblastic neoplasia because they can locally invade and distantly metastasise, but, biologically, they represent abnormally formed placental tissues rather than true neoplasia. Therefore, from the perspectives of pathobiology, gestational trophoblastic diseases can be broadly divided into three groups (panel), modified from the 2003 WHO classification1: benign trophoblastic lesions (placental-site nodule and exaggerated placental reaction), which are non-neoplastic lesions; hydatidiform moles (complete, partial, and invasive moles), which are
Figure 1: ETT (arrow) in a hysterectomy specimen of a 32-year-old woman ETT was first diagnosed based on endometrial curettage and patient subsequently underwent total hysterectomy. 1 year after surgery, metastatic ETT developed in the lung and was treated with chemotherapy and lung resection. A second recurrent tumour, resistant to prior chemotherapy, developed 1 year later.
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aberrant placental derivatives; and true gestational trophoblastic neoplasia.2,3 The term gestational trophoblastic neoplasia encompasses a group of interrelated but distinct tumours that include choriocarcinoma, placental-site trophoblastic tumour (PSTT), and epithelioid trophoblastic tumour (ETT; figure 1). Gestational trophoblastic neoplasms are unique among human neoplastic diseases because they are genetically related to fetal tissues (ie, trophoblastic cells from placentas) and, therefore, represent semiallografts in patients. Although there has been substantial interest in exploring the pathogenesis of these neoplasms, studies on their molecular aspects are challenging for several reasons. First, gestational trophoblastic neoplasms are rare diseases and the cumulative incidence rate for gestational choriocarcinoma, for example, continues to decline. As a result, tissue specimens, especially those from fresh tumours, are scarce for molecular analysis. Second, many metastatic gestational trophoblastic neoplasms, which are clinically classified under the rubric of persistent gestational trophoblastic diseases are not usually obtained for pathological classification and characterisation. Therefore, their natures remain intriguing, because whether persistent gestational trophoblastic disease represents an invasive hydatidiform mole, a metastatic choriocarcinoma, PSTT, or ETT is not known. An absence of such clinicopathological correlation compromises attempts to further understand the biological nature of gestational trophoblastic neoplasia. Third, because of the above reasons, reagents, such as cell lines and animal models, are limited for mechanistic studies. For example, only a few choriocarcinoma cell lines and one PSTT cell line are currently available for molecular and cell-biology studies. Despite these challenges, progress has been made in recent years towards a better understanding of the molecular aetiology underlying the development of these neoplasms. This review will briefly summarise recent http://oncology.thelancet.com Vol 8 July 2007
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Panel: Gestational trophoblastic diseases Benign trophoblastic lesions Exaggerated placental reaction Placental-site nodule Hydatidiform moles (abnormally formed placentas) Complete mole Partial mole Invasive mole Trophoblastic neoplasia Choriocarcinoma Placental-site trophoblastic tumour Epithelioid tophoblastic tumour
advances and highlight the molecules that could potentially be used for therapeutic targets to treat metastatic gestational trophoblastic neoplasia.
Pathogenesis of trophoblastic tumours Previous clinical, pathological, and molecular studies have provided fundamental insights into the pathogenesis of hydatidiform moles,4 but the molecular and cellular basis in the development of true gestational trophoblastic neoplasia remain poorly understood. Gestational trophoblastic neoplasia has long been thought to be a homogeneous group of diseases arising from neoplastic transformation of trophoblastic cells. However, recent clinicopathological studies have provided new evidence that there are at least three distinct types of gestational trophoblastic neoplasm including the most common type, choriocarcinoma, and the less common ones, PSTT and ETT. Molecular analysis of gestational trophoblastic neoplasia is largely based on the characterisation of gene-expression profiles in various types of these neoplasms and the reference of their unique gene-expression patterns to different trophoblastic subpopulations in healthy early placentas.2 The main conclusion from these studies is that, after neoplastic transformation of trophoblastic stem cells, presumably cytotrophoblast stem cells, specific differentiation programmes dictate the type of trophoblastic tumour that develops (figure 2).5 Of interest, is the fact that these patterns of differentiation in gestational trophoblastic neoplasia recapitulate the stages of early placental development.2,3,5–7 Choriocarcinoma is composed of variable amounts of neoplastic cytotrophoblast, syncytiotrophoblast, and extravillous (intermediate) trophoblast and resembles the previllous blastocyst, which is composed of a similar mixture of trophoblastic subpopulations. By contrast, the neoplastic cytotrophoblast in PSTT differentiates mainly into extravillous (intermediate) trophoblastic cells in an implantation site whereas the neoplastic cytotrophoblast in ETT differentiates into chorionic-type extravillous (intermediate) trophoblastic cells in the chorion laeve.5 http://oncology.thelancet.com Vol 8 July 2007
According to this model, choriocarcinoma is the most primitive trophoblastic tumour whereas PSTT and ETT are relatively more differentiated. This hypothesis explains the existence of gestational trophoblastic neoplasia with a mixed histological feature, including choriocarcinoma, PSTT, and ETT. This new model might also help to explain the findings previously reported by Mazur8 who described ETT that occurred after intense chemotherapy of metastatic choriocarcinomas in the lung. In the patients described by Mazur, it is plausible that chemotherapeutic agents allow choriocarcinoma cells to differentiate into an ETT phenotype, which is more refractory to chemotherapy than the original choriocarcinoma counterpart. As a result, ETT predominated the choriocarcinoma after chemotherapy. Alongside studies of the histogenesis of gestational trophoblastic neoplasia, several studies have focused on the biology of these neoplasms by characterising the expressions of genes that are important in tumorigenesis. The pathogenesis of each type of neoplasm will now be summarised. Furthermore, advances in the pathogenesis of hydatidiform moles will be briefly highlighted because hydatidiform moles represent the precursor lesions in some gestational trophoblastic neoplasms and invasive hydatidiform moles are, sometimes, clinically considered as gestational trophoblastic neoplasia because they can locally invade and distantly metastasize.
Choriocarcinoma Gestational choriocarcinoma is a highly malignant epithelial tumour that can be associated with any type of gestational event, most often a complete hydatidiform mole.4 Several molecular studies have been done to establish the expression of tumour-associated proteins in choriocarcinoma. In the P53 pathway, overexpression of
Syncytiotrophoblast
Cytotrophoblast
Normal trophoblast Extravillous trophoblast (intermediate trophoblast) at implantation site
Extravillous trophoblast (intermediate trophoblast) at chorion laeve
Neoplastic transformation
Trophoblastic neoplasia
Placental-site trophoblastic tumour
Choriocarcinoma
Epithelial trophoblastic tumour
Figure 2: Proposed model of pathogenesis for gestational trophoblastic neoplasia
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the P53 protein has been detected in choriocarcinoma,9 but mutational analysis of TP53 has failed to show somatic mutations.10–12 The P53-associated protein, MDM2, has also been seen to be overexpressed in choriocarcinomas.9,13 Because upregulated MDM2 protein is associated with various human cancers, overexpression of MDM2 in choriocarcinomas might overcome growth suppression by wild-type P53 proteins and, therefore, contribute to the development of choriocarcinoma. To show the biological role of MDM2 in choriocarcinoma, Wang and co-workers14 used an MDM2 antisense oligonucleotide and showed a synergistic antitumour effect with DNA-damaging agents in JAR choriocarcinoma xenografts in mice. By use of a PCR-based subtracted fragmentary cDNA library between normal chorionic villi and a choriocarcinoma cell line, Asanoma and colleagues15,16 identified a novel homeobox gene designated NECC1 (located on 4q11-q12) that might be involved in the development of choriocarcinoma. In this study, the expression of NECC1 was lost in all choriocarcinoma cell lines and most choriocarcinoma tissues, but healthy adult tissues, including trophoblastic cells in healthy placentas, expressed abundant NECC1. The investigators further showed that engineered expression of NECC1 in choriocarcinoma cell lines suppressed tumorigenicity and induced terminal differentiation.15,16 Other genes that are potentially involved in the development of choriocarcinoma include epidermal growth factor receptor (EGFR),17 DOC-2/hDab2 (a candidate tumour suppressor gene),18,19 putative tumour suppressor loci at chromosomes 7p12-7q11.2320 and 8p12-p21,21 and the gene for ras GTPase activating protein.22 Synergistic upregulation of c-MYC, c-ERB-2, c-FMS, and BCL-2 oncoproteins have also been suggested to have an important role in the pathogenesis of choriocarcinoma.10 Conversely, promoter hypermethylation of E-cadherin, HIC-1, P16, and TIMP3 has been frequently seen in choriocarcinomas, suggesting that downregulation of these genes by promoter methylation could be involved in the development of choriocarcinomas.23 In addition to the candidate approaches described above, Vegh and colleagues24 compared the gene-expression pattern between healthy trophoblast and choriocarcinoma cell lines using cDNA microarrays. In their study, one of the downregulated genes in choriocarcinoma cells, HSP-27, was selected for characterisation because the HSP-27 protein has been implicated in the development of several human neoplastic diseases.25 The investigators noted that choriocarcinoma tissues had a lower level of HSP-27 expression compared with trophoblastic cells in early placentas. This downregulation of HSP-27 might contribute to the high sensitivity of choriocarcinomas to chemotherapeutic agents because HSP-27 proteins have been shown to contribute to drug resistance in malignant cells.26,27 644
The development of a choriocarcinoma might also be related to tumour microenvironment—eg, immune regulation and stromal tissues. HLA-G, a non-classic MHC class I molecule, has been shown to participate in immune regulation and response. The membrane-bound and secreted HLA-G molecules are able to inactivate the local immune response and, therefore, help tumour cells escape from immune surveillance. Trophoblastic cells in normal placentas and gestational trophoblastic neoplasms have been shown to upregulate HLA-G expression.28,29 In fact, in the human cancers analysed, gestational trophoblastic neoplasms, including choriocarcinoma, contain the highest levels of HLA-G expression.28 Therefore, HLA-G expressed in choriocarcinomas might assist tumour cells to escape from the host immune recognition and promote tumour growth.29 Another explanation for tumour growth could be matrix metalloproteinases (MMP) and tissue inhibitors of metalloproteinases (TIMPs), which have a key role in tumour invasion. MMP is a zinc-dependent proteinase that degrades and remodels the extracellular matrix surrounding tumour cells. Choriocarcinomas have been shown to express increased concentrations of MMP and decreased concentrations of TIMP to help with tumour invasion and metastasis,30 and this finding could explain why choriocarcinoma is highly metastatic if left untreated.
PSTT and ETT PSTT and ETT are rare gestational trophoblastic neoplasms and, so far, have not been well studied at the molecular level, although both tumours have been described in the published work on diagnostic pathology.2 Both forms of these neoplasms can be derived from any type of gestational event, including complete and partial hydatidiform moles.7,31–33 The origin of both tumours has puzzled investigators since their discovery because, unlike choriocarcinomas, PSTTs and ETTs can develop long after the prior gestational events. The trophoblastic nature of PSTT and ETT has been shown by molecular genetic analysis, which shows that they contain new (paternal) alleles not present in adjacent healthy uterine tissue.34–36 Additionally, both tumours express trophoblastassociated markers, including HLA-G and HSD3B1.29 On the basis of published work, most cases of PSTT and ETT are thought to be benign, especially for those tumours that are confined to the uterus, but about 15–25% of cases are malignant and present with local invasion and distant metastasis.37–40 In some cases, recurrent or metastatic PSTT and ETT can occur in patients long after the initial treatment but, unfortunately, analysis of histopathological features and molecular changes has not provided reliable markers to predict the long-term clinical behaviour of PSTT and ETT.41 By contrast to the normal implantation site where invasion of the extravillous (intermediate) trophoblast is tightly regulated and confined to the inner third of the http://oncology.thelancet.com Vol 8 July 2007
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myometrium, tumour cells of PSTT and ETT are highly invasive and infiltrate deeply into the myometrium. Although PSTT and ETT share similar clinical features, careful examination of tumour histology and geneexpression patterns shows that PSTT and ETT are composed of different extravillous (intermediate) trophoblastic cells.3,7,38 During neoplastic transformation, the tumour cells in PSTT and ETT are likely to assume unique differentiation programmes toward the extravillous (intermediate) trophoblast in the implantation site and the extravillous (intermediate) trophoblast in the chorion laeve, respectively (figure 2). Molecularly, PSTT is associated with abnormal expression of cell-cycle regulatory genes, including those for cyclins, cyclin-dependent kinases, and P53.42 Most PSTTs are diploid, based on flow cytometric DNA analysis, but PSTTs might also be triploid and, thus, be derived from a partial hydatidiform mole.31 ETT expresses markers of epithelial cells, including cytokeratin, epithelial membrane antigen, E-cadherin, and EGFR, and this finding is compatible with their characteristic epithelioid growth pattern. The P63 gene, a transcription factor belonging to the P53 family, is expressed in ETT but not in PSTT. P63 has various isoforms that are classified into two groups designated as TAP63 and ∆NP63 isoforms.38 The cytotrophoblast on chorionic villi has been shown to express the ∆NP63 isoform, whereas extravillous (intermediate) trophoblastic cells in the chorion laeve and ETT express the TAP63.38 Further studies are needed to delineate the functional role of TAP63 in ETT. Expression of cyclin E is found in ETT but not in the extravillous (intermediate) trophoblastic cells of the chorion laeve, the normal counterpart of ETT.43 This finding suggests that cyclin E probably plays a part in neoplastic transformation of ETT because an oncogenic role of cyclin E has been shown in other neoplastic diseases.
Hydatidiform moles Hydatidiform moles include complete, partial, and invasive moles, the latter representing a hydatidiform mole that becomes invasive or is exported to a distant anatomical site. Although hydatidiform moles are not classed as true gestational trophoblastic neoplasia, postmolar gestational trophoblastic neoplasia can be seen in many patients with hydatidiform moles, especially complete moles. This finding indicates that hydatidiform moles are the precursor lesions of some gestational trophoblastic neoplasms. Because of the close relationship of these moles with gestational trophoblastic neoplasia, their pathogenesis will be briefly highlighted. Advances in cytogenetics, and in the molecular and immunohistochemical characterisation of hydatidiform moles, have shown a distinct pathogenesis in complete and partial moles. Most complete moles are diploid and http://oncology.thelancet.com Vol 8 July 2007
usually have the karyotype 46,XX with both sets of chromosomes originating from the paternal complement as a result of haploid genome (23X) duplication. This chromosomal composition is referred to as androgenetic, meaning that genetic material is derived exclusively from paternal DNA. Partial moles, however, are generally triploid (XXY) with two sets of chromosomes from the paternal complement and a haploid set from the maternal genetic content. This triploid chromosomal composition is known as diandric or biparental. Understanding the cytogenetic constituents that characterise complete and partial moles is fundamental for the future study of how maternally or paternally imprinted genes contribute to different types of molar pregnancy. Despite the well-established cytogenetic findings associated with different hydatidiform moles, the molecular aetiology underlying the development of molar pregnancy remains elusive. One of the most exciting studies showed that MALP7 is mutated in patients with familial and recurrent biparental moles.44 MALP7 encodes a member of the CATERPILLER protein family that is potentially involved in inflammatory and apoptotic processes. More work is needed to elucidate the possible mechanisms of mutant NALP7 proteins in the pathogenesis of hydatidiform moles, such as during oogenesis and implantation. Additionally, several oncogenes and tumour-suppressor genes have been studied in complete moles. Similar to choriocarcinomas, synergistic upregulation of c-MYC, c-ERB-2, c-FMS, and BCL-2 oncoproteins has been suggested in the pathogenesis of complete moles.10 Mutational analysis of K-ras and P53 did not show mutations in complete moles, but overexpression of P53 and MDM2 proteins has been recorded.9 Furthermore, the role of promoter methylation has been studied in hydatidiform moles and gestational trophoblastic neoplasia. Frequent promoter hypermethylation and decreased expression of PTEN, CDH1, HIC-1, and CDKN2A have been found in hydatidiform moles compared with healthy placentas,23,45 suggesting that inactivating genes that might participate in tumour suppression by promoter methylation could be one of the mechanisms in the development of molar pregnancy. Furthermore, promoter hypermethylation of CDKN2A alone, or combined with CDH1, was significantly correlated with subsequent development of gestational trophoblastic neoplasia.23
New molecular targets with therapeutic potential Choriocarcinoma is among the few human cancers that are highly responsive to chemotherapy and, unlike other malignant tumours, metastatic choriocarcinomas are potentially curable by combined chemotherapy and adjuvant surgical procedures.46,47 Despite this fact, a small but significant proportion of choriocarcinoma patients develop recurrent diseases after primary treatment.48,49 By contrast with choriocarcinoma, PSTT and ETT are generally 645
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more refractory to conventional chemotherapy. Patients with PSTT and ETT might develop metastatic diseases and some patients will have terminal disease.7,37 Therefore, new therapeutic regimens are needed to reduce the toxic effects associated with current chemotherapy—especially the combination regimens, ie, etoposide, methotrexate, dactinomycin; etoposide, methotrexate, dactinomycin, cyclophosphamide, vincristine; and etoposide, cisplatin, etoposide, methotrexate, and dactinomycin—and to salvage the occasional non-operable patients with metastatic chemoresistant diseases. Target-based treatment designed to inactivate molecular pathways that are essential for tumour-cell growth and survival has shown clinical promise. Unlike standard chemotherapy, which affects most proliferating cells, inhibitors that target specific pathways selectively eliminate tumour cells to achieve maximum therapeutic effects and minimum adverse side-effects. These new anticancer agents include gefitinib, a small kinase inhibitor that targets EGFR, and trastuzumab, which is a humanised antibody that targets ERBB2 receptors. Because the merits of molecular targeting have been shown by clinical studies, targeted treatment might also be applicable to treat metastatic gestational trophoblastic neoplasms. In the following sections, several molecular targets that are expressed in gestational trophoblastic neoplasia tissues and cell lines are discussed (table 1). However, there are many other inhibitors that target emerging cancerassociated molecules and new therapeutic approaches, such as anti-human chorionic gonadotropin antibody treatment and immunotherapy, might also prove useful as new therapeutics in gestational trophoblastic neoplasia in future.
c-MYC proto-oncogene The activation and overexpression of the c-MYC protooncogene is one of the most frequent molecular changes in human cancers.50 In trophoblastic lesions, high levels of c-MYC expression have been detected in choriocarcinomas and complete hydatidiform moles, and an increased expression of c-MYC proteins might suppress differentiation and enhance cellular proliferation in gestational trophoblastic neoplasia.10 Direct evidence of tumour dependence on c-MYC for growth and survival comes from elegant studies showing that induction of c-MYC expression results in tumour formation, and that the blocking c-MYC activation generally leads to tumour stasis or regression.51,52 In recent years, the targeting c-MYC oncoproteins has been proposed as a new anticancer regimen.53,54 For example, several strategies that aim to inactivate c-MYC function and expression are being developed. These approaches use: inhibitors that block c-MYC expression, eg, triple-helix-forming oligonucleotides; molecules that disrupt MYC-MAX interaction; agents that block the functions of c-MYC-regulated genes, eg, cationic porphyrins and ornithine decarboxylase; and antisense 646
Molecular targets
Possible drugs
c-MYC
Antisense oligonucleotides
EGFR
Cetuximab, gefitinib, erlotinib
MAPK
CI-1040, PD59089
MMP
Marimastat
mTOR
AP23573, RAD001, CCI-779
MAPK=mitogen-activated protein kinase. mTOR=mammalian target of rapamycin.
Table 1: Molecular targets and examples of possible clinical drugs for gestational trophoblastic neoplasia
oligonucleotides and siRNA that silence c-MYC expression.54,55 Some of the antisense oligonucleotides have successfully completed phase I clinical trials and are at an advanced stage of drug development. Therefore, future patients with metastatic gestational trophoblastic neoplasia could benefit from anti-MYC agents in combination with conventional treatment.
EGFR EGFR is a transmembrane-receptor tyrosine kinase that belongs to the ERBB-related kinase family. Many epithelial tumours overexpress EGFR and harbour somatic mutations, leading to constitutive activation of the EGFR pathway.56 High levels of EGFR expression have been detected in choriocarcinomas, PSTT, and ETT.7,17,57 Additionally, the expression levels of EGFR in choriocarcinomas and complete hydatidiform moles were significantly higher than those of trophoblastic cells in healthy placentas and partial hydratidiform moles.17 Further analysis showed that intense EGFR immunoreactivity in the extravillous (intermediate) trophoblastic cells of a complete mole was seen to be significantly correlated with the development of a persistent postmolar gestational trophoblastic neoplasm.17 Expression of ERBB-3 and ERBB-4, however, did not differ between gestational trophoblastic neoplasms and healthy placental tissues.17 The anti-EGFR regimen has become the prototype of target-based treatment for cancer. Clinical and survival benefits of anti-EGFR treatment have been shown in patients with non-small-cell lung cancer and other neoplastic diseases. The repertoire of cancer types that might respond to anti-EGFR treatment is expanding, including head and neck and pancreatic carcinomas. Drugs that target EGFR, including cetuximab, gefitinib, and erlotinib, have emerged as effective treatments against various malignant diseases. Therefore, patients with metastatic gestational trophoblastic neoplasia might also benefit from anti-EGFR treatment in combination with other anticancer agents.
Mitogen-activated protein kinase Mitogen-activated protein kinase (MAPK) has a role in signal transduction, and activation (phosphorylation) of http://oncology.thelancet.com Vol 8 July 2007
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MAPK results in the regulation of downstream effectors to coordinate various cellular activities, including proliferation, differentiation, apoptosis, angiogenesis, and migration. Aberrant molecular changes that result in the constitutive activation of MAPK contribute to tumour development, with activation being caused by either a genetic or epigenetic event. For example, activating mutations in KRAS and BRAF, the upstream genes of MAPK, have been found in various human carcinomas.58,59 Additionally, MAPK activation can result from growth-factor stimulation in the tumour microenvironment.60 Kobel and co-workers61 studied MAPK activation in PSTT tissues using immunohistochemistry. They showed that the activated (phosphorylated) form of MAPK was expressed in 84% of cases, but not in healthy extravillous (intermediate) trophoblastic cells, the normal counterparts of PSTT.61 However, because the total concentration of MAPK was similar between PSTT and healthy extravillous (intermediate) trophoblastic cells, the investigators proposed that hyperactive MAPK was not likely to be the result of MAPK-gene upregulation, but rather a result of other epigenetic events that lead to its activation. To characterise the biological role of MAPK activation in PSTT, they established the first PSTT cell line, implantation-site trophoblastic cells, in which the cells were highly motile and invasive in vitro. Treatment with the mitogene inhibitors, CI-1040 and PD59089, which prevent activation of MAPK, significantly reduced the motility and invasion of IST-2 cells in vitro.61 These findings suggest a functional role of MAPK activation in the motility and invasion of PSTT cells. Because CI-1040 and other mitogen inhibitors that prevent MAPK activation have already been tested in clinical trials,62–64 these inhibitors could also be assessed for treating patients with metastatic PSTT who have not responded to conventional treatment. Whether or not inhibitors of the MAPK pathway can reduce the invasiveness and metastatic potential of PSTT would be interesting to learn.
Mammalian target-of-rapamycin (mTOR) mTOR is a member of the phosphatidyl inositol 3´ kinase (PI3K) family and is one of the key molecules that participate in the cellular signalling network. On stimulation by miscellaneous growth signals, a cascade of signalling occurs from PI3K, Akt, and mTOR to P70S6 kinase, resulting in a variety of cellular responses, including cellular migration and proliferation. Osteopontin, a cell-surface phosphorylated glycoprotein, represents one of the signals that trigger activation of the PI3K/mTOR/P70S6 pathway and has been known to have an important role during implantation by aiding attachment of the trophectoderm of a blastocyst to the surface epithelium of the endometrium.65 In gestational trophoblastic neoplasia, osteopontin is highly expressed in choriocarcinoma tissues and the derived cell lines.66,67 http://oncology.thelancet.com Vol 8 July 2007
This glycoprotein is co-localised with an adhesion molecule, called carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1), in the extravillous (intermediate) trophoblast of a healthy placenta and also, probably, in choriocarcinoma cells.68 The expression of osteopontin has been shown to enhance the invasion of CAECAM1-expressing trophoblastic cells.68 Likewise, expression of highly phosphorylated osteopontin activates the PI3K/mTOR/P70S6 kinase pathway and triggers migration of CAECAM1-expressing trophoblastic cells in choriocarcinoma cells grown in culture. Therefore, this tumour phenotype could be suppressed by the compounds LY294002 and rapamycin, which inhibit PI3K and mTOR, respectively.66 Over the past years, several mTOR inhibitors have been developed and tested in preclinical tumour models,69,70 and, more importantly, several of these inhibitors, including AP23573, RAD001, and CCI-779, have been tested in clinical trials as single agents or in combination with other drugs in patients with solid or haematological neoplastic diseases.71,72 Ongoing clinical studies involving mTOR inhibitors will, hopefully, identify the efficacy of these drugs in treating patients with cancer and provide the foundation for future use of mTOR inhibitors in patients with gestational trophoblastic neoplasia.
Matrix metalloproteinase (MMP) As previously discussed, the upregulation of MMP and downregulation of TIMP in choriocarcinomas suggest that patients with metastatic, chemoresistant gestational trophoblastic neoplasia might benefit from MMP inhibitors. Marimastat is an MMP inhibitor and preclinical studies in animal-tumour models have shown that MMP inhibitors restrict the growth of solid tumours, inhibit metastatic potential, and block tumour angiogenesis. This inhibitor is an orally bioavailable drug that has also been assessed in clinical trials of patients with advanced non-small-cell lung cancer, relapsed prostate cancer, glioblastoma multiforme, and several other types of cancer. In general, modest toxic effects have been reported, on the basis of several phase I clinical trials,73 and marimastat has been shown to delay progression in patients with biochemically relapsed prostate cancer.74 Further studies need to be done to establish the efficacy of marimastat and other new MMP inhibitors in treating metastatic choriocarcinomas, and to find out if the clinical outcome can be improved in patients with choriocarcinoma who do not respond to conventional chemotherapy.
Promises and challenges of target-based treatment The molecules discussed above have been shown to be expressed in neoplastic trophoblastic cells and, thus, their inhibitors represent attractive new therapeutics in chemoresistant gestational trophoblastic neoplasia. However, there are several challenges ahead before 647
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molecular target-based treatment becomes a reality for this group of diseases. First, the expression frequency of the target genes in a large series of neoplastic tissues is not yet known, because such clinical samples are rare for study. Furthermore, studies of gestational trophoblastic diseases have primarily focused on hydatidiform moles and choriocarcinoma. PSTT and ETT have rarely been studied so knowledge of these forms of gestational trophoblastic neoplasia is limited. Second, it is crucial to question whether the anticancer effects seen in preclinical models using gestational trophoblastic neoplasia cell lines can be translated to treatment efficacy in vivo. Third, the clinical usefulness of applying molecular inhibitors in gestational trophoblastic neoplasia depends on careful correlation studies between clinical response and the genetic and molecular changes of genes and the pathways they control, respectively. For example, mutational analysis of EGFR and members of the PI3K and MAPK pathways would be important, in addition to the assessment of their gene-expression levels. The results of such genotype–phenotype correlation studies would be essential for the development of tailored treatment in order to keep potential side effects to a minimum and optimise therapeutic effects. Fourth, given the fact that gestational trophoblastic neoplasia is rare and most patients can be cured by the current treatment, recruitment of patients with this disease in clinical trials can be challenging because only a limited number of patients qualify and are available for such studies. Nevertheless, drugs that inhibit the expression or function of the molecular targets have been assessed in both preclinical and clinical trials of various cancer types. With results from ongoing studies soon to emerge and new inhibitors in the development pipeline, future patients with gestational trophoblastic neoplasia should benefit from new waves of target-based treatment that have been primarily developed to treat other types of cancer. Effective multicentre collaborative networks should be developed or better orchestrated to aid with these clinical studies and to share valuable clinical specimens, in order to characterise emerging targets with therapeutic potential in gestational trophoblastic neoplasia. With this continuous effort, we will be wellpositioned to further establish the pathogenesis of this rare but intriguing disease and offer a better treatment alternative for patients.
Conclusion Recent advances in molecular studies of trophoblastic cells in healthy placentas and in gestational trophoblastic neoplasms have indicated that these neoplasms recapitulate normal trophoblast differentiation seen in early developing placentas. Relating gestational trophoblastic neoplasia with normal trophoblastic differentiation programmes highlights the fundamental nature of this group of diseases and provides the framework for future molecular studies. These new findings not only shed 648
Search strategy and selection criteria Published and unpublished data for this review were identified by searching PubMed. Key words used to identify papers included, “trophoblast”, “gestation”, “neoplasm”, “metastasis”, “target”, “therapy”, “pathogenesis”, “choriocarcinoma”, “placenta”, “mole”, and “epithelioid”. The search strategy was not limited by year of publication, but only articles published in English were selected.
light on the pathogenesis of gestational trophoblastic neoplasia but also provide an array of candidate molecules with therapeutic potential. Characterisation of the molecular changes that have been targeted to treat several other human cancers in gestational trophoblastic neoplasia would open an exciting avenue for target-based treatment of metastatic forms of this disease that are refractory to conventional treatment. Conflicts of interest The author declared no conflict of interest. References 1 Genest DR, Berkowitz RS, Fisher RA. Gestational trophoblastic disease. Lyon: IARC press, 2003. 2 Shih IeM, Kurman RJ. Molecular basis of gestational trophoblastic diseases. Curr Mol Med 2002; 2: 1–12. 3 Shih IM, Kurman RJ. The pathology of intermediate trophoblastic tumors and tumor-like lesions. Int J Gynecol Pathol 2001; 20: 31–47. 4 Soper JT. Gestational trophoblastic disease. Obstet Gynecol 2006; 108: 176–87. 5 Mao TL, Kurman RJ, Huang CC, Lin MC, Shih IM. Immunohistochemistry of choriocarcinoma: an aid in differential diagnosis and in elucidating pathogenesis. Am J Surg Pathol 2007; (in press). 6 Shih IM, Seidman JD, Kurman RJ. Placental site nodule and characterization of distinctive types of intermediate trophoblast. Hum Pathol 1999; 30: 687–94. 7 Shih IM, Kurman RJ. Epithelioid trophoblastic tumor: a neoplasm distinct from choriocarcinoma and placental site trophoblastic tumor simulating carcinoma. Am J Surg Pathol 1998; 22: 1393–403. 8 Mazur MT. Metastatic gestational choriocarcinoma. Unusual pathologic variant following therapy. Cancer 1989; 63: 1370–77. 9 Fulop V, Mok SC, Genest DR, Gati I, Doszpod J, Berkowitz RS. p53, p21, Rb and mdm2 oncoproteins. Expression in normal placenta, partial and complete mole, and choriocarcinoma. J Reprod Med 1998; 43: 119–27. 10 Fulop V, Mok SC, Genest DR, Szigetvari I, Cseh I, Berkowitz RS. c-myc, c-erbB-2, c-fms and bcl-2 oncoproteins. Expression in normal placenta, partial and complete mole, and choriocarcinoma. J Reprod Med 1998; 43: 101–10. 11 Chen CA, Chen YH, Chen TM, et al. Infrequent mutation in tumor suppressor gene p53 in gestational trophoblastic neoplasia. Carcinogenesis 1994; 15: 2221–23. 12 Shi Y-F, Xie X, Zhao C-L, et al. Lack of mutation in tumoursuppressor gene p53 in gestational trophoblastic tumours. Br J Cancer 1996; 73: 1216–19. 13 Landers JE, Haines DS, Strauss JF 3rd, George DL. Enhanced translation: a novel mechanism of mdm2 oncogene overexpression identified in human tumor cells. Oncogene 1994; 9: 2745–50. 14 Wang H, Zeng X, Oliver P, et al. MDM2 oncogene as a target for cancer therapy: an antisense approach. Int J Oncol 1999; 15: 653–60. 15 Asanoma K, Kato H, Inoue T, Matsuda T, Wake N. Analysis of a candidate gene associated with growth suppression of choriocarcinoma and differentiation of trophoblasts. J Reprod Med 2004; 49: 617–26.
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Asanoma K, Matsuda T, Kondo H, et al. NECC1, a candidate choriocarcinoma suppressor gene that encodes a homeodomain consensus motif. Genomics 2003; 81: 15–25. Tuncer ZS, Vegh GL, Fulop V, Genest DR, Mok SC, Berkowitz RS. Expression of epidermal growth factor receptor-related family products in gestational trophoblastic diseases and normal placenta and its relationship with development of postmolar tumor. Gynecol Oncol 2000; 77: 389–93. Fulop V, Colitti CV, Genest D, et al. DOC-2/hDab2, a candidate tumor suppressor gene involved in the development of gestational trophoblastic diseases. Oncogene 1998; 17: 419–24. Fulop V, Mok SC, Berkowitz RS. Molecular biology of gestational trophoblastic neoplasia: a review. J Reprod Med 2004; 49: 415–22. Matsuda T, Sasaki M, Kato H, et al. Human chromosome 7 carries a putative tumor suppressor gene(s) involved in choriocarcinoma. Oncogene 1997; 15: 2773–81. Burke B, Sebire NJ, Moss J, et al. Evaluation of deletions in 7q11.2 and 8p12-p21 as prognostic indicators of tumour development following molar pregnancy. Gynecol Oncol 2006; 103: 642–48. Stahle-Backdhal M, Inoue M, Zedenius J, et al. Decreased expression of Ras GTPase activating protein in human trophoblastic tumors. Am J Pathol 1995; 146: 1073–78. Xue WC, Chan KY, Feng HC, et al. Promoter hypermethylation of multiple genes in hydatidiform mole and choriocarcinoma. J Mol Diagn 2004; 6: 326–34. Vegh GL, Fulop V, Liu Y, et al. Differential gene expression pattern between normal human trophoblast and choriocarcinoma cell lines: downregulation of heat shock protein-27 in choriocarcinoma in vitro and in vivo. Gynecol Oncol 1999; 75: 391–96. Ciocca DR, Oesterreich S, Chamness GC, McGuire WL, Fuqua SA. Biological and clinical implications of heat shock protein 27,000 (Hsp27): a review. J Natl Cancer Inst 1993; 85: 1558–70. Richards EH, Hickey E, Weber L, Master JR. Effect of overexpression of the small heat shock protein HSP27 on the heat and drug sensitivities of human testis tumor cells. Cancer Res 1996; 56: 2446–51. Garrido C, Ottavi P, Fromentin A, et al. HSP27 as a mediator of confluence-dependent resistance to cell death induced by anticancer drugs. Cancer Res 1997; 57: 2661–67. Shih IM. Application of human leukocyte antigen-G expression in the diagnosis of human cancer. Hum Immunol 2007; 68: 272–76. Singer G, Kurman RJ, McMaster M, Shih IeM. HLA-G immunoreactivity is specific for intermediate trophoblast in gestational trophoblastic disease and can serve as a useful marker in differential diagnosis. Am J Surg Pathol 2002; 26: 914–20. Vegh GL, Selcuk Tuncer Z, Fulop V, Genest DR, Mok SC, Berkowitz RS. Matrix metalloproteinases and their inhibitors in gestational trophoblastic diseases and normal placenta. Gynecol Oncol 1999; 75: 248–53. Palmieri C, Fisher RA, Sebire NJ, et al. Placental site trophoblastic tumour arising from a partial hydatidiform mole. Lancet 2005; 366: 688. Felemban AA, Bakri YN, Alkharif HA, Altuwaijri SM, Shalhoub J, Berkowitz RS. Complete molar pregnancy. Clinical trends at King Fahad Hospital, Riyadh, Kingdom of Saudi Arabia. J Reprod Med 1998; 43: 11–13. Moore-Maxwell CA, Robboy SJ. Placental site trophoblastic tumor arising from antecedent molar pregnancy. Gynecol Oncol 2004; 92: 708–12. Oldt RJ 3rd, Kurman RJ, Shih IeM. Molecular genetic analysis of placental site trophoblastic tumors and epithelioid trophoblastic tumors confirms their trophoblastic origin. Am J Pathol 2002; 161: 1033–37. Fisher RA, Paradinas FJ, Newlands ES, Boxer GM. Genetic evidence that placental site trophoblastic tumours can originate from a hydatidiform mole or a normal conceptus. Br J Cancer 1992; 65: 355–58. Arima T, Imamura T, Sakuragi N, et al. Malignant trophoblastic neoplasms with different modes of origin. Cancer Genet Cytogenet 1995; 85: 5–15. Chang YL, Chang TC, Hsueh S, et al. Prognostic factors and treatment for placental site trophoblastic tumor- report of 3 cases and analysis of 88 cases. Gynecol Oncol 1999; 73: 216–22.
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Shih IM, Kurman RJ. p63 expression is useful in the distinction of epithelioid trophoblastic and placental site trophoblastic tumors by profiling trophoblastic subpopulations. Am J Surg Pathol 2004; 28: 1177–83. Shih IeM, Kurman RJ. Molecular basis of gestational trophoblastic diseases. Curr Mol Med 2002; 2: 1–12. Shih I-M, Mazur MT, Kurman RJ. Gestational trophoblastic disease. In: Kurman RJ, ed. Blaustein’s pathology of the female genital tract, 5th edn. New York: Springer-Verlag, 2002: 1193–247. Shih IM, Kurman RJ. Placental site trophoblastic tumor--past as prologue. Gynecol Oncol 2001; 82: 413–14. Ichikawa N, Zhai YL, Shiozawa T, et al. Immunohistochemical analysis of cell cycle regulatory gene products in normal trophoblast and placental site trophoblastic tumor. Int J Gynecol Pathol 1998; 17: 235–40. Mao TL, Seidman JD, Kurman RJ, Shih IeM. Cyclin E and p16 immunoreactivity in epithelioid trophoblastic tumor–an aid in differential diagnosis. Am J Surg Pathol 2006; 30: 1105–10. Murdoch S, Djuric U, Mazhar B, et al. Mutations in NALP7 cause recurrent hydatidiform moles and reproductive wastage in humans. Nat Genet 2006; 38: 300–02. Chen H, Ye D, Xie X, Lu W, Zhu C, Chen X. PTEN promoter methylation and protein expression in normal early placentas and hydatidiform moles. J Soc Gynecol Investig 2005; 12: 214–17. Cagayan MS, Lu-Lasala LR. Management of gestational trophoblastic neoplasia with metastasis to the central nervous system: a 12-year review at the Philippine General Hospital. J Reprod Med 2006; 51: 785–92. Lurain JR, Singh DK, Schink JC. Primary treatment of metastatic high-risk gestational trophoblastic neoplasia with EMA-CO chemotherapy. J Reprod Med 2006; 51: 767–72. Ngan HY, Tam KF, Lam KW, Chan KK. Relapsed gestational trophoblastic neoplasia: a 20-year experience. J Reprod Med 2006; 51: 829–34. Newlands ES. The management of recurrent and drug-resistant gestational trophoblastic neoplasia (GTN). Best Pract Res Clin Obstet Gynaecol 2003; 17: 905–23. Dang CV, O’Donnell KA, Zeller KI, Nguyen T, Osthus RC, Li F. The c-Myc target gene network. Semin Cancer Biol 2006; 16: 253–64. Jonkers J, Berns A. Oncogene addiction: sometimes a temporary slavery. Cancer Cell 2004; 6: 535–38. Yu D, Dews M, Park A, Tobias JW, Thomas-Tikhonenko A. Inactivation of Myc in murine two-hit B lymphomas causes dormancy with elevated levels of interleukin 10 receptor and CD20: implications for adjuvant therapies. Cancer Res 2005; 65: 5454–61. Vita M, Henriksson M. The Myc oncoprotein as a therapeutic target for human cancer. Semin Cancer Biol 2006; 16: 318–30. Ponzielli R, Katz S, Barsyte-Lovejoy D, Penn LZ. Cancer therapeutics: targeting the dark side of Myc. Eur J Cancer 2005; 41: 2485–501. Nilsson JA, Keller UB, Baudino TA, et al. Targeting ornithine decarboxylase in Myc-induced lymphomagenesis prevents tumor formation. Cancer Cell 2005; 7: 433–44. Mendelsohn J, Baselga J. Epidermal growth factor receptor targeting in cancer. Semin Oncol 2006; 33: 369–85. Muller-Hocker J, Obernitz N, Johannes A, Lohrs U. P53 gene product and EGF-receptor are highly expressed in placental site trophoblastic tumor. Hum Pathol 1997; 28: 1302–06. Singer G, Oldt R 3rd, Cohen Y, et al. Mutations in BRAF and KRAS characterize the development of low-grade ovarian serous carcinoma. J Natl Cancer Inst 2003; 95: 484–86. Cohen Y, Xing M, Mambo E, et al. BRAF mutation in papillary thyroid carcinoma. J Natl Cancer Inst 2003; 95: 625–27. Satyamoorthy K, Li G, Gerrero MR, et al. Constitutive mitogenactivated protein kinase activation in melanoma is mediated by both BRAF mutations and autocrine growth factor stimulation. Cancer Res 2003; 63: 756–59. Kobel M, Pohl G, Schmitt WD, Hauptmann S, Wang TL, Shih IeM. Activation of mitogen-activated protein kinase is required for migration and invasion of placental site trophoblastic tumor. Am J Pathol 2005; 167: 879–85.
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Rinehart J, Adjei AA, Lorusso PM, et al. Multicenter phase II study of the oral MEK inhibitor, CI-1040, in patients with advanced nonsmall-cell lung, breast, colon, and pancreatic cancer. J Clin Oncol 2004; 22: 4456–62. Lorusso PM, Adjei AA, Varterasian M, et al. Phase I and pharmacodynamic study of the oral MEK inhibitor CI-1040 in patients with advanced malignancies. J Clin Oncol 2005; 23: 5281–93. Thompson N, Lyons J. Recent progress in targeting the Raf/MEK/ ERK pathway with inhibitors in cancer drug discovery. Curr Opin Pharmacol 2005; 5: 350–56. Martin PM, Sutherland AE. Exogenous amino acids regulate trophectoderm differentiation in the mouse blastocyst through an mTOR-dependent pathway. Dev Biol 2001; 240: 182–93. Al-Shami R, Sorensen ES, Ek-Rylander B, Andersson G, Carson DD, Farach-Carson MC. Phosphorylated osteopontin promotes migration of human choriocarcinoma cells via a p70 S6 kinasedependent pathway. J Cell Biochem 2005; 94: 1218–33. Briese J, Oberndorfer M, Schulte HM, Loning T, Bamberger AM. Osteopontin expression in gestational trophoblastic diseases: correlation with expression of the adhesion molecule, CEACAM1. Int J Gynecol Pathol 2005; 24: 271–76. Briese J, Oberndorfer M, Patschenik C, et al. Osteopontin is colocalized with the adhesion molecule CEACAM1 in the extravillous trophoblast of the human placenta and enhances invasion of CEACAM1-expressing placental cells. J Clin Endocrinol Metab 2005; 90: 5407–13.
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Gestational trophoblastic neoplasia—pathogenesis and potential therapeutic targets Ie-Ming Shih Lancet Oncol 2007; 8: 642–50 Departments of Pathology, Oncology, and Gynecology and Obstetrics, Johns Hopkins Medical Institutions, Baltimore, MD, USA (I-M Shih MD) Correspondence to: Dr Ie-Ming Shih [email protected]
Gestational trophoblastic neoplasia comprises a unique group of human neoplastic diseases that derive from fetal trophoblastic tissues and represent semiallografts in patients. This group is composed of choriocarcinoma, placentalsite trophoblastic tumour, and epithelioid trophoblastic tumour, and many forms are derived from the precursor lesions, hydatidiform moles. Although most patients with gestational trophoblastic neoplasia are cured by chemotherapy and tumour resection, some patients suffer from metastatic diseases that are refractory to conventional chemotherapy. Therefore, new therapeutic regimens are needed to reduce the toxic effects associated with current chemotherapy and to salvage the occasional non-operable patients with recurrent and chemoresistant disease. Until the fundamental biology of gestational trophoblastic neoplasia becomes more clearly understood, development of a new treatment will remain empirical. This review will briefly summarise the recent advances in understanding the molecular aetiology of this group of diseases and highlight the molecules that can be potentially used for therapeutic targets to treat metastatic gestational trophoblastic neoplasia.
Introduction Gestational trophoblastic diseases represent a spectrum of related disorders including benign trophoblastic lesions, premalignant hydatidiform moles, clinically malignant invasive hydatidiform moles, and neoplastic diseases. Hydatidiform moles, especially the invasive moles, are sometimes clinically considered as gestational trophoblastic neoplasia because they can locally invade and distantly metastasise, but, biologically, they represent abnormally formed placental tissues rather than true neoplasia. Therefore, from the perspectives of pathobiology, gestational trophoblastic diseases can be broadly divided into three groups (panel), modified from the 2003 WHO classification1: benign trophoblastic lesions (placental-site nodule and exaggerated placental reaction), which are non-neoplastic lesions; hydatidiform moles (complete, partial, and invasive moles), which are
Figure 1: ETT (arrow) in a hysterectomy specimen of a 32-year-old woman ETT was first diagnosed based on endometrial curettage and patient subsequently underwent total hysterectomy. 1 year after surgery, metastatic ETT developed in the lung and was treated with chemotherapy and lung resection. A second recurrent tumour, resistant to prior chemotherapy, developed 1 year later.
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aberrant placental derivatives; and true gestational trophoblastic neoplasia.2,3 The term gestational trophoblastic neoplasia encompasses a group of interrelated but distinct tumours that include choriocarcinoma, placental-site trophoblastic tumour (PSTT), and epithelioid trophoblastic tumour (ETT; figure 1). Gestational trophoblastic neoplasms are unique among human neoplastic diseases because they are genetically related to fetal tissues (ie, trophoblastic cells from placentas) and, therefore, represent semiallografts in patients. Although there has been substantial interest in exploring the pathogenesis of these neoplasms, studies on their molecular aspects are challenging for several reasons. First, gestational trophoblastic neoplasms are rare diseases and the cumulative incidence rate for gestational choriocarcinoma, for example, continues to decline. As a result, tissue specimens, especially those from fresh tumours, are scarce for molecular analysis. Second, many metastatic gestational trophoblastic neoplasms, which are clinically classified under the rubric of persistent gestational trophoblastic diseases are not usually obtained for pathological classification and characterisation. Therefore, their natures remain intriguing, because whether persistent gestational trophoblastic disease represents an invasive hydatidiform mole, a metastatic choriocarcinoma, PSTT, or ETT is not known. An absence of such clinicopathological correlation compromises attempts to further understand the biological nature of gestational trophoblastic neoplasia. Third, because of the above reasons, reagents, such as cell lines and animal models, are limited for mechanistic studies. For example, only a few choriocarcinoma cell lines and one PSTT cell line are currently available for molecular and cell-biology studies. Despite these challenges, progress has been made in recent years towards a better understanding of the molecular aetiology underlying the development of these neoplasms. This review will briefly summarise recent http://oncology.thelancet.com Vol 8 July 2007
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Panel: Gestational trophoblastic diseases Benign trophoblastic lesions Exaggerated placental reaction Placental-site nodule Hydatidiform moles (abnormally formed placentas) Complete mole Partial mole Invasive mole Trophoblastic neoplasia Choriocarcinoma Placental-site trophoblastic tumour Epithelioid tophoblastic tumour
advances and highlight the molecules that could potentially be used for therapeutic targets to treat metastatic gestational trophoblastic neoplasia.
Pathogenesis of trophoblastic tumours Previous clinical, pathological, and molecular studies have provided fundamental insights into the pathogenesis of hydatidiform moles,4 but the molecular and cellular basis in the development of true gestational trophoblastic neoplasia remain poorly understood. Gestational trophoblastic neoplasia has long been thought to be a homogeneous group of diseases arising from neoplastic transformation of trophoblastic cells. However, recent clinicopathological studies have provided new evidence that there are at least three distinct types of gestational trophoblastic neoplasm including the most common type, choriocarcinoma, and the less common ones, PSTT and ETT. Molecular analysis of gestational trophoblastic neoplasia is largely based on the characterisation of gene-expression profiles in various types of these neoplasms and the reference of their unique gene-expression patterns to different trophoblastic subpopulations in healthy early placentas.2 The main conclusion from these studies is that, after neoplastic transformation of trophoblastic stem cells, presumably cytotrophoblast stem cells, specific differentiation programmes dictate the type of trophoblastic tumour that develops (figure 2).5 Of interest, is the fact that these patterns of differentiation in gestational trophoblastic neoplasia recapitulate the stages of early placental development.2,3,5–7 Choriocarcinoma is composed of variable amounts of neoplastic cytotrophoblast, syncytiotrophoblast, and extravillous (intermediate) trophoblast and resembles the previllous blastocyst, which is composed of a similar mixture of trophoblastic subpopulations. By contrast, the neoplastic cytotrophoblast in PSTT differentiates mainly into extravillous (intermediate) trophoblastic cells in an implantation site whereas the neoplastic cytotrophoblast in ETT differentiates into chorionic-type extravillous (intermediate) trophoblastic cells in the chorion laeve.5 http://oncology.thelancet.com Vol 8 July 2007
According to this model, choriocarcinoma is the most primitive trophoblastic tumour whereas PSTT and ETT are relatively more differentiated. This hypothesis explains the existence of gestational trophoblastic neoplasia with a mixed histological feature, including choriocarcinoma, PSTT, and ETT. This new model might also help to explain the findings previously reported by Mazur8 who described ETT that occurred after intense chemotherapy of metastatic choriocarcinomas in the lung. In the patients described by Mazur, it is plausible that chemotherapeutic agents allow choriocarcinoma cells to differentiate into an ETT phenotype, which is more refractory to chemotherapy than the original choriocarcinoma counterpart. As a result, ETT predominated the choriocarcinoma after chemotherapy. Alongside studies of the histogenesis of gestational trophoblastic neoplasia, several studies have focused on the biology of these neoplasms by characterising the expressions of genes that are important in tumorigenesis. The pathogenesis of each type of neoplasm will now be summarised. Furthermore, advances in the pathogenesis of hydatidiform moles will be briefly highlighted because hydatidiform moles represent the precursor lesions in some gestational trophoblastic neoplasms and invasive hydatidiform moles are, sometimes, clinically considered as gestational trophoblastic neoplasia because they can locally invade and distantly metastasize.
Choriocarcinoma Gestational choriocarcinoma is a highly malignant epithelial tumour that can be associated with any type of gestational event, most often a complete hydatidiform mole.4 Several molecular studies have been done to establish the expression of tumour-associated proteins in choriocarcinoma. In the P53 pathway, overexpression of
Syncytiotrophoblast
Cytotrophoblast
Normal trophoblast Extravillous trophoblast (intermediate trophoblast) at implantation site
Extravillous trophoblast (intermediate trophoblast) at chorion laeve
Neoplastic transformation
Trophoblastic neoplasia
Placental-site trophoblastic tumour
Choriocarcinoma
Epithelial trophoblastic tumour
Figure 2: Proposed model of pathogenesis for gestational trophoblastic neoplasia
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the P53 protein has been detected in choriocarcinoma,9 but mutational analysis of TP53 has failed to show somatic mutations.10–12 The P53-associated protein, MDM2, has also been seen to be overexpressed in choriocarcinomas.9,13 Because upregulated MDM2 protein is associated with various human cancers, overexpression of MDM2 in choriocarcinomas might overcome growth suppression by wild-type P53 proteins and, therefore, contribute to the development of choriocarcinoma. To show the biological role of MDM2 in choriocarcinoma, Wang and co-workers14 used an MDM2 antisense oligonucleotide and showed a synergistic antitumour effect with DNA-damaging agents in JAR choriocarcinoma xenografts in mice. By use of a PCR-based subtracted fragmentary cDNA library between normal chorionic villi and a choriocarcinoma cell line, Asanoma and colleagues15,16 identified a novel homeobox gene designated NECC1 (located on 4q11-q12) that might be involved in the development of choriocarcinoma. In this study, the expression of NECC1 was lost in all choriocarcinoma cell lines and most choriocarcinoma tissues, but healthy adult tissues, including trophoblastic cells in healthy placentas, expressed abundant NECC1. The investigators further showed that engineered expression of NECC1 in choriocarcinoma cell lines suppressed tumorigenicity and induced terminal differentiation.15,16 Other genes that are potentially involved in the development of choriocarcinoma include epidermal growth factor receptor (EGFR),17 DOC-2/hDab2 (a candidate tumour suppressor gene),18,19 putative tumour suppressor loci at chromosomes 7p12-7q11.2320 and 8p12-p21,21 and the gene for ras GTPase activating protein.22 Synergistic upregulation of c-MYC, c-ERB-2, c-FMS, and BCL-2 oncoproteins have also been suggested to have an important role in the pathogenesis of choriocarcinoma.10 Conversely, promoter hypermethylation of E-cadherin, HIC-1, P16, and TIMP3 has been frequently seen in choriocarcinomas, suggesting that downregulation of these genes by promoter methylation could be involved in the development of choriocarcinomas.23 In addition to the candidate approaches described above, Vegh and colleagues24 compared the gene-expression pattern between healthy trophoblast and choriocarcinoma cell lines using cDNA microarrays. In their study, one of the downregulated genes in choriocarcinoma cells, HSP-27, was selected for characterisation because the HSP-27 protein has been implicated in the development of several human neoplastic diseases.25 The investigators noted that choriocarcinoma tissues had a lower level of HSP-27 expression compared with trophoblastic cells in early placentas. This downregulation of HSP-27 might contribute to the high sensitivity of choriocarcinomas to chemotherapeutic agents because HSP-27 proteins have been shown to contribute to drug resistance in malignant cells.26,27 644
The development of a choriocarcinoma might also be related to tumour microenvironment—eg, immune regulation and stromal tissues. HLA-G, a non-classic MHC class I molecule, has been shown to participate in immune regulation and response. The membrane-bound and secreted HLA-G molecules are able to inactivate the local immune response and, therefore, help tumour cells escape from immune surveillance. Trophoblastic cells in normal placentas and gestational trophoblastic neoplasms have been shown to upregulate HLA-G expression.28,29 In fact, in the human cancers analysed, gestational trophoblastic neoplasms, including choriocarcinoma, contain the highest levels of HLA-G expression.28 Therefore, HLA-G expressed in choriocarcinomas might assist tumour cells to escape from the host immune recognition and promote tumour growth.29 Another explanation for tumour growth could be matrix metalloproteinases (MMP) and tissue inhibitors of metalloproteinases (TIMPs), which have a key role in tumour invasion. MMP is a zinc-dependent proteinase that degrades and remodels the extracellular matrix surrounding tumour cells. Choriocarcinomas have been shown to express increased concentrations of MMP and decreased concentrations of TIMP to help with tumour invasion and metastasis,30 and this finding could explain why choriocarcinoma is highly metastatic if left untreated.
PSTT and ETT PSTT and ETT are rare gestational trophoblastic neoplasms and, so far, have not been well studied at the molecular level, although both tumours have been described in the published work on diagnostic pathology.2 Both forms of these neoplasms can be derived from any type of gestational event, including complete and partial hydatidiform moles.7,31–33 The origin of both tumours has puzzled investigators since their discovery because, unlike choriocarcinomas, PSTTs and ETTs can develop long after the prior gestational events. The trophoblastic nature of PSTT and ETT has been shown by molecular genetic analysis, which shows that they contain new (paternal) alleles not present in adjacent healthy uterine tissue.34–36 Additionally, both tumours express trophoblastassociated markers, including HLA-G and HSD3B1.29 On the basis of published work, most cases of PSTT and ETT are thought to be benign, especially for those tumours that are confined to the uterus, but about 15–25% of cases are malignant and present with local invasion and distant metastasis.37–40 In some cases, recurrent or metastatic PSTT and ETT can occur in patients long after the initial treatment but, unfortunately, analysis of histopathological features and molecular changes has not provided reliable markers to predict the long-term clinical behaviour of PSTT and ETT.41 By contrast to the normal implantation site where invasion of the extravillous (intermediate) trophoblast is tightly regulated and confined to the inner third of the http://oncology.thelancet.com Vol 8 July 2007
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myometrium, tumour cells of PSTT and ETT are highly invasive and infiltrate deeply into the myometrium. Although PSTT and ETT share similar clinical features, careful examination of tumour histology and geneexpression patterns shows that PSTT and ETT are composed of different extravillous (intermediate) trophoblastic cells.3,7,38 During neoplastic transformation, the tumour cells in PSTT and ETT are likely to assume unique differentiation programmes toward the extravillous (intermediate) trophoblast in the implantation site and the extravillous (intermediate) trophoblast in the chorion laeve, respectively (figure 2). Molecularly, PSTT is associated with abnormal expression of cell-cycle regulatory genes, including those for cyclins, cyclin-dependent kinases, and P53.42 Most PSTTs are diploid, based on flow cytometric DNA analysis, but PSTTs might also be triploid and, thus, be derived from a partial hydatidiform mole.31 ETT expresses markers of epithelial cells, including cytokeratin, epithelial membrane antigen, E-cadherin, and EGFR, and this finding is compatible with their characteristic epithelioid growth pattern. The P63 gene, a transcription factor belonging to the P53 family, is expressed in ETT but not in PSTT. P63 has various isoforms that are classified into two groups designated as TAP63 and ∆NP63 isoforms.38 The cytotrophoblast on chorionic villi has been shown to express the ∆NP63 isoform, whereas extravillous (intermediate) trophoblastic cells in the chorion laeve and ETT express the TAP63.38 Further studies are needed to delineate the functional role of TAP63 in ETT. Expression of cyclin E is found in ETT but not in the extravillous (intermediate) trophoblastic cells of the chorion laeve, the normal counterpart of ETT.43 This finding suggests that cyclin E probably plays a part in neoplastic transformation of ETT because an oncogenic role of cyclin E has been shown in other neoplastic diseases.
Hydatidiform moles Hydatidiform moles include complete, partial, and invasive moles, the latter representing a hydatidiform mole that becomes invasive or is exported to a distant anatomical site. Although hydatidiform moles are not classed as true gestational trophoblastic neoplasia, postmolar gestational trophoblastic neoplasia can be seen in many patients with hydatidiform moles, especially complete moles. This finding indicates that hydatidiform moles are the precursor lesions of some gestational trophoblastic neoplasms. Because of the close relationship of these moles with gestational trophoblastic neoplasia, their pathogenesis will be briefly highlighted. Advances in cytogenetics, and in the molecular and immunohistochemical characterisation of hydatidiform moles, have shown a distinct pathogenesis in complete and partial moles. Most complete moles are diploid and http://oncology.thelancet.com Vol 8 July 2007
usually have the karyotype 46,XX with both sets of chromosomes originating from the paternal complement as a result of haploid genome (23X) duplication. This chromosomal composition is referred to as androgenetic, meaning that genetic material is derived exclusively from paternal DNA. Partial moles, however, are generally triploid (XXY) with two sets of chromosomes from the paternal complement and a haploid set from the maternal genetic content. This triploid chromosomal composition is known as diandric or biparental. Understanding the cytogenetic constituents that characterise complete and partial moles is fundamental for the future study of how maternally or paternally imprinted genes contribute to different types of molar pregnancy. Despite the well-established cytogenetic findings associated with different hydatidiform moles, the molecular aetiology underlying the development of molar pregnancy remains elusive. One of the most exciting studies showed that MALP7 is mutated in patients with familial and recurrent biparental moles.44 MALP7 encodes a member of the CATERPILLER protein family that is potentially involved in inflammatory and apoptotic processes. More work is needed to elucidate the possible mechanisms of mutant NALP7 proteins in the pathogenesis of hydatidiform moles, such as during oogenesis and implantation. Additionally, several oncogenes and tumour-suppressor genes have been studied in complete moles. Similar to choriocarcinomas, synergistic upregulation of c-MYC, c-ERB-2, c-FMS, and BCL-2 oncoproteins has been suggested in the pathogenesis of complete moles.10 Mutational analysis of K-ras and P53 did not show mutations in complete moles, but overexpression of P53 and MDM2 proteins has been recorded.9 Furthermore, the role of promoter methylation has been studied in hydatidiform moles and gestational trophoblastic neoplasia. Frequent promoter hypermethylation and decreased expression of PTEN, CDH1, HIC-1, and CDKN2A have been found in hydatidiform moles compared with healthy placentas,23,45 suggesting that inactivating genes that might participate in tumour suppression by promoter methylation could be one of the mechanisms in the development of molar pregnancy. Furthermore, promoter hypermethylation of CDKN2A alone, or combined with CDH1, was significantly correlated with subsequent development of gestational trophoblastic neoplasia.23
New molecular targets with therapeutic potential Choriocarcinoma is among the few human cancers that are highly responsive to chemotherapy and, unlike other malignant tumours, metastatic choriocarcinomas are potentially curable by combined chemotherapy and adjuvant surgical procedures.46,47 Despite this fact, a small but significant proportion of choriocarcinoma patients develop recurrent diseases after primary treatment.48,49 By contrast with choriocarcinoma, PSTT and ETT are generally 645
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more refractory to conventional chemotherapy. Patients with PSTT and ETT might develop metastatic diseases and some patients will have terminal disease.7,37 Therefore, new therapeutic regimens are needed to reduce the toxic effects associated with current chemotherapy—especially the combination regimens, ie, etoposide, methotrexate, dactinomycin; etoposide, methotrexate, dactinomycin, cyclophosphamide, vincristine; and etoposide, cisplatin, etoposide, methotrexate, and dactinomycin—and to salvage the occasional non-operable patients with metastatic chemoresistant diseases. Target-based treatment designed to inactivate molecular pathways that are essential for tumour-cell growth and survival has shown clinical promise. Unlike standard chemotherapy, which affects most proliferating cells, inhibitors that target specific pathways selectively eliminate tumour cells to achieve maximum therapeutic effects and minimum adverse side-effects. These new anticancer agents include gefitinib, a small kinase inhibitor that targets EGFR, and trastuzumab, which is a humanised antibody that targets ERBB2 receptors. Because the merits of molecular targeting have been shown by clinical studies, targeted treatment might also be applicable to treat metastatic gestational trophoblastic neoplasms. In the following sections, several molecular targets that are expressed in gestational trophoblastic neoplasia tissues and cell lines are discussed (table 1). However, there are many other inhibitors that target emerging cancerassociated molecules and new therapeutic approaches, such as anti-human chorionic gonadotropin antibody treatment and immunotherapy, might also prove useful as new therapeutics in gestational trophoblastic neoplasia in future.
c-MYC proto-oncogene The activation and overexpression of the c-MYC protooncogene is one of the most frequent molecular changes in human cancers.50 In trophoblastic lesions, high levels of c-MYC expression have been detected in choriocarcinomas and complete hydatidiform moles, and an increased expression of c-MYC proteins might suppress differentiation and enhance cellular proliferation in gestational trophoblastic neoplasia.10 Direct evidence of tumour dependence on c-MYC for growth and survival comes from elegant studies showing that induction of c-MYC expression results in tumour formation, and that the blocking c-MYC activation generally leads to tumour stasis or regression.51,52 In recent years, the targeting c-MYC oncoproteins has been proposed as a new anticancer regimen.53,54 For example, several strategies that aim to inactivate c-MYC function and expression are being developed. These approaches use: inhibitors that block c-MYC expression, eg, triple-helix-forming oligonucleotides; molecules that disrupt MYC-MAX interaction; agents that block the functions of c-MYC-regulated genes, eg, cationic porphyrins and ornithine decarboxylase; and antisense 646
Molecular targets
Possible drugs
c-MYC
Antisense oligonucleotides
EGFR
Cetuximab, gefitinib, erlotinib
MAPK
CI-1040, PD59089
MMP
Marimastat
mTOR
AP23573, RAD001, CCI-779
MAPK=mitogen-activated protein kinase. mTOR=mammalian target of rapamycin.
Table 1: Molecular targets and examples of possible clinical drugs for gestational trophoblastic neoplasia
oligonucleotides and siRNA that silence c-MYC expression.54,55 Some of the antisense oligonucleotides have successfully completed phase I clinical trials and are at an advanced stage of drug development. Therefore, future patients with metastatic gestational trophoblastic neoplasia could benefit from anti-MYC agents in combination with conventional treatment.
EGFR EGFR is a transmembrane-receptor tyrosine kinase that belongs to the ERBB-related kinase family. Many epithelial tumours overexpress EGFR and harbour somatic mutations, leading to constitutive activation of the EGFR pathway.56 High levels of EGFR expression have been detected in choriocarcinomas, PSTT, and ETT.7,17,57 Additionally, the expression levels of EGFR in choriocarcinomas and complete hydatidiform moles were significantly higher than those of trophoblastic cells in healthy placentas and partial hydratidiform moles.17 Further analysis showed that intense EGFR immunoreactivity in the extravillous (intermediate) trophoblastic cells of a complete mole was seen to be significantly correlated with the development of a persistent postmolar gestational trophoblastic neoplasm.17 Expression of ERBB-3 and ERBB-4, however, did not differ between gestational trophoblastic neoplasms and healthy placental tissues.17 The anti-EGFR regimen has become the prototype of target-based treatment for cancer. Clinical and survival benefits of anti-EGFR treatment have been shown in patients with non-small-cell lung cancer and other neoplastic diseases. The repertoire of cancer types that might respond to anti-EGFR treatment is expanding, including head and neck and pancreatic carcinomas. Drugs that target EGFR, including cetuximab, gefitinib, and erlotinib, have emerged as effective treatments against various malignant diseases. Therefore, patients with metastatic gestational trophoblastic neoplasia might also benefit from anti-EGFR treatment in combination with other anticancer agents.
Mitogen-activated protein kinase Mitogen-activated protein kinase (MAPK) has a role in signal transduction, and activation (phosphorylation) of http://oncology.thelancet.com Vol 8 July 2007
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MAPK results in the regulation of downstream effectors to coordinate various cellular activities, including proliferation, differentiation, apoptosis, angiogenesis, and migration. Aberrant molecular changes that result in the constitutive activation of MAPK contribute to tumour development, with activation being caused by either a genetic or epigenetic event. For example, activating mutations in KRAS and BRAF, the upstream genes of MAPK, have been found in various human carcinomas.58,59 Additionally, MAPK activation can result from growth-factor stimulation in the tumour microenvironment.60 Kobel and co-workers61 studied MAPK activation in PSTT tissues using immunohistochemistry. They showed that the activated (phosphorylated) form of MAPK was expressed in 84% of cases, but not in healthy extravillous (intermediate) trophoblastic cells, the normal counterparts of PSTT.61 However, because the total concentration of MAPK was similar between PSTT and healthy extravillous (intermediate) trophoblastic cells, the investigators proposed that hyperactive MAPK was not likely to be the result of MAPK-gene upregulation, but rather a result of other epigenetic events that lead to its activation. To characterise the biological role of MAPK activation in PSTT, they established the first PSTT cell line, implantation-site trophoblastic cells, in which the cells were highly motile and invasive in vitro. Treatment with the mitogene inhibitors, CI-1040 and PD59089, which prevent activation of MAPK, significantly reduced the motility and invasion of IST-2 cells in vitro.61 These findings suggest a functional role of MAPK activation in the motility and invasion of PSTT cells. Because CI-1040 and other mitogen inhibitors that prevent MAPK activation have already been tested in clinical trials,62–64 these inhibitors could also be assessed for treating patients with metastatic PSTT who have not responded to conventional treatment. Whether or not inhibitors of the MAPK pathway can reduce the invasiveness and metastatic potential of PSTT would be interesting to learn.
Mammalian target-of-rapamycin (mTOR) mTOR is a member of the phosphatidyl inositol 3´ kinase (PI3K) family and is one of the key molecules that participate in the cellular signalling network. On stimulation by miscellaneous growth signals, a cascade of signalling occurs from PI3K, Akt, and mTOR to P70S6 kinase, resulting in a variety of cellular responses, including cellular migration and proliferation. Osteopontin, a cell-surface phosphorylated glycoprotein, represents one of the signals that trigger activation of the PI3K/mTOR/P70S6 pathway and has been known to have an important role during implantation by aiding attachment of the trophectoderm of a blastocyst to the surface epithelium of the endometrium.65 In gestational trophoblastic neoplasia, osteopontin is highly expressed in choriocarcinoma tissues and the derived cell lines.66,67 http://oncology.thelancet.com Vol 8 July 2007
This glycoprotein is co-localised with an adhesion molecule, called carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1), in the extravillous (intermediate) trophoblast of a healthy placenta and also, probably, in choriocarcinoma cells.68 The expression of osteopontin has been shown to enhance the invasion of CAECAM1-expressing trophoblastic cells.68 Likewise, expression of highly phosphorylated osteopontin activates the PI3K/mTOR/P70S6 kinase pathway and triggers migration of CAECAM1-expressing trophoblastic cells in choriocarcinoma cells grown in culture. Therefore, this tumour phenotype could be suppressed by the compounds LY294002 and rapamycin, which inhibit PI3K and mTOR, respectively.66 Over the past years, several mTOR inhibitors have been developed and tested in preclinical tumour models,69,70 and, more importantly, several of these inhibitors, including AP23573, RAD001, and CCI-779, have been tested in clinical trials as single agents or in combination with other drugs in patients with solid or haematological neoplastic diseases.71,72 Ongoing clinical studies involving mTOR inhibitors will, hopefully, identify the efficacy of these drugs in treating patients with cancer and provide the foundation for future use of mTOR inhibitors in patients with gestational trophoblastic neoplasia.
Matrix metalloproteinase (MMP) As previously discussed, the upregulation of MMP and downregulation of TIMP in choriocarcinomas suggest that patients with metastatic, chemoresistant gestational trophoblastic neoplasia might benefit from MMP inhibitors. Marimastat is an MMP inhibitor and preclinical studies in animal-tumour models have shown that MMP inhibitors restrict the growth of solid tumours, inhibit metastatic potential, and block tumour angiogenesis. This inhibitor is an orally bioavailable drug that has also been assessed in clinical trials of patients with advanced non-small-cell lung cancer, relapsed prostate cancer, glioblastoma multiforme, and several other types of cancer. In general, modest toxic effects have been reported, on the basis of several phase I clinical trials,73 and marimastat has been shown to delay progression in patients with biochemically relapsed prostate cancer.74 Further studies need to be done to establish the efficacy of marimastat and other new MMP inhibitors in treating metastatic choriocarcinomas, and to find out if the clinical outcome can be improved in patients with choriocarcinoma who do not respond to conventional chemotherapy.
Promises and challenges of target-based treatment The molecules discussed above have been shown to be expressed in neoplastic trophoblastic cells and, thus, their inhibitors represent attractive new therapeutics in chemoresistant gestational trophoblastic neoplasia. However, there are several challenges ahead before 647
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molecular target-based treatment becomes a reality for this group of diseases. First, the expression frequency of the target genes in a large series of neoplastic tissues is not yet known, because such clinical samples are rare for study. Furthermore, studies of gestational trophoblastic diseases have primarily focused on hydatidiform moles and choriocarcinoma. PSTT and ETT have rarely been studied so knowledge of these forms of gestational trophoblastic neoplasia is limited. Second, it is crucial to question whether the anticancer effects seen in preclinical models using gestational trophoblastic neoplasia cell lines can be translated to treatment efficacy in vivo. Third, the clinical usefulness of applying molecular inhibitors in gestational trophoblastic neoplasia depends on careful correlation studies between clinical response and the genetic and molecular changes of genes and the pathways they control, respectively. For example, mutational analysis of EGFR and members of the PI3K and MAPK pathways would be important, in addition to the assessment of their gene-expression levels. The results of such genotype–phenotype correlation studies would be essential for the development of tailored treatment in order to keep potential side effects to a minimum and optimise therapeutic effects. Fourth, given the fact that gestational trophoblastic neoplasia is rare and most patients can be cured by the current treatment, recruitment of patients with this disease in clinical trials can be challenging because only a limited number of patients qualify and are available for such studies. Nevertheless, drugs that inhibit the expression or function of the molecular targets have been assessed in both preclinical and clinical trials of various cancer types. With results from ongoing studies soon to emerge and new inhibitors in the development pipeline, future patients with gestational trophoblastic neoplasia should benefit from new waves of target-based treatment that have been primarily developed to treat other types of cancer. Effective multicentre collaborative networks should be developed or better orchestrated to aid with these clinical studies and to share valuable clinical specimens, in order to characterise emerging targets with therapeutic potential in gestational trophoblastic neoplasia. With this continuous effort, we will be wellpositioned to further establish the pathogenesis of this rare but intriguing disease and offer a better treatment alternative for patients.
Conclusion Recent advances in molecular studies of trophoblastic cells in healthy placentas and in gestational trophoblastic neoplasms have indicated that these neoplasms recapitulate normal trophoblast differentiation seen in early developing placentas. Relating gestational trophoblastic neoplasia with normal trophoblastic differentiation programmes highlights the fundamental nature of this group of diseases and provides the framework for future molecular studies. These new findings not only shed 648
Search strategy and selection criteria Published and unpublished data for this review were identified by searching PubMed. Key words used to identify papers included, “trophoblast”, “gestation”, “neoplasm”, “metastasis”, “target”, “therapy”, “pathogenesis”, “choriocarcinoma”, “placenta”, “mole”, and “epithelioid”. The search strategy was not limited by year of publication, but only articles published in English were selected.
light on the pathogenesis of gestational trophoblastic neoplasia but also provide an array of candidate molecules with therapeutic potential. Characterisation of the molecular changes that have been targeted to treat several other human cancers in gestational trophoblastic neoplasia would open an exciting avenue for target-based treatment of metastatic forms of this disease that are refractory to conventional treatment. Conflicts of interest The author declared no conflict of interest. References 1 Genest DR, Berkowitz RS, Fisher RA. Gestational trophoblastic disease. Lyon: IARC press, 2003. 2 Shih IeM, Kurman RJ. Molecular basis of gestational trophoblastic diseases. Curr Mol Med 2002; 2: 1–12. 3 Shih IM, Kurman RJ. The pathology of intermediate trophoblastic tumors and tumor-like lesions. Int J Gynecol Pathol 2001; 20: 31–47. 4 Soper JT. Gestational trophoblastic disease. Obstet Gynecol 2006; 108: 176–87. 5 Mao TL, Kurman RJ, Huang CC, Lin MC, Shih IM. Immunohistochemistry of choriocarcinoma: an aid in differential diagnosis and in elucidating pathogenesis. Am J Surg Pathol 2007; (in press). 6 Shih IM, Seidman JD, Kurman RJ. Placental site nodule and characterization of distinctive types of intermediate trophoblast. Hum Pathol 1999; 30: 687–94. 7 Shih IM, Kurman RJ. Epithelioid trophoblastic tumor: a neoplasm distinct from choriocarcinoma and placental site trophoblastic tumor simulating carcinoma. Am J Surg Pathol 1998; 22: 1393–403. 8 Mazur MT. Metastatic gestational choriocarcinoma. Unusual pathologic variant following therapy. Cancer 1989; 63: 1370–77. 9 Fulop V, Mok SC, Genest DR, Gati I, Doszpod J, Berkowitz RS. p53, p21, Rb and mdm2 oncoproteins. Expression in normal placenta, partial and complete mole, and choriocarcinoma. J Reprod Med 1998; 43: 119–27. 10 Fulop V, Mok SC, Genest DR, Szigetvari I, Cseh I, Berkowitz RS. c-myc, c-erbB-2, c-fms and bcl-2 oncoproteins. Expression in normal placenta, partial and complete mole, and choriocarcinoma. J Reprod Med 1998; 43: 101–10. 11 Chen CA, Chen YH, Chen TM, et al. Infrequent mutation in tumor suppressor gene p53 in gestational trophoblastic neoplasia. Carcinogenesis 1994; 15: 2221–23. 12 Shi Y-F, Xie X, Zhao C-L, et al. Lack of mutation in tumoursuppressor gene p53 in gestational trophoblastic tumours. Br J Cancer 1996; 73: 1216–19. 13 Landers JE, Haines DS, Strauss JF 3rd, George DL. Enhanced translation: a novel mechanism of mdm2 oncogene overexpression identified in human tumor cells. Oncogene 1994; 9: 2745–50. 14 Wang H, Zeng X, Oliver P, et al. MDM2 oncogene as a target for cancer therapy: an antisense approach. Int J Oncol 1999; 15: 653–60. 15 Asanoma K, Kato H, Inoue T, Matsuda T, Wake N. Analysis of a candidate gene associated with growth suppression of choriocarcinoma and differentiation of trophoblasts. J Reprod Med 2004; 49: 617–26.
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