Prognostic and predictive factors in prostate cancer

C ANCER TREATMENT REVIEWS 2001; 27: 143–151 doi: 10.1053/ctr v.2001.0208, available online at http://www.idealibr ar y.com on

Prognostic and predictive factors in prostate cancer F. C. Hamdy Section of Urology, University of Sheffield, Sheffield, U.K. Prostate cancer is a major public health problem in the Western world, and the second most common male malignancies in the European Union. Detection of the disease is possible at an early stage, using serum prostate specific antigen measurement and prostatic biopsies. To date, however, screening for prostate cancer has not been shown to be of benefit to patients in improving outcome.This is compounded by uncertainties surrounding treatment efficacy, as more men appear to die with prostate cancer than from it. Studies addressing these issues are underway in Europe and the U.S.A. Clinicians are currently unable to advise their patients with any degree of certainty as to the appropriateness of treatment for prostate cancer, because of their inability to differentiate tumours that will progress from those that will remain quiescent. This article reviews the various clinical, pathological and experimental markers available, and their value in providing prognostic information, which may assist clinicians and patients in making management decisions. Further research is still required to understand the biological behaviour of prostate cancer and to assess the value of screening and treatment efficacy in order to advise patients, clinicians and health care systems accordingly. © 2001 Harcourt Publishers Ltd

CLINICAL IMPORTANCE OF EARLY PROSTATE CANCER Prostate cancer is the second most common male malignancy in the European Union with approximately 100 000 men newly diagnosed and 40 000 dying from the disease every year (Figure 1) (1). The disease is undoubtedly a major public health problem, compounded by disturbing controversies surrounding its management. It is clear from epidemiological studies that more men die with prostate cancer than from it. Current investigational methods do not allow clinicians to differentiate with certainty tumours which will progress from those which will remain quiescent. This leads to difficult decision-making processes between patients and concerned clinicians, with potential over- and under treatment of the disease. These difficulties were aggravated

Address for correspondence: Freddie C. Hamdy, Professor and Head of Urology, Section of Urology, Royal Hallamshire Hospital, Glossop Road, Sheffield S10 2JF, U.K. 0305-7372/01/030143 + 09 $35.00/0

over the past decade by the establishment of serum prostate specific antigen (PSA) measurement as a valuable screening tool to detect early prostate cancer. Whilst the issue of treatment efficacy has yet to be resolved by large randomized controlled trials, screening is being practised regularly in many Western countries, with no evidence of its benefits in improving survival and outcome in patients found to have prostate cancer. Screening is currently being addressed by a large randomized controlled trial in Europe; results are emerging slowly and a final outcome is expected in the next 5–10 years. The clinical picture in the advanced stages is dominated by the problem of bone metastases, which occur in 85% of patients. They represent the most important cause of morbidity, with pain requiring substantial analgesia, and subsequent complications including pathological fractures and spinal compression. The development of bone metastases occurs at a constant rate of approximately 8% per year in patients with advanced prostate cancer, reaching 40% at 5 years. There is a remarkable affinity between prostate cancer cells and bone, which © 2001 HARCOURT PUBLISHERS LTD

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Histological prostate cancer

Normal epithelium

Prostatic intraepithelial neoplasia (PIN)

Locally advanced prostate cancer

Clinically advanced prostate cancer

Metastatic disease

Hormone refractory disease

† Figure 1 Clinico-pathological course of prostatic neoplasia.

leads to this predominant pattern of metastasis, which is most frequently osteoblastic in nature, unlike most other malignancies. This remains largely unexplained (Figure 2).

prostate cancer are found to have extracapsular disease following prostatectomy.

Conventional treatment of metastatic disease Conventional treatment of early disease There are three treatment options in early prostate cancer: watchful waiting, radical radiotherapy (including brachytherapy) and radical prostatectomy. To date there have been no valid randomized controlled trials comparing these treatment modalities. Conventionally, watchful waiting is offered to men with low grade low volume disease, with a life expectancy less than 10 years, whilst surgery is the preferred treatment option for younger, fitter men, and radiotherapy is given to the intermediate group of men who are less fit or otherwise unwilling to undergo surgery. These decisions, however, are not based on evidence, but result from individual biases and choices made by informed patients and clinicians. Complications of surgery and radiotherapy include erectile dysfunction and various degrees of incontinence. Staging methods have their limitations, and up to 50% of men with clinically confined

Since the work of Charles Huggins over 50 years ago, androgen ablation remains the treatment of choice in men with metastatic prostate cancer. However, approximately 15% of patients will not respond to

Latent TGFE

Active TGFE

Inactive fragments PTHrP Tumour cell

Proteases IGF FGFs, BMPs, ET-1

Figure 2 Mechanisms of osteoblastic metastases.TGFE = transforming growth factor, IGF = insulin-like growth factor, FGF = fibroblast growth factor, BMP = bone morphogenic protein, ET = endothelin.

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SKELETAL CONSEQUENCES OF TREATMENT Strictly speaking, skeletal consequences apply only to hormonal therapy for metastatic disease. For many years, evidence for administering early antiandrogen treatment in patients with advanced but asymptomatic prostate cancer was not available, and many men received deferred hormonal treatment. More recently, the MRC UK study of immediate vs deferred hormonal treatment in these patients suggested that patients would benefit from immediate treatment, and would suffer less morbidity from the disease. At the same time, reports appeared suggesting that long-term anti-androgen treatment in men with prostate cancer induced osteoporosis, and the risk of osteoporosis-related fractures, with almost 2000 cases yearly in the U.S.A. (2). With insidious PSA screening, there is a continuous stage migration of newly diagnosed patients, with lower volume locally advanced disease being detected in men with a higher life expectancy. Should all these men be treated with early androgen deprivation using orchidectomy or LHRH analogues, they will be likely to develop osteoporosis-related complications (Figure 3). It has been suggested recently that this could be avoided by administration of low-dose diethylstilboestrol (1 mg daily) which has insignificant cardiovascular thrombo-embolic complications. A further hypothesis suggests the use of simultaneous bisphosphonate therapy with androgen deprivation as a prophylactic measure against osteoporosis in these patients. These hypotheses will

50% Cumulative incidence

treatment, and the majority will relapse within 2–3 years of initiation of treatment. Chemotherapy is ineffective. Radiotherapy can be given as a palliative measure, either as hemibody irradiation, or as ‘spotwelding’ to treat painful metastatic foci in bone. Strontiumx 89 can be administered intravenously and appears to have similar efficacy to external beam irradiation. Finally, simple and complex analgesic regimens can be used, including combinations of non-steroidal anti-inflammatory agents and opiates. In addition, bisphosphonate compounds, which prevent bone resorption by inhibiting osteoclasts, appear to have a beneficial effect in relieving bone pain in patients with skeletal metastases from prostate cancer. More recently, the compounds have been tested prohylactically in clinical trials in an attempt to delay/prevent the appearance of bone lesions in patients with locally advanced disease. Despite these various treatment strategies, the morbidity caused by skeletal metastases in prostate cancer remains considerable.

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(4)

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(7) (8)

30% (11)

20% (20) (13) (30)

10% 0%

(49) (40) (114) (88) (162) (151) (142)

1

2

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5

(20)

(13)

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(54) (38)

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Figure 3 Cumulative incidence of osteoporotic fractures in men with prostate cancer, treated with ( __●__) or without (____ ● ) orchidectomy (n = 235). (Adapted with permission (42).)

require to be tested in the context of large-scale randomized controlled trials.

CLINICAL, BIOLOGICAL AND PATHOLOGICAL PROGNOSTIC FACTORS Clinical variables which have been shown to have a strong prognostic value include stage, Gleason grade, serum PSA level and the extent of tumour on systematic prostatic biopsy. Pathological markers include Gleason score of the final prostate specimen, stage and surgical margin status.

Digital rectal examination (DRE) The experienced urological index finger has been used for many years to palpate the prostate in an attempt to detect pathological changes within the gland, a practice dating back several centuries. Interobserver variability, however, is an important limitation of the technique, and is well documented. The sensitivity in predicting organ-confined cancer is approximately 50%, and specificity is 80% (3). Evaluation of DRE in staging prostate cancer is undoubtedly enhanced in combination with other variables, which we will examine in turn.

Transrectal ultrasound of the prostate (TRUS) Transrectal ultrasonography of the prostate remains a poor predictor of a pathological stage in early prostate cancer, despite technological advances in imaging quality. Alone, its performance is no better that DRE, with a positive predictive value of approximately 50%. The greatest value of TRUS, however, is in combination with other parameters, including

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DRE and serum PSA measurements. In addition, it is extremely valuable in accurate needle placement for guided biopsies of the prostate.

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aggressiveness of prostate cancer, and has the potential to predict tumour grade (6). The usefulness of free/total PSA ratios in routine clinical practice, however, remains to be determined.

Sextant biopsies of the prostate Combined parameters It has been postulated, recently, that the number and percentage of positive needle core biopsies involved may predict the extent of the disease in clinically localized prostate cancer. Ackerman et al. (4) suggested that the best combination was to use PSA density (PSAD), total PSA and the number of positive cores. A more recent report by Wills et al. (5) examined the value of number of positive cores, bilaterality and serum PSA, suggesting the best prediction is achieved by combining Gleason score and number of positive biopsies.

Prostate specific antigen Prostate specific antigen (PSA) is a serine protease and organ-specific glycoprotein (molecular weight 34 000) which originates in the cytoplasm of ductal cells of the prostate. It is responsible for liquefaction of seminal fluid. The measurement of serum PSA concentrations is now well established as a useful investigation in the diagnosis and follow-up of patients with prostate cancer. The greatest limitation of PSA is that it is tissue- and not tumour-specific in the prostate. However, PSA concentrations are the best overall predictor of bone scan findings and can be used as a screening test for prostate cancer. PSA circulates in blood mainly bound to protease inhibitors, including alpha-1-antichymotrypsin (ACT) and alpha-2-macroglobulin (AMG); only a small fraction of the total PSA exists in a free state. While AMG encapsulates all epitopes of the PSA protein, ACT leaves some exposed; therefore immunoassay techniques have been developed to assess free PSA and PSA bound to ACT but not to AMG. Recent reports suggest that the free/total PSA ratio in patients with BPH is significantly higher than in prostate cancer, but its role is not yet fully established in diagnosing the disease. Several studies report various optimal cut-off levels, largely due to the different nature of the assays used. A recent large study using the Hybritech assay (Hybritech Inc, U.S.A.), demonstrated a sensitivity of 90% in diagnosing prostate cancer in the total PSA range of 2.6 to 4.0 ng/ml, whilst sparing approximately 18% of patients from having prostatic biopsies. Recent evidence suggests that PSA ratio inversely correlates the to

The most extensive analysis of combinations of different parameters to predict pathological stage in early prostate cancer has been undertaken by Partin et al. (7). The factors included clinical stage, Gleason score and serum PSA, and led to the development of nomograms. These can be used to place patients in different ‘pigeon holes’ according to specified criteria. The nomograms, however, are complex for day-to-day use in the clinic, and are limited by the fact that there is more than a 25% probability of patients qualifying for more that one of the pathological stages. Table 1 summarizes the essential grouping related to pathological predictors of disease-free survival (DFS) 10 years after radical prostatectomy (8, 9):

Bone metabolic markers in advanced disease The most commonly used marker of increased metabolic bone activity is serum alkaline phosphatase measurement. It has been suggested that bone alkaline phosphatase (BAP) is a more sensitive and specific marker than total alkaline phosphatase (TAP) (10). In addition, BAP was shown to enhance the clinical utility of PSA in staging prostate cancer, avoiding up to 32% of bone scans in detecting skeletal metastases (11). As there appears to be a combination of bone resorption and osteoblastic activity in prostate cancer skeletal metastases, the use of bone formation and resorption markers is appropriate. This was assessed in a recent study using a combination of osteocalcin and bone alkaline phosphatase activity (bone formation), as well as deoxypyridinoline and pyridinoline cross-linked carboxyterminal telopeptide of type I collagen (ICTP) (bone resorption). Patients with and without evidence of skeletal metastases on bone scanning were studied. Levels of both sets of markers were elevated and were as effective as bone scans in the detection of metastases. ICTP was a better indicator of skeletal disease extent than PSA; bone alkaline phosphatase was more sensitive than total alkaline phosphatase, and there was accelerated bone resorption evidenced by increased urinary hydroxyproline levels, bone histomorphometry and radiological lytic appearances (12).

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TABLE 1 Grouping of pathological predictors of disease-free survival (DFS) 10 years after radical prostatectomy Good: >75% DFS

Intermediate: 50–74% DFS

Poor: <50% DFS

Gleason score 2–4

Gleason 5–6, capsular penetration, positive margin

Gleason score 7, extensive capsular penetration, positive margin

Gleason score 5–6, organ confined

Gleason score 7, organ confined

Gleason score 8+

Gleason score 5–6, focal capsular penetration + positive margin, or extensive capsular penetration + negative margin

Gleason score 7, focal capsular penetration + positive margin, or extensive capsular penetration + negative margin

positive seminal vesicles Positive lymph nodes

Tumours found as a consequence of PSA screening (T1c) and TRUS biopsy can be categorized according to the following criteria: PSA density of 0.15 ng/ml/g or over ● Gleason score of 7 or over ● Three or more cores involved with cancer ● 50% or more involvement of any core with cancer On pathological examination, tumours treated with radical prostatectomy (RP) can thus be categorized into the following groups: ● Insignificant: organ-confined, 0.2 cc or less, Gleason score <7 (17% of men receiving RP) ● Minimal: organ-confined, 0.2–0.5 cc, Gleason score <7 (12% of men receiving RP) ● Moderate: 0.5 cc or more, or capsular penetration + Gleason score <7 (52% of men receiving RP) ● Advanced: Gleason score >7, capsular penetration or positive margins, seminal vesicle or node involvement (19% of men receiving RP).

MOLECULAR PROGNOSTIC MARKERS Molecular staging In recent years, new technology, including flow cytometry and the reverse transcription polymerase chain reaction (RT-PCR), has given researchers the opportunity of detecting circulating tumour cells with high levels of sensitivity. In that sense, prostate cancer has a significant advantage over other malignancies, due to the specificity of PSA as a reliable marker. However, PSA is not tumour-specific. A PSA-positive cell in the peripheral blood, therefore, does not necessarily mean a prostate tumour cell, but a cell expressing PSA which is likely to be of prostatic origin, especially if the cell is found to express the gene constitutively, i.e. mRNA for PSA. Based on these principles, analytical flow cytometry and RT-PCR have been used in an attempt to detect and isolate circulating tumour cells from patients with prostate cancer. Studies have shown that although quantification of circulating PSA-positive cells by FC was a better predictor of skeletal metastases than isotope bone scanning, the majority of these

cells were not of prostatic origin (13, 14). RT-PCR methods, however, are considerably more sensitive in detecting circulating PSA-positive cells, relying on the identification of mRNA for PSA—an unequivocal proof that the cells are of prostatic origin. A number of studies have demonstrated the ability of RT-PCR to detect circulating prostate cells in patients with apparently localized disease undergoing radical prostatectomy. Other studies found a strong correlation between a positive PCR reaction, capsular tumour penetration and positive surgical margins, suggesting the potential of this technique to be used for ‘molecular staging’ of prostate cancer (15). Still other workers applied the same technique but their results show inconsistency and wide variability. The authors of all these studies assume that these PSA-positive cells are endowed with metastatic propensity, despite the fact that the results only demonstrate the presence of cells of prostatic origin. More than 16 institutions in the U.S.A. are currently evaluating the role of RT-PCR in prostate cancer and, of those, only two have reported its utility in staging (16). More recently, attempts have been made to quantify mRNA for PSA in the blood stream of patients with clinically localized disease, and correlations with pathological staging are being assessed

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(17). The role of detecting circulating PSA-positive cells in the circulation of patients with prostate cancer remains unclear.

Apoptosis regulating genes in prostate cancer

F. C . HAMDY

against Factor VIII to identify endothelial cells. Increasing MVD correlates with increasing Gleason score and the presence of metastases; it is an independent predictor of progression after radical prostatectomy for Gleason score 5 to 7 tumour (23).

bcl-2

Vascular endothelial growth factor

In the prostate bcl-2 is normally expressed in basal epithelial cells, seminal vesicles and ejaculatory ducts. In primary prostate cancer bcl-2 is expressed in around 25% of cases. bcl-2 overexpression is associated with increasing tumour stage and the development of hormone refractory disease (18). In a recent study comparing high grade prostatic intrapeilthelial neoplasia (HGPIN), prostate cancer and benign prastatic hyperplasia (BPH), bcl-2 expression was highest in HGPIN (19).

Vascular endothelial growth factor (VEGF) is a potent inducer of endothelial cell growth and is expressed in a variety of tumours. VEGF expression is increased in prostate cancer compared with benign prostatic epithelium (24).

p53 In benign prostatic epithelium p53 positivity is absent. In primary prostate cancer p53 nuclear positivity is present in about 20% of cases. p53 protein accumulation appears to be a late event and associated with an advanced stage, a high Gleason tumour grade, hormonal resistance, poor survival DNA aneuploidy and a high cell proliferation rate. The combination of bcl-2 overexpression and p53 nuclear protein accumulation in human prostate cancer has been shown to correlate with the development of hormone refractory disease and these are independent prognostic markers for post-radical prostatectomy recurrence (18, 20).

pp32 pp32 is a nuclear phosphoprotein and tumour suppressor gene, which is expressed in nearly 90% of clinically significant prostate cancers. Recent work suggests that pp32 expression is altered in prostate cancer, with benign prostatic tissue expressing pp32 and carcinomas expressing the variants pp32r1 and pp32r2 (21, 22). This switch from pp32 to its variants may be a significant causative event in the development of prostate cancer.

Angiogenesis Microvessel density (MVD) The quantification of new microvessels within a tumour is commonly performed using antibodies

Growth factors Transforming growth factor-beta 1 (TGF-β1) TGFβ-1 and -2 have been implicated in the development of prostatic disease. TGF-β1 has been detected immunohistochemically in both human prostatic stromal and epithelial cells and TGF-β2 mRNA identified in normal and malignant human prostate. The addition of TGF-β1 to cultured prostatic epithelial and stromal cells inhibits proliferation (25).

Bone morphogenetic proteins (BMPs) A number of studies have shown an association between BMP expression and skeletal metastases in prostate cancer. BMP-6, in particular, is expressed in the majority of primary prostate cancers with established skeletal secondaries, and rarely in localized disease. Primary and secondary prostate cancer expresses BMP-6, which is found infrequently in skeletal metastases from other human malignancies. BMP-6 may have a role in the initiation of skeletal secondaries and the osteoblastic reaction commonly seen in these deposits (26). More recently, BMP-6 expression in primary prostate cancer was shown to have prognostic value in patients undergoing radical prostatectomy, predicting biochemical, clinical progression and reduced survival (27).

Fibroblast growth factors (FGFs) Basic FGF (bFGF-2) is secreted by prostatic fibroblasts in response to androgen and acts in an autocrine fashion to stimulate fibroblast cell growth (28). Stromal-derived keratinocyte growth factor (KGF/ FGF-7) is upregulated in hormone resistant prostate cancer and has a role as a potential paracrine growth factor on epithelial cells (29). KGF has a potent mitogenic action on epithelial cells and has been suggested to act as an androgen-regulated mediator of

PROGNOSIS AND PREDICTION IN PROSTATE C ANCER

epithelial cell growth. A similar paracrine action applies to FGF-8 (androgen-induced growth factor). FGF-8 is secreted in response to androgens and can stimulate the growth of both epithelial and fibroblast cells.

Insulin-like growth factors (IGFs) IGF-1 and -2 are important mitogens that mediate normal and neoplastic cell growth. The IGFs bind to specific receptors, designated type I and II IGF receptors (IGFR). Type I IGFR is a transmembrane hetero-tetramer tyrosine kinase which primarily mediates the mitogenic actions of IGFs. IGFs are two of the most abundant growth factors in bone, the preferential site for metastatic prostate cancer. Type I IGFR is expressed by prostate cancer cells which could facilitate the development of bone metastases. IGFs also have high affinity for a family of at least six IGF binding proteins (IGFBPs) which act to regulate their bioavailability (30). The levels of circulating IGFBPs is regulated by endocrine factors and by specific proteases that cleave IGFBPs to small inactive peptides. IGFBPs are believed to modulate proliferative and mitogenic effects of IGFs as well as modulate cell growth independently of IGF. Although all IGFBPs have high affinity for IGFs, IGFBP-3 is the major transporter of IGFs in serum. A number of studies have suggested that IGFBPs may be involved in growth modulation of prostate malignancy. One study showed elevated IGFBP-2 and decreased IGFBP-3 in patients with prostate cancer (31).

Epidermal growth factor (EGF) Members of the EGF family play a role in modulation of prostatic growth. Withdrawal of androgen from the rodent prostate leads to reduced expression of EGF, which is a potent mitogen for epithelial cells (32). Thus the continued presence of androgens within the prostate helps maintain epithelial cell proliferation mediated through the expression of EGF.

Her-2/neu Her-2/neu is a member of the EGF family of receptor kinases, and is normally expressed in prostate epithelial cells. Some studies have shown that Her2/neu was overexpressed in a subset of prostate cancer patients, and serum levels of Her2 extracellular domain have been correlated with hormonerefractory disease. Recent work suggests that Her2/neu overexpression is involved with the emergence of hormone resistant prostate cancer in vivo, by modu-

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lating the response of the androgen receptor to low doses of androgen (33).

IL-6 Recent clinical studies have shown that elevated serum levels of IL-6 in prostate cancer patients correlate strongly with the presence of skeletal metastases (34). Studies using experimental model systems also suggest that IL-6 may have a variety of roles in prostate cancer.

Bone sialoprotein Bone sialoprotein (BSP), a bone matrix protein, was found recently to be overexpressed in prostate cancer patients who demonstrate biochemical progression following treatment (35). There are no data as yet to link BSP expression in serum of patients with development of skeletal metastases, and studies to evaluate this protein as a predictive serum marker are underway.

Cell adhesion E-cadherin Immunocytochemistry performed on human prostate cancer samples showed a general reduction of E-cadherin expression in high grade tumours associated with aberrant staining. This aberrant E-cadherin staining is proving to be a powerful predictor of poor outcome, both in terms of disease progression and patient survival (36).

CD44 CD44 is expressed on the plasma membrane of prostatic glandular cells. It is involved in cell adhesion since it acts as a receptor for the extracellular matrix components hyaluronic acid and osteopontin. CD44 is believed to play a major role in tumour metastases; alternative splice variants of the receptor differ in their capacity to enhance or decrease metastatic potential. In human prostate cancer CD44 downregulation is correlated with high tumour grade, aneuploidy and distant metastases (37).

Tumour ploidy and nuclear morphometry Tumours can be broadly classified as diploid, tetraploid or aneuploid, with accompanying variations in view of the well-documented heterogeneity

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of prostatic adenocarcinoma. Several reports correlating DNA ploidy with prognosis in prostate cancer have emerged in the last three decades. The results are conflicting, and only half the studies published confirm ploidy to be an independent prognostic marker (38).

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The ratio of MMP-2 to TIMP-2 expression in primary prostate cancer has been shown recently to correlate with histological grade and invasive disease (41).

REFERENCES Genetic factors Most known oncogenes and suppressor genes have been screened for their importance in primary prostate cancer but no common mutations have been identified. p53 mutations are rare in early prostate cancer but have been observed in almost 50% of advanced, metastatic disease. Mutations in the retinoblastoma (Rb) gene and deletion or methylation of p16INK4a (CDKN2), two genes intimately linked to cell cycle progression, have been described in a few prostate tumours and cell lines. Allelic loss, defined by the absence of one of the two copies of an autosomal locus present in somatic cells, commonly occurs in prostate cancer. Loss of heterozygosity (LOH) and comparative genomic hybridization analyses have revealed frequent loss of genetic material from chromosome regions 7q, 8p, 10pq, 13q, 16q, 17p and 18q in primary and metastatic prostate cancer. More recently, specific gene loci have been identified as metastatic suppressors in prostate cancer. Introduction of the genes for KAI1 (chromosome 11p11.2), E-cadherin (chromosome 16q22) or CD44 (chromosome 11p13) into prostate cancer cells have been shown to suppress metastatic ability. A number of studies revealed a familial clustering for prostate cancer. A risk factor of between two and three has been indicated for a relative of a prostate cancer patient acquiring the disease. This risk factor depends on the relationship within the family, a first degree relative (brother, father) presents the highest risk. Additionally, the risk factor increases if two or more family members have the disease. Hereditary prostate cancer, which can be separated from familial prostate cancer, has been reported to account for some 9% of all prostate cancer and more than 40% of early onset disease (39). Recent work provides strong evidence for a major susceptibility locus for prostate cancer on chromosome 1(1q24–25).

Matrix metalloproteinases and their inhibitors A number of malignancies including prostate cancer have been shown to differentially express MMPs which correlated strongly with aggressive disease (40). MMPs are tightly regulated by their inhibitors called TIMPs (tissue inhibitors of metalloproteinases).

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