The genetics of gynaecological cancer

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The genetics of gynaecological cancer

D. G. R. Evans

combination of loss of function of tumour suppressor genes and activation of oncogenes is usually involved. The particular combination and order may alter the histological as well as invasive nature of the cancer. There is now evidence that a minority of people who develop common cancers have inherited a faulty gene which puts them at a high risk of malignancy. This is not usually recognised as a syndrome in a particular individual, apart from their family history. Adenocarcinomas are more likely than carcinomas of squamous epithelium to have a strong hereditary component with between 4 and 10% of all breast, ovarian and colon cancer resulting from an inherited gene defect. The discovery of germ line (inherited) mutations in the p53 gene on the short arm of chromosome 17 in families with a peculiar combination of early onset and multiple tumours was the first proven example of this. 1

In common with all other malignancies, gynaecological cancers are caused by the accumulation of mutations in growth regulating genes. These mutations may be inherited in a minority of individuals with ovarian or endometrial cancer and are caused predominantly by viral agents in cervical cancer. Hereditary predisposition to ovarian cancer is often found in conjunction with breast cancer and the major predisposing gene BRCA1 has now been identified. Susceptibility to endometrial cancer may be inherited as part of a familial predisposition to colorectal cancer and four genes causing this pattern of cancers in families have recently been identified as DNA repair genes. Hormonal factors contribute to genetic changes in both the endometrium and ovary and drugs which have oestrogenic activity may alter the risks of developing malignancies at these sites. Recent years have seen an enormous improvement in our understanding of the mechanisms of carcinogenesis. Most cancers require a number of genetic changes in a single cell before an invasive turnout results. These changes occur predominantly in two types of growth regulatory gene. An oncogene if activated by a mutation will accelerate cell growth and division. In contrast, most mutations in turnout suppressor genes inactivate the growth suppressor function and again this leads to proliferation. However, the classical model for a growth suppressor gene inactivation requires loss of both functional copies of the gene. In practice this will not always be the case as some mutations will produce a protein product which interferes with the normal 'wild type' protein from the normally functioning copy (allele) of the gene. Few gynaecological turnouts are likely to be caused purely by the loss of two tumour suppressor genes as in the classical model of retinoblastoma, and the number of changes probably varies between four and 10. A

Hereditary predisposition Much of the work to determine the molecular mechanisms for tumorigenesis in cancer has concentrated on isolating the genes causing aggregations of cancer in families. This approach allows scientists to use samples from large kindreds with cancer to try to link the cancer risk to a particular chromosome region. By using several molecular approaches, such as deletion mapping in familial and sporadic tumours, it is then possible to close in on the region containing the tumour suppressor gene, or another susceptibile gene. It is then usually possible to sequence the genes in that region to determine which one is involved in tumorigenesis. Tumour suppressor genes can also be identified by starting with tumour material. By using genetic markers it is possible to search for regions of chromosomes which have been deleted (gone missing) in the tumour, but are present in the constitutional DNA. This is called loss of constitutional heterozygosity (LOH).

D. G. R. Evans, D e p a r t m e n t of Medical Genetics, St Mary's Hospital, Manchester M13 OJH, U K

Current Obstetrics & Gynaecology (1995)5, 201~05

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202 CURRENT OBSTETRICSAND GYNAECOLOGY

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chromosome3 DNA mismatch repair gene hMLH 1. This shows the predilictionto proximalcolorectalcancer and multiple tumours. +6age at death in years; Dia - age at diagnosis; blacked in circlesrepresentaffectedwomen.

Ovarian cancer

There have been many reports of familial aggregation of ovarian cancer dating back to at least 1950. 2 Increased risk of ovarian malignancy may be inherited as part of several genetic conditions. Gorlin syndrome, Peutz-Jeghers syndrome and XY females are all at heightened risk. In addition to this, ovarian cancer is known to be part of the Lynch type II cancer family syndrome and also breast/ovarian aggregation. There are also several reports of familial site specific ovarian cancer, 3 but many contain cases of breast and other malignancies. The association of breast and ovarian cancer in both family reports and epidemiological studies of breast and ovarian cancer 4 suggests the presence of an autosomal dominant cancer predisposing gene. Tumour studies have shown L O H on a number of chromosomal regions in ovarian cancer: 3p, 6q, 1 lp, 13q, 17p, 17q and Xp. 5 However, more recently the search has narrowed down using data from linkage studies in large families. Following the discovery of linkage of breast cancer to 17q, Narod et al 6 undertook linkage on five families with breast/ovarian aggregation. They found that three of the families were linked to a locus at 17q12-q23 and their additive lod (probability of linkage) scores reached statistical significance. Further data from international collaboration suggests that close to 100% of familial breast and ovarian cancer may be linked to this locus now labelled BRCAI.7 However, it cannot be assumed that all families are caused by BRCA1 particularly if they contain cases of male breast cancer which are more likely to be due to a new susceptibility gene labelled

BRCA2 on chromosome 13. 7 BRCA1 has recently been localised and cloned, 8 opening the way to direct mutation analysis in families. Whether site specific ovarian cancer is a specific entity is becoming increasingly unclear, as many apparently site specific families go on to develop breast cancer? More recent linkage analysis in apparent site-specific ovarian cancer has also confirmed linkage in the great majority to BRCA1. 9 It seems likely that if there is one locus implicated on the long arm of chromosome 17 that mutations within it will lead to a predisposition to both breast and ovarian cancer. Whether this predisposition is mainly to ovary or breast may depend on the nature of the mutation. The recent cloning of BRCA1 will eventually answer these questions and allow presymptomatic testing of unaffected 'at risk' individuals, so that screening with ultrasound and prophylactic surgery can be targeted at those really at risk. Another tumour association with ovarian cancer is that of bowel cancer. Ovarian cancer is part of the Lynch cancer family syndrome or Hereditary N o n Polyposis Colon Cancer (HNPCC) type 2 of which bowel cancer is the predominant feature (Fig. 1). This may account for the association of bowel and ovary in double primary studies l° as well as in many family reports. However, as endometrial cancer is a more prominent component of this hereditary predisposition, it is discussed more fully under that heading.

Histology The evidence for hereditary predisposition is almost entirely for epithelial ovarian cancer. In fact, recent evidence suggests that it is particularly serous rather than mucinous tumours which are associated with the familial forms of the disease, i° There is little or no evidence that germ cell tumours outside those occurring in XY females have a hereditary basisJ ~ This is clearly important as ordinarily early age at onset is a strong indicator of hereditary predisposition and as germ cell tumours occur at particularly young ages, histological verification is important for accurate estimation of risks.

Other sites Primary carcinoma of the peritoneum and fallopian tube may occur as part of hereditary predisposition to ovarian cancer, but these tumours are rare even though there are anecdotal reports of their occurence after oophorectomy.

Risk estimation In 1986, 5172 women developed ovarian cancer in the U K and around 1 in 70 women develop ovarian cancer in an average lifetime. The risk of the disease is increased 3 4 fold with one affected first degree relative?

THE GENETICS OF GYNAECOLOGICAL CANCER

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inheriting the predisposition. In families with a predominant history of ovarian cancer, the risk associated with the gene is about 800/0.12,13 Therefore, the risk of ovarian cancer if there are two affected close relatives is a little greater than 1 in 4. However, in some families with clear dominant inheritance, the risk equates to a 1 in 2 risk of inheriting a predisposing gene and a 40% lifetime risk of the disease. These families are rare and a more common situation is when there is a combination of breast and ovarian cancer (Fig. 2). Unless the family is sufficiently large to determine the relative risks of each tumour type an average risk of 60% (30% risk of ovarian cancer for a I st degree relative) would be the appropriate compromise. TMHowever, molecular studies may yet resolve the issue in many families. Risk of ovarian cancer in HNPCC families is yet to be established, but a likely range is shown in Table 1. Modifications of risk should take into account hormonal factors such as number of ovulatory cycles (OCP use, age at menarche and menopause, pregnancies) as well as infertility and infertility treatment.

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Fig. 2,--Small nuclear family showing autosomal dominant inheritance of susceptibility to breast and ovarian cancer. 92% of such families are thought to be due to mutations in BRCA1. Blacked in quadrants represent women affected with breast or ovarian cancer; d - age at death.

This amounts to around a 1 in 20 risk of the disease. However, the risk is higher if the disease has occurred at a younger age. 12, 13 Where two close relatives have developed ovarian cancer there is a very high likelihood of a dominant predisposition& This probably amounts to about a 2 in 3 chance of being hereditary and implies a 1 in 3 risk of other first degree relatives

Screening Screening by bimanual examination, serum markers and/or ovarian ultrasound, with or without colour doppler, has been advocated. Bimanual examination has poor sensitivity, as does screening with CA125 alone. While ovarian ultrasound with doppler carried out on equipment with a high resolution is likely to have very high sensitivity, many women may end up with surgery for benign disease due to the poor specificity of the technique. It is probably appropriate only to screen those women who are at a very high risk. However, the introduction of multiple marker screening including OVX1 may become a more appropriate first line screen. 15

Table--Localised and cloned genes predisposing to either ovarian or endometrial cancer or both Gene

Location

Ovarian cancer

Lt risk %

Endometrial

of familial

Lt risk %

of familial

Other cancer risk

BRCA1

17q

60%

60-80%

Small

<5%

Breast, prostate, bowel

BRCA2

13q

10-20%

< 5%

Small

< 5%

Breast, male breast

p53

17p

Small

<1%

Small

<1%

Sarcoma, glioma, breast

hMSH2

2p

Variable

5-10%

20-40%

40%

Bowel, gastric, ureter

hMLH1

3p

Variable

1-5%

2040%

30%

Bowel, gastric, ureter

hPMS 1

2q

Variable

1-5%

20~40%

< 10%

Bowel, gastric, ureter

hPMS2

7q

variable

1-5%

20-40%

< 10%

Bowel, gastric, ureter

Lt risk - lifetime risk of either ovarian or endometrial cancer; small - ovarian/endometrial cancer has been described in families, but no absolute evidence of an increased risk.

204 CURRENTOBSTETRICSAND GYNAECOLOGY E n d o m e t r i a l cancer

Little is known about the molecular events which give rise to endometrial cancer. Various groups have looked at the expression of oncogenes in the tumours and for specific mutations therein. The only tumour suppressor gene that has come under any detailed scrutiny is p53, but this gene has been studied in just about every tumour occurring in humans. Some work has focused on the prognostic significance of p53 and oncogene involvement. However, apart from this limited work there has been little published material on D N A involvement at other chromosomal locations. This is surprising in that more than 50 tumour suppressor genes have been implicated in other tumours by loss of chromosomal regions in the tumours. Involvement of p53 is not as frequent as in other tumours either by mutation analysis, loss of heterozygosity studies or overexpression by immunohistochemical studiesJ 6 Evidence would also suggest that p53 involvement is a relatively late eventJ 7 Late events, if consistent, could act as poor prognostic indicators and this appears to be the case for C-myc. Unsurprisingly, D N A ploidy analysis, which detects large scale genetic change, is also associated with poor prognosis. In contrast, Ki-ras would appear to be an early event as it is involved even in hyperplastic lesions. 18

may well indicate that the individual carries a H N P C C mutation.

Risk estimation It is not yet possible to arrive at accurate risk estimates for endometrial cancer, but HNPCC mutation carriers may be up to a 40% lifetime risk of the disease and therefore first degree relatives at approximately, a 20% riskJ 9 Whether site specific endometrial families will eventually also be shown to be part of H N P C C remains to be seen, but the precedent of apparently site specific ovarian cancer families being due to BRCA1 mutations suggests that this may well be the case. The absence of good epidemiological studies of endometrial cancer make it hard to assess relative risk outside dominant families, but it is likely that it will be increased in a similar way to ovarian cancer.

Hormonal factors Parity and number and frequency of ovulatory cycles also play a part in endometrial cancer risk and unopposed oestrogen use after oophorectomy gives a heightened risk of the disease. Recent evidence would suggest that tamoxifen gives rise to endometrial proliferation and eventually endometrial carcinoma. However, nothing is known of the progression of these tumours at the molecular level.

Hereditary disease

Screening

In 1986, 3767 women developed endometrial cancer in the UK. Perhaps 1 in 80 women would be expected to develop the disease in their lifetime. Little is known of the proportion of this due to hereditary disease, however a best approximation would be that 5% were caused by highly penetrant dominant genes mostly related to familial colorectal cancer. Endometrial cancer is part of the so called Lynch type II cancer family syndrome, where it is the second most common malignancy after colorectal cancer, but is also associated with cancer of the ovary, breast, upper GI tract and upper urological tract. There is some evidence of heterogeneity with respect to relative risk of endometrial cancer between these families.19.2oThere is also some evidence for an apparent site specific predisposition to endometrial cancer. 21 There are now at least four loci for familial colorectal cancer in which endometrial cancer risk is high, on chromosome 2p, 22 chromosome 3p, 23 7p and 2q. 24 These genes are a new breed of cancer gene in that they do not appear to be turnout suppressor genes, but are D N A mismatch repair genes. 2124 Their involvement gives rise to an inability to repair minor D N A damage which is found particularly in the tumours as micro-sattelite instability. This is referred to as replication error and can often be seen in the tumours of affected cases from H N P C C families. Indeed, identification of replication error in a turnout

Endometrial ultrasound and hysteroscopy and biopsy have been suggested as screening measures, but their effectiveness is yet to be established. Identification of high risk individuals from family history, and eventually from identification of individuals who carry H N P C C mutations, may soon be able to target women more effectively for screening and also for preventative surgery.

Molecular testing As can be seen from the Table there are now known to be a number of genes which predispose to ovarian and endometrial cancer. At present, genetic tests can only be really offered if the causative mutation has been established in an affected individual in a family. The large number and size of genes involved, and the diversity of the mutations, makes it almost impossible to offer testing outside this situation at the present time. However, with improvements in technology and the sensitivity of mutation detection, it should become possible to eventually test women who wish to know if they are at high or normal risk for these cancers. Cervical cancer

In common with the rest of the female genital tract there is no evidence to suggest major predisposing

THE GENETICS OF GYNAECOLOGICAL CANCER

genes t o a c c o u n t for a g g r e g a t i o n o f c e r v i c a l c a n c e r in families. I t is m o r e likely t h a t t h e s e a g g r e g a t i o n s are d u e to s h a r e d e n v i r o n m e n t a l insults s u c h as e x p o s u r e t o p a p i l l o m a virus. H o w e v e r , g e n e s w h i c h m a y m a k e i n d i v i d u a l s m o r e s u s c e p t i b l e to t h e effects o f p a p i l l o m a v i r u s c a n n o t b e c o u n t e d out.

Conclusion T h e last 5 y e a r s h a s seen a n e n o r m o u s a d v a n c e in o u r u n d e r s t a n d i n g o f c a n c e r a n d its f a m i l i a l elements. A g r e a t d e a l o f this k n o w l e d g e derives f r o m t h e s t u d y o f r a r e c a n c e r p r e d i s p o s i n g s y n d r o m e s . T h i s r e s e a r c h is n o t e s o t e r i c b e c a u s e the c l o n i n g o f t h e s e genes will not only benefit the small proportion of people who suffer f r o m t h e s e c o n d i t i o n s b u t also t h o s e w h o suffer f r o m t h e c o m m o n c a n c e r s o c c u r r i n g in t h e s e syndromes. M o l e c u l a r analysis will a l l o w b e t t e r t a r g e t i n g of screening and preventative measures and may event u a l l y h e r a l d n e w f o r m s o f g e n e t i c therapies.

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