Prospects for genetic diagnosis of inherited predisposition to cancer

Prospects for genetic diagnosis of inherited predisposition to cancer

98 TIBTECH - APRIL 1990 [Vol. 8] 14 Privalov, P. L. (1979) Adv. Protein Chem. 33, 167-241 15 Becktel, W. J. and Schellman, J. A. (1987) Biopolymers ...

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14 Privalov, P. L. (1979) Adv. Protein Chem. 33, 167-241 15 Becktel, W. J. and Schellman, J. A. (1987) Biopolymers 26, 1859-1877 16 Chen, B. and Schellman, J. A. (1989) Biochemistry 28, 685-691 17 Privalov, P. L., Gricko, Y., Venyaminov, S. Y. and Kutyshenko (1986) J. Mol. Biol. 190, 487-498 18 Baldwin, R. L. (1986) Proc. NatlAcad. Sci. USA 83, 8069-8072 19 Privalov, P. L. and Gill, S. J. (1988) Adv. Protein Chem. 39, 191-234 20 Alber, T. (1989) Annu. Rev. Biochem. 58, 765-798 21 Shirley, B. A., Stanssens, P., Steyaert, J. and Pace, C. N. (1989) J. Biol. Chem. 264, 11621-11625 22 States, D. J., Creighton, T. E., Dobson, C. M. and Karplus, M. (1987) J. Mol. Biol. 195, 731-739 23 Johnson, R. E., Adams, P. and Rnpley, J. A. (1978) Biochemistry 17, 1479-1484

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24 Lin, S. H., Konishi, Y., Denton, M. E. and Scheraga, H. A. (1984) Biochemistry 23, 5504-5512 25 Goto, Y., Tsunenaga, M., Kawata, Y. and Hamaguchi, K. (1987)J. Biochem. 101,319-329 26 Pace, C. N., Grimsley, G. R., Thomson, J. A. and Barnett, B. J. (1988) J. Biol. Chem. 263, 11820-11825 27 Matsumura, M., Signor, G. and Matthews, B. W. (1989) Nature 342, 291-293 28 Pace, C. N. and Grimsley, G. R. (1988) Biochemistry 27, 3242-3246 29 Mitani, M., Harushima, Y., Kuwajima, K., Ikeguchi, M. and Sngai, S. (1986)J, Biol. Chem. 261, 8824-8829 30 Pakula, A. A. and Sauer, R. T. (1989) Proteins Struct. Funct. Genet. 5, 202-210 31 Des, G., Hickey, D. R., McLendon, D., McLendon, G. and Sherman, F. (1989) Proc. Nat] Acad. Sci. USA 86,

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Prospects for genetic diagnosis of inherited predisposition to cancer Bruce A. J. Ponder The best long-term prospects for the reduction in the number of deaths due to cancer lie with screening and prevention rather than with therapy of the advanced disease. Screening efficiency will be improved by the identification of individuals at inherited high risk of cancer. Understanding the m e c h a n i s m of predisposition may provide the basis for rational attempts at prevention, as well as for new approaches to therapy. Cancer is a genetic disease at the level of the s o m a t i c cell. Several s e q u e n t i a l events are required to t u r n a n o r m a l cell into a cancer cell, a n d m o s t or all of these i n v o l v e m u t a t i o n s - for e x a m p l e , the a c t i v a t i o n of o n c o g e n e s or the loss of s u p p r e s s o r genes. T h e idea of cancer as a genetic disease at the level of the g e r m l i n e is less wellestablished, but it fits easily into the s a m e scheme. I n d i v i d u a l s w i t h in-

B. A. J. Ponder is at the CRC Human Cancer Genetics Research Group, Department of Pathology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QP, UK. © 1990, Elsevier Science Publishers Ltd (UK)

herited p r e d i s p o s i t i o n to c a n c e r m a y h a v e inherited: • one of the m u t a t i o n a l events a l r e a d y c o m p l e t e d in the germline; • a trait w h i c h m a k e s one of these m u t a t i o n a l steps m o r e likely to happen (e.g. a polymorphism in m e t a b o l i s m of an e x o g e n o u s c h e m i c a l w h i c h increases its conv e r s i o n to a carcinogen); or • a trait w h i c h m a k e s the consequences of one of the steps in carcinogenesis m o r e severe in t e r m s of p r o g r e s s i o n to c a n c e r (e.g. a p o l y m o r p h i s m in e n d o c r i n e m e t a b o l i s m w h i c h favours the proliferation of an altered p r e - c a n c e r o u s cell).

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496-499 32 Kellis, J. T., Nyberg, K. and Fersht, A. R. (1989) Biochemistry 28, 4914-4922 33 Yutani, K., Ogasahara, K., Tsujita, T. and Sugino, Y. (1987) Proc. Natl Acad. Sci. USA 84, 4441-4444 34 Matsumura, M., Becktel, W. J., Levitt, M. and Matthews, B. W. (1989) Proc. Natl Acad. Sci. USA 86, 6562-6566 35 Pace, C. N. and McGrath, T. (1980) J. Biol. Chem. 255, 3862-3865 36 Kostrewa, D., Choe, H-W., Heinemann, U. and Saenger, W. (1989) Biochemistry 28, 7592-7599 37 Acharya, K. R., Stuart, D. I., Walker, N. P. C., Lewis, M. and Phillips, D. C. (1989) J. Mol. Biol. 208, 99-127 38 Pantoliano, M. W. et el. (1988) Biochemistry 27, 8311-8317 39 Kuroki, R., Taniyama, Y., Nakmura, H., Kikuchi, M. and Ikehara, M. (1989) Proc. Natl Acad. Sci. USA 86, 6903-6907

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The great difference in investigating cancer c o m p a r e d with, for e x a m p l e , i n h e r i t e d p r e d i s p o s i t i o n to c a r d i o v a s c u l a r disease (where the genes i n v o l v e d in l i p o p r o t e i n m e t a b o l i s m w e r e an o b v i o u s target), is that there are a l m o s t no clues as to w h a t a n y of the p r e d i s p o s i n g genes for cancer m i g h t be. In the cancers w h e r e i n h e r i t e d risk is sufficient to cause o b v i o u s familial clustering, it is p o s s i b l e to e x p l o i t the clustering to m a k e an e m p i r i c a l search for the genes, u s i n g the t e c h n i q u e s of genetic linkage 1. Each of a series of D N A m a r k e r s is tested for c o - i n h e r i t a n c e of one allele of a restriction f r a g m e n t length p o l y m o r p h i s m w i t h the c a n c e r in several m e m b e r s of a family. If a m a r k e r is found whose inheritance coincides w i t h that of the cancer, the m a r k e r locus m u s t lie close to the cancerd e t e r m i n i n g gene on the s a m e chromosome, otherwise they would probably have been separated by genetic recombination in their passage t h r o u g h the family. F a m i l i e s w i t h several affected i n d i v i d u a l s are n e e d e d for this t e c h n i q u e to work. A significant c o m p o n e n t of i n h e r i t e d p r e d i s p o s i t i o n to c a n c e r m a y , h o w ever, o c c u r outside o b v i o u s ' c a n c e r families' (see below) 2. Here, genetic linkage c a n n o t be a p p l i e d . All that can be d o n e is to guess the genes that

TIBTECH- APRIL1990[Vol.8] ~Table

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I

Classification o f genetic p r e d i s p o s i t i o n to cancer by strength o f f a m i l i a l clustering

Class

Examples

Inherited cancer syndromes

Familial polyposis; Multiple endocrine neoplasia types 1 and 2; Von HippeI-Lindau syndrome. Breast cancer; Ovarian cancer; Nonpolyposis colorectal cancer. Metabolic polymorphisms determining response to exogenous or endogenous carcinogens.

Familial clusters of cancer Genetic predisposition without evident familial clustering (still hypothetical)

may be involved, or their phenotypes, and test these guesses empirically in case-control studies of individuals with or without cancer. Finding the predisposing genes and recognition of inherited predisposition depends, therefore, upon the degree of familial clustering. From this practical viewpoint, cancers can be classified into three groups (Table 1). The features of each group, and their implications for genetic diagnosis, will be discussed in turn.

Inherited cancer syndromes In these syndromes, which account for perhaps 2% of all cancer incidence, there is strong predisposition consistent with the effect of a single autosomal dominant gene (Fig. 1; for review, see Refs 3, 4). Family members who have inherited the gene have a high probability (usually at least 50% lifetime risk) of developing one or more specific cancers which are characteristic of each syndrome. A child of an affected parent is at 50% risk of inheriting the

'cancer gene'. Therefore, the most recent generations of such a family are a high risk group on whom, provided there is effective treatment, screening should be concentrated. In most of these syndromes, development of the cancer is preceded by a characteristic phenotypic abnormality in one or a few tissues (generally the tissues in which the cancer will develop): for example, C-cell hyperplasia 5 or colonic polyps (Fig. 2; Ref. 6). These phenotypes are important for two reasons. First, they signal clearly which cancers have a familial basis and which do not: in common cancers such as breast and ovarian where such marker phenotypes have not been found, the distinction between family clusters which are due to chance and those which represent true predisposition can be very difficult (see below). Second, they indicate, before the cancers develop, which family members are gene carriers and therefore at risk, and which are not. Such marker phenotypes therefore provide the basis for screening tests in the

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Pedigree of family with familial polyposis of the colon. Affected individuals are indicated by black (cancer) or hatched (polyps only) symbols. The family was recognized when the individual in generation III (arrowed) presented with colonic cancer and was found to have multiple colonic polyps. Subsequently, other members of the family (indicated by asterisk) have been screened by examination of the bowel through a colonoscope. Three were found already to have cancer; five siblings and a cousin in generation IV had polyps only and were treated by prophylactic colectomy. A line through the symbol indicates the individual died. (d.37 = died age 37; Ca 28 = cancer diagnosed age 28.)

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--Fig. 2 management of high-risk families 7,8. The mapping of the genetic loci for six of the inherited cancer syndromes by genetic linkage, in the space of just over a year (Table 2; Ref. 9), testifies to the pace of research once the necessary technical development in this case, an outline human genome map - has been achieved. For the most part, the mapping is still based on simple two-allele polymorphisms and these markers are still a few recombination units away from the disease gene locus. Nevertheless, there is already the possibility of using the linked genetic markers for genetic diagnosis of gene carriers in families. The further development of genetic diagnosis using these markers involves both technical and biological problems. The technical requirements are straightforward: closer markers, markers which lie either side of the gene, and multi-allele polymorphisms will improve the accuracy of prediction and the proportion of families in which prediction is possible (e.g. Ref. 10). Ultimately, probes which can detect the mutation in the predisposing gene itself will be required. Probes are already available for retinoblastoma ~. Such probes have several advantages:

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• they provide the greatest accuracy of diagnosis; • if different mutations are associated with different behaviour of the disease, probes which recognize each mutation will provide additional prognostic information; • they do not have the limitation, inherent in the use of linked markers, that two or more affected family members must be available to provide the linkage 'phase' on which to base prediction. The biological problems in the application of genetic diagnosis are more subtle. Figure 3 shows the use of a linked probe (14.34), to predict the inheritance of the predisposing gene for multiple endocrine neoplasia type 2 (MEN 2) in a family. There are 3 types of MEN 2: MEN 2A, MEN 2B and MTC-only, each with specific features, probably due to different mutations at the MEN2 locus. (Nomenclature: MEN 2A, disease; MEN2A, gene.) People carry-

(a) Polyps in colon of individual with familial polyposis. Two large polyps and a number of smaller ones are shown arising from the surface of the colonic epithelium. Bar = 5 mm. (Photograph provided by the ICRF Colorectal Cancer Unit, St. Mark's Hospital.) (b) Focus of C-cell hyperplasia in the thyroid of a patient with MEN 2A. A 41~m paraffin section of thyroid stained immunochemically for calcitonin. A hyperplastic focus of C-cells (arrowed) is seen in the centre of the photograph, lying between, and extending into adjacent thyroid follicles. F indicates the lumen of the follicle on the right. Bar = 100 Hm.

ing the MEN2A gene develop C-cell hyperplasia as a prelude to developing medullary thyroid carcinoma. (C-cells are located in the thyroid and produce calcitonin - hyperplasia results in elevated production of calcitonin.) The 7-year-old boy is predicted to be a gene carrier; the prediction was confirmed by a grossly abnormal plasma calcitonin level,, reflecting advanced C-cell hyperplasia 5 (Fig. 2) or early tumour; he underwent thyroidectomy with removal of a 6 mm tumour. The

decision for surgery was easy because the raised calcitonin indicated the development of the tumour: but what if no such marker phenotype had been available? Figure 4 shows the age-related probability that a carrier of the MEN2A gene will develop disease sufficient to take him to a doctor 8. Hardly anyone has symptomatic disease before the age of 20 years: 40% of gene carriers are still clinically unaffected at the age of 70 years. Without the confirmatory biochemical evidence that a tumour

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~Table 2 M a p p i n g o f loci o f predisposition to inherited cancer syndromes Syndrome

Chromosome

Retinoblastoma Wilms tumour •Familial polyposis /Multiple endocrine neoplasia type 1 All mapped ] M u l t i p l e endocrine neoplasia type 2 between | V o n HippeI-Lindau syndrome 1987 and 1988|Neurofibromatosis type 1 ~,Neurofibromatosis type 2 Familial melanoma

13q 11p a 5q 11q 10 3p 17q 22 l p (unconfirmed)

aThe locus for the familial form of Wilms tumour maps to another chromosomal region, still not identified.

was developing; a decision for thyroidectomy in this 7-year-old based on genetic diagnosis alone would have been difficult. On the other hand, the prediction from Fig. 3 that the boy's three sisters aged 15, 13 and 3 years are not carriers of the gene is very helpful. Because of the variation in pace of development of the tumour in different individuals 8, a negative result from the current biochemical screening test in childhood is only weak evidence against carriage of the predisposing gene. The biochemical testing (which is rather uncomfortable and provokes a good deal of anxiety) must therefore be repeated every year from the age of 5 until the mid-20s, and at longer intervals until the age of 40 years, before one can be confident that the individual is not a gene carrier. A negative prediction using DNA markers, however, can already reduce the risk to around 2% in the best situation (and will reduce it below 1% as markers improve), on the basis of a single blood sample in early childhood. The most straightforward use of DNA prediction in the inherited cancer syndromes at present is therefore to exclude family members from risk, and so (at a level of risk which will vary from case to case) from the necessity of screening. On the other hand, because of the variation in expression of predisposing genes within and between families, genetic diagnosis of the presence of the predisposing gene may still leave considerable uncertainty about the probability and timing of the development of cancer. Clinical decisions must balance this uncertainty against the severity of the treatment. So long as prophylactic treatment for those at risk in the inherited cancer syndromes remains

largely surgical, a genetic diagnosis of the inherited gene by itself, without the implication of certain and imminent disease, is likely to

lead to more intensive screening rather than directly to treatment. F a m i l i a l clusters o f c a n c e r

This group includes cancers which have a tendency to cluster in families, but where the genetic basis of the clustering is not at present so clearly defined as in the inherited cancer syndromes. The degree of clustering is measured by the relative risk (RR) of a specific cancer occurring in siblings of a patient, compared with the risk in the general population. For most cancers, including breast and ovarian, the RR is around two- to three-fold higher. This clustering could be due to either environmental or genetic effects; but analysis of the patterns of cancer among relatives

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i Genetic prediction in multiple endocrine neoplasia type 2 (MEN 2A) using a linked DNA marker. An autoradiograph of a Southern blot of DNA from family members, hybridized with a cosmid DNA probe (14.34) closely linked to the MEN2A locus on chromosome 10. The alleles recognized by the probe are A and B. The bands immediately below B are non-polymorphic DNA fragments which also hybridize with the probe. The affected grandfather (see pedigree at top of gel) is homozygous BB. One of his B alleles, which must be linked to the M E N 2A gene, is inherited by his affected daughter (genotype AB). As her husband is AA, each of their children must have received an A from the father: the inheritance of A or B from their mother predicts the inheritance of predisposition to the disease.

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favours a genetic explanation t2. The relative risk of two- to three-fold is, of course, an average figure derived from the whole population. We need to know whether the risk is evenly distributed, or whether this average figure conceals a small number of families at much greater risk. In the inherited cancer syndromes, the high-risk families could be identified by their characteristic phenotypes (e.g. polyps in familial polyposis), but for the familial cancers, no such marker phenotypes are available. Data from a current populationbased study of ovarian cancer 13 illustrate how the clustering can be a n a l y s e d . Because ovarian cancer tends to present early in predisposed individuals, one can ask whether the risk is concentrated in sisters of patients who were diagnosed at younger ages. The risk is indeed somewhat greater (Table 3), but this still does not define a group at very high risk: only i in 40 of the sisters of a patient diagnosed before age 50 will themselves develop and die from ovarian cancer by age 70. Another clue to a high-risk group might be

a more extensive family history, and, indeed, women in whose family there are already two close relatives with ovarian cancer are a high-risk group. The limited data available so far suggest that these women may have a lifetime probability of ovarian cancer of 30-40%. This figure, and the distribution of ovarian cancers in families with multiple cases, are consistent with predisposition by a single autosomal dominant gene with incomplete penetrance, leading to cancer in 60-80% of those who inherit it. (Note, however, that proof of this must await the demonstration of genetic linkage in families.)

If a dominant gene is assumed, clinical risk estimates can be offered to members of these families as part of genetic counselling 14. Without a genetic or phenotypic marker of predisposition, however, no woman can be excluded from risk, nor can anyone be assigned a risk greater than 50%. A marker would allow predisposed families to be distinguished from chance aggregates of ovarian cancer, and women who had not inherited the gene to be reassured and excluded both from screening and from the prophylactic oophorectomy which is currently recommended (because it is uncertain whether screening is effective) once childbearing is complete. Because the penetrance of the predisposing gene is incomplete (and it is not expressed in males), a large proportion of the women who have ovarian cancer because they are predisposed will not have an affected relative. Because there is no marker gene or phenotype, they will escape recognition. Two questions follow: • should the relatives of even apparently isolated cases of ovarian cancer be offered screening in order to include as many as possible of those who may be predisposed; and • what proportion of all ovarian cancers might be due to the strong predisposing gene that is supposed to be present in the families with several cases, and to what extent, therefore, can concentration on these predisposed individuals address the problem of ovarian cancer in general? Table 3 shows that the risk of ovarian cancer by age 70 in the close relatives of all women who were diagnosed below age 50 (which includes some in multiple case families) is only 2.4%, or three times

--Table 3. Cumulative risk o f death from ovarian cancer with increasing age Cumulative risk of death (percent population) w i t h increasing age Age

40 yr

50 yr

60 yr

70 yr

General population Sisters of patient diagnosed before age 50 years

0.04 0.24

0.16 1.0

0.44 1.6

0.83 2.4

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Possible effects of hypothetical cancer-predisposing genes causing the concentration of cancer incidence in a minority of the population a Gene frequency

P r o p o r t i o n of Riskto population carriervs w h o are non-carrier carriers

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0.1

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95

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Recessive

0.5

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aThese major effects may not necessarily give rise to obvious familial clustering, as shown bythe small relative risks (RR) in siblings. (Adapted from Ref. 2.) b In homozygote.

the general-population risk. Concentrating screening on these relatives does, therefore, offer a small gain in efficiency over screening the whole population, but still only about 1 in 1000 women in this group will develop ovarian cancer each year. Table 3 also shows that the excess risk of ovarian cancer by age 70 in the close relatives of patients, compared with the risk in the general population, is about 1.6%. If all of this excess were due to the inheritance by these relatives of the single putative predisposing gene, it would imply that about 4% of the affected index cases had the gene (since it is dominant, 2% of their relatives would have it, and 80% penetrance in them would give an incidence of 1.6%). This means that only about 1 in 25 of ovarian cancers diagnosed below age 50 can be attributed to the predisposing gene. (If the gene is less completely penetrant, the proportion may be a little higher, but almost certainly less than 10%.) Thus, while the ovarian cancer families are clearly a very high risk group that require special consideration, screening targeted by family history does not appear to be a solution to the public health problem of ovarian cancer. It follows that a genetic marker for the predisposing gene, while it would undoubtedly be valuable in the management of the high risk group, would have a limited impact on the disease as a whole.

Inherited predisposition without evident family clustering So far, only the possible role of a rare single gene of incomplete but fairly high penetrance has been considered. Part of the increased relative risk in siblings could, alter-

natively, be explained by a relatively common predisposing gene or genes with m u c h lower penetrance. If individuals with the gene had only a 1 in 10 chance of developing cancer, they would still be at substantially increased risk, but extensive family clusters would be rare and the predisposition would be hard to recognize. Such a gene could nevertheless result in most of the incidence of a particular cancer being concentrated in a minority of the population, with important implications for screening and prevention. The theoretical arguments have been set out by Peto 2. These show clearly that a common predisposing gene for a common cancer can result in quite striking concentrations of cancer incidence in a minority of the population, without drawing attention to itself by causing extensive familial aggregation. Examples of the effects that could be seen for dominant or recessive predisposing genes are given in Table 4. A study 15 in which colonic polyps were tested as a marker phenotype for a common gene predisposing to colonic cancer (separate from the inherited cancer syndrome polyposis coli) provides support for these calculations. Colonic cancer tends to cluster in families but, like breast and ovarian cancer, only a small minority of cases have a family history which clearly signals predisposition. When the relatives of isolated cases of colonic cancer are systematically examined for colonic polyps, however, they are found to have a significantly increased number compared to controls. The data can be interpreted to show that the majority of colon cancer incidence may be concentrated in a minority of the

population who are predisposed and who, in this case, can be recognized by the phenotype of colonic polyps. If these conclusions can be confirmed, they have obvious and very important implications for the reduction of cancer deaths in a much larger proportion of the population than are included in the recognizable familial syndromes. First, screening might be concentrated on a minority of the population which contains most of the risk. Second, elucidation of the mechanism of predisposition may in some cases provide opportunities for prevention. For example, the wellknown examples of polymorphism in metabolism of drugs suggest that at least some instances of genetic predisposition to cancer will be due to comparable polymorphisms in metabolism of exogenous or endogenous carcinogens (for examples and discussion, see Ref. 16). If so, elucidation of the mechanisms may lead to better identification of the carcinogens - about which information is still surprisingly scanty and thence to methods of prevention by avoiding them or altering their metabolism. Exciting though this prospect is, it remains largely hypothetical. Examples of metabolic polymorphism which appear to be associated with increased cancer risk have been described 17,18, but they are still few and awaiting confirmation. Progress is slow because, in the absence of familial clusters, the empirical methods of genetic linkage cannot be used to find the genes. Candidate genes or phenotypes must be guessed at, and tested in case-control studies. With a hundred hypotheses to be tested and no strong clue where to start, this is a laborious business.

Conclusion and prospects In most of the inherited cancer syndromes, genetic diagnosis using linked markers is already feasible; and it will probably soon become so in at least some of the common familial cancers such as breast cancer. Together, these may account for about 5% of cancer incidence. In these cases, the practical value of the diagnosis at present is to identify those not at risk. Further progress is possible in several directions: • better definition of marker phenotypes will improve the recognition of

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those at i n h e r i t e d risk and m a k e easier genetic linkage studies to find the genes;

• if the m e c h a n i s m of predisp o s i t i o n is u n d e r s t o o d it m a y be possible to use this k n o w l e d g e to design strategies to retard or p r e v e n t the d e v e l o p m e n t of cancer, as an alternative to the p r o p h y l a c t i c surgery w h i c h will m o s t l y o n l y be acceptable to those at v e r y high risk.

phisms in h o r m o n e or c a r c i n o g e n m e t a b o l i s m - are likely to be vital in providing clues to the recognition of these groups. W i t h o u t clear family clusters, a genetic linkage a p p r o a c h will be difficult, although if highly p o l y m o r p h i c DNA markers can be defined at regular intervals along the h u m a n gene map, an empirical a p p r o a c h using pairs of affected siblings might just succeed. (It m a y also be w o r t h investigating the predisposing loci identified in the inherited cancer s y n d r o m e s , in case 'weak' alleles at these loci are responsible for low-level predisposition in the population.) Because the lifetime risk of c a n c e r in these individuals is still low, and their n u m b e r s will be large, invasive or e x p e n s i v e measures such as surgery will not be acceptable. In cancers w h e r e screening is effective, this can be m a d e more efficient. T h e longterm goal, however, m u s t be the same as for the familial clusters of cancer: to use genetic p r e d i s p o s i t i o n to discover the e n v i r o n m e n t a l or endogenous factors w h i c h interact w i t h the genotype to cause cancer, and use these as a m e a n s to prevention.

Further ahead, but p o t e n t i a l l y m o r e i m p o r t a n t in terms of the n u m b e r s affected, it m a y be possible to identify individuals genetically p r e d i s p o s e d at a lower level, w h o c o m p r i s e a substantial fraction of the c a n c e r i n c i d e n c e in the p o p u l a t i o n . Once again, c a n d i d a t e p h e n o t y p e s w h e t h e r preneoplastic lesions s u c h as polyps, or metabolic p o l y m o r -

References 1 White, R. (1985) Trends Genet. 1, 177-180 2 Peto, J. (1980) Cancer Incidence in Defined Populations, Banbury Report, 4 (Cairns, J., Lyon, J. L. and Skolnick, M., eds), pp. 203-213, Cold Spring Harbor Laboratory 3 Harnden, D., Morten, J. and Feather-

• better definition of p r e n e o p l a s t i c lesions m a y i m p r o v e screening and the p o w e r to predict w h i c h predisp o s e d individuals are at i m m e d i a t e risk; • cloning of the p r e d i s p o s i n g genes will allow direct assay for the m u t a n t gene in individuals at risk and so genetic diagnosis w i t h o u t the n e e d for linkage in family members; • it m a y be possible to identify other genes w h i c h m o d i f y the e x p r e s s i o n of a p r e d i s p o s i n g gene, and so to predict w h e t h e r a p r e d i s p o s e d individual will be severely or m i l d l y affected; and

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