1363
EDITORIALS
Duchenne/Becker muscular dystrophy, for example.6 However, it has never been established with certainty that the osteomas, skin tags, and desmoid tumours that characterise Gardner’s syndrome actually breed true; it seems more likely that their identification in APC patients depends on how intensively they are
sought.1
Molecular secrets of colorectal cancer
Adenomatous polyposis coli (APC) is in many ways an ideal condition for the application of "reverse genetics"-elucidation of the pathogenesis of a heritable disease by characterisation of the relevant gene, starting from a knowledge only of its approximate position in the genome. The inherent attractions of APC in this respect are that it is a moderately common condition compatible with large family size and that it follows a very clear autosomal dominant pattern of transmission.1 Moreover, following the doctrine attributed mainly to Knudson, it can be predicted that where a genetic lesion predisposes to familial cancer, the same gene will be found to be involved in a proportion of sporadic tumours of the same histological type.2 Since sporadic colorectal cancer is even commoner than APC, there are plenty of opportunities to search in tumour tissue for consistent sites of molecular change, one of which is likely to identify the APC locus. In practice, both linkage studies in APC families and deletion analysis in sporadic colon carcinoma have amply fulfilled the expectations of their proponents. A crucial single case-report of association between APC (behaving as a new mutation) and a visible deletion on the long arm (q) of chromosome 5 provided the first clue as to where the search should concentrate.3 Genetic linkage to molecular markers in 5q21-22 was very rapidly established and, equally important, it was shown that classic APC and Gardner’s syndrome (thought to be distinguishable by the presence of extracolonic lesions in the latter) map to the same region.4,s This finding meant that information from all families could be pooled for more accurate localisation of the gene. It also raised the question of whether different mutations within the same gene might result in different degrees of severity of the disorder, as in
The same caveat probably does not apply to another extracolonic manifestation of APC/Gardner’s syndrome, congenital hypertrophy of the retinal pigment epithelium, which behaves as a good marker for the genetic lesion in most families but is completely absent in a minority.7Furthermore, Leppert and colleagues have described a large family with a hereditary predisposition to colonic polyps, leading to cancer if not removed. Although polyps did not arise in the massive numbers required for a diagnosis of APC, the condition in that family was linked to the APC locus.9 There is therefore ample evidence for a spectrum of phenotypes that could be associated with different mutations. Similar speculation has arisen from findings in sporadic colon cancers. As more and more polymorphic markers for the 5q2l-22 region have become available, the picture that emerges is of a common region of deletion, centred on the site of the APC gene, as defined by family studies.10,11 In a large series of tumours, the actual frequency of such deletions increases as the markers used approach the APC locus and over 50% of tumours show loss of heterozygosity with the probe L5-71-3, which seems to represent the apex of the "deletion peak". When a tumour retains heterozygosity for L5-71-3 but shows loss of another marker in the same region, the deletion may be on either the centromeric or the telomeric side. This observation suggests that L5-71-3 is within a region of the genome that is critical for the development of colorectal cancer and begs the question of whether L5-71-3 identifies a single very large gene or a cluster of functionally related genes.li In fact L5-71-3 includes part of an exon (coding region) belonging to a gene expressed in colonic epithelium. An initial screen showed that this gene is mutated in an appreciable proportion of colon cancers. The exact figure is not yet known since only about one third of the coding sequence has been analysed so far. However, on the strength of these findings, it was named MCC (mutated in colon cancer) and for a time was the favoured candidate for the polyposis gene. 12 That status was undermined when diligent searches of a large number of APC families yielded no constitutional mutations in MCC and when at least two APC patients were found to have inherited deletions very close to, but not extending into, MCC. Two groups have now reported the results of intensive molecular mapping of this region.13-16 In all, five new genes have been detected within a stretch of about four million base pairs of DNA. Four of these appear to be unaltered in colon cancer but the fifth, which lies only a hundred thousand bases telomeric to MCC, is also
1364
mutated in
some
sporadic
tumours
and,
most
importantly, constitutional mutations occur in families with APC, the mutant form being inherited with the disease in those kindreds where the analysis has been completed.15 This extremely large gene (named, not surprisingly, APC) encodes a correspondingly large protein expressed in many tissues, including colonic epithelium. Its function and the cellular localisation of its protein product are currently unknown, as is its relation, if any, with the MCC gene product. However, physical clustering of functionally and/or structurally related genes is well known and it seems unlikely that two genes implicated in colon cancer should be juxtaposed merely by chance. One very interesting observation is that an identical point mutation was found in two unrelated APC patients, one of whom had a desmoid tumour while the other had "no evidence of extracolonic disease". 14 This finding apparently confirms that the distinction between APC and Gardner’s syndrome has no simple genetic basis, although the possibility of a second, as yet undetected, mutation in one of the patients is not formally excluded. The underlying molecular lesion in Leppert’s APC-linked non-polyposis colorectal carcinoma family9 has still to be reported. Paradoxically perhaps, precise identification of mutations in individual APC families will not contribute significantly to presymptomatic screening of APC family members, at least in the short term. Such a range of informative flanking markers is now available that accurate risk assignments can be made in the great majority of instances by use of probes from this panell rather than by the complex, timeconsuming, and expensive procedure of searching throughout a very large gene for what may be a single base substitution. This is not the end of the hereditary polyposis story but the beginning of a new and exciting phase that holds out the prospect of understanding the pathogenesis of colorectal cancer at the most fundamental level. Similar progress from gene localisation to isolation and characterisation is being made in increasing numbers of inherited disorders. Nevertheless, as a counter to any suggestion that it may have become a routine procedure, if is worth noting that five years have elapsed since the APC gene was mapped to 5q while the four simultaneous publications reporting its identification have a combined authorship of forty-seven. Murday V, Slack J. Inherited disorders associated with colorectal cancer. Cancer Surv 1989; 8: 139-57. 2. Knudson AG. Hereditary cancers: clues to mechanisms of carcinogenesis. Br J Cancer 1989; 59: 661-66. 3. Herrera L, Kakatis S, Gibas L, Pietryak E, Sandberg AA. Gardner syndrome in a man with an interstitial deletion of 5q. Am J Med Genet 1.
1986; 25: 473-76. 4. Bodmer WF, Bailey CJ, Bodmer J, et al. Localisation of the gene for familial adenomatous polyposis on chromosome 5. Nature 1987; 328: 614-16. 5. Leppert M, Dobbs M, Scambler P, et al. The gene for familial polyposis coli maps to the long arm of chromosome 5. Science 1987; 238 1411-13.
6. Monaco AP, Kunkel LM. A giant locus for the Duchenne and Becker muscular dystrophy gene. Trends Genet 1987; 3: 33-37. 7. Lyons LA, Lewis RA, Strong LC, Zuckerbrod S, Ferrell RE. A general study of Gardner syndrome and congenital hypertrophy of the retinal pigment epithelium. Am J Hum Genet 1988; 42: 290-96. 8. Polkinghorne PJ, Ritchie S, Neale K, Schoeppner G, Thomson JPS, Jay BS. Pigmented lesions of the retinal pigment epithelium and familial adenomatous polyposis. Eye 1990; 4: 216-21. 9. Leppert M, Burt R, Hughes JP, et al. Genetic analysis of an inherited predisposition to colon cancer in family with a variable number of adenomatous polyps. N Engl J Med 1990; 332: 904-08. 10. Ashton-Rickardt PG, Dunlop MG, Nakamura Y, et al. High frequency of APC loss in sporadic colorectal cancer due to breaks clustered in 5q21-22. Oncogene 1989; 4: 1169-74. 11. Ashton-Rickardt PG, Wyllie AH, Bird CC, et al. MCC, a candidate familial polyposis gene in 5q21, shows frequent allele loss in colorectal and lung cancer. Oncogene 1991; 6: 1881-86. 12. Kinzler KW, Nilbert MC, Vogelstein B, et al. Identification of a gene located at chromosome 5q21 that is mutated in colorectal cancers. Science 1991; 251: 1366-70. 13. Kinzler KW, Nilbert MC, Su L-K, et al. Identification of FAP locus genes from chromosome 5q21. Science 1991; 253: 661-65. 14. Nishisho I, Nakamura Y, Miyoshi Y, et al. Mutations of chromosome 5q21 genes in FAP and colorectal cancer patients. Science 1991; 253: 665-69. 15. Groden J, Thliveris A, Samowitz W, et al. Identification and characterisation of the familial adenomatous polyposos gene. Cell 1991. 66: 589-600. 16. Joslyn G, Carlson, Thlieveries A, et al. Identification of deletion mutations and three new genes at the familial polyposis locus. Cell 1991; 66: 601-13. 17. Dunlop MG, Wyllie AM, Steel CM, Piris J, Evans HJ. Linked DNA markers for presymptomatic diagnosis of familial adenomatous polyposis. Lancet 1991; 337: 313-16.
Predictability and When
coronary disease
patient has severe angina that does not medical therapy, the decision to proceed to a coronary artery bypass graft is usually straightforward. But what of the patient with very few symptoms who has angiographically proven coronary disease; can we determine when surgery will prolong life or prevent myocardial infarction? Theoretically, if we could predict both the natural history of the coronary lesions and the outcome of surgery, we could calculate the risk/benefit ratio of bypass grafting. In the first decade of coronary artery surgery this information was not available, so the risk/benefit equation had to be solved empirically by the three main coronary artery surgery trials.1 Are we now in a better position to predict the behaviour of individual coronary lesions? Three factors must be considered: (a) progression of disease; (b) effect of collaterals; and (c) identification of culprit lesions. In general, coronary artery disease is progressive, severe lesions are more likely to progress than mild stenoses,2 and chronic occlusion occurs 3 more commonly in arteries with tight stenoses.3 Nevertheless, there is considerable variation in the behaviour of individual lesions. In a 3-year follow-up study, only 13% of severe coronary stenoses progressed to total occlusion.3When progression is identified, the rate of change is unpredictable. Thus, when Bruschke’s4 team examined 168 patients by angiography on three occasions, thereby providing two intervals for analysis, only 32 patients showed atheroma progression in both intervals and in only 9 of respond
a
to
EDITORIALS
Duchenne/Becker muscular dystrophy, for example.6 However, it has never been established with certainty that the osteomas, skin tags, and desmoid tumours that characterise Gardner’s syndrome actually breed true; it seems more likely that their identification in APC patients depends on how intensively they are
sought.1
Molecular secrets of colorectal cancer
Adenomatous polyposis coli (APC) is in many ways an ideal condition for the application of "reverse genetics"-elucidation of the pathogenesis of a heritable disease by characterisation of the relevant gene, starting from a knowledge only of its approximate position in the genome. The inherent attractions of APC in this respect are that it is a moderately common condition compatible with large family size and that it follows a very clear autosomal dominant pattern of transmission.1 Moreover, following the doctrine attributed mainly to Knudson, it can be predicted that where a genetic lesion predisposes to familial cancer, the same gene will be found to be involved in a proportion of sporadic tumours of the same histological type.2 Since sporadic colorectal cancer is even commoner than APC, there are plenty of opportunities to search in tumour tissue for consistent sites of molecular change, one of which is likely to identify the APC locus. In practice, both linkage studies in APC families and deletion analysis in sporadic colon carcinoma have amply fulfilled the expectations of their proponents. A crucial single case-report of association between APC (behaving as a new mutation) and a visible deletion on the long arm (q) of chromosome 5 provided the first clue as to where the search should concentrate.3 Genetic linkage to molecular markers in 5q21-22 was very rapidly established and, equally important, it was shown that classic APC and Gardner’s syndrome (thought to be distinguishable by the presence of extracolonic lesions in the latter) map to the same region.4,s This finding meant that information from all families could be pooled for more accurate localisation of the gene. It also raised the question of whether different mutations within the same gene might result in different degrees of severity of the disorder, as in
The same caveat probably does not apply to another extracolonic manifestation of APC/Gardner’s syndrome, congenital hypertrophy of the retinal pigment epithelium, which behaves as a good marker for the genetic lesion in most families but is completely absent in a minority.7Furthermore, Leppert and colleagues have described a large family with a hereditary predisposition to colonic polyps, leading to cancer if not removed. Although polyps did not arise in the massive numbers required for a diagnosis of APC, the condition in that family was linked to the APC locus.9 There is therefore ample evidence for a spectrum of phenotypes that could be associated with different mutations. Similar speculation has arisen from findings in sporadic colon cancers. As more and more polymorphic markers for the 5q2l-22 region have become available, the picture that emerges is of a common region of deletion, centred on the site of the APC gene, as defined by family studies.10,11 In a large series of tumours, the actual frequency of such deletions increases as the markers used approach the APC locus and over 50% of tumours show loss of heterozygosity with the probe L5-71-3, which seems to represent the apex of the "deletion peak". When a tumour retains heterozygosity for L5-71-3 but shows loss of another marker in the same region, the deletion may be on either the centromeric or the telomeric side. This observation suggests that L5-71-3 is within a region of the genome that is critical for the development of colorectal cancer and begs the question of whether L5-71-3 identifies a single very large gene or a cluster of functionally related genes.li In fact L5-71-3 includes part of an exon (coding region) belonging to a gene expressed in colonic epithelium. An initial screen showed that this gene is mutated in an appreciable proportion of colon cancers. The exact figure is not yet known since only about one third of the coding sequence has been analysed so far. However, on the strength of these findings, it was named MCC (mutated in colon cancer) and for a time was the favoured candidate for the polyposis gene. 12 That status was undermined when diligent searches of a large number of APC families yielded no constitutional mutations in MCC and when at least two APC patients were found to have inherited deletions very close to, but not extending into, MCC. Two groups have now reported the results of intensive molecular mapping of this region.13-16 In all, five new genes have been detected within a stretch of about four million base pairs of DNA. Four of these appear to be unaltered in colon cancer but the fifth, which lies only a hundred thousand bases telomeric to MCC, is also
1364
mutated in
some
sporadic
tumours
and,
most
importantly, constitutional mutations occur in families with APC, the mutant form being inherited with the disease in those kindreds where the analysis has been completed.15 This extremely large gene (named, not surprisingly, APC) encodes a correspondingly large protein expressed in many tissues, including colonic epithelium. Its function and the cellular localisation of its protein product are currently unknown, as is its relation, if any, with the MCC gene product. However, physical clustering of functionally and/or structurally related genes is well known and it seems unlikely that two genes implicated in colon cancer should be juxtaposed merely by chance. One very interesting observation is that an identical point mutation was found in two unrelated APC patients, one of whom had a desmoid tumour while the other had "no evidence of extracolonic disease". 14 This finding apparently confirms that the distinction between APC and Gardner’s syndrome has no simple genetic basis, although the possibility of a second, as yet undetected, mutation in one of the patients is not formally excluded. The underlying molecular lesion in Leppert’s APC-linked non-polyposis colorectal carcinoma family9 has still to be reported. Paradoxically perhaps, precise identification of mutations in individual APC families will not contribute significantly to presymptomatic screening of APC family members, at least in the short term. Such a range of informative flanking markers is now available that accurate risk assignments can be made in the great majority of instances by use of probes from this panell rather than by the complex, timeconsuming, and expensive procedure of searching throughout a very large gene for what may be a single base substitution. This is not the end of the hereditary polyposis story but the beginning of a new and exciting phase that holds out the prospect of understanding the pathogenesis of colorectal cancer at the most fundamental level. Similar progress from gene localisation to isolation and characterisation is being made in increasing numbers of inherited disorders. Nevertheless, as a counter to any suggestion that it may have become a routine procedure, if is worth noting that five years have elapsed since the APC gene was mapped to 5q while the four simultaneous publications reporting its identification have a combined authorship of forty-seven. Murday V, Slack J. Inherited disorders associated with colorectal cancer. Cancer Surv 1989; 8: 139-57. 2. Knudson AG. Hereditary cancers: clues to mechanisms of carcinogenesis. Br J Cancer 1989; 59: 661-66. 3. Herrera L, Kakatis S, Gibas L, Pietryak E, Sandberg AA. Gardner syndrome in a man with an interstitial deletion of 5q. Am J Med Genet 1.
1986; 25: 473-76. 4. Bodmer WF, Bailey CJ, Bodmer J, et al. Localisation of the gene for familial adenomatous polyposis on chromosome 5. Nature 1987; 328: 614-16. 5. Leppert M, Dobbs M, Scambler P, et al. The gene for familial polyposis coli maps to the long arm of chromosome 5. Science 1987; 238 1411-13.
6. Monaco AP, Kunkel LM. A giant locus for the Duchenne and Becker muscular dystrophy gene. Trends Genet 1987; 3: 33-37. 7. Lyons LA, Lewis RA, Strong LC, Zuckerbrod S, Ferrell RE. A general study of Gardner syndrome and congenital hypertrophy of the retinal pigment epithelium. Am J Hum Genet 1988; 42: 290-96. 8. Polkinghorne PJ, Ritchie S, Neale K, Schoeppner G, Thomson JPS, Jay BS. Pigmented lesions of the retinal pigment epithelium and familial adenomatous polyposis. Eye 1990; 4: 216-21. 9. Leppert M, Burt R, Hughes JP, et al. Genetic analysis of an inherited predisposition to colon cancer in family with a variable number of adenomatous polyps. N Engl J Med 1990; 332: 904-08. 10. Ashton-Rickardt PG, Dunlop MG, Nakamura Y, et al. High frequency of APC loss in sporadic colorectal cancer due to breaks clustered in 5q21-22. Oncogene 1989; 4: 1169-74. 11. Ashton-Rickardt PG, Wyllie AH, Bird CC, et al. MCC, a candidate familial polyposis gene in 5q21, shows frequent allele loss in colorectal and lung cancer. Oncogene 1991; 6: 1881-86. 12. Kinzler KW, Nilbert MC, Vogelstein B, et al. Identification of a gene located at chromosome 5q21 that is mutated in colorectal cancers. Science 1991; 251: 1366-70. 13. Kinzler KW, Nilbert MC, Su L-K, et al. Identification of FAP locus genes from chromosome 5q21. Science 1991; 253: 661-65. 14. Nishisho I, Nakamura Y, Miyoshi Y, et al. Mutations of chromosome 5q21 genes in FAP and colorectal cancer patients. Science 1991; 253: 665-69. 15. Groden J, Thliveris A, Samowitz W, et al. Identification and characterisation of the familial adenomatous polyposos gene. Cell 1991. 66: 589-600. 16. Joslyn G, Carlson, Thlieveries A, et al. Identification of deletion mutations and three new genes at the familial polyposis locus. Cell 1991; 66: 601-13. 17. Dunlop MG, Wyllie AM, Steel CM, Piris J, Evans HJ. Linked DNA markers for presymptomatic diagnosis of familial adenomatous polyposis. Lancet 1991; 337: 313-16.
Predictability and When
coronary disease
patient has severe angina that does not medical therapy, the decision to proceed to a coronary artery bypass graft is usually straightforward. But what of the patient with very few symptoms who has angiographically proven coronary disease; can we determine when surgery will prolong life or prevent myocardial infarction? Theoretically, if we could predict both the natural history of the coronary lesions and the outcome of surgery, we could calculate the risk/benefit ratio of bypass grafting. In the first decade of coronary artery surgery this information was not available, so the risk/benefit equation had to be solved empirically by the three main coronary artery surgery trials.1 Are we now in a better position to predict the behaviour of individual coronary lesions? Three factors must be considered: (a) progression of disease; (b) effect of collaterals; and (c) identification of culprit lesions. In general, coronary artery disease is progressive, severe lesions are more likely to progress than mild stenoses,2 and chronic occlusion occurs 3 more commonly in arteries with tight stenoses.3 Nevertheless, there is considerable variation in the behaviour of individual lesions. In a 3-year follow-up study, only 13% of severe coronary stenoses progressed to total occlusion.3When progression is identified, the rate of change is unpredictable. Thus, when Bruschke’s4 team examined 168 patients by angiography on three occasions, thereby providing two intervals for analysis, only 32 patients showed atheroma progression in both intervals and in only 9 of respond
a
to
