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Molecular basis for subdividing hereditary colon cancer?
  1. W M Grady
  1. Correspondence to:
    Dr W M Grady
    Clinical Research Division, Fred Hutchinson Cancer Research Center, 1100 Fairview Ave N, D4-100, Seattle, WA 98109, USA; wgradyfhcrc.org

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Much progress has been made in our understanding of the molecular basis of familial colorectal cancer syndromes. Molecular characterisation of cancer family syndromes will ultimately be the most accurate way of defining hereditary non-polyposis colorectal cancer-like cancer family syndromes and will provide more accurate information regarding cancer risk and optimal cancer surveillance regimens

Colorectal cancer (CRC) is estimated to affect over 1 million people and to cause over 528 000 deaths worldwide each year (Globocan, 2002). In the USA, the cumulative lifetime risks of CRC and death from CRC are approximately 5–6% and 2.5%, respectively.1 Most colon cancers occur in individuals over the age of 50 years and are believed to develop as a consequence of environmental carcinogen exposure and genetic factors.2,3 However, approximately 3–5% of all colon cancers occur as a direct consequence of highly penetrant germline mutations which cause hereditary colon cancer syndromes, such as familial adenomatous polyposis (FAP), hereditary non-polyposis colon cancer (HNPCC), juvenile polyposis syndrome, and Peutz-Jeghers syndrome.4–6

HNPCC is the most common hereditary CRC syndrome and the subject of a study by Mueller-Koch and colleagues7 in this issue of Gut that characterises the cancer risks of families that meet the clinical definition for HNPCC but who do not have any of the molecular features that have come to define this syndrome (see page 1733). The study by Mueller-Koch and colleagues7 is remarkable because it demonstrates the progress that has been made in our understanding of the molecular basis of familial CRC syndromes. In fact, since the discovery of APC germline mutations as the major cause of FAP and of MLH1 and MSH2 germline mutations as the cause of most cases of HNPCC, it has become increasingly recognised that the clinical presentation of these families with hereditary cancers is often ambiguous.4,8 In fact, because of the growing appreciation that family history and/or presentation of the proband may not accurately reveal the true molecular nature of many cancer families (that is, germline mutation in APCvMLH1, etc), it is now standard practice during the evaluation of individuals with possible hereditary CRC to perform molecular characterisation of the colon neoplasms and/or germline mutation testing. Germline mutation testing not only identifies the specific genetic factor responsible for the cancer risk in these families but also provides accurate predictive information for the risk of colon cancer and extracolonic cancers in these family members.

With regard to HNPCC, in the last 2–3 years there have been several notable advances in our understanding of the molecular nature of this clinical syndrome, which have allowed us to subdivide these HNPCC families by cancer risk on the basis of the underlying germline mutation. In order to appreciate the significance of these advances it is helpful to review the clinical aspects of classic HNPCC. The central clinical features of HNPCC are familial clustering of HNPCC associated tumours (that is, CRC, gastric cancer, endometrial cancer, cancer of the small bowel, renal cancer, and cancer of the ureter) and an early age of onset of these tumours. Classical HNPCC is inherited in an autosomal dominant fashion and is 80% penetrant. Thus the lifetime risk for colon cancer is 80% by age 70 years, and the mean age of diagnosis of colon cancer in an HNPCC individual is 44–48 years, compared with 64 years for sporadic colon cancer.9,10 The majority of HNPCC patients (60–80%) present with colon cancers arising proximal to the splenic flexure but it is important to recognise that HNPCC tumours do occur on the left side of the colon. Approximately 10% of patients will have synchronous (simultaneous onset of two or more distinct tumours separated by normal bowel) or metachronous (non-anastamotic new tumours developing at least six months after the initial diagnosis) colon cancers at the time of diagnosis.11 Furthermore, in 45% of affected individuals, multiple synchronous and/or metachronous CRCs will occur within 10 years of resection of an initial colon cancer, underscoring the importance of establishing a diagnosis in an individual with suspected HNPCC at the time of colon cancer detection so that appropriate surgical treatment can be offered at that time.12,13

In addition to colon cancer, HNPCC family members are at a substantially increased risk of extracolonic cancers. The four most common extracolonic cancers include (in descending order) endometrial, ovarian, gastric, and transitional cell carcinoma of the uroepithelial tract (bladder, kidney, and ureter).14 Endometrial cancer is the most common extracolonic malignancy associated with HNPCC.15–19 Women with HNPCC are at a 10-fold increased risk of endometrial cancer and are usually diagnosed between the ages of 40 and 60 years or 15 years earlier than the general population.20 The estimated cumulative risk by age 70 years is 40–50%.21–23

This understanding of the clinical features of HNPCC has been complemented by dramatic advances in our understanding of the molecular genetics of the syndrome. Our first insight into the molecular characterisation of these tumours came in the early 1990s when it was recognised that the tumours occurring in HNPCC patients had a characteristic molecular change called microsatellite instability (MSI).24 This finding was quickly followed by the discovery that a class of genes that regulate DNA mutation mismatch repair (MMR) activity and DNA microsatellite stability in cells are responsible for many of the cases of HNPCC.8,25–34 The DNA mismatch repair system (also known as the mutation mismatch repair (MMR) system) consists of a complex of proteins that recognise and repair base pair mismatches that occur during DNA replication. At the molecular level, MMR genes encode proteins that are responsible for correcting DNA nucleotide base mispairs and small insertions or deletions that frequently occur during DNA replication.35,36 Thus MMR proteins function as “DNA caretakers” to maintain the fidelity of genomic DNA during DNA replication.

To date, germline mutations in six of these MMR genes have been demonstrated to be prominent causes of either HNPCC or atypical HNPCC. These genes include MLH1, MSH2, MSH6, PMS2, MLH3, and PMS1, in decreasing frequency of occurrence.12,21,37,38 Two genes that were previously implicated as the cause of HNPCC in some families, EXO1 and TGFBR2, have been recently shown to be unlikely causes of HNPCC.39–41 Although six MMR genes have been identified to date to play a role in causing HNPCC, MSH2 (chromosome 2p16), MLH1 (chromosome 3p21), and MSH6 (chromosome 2p16) account for >95% of the germline mutations in those families found to have a defined genetic aetiology.9,42 Other identified MMR genes, PMS1 (chromosome 7p22), PMS2 (chromosome 7p22), and MLH3 (chromosome 14q24.3), account for the other <5% of HNPCC cases.43 There are also reports of constitutional aberrant methylation of MLH1 as the cause of cancer predisposition syndromes, although this mechanism does not appear to be a common cause of HNPCC.44,45

Germline mutations that occur in MSH2 and MLH1 are widely distributed throughout either gene. MSH2 possesses 16 exons and spans 73 kb, and MLH1 has 19 exons and spans 58 kb. Mutations that occur in either gene tend to be missense and nonsense mutations, inframe deletions, large genomic deletions, and putative splice site mutations. Notable progress has been made recently in defining the role of intragenic deletions and missense mutations in these genes, particularly in MSH2 and MLH1. Initially, the majority of deleterious mutations identified in these genes were found to be missense, nonsense, or frameshift mutations, which are the types of mutations that can be readily detected by the mutation detection technique first used to assess these genes, DNA sequencing. However, the use of newer and more sophisticated mutation detection techniques, such as multiplex ligation dependent probe amplification and conversion technology, has revealed that a substantial proportion of MLH1 and MSH2 germline mutations are actually genomic rearrangements.46–48 These types of mutations are missed by DNA sequencing because they are masked by the wild-type allele present in the cells. Furthermore, it is now appreciated through the use of mutation detection techniques employing conversion of haploidy techniques that germline mutations in PMS2 are more common than previously believed.49 Paralogous genes interfered with PMS2 mutation detection by DNA sequencing resulting in a lack of appreciation of the frequency of these mutations in HNPCC families. Paralogous genes mask the ability to identify mutations in PMS2 because the paralogous genes share sequence identity with the 5′ or 3′ ends of PMS2 and can consequently generate “false” wild-type results when mutation analysis in PMS2 is performed using DNA sequencing. The advances in mutation detection techniques have solved one of the mysteries of HNPCC, namely the genetic aetiology of HNPCC families with microsatellite unstable tumours but no detectable germline mutations in any of the MMR genes. It was previously suspected that these families might have mutations in novel genes but it is now known that the majority of these families have mutations in the known MMR genes that were missed by mutation detection techniques using DNA sequencing.

The discovery of different genes (termed locus heterogeneity) and different mutations in these genes (termed allelic heterogeneity) as the cause of HNPCC has led to efforts to determine genotype:phenotype correlations in HNPCC families with differing germline mutations in MMR genes. Notably, compared with families with germline mutations in MLH1 or MSH2, families with MSH6 germline mutations have a later age of onset of CRC (54 years v 44 years), and women in these families have a lower risk of CRC (30% by 71 years of age) but a high risk of endometrial cancer (71% of women by 71 years of age).50,51 With regard to risk of transitional cell carcinoma, some studies have shown that only carriers of MSH2 mutations appear to have a significantly increased risk of cancer in the urinary tract (relative risk of 75.3).23,52 In fact, overall, the relative risk of gastric cancer, ovarian cancer, and cancer of the urinary tract has been shown to be higher in patients with mutations in MSH2 compared with MLH1.23 Furthermore, polymorphisms in TP53, CCND1 (the gene for cyclin D1), and NAT2 appear to associate with earlier age of onset for CRC than is seen in typical HNPCC families, demonstrating another level of molecular subdivision of these families that is likely to become more prominent in the future.53–55 Thus, not surprisingly, our ability to predict the risk of CRC and extracolonic cancers has been improved by our ability to subdivide HNPCC by germline mutation status.

An important caveat that is worth mentioning is that our ability to identify these mutations has outpaced our ability to determine which mutations are deleterious and which are uncommon but innocent polymorphisms. Mutations that are not clearly deleterious are termed “variants of uncertain significance” and an understanding of the clinical significance of these variants will rely on the sharing of mutation analysis results in mutation registries, such as the International Collaborative Group of Hereditary Nonpolyposis Colorectal Cancer (ICG-HNPCC) database (http://www.nfdht.nl).42 In addition, the use of conversion technology in mutation detection assays may permit the reclassification of some of these variants to deleterious mutations.46

Mueller-Koch and colleagues7 have added to this growing body of knowledge that demonstrates the power of molecular characterisation of cancer family syndromes to ultimately be the most accurate way to define HNPCC-like cancer family syndromes. Mueller-Koch et al have shown that there is a subset of families that meet the Amsterdam criteria for HNPCC, which are the most strict clinical criteria for this syndrome, but who do not have detectable molecular changes that define this syndrome (that is, germline mutations in any of the genes implicated in this syndrome or tumours with MSI, a hallmark molecular change observed in cancers arising in HNPCC families). These authors have found that the CRCs in these families have a later age of onset and are more commonly located in the distal colon than is seen in HNPCC families with germline mutations in MLH1 or MSH2, the most common genes affected in classic HNPCC. Furthermore and of substantial clinical importance, these family members appear to have a slower adenoma to carcinoma progression sequence and lower risk of extracolonic cancer than that seen in HNPCC. These findings are congruent with those from other investigators who have characterised these “MMR mutation negative” HNPCC families. In a study published this year by Lindor et al, that corroborates the results of Mueller-Koch et al, these HNPCC-like familial aggregations of colon cancer were termed familial colorectal cancer type X.46,56,57 As has been true of recent progress in HNPCC, it is predicted that identification of the molecular mechanisms responsible for colon cancers in these families with familial colorectal cancer type X will provide more accurate information regarding cancer risk and optimal cancer surveillance regimens. Interestingly, assessment of chromosomal instability, TP53 mutations, and β-catenin localisation in the tumours of these familial colorectal cancer type X patients has revealed unique patterns of alterations, suggesting that novel predisposition genes will be found in these families.58 Characterisation of these “MMR mutation negative” HNPCC families and also of the phenotype of HNPCC families with different germline MMR gene mutations continues to usher in an era in which the molecular aetiology of the cancer family syndrome will be the primary tool for assigning cancer risk and designing cancer prevention programmes.

Acknowledgments

This work was supported by the Damon Runyon Cancer Research Foundation (Damon-Runyon/Lilly Clinical Investigator Award), Department of Veterans Affairs R&D Service (VA Advanced Research Career Development Award), and Edward Mallinckrodt Jr Foundation (Mallinckrodt Scholar Award) to WMG.

Much progress has been made in our understanding of the molecular basis of familial colorectal cancer syndromes. Molecular characterisation of cancer family syndromes will ultimately be the most accurate way of defining hereditary non-polyposis colorectal cancer-like cancer family syndromes and will provide more accurate information regarding cancer risk and optimal cancer surveillance regimens

REFERENCES

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Footnotes

  • Conflict of interest: None declared.

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