Understanding familial colorectal cancer—finding the corner pieces and filling in the center of the puzzle

Abstract

Colorectal cancer (CRC) is the most common malignancy managed by the gastroenterologist. CRC is caused by a complex interplay between host susceptibility and environmental exposure. When considering worldwide variations in CRC incidence, environmental exposures—presumably dietary—are probably the most important determinants of risks. However, when we evaluate our patients, genetic factors are of greater importance in assigning risk of CRC. CRC is a common disease, affecting more than 5% of Americans, so a substantial proportion of patients will have a positive family history of this disease. Only a small proportion of those positive family histories represent a syndromic, clinically identifiable disease entity such as familial adenomatous polyposis (FAP) or Lynch syndrome (also called hereditary nonpolyposis colorectal cancer [HNPCC]. The latter are recognizable entities because those affected have very strong family histories of cancer and develop tumors 2–3 decades earlier than the general population. These syndromic forms of familial CRC are uncommon, and probably account for <5% of all cases of colon cancer. Less impressive familial clusters of colon cancer have several possible explanations. First, families have shared environmental exposures and, if sufficiently powerful, these may confer an increased risk of cancer. If one encountered a familial cluster of lung cancer, the most obvious question to ask would be how many in the family were smokers. Second, there may be additional genetic diseases—yet to be discovered—that cause familial CRC. Finally, it is possible that some familial CRC is caused by mutations in the known genes responsible for syndromic CRC but which are weaker or attenuated (Table 1). These diseases overlap clinically and can be confusing. Two articles in this issue of Gastroenterology shed new light on this issue and will have direct impact on the clinical treatment of patients.Table 1Familial CRC SyndromesClinical PhenotypeDiseaseGermline mutationMutational signature in tumorsRecurrent adenomatous polyps and CRCFAPAPC geneStop codon between 1250–1464 (profuse polyposis)10Nagase H. Miyoshi Y. Horii A. Aoki T. Ogawa M. Utsunomiya J. Baba S. Sasazuki T. Nakamura Y. Correlation between the location of germ-line mutations in the APC gene and the number of colorectal polyps in familial adenomatous polyposis patients.Cancer Res. 1992; 52: 4055-4057PubMed Google ScholarStop codon flanking 1250–1464 (milder polyposis)Attenuated FAPAPC geneStop codon at 5′ or 3′ extremes of APC11Spirio L. Olschwang S. Groden J. Robertson M. Samowitz W. Joslyn G. Gelbert L. Thliveris A. Carlson M. Otterud B. et al.Alleles of the APC gene an attenuated form of familial polyposis.Cell. 1993; 75: 951-957Abstract Full Text PDF PubMed Scopus (536) Google Scholar, 12Knudsen A.L. Bisgaard M.L. Bulow S. Attenuated familial adenomatous polyposis (AFAP). A review of the literature.Fam Cancer. 2003; 2: 43-55Crossref PubMed Scopus (252) Google ScholarMYH geneMultiple different G:C → T:A mutations16Al Tassan N. Chmiel N.H. Maynard J. Fleming N. Livingston A.L. Williams G.T. Hodges A.K. Davies D.R. David S.S. Sampson J.R. Cheadle J.P. Inherited variants of MYH associated with somatic G C → T:A mutations in colorectal tumors.Nat Genet. 2002; 30: 227-232Crossref PubMed Scopus (1081) Google ScholarEarly onset cancers of the colon, rectum, endometrium, stomach, ovary, renal collecting system, etc.Lynch syndromehMSH2MSI-H20Boland C.R. Thibodeau S.N. Hamilton S.R. Sidransky D. Eshleman J.R. Burt R.W. Meltzer S.J. Rodriguez-Bigas M.A. Fodde R. Ranzani G.N. Srivastava S. A National Cancer Institute Workshop on Microsatellite Instability for cancer detection and familial predisposition development of international criteria for the determination of microsatellite instability in colorectal cancer.Cancer Res. 1998; 58: 5248-5257PubMed Google ScholarhMLH1MSI-HAttenuated Lynch syndromehMSH6MSI-H (86%)MSI-L (14%)MSI-H, microsatellite instability-high; MSI-L, microsatellite instability-low.20Boland C.R. Thibodeau S.N. Hamilton S.R. Sidransky D. Eshleman J.R. Burt R.W. Meltzer S.J. Rodriguez-Bigas M.A. Fodde R. Ranzani G.N. Srivastava S. A National Cancer Institute Workshop on Microsatellite Instability for cancer detection and familial predisposition development of international criteria for the determination of microsatellite instability in colorectal cancer.Cancer Res. 1998; 58: 5248-5257PubMed Google Scholar. Open table in a new tab MSI-H, microsatellite instability-high; MSI-L, microsatellite instability-low.20Boland C.R. Thibodeau S.N. Hamilton S.R. Sidransky D. Eshleman J.R. Burt R.W. Meltzer S.J. Rodriguez-Bigas M.A. Fodde R. Ranzani G.N. Srivastava S. A National Cancer Institute Workshop on Microsatellite Instability for cancer detection and familial predisposition development of international criteria for the determination of microsatellite instability in colorectal cancer.Cancer Res. 1998; 58: 5248-5257PubMed Google Scholar. Fifty years ago, the only form of hereditary colon cancer known was FAP. This was recognized because of its distinctive phenotype. Anyone who had an opportunity to observe a colon filled with thousands of adenomatous polyps would recognize that something was amiss. In 1987, the gene for FAP was mapped to chromosome 5q,1Bodmer W.F. Bailey C.J. Bodmer J. Bussey H.J. Ellis A. Gorman P. Lucibello F.C. Murday V.A. Rider S.H. Scambler P. et al.Localization of the gene for familial adenomatous polyposis on chromosome 5.Nature. 1987; 328: 614-616Crossref PubMed Scopus (1146) Google Scholar and, in 1991, 2 groups simultaneously reported that mutations in, or deletions of, the adenomatous polyposis coli (APC) gene gave rise to FAP.2Kinzler K.W. Nilbert M.C. Su L.K. Vogelstein B. Bryan T.M. Levy D.B. Smith K.J. Preisinger A.C. Hedge P. McKechnie D. et al.Identification of FAP locus genes from chromosome 5q21.Science. 1991; 253: 661-665Crossref PubMed Scopus (2007) Google Scholar, 3Groden J. Thliveris A. Samowitz W. Carlson M. Gelbert L. Albertsen H. Joslyn G. Stevens J. Spirio L. Robertson M. et al.Identification and characterization of the familial adenomatous polyposis coli gene.Cell. 1991; 66: 589-600Abstract Full Text PDF PubMed Scopus (2403) Google Scholar Once this gene was identified, it was subsequently confirmed that the so-called variants of FAP, including Gardner syndrome, Turcot syndrome, and other variations that carry long-forgotten eponyms, were all caused by mutations in the APC gene. Thus, in the initial phase of discovery, it is possible to lump a variety of slightly heterogeneous conditions under one umbrella. Finding the APC gene represented the first corner piece in the familial colon cancer puzzle. However, an even larger group of patients with familial colon cancer had no defects in the APC gene but, instead, had disease with a modest number of adenomatous polyps that rapidly evolved into cancers at an early age. Lynch syndrome (or HNPCC), was linked to germline defects in the DNA mismatch repair (MMR) genes by 3 groups that worked independently. A group lead by Perucho4Ionov Y. Peinado M.A. Malkhosyan S. Shibata D. Perucho M. Ubiquitous somatic mutations in simple repeated sequences reveal a new mechanism for colonic carcinogenesis.Nature. 1993; 363: 558-561Crossref PubMed Scopus (2410) Google Scholar recognized a unique mutational signature in a subset of approximately 12%–15% of CRCs. Independently, Thibodeau et al.5Thibodeau S.N. Bren G. Schaid D. Microsatellite instability in cancer of the proximal colon.Science. 1993; 260: 816-819Crossref PubMed Scopus (2805) Google Scholar at the Mayo Clinic made the same observation. An international collaboration led by both Vogelstein at Johns Hopkins and de la Chapelle from Finland first linked Lynch syndrome to a genetic locus on chromosome 2p,6Peltomaki P. Aaltonen L.A. Sistonen P. Pylkkanen L. Mecklin J.P. Jarvinen H. Green J.S. Jass J.R. Weber J.L. Leach F.S. et al.Genetic mapping of a locus predisposing to human colorectal cancer.Science. 1993; 260: 810-812Crossref PubMed Scopus (808) Google Scholar and then, while looking for allelic loses at this locus (assuming it would be a tumor-suppressor gene), also found the mutational signature which we now call microsatellite instability (MSI).7Aaltonen L.A. Peltomaki P. Leach F.S. Sistonen P. Pylkkanen L. Mecklin J.P. Jarvinen H. Powell S.M. Jen J. Hamilton S.R. et al.Clues to the pathogenesis of familial colorectal cancer.Science. 1993; 260: 812-816Crossref PubMed Scopus (2574) Google Scholar Although it was not obvious to the tumor biologist, yeast geneticists recognized the mutational spectrum as one that would be caused by loss of the DNA MMR genes. In less than a year, 2 groups independently found that the MSH2 (also called hMSH2, indicating the human gene) and MLH1 genes were responsible for most of Lynch syndrome.8Fishel R. Lescoe M.K. Rao M.R. Copeland N.G. Jenkins N.A. Garber J. Kane M. Kolodner R. The human mutator gene homolog MSH2 and its association with hereditary nonpolyposis colon cancer.Cell. 1993; 75: 1027-1038Abstract Full Text PDF PubMed Scopus (2603) Google Scholar, 9Leach F.S. Nicolaides N.C. Papadopoulos N. Liu B. Jen J. Parsons R. Peltomaki P. Sistonen P. Aaltonen L.A. Nystrom-Lahti M. et al.Mutations of a mutS homolog in hereditary nonpolyposis colorectal cancer.Cell. 1993; 75: 1215-1225Abstract Full Text PDF PubMed Scopus (2102) Google Scholar Thus, the second major corner piece of familial colon cancer was found. It is only after one identifies corner pieces that one can begin to fill in the center of the puzzle. The first of the center pieces was the recognition that the intensity of the phenotype of FAP is determined in large part by the location of the mutation in the APC gene. Specifically, virtually all mutations that inactivate APC are either premature stop codons or deletions of the entire gene. However, premature stop codons that occur in the middle of the APC gene (specifically, between codons 1250 and 1464) caused profuse polyposis, with an average of 5000 polyps.10Nagase H. Miyoshi Y. Horii A. Aoki T. Ogawa M. Utsunomiya J. Baba S. Sasazuki T. Nakamura Y. Correlation between the location of germ-line mutations in the APC gene and the number of colorectal polyps in familial adenomatous polyposis patients.Cancer Res. 1992; 52: 4055-4057PubMed Google Scholar Mutations that are upstream (i.e., toward the 5′ end of the gene) or downstream (i.e., toward the 3′ end of the gene) of this region are associated with a less severe form of the disease, having an average of 1000 polyps in the colon, and a later onset of disease. An attenuated form of the disease occurs when mutations occur at the extreme 5′ end of the gene,11Spirio L. Olschwang S. Groden J. Robertson M. Samowitz W. Joslyn G. Gelbert L. Thliveris A. Carlson M. Otterud B. et al.Alleles of the APC gene an attenuated form of familial polyposis.Cell. 1993; 75: 951-957Abstract Full Text PDF PubMed Scopus (536) Google Scholar at the 3′ extremes of the gene, and in exon 9.12Knudsen A.L. Bisgaard M.L. Bulow S. Attenuated familial adenomatous polyposis (AFAP). A review of the literature.Fam Cancer. 2003; 2: 43-55Crossref PubMed Scopus (252) Google Scholar This came to be called attenuated FAP. Most of the patients had a very modest number of polyps (<50–100) and the polyps occurred later in life. Classic FAP has a median onset for the first polyp at age 16 years; the polyps develop 1–2 decades later in attenuated FAP. It was subsequently shown that the mechanism responsible for the attenuated phenotype for the 5′ mutations was an internal ribosomal entry site that permitted bypassing of the premature stop codons.13Heppner G.K. Trzepacz C. Tuohy T.M. Groden J. Attenuated APC alleles produce functional protein from internal translation initiation.Proc Natl Acad Sci U S A. 2002; 99: 8161-8166Crossref PubMed Scopus (58) Google Scholar The APC gene is considered the gatekeeper for colorectal epithelial growth14Kinzler K.W. Vogelstein B. Lessons from hereditary colorectal cancer.Cell. 1996; 87: 159-170Abstract Full Text Full Text PDF PubMed Scopus (4274) Google Scholar, 15Kinzler K.W. Vogelstein B. Cancer-susceptibility genes. Gatekeepers and caretakers.Nature. 1997; 386: 761-763Crossref PubMed Scopus (1008) Google Scholar because the protein product of this gene is responsible for limiting intracellular concentrations of β-catenin, which promotes epithelial growth in the colon. At the appropriate time, the APC gene is expressed, which terminates growth. The APC gene is a tumor-suppressor gene and its inactivation is associated with the onset of the adenomatous polyp. However, each cell has 2 APC alleles (i.e., one paternal and one maternal), and only one is necessary for normal cell behavior. Thus, most colorectal neoplasms have two alterations or mutations in the APC gene. Individuals with FAP have 1 inactivated APC allele in every cell; a second mutational event is acquired for loss of function, and the mechanism by which this occurs depends on the environmental exposure involved. Thus, if one were to analyze 10 neoplasms from a patient with FAP, one would find the same germline mutation in every lesion, and 10 unique mutations. The disease is inherited as an autosomal-dominant disease and each offspring who inherits the mutant allele is at an extremely high risk of developing multiple polyps, beginning in the teenage years. Some patients have multiple adenomatous polyps and are at high risk of cancer, but their germline has 2 normal (wild-type) copies of the APC gene. In 2002, Al-Tassan et al.16Al Tassan N. Chmiel N.H. Maynard J. Fleming N. Livingston A.L. Williams G.T. Hodges A.K. Davies D.R. David S.S. Sampson J.R. Cheadle J.P. Inherited variants of MYH associated with somatic G C → T:A mutations in colorectal tumors.Nat Genet. 2002; 30: 227-232Crossref PubMed Scopus (1081) Google Scholar analyzed polyps from patients who had multiple adenomatous polyps but 2 wild-type copies of the APC gene in the germ line. They found that each of the colorectal adenomas had 2 unique inactivating mutations in the APC genes. The mutational spectrum (specifically, a predominance of G:C → T:A mutations) led these investigators to speculate that a defect in the base excision repair (BER) system might be involved. It was subsequently shown that many of the patients who have a clinical phenotype of mild or attenuated FAP, but no germline mutations in the APC gene, have defects in their MYH genes that would cause inactivation of BER. This disease, which may be called MYH polyposis, is caused by 2 inactivating mutations in the MYH genes.17Sieber O.M. Lipton L. Crabtree M. Heinimann K. Fidalgo P. Phillips R.K. Bisgaard M.L. Orntoft T.F. Aaltonen L.A. Hodgson S.V. Thomas H.J. Tomlinson I.P. Multiple colorectal adenomas, classic adenomatous polyposis, and germ-line mutations in MYH.N Engl J Med. 2003; 348: 791-799Crossref PubMed Scopus (737) Google Scholar, 18Sampson J.R. Dolwani S. Jones S. Eccles D. Ellis A. Evans D.G. Frayling I. Jordan S. Maher E.R. Mak T. Maynard J. Pigatto F. Shaw J. Cheadle J.P. Autosomal recessive colorectal adenomatous polyposis due to inherited mutations of MYH.Lancet. 2003; 362: 39-41Abstract Full Text Full Text PDF PubMed Scopus (364) Google Scholar Patients with MYH polyposis have intrinsic genomic instability in every cell of the body because of inactivation of BER. This is analogous to the disease xeroderma pigmentosa, which is an extreme sensitivity to sunlight that is caused by inheriting biallelic inactivation of genes responsible for nucleotide excision repair. Because DNA damage induced by sunlight requires nucleotide excision repair to be repaired, patients with xeroderma pigmentosa are at risk of extreme sunburns and can develop multiple skin cancers before adolescence. Apparently, because of the unique environmental stress to which the colonic epithelium is exposed and the structure of the APC gene, patients with an inactive BER system are at great risk of acquiring biallelic mutations that inactivate the APC genes, which consequently gives rise to adenomatous polyps. Some proportion of the healthy population carries one of these inactivating mutations. However, to develop MYH polyposis, one must inherit a mutation from each of the parents, and, being an autosomal-recessive disease, only 1 of 4 offspring from a pair of carriers will have MYH polyposis. Affected individuals will appear to have a sporadic case of FAP, and, if one assumed that all cases of FAP were caused by germline mutations of the APC gene, one would be very puzzled. The parents and the children would not share the phenotype, and only 25% of the siblings of an affected individual would have the disease. Understanding MYH polyposis has given us a second large interior piece of our puzzle. The biochemistry of the DNA MMR system predicted from the beginning that the major DNA MMR genes (MSH2 and MLH1) would be responsible for the most severe forms of the disease.19Acharya S. Wilson T. Gradia S. Kane M.F. Guerrette S. Marsischky G.T. Kolodner R. Fishel R. hMSH2 forms specific mispair-binding complexes with hMSH3 and hMSH6.Proc Natl Acad Sci U S A. 1996; 93: 13629-13634Crossref PubMed Scopus (467) Google Scholar Interestingly, patients who develop CRC in this setting do not have antecedent polyposis, but they still tend to develop their cancers in their early 40s, which is the same as seen in FAP. One can determine that a tumor has developed because of this disease by finding MSI in the DNA extracted from the tumor—a phenomenon that is uniquely linked to defective DNA MMR activity.20Boland C.R. Thibodeau S.N. Hamilton S.R. Sidransky D. Eshleman J.R. Burt R.W. Meltzer S.J. Rodriguez-Bigas M.A. Fodde R. Ranzani G.N. Srivastava S. A National Cancer Institute Workshop on Microsatellite Instability for cancer detection and familial predisposition development of international criteria for the determination of microsatellite instability in colorectal cancer.Cancer Res. 1998; 58: 5248-5257PubMed Google Scholar The DNA MMR system is a complex family of proteins that detect and repair errors that occur during DNA synthesis. Certain members of the DNA MMR system are not absolutely required for DNA MMR activity. One of these is the MSH6 protein, which can be at least partially replaced by MSH3, the product of a closely related gene. In 1996, it was discovered that germline mutations in MSH6 cause an attenuated form of Lynch syndrome in humans.21Miyaki M. Konishi M. Tanaka K. Kikuchi-Yanoshita R. Muraoka M. Yasuno M. Igari T. Koike M. Chiba M. Mori T. Germline mutation of MSH6 as the cause of hereditary nonpolyposis colorectal cancer.Nat Genet. 1997; 17: 271-272Crossref PubMed Scopus (594) Google Scholar, 22Wijnen J. De Leeuw W. Vasen H. van der K.H. Moller P. Stormorken A. Meijers-Heijboer H. Lindhout D. Menko F. Vossen S. Moslein G. Tops C. Brocker-Vriends A. Wu Y. Hofstra R. Sijmons R. Cornelisse C. Morreau H. Fodde R. Familial endometrial cancer in female carriers of MSH6 germline mutations.Nat Genet. 1999; 23: 142-144Crossref PubMed Scopus (366) Google Scholar It was initially noted that patients with these mutations suffered from some degree of familial cancer. However, the disease was incompletely penetrant (i.e., not everyone with the mutation developed cancer), and the cancers occurred later in life, the median onset over age 60, which is intermediate between the age when CRC develops in Lynch syndrome and the median age of sporadic CRCs (69–70 years old in the United States). Furthermore, Kolodner et al.23Kolodner R.D. Tytell J.D. Schmeits J.L. Kane M.F. Gupta R.D. Weger J. Wahlberg S. Fox E.A. Peel D. Ziogas A. Garber J.E. Syngal S. Anton-Culver H. Li F.P. Germ-line msh6 mutations in colorectal cancer families.Cancer Res. 1999; 59: 5068-5074PubMed Google Scholar showed that more than 8% of families with a cluster of CRC who did not meet the Amsterdam criteria for classic Lynch syndrome had germline mutations in the MSH6 gene. Thus, another critical interior piece of the familial colon cancer puzzle was found. In this issue of Gastroenterology, Wang et al.24Wang L. Baudhuin L.M. Boardman L.A. Steenblock K.J. Peterson G.M. Halling K.C. French A.J. Johnson R.A. Burgart L.J. Rabe K. Lindor N.M. Thibodeau S.N. MYH mutations in patients with attenuated and classic polyposis and with young-onset colorectal cancer without polyps.Gastroenterology. 2004; 127: 9-16Abstract Full Text Full Text PDF PubMed Scopus (194) Google Scholar from the Mayo Clinic have made important observations regarding the phenotype caused by homozygous mutations in the MYH genes. They used a rapid throughput technique to screen for the 2 most prevalent MYH mutations in whites, Y165C and G382D. There are other mutations that may occur in this gene, so their findings represent a minimal estimate of the impact of this disease in the population. Because this is an autosomal-recessive disease, the expectation is that some proportion of the population will carry one mutated allele but will be unaffected. It is not clear what is the population frequency of MYH mutations. However, Wang et al.24Wang L. Baudhuin L.M. Boardman L.A. Steenblock K.J. Peterson G.M. Halling K.C. French A.J. Johnson R.A. Burgart L.J. Rabe K. Lindor N.M. Thibodeau S.N. MYH mutations in patients with attenuated and classic polyposis and with young-onset colorectal cancer without polyps.Gastroenterology. 2004; 127: 9-16Abstract Full Text Full Text PDF PubMed Scopus (194) Google Scholar found that 1.3% of patients with a normal colonoscopic examination carried a monoallelic germline mutation. Interestingly, 2.3% of patients who had CRC had a monoallelic mutation, which results in an odds ratio of 1.7 (with wide confidence intervals). Thus, it would appear that MYH mutations in the germline play some role in increasing one’s risk of CRC, but, certainly, this does not fill up the centerpieces of the familial colon cancer puzzle. Curiously, monoallelic MYH mutations are slightly more common in CRC patients older than 50 than in those who are younger. It was far more interesting that there was a stepwise increase in biallelic mutations among the patients with increasing numbers of multiple adenomatous polyps. No patient with fewer than 20 polyps had biallelic mutations in MYH. However, this was found in 13.6% of those with 20–49 polyps and in 20% of those with ≥50 polyps. There was no overlap between the mutational spectrum seen in Lynch syndrome (i.e., MSI) and the multiple mutations seen in the absence of BER (i.e., MYH polyposis). Thus, biallelic mutations in the MYH gene are an important cause of multiple adenomatous polyps in individuals who do not have a germline mutation in the APC gene. Furthermore, this helps explain the individuals with multiple adenomatous polyps in the apparent absence of a family history. As is the case with autosomal-recessive disease, one expects illness among siblings in the absence of illness in the parents. We now have a second mechanistic explanation for attenuated FAP. In the first of these, the germline mutation in the APC gene can be partially minimized by the ability of the ribosome to skip over the premature stop codon. In the case of MYH polyposis, a generalized form of genomic instability accelerates the acquisition of APC mutations. Both develop multiple colorectal adenomas at an early age. Hendriks et al.25Hendricks Y.M.C. Wagner A. Morreau H. Menko F. Stormorken A. Quehenberger F. Sandkuijl L. Moller P. Genuardi M. van Houwelingen H. Tops C. van Puijenbroek M. Verkuijlen P. Kenter G. van Mil A. Meijers-Heijboer H. Tan G.B. Breuning M.H. Fodde R. Wijnen J.Th. Bröcker-Vriends A.H.J.T. Vasen H. Cancer risk in hereditary nonpolyposis colorectal cancer due to MSH6 mutations impact on counseling and surveillance.Gastroenterology. 2004; 127: 17-25Abstract Full Text Full Text PDF PubMed Scopus (338) Google Scholar have determined the form of hereditary colon cancer that results from germline mutations in MSH6. This work was accomplished by a collaboration among several European hereditary cancer registries, led by a group from The Netherlands. They collected a total of 20 families with truncating germline mutations in the MSH6 gene. Truncating mutations (i.e., those caused by premature stop codons) are valuable for study because they unambiguously abrogate the function of a gene; missense mutations of a gene may or may not alter its function and are more difficult to interpret. This permitted them to study the phenotype in a total of 146 carriers of this disease, including 57 men and 87 women. Outcomes in these families were compared with Lynch syndrome families carrying mutations in the MSH2 and MLH1 genes. Germline mutations in MSH6 result in an attenuated phenotype—of sorts. The risk of all Lynch syndrome-related tumors was significantly lower in those with MSH6 mutations than in those with MLH1 or MSH2 mutations. However, although the cumulative risks of these cancers were delayed in time, by age 70, people with MSH6 mutations had the same risk of cancer—which exceeded 80%. Interestingly, the specific risk of CRC in men with MSH6 mutations was slightly (but not significantly) lower than with MSH2 or MLH1 mutations. The mean age for diagnosis of CRC in men with MSH6 mutations was 55 years, versus 43–44 years for the other 2 genes. However, women with germline mutations in MSH6 had a significantly reduced risk of CRC, developing this disease at a mean age of 57 years. On the other hand, the risk of endometrial cancer, although delayed in onset (median age, 54 years), was actually a significantly greater problem than with MSH2 and MLH1 mutations. For women, germline mutations in MSH6 cause hereditary endometrial cancer, complicated by moderately high risks of CRC. Hendriks et al.30 also showed that the use of MSI testing and immunohistochemistry were useful diagnostic tests for this disease, being 86% and 90% sensitive, respectively. Furthermore, research laboratories can detect all cases of MSH6 CRCs if they include both high (MSI-H) and low levels of MSI (MSI-L) in their diagnostic criteria,20Boland C.R. Thibodeau S.N. Hamilton S.R. Sidransky D. Eshleman J.R. Burt R.W. Meltzer S.J. Rodriguez-Bigas M.A. Fodde R. Ranzani G.N. Srivastava S. A National Cancer Institute Workshop on Microsatellite Instability for cancer detection and familial predisposition development of international criteria for the determination of microsatellite instability in colorectal cancer.Cancer Res. 1998; 58: 5248-5257PubMed Google Scholar and astute pathologists will be able to identify 95% of these tumors by use of an immunohistochemical panel staining for MSH2, MSH6, MLH1, and PMS2. Thus, MSH6 mutations cause attenuated Lynch syndrome, with some unexpected twists in the story. The Lynch syndrome-associated cancers have a later onset but, as these patients reach age 70 (as most should with proper treatment), the risks of cancer are the same as those with mutations in the MSH2 and MLH1 genes. The onset of CRC occurs later, and women have a major lifetime risk of endometrial cancer. The puzzle of hereditary CRC is being slowly assembled, piece by piece. It was because of the identification of the critical corner pieces (i.e., the linkage of the APC gene to FAP and the linkage of the DNA MMR genes to Lynch syndrome) that these inner pieces are now being assembled. Attenuated FAP can occur either by a mutation at the extreme ends of the APC gene or by biallelic mutations in the MYH gene. The latter event completely changes the genetic complexion of FAP. MYH probably accounts for approximately 30% of attenuated polyposis, whereas germline mutations at the extreme ends of the APC gene account for approximately 10%. This has implications for screening and diagnostic strategies. More importantly for the genetic counselor, MYH polyposis is an autosomal-recessive condition, and counseling must take into account the reproductive partner because affected individuals are only at risk of reproducing the phenotype if the partner is a mutation carrier. To the molecular biologist, APC is a gatekeeper and MYH is a caretaker gene.15Kinzler K.W. Vogelstein B. Cancer-susceptibility genes. Gatekeepers and caretakers.Nature. 1997; 386: 761-763Crossref PubMed Scopus (1008) Google Scholar In both MYH polyposis and Lynch syndrome, the germline mutations provide the genomic instability for the subsequent mutations that are the proximate causes of neoplasia. MYH mutations produce a loss of BER, whereas Lynch syndrome mutations produce defects in the DNA MMR system. Attenuated Lynch syndrome may be caused by a germline mutation in one of the minor DNA MMR genes, MSH6. Curiously, this delays the onset of cancer, but, when such patients reach age 70, the patients have the same very high risk of cancer as those families with germline mutations in MSH2 and MLH1. Physicians must be particularly concerned about familial endometrial cancer in MSH6 families and be aware that familial collections of tumors at older ages may have a genetic basis. The future is likely to bring additional unexpected findings to this story. Unquestionably, new genes will be found that increase the risks of developing cancer, and many of these will be higher prevalence, lower penetrance genes, similar to what has been reported this month in Gastroenterology.