A family with juvenile polyposis linked to the BMPR1A locus: Cryptic mutation or closely linked gene?

BACKGROUND AND AIM: Familial juvenile polyposis syndrome (JPS) is a rare autosomal dominant condition in which patients develop hamartomatous gastrointestinal polyps with malignant potential. Pathogenic germline mutations in both the SMAD4 and BMPR1A genes involved in the transforming growth factor beta pathway account for 40% of cases of JPS. Genetic heterogeneity remains evident, as the balance of cases is not accounted for by mutations in these genes. The aim of this study was to determine the mutation responsible in a family with juvenile polyposis. METHODS: An Australian Caucasian family with juvenile polyposis have attended and followed surveillance plans through the Familial Bowel Cancer Clinic, The Royal Melbourne Hospital. A pedigree of the family was constructed with attention to the mixed phenotypic expression of polyps in affected members. Genetic testing for SMAD4 and BMPR1A mutations in germline DNA and linkage analysis to SMAD4, BMPR1A and 15q14 (CRAC1 locus) were performed. RESULTS: There were no pathogenic mutations in SMAD4 and BMPR1A. There was no linkage to SMAD4 or 15q14 (CRAC1 locus). Linkage analysis suggested a cryptic BMPR1A mutation or the presence of another gene in close proximity to the BMPR1A locus. Two additional candidate genes in the region of linkage (PTEN and MINPP1) were excluded. CONCLUSION: Most affected members of this Australian Caucasian family demonstrate a phenotype of mixed polyps: juvenile polyps, adenomas and/or hyperplastic polyps. Cloning of a potentially responsible gene closely linked to the BMPR1A locus or a cryptic mutation in BMPR1A may offer valuable insights into the pathogenesis of JPS.     

Characterization of APC exon 15 germ-line mutation in FAP family with severe phenotype showing extracolonic symptoms

The adenomatous polyposis coli (APC) gene plays a crucial role in colorectal carcinogenesis. Germ-line mutations of APC gene give rise to familial adenomatous polyposis coli (FAP) - autosomal dominant syndrome manifesting hundreds to thousands of colorectal polyps, if untreated with malignant progression. We have used the techniques of heteroduplex analysis (HDA), protein truncation test (PTT), single strand conformation polymorphism (SSCP) and DNA sequencing for the identification and detailed positional analysis of mutations in IFAP family with the expressive phenotype characterized by polyposis and extracolonic lesions. Detailed analysis revealed a 5bp deletion in a mutation cluster region (MCR) in exon 15 of APC gene in codon 1308. Two screened members of the FAP family exhibited this novel mutation.     

Novel Germline APC Mutations in Swedish Patients with Familial Adenomatous Polyposis and Gardner Syndrome

Background: Familial adenomatous polyposis (FAP) is a familial cancer syndrome in which affected individuals develop multiple adenomatous polyps and are thereby at greatly increased risk of developing colorectal cancer. Gardner syndrome is a variant of FAP, in which the patients also develop extraintestinal tumors, in particular osteomas and desmoid tumors. An attenuated form of the disease (AFAP) is associated with fewer polyps, but still a high risk for colorectal cancer. Germline mutations in the adenomatosis polyposis coli (APC) gene cause FAP and Gardner syndrome and have recently been associated also with the development of AFAP. Methods: We have analysed the entire APC gene for germline mutations in 7 patients with FAP and in 6 patients with suspected AFAP. Mutation screening was performed by direct sequencing of exons 1-14 and using the protein truncation test for analysis of exon 15. Results: Novel disease-causing germline mutations, all of which resulted in truncation of the APC protein, were identified in 6 of the 7 patients with FAP or Gardner syndrome. No APC mutation was detected in any of the 6 patients with suspected AFAP. Conclusions: This study reports novel FAP- and Gardner syndrome-causing mutations in the APC gene. The lack of APC mutations in patients with multiple polyps at young age indicates that other genetic defects may cause this phenotype.     

Novel germline APC mutations in Swedish patients with familial adenomatous polyposis and Gardner syndrome

BACKGROUND: Familial adenomatous polyposis (FAP) is a familial cancer syndrome in which affected individuals develop multiple adenomatous polyps and are thereby at greatly increased risk of developing colorectal cancer. Gardner syndrome is a variant of FAP, in which the patients also develop extraintestinal tumors, in particular osteomas and desmoid tumors. An attenuated form of the disease (AFAP) is associated with fewer polyps, but still a high risk for colorectal cancer. Germline mutations in the adenomatosis polyposis coli (APC) gene cause FAP and Gardner syndrome and have recently been associated also with the development of AFAP. METHODS: We have analysed the entire APC gene for germline mutations in 7 patients with FAP and in 6 patients with suspected AFAP. Mutation screening was performed by direct sequencing of exons 1-14 and using the protein truncation test for analysis of exon 15. RESULTS: Novel disease-causing germline mutations, all of which resulted in truncation of the APC protein, were identified in 6 of the 7 patients with FAP or Gardner syndrome. No APC mutation was detected in any of the 6 patients with suspected AFAP. CONCLUSIONS: This study reports novel FAP- and Gardner syndrome-causing mutations in the APC gene. The lack of APC mutations in patients with multiple polyps at young age indicates that other genetic defects may cause this phenotype.     

Singapore familial adenomatous polyposis (FAP) patients with classical adenomatous polyposis but undetectable APC mutations have accelerated cancer progression

OBJECTIVES: Germline mutation in adenomatous polyposis coli (APC) is detected in up to 80% of familial adenomatous polyposis (FAP) patients worldwide. In this study, we evaluated clinical features and APC mutations of Singapore FAP patients and contrasted genotype-phenotype correlation with Caucasians from other regions of the world and between FAP patients with and without detectable APC mutations. METHODS: We screened 242 members from 57 unrelated FAP families using a combination of cDNA protein truncation test, multiplex ligation-dependent probe amplification, and differential expression techniques. RESULTS: APC germline mutations were detected in 50 families. In contrast to Caucasians, fundic gland polyposis in Singapore patients was associated with APC mutations throughout the coding region and osteomas were also not confined to codon 767-1573. There was also no FAP-associated hepatoblastoma or medullablastoma. APC mutation-negative patients from four families with mixed (adenomatous/hyperplastic/atypical juvenile) polyps were subsequently reclassified as hereditary mixed polyposis syndrome (HMPS) patients. APC mutation-negative patients with classical adenomatous polyposis were negative for MYH, beta-catenin, and Axin 1 mutations. These patients had a significantly older age at diagnosis (P < 0.001) and more colorectal cancers (P= 0.017) than patients with APC mutations. CONCLUSIONS: We achieved a 94% (50/53) APC mutation detection rate via a combination of techniques, suggesting that the current detection rate is probably not exhaustive. Singapore patients have some features similar to and other features distinct from Caucasians. Furthermore, APC mutation-negative patients have accelerated cancer progression that merits closer surveillance.     

APC gene mutations and extraintestinal phenotype of familial adenomatous polyposis.

BACKGROUND: Familial adenomatous polyposis (FAP) is caused by germline mutation of the adenomatous polyposis coli (APC) gene on chromosome 5q. AIMS: This study assessed genotype-phenotype correlations for extraintestinal lesions in FAP. METHODS: Mutations of the APC gene were compared with the occurrence of seven extraintestinal manifestations in 475 FAP patients from 51 families. The frequency of manifestations was adjusted for different ages of patients using person years of exposure. In pedigrees without identified APC gene mutation, analysis of linkage to chromosome 5q and/or assessment of neoplasms for replication errors characteristic of mutation in mismatch repair genes were performed. RESULTS: FAP patients from the 42 families (82%) with identified mutations of the APC gene had more frequent expression of extraintestinal manifestations than affected individuals without identified mutations (risk ratio 1.2-4.0; significant difference for cutaneous cysts). The presence of a cutaneous cyst or extraintestinal cancer significantly increased the likelihood of detection of a mutation in the APC gene (94% and 92% respectively; p < 0.05). In patients without identified APC gene mutation, linkage to the APC gene was found in one large family (lod = 5.1, theta 0.01), and replication error phenotype was absent in all 24 neoplasms from 16 members of these nine pedigrees. Expression of pigmented ocular fundus lesions was strongly associated with mutations in codons 541-1309, but no other extraintestinal manifestations were related to mutation position. Multiplicity of extraintestinal manifestations was high with mutation in codons 1465, 1546, and 2621. CONCLUSIONS: Patients with the colorectal phenotype of FAP but no extraintestinal manifestations may have non-truncating mutations of the APC gene or mutation in a gene other than APC or mismatch repair genes. The site of APC gene mutation is associated with pigmented ocular fundus lesions (codons 542-1309) and predisposition to multiplicity of extraintestinal manifestations (codons 1465, 1546, and 2621).     

Analysis of PTEN Gene Mutations in Korean Patients With Cowden Syndrome and Polyposis Syndrome

PURPOSE PTEN (phosphatase and tensin homologue deleted in chromosome 10) is a candidate tumor suppressor gene. Mutations of this gene are responsible for PTEN hamartoma tumor syndromes, including Cowden syndrome, Bannayan-Riley-Ruvalcaba syndrome, Proteus syndrome, and Proteus -like syndromes. Recently, PTEN mutations were identified in several human neoplasms. We analyzed the DNA of various organs and lesions in Korean patients with Cowden syndrome, their family members, and patients with familial adenomatous polyposis for germline or somatic PTEN mutations.METHODS The 11 patients included in this study were 5 patients with Cowden syndrome, 4 of their family members, and 2 patients with familial adenomatous polyposis. Deletions and mutations in exons 1 to 9 of the PTEN gene were evaluated by polymerase chain reaction-single strand conformation polymorphism and sequencing analysis in esophageal acanthosis, gastric polyps, colonic polyps, skin lesions, and peripheral blood mononuclear cells. To exclude common polymorphisms, 240 controls were tested.RESULTS All patients with Cowden syndrome showed several to numerous polyps in the gastrointestinal tract. A missense mutation at codon 217 (GTC to GAC, Val to Asp) in exon 7 was identified in one Cowden syndrome patient, and a nonsense mutation at codon 211 (TGC to TGA, Cys to stop) in exon 6 was identified in a second patient. Identical mutations were found in all tissue samples, including colonic polyps, from each patient. No PTEN mutations were found in their family members or in any patient with familial adenomatous polyposis. None of tested controls contained a mutation.CONCLUSIONS We have identified two new germline PTEN mutations in Korean patients with Cowden syndrome. Mutations in the introns and regulatory regions of the PTEN gene may be present in additional patients with Cowden syndrome and polyposis syndrome.Supported by a grant (Grant number 2003-261) from the Asan Institute for Life Sciences, Seoul, Korea.Reprints are not available.Presented at the meeting of International Gastrointestinal Bioregulation Conference, Hyogo, Japan, March 27, 2004.     

Familial adenomatous polyposis: more evidence for disease diversity and genetic heterogeneity

Familial adenomatous polyposis (FAP) is characterised by the presence of profuse colonic carpeting of adenomas throughout the entire colon and rectum. The genetic basis of FAP has been shown to be primarily associated with germline mutations in the APC gene. Notwithstanding, several reports have been published indicating that there is genetic heterogeneity in FAP and that the most likely explanation is the existence of another gene. In this report we further delineate the genotype/phenotype correlation in families that harbour germline mutations in the APC gene and identify some previously unreported changes in the APC gene which predispose to an attenuated disease phenotype. From 53 index patients diagnosed with either FAP or attenuated FAP, 27 harboured changes in the APC gene. The remaining 26 patients were further subgrouped according to their colonic phenotype. There were nine patients with a mixed hyperplastic/adenomatous colonic phenotype and there were 17 patients with an adenomatous colonic phenotype. Evaluation of the disease characteristics of these patients and their families is presented which may aid in the identification of new genes associated with colonic polyposis.     

Genetic and clinical characterisation of familial adenomatous polyposis: a population based study

BACKGROUND: Familial adenomatous polyposis (FAP) is a rare autosomal dominantly inherited disease predisposing to colon cancer and caused by germline mutations in the APC (adenomatous polyposis coli) gene. AIMS: We conducted a population based study to evaluate the prevalence and clinical implications of APC mutations among Finnish FAP kindreds. A possible founder effect in parallel with previous observations in hereditary non-polyposis colon cancer (HNPCC) was addressed. PATIENTS: Affected individuals from 65 kindreds were included. METHODS: The APC gene was screened for mutations using the protein truncation test and heteroduplex analysis. Haplotype analysis was performed with four flanking microsatellite markers. Families that failed to show any mutations were scrutinised with Southern blot hybridisation and allelic expression analysis. RESULTS: Thirty eight different germline mutations in APC were identified in 47 kindreds (72%). The majority of these mutations were novel and unique to each family. Although sharing the classical polyposis phenotype, families without detectable APC mutations differed from mutation positive families in the following respects: firstly, mean age at polyposis diagnosis was higher (38.6 years (48 individuals) v 30.0 years (140 individuals); p=0.001); and secondly, the proportion of kindreds lacking extracolonic disease was higher (6/18 v. 5/47; p=0.04). CONCLUSIONS: Our results may pave the way for predictive testing in mutation positive families and should stimulate further molecular studies in mutation negative families. No founder effect was observed, which is in contrast with HNPCC in the same population.     

Familial adenomatous polyposis patients without an identified APC germline mutation have a severe phenotype

BACKGROUND: Development of more than 100 colorectal adenomas is diagnostic of the dominantly inherited autosomal disease familial adenomatous polyposis (FAP). Germline mutations can be identified in the adenomatous polyposis coli (APC) gene in approximately 80% of patients. The APC protein comprises several regions and domains for interaction with other proteins, and specific clinical manifestations are associated with the mutation assignment to one of these regions or domains. AIMS: The phenotype in patients without an identified causative APC mutation was compared with the phenotype in patients with a known APC mutation and with the phenotypes characteristic of patients with mutations in specific APC regions and domains. PATIENTS: Data on 121 FAP probands and 149 call up patients from 70 different families were extracted from the Danish Polyposis register. METHODS: Differences in 16 clinical manifestations were analysed according to the patient's mutational status. Two sided independent t sample test, two sided chi(2) test, and odds ratios were calculated. RESULTS: Patients without identified APC mutations had a unique and severe phenotype, which was roughly described as: young age at diagnosis and subsequent death in spite of development of few colorectal adenomas; low risk of involvement of the upper gastrointestinal tract, as reflected by a low mean Spigelman stage, and a low risk of fundic gland polyposis. Finally, they had significantly fewer affected family members, although they do not themselves more often represent an isolated case. CONCLUSIONS: The severe phenotype should be considered when counselling FAP families in which attenuated FAP is excluded and in which a causative APC mutation has not been identified.     

Desmoid tumours in familial adenomatous polyposis.

Desmoids are rare, benign fibromatous lesions, which can arise in patients with familial adenomatous polyposis (FAP), a disorder caused by germline adenomatous polyposis coli (APC) gene mutation. This study investigated the risk of desmoids in FAP, the relation between specific APC gene mutations and desmoid formation, and the clinical characteristics of FAP patients with desmoids. Eighty three of 825 FAP patients (10%) from 49 of 161 kindreds (30%) had desmoids. The absolute risk of desmoids in FAP patients was 2.56/1000 person years; comparative risk was 852 times the general population. APC gene mutations were similar in families with and without desmoids. The female/male ratio was 1.4 (p = NS). Previous abdominal surgery was noted in 68% of patients with abdominal desmoids (55% developed within five years postoperatively). Desmoid risk in FAP family members of a desmoid patient was 25% in first degree relatives v 8% in third degree relatives. Desmoids are a comparatively common complication of FAP associated with surgical trauma and familial aggregation. Desmoid development was not linked to specific APC gene mutations and was not found predominantly in women. Studies of chemopreventive therapy, given within five years after abdominal surgery, should be considered in FAP patients with a family history of desmoid disease.     

Serrated adenoma in familial adenomatous polyposis: relation to germline APC gene mutation

BACKGROUND: Serrated adenoma is a precursor of colorectal cancer. AIM: To clarify possible genotype-phenotype correlations of serrated adenomas in familial adenomatous polyposis (FAP). Patients: Eleven patients from eight families with FAP. METHODS: We performed total colonoscopy with multiple biopsies in patients. Neoplasia with a serrated glandular structure was regarded as a serrated adenoma. In each patient, germline mutations of the APC gene were determined. Colonic phenotype was compared with germline mutations of the APC gene. RESULTS: Serrated adenomas were found in three patients. These patients had macroscopic polyps <100 in number. Pedigrees with serrated adenomas had the truncating germline APC mutation at codon 161, 332, or 1556 while in the other pedigrees mutations were found between codons 554 and 1324. CONCLUSIONS: In FAP, serrated adenoma may be a phenotype characteristic of the attenuated form.     

Analysis of genetic and phenotypic heterogeneity in juvenile polyposis

BACKGROUND: Juvenile polyposis syndrome (JPS) is characterised by gastrointestinal (GI) hamartomatous polyposis and an increased risk of GI malignancy. Juvenile polyps also occur in the Cowden (CS), Bannayan-Ruvalcaba-Riley (BRRS) and Gorlin (GS) syndromes. Diagnosing JPS can be problematic because it relies on exclusion of CS, BRRS, and GS. Germline mutations in the PTCH, PTEN and DPC4 (SMAD4) genes can cause GS, CS/BRRS, and JPS, respectively. AIMS: To examine the contribution of mutations in PTCH, PTEN, and DPC4 (SMAD4) to JPS. METHODS: Forty seven individuals from 15 families and nine apparently sporadic cases with JPS were screened for germline mutations in DPC4, PTEN, and PTCH. RESULTS: No patient had a mutation in PTEN or PTCH. Five different germline mutations were detected in DPC4; three of these were deletions, one a single base substitution creating a stop codon, and one a missense change. None of these patients had distinguishing clinical features. CONCLUSIONS: Mutations in PTEN and PTCH are unlikely to cause juvenile polyposis in the absence of clinical features indicative of CS, BRRS, or GS. A proportion of JPS patients harbour DPC4 mutations (21% in this study) but there remains uncharacterized genetic heterogeneity in JPS.     

Germline and somatic mutations in exon 15 of the APC gene and K-ras mutations in duodenal adenomas in patients with familial adenomatous polyposis.

BACKGROUND: In sporadic colorectal adenomas mutations in the adenomatous polyposis gene (APC) are among the first gene aberrations to appear. In familial adenomatous polyposis (FAP) the patients already have a germline mutation in the APC gene. To investigate the natural history of duodenal adenomas in FAP patients, we examined germline and somatic mutations of the APC gene and K-ras mutations in these lesions. METHODS: Frozen sections from 54 duodenal polyps from 31 FAP patients were used to histologically verify the presence of adenomatous growth in the mucosa; the rest of each biopsy specimen was processed for DNA extraction. APC exon 15 was investigated with the protein truncation test (PTT), using four overlapping polymerase chain reaction (PCR) fragments, and samples showing an APC mutation were thereafter sequenced. The adenomas were examined for K-ras mutations by use of a combination of the 'enriched PCR method' and temporal temperature gradient electrophoresis. RESULTS: APC germline mutations in exon 15 were found in 19 of 31 (61%) patients, whereas somatic mutations were localized to 12 of 54 (22%) duodenal adenomas. In seven adenomas both the germline and the somatic mutations were found, whereas five small adenomas showed somatic mutations only. There was no tendency for more mutations to be detected in large and severely dysplastic adenomas compared with small and mildly dysplastic ones. K-ras mutations were found in four (7%) duodenal adenomas. CONCLUSIONS: The low rate of somatic APC and K-ras mutations in duodenal adenomas may indicate another neoplastic pathway than in FAP adenomas of the large bowel, or that a modifier gene is cosegregating with the disease.     

Do we know all there is to know about Familial Adenomatous Polyposis?

Familial Adenomatous Polyposis (FAP) and Attenuated FAP (AFAP) are caused by a germline mutation in the Adenomatous polyposis coli (APC) gene. Recently, a new pathway characterized by a biallelic mutation in the MYH gene, with a recessive model of inheritance was discovered for this inherited syndrome. This report describes a Tunisian patient with an attenuated FAP phenotype, presenting seven colon polyps and an adenocarcinoma but no detectable germline mutations in the FAP target genes. A well known somatic mutation was found in the APC mutation cluster region (MCR). This case shows that further studies are needed to fully understand all the pathways of the FAP syndrome.     

A double germline mutations in the APC and p53 genes

Germline mutation in the APC gene is required for the initiation of the development of familial adenomatous polyposis (FAP). According to Fearon and Vogelstein model, further somatic mutations in the K-ras oncogene, DCC gene and p53 tumor suppressor gene are prerequisite for development of colon carcinoma. We have found that the germline mutations in the DNA isolated from lymphocytes of an 18 years old girl with extraordinary expressive phenotype in codons 1060-1061 of the APC gene result in truncation of the APC protein. The mutation in codons 12 and 13 of the K-ras oncogene was not detected, but another germline mutation was found in codon 210 of the p53 gene. Furthermore, no one of these germline mutations was detected in the DNA of peripheral blood lymphocytes of the patient's 21 years old healthy sister. Until now, there has been no evidence about the expressive phenotype due to mutation in codons 1060-1061 of the APC gene; the role of germline missense mutation in codon 210 of the p53 gene in the FAP malignant process remains to be elucidated too. The effect of the combination of germline mutation in two different tumor suppressor genes in the progress of disease is discussed.     

Multiple desmoid tumors in a patient with familial adenomatous polyposis caused by the novel W421X mutation

Familial adenomatous polyposis (FAP) is a rare syndrome characterized by the presence of hundreds to thousands of colorectal adenomas and is responsible for less than 1% of all colorectal cancers. The syndrome is also characterized by extra-colorectal features including amongst others upper gastrointestinal tract polyps and desmoid tumors. The syndrome is inherited by an autosomal dominant gene, the adenomatous polyposis coli (APC) gene. We present the physical history, clinical presentation, diagnosis and treatment of a patient with a novel germline APC mutation, the W421X mutation, which resulted in FAP presenting with about a hundred colorectal polyps, gastric hyperplastic polyps and multiple aggressive intra-abdominal and extra-abdominal desmoid tumors.     

Screening SMAD1, SMAD2, SMAD3, and SMAD5 for germline mutations in juvenile polyposis syndrome

BACKGROUND AND AIMS: Juvenile polyps occur in several Mendelian disorders, whether in association with gastrointestinal cancer alone (juvenile polyposis syndrome, JPS) or as part of known syndromes (Cowden, Gorlin, and Bannayan-Zonana) in association with developmental abnormalities, dysmorphic features, or extraintestinal tumours. Recently, some JPS families were shown to harbour germline mutations in the SMAD4 (DPC4) gene, providing further evidence for the importance of the TGFbeta signalling pathway in colorectal cancer. There remains, however, considerable, unexplained genetic heterogeneity in JPS. Other members of the SMAD family are excellent candidates for JPS, especially SMAD2 (which, like SMAD4, is mutated somatically in colorectal cancers), SMAD3 (which causes colorectal cancer when "knocked out" in mice), SMAD5, and SMAD1. METHODS: SMAD1, SMAD2, SMAD3, and SMAD5 were screened for germline mutations in 30 patients with JPS and without SMAD4 mutations. RESULTS: No mutations were found in any of these genes. A G-A C89Y polymorphism with possible effects on protein function was found in SMAD3, but the frequencies of the G and A alleles did not differ between patients with JPS and controls. CONCLUSIONS: It remains to be determined whether or not this polymorphism is involved in a minor predisposition to colorectal or other carcinomas. SMAD4 may be the only member of the SMAD family which causes JPS when mutant in the germline. The other genes underlying JPS remain to be identified.     

Hepatoblastoma and APC gene mutation in familial adenomatous polyposis.

BACKGROUND: Hepatoblastoma is a rare, rapidly progressive, usually fatal childhood malignancy, which if confined to the liver can be cured by radical surgical resection. An association between hepatoblastoma and familial adenomatous polyposis (FAP), which is due to germline mutation of the APC (adenomatous polyposis coli) gene, has been confirmed, but correlation with site of APC mutation has not been studied. AIM: To analyse the APC mutational spectrum in FAP families with hepatoblastoma as a possible basis to select kindreds for surveillance. PATIENTS: Eight patients with hepatoblastoma in seven FAP kindreds were compared with 97 families with identified APC gene mutation in a large Registry. METHODS: APC gene mutation was evaluated by RNase protection assay or in vitro synthesis protein assay. The chi 2 test and correlation were used for data analysis. RESULTS: APC gene mutation was identified in all seven FAP kindreds in which an at risk member developed hepatoblastoma. A male predominance was noted (six of eight), similar to literature cases (18 of 25, p < 0.01. Mutations were restricted to codons 141 to 1230, but no significant difference in site of mutation between pedigrees with and without hepatoblastoma was identified. CONCLUSIONS: Hepatoblastoma occurs primarily in boys in FAP kindreds and is associated with germline APC mutation in the 5' end of the gene. However, the site of APC mutation cannot be used to predict occurrence of this extracolonic cancer in FAP pedigrees.     

The multiple colorectal adenoma phenotype and MYH, a base excision repair gene.

We summarize the genetic and clinical features of the colorectal adenomas and cancers that occur in MYH-associated polyposis (MAP). MAP results from biallelic germline mutations in the base excision repair gene, mutY homologue (MYH). MAP has a phenotype that is often indistinguishable from classical or attenuated familial adenomatous polyposis (FAP), but the former is inherited as a recessive condition, whereas the latter is a dominantly inherited disease caused by germline mutations of the APC gene. MYH mutations seem to act by increasing the frequency of somatic APC mutations. MAP tumors may then progress to cancer along a distinct genetic pathway. MAP occurs in several different ethnic groups, the mutation spectrum appearing to differ among groups. It remains unknown, however, as to why carriers of MYH mutations specifically develop tumors of the gastrointestinal tract. In general, carriers of biallelic MYH mutations should be treated and followed up as for FAP patients with a similar phenotype. Relatives of MAP patients should be counseled as for any other recessive condition, although it remains possible that carriers of single mutations are at a modestly increased risk of colorectal cancer.