The APCI1307K allele and cancer risk in a community-based study of Ashkenazi Jews.

Mutations in APC are classically associated with familial adenomatous polyposis (FAP), a highly penetrant autosomal dominant disorder characterized by multiple intestinal polyps and, without surgical intervention, the development of colorectal cancer (CRC). APC is a tumour-suppressor gene, and somatic loss occurs in tumours. The germline T-to-A transversion responsible for the APC I1307K allele converts the wild-type sequence to a homopolymer tract (A8) that is genetically unstable and prone to somatic mutation. The I1307K allele was found in 6.1% of unselected Ashkenazi Jews and higher proportions of Ashkenazim with family or personal histories of CRC (ref. 2). To evaluate the role of I1307K in cancer, we genotyped 5,081 Ashkenazi volunteers in a community survey. Risk of developing colorectal, breast and other cancers were compared between genotyped I1307K carriers and non-carriers and their first-degree relatives.

Familial colorectal cancer in Ashkenazim due to a hypermutable tract in APC.

Approximately 130,000 cases of colorectal cancer (CRC) are diagnosed in the United States each year, and about 15% of these have a hereditary component. Two well-defined syndromes, familial adenomatous polyposis (FAP) and hereditary non-polyposis colorectal cancer (HNPCC), account for up to 5% of the total new cases of CRC. Truncating APC mutations are responsible for FAP, and defective mismatch repair genes cause HNPCC. However, the genes responsible for most of the familial cases are unknown. Here we report a mutation (T to A at APC nucleotide 3920) found in 6% of Ashkenazi Jews and about 28% of Ashkenazim with a family history of CRC. Rather than altering the function of the encoded protein, this mutation creates a small hypermutable region of the gene, indirectly causing cancer predisposition.

Genotyping by mass spectrometric analysis of short DNA fragments.

A method has been developed to produce small DNA fragments from PCR products for analysis of defined DNA variations by mass spectrometry. The genomic region to be analyzed is PCR-amplified with primers containing a sequence for the type IIS restriction endonuclease Bpml. Bpml digestion of the resultant PCR products yields fragments as small as seven bases, which are then analyzed by electrospray ionization mass spectrometry. The approach was validated using seven different variants within the APC tumor suppressor gene, in which a perfect correlation was obtained with DNA sequencing. Both the sense and antisense strands were analyzed independently, and several variants can be analyzed simultaneously. These results provide the basis for a generally applicable and highly accurate method that directly queries the mass of variant DNA sequences.

Pathogenesis of adenocarcinoma in Peutz-Jeghers syndrome.

Peutz-Jeghers syndrome (PJS) is an autosomal dominant condition characterized by intestinal hamartomatous polyps, mucocutaneous melanin deposition, and increased risk of cancer. Families with PJS from the Johns Hopkins Polyposis Registry were studied to identify the molecular basis of this syndrome and to characterize the pathogenesis of gastrointestinal hamartomas and adenocarcinomas in PJS patients. Linkage analysis in the family originally described by Jeghers in 1949 and five other families confirmed linkage to 19p13.3 near a recently identified gene responsible for PJS. Germ-line mutations in this gene, STK11, were identified in all six families by sequencing genomic DNA. Analysis of hamartomas and adenocarcinomas from patients with PJS identified loss of heterozygosity (LOH) of 19p markers near STK11 in 70% of tumors. Haplotype analysis indicated that the retained allele carried a germ-line mutation, confirming that STK11 is a tumor suppressor gene. LOH of 17p and 18q was identified in an adenocarcinoma but not in hamartomas, implying that allelic loss of these two regions corresponds to late molecular events in the pathogenesis of cancer in PJS. The adenocarcinomas showing 17p LOH also demonstrated altered p53 by immunohistochemistry. None of the 18 PJS tumors showed microsatellite instability, LOH on 5q near APC, or mutations in codons 12 or 13 of the K-ras proto-oncogene. These data provide evidence that STK11 is a tumor suppressor gene that acts as an early gatekeeper regulating the development of hamartomas in PJS and suggest that hamartomas may be pathogenetic precursors of adenocarcinoma. Additional somatic mutational events underlie the progression of hamartomas to adenocarcinomas, and some of these somatic mutations are common to the later stages of tumor progression seen in the majority of colorectal carcinomas.

Analysis of masked mutations in familial adenomatous polyposis.

Familial adenomatous polyposis (FAP) is an autosomal-dominant disease characterized by the development of hundreds of adenomatous polyps of the colorectum. Approximately 80% of FAP patients can be shown to have truncating mutations of the APC gene. To determine the cause of FAP in the other 20% of patients, MAMA (monoallelic mutation analysis) was used to independently examine the status of each of the two APC alleles. Seven of nine patients analyzed were found to have significantly reduced expression from one of their two alleles whereas two patients were found to have full-length expression from both alleles. We conclude that more than 95% of patients with FAP have inactivating mutations in APC and that a combination of MAMA and standard genetic tests will identify APC abnormalities in the vast majority of such patients. That no APC expression from the mutant allele is found in some FAP patients argues strongly against the requirement for dominant negative effects of APC mutations. The results also suggest that there may be at least one additional gene, besides APC, that can give rise to FAP.

Reversal of apoptosis by the leukaemia-associated E2A-HLF chimaeric transcription factor.

The E2A-HLF (for hepatic leukaemia factor) fusion gene, formed by action of the t(17;19) (q22;p13) chromosomal translocation, drives the leukaemic transformation of early B-cell precursors, but the mechanism of this activity remains unknown. Here we report that human leukaemia cells carrying the translocation t(17;19) rapidly died by apoptosis when programmed to express a dominant-negative suppressor of the fusion protein E2A-HLF, indicating that the chimaeric oncoprotein probably affects cell survival rather than cell growth. Moreover, when introduced into murine pro-B lymphocytes, the oncogenic E2A-HLF fusion protein reversed both interleukin-3-dependent and p53-mediated apoptosis. The close homology of the basic region/leucine zipper (bZIP) DNA-binding and dimerization domain of HLF to that of the CES-2 cell-death specification protein of Caenorhabditis elegans suggests a model of leukaemogenesis in which E2A-HLF blocks an early step within an evolutionarily conserved cell-death pathway.

Variable phenotype of familial adenomatous polyposis in pedigrees with 3' mutation in the APC gene.

BACKGROUND: Germline mutation in the adenomatous polyposis coli (APC) gene on chromosome 5 causes familial adenomatous polyposis. "Attenuated" phenotype has been reported with mutation in the 5' end of the gene (5' to codon 158), but genotype-phenotype relations at the 3' end (3' to codon 1596) have not been described fully. AIMS: To describe and compare colorectal and extracolonic phenotypes in a case series of families with mutation in the 3' end of the APC gene. METHODS: Thirty one at risk or affected members from four families with a mutation in the APC gene located at codon 1979 or 2644 were evaluated. RESULTS: Variable intrapedigree colorectal phenotype was observed: some members at older age had oligopolyposis (fewer than one hundred colorectal adenomas) whereas other members had classic polyposis at young age. Colorectal cancer was diagnosed at older mean age (50 (7) years) in the four families than in classic FAP pedigrees (39 (14) years). Extracolonic lesions characteristic of FAP occurred with 3' APC mutations, but variability in intrapedigree and interpedigree extracolonic phenotype and dissociation of severity of extracolonic manifestations from number of colorectal polyps was noted. CONCLUSIONS: Families with 3' mutations of the APC gene exhibit variable intrapedigree phenotype similar to the heterogeneity noted in families with proximal 5' mutations. Genotyping of FAP and oligopolyposis pedigrees can guide appropriate surveillance of the upper and lower gastrointestinal tract in affected members.

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).