Refining the relation between 'first hits' and 'second hits' at the APC locus: The 'loose fit' model and evidence for differences in somatic mutation spectra among patients

Abstract

The site of the 'first hit' in the APC tumour suppressor gene determines the type of the 'second hit', both in familial adenomatous polyposis (FAP) and sporadic colorectal tumours. Mutations near codon 1300 are associated with loss of heterozygosity (LOH) of the wild-type allele; other tumours tend to have two protein-truncating mutations. In this study, we have confirmed and refined the LOH-associated region in colorectal FAP: allelic loss in adenomatous polyps tended to occur when the germline mutation lay in the region of the APC gene between the first and second beta-catenin degradation repeats (codons 1285-1378). LOH generally occurred by mitotic recombination, leaving two identical all..