Whole-gene APC deletions cause classical familial adenomatous polyposis, but not attenuated polyposis or "multiple" colorectal adenomas.
Sieber OM., Lamlum H., Crabtree MD., Rowan AJ., Barclay E., Lipton L., Hodgson S., Thomas HJW., Neale K., Phillips RKS., Farrington SM., Dunlop MG., Mueller HJ., Bisgaard ML., Bulow S., Fidalgo P., Albuquerque C., Scarano MI., Bodmer W., Tomlinson IPM., Heinimann K.
Familial adenomatous polyposis (FAP) is a dominantly inherited colorectal tumor predisposition that results from germ-line mutations in the APC gene (chromosome 5q21). FAP shows substantial phenotypic variability: classical polyposis patients develop more than 100 colorectal adenomas, whereas those with attenuated polyposis (AAPC) have fewer than 100 adenomas. A further group of individuals, so-called "multiple" adenoma patients, have a phenotype like AAPC, with 3-99 polyps throughout the colorectum, but mostly have no demonstrable germ-line APC mutation. Routine mutation detection techniques fail to detect a pathogenic APC germ-line mutation in approximately 30% of patients with classical polyposis and 90% of those with AAPC/multiple adenomas. We have developed a real-time quantitative multiplex PCR assay to detect APC exon 14 deletions. When this technique was applied to a set of 60 classical polyposis and 143 AAPC/multiple adenoma patients with no apparent APC germ-line mutation, deletions were found exclusively in individuals with classical polyposis (7 of 60, 12%). Fine-mapping of the region suggested that the majority (6 of 7) of these deletions encompassed the entire APC locus, confirming that haploinsufficiency can result in a classical polyposis phenotype. Screening for germ-line deletions in APC mutation-negative individuals with classical polyposis seems warranted.