Genetics of FAP associated tumours

Consistent with the 2-hit model of carcinogenesis by tumour suppressor genes, the wild type APC allele is lost or mutated in the vast majority of FAP associated tumours, including colorectal ade-nomatous polyps and carcinoma, desmoid tumours {1245}, medulloblas-toma {202}, gastroduodenal tumours {1949}, thyroid carcinoma {822} and hepatoblastoma {980}. Each colorectal adenomatous polyp is a premalignant lesion that may progress to carcinoma in an unpredictable fashion. In addition to APC mutations, colon carcinomas in FAP patients contain somatic mutations that are similar to those found in the most fre quent type of sporadic colon cancers not associated with replication errors. TP53 mutation and 17p allele loss have been observed in 40% of invasive carcinomas {910}. However, in some families TP53 may not be involved {30}. Loss of alleles on chromosome 18 and 22 were observed in 46% and 33% respectively. The KRAS mutation frequency increases from 11% inmoderately to 36% in severely dysplastic adenomas {30}. KRAS mutations may potentiate cyclin D1 transcription {680}. Interestingly, the type of APC germline mutation may influence the mode of inactivation of the second APC allele {30}.

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