Hereditary Colorectal Cancer; Identification, Characterization and Classification of Mutations

Abstract

Hereditary factors are thought to play are role in 20-30% of all colorectal cancers Around 6% are found as high penetrant disease-causing mutations in genes correlated to hereditary polyposis or hereditary non-polyposis syndromes. The aim of this thesis was to identify new causative genes and variants and also mutation mechanisms in families presenting a polyposis, atypical polyposis or nonpolyposis CRC phenotype. In classical familial adenomatous polyposis (FAP) 100% of the disease-causing mutations were found in patients from the Swedish Polyposis Registry. The mutation underlying the lowered expression of the APC gene in one family was identified by SNP array analysis, the mutation was a split deletion of 61Kb including half of the promoter 1B. Investigation of the significance of this promoter for expression of the APC gene demonstrated considerably higher expression compared with the established promoter 1A. In order to establish a sensitive method for mosaic-mutation detection a comparison of mutation detection methods was performed. Low frequency mosaic mutations were detected down to 1 % by use of massively parallel sequencing (MPS). Whole exome sequencing in four families with attenuated FAP (AFAP), atypical polyposis or non-polyposis syndromes identified two high penetrant disease-causing mutations. One was found in the upstream regulatory region of GREM1and one in the exonuclease domain of POLE. Variants in low-penetrant genes possibly contributing to CRC development were also proposed from the exome sequencing and gene specific analyses of 107 patients. Sixty-seven of these patients were analyzed in a panel of 19 selected CRC predisposing genes. Truncating mutations were found in the BMPR1A and SMAD4 genes in patients with a classical FAP, atypical FAP or non-polyposis phenotype. Classification of found non-synonymous variants was also performed. In summary, using a combination of different molecular screening techniques, 100% of diseasecausing mutations in classical FAP can be found. With MPS it is possible to detect low-frequency mosaic mutations down to 1% by absolute quantification. Whole exome analyses identified mutations in the new causative genes POLE and GREM1. It was also concluded that patients without identified mutations, based on phenotypical CRC classification, can have mutations in genes not included in the primary routine analysis. These results will lead to improved mutation detection analysis for diagnostic and carrier testing.