Hereditary colorectal cancer : assessment of genotype-phenotype correlations and analysis of rare susceptibility genes in familial adenomatous polyposis (FAP) and hereditary nonpolyposis colorectal cancer (HNPCC)

摘要

Each year 3500 people in Switzerland are diagnosed with colorectal cancer.udApproximately 20 percent of all affected patients have two or more first orudsecond-degree relatives with colorectal cancer (at-risk family members).udAbout five percent of these are inherited in an autosomal dominant manner.udThis thesis has focused on genotype-phenotype correlations in two hereditaryudcolorectal cancer syndromes, familial adenomatous polyposis (FAP) andudhereditary nonpolyposis colorectal cancer (HNPCC). In addition, rareudsusceptibility genes were analyzed: MYH in FAP and PMS2 and MSH3 inudHNPCC. The works encompassed investigations of a consecutive series ofud101 Swiss polyposis patients and establishment of genotype-phenotypeudcorrelations, delineation of somatic APC alterations in attenuated familialudadenomatous polyposis (AFAP), genetic characterization of the MYH geneudrecently associated with a multiple colorectal adenoma and carcinomaudphenotype, and finally, the assessment of the role of rarely mutated mismatchudrepair genes PMS2 and MSH3 in HNPCC.udIn the first part of the thesis, phenotypic differences between APC germlineudmutation carriers and APC/MYH mutation-negative individuals in audconsecutive cohort of 101 FAP patients were characterized. Furthermore, weudwanted to assess possible genotype-phenotype correlations in APC mutationudcarriers. In our study population, no genotype-phenotype correlations withudregard to polyp number or extracolonic disease manifestations could beudestablished. The data challenge the prevailing view on genotype-phenotypeudcorrelations and advise great caution when basing clinical managementuddecisions for an individual patient on the site of the APC germline mutation.udIn the second part of the thesis 235 tumors from 35 AFAP patients out ofud16 families were screened for APC mutations to find out the somatic APCudmutation spectrum, to determine phenotypic differences among AFAPudfamilies, and to delineate the pathways of somatic APC mutation in AFAP. Itudhas been shown that colonic polyp number varies greatly among AFAPudpatients, but members of the same family tended to have more similaruddisease severity. 5’-mutants generally had more polyps than the other patients. In some polyps bi-allelic changes (“third hits”) have been found,udwhich probably initiated tumorigenesis. Taken together, AFAP isudphenotypically and genetically heterogeneous and modifier genes may beudacting on the AFAP phenotype.udBiallelic changes in the MYH gene have been shown to predispose to audmultiple adenoma and carcinoma phenotype. In the third part of the thesis, 79udunrelated APC-negative Swiss polyposis patients were screened for germlineudmutations in MYH to assess the frequency of MYH mutations and to identifyudphenotypic differences between MYH mutation carriers and APC/MYHudmutation-negative polyposis patients. Colorectal cancer was significantly moreudfrequent in biallelic as compared to monoallelic mutation carriers or thoseudwithout MYH alterations. With regard to other phenotypic properties (age ofudonset, extracolonic disease manifestations), it is virtually impossible touddiscriminate biallelic from monoallelic MYH mutation carriers and MYHudmutation-negative polyposis patients.udIn HNPCC alterations in PMS2 have been documented only in extremelyudrare cases. In the fourth part of the thesis, DNAs of colorectal cancer patientsudwith immunohistochemically proven loss of PMS2 in the tumor (n = 16) wereudscreened for PMS2 germline mutations. It was possible to identifyudheterozygous PMS2 germline mutations in six patients. To detect germlineudmutations in the remaining 10 patients, additional mutation screening methodsud(cDNA sequencing and MLPA technique) have been applied. In conclusion itudwas shown that PMS2 defects account for a small but significant proportion ofudCRCs.udIn the fifth part of the thesis MSH3, a MMR gene, which has thus far notudbeen implicated in HNPCC, has been investigated in a 46 years old colorectaludcancer patient with immunohistochemical loss of MSH3 only. A MSH3udmissense mutation (c.2383C>T, p.Arg795Trp) was identified and the possibleudpathogenicity of the alteration was assessed. It was found that the mutation isudpresent in a hemizygous state in the tumor. Furthermore, 100 healthyudprobands did not carry the alteration and sequence and amino acid alignmentudwith vertebrates showed that it is located in a conserved region of the gene.udTaken together, our findings indicate that the alteration in MSH3 may indeedudbe pathogenic.