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)

Each year 3500 people in Switzerland are diagnosed with colorectal cancer.
Approximately 20 percent of all affected patients have two or more first or
second-degree relatives with colorectal cancer (at-risk family members).
About five percent of these are inherited in an autosomal dominant manner.
This thesis has focused on genotype-phenotype correlations in two hereditary
colorectal cancer syndromes, familial adenomatous polyposis (FAP) and
hereditary nonpolyposis colorectal cancer (HNPCC). In addition, rare
susceptibility genes were analyzed: MYH in FAP and PMS2 and MSH3 in
HNPCC. The works encompassed investigations of a consecutive series of
101 Swiss polyposis patients and establishment of genotype-phenotype
correlations, delineation of somatic APC alterations in attenuated familial
adenomatous polyposis (AFAP), genetic characterization of the MYH gene
recently associated with a multiple colorectal adenoma and carcinoma
phenotype, and finally, the assessment of the role of rarely mutated mismatch
repair genes PMS2 and MSH3 in HNPCC.
In the first part of the thesis, phenotypic differences between APC germline
mutation carriers and APC/MYH mutation-negative individuals in a
consecutive cohort of 101 FAP patients were characterized. Furthermore, we
wanted to assess possible genotype-phenotype correlations in APC mutation
carriers. In our study population, no genotype-phenotype correlations with
regard to polyp number or extracolonic disease manifestations could be
established. The data challenge the prevailing view on genotype-phenotype
correlations and advise great caution when basing clinical management
decisions for an individual patient on the site of the APC germline mutation.
In the second part of the thesis 235 tumors from 35 AFAP patients out of
16 families were screened for APC mutations to find out the somatic APC
mutation spectrum, to determine phenotypic differences among AFAP
families, and to delineate the pathways of somatic APC mutation in AFAP. It
has been shown that colonic polyp number varies greatly among AFAP
patients, but members of the same family tended to have more similar
disease severity. 5’-mutants generally had more polyps than the other patients. In some polyps bi-allelic changes (“third hits”) have been found,
which probably initiated tumorigenesis. Taken together, AFAP is
phenotypically and genetically heterogeneous and modifier genes may be
acting on the AFAP phenotype.
Biallelic changes in the MYH gene have been shown to predispose to a
multiple adenoma and carcinoma phenotype. In the third part of the thesis, 79
unrelated APC-negative Swiss polyposis patients were screened for germline
mutations in MYH to assess the frequency of MYH mutations and to identify
phenotypic differences between MYH mutation carriers and APC/MYH
mutation-negative polyposis patients. Colorectal cancer was significantly more
frequent in biallelic as compared to monoallelic mutation carriers or those
without MYH alterations. With regard to other phenotypic properties (age of
onset, extracolonic disease manifestations), it is virtually impossible to
discriminate biallelic from monoallelic MYH mutation carriers and MYH
mutation-negative polyposis patients.
In HNPCC alterations in PMS2 have been documented only in extremely
rare cases. In the fourth part of the thesis, DNAs of colorectal cancer patients
with immunohistochemically proven loss of PMS2 in the tumor (n = 16) were
screened for PMS2 germline mutations. It was possible to identify
heterozygous PMS2 germline mutations in six patients. To detect germline
mutations in the remaining 10 patients, additional mutation screening methods
(cDNA sequencing and MLPA technique) have been applied. In conclusion it
was shown that PMS2 defects account for a small but significant proportion of
CRCs.
In the fifth part of the thesis MSH3, a MMR gene, which has thus far not
been implicated in HNPCC, has been investigated in a 46 years old colorectal
cancer patient with immunohistochemical loss of MSH3 only. A MSH3
missense mutation (c.2383C>T, p.Arg795Trp) was identified and the possible
pathogenicity of the alteration was assessed. It was found that the mutation is
present in a hemizygous state in the tumor. Furthermore, 100 healthy
probands did not carry the alteration and sequence and amino acid alignment
with vertebrates showed that it is located in a conserved region of the gene.
Taken together, our findings indicate that the alteration in MSH3 may indeed
be pathogenic.