Hereditary colorectal cancer : clinical and biological consequences of known and new genes, as well as modifiers

Each year 3500 people in Switzerland are diagnosed with colorectal cancer, approximately 51.8 and 34.3 per 100’000 inhabitants for males and females, respectively. Those patients with a familial risk ie. they haveor more first or second degree relatives with colorectal cancer, account for approximately 20 percent of all affected patients, whereas roughlyto 10 percent of the total annual burden of colorectal cancer is mendelian in nature – that is, it is inherited in an autosomal dominant manner. This thesis has focused on genotype-phenotype correlations in two hereditary colorectal cancer syndromes, hereditary nonpolyposis colorectal cancer (HNPCC) and familial adenomatous polyposis (FAP) in an attempt to optimise the selection criteria for affected individuals, to establish the sensitivity and specificity of different screening methods, to investigate a relatively new gene associated with a multiple colorectal adenoma and carcinoma phenotype and to assess the role of a modifier gene locus on chromosome 1p33-36. Since only limited data are available which detail the value of the different HNPCC referral criteria in combination with microsatellite instability (MSI) testing and various mutation screening methods, 222 unrelated Swiss patients were studied in order to (i) assess the phenotypic and molecular differences between patients belonging to different referral criteria groups, and (ii) determine the diagnostic accuracy of the criteria and screening procedures employed in identifying individuals with mismatch repair (MMR) gene alterations. The Bethesda Guidelines (BG) proved to be of superior sensitivity and diagnostic accuracy compared to Amsterdam Criteria I/II (AC I/II) alone, in identifying patients with MMR gene alterations. Based on the evaluation of the different screening techniques employed in this study, it is suggested that MSI analysis combined with immunohistochemistry testing and subsequent mutational analysis of the positively scored individuals encompassing both a DNA and a mRNA-based technique, should be conducted for optimal rates of mutation detection. Investigations subsequently continued in attempts to further characterise the phenotype of Swiss HNPCC patients by comparing 46 MMR gene mutation carriers to 84 gene alteration negative individuals in order to ultimately aid the identification of HNPCC individuals and MMR gene mutation carriers. Ninety-four percent of the mutation positive patients were classified by referral criteria (AC or BG) compared to only 76% of mutation negative individuals. Mutation positive patients were also younger at the time of their CRC diagnosis, had more often proximally located CRCs, a higher prevalence of syn-/metachronous CRCs and more frequently extracolonic
manifestations. Using such phenotypic differences to distinguish mutation positive from mutation
negative individuals, clinicians may be aided in their preselection of patients for genetic
surveillance, mutation screening and subsequently, genetic counselling.
In light of results from recent studies, implicating germline mutations in MYH with a multiple
colorectal adenoma and carcinoma phenotype, it was the purpose of this study to further correlate
MYH germline mutations with Swiss APC-negative individuals (n=65) and establish any
genotype-phenotype correlations to aid in the optimisation of clinical screening and prevention
strategies. An optimised protocol for the rapid and sensitive mutation analysis of MYH via high
performance liquid chromatography (DHPLC) was established. Thirteen (20%) individuals were
identified as MYH mutation carriers, 7 (54%) of which had biallelic mutations. Aside from
previously reported mutations, 3 apparently novel gene alterations were established in 3 patients
with a multiple adenoma phenotype. The phenotypical characteristics of all patients investigated
were similar, with no statistically significant correlations to genotype, hence, clinicians and
counsellors are advised to screen for MYH mutations in patients displaying tens to hundreds of
colorectal adenomas, and a family history consistent only with recessive inheritance.
FAP patients typically display considerable inter- and intra-familial phenotypic heterogeneity,
which represents a major problem in genetic counselling of APC mutation carriers. The Min
mouse model indicated a putative disease modifier locus on chromosome 4, which is syntenic to
human chromosome 1p35-36. Furthermore, germline mutations in the base-excision repair gene
MYH, which maps to the 1p33-34 region, have been described in patients with multiple
adenomas, pointing to a possible role as disease modifier in FAP. Here, the re-assessment of one
of the largest FAP kindreds published, which was previously used in linkage mapping of 1p35-
36, is documented. Using the latest available clinical information, additional mutation carriers
and polymorphic markers, fine-mapping of the critical region as well as mutation analysis of the
MYH gene were performed. These investigations significantly excluded (i) the 1p33-36 region as
a modifier locus and (ii) MYH as a modifier gene for extracolonic disease in this FAP kindred.
The results indicate that linkage analysis of further putative candidate regions is necessary to
identify a disease modifier locus in FAP.