|dc.description.abstract||Colorectal cancer is one of the most common cancer types, and a leading cause of cancer-related deaths. Cases can be divided into two main molecular phenotypes: those with chromosomal instability (CIN) and those with microsatellite instability (MSI), which occur at a frequency of 85% and 15%, respectively. The focus of this study was on MSI tumors, which have defective mismatch repair systems, and on the multitudinous insertions and deletions in MSI tumor DNA which are the result of this defect.
The primary purpose of the thesis was to serve as a pilot project for later work on a consecutive, clinically representative tumor series, with the goal of identifying genetic markers that can distinguish between patients within the microsatellite unstable group of colorectal cancers. Firstly, we wished to establish an assemblage of genes which could reasonably be assumed to be targets of mismatch repair deficiency, i.e. that they were more frequently subject to insertions or deletions than comparable sequences. Secondly, we wanted to see whether there were, among the above targets, genes which either singly or in company appeared to affect patient outcome depending on their mutational status.
A search of the available scientific literature for genes which had been analyzed for, almost uniquely, frameshift mutations in MSI tumors yielded 162 candidates. Typically, studies were either of few genes in a sizeable tumor series or of several to many genes in a smaller group of tumors. To improve upon this situation we selected one fourth of all published candidate target genes for experimental analysis in a large series (n=94) of confirmed MSI colorectal carcinomas. Of the 43 genes we analyzed, 34 had a mononucleotide repeat of eight or more bases in the coding sequence. The results provided confirmation of the target status for many genes, among them certain well-known targets such as TGFβRII, MSH3, E2F4 and CASP5. The mutation frequencies lay within the published range for most genes, with the exception of ACVR2, CASP5, MSH2, RAD50, AC1, EPHB2, SEMG1 and SPINK5. The histone acetyl transferase EP300 had never before been examined for this type of mutation in primary tumor DNA, and proved to be a low-frequency, but nevertheless noteworthy, target in MSI colorectal cancers. No significant covariance of genes as assessed by hierarchical clustering was found which did not depend on mutation frequency alone. One could, however, note a suggestive pattern of association between AC1 and the cell cycle inhibitory E2F4, as well as for a group containing OGT, a possible global regulator of the cell cycle; UVRAG, an autophagy-promoting tumor suppressor, and the proto-oncogene TCF4. This group is particularly interesting in that it encompasses an exemplar of each of the main types of cancer-related genes, save a DNA repair component.
For clinical association studies we used the Norwegian consecutive subset of our main MSI series; ten year follow-up was available for these patients. Intriguingly, a single gene of intermediate mutation frequency showed a robust association with long-term patient survival. SLC23A2, which encodes a sodium/vitamin C cotransporter, was significantly associated with poor prognosis when mutated, and showed indications of being additionally informative with regard to clinical staging. This is striking when one takes into account the limited number of patients included (n=35) and that the patients with MSI tumors generally have a better prognosis with respect to the majority of colorectal cancer patients. This gene and others of suggested but unvalidated prognostic value are to be included in a planned study of a ten year cohort consisting of nearly one thousand patients from a single Oslo hospital.||nor