Germline methylation of the MSH2 promoter as a cause of Hereditary Non-Polyposis Colorectal Cancer

Abstract

Promoter methylation mediated silencing of the DNA mismatch repair gene, MLH1, has been observed mostly as a somatic event in sporadic colorectal cancer (CRC) but rare Hereditary Non-Polyposis colorectal cancer (HNPCC) patients with germline methylation of MLH1 promoter have been reported. So far methylation of MSH2 promoter has never been observed in CRC. We have identified a family who satisfied Bethesda criteria 3, with two siblings who developed CRC at the age of 30 and 40 years respectively, and a third sibling who developed adenocarcinoma-in-situ of the endometrium at the age of 43 years. Both CRCs displayed loss of MSH2 protein by immunohistochemistry and available CRC tissue from one case demonstrated high level microsatellite instability (MSI-H). Analysis for MSH2 mutation by protein-truncation test and exon-by-exon sequencing revealed no germline mutation but presence of a somatic MSH2 mutation with retention of heterozygosity in the cancer tissue of the index patient. Methylation of the MSH2 promoter, however, was found by methylation-specific PCR in the cancer tissue, which were further confirmed by direct bisulphite genomic sequencing. Similar analysis of a batch of MSI-H CRC with MSH2 protein loss did not reveal the presence of any methylated products. Furthermore, analysis of the constitutional DNA extracted from the blood and normal colonic mucosae in the index patient and her 5 siblings all showed the presence of the methylated MSH2 promoter products. Haplotype analysis using microsatellite markers within and flanking the MSH2 gene in these 6 siblings revealed 3 haplotypes in total, with all siblings sharing a single common haplotype that is associated with cancer development in three siblings so far. Our results suggest that germline methylation of MSH2 promoter co-segregate with a single MSH2 haplotype and this is associated with a high risk of development of HNPCC-related cancers. Further study for potential alteration in the DNA structure flanking the MSH2 gene in this family may reveal novel mechanisms for epigenetic germline inheritance.