Study of the mechanisms underlying heritable germline epimutation of MSH2 in a Hereditary Nonpolyposis Colorectal Cancer family

Abstract

Colorectal cancer (CRC) is one of the most common cancers world-wide. Among various forms of familial cancers, Hereditary Nonpolyposis Colorectal Cancer (HNPCC) is the most frequent type of hereditary cancer syndromes. It is well known that inactivation of DNA mismatch repair (MMR) genes are responsible for HNPCC. Germline mutations in either the MSH2 or the MLH1 genes can be detected in the majority of HNPCC patients. Alternatively, methylation of the MLH1 promoter in the germline have been observed in a few individuals with HNPCC. More recently, we have first identified a family with inheritance in three successive generations, of germline allele-specific and mosaic hypermethylation of the MSH2 gene promoter, without evidence of DNA mismatch repair gene mutation. Three siblings carrying the germline methylation developed early-onset colorectal (CRC) or endometrial cancers, all with microsatellite instability and MSH2 protein loss [Chan et al Nat Genet 2006]. To further elucidate the underlying mechanism of this germline epimutation, we have performed linkage analysis using microsatellite markers flanking the MSH2 gene. Based on the presence of recombination events, we have successfully narrowed down the chromosomal region to less than 20Mb that is linked with the occurrence of germline MSH2 methylation. Methylation status of the genes with CpG island within this region was examined. The promoter of all genes located upstream of MSH2 were unmethylated. The MSH6 gene which is located downstream of MSH2 also showed no sign of methylation. Our result suggests a possible existence of a cis-acting element that may contribute to the MSH2 methylation. Further study of this chromosomal region may reveal novel mechanisms involved in the regulation of methylation in humans.