Redundant DNA methylation in colorectal cancers of Lynch‐syndrome patients

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Abstract
Lynch syndrome is an inherited disease resulting predominantly in colorectal cancer (CRC). The crucial cause is DNA mismatch repair (MMR) malfunction that is associated mostly with MLH1 or MSH2 germline mutations. A significant hallmark of repair defects is a high level of instability in microsatellites (MSI‐H). In many sporadic unstable CRCs, the MLH1 gene is inactivated by promoter hypermethylation in addition to extensive promoter methylation in many tumor‐suppressor genes known as CpG island methylation phenotype (CIMP). To investigate the possible role of epigenetic alterations in causing MMR deficiency and thereby Lynch syndrome, we evaluated the MLH1 specific and global hypermethylation in hereditary CRCs. Of 22 Lynch‐syndrome‐related CRCs, 18 (81.8%) demonstrated various levels of DNA methylation; of these, 14 (63.6%) and 4 (18.2%) were methylated in distal and both distal and proximal regions of the MLH1 promoter, respectively. However, only 7/18 (38.9%) of results were confirmed by bisulfite sequencing. Similar methylation patterns in tumors and frequently in matched normal DNA were found in twelve and four patients with MLH1 and MSH2 alterations documented by the absence of protein or presence of germline mutation, respectively. Moreover, the same results were observed in five stable CRCs. None of 22 Lynch‐syndrome‐related tumors presented CIMP in contrast to 3/10 (30%) stable carcinomas. The rather randomly distributed weak methylation patterns in hereditary CRCs indicate that epigenetic events are redundant in Lynch‐syndrome etiology, in contrast to the widespread DNA methylation in sporadic unstable CRCs. These methylation‐profile differences can lead to more effective molecular diagnosis of Lynch syndrome.