The profile of hMLH1 methylation and microsatellite instability in colorectal and non-small cell lung cancer

Microsatellite instability (MSI) is caused mainly by dysfunction of hMLH1, where aberrant hypermethylation (HM) of its promoter region is involved. Previously, we suggested that HM in the proximal region of the hMLH1 promoter plays a critical role in progression of gastric cancer with MSI and this specific region should be analyzed for diagnostic use of hMLH1 HM. We expanded the analyses of hMLH1 HM and MSI phenotype to sporadic colorectal cancer (CRC) and non-small cell lung cancer (NSCLC) to further evaluate the diagnostic value of hMLH1 HM. A total of 174 CRC and 94 NSCLC samples were used for hMLH1 methylation analysis by real-time methylation-specific PCR. Methylation levels were measured in three distinct regions of the promoter, designated as hMLH1-A, hMLH1-B, and hMLH1-C from distal to proximal. MSI phenotype was determined using five microsatellite markers, BAT25, BAT26, D2S123, D5S346, and D17S250. Methylation profile of the hMLH1 promoter varies between CRC and NSCLC. High methylation levels were observed in a group of CRC samples. Consequently, three patterns of methylation in the hMLH1 promoter regions were found: 1) low methylation level in all regions, 2) high methylation level in hMLH1-A with low methylation level in hMLH1-C, 3) high methylation level in all regions. In contrast, only one NSCLC showed high methylation level in hMLH1-A. Of the 134 CRCs examined, 14 (10.4%) showed MSI phenotype. No MSI phenotype was found in the initial 80 NSCLCs analyzed. Eight (57.1%) of 14 CRC with MSI showed HM in hMLH1-C, which was linked exclusively with MSI phenotype. However, the HM in hMLH1-A or -B was not sufficient for MSI. CRC with MSI phenotype was significantly more frequent in older patients and in the proximal colon, and was more evident in cases with hMLH1-C HM. The results suggested that hMLH1 HM cannot be used as an alternative diagnostic marker of MSI phenotype in sporadic CRC and NSCLC. CRC with MSI might have clinicopathologically distinct subgroups according to hMLH1-C HM status. The observed profiles of hMLH1 methylation and MSI in gastric cancer, CRC, and NSCLC were quite different from each other, facilitating the better understanding of the pathogenesis of these cancers.