Extensive methylation of hMLH1 promoter region predominates in proximal colon cancer with microsatellite instability

Methylation of the hMLH1 promoter region has been suggested to cause microsatellite instability (MSI) in sporadic colorectal carcinoma (CRC). We studied the methylation profile in a wide region of the hMLH1 promoter and compared with the hMLH1 protein expression and MSI status in 88 cases of sporadic CRC.

Na-bisulfite treatment and polymerase chain reaction single-strand conformation polymorphism analysis was performed using 5 sets of polymerase chain reaction primers spanning the promoter region of the hMLH1 to examine methylation status. Results were compared with immunostaining using anti-hMLH1 monoclonal antibody and MSI status of the tumor samples.

Methylation status was classified as full or partial methylation. Full methylation indicates the methylation of all CpG sites in the examined regions. Methylation of the hMLH1 promoter was observed in 88.9% (16 of 18) of CRCs showing high frequency MSI (MSI-H), among which 89% (14 of 16) had full methylation with reduced hMLH1 protein expression. All cases showing full methylation were proximal colon tumors with MSI-H. In cases with partial methylation, only the upstream region of the hMLH1 promoter was methylated. Partial methylation was also shown in 33.3% (6 of 18) of the normal mucosa of MSI-H cases. Frequencies of methylation were significantly correlated with female gender (P = 0.0009) and aging (P = 0.007).

Full methylation of the hMLH1 promoter region and subsequent gene inactivation may play a crucial role in the carcinogenesis of MSI-H CRCs in the proximal colon. Methylation upstream of the hMLH1 promoter appears to be an early event in the carcinogenesis of MSI-H tumors.