Densely methylated MLHI promoter correlates with decreased mRNA expression in sporadic colorectal cancers

It has been reported that MLHI is silenced by promoter methylation, and that this phenomenon is associated with microsatellite instability (MSI) in sporadic colorectal cancer (CRC). To clarify the significance of MLHI promoter methylation in sporadic CRC, we examined the correlation between methylation status over the entire promoter region and mRNA expression in cases showing high-frequency MSI (MSI-H). MLHI promoter methylation was analyzed using the bisulfite modification sequencing in 48 MSI-H cases. We also screened for somatic mutation, loss of heterozygosity, and immunohistochemical staining of MLHI . The results showed that methylation patterns could be subdivided into three types: methylation of more than 80% of the CpG sites analyzed (type 1 methylation), methylation of less than 20% (type 2 methylation), and methylation mainly in the region 500 to 921 bases upstream from the translation start site (type 3 methylation). Of the three types, only type 1 methylation correlated with decreased mRNA expression. The frequency of type 1 methylation was significantly higher in cases involving the proximal colon (66.7%, 18/27) compared to that of the distal colon and rectum (23.8%, 5/21, P = 0.004). Immunohistochemical staining of MSI-H cases showed that decreased MLHI was found in 77.1% (37/48). Of the cases with decreased MLHI, type 1 methylation was present in 59.5% (22/37). Overall, our data suggested that the type I methylation pattern may affect MLHI mRNA expression, such that the majority of MSI-H cases in sporadic CRC, especially proximal colon cancer, exhibited type 1 methylation. (C) 2002 Wiley-Liss, Inc.