Methylation profile of the MLH1 promoter region and their relationship to colorectal carcinogenesis

Methylation of the MLHI promoter region has been suggested to be a principal mechanism of, gene inactivation in sporadic microsatellite instability (MSI)-positive colorectal carcinoma. Recently, we have shown a novel methylation profile of the MLHI promoter region (i.e., full, partial, and no methylation), among which full methylation was strongly associated with MSI. In this study, to confirm whether methylation requires the involvement of both alleles, we studied the MLHI promoter region concerning the methylation profile and allelic loss. Furthermore, we studied correlations of methylation profiles with genetic alternations such as loss of heterozygosity (LOH) of the TP53 locus and KRAS mutation. Eighty-eight tumors were classified as full (n = 14), partial (n = 26), and no methylation (n = 48). Full methylation was observed in 78% (14/18) of high-frequency MSI, in which all CpG sites in the promoter region were methylated. Full methylation differed significantly from partial methylation regarding absence of TPS3 LOH (0/12) and KRAS mutation (0/14). In cases with full methylation, we could show biallelic methylation by use of a single-base nucleotide polymorphism in the promoter. However, this did not accompany LOH of the MLHI locus. In contrast, there were no significant differences in molecular features between partial and no methylation, except for low frequencies of LOH of the MLHI locus (P = 0.02). In conclusion, biallelic extensive methylation of the MLHI promoter region plays a significant role in gene inactivation and is independent of KRAS mutation and TP53 LOH.