Promoter methylation is believed to inactivate the expression of hMLH1. This process has been implicated in the tumorigenesis of oral squamous cell carcinoma (OSCC). Thus, the aim of this study was to determine the profile of hMLH1 methylation and protein expression in OSCC. The matched case-control study included 50 OSCC cases and 200 controls, with a median of age 64 (Q₁-Q₃ 54-71) years. Protein expression was determined by immunohistochemical staining, and hMLH1 gene promoter methylation was analyzed by methylation-specific polymerase chain reaction (MSP). A conditional logistic regression model for risk factors was built for OSCC cases and matched controls. Promoter methylation of hMLH1 was detected in 38 (76%) OSCC cases, but in none of the control samples. Of the 38 OSCC samples with promoter methylation, 12 (32%) were negative for hMLH1 protein, and corresponded to early clinical stages (10 in stage II and 2 in stage I). All 12 unmethylated samples showed positive stain for hMLH1. Multiple logistic regression analysis showed an OR of 16.54 (IC 95%: 1.69-161.68, p=0.016) for methylation of the hMLH1 gene and early stages of OSCC, adjusting by gender and tobacco use. This study showed a high frequency of hMLH1 promoter methylation that occurred in most of the early stage cases and in about half of the late stage cases. It is proposed that hMLH1 promoter methylation is an early event that is maintained during tumor progression.
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