Hypermethylation of the MLH1 gene has been described in many kinds of human cancers with microsatellite instability (MSI). However, it is not clear whether the same mechanism occurs in hematological malignancies. Genomic DNA was extracted from 31 patients with adult T-cell leukemia/lymphoma (ATL), 9 patients with acute lymphoblastic leukemia (ALL) who had MSI, and 12 leukemia and lymphoma cell lines with MSI. Aberrant methylation of the MLH1 gene was found in 2/31 (6%) ATL patients, and in 1/12 (8%) cell lines with MSI. MLH1 promoter was not methylated in either of the twelve peripheral blood samples from normal individuals or ALL samples. The MLH1 gene was expressed in the normal peripheral blood samples, but not in the MLH1-methylated cell line KCL22. Demethylation with 5-Azacytidine treatment restored MLH1 expression in the KCL22 cell line. Methylation of the MSH2 gene was not found in any of the samples. Our data show that hypermethylation of the MLH1 gene is occasionally involved in the pathogenesis of hematological malignancies, but is not always associated with MSI.

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