Biallelic inactivation of hMLH 1 by epigenetic gene silencing, a novel mechanism causing human MSI cancers

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 95; no. 15; pp. 8698 - 8702
• Main Authors: Veigl, Martina L., Kasturi, Lakshmi, Olechnowicz, Joseph, Ma, AiHong, Lutterbaugh, James D., Periyasamy, Sumudra, Li, Guo-Min, Drummond, James, Modrich, Paul L., Sedwick, W. David, Markowitz, Sanford D.
• Format: Journal Article
• Language: English
• Published: 21-07-1998
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.95.15.8698

Mutations of DNA mismatch repair genes, including the
hMLH1
gene, have been linked to human colon and other cancers in which defective DNA repair is evidenced by the associated instability of DNA microsatellite sequences (MSI). Germ-line
hMLH1
mutations are causally associated with inherited MSI colon cancer, and somatic mutations are causally associated with sporadic MSI colon cancer. Previously however, we demonstrated that in many sporadic MSI colon cancers
hMLH1
and all other DNA mismatch repair genes are wild type. To investigate this class of tumors further, we examined a group of MSI cancer cell lines, most of which were documented as established from antecedent MSI-positive malignant tumors. In five of six such cases we found that hMLH1 protein was absent, even though
hMLH1
-coding sequences were wild type. In each such case, absence of hMLH1 protein was associated with the methylation of the
hMLH1
gene promoter. Furthermore, in each case, treatment with the demethylating agent 5-azacytidine induced expression of the absent hMLH1 protein. Moreover, in single cell clones,
hMLH1
expression could be turned on, off, and on again by 5-azacytidine exposure, washout, and reexposure. This epigenetic inactivation of
hMLH1
additionally accounted for the silencing of both maternal and paternal tumor
hMLH1
alleles, both of which could be reactivated by 5-azacytidine. In summary, substantial numbers of human MSI cancers appear to arise by
hMLH1
silencing via an epigenetic mechanism that can inactivate both of the
hMLH1
alleles. Promoter methylation is intimately associated with this epigenetic silencing mechanism.