ΔDNMT3B variants regulate DNA methylation in a promoter-specific manner

DNA methyltransferase 313 (DNMT3B) is critical in de novo DNA methylation during development and tumorigenesis. We recently reported the identification of a DNMT3B subfamily, Delta DNMT3B, which contains at least seven variants, resulting from alternative pre-mRNA splicing. Delta DNMT3Bs are the predominant expression forms of DNMT3B in human lung cancer. A strong correlation was observed between the promoter methylation of RASSF1A gene but not p16 gene (both frequently inactivated by promoter methylation in lung cancer) and expression of Delta DNMT3B4 in primary lung cancer, suggesting a role of Delta DNMT3B in regulating promoterspecific methylation of common tumor suppressor genes in tumorigenesis. In this report, we provide first experimental evidence showing a direct involvement of Delta DNMT3B4 in regulating RASSF1A promoter methylation in human lung cancer cells. Knockdown of Delta DNMT3B4 expression by small interfering RNA resulted in a rapid demethylation of RASSF1A promoter and reexpression of RASSF1A mRNA but had no effect on p16 promoter in the lung cancer cells. Conversely, normal bronchial epithelial cells with stably transfected,Delta DNMT3B4 gained an increased DNA methylation in RASSF1A promoter but not p16 promoter. We conclude that promoter DNA methylation can be differentially regulated and Delta DNMT3Bs are involved in regulation of such promoterspecific de novo DNA methylation.