Tumor Suppressor Function of Dnmt3b in Hematologic Malignancies Critically Depends on Its Catalytic Activity

DNA methylation regulates gene transcription and it is involved in various physiological processes in mammals including development and hematopoiesis. It is catalyzed by several DNA methyltransferases, including DNMT1, DNMT3A and DNMT3B that not only mediate methylation but also interact with a number of repressive proteins, thereby contributing to transcriptional repression in methylation-independent manner. Dnmt3b is critical gene in mouse embryogenesis whose loss results in embryonic lethality around E13.5. It also functions as a tumor suppressor in lymphoid and myeloid malignancies in mice. The extent to which Dnmt3b's catalytic activity is involved in these processes is unclear.

To address the role of catalytic activity in embryogenesis and tumorigenesis, we generated mice carrying a catalytically inactive allele of Dnmt3b (Dnmt3b^CI) by introducing a double amino acid substitution in enzyme active center (P656V; C657D) in endogenous Dnmt3b locus. By interbreeding Dnmt3b^+/CI mice we found that homozygous mutant Dnmt3b^CI/CImice survived embryonic development and had a long life span. Such results demonstrated that catalytic activity of Dnmt3b is, surprisingly, dispensable for both pre- and post-natal development. The presence of catalytically inactive Dnmt3b rescued most methylation changes observed in the absence of Dnmt3b in embryogenesis suggesting that Dnmt3b plays a role of accessory protein for methylation mediated by other Dnmts. In contrast, the absence of Dnmt3b's catalytic activity promoted MYC/Ras-induced transformation of mouse embryonic fibroblasts in vitro, and accelerated both MYC-induced T-cell lymphomagenesis and MLL-AF9-induced acute myeloid leukemia. Global gene expression and methylation profiling revealed a number of deregulated events specific for MYC;Dnmt3b^CI/CI, resulting in aberrant activation of IL7-R and STAT3 and downregulation of putative tumor suppressor genes including Cdkn2c and Dusp6.

Altogether, our data suggest that Dnmt3b's methyltransferase activity is dispensable for mouse development but critical to prevent hematologic malignancies by controlling events involved in cellular transformation.