Invited Speaker

Application of Direct DNMT3A Inhibition to the Development of Inhibitors for Cancer-specific CpG Island Methylation
Hirohide Yoshikawa
Japan

De novo DNA methylation is known to occur during cancer development. Various tumor suppressor genes are silenced in association with aberrant CpG island methylation. DNA methyltransferase 3 (Dnmt3) is responsible for de novo DNA methylation during embryonic development. The growing number of reports strongly suggests that DNMT3 mediate aberrant CpG island methylation in human cancer at least in part. We found that sal-like 3 (SALL3) is a novel inhibitory factor for DNMT3A. SALL3 binds to DNMT3A by a direct interaction between the double zinc finger (DZF) motif of SALL3 and the PWWP domain of DNMT3A. SALL3 expression reduces DNMT3A-mediated CpG island methylation in cell culture and in vitro. CpG island methylation is enhanced in SALL3 depleted cells. Consistently, DNMT3A from SALL3 depleted cells increases methyltransferase activity in vitro. Binding of DNMT3A to chromatin is reduced or increased by SALL3 expression or depletion, respectively, accounting for the mechanism by which SALL3 inhibits DNMT3A-mediated CpG island methylation. The mutant SALL3 with a defective DZF motif is unable to bind to DNMT3A, thereby failing to inhibit DNMT3A-mediated de novo CpG island methylation. Interestingly, SALL3 expression was undetectable in some hepatoma cells, suggesting a possibility that inactivation of SALL3 lead to enhancement of DNMT3A activity, resulting in acceleration of CpG island methylation in cancer. Our findings identify a novel function of the PWWP domain in DNMT3 through which SALL3 is able to inhibit DNMT3A activity. This striking function of the PWWP domain will be useful to manipulate DNMT3A activity. In particular, targeting the PWWP domain is one of the promising strategies for suppression of tumor development by protecting the genome from de novo DNA methylation. The interaction between the DZF motif and the PWWP domain provides significant insight into negative regulation of DNMT3A activity. The PWWP peptides or the analogues such as chemical molecules would be expected to inactivate DNMT3A regardless of the SALL3 expression status.