While selective binding of the tandem tudor domains of p53 binding protein 1 (53BP1) to dimethylated histone H4 lysine 20 (H4K20me2) at DNA double strand breaks (DSBs) is well-documented to be an essential event for proficient DSB repair, the enzyme(s) responsible for H4K20 methylation at DSBs remained unknown. Contrary to previous reports implicating the MMSET/NSD2 methyltransferase in this pathway, we demonstrate that MMSET is dispensable for H4K20 methylation, 53BP1 accumulation and DSB repair. Alternatively, we discovered that the PR-Set7 H4K20 monomethyltransferase (H4K20me1) is an essential component of nonhomologous end joining (NHEJ) which functions early and directly in DSB repair. PR-Set7 was found to mobilize immediately to DSBs and PR-Set7 enzymatic activity was necessary for H4K20 methylation, 53BP1 accumulation and proficient DSB repair. Identification of PR-Set7-associated proteins revealed interactions between PR-Set7 and several NHEJ repair proteins. Consistent with these findings, the rapid recruitment of PR-Set7 to induced DSBs was dependent on direct binding to the NHEJ Ku70/Ku86 heterodimer. PR-Set7-mediated H4K20me1 was found to be necessary, but not sufficient, for H4K20me2 and 53BP1 recruitment at induced DSBs and an undamaged ectopic locus. In addition, we also discovered that the Suv4-20 H4K20 dimethyltransferases mobilize to DSBs and were required for H4K20me2 and 53BP1 accumulation. Importantly, Suv4-20 was not sufficient for H4K20me2 and 53BP1 localization at induced DSBs or an undamaged ectopic locus in cells lacking a catalytically competent PR-Set7. Therefore, these results illuminate an orchestrated molecular pathway whereby PR-Set7-mediated H4K20me1 “primes” the DSB to expedite subsequent catalysis by Suv4-20 for the generation of H4K20me2 at the lesion necessary for 53BP1 binding. The collective findings identify PR-Set7 as an essential early component of NHEJ and reveal a novel temporal PR-Set7-dependent pathway that facilitates H4K20me2 and 53BP1 accumulation at DSBs necessary for proficient DSB repair. Furthermore, these results and preliminary studies indicate that small molecule inhibitors targeting PR-Set7 and/or Suv4-20 will likely be clinically effective radio- and chemo-sensitizing agents in cancer treatment.

Citation Format: Creighton T. Tuzon, Tanya M. Spektor, Lauren M. Congdon, Xiaodong Kong, Peggie Cheung, Alex J. Kuo, Kyoko Yokomori, Or Gozani, Judd C. Rice. Orchestrated recruitment of histone methyltransferases to DNA double strand breaks facilitates 53BP1 binding and proficient repair. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-133. doi:10.1158/1538-7445.AM2014-LB-133

©2014 American Association for Cancer Research.
2014