Hyperthermia is the most effective chemo- and radio-sensitizer in use clinically for containing tumor growth. Hyperthermia is proposed to induce degradation of BRCA2, the protein involved in homologous recombination (HR) repair of DNA double strand breaks and, thus sensitizes cells to poly (ADP-ribose) polymerase (PARP)-1 inhibition. We and other laboratories have reported that hyperthermia inhibits DNA damage repair. Since HR includes single-strand annealing, gene conversion and break-induced replication, it is critical for DNA replication during recovery of stalled replication forks, but it is not yet clear how hyperthermia affects these important steps in HR.

Here we report that SMARCAD1 depletion increases hyperthermia induced cell death and chromosome damage in irradiated S-phase cells. Combined SMARCAD1 depletion and hyperthermia treatment decreased repair and recruitment of repairosome proteins to ISce-1 induced DSBs as well as decreased IR-induced repairosome foci formation. This effect was limited to S-phase cells, where homologous recombination is up regulated and the predominant pathway of DSB repair. Hyperthermia decreased the quantity of 5′ single stranded DNA intermediates at endonuclease-generated DNA break sites, an effect that was further decreased by SMARCAD1 depletion. Consistent with these results, simultaneous SMARCAD1 depletion and hyperthermia increased replication fork stalling and affected firing of new replication origins. The results presented here support a mechanism by which SMARCAD1 depletion and hyperthermia mediate radiosensitization through inhibition of the DNA resection step that precedes strand homology search during HR repair of IR-induced DSBs.

Citation Format: Sharmistha Chakraborty, Clayton R. Hunt, Shashank Hambarde, Kalpana Mujoo, Nobuo Horikoshi, Raj K. Pandita, Abid Mattoo, Bin Teh, Tej K. Pandita. SMARCAD1 depletion enhances hyperthermia-mediated radiosensitization by decreasing resection and enhancing stalled replication forks. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2756.