Abstract 3519: The c-Myc oncogene over-activates dormant replication origins and sensitizes cancer cells to chemotherapy drugs

Abstract Several clinical studies have shown a correlation between elevated c-Myc levels and a better response to chemotherapy. Although elevated c-Myc might indirectly cause such chemosensitivity, it is possible that a direct effect is occurring due to underlying molecular mechanisms. The purpose of the current study was to determine whether c-Myc overexpression could acutely cause chemosensitivity. The c-Myc protein is known to activate DNA replication origins, but the mechanism for this is unknown. Origins are regulated by the loading of MCM2-7 hexamers, which form the core of the helicase that starts the initiation of replication. Mammalian cells load more MCM complexes than are necessary to complete one round of replication. The excess MCM complexes function as backups to allow recovery of replication when forks stall during drug treatment. Consistent with this, we found that loss of Mcm4 or Mcm7 sensitizes PDAC cell lines to 5-FU and Gemcitabine, two S phase stress drugs that are used to treat PDAC patients but are not highly effective on their own. Similar to depletion of backup MCM complexes via loss of Mcm4 or Mcm7, elevated c-Myc expression over-activates backup MCMs, leaving fewer available to recover from drug treatment. Cells containing overexpressed c-Myc display increased chemosensitivity to Gemcitabine that is not seen in cells containing normal c-Myc levels or a non-functional c-Myc mutant. These results demonstrate that reduction in backup MCM complexes, either by direct downregulation or by oncogene driven over-activation (via c-Myc), render cells sensitive to chemotherapy drugs. These results suggest that development of drugs that target MCM functionality may be useful in combination with S-phase stress drugs currently used in the clinic. These results also demonstrate that c-Myc has the ability to acutely cause chemosensitivity and c-Myc elevated tumors may respond better to chemotherapy due to the c-Myc effects on MCM complexes. Citation Format: Victoria Bryant, Mark G. Alexandrow. The c-Myc oncogene over-activates dormant replication origins and sensitizes cancer cells to chemotherapy drugs. [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 3519. doi:10.1158/1538-7445.AM2014-3519