Abstract P6-03-09: Targeting the c-myc/E2F1 pathway in TNBC promotes a DNA damage dependent synthetic lethality

Abstract

Abstract The c-myc/E2F1 pathway is highly upregulated in TNBC and dictates increased genomic instability, a higher frequency of relapse and poor overall survival. C-myc is a potent oncogene with overexpression activity that influences several pathways including metabolism, DNA replication, DNA damage, cell cycle and apoptosis. The essential role of c-myc in normal cells has made therapeutic intervention elusive. However the role of c-myc influencing cell cycle progression has demonstrated that CDK inhibition downregulates c-myc expression in TNBC breast cancer cells. We hypothesize that CDK inhibition downregulates c-myc expression resulting in an accumulation of DNA damage thus increasing the susceptibility of TNBC cells to PARP inhibition. A panel of TNBC cell lines was assessed for c-myc/E2F1 pathway expression and downregulation in the presence of CDK inhibitor Dinaciclib. An MTT-based High Throughput Survival Assay (HTSA) was utilized to assess growth inhibition response to drug treatment in TNBC cells. FACS analysis was performed on Dinaciclib treated TNBC cell lines to asses cell cycle response to CDK inhibition. DNA damage and Homologous Recombination DNA repair was assessed by γH2AX, BRCA1, RAD51 foci formation by immunofluorescence. Lentiviral knockdown of c-myc, E2F1, BRCA1 and RAD51 were performed to assess susceptibility to PARP inhibition. Combination treatment of Dinaciclib + PARP inhibitor MK4827 was assessed via combination index values calculated by Calcusyn. CDK inhibition resulted in downregulation of both c-myc and E2F1 in all TNBC cells. The pan-CDK inhibitor Dinaciclib universally inhibited proliferation in all TNBC cell lines in the presence of the drug however drug removal allowed TNBC cells to recover a normal proliferative profile. Dinaciclib induction increased γH2AX foci formation while decreasing BRCA1 and RAD51 foci formation resulting in an increase in DNA damage induced cell death that correlated with c-myc downregulation. CDK inhibition correlated with downregulation of c-myc/E2F1 target genes. Lentiviral knockdown of c-myc and not E2F1 induced susceptibility to PARP inhibitor MK4827. Combination CDK + PARP inhibitor resulted in a dose dependent synthetic lethal increase in PARP inhibitor efficacy in both BRCA1 mutant and wild-type cells. Together these results suggest that c-myc downregulation via CDK inhibition in combination with PARP inhibition may pose a novel combination therapy for TNBC patients. Citation Format: Jason P Carey, Khandan Keyomarsi. Targeting the c-myc/E2F1 pathway in TNBC promotes a DNA damage dependent synthetic lethality [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P6-03-09.