Abstract 4045: Decreased Fanconi anemia gene expression contributes to efficacy of PARP and DNMT inhibitor combination therapy in triple negative breast cancer
Authors: McLaughlin Lena J. , Li Huili , Nagaria Pratik , Baylin Stephen B. , Zahnow Cynthia A. , Rassool Feyruz V. 
Published: July 2017

Poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi) have efficacy in a sub-set of triple negative breast cancers (TNBCs) with inherited mutations in DNA double strand break repair (DSBR) genes, such as the BRCA1/2 genes in homologous recombination (HR), through synthetic lethality. However, PARP inhibitors have failed for the majority of sporadic TNBCs with intact BRCA1 genes. Therefore, novel targeted therapies must be explored. We have recently reported that PARPi Talazoparib in combination with DNA methyl transferase inhibitors (DNMTi)s azacytidine (AZA) or decitabine (DAC) have efficacy in sporadic TNBCs in vitro and in vivo. PARPi act not only to catalytically inhibit PARP but also to trap PARP at DNA single strand breaks (SSB), leading to suicidal DNA-protein crosslinks (DPC). We also showed that Talazoparib in combination with AZA/DAC increases PARP trapping in DNA, leading to increased and persistent levels of lethal DSBs, suggesting that DSBR may also be impaired with this treatment. HR plays an important role in resolving DPC in mammalian cells and Fanconi anemia (FA)-dependent HR has been previously shown to resolve AZA-induced replication lesions. Herein, we determined whether HR is impaired with combination drug treatment using a chromosomally integrated GFP-based reporter in the TNBC cell line, MDA-MB-231. HR activity was significantly (p&lt;0.05) decreased with AZA or AZA/Talazoparib combination treatment, suggesting that AZA mediates decreased HR activity. We next questioned whether subsets of HR genes are indirectly down-regulated by the epigenetic reprogramming effects of DNMTis, thereby contributing to the efficacy of DNMTi and PARPi combination therapy. Microarray analysis was performed in multiple TNBC cell lines (including MDA-MB-231, MDA-MB-468, and SUM159PT) post DNMTi treatment and showed significant (p&lt;0.05) decreases in expression of FANCD2, FANCC, and FANCE that could potentially generate a synthetic lethality when combined with PARPi, as has recently been reported. Decreased FA gene expression was validated by qPCR of mRNA and western analysis of proteins. Moreover, treatment of TNBC with DNA crosslinking agents, which require FA-dependent repair, have increased sensitivity post DNMTi treatment. These data suggest that decreased FA gene expression contributes to the efficacy of PARPi/DNMTi treatment in TNBC. Work is now underway to determine whether depletion of FA proteins can increase PARP trapping in PARPis treated TNBCs, leading to increased levels of cytotoxic DSBs.

Citation Format: Lena J. McLaughlin, Huili Li, Pratik Nagaria, Stephen B. Baylin, Cynthia A. Zahnow, Feyruz V. Rassool. Decreased Fanconi anemia gene expression contributes to efficacy of PARP and DNMT inhibitor combination therapy in triple negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4045. doi:10.1158/1538-7445.AM2017-4045