Abstract 3536: BRCA1 promoter methylation and homologous recombination repair status in primary chronic myeloid neoplasms.

Chronic myeloid neoplasms (CMNs) are characterized by excessive expansion of terminally differentiated blood cells arising from myeloid progenitors. This group of hematologic clonal disorders includes the BCR-ABL1-negative myeloproliferative neoplasms (MPNs), myelodysplastic syndromes (MDS) and MPN/MDS disorders with features overlapping both subtypes. CMN patients are at risk of developing acute myeloid leukemia (AML), however curative therapy is lacking with the exception of allogeneic stem cell transplant. Since leukemic transformation portends a dismal treatment outcome and survival, there is a need to identify novel molecular events driving disease progression as a first step in effective targeted therapy. Previously, we achieved complete clearance of leukemic blasts in several CMN patients who had progressed onto AML when treated with poly (ADP-ribose) polymerase (PARP) inhibitor ABT-888, carboplatin and topotecan (McDevitt et al., ASH 2011 and Pratz et al., ASH 2011). PARP inhibitors block single stand break repair which cumulates in an increase in DSBs that cannot be repaired in tumor cells defective in homologous recombination (HR) repair, resulting in the exquisite death response of the malignant clone. In this study, we investigated whether this favorable response can be attributed to defective DNA double-stranded break (DSB) repair resulting from epigenetic silencing of HR genes. To this end, we evaluated the promoter methylation of HR genes and then assessed DNA DSB repair status in CMN patients. We detected BRCA1 promoter hypermethylation in 10% of 99 unique MPN patient samples using quantitative methylation-specific PCR (qMSP) and confirmed decreased BRCA1 transcript expression in several samples. We did not observe DNA promoter methylation of other HR repair genes (Fanconi Anemia genes, Bloom, ATM, ATR, BRCA2). In addition, we characterized the functional DSB repair status in primary mononuclear cells from CMN patients by measuring Rad51 foci formation after irradiation and identified a subset of patients with defective HR repair. Ongoing studies of additional CMN patient samples are underway to further link HR repair gene promoter hypermethylation, transcript expression and functional Rad51 foci formation. These findings will also be compared to karyotype information and in vitro PARP inhibition sensitivity results. By presenting evidence of DNA repair defect that can be linked to epigenetic silencing of HR repair genes in CMN patients, we highlight defective DSB repair as a novel mechanism driving disease progression in myeloid malignancies and also propose examining the DNA promoter methylation status of HR genes as a biomarker to identify patients who will respond favorably to PARP inhibition. Citation Format: Weijie Poh, Robert L. Dilley, Alison R. Moliterno, Keith W. Pratz, Michael A. McDevitt, James G. Herman. BRCA1 promoter methylation and homologous recombination repair status in primary chronic myeloid neoplasms. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3536. doi:10.1158/1538-7445.AM2013-3536