Purpose: Defective homologous recombination (HR) has been reported in multiple myeloid disorders, suggesting a shared dysregulated pathway in these diverse malignancies. Since targeting HR-defective cancers with poly(ADP-ribose) polymerase inhibition (PARPi) has yielded clinical benefit, improved understanding of HR defects is needed to implement this treatment modality. Experimental Design: We used an ex vivo irradiation-based assay to evaluate HR repair, HR gene promoter methylation and mRNA expression in primary myeloid neoplastic cells. In vitro BRCA1 gene silencing was achieved to determine the consequences on HR repair, sensitivity to poly(ADP-ribose) polymerase inhibition and expression of miR-155, an oncogenic miRNA. Results: Impaired HR repair was frequently detected in myeloid neoplasm samples (9/21, 43%), and was linked to promoter methylation-mediated transcriptional repression of BRCA1, which was not observed for other members of the HR pathway (BRCA2, ATM, ATR, FANC-A). In vitro BRCA1 knockdown increased sensitivity to PARP inhibition, and BRCA1 expression is inversely correlated with miR-155 expression, a finding reproduced in vitro with BRCA1 knockdown. Increased miR-155 was associated with PU.1 and SHIP1 repression, known myeloid differentiation factors that are frequently down-regulated during leukemic transformation. Conclusions: This study demonstrates frequent defective HR, associated with BRCA1 epigenetic silencing, in a broad range of myeloid neoplasms. The increased prevalence of BRCA1 promoter methylation, resulting in repressed BRCA1, may have an additional role in leukemogenesis by increasing miR-155 expression, which then inhibits transcription factors associated with normal myeloid differentiation. Further study of HR defects may facilitate the identification of HR defective myeloid neoplasms sensitive to PARPi.
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