<i>BRCA1</i> Promoter Methylation Is Linked to Defective Homologous Recombination Repair and Elevated <i>miR-155</i> to Disrupt Myeloid Differentiation in Myeloid Malignancies.

Abstract

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