Homologous Recombination Regulation via CtIP Expression Modulated by Transcription and mRNA Stability

Abstract

Homologous recombination (HR) is a critical DNA repair pathway that maintains genomic integrity by accurately repairing double-strand break (DSB). CtIP is a key protein involved in DNA end resection, a pivotal early step in HR. While the functional role of CtIP in HR is well established, the upstream regulation of its expression remains poorly understood. In this study, I identify novel regulatory mechanisms controlling CtIP expression at both the transcriptional and post-transcriptional levels, revealing an additional layer of regulation in the HR pathway. At the transcriptional level, I demonstrate that BRD4 regulates CtIP expression. Inhibition of BRD4 with the small molecule UNI66 reduces BRD4 occupancy at CtIP promoter and enhancer regions, leading to decreased CtIP expression and compromised HR activity. This HR deficiency sensitizes cells to PARP inhibitors, suggesting that BRD4 inhibition could be leveraged as a combinatorial therapeutic strategy to overcome resistance in PARP inhibitor-refractory cancers. In addition to transcriptional control, I identify a post-transcriptional regulatory mechanism mediated by PCID2, a component of the TREX-2 mRNA export complex. Loss of PCID2 significantly reduces CtIP mRNA stability, thereby diminishing CtIP protein levels and impairing HR. I found that PCID2 is essential for the stability of CtIP mRNA. Together, these findings uncover novel transcriptional and post-transcriptional regulatory mechanisms of CtIP expression, highlighting a multilayered regulatory network that ensures proper HR function.