Identification of BRD4 as a Synthetic Lethal Gene to Treat MCPH1 deficient Ovarian Cancer

Abstract

Background: Ovarian cancer (OC) and endometrial carcinoma (EC) are aggressive, heterogeneous gynaecological cancers. MCPH1 is a tumour suppressor in several cancers and is reported to be downregulated and mutated in OC and EC. BRD4 (Bromodomain Containing 4) plays a vital role in gene regulation and chromatin remodelling. BRD4 overexpression has been implicated in tumorigenesis and therapeutic resistance in several cancers, including OC. Targeting synthetic lethality (SL), involving mutations in two functionally related pathway genes, is a promising therapy for OC. In EC, immune-checkpoint inhibitors (ICIs) have become important treatment options, particularly for tumours with deficient mismatch repair or microsatellite instability, which are associated with higher response rates to PD- 1/PD-L1 blockade. Objectives: This study investigates whether BRD4 inhibition with siMCPH1 knockdown in OC cells is a potential SL target and explores the underlying mechanisms. The association between MCPH1 and BRD4 protein expression and clinical markers in OC and EC are also examined. Methods: Previous siRNA screen of two high-throughput screens identified BRD4 as a potential MCPH1 SL target. A colony-forming assay was used to determine the reduction in cell viability. Mechanistic studies, including flow cytometry, DAPI staining and RNA-Seq analysis, identified the pathways involved. To determine the clinical relevance of MCPH1 and BRD4 protein expression, 431 OC and 376 EC samples were investigated. Results: siMCPH1 knockdown combined with JQ1 inhibitor treatment showed a significant decrease in viability. Apoptosis was significantly increased in siMCPH1 SKOV-3 cells treated with JQ1. An increase in premature chromosome condensation was identified in SKOV-3 (10.33%) and OVCA-433 (16.96%) cell lines. RNA-Seq identified potential SL pathways, including extracellular matrix organisation, apoptosis, p53 signaling, angiogenesis, chromatin remodelling. Low MCPH1 protein expression in OC cases is correlated with reduced overall survival (OS) (p=0.003), progression-free survival (PFS) (p<0.001) and BRCA status (p=0.004). 13.22% of OC samples with low MCPH1 and high BRD4 expression, demonstrated a targeted group of SL treatment. By contrast, in EC, high MCPH1 (p=0.034) and BRD4 (p=0.044) expression were associated with reduced disease-specific survival and correlated with immune-checkpoint markers. Conclusion: This study suggests that BRD4 is a promising SL target for MCPH1- deficient OC. MCPH1 and BRD4 could serve as potential prognostic markers for OC and EC and a possible ICI therapy.