Characterization of a <i>RAD51C</i> -silenced high-grade serous ovarian cancer model during development of PARP inhibitor resistance

Abstract

Acquired PARP inhibitor (PARPi) resistance in <i>BRCA1</i>- or <i>BRCA2</i>-mutant ovarian cancer often results from secondary mutations that restore expression of functional protein. <i>RAD51C</i> is a less commonly studied ovarian cancer susceptibility gene whose promoter is sometimes methylated, leading to homologous recombination (HR) deficiency and PARPi sensitivity. For this study, the PARPi-sensitive patient-derived ovarian cancer xenograft PH039, which lacks HR gene mutations but harbors <i>RAD51C</i> promoter methylation, was selected for PARPi resistance by cyclical niraparib treatment <i>in vivo</i>. PH039 acquired PARPi resistance by the third treatment cycle and grew through subsequent treatment with either niraparib or rucaparib. Transcriptional profiling throughout the course of resistance development showed widespread pathway level changes along with a marked increase in <i>RAD51C</i> mRNA, which reflected loss of <i>RAD51C</i> promoter methylation. Analysis of ovarian cancer samples from the ARIEL2 Part 1 clinical trial of rucaparib monotherapy likewise indicated an association between loss of <i>RAD51C</i> methylation prior to on-study biopsy and limited response. Interestingly, the PARPi resistant PH039 model remained platinum sensitive. Collectively, these results not only indicate that PARPi treatment pressure can reverse <i>RAD51C</i> methylation and restore RAD51C expression, but also provide a model for studying the clinical observation that PARPi and platinum sensitivity are sometimes dissociated.