A RAD 51 assay feasible in routine tumor samples calls PARP inhibitor response beyond BRCA mutation

Abstract

Poly(ADP‐ribose) polymerase (PARP) inhibitors (PARPi) are effective in cancers with defective homologous recombination DNA repair (HRR), including BRCA1/2‐related cancers. A test to identify additional HRR‐deficient tumors will help to extend their use in new indications. We evaluated the activity of the PARPi olaparib in patient‐derived tumor xenografts (PDXs) from breast cancer (BC) patients and investigated mechanisms of sensitivity through exome sequencing, BRCA1 promoter methylation analysis, and immunostaining of HRR proteins, including RAD51 nuclear foci. In an independent BC PDX panel, the predictive capacity of the RAD51 score and the homologous recombination deficiency (HRD) score were compared. To examine the clinical feasibility of the RAD51 assay, we scored archival breast tumor samples, including PALB2‐related hereditary cancers. The RAD51 score was highly discriminative of PARPi sensitivity versus PARPi resistance in BC PDXs and outperformed the genomic test. In clinical samples, all PALB2‐related tumors were classified as HRR‐deficient by the RAD51 score. The functional biomarker RAD51 enables the identification of PARPi‐sensitive BC and broadens the population who may benefit from this therapy beyond BRCA1/2‐related cancers.

Reversion and non-reversion mechanisms of resistance to PARP inhibitor or platinum chemotherapy inBRCA1/2-mutant metastatic breast cancer

“…Our results suggest that immunohistochemical staining for RAD51 foci offers real-time assessment of a tumor’s HR proficiency, and correlates with response and resistance to PARPi/platinum therapy. The feasibility of RAD51 foci staining in human tumors without antecedent exposure to DNA damaging agents has been previously demonstrated in two small cohorts, 25,42 and our results offer further proof-of-concept. The presence of RAD51 foci has been shown to correlate with decreased efficacy of PARP inhibition in patient derived xenografts and in a cohort of 8 patients with germline BRCA1/2 mutation.…”

“…The presence of RAD51 foci has been shown to correlate with decreased efficacy of PARP inhibition in patient derived xenografts and in a cohort of 8 patients with germline BRCA1/2 mutation. 25,42 However, we demonstrate here for the first time that the presence or absence of RAD51 staining changes over time as predicted with HR-restoring mechanisms of resistance. Taken together, our data along with previous results strongly suggest that RAD51 staining should be investigated as a predictive biomarker in larger cohorts of BRCA1/2 -deficient patients treated with PARPi/platinum.…”

PARP Inhibitors: Extending Benefit Beyond BRCA-Mutant Cancers

“…The mechanisms have been extensively previously reviewed (7,78,79), and include reversion to wild-type mutations in BRCA and other HRR genes, promoter demethylation of suppressed DDR genes, mitigation of replication stress, mutations in PARP itself, and/or drug efflux pumps, among others. This overarching mechanism of HRR restoration has been highlighted in multiple preclinical PDX studies of PARP inhibition in TNBC (15,(80)(81)(82), where functionality of HRR in virtually all cases was implied by the presence of RAD51 foci in untreated tumor samples, suggesting this could represent a useful clinical biomarker for PARP inhibitor response and/or resistance. Emerging data also suggest that these olaparibresistant cancer models can be resensitized to olaparib when combined with a WEE1 inhibitor or an ATR inhibitor (67,73,(83)(84)(85).…”

PARP and PARG inhibitors in cancer treatment

“…Moreover, BRCA1/2, ATM, and CHEK2 are the most frequently mutated DNA repair genes in somatic cancer cells (Heeke et al 2018). In addition to being directly inactivated by mutation, BRCA1 and RAD51C were also found to be down-regulated through promoter hypermethylation in breast and ovarian cancer (Chiang et al 2006;The Cancer Genome Atlas Research Network 2011;Lips et al 2013;Timms et al 2014;Polak et al 2017;Bernards et al 2018;Castroviejo-Bermejo et al 2018;Kondrashova et al 2018). BRCA1 promoter methylation confers the same degree of sensitivity to PARP inhibitors as BRCA1 mutations (Veeck et al 2010).…”