Introduction:

PARP inhibitors kill cancer cells by harnessing the synthetic lethality between PARP1 and BRCA1/2 thereby impairing DNA damage detection and repair ^1,2. Reversion mutations, secondary mutations that restore wild-type function to the BRCA gene, have been shown to play a significant role in clinical resistance to PARP inhibitor therapy ^{6, 7, 8, 9}. However, the frequency and optimal detection strategy of BRCA reversion mutations are still not well understood. We report the results of extensive genomic interrogation of a BRCA2-mutant prostate cancer patient with acquired resistance to combination therapy including a PARP inhibitor mediated by multiple BRCA2 reversion mutations.

Methods:

We completed extensive genomic interrogation of both cfDNA and tissue pre and post PARP inhibitor therapy and postmortem in a prostate cancer patient with a germline BRCA2 mutation. The patient provided written informed consent for genomic sequencing of tumor and cfDNA, and review of medical records for detailed demographic, pathologic, and clinical data as part of an institutional IRB-approved research protocol. Whole exome sequencing was performed on biopsy samples and post-mortem tissue, and targeted next-generation sequencing on serial cfDNA samples.

Results:

Genomic analysis revealed ten unique BRCA2 reversion mutations, functioning by either restoring the reading frame or excising the germline mutation. These ten reversion mutations were identified across ten different sample sites collected post-PARP inhibitor therapy, including samples collected pre and post mortem. While several of the reversion mutations were private to a specific site, all tumors contained at least one mutation. Notably, post-PARP inhibitor cfDNA did not identify any reversion mutations, and only two of these ten reversion mutations were detected in post-mortem cfDNA.

Conclusion:

This case is the most extensive analysis of autopsy and longitudinal clinical specimens for PARP inhibitor acquired resistance in BRCA-mutant metastatic castration resistant prostate cancer (mCRPC) to date, and a remarkable demonstration of spatial heterogeneity and convergent evolution. The failure of cfDNA to appropriately reflect the intratumoral heterogeneity of reversion mutations in the case of a mCRPC patient is both unexpected and clinically impactful given its contradiction to recent research ^{8, 9}. Furthermore, our study reaffirms the importance of genomic interrogation and detection of resistance mechanisms at the time of progression to PARP inhibitors to inform therapeutic decision making.

Funding: National Cancer Institute (NCI) under the MSK Cancer Center Support Grant/Core Grant (P30 CA008748) and Gray Foundation