Abstract P1-06-06: Direct ex vivo observation of homologous recombination defect reversal after DNA damaging chemotherapy in metastatic breast cancer patients
Authors: Meijer TG , Verkaik NS , Van Deurzen CH , Dubbink HJ , Den Toom TD , Dinjens WN , Kanaar R , Van Gent DC , Jager A 
Published: May 2019

Abstract
Introduction
Better predictive biomarkers for response to Poly ADP-Ribose inhibitors (PARPi) are required, since on the one hand evidence is emerging that PARPi are also effective beyond germline BRCA mutated (gBRCAm) cancers and on the other hand gBRCAm cancers can become resistant to PARPi. Therefore, we previously developed a functional homologous recombination (HR) assay exploiting the formation of RAD51 foci in proliferating cells after ex vivo irradiation of fresh primary breast cancer (BrC) tissue (n=148): the REpair CAPacity (RECAP) test. The aim of the current study is to molecularly characterize real-time HR deficient (HRD) tumors and explore the utility of RECAP as a predictive biomarker for PARPi treatment in metastatic BrCs.
Material and method
Patients with advanced or recurrent BrC with easily accessible metastases were eligible. Fresh tissue biopsies from metastatic BrC lesions were collected in customized medium, irradiated with 5 Gy and cultured for 2 hours. Molecular characterization of functional HRD biopsies as well as platinum/PARP resistant biopsies was performed by targeted sequencing (BRCA1/2, TP53, CHEK2, PALB2), BRCA1 promoter methylation analysis and multiplex ligation-dependent probe amplification (MLPA) analysis of BRCA1 and BRCA2 to identify large rearrangements.
Results
41 biopsies were derived from 38 patients with recurrent or metastatic BrC. The RECAP test had a high success rate (93%) when performed on core needle or punch biopsies, and test results were available within 1 week. HRD was detected in 13 out of 41 biopsies (32%), among which 5 were gBRCAm, indicating that the RECAP test identifies more patients who may benefit from PARPi treatment than gBRCA analysis only. Among the 8 non-gBRCAm HRD tumors was one tumor with a germline PALB2 mutation, one with BRCA1 promoter hypermethylation and two with somatic variants of unkown significance (VUSes) in BRCA2. In three gBRCAm patients BRCA reversion was detected, as the HRD tumors became HR proficient (HRP) after showing in vivo progressive disease (PD) on cisplatin/PARPi treatment. One of these patients obtained a secondary BRCA1 mutation that restored the open reading frame and led to production of full-length BRCA1 protein, while the causative molecular event in the other patients is still elusive.
Conclusion
The RECAP test is a robust and reproducible HRD test which identifies approximately 60% more potential candidates for PARPi treatment, as 40% of HRD tumors were caused by gBRCAm. Due to its functional character, the RECAP test reflects the real-time HR status regardless of BRCA mutational status and therefore detects HR reversal upon therapy resistance. Thus, RECAP shows great potential as a predictive biomarker for PARPi treatment of metastatic BrC.
Citation Format: Meijer TG, Verkaik NS, Van Deurzen CH, Dubbink HJ, Den Toom TD, Dinjens WN, Kanaar R, Van Gent DC, Jager A. Direct ex vivo observation of homologous recombination defect reversal after DNA damaging chemotherapy in metastatic breast cancer patients [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P1-06-06.