Therapeutic relevance of homologous recombination repair (HRR) pathway variants in metastatic colorectal cancer (mCRC).

Abstract

677 Background: Intact HRR pathway genes play a critical role in repairing double-stranded DNA breaks. Genomic alterations (GA) in several HRR pathway genes have been well characterized, demonstrating prognostic and predictive significance. However, there are many variants of unknown significance (VUS) in the HRR pathway genes that need better characterization. Methods: Patients (pts) with mCRC that harbored VUSs in HRR pathway genes (HRR- VUS; BRCA1/2, ATM, ATR, RAD50, RAD51, PALB2, CHEK1, CHEK2) and treated with front line platinum therapy were identified by review of our institutional molecular data base (MDB) and EMR. Time to 2nd line therapy (TT2L) and Overall Survival (OS) were calculated. DNA was extracted from formalin fixed paraffin embedded clinical specimens and Next Generation Sequencing (NGS) was performed on hybrid-capture, adaptor ligation based libraries to a mean coverage depth of > 600 unique reads utilizing the Foundation Medicine NGS platform. Results: Among the 873 pts in our institutional MDB, 96 (11%) had mCRC, 20 (21%) harbored HRR pathway GAs, 15 (16%) were categorized as VUSs. HRR-VUS pts had a median age of 57 at diagnosis, 5 (33%) were male, 3 (20%) were right-sided. Distribution of HRR-VUSs in this cohort is summarized in table 1. All pts with HRR-VUSs were micro-satellite stable (MSS), median TMB was 4.5 (range 0 – 8.1). Median TT2L and OS in mCRC pts with HRR-VUSs were 8.0 and 43 months (m) respectively. Two pts with HRR-VUSs had TT2L > 24 m and OS > 40 m; one of them had a BRCA1P1464A mutation, while the other had a BRCA2 amplification. Conclusions: BRCA2, ATM and BRCA1 were the HRR genes that harbored VUSs most frequently in pts with mCRC. Majority with HRR-VUSs had left sided primaries, were MSS and had low TMB. BRCA1P1464A mutation in the ATM binding domain needs further characterization on account of prolonged TT2L with platinum based chemotherapy. Patients with HRR-VUSs that are predictive of benefit with platinum based chemotherapies could be considered for maintenance therapy with PARP inhibitors [Table: see text]