Genomic characterization of ATM alterations in advanced pancreatic ductal adenocarcinoma (PDAC)

396 Background: BRCA1/2 and PALB2 are genes critical to the faithful repair of double strand breaks through the homologous recombination repair (HRR) pathway. Alterations in these genes serve as predictive biomarkers to both platinum and PARP inhibitors. Ataxia-telangiectasia mutated ( ATM) is also indirectly involved in HRR; however, its role as a predictive biomarker to DNA damage response agents is debated. Herein we evaluated the genomic characteristics and clinical outcomes of patients with ATM alterations on the Comprehensive Molecular Characterization of Advanced Ductal Pancreas Adenocarcinoma for Better Treatment Selection (COMPASS) trial. Methods: Patients on this study undergo a biopsy for whole genome sequencing (WGS) and RNA sequencing prior to chemotherapy; those with germline variants in ATM were reviewed by a genetics counsellor and defined as pathogenic, likely pathogenic, variant of unknown significance (VUS) or benign/likely benign. Genomic characteristics were reviewed and published classifiers of homologous recombination deficiency (HRD) were applied to all cases and included the percentage of substitution base signature (SBS) 3, the HRDetect score, the computed algorithm of large scale transitions, telomeric allelic imbalances and loss of heterozygosity (LOH), otherwise known as the genomic instability score (GIS). Results: As of January 2020, 304 patients were enrolled and 245 patients had both WGS and clinical data available. 86 germline variants in ATM were present in 70 patients. The majority of these (80%) were classified as benign or likely benign. 10 VUS were detected and 4 patients (2%) had pathogenic/likely pathogenic variants (PV). Of these 4 patients, LOH or a second somatic hit was evident in 1 case. Upon review of the PVs and VUS, SBS were consistent with typical PDAC and tumour mutational burden was low. HRDetect scores were low ( < 0.1) for 13/14 cases with either a VUS or PV; one VUS without biallelic loss, had a high HRDetect score, with presence of SBS 3 and a high GIS. This particular case was also found to have a tandem duplicator phenotype. None of the 4 cases with PV had evidence of HRD. Furthermore all four were treated with platinum based regimens without evidence of response. Conclusions: In a large series of sequenced pancreatic cancers, the presence of pathogenic germline variants in ATM was rare, with none of the cases demonstrating evidence of HRD. This suggests that this population is unlikely to benefit from PARP inhibition.