BRCA1 and RAD51C promotor methylation in human resectable pancreatic adenocarcinoma

Background: Homozygous Recombination Deficiency (HRD) is associated with sensitivity to PARP-inhibitors (PARPi) in different cancer types. In pancreatic adenocarcinoma (PA) the main cause of HRD is BRCA1/2 germline mutation and patients with mutations in BRCA1/2 may benefit from PARPi. Recently other mechanisms leading to HRD were described in different cancer types, including gene mutations and epigenetic changes such as promoter hypermethylation. In PA, BRCA1 promoter hypermethylation, a known mechanism of gene silencing, was recently described. However, results are discordant between North American studies (0.7% of PA) and Asian ones (up to 60% of PA) and the association with HRD is not clear.

Methods: Here, we developed 2 quantifications methods to explore BRCA1 and RAD51C promoter methylation in a series of 121 Formalin Fixed-Paraffin-Embedded (FFPE) specimens from resected PA without neoadjuvant treatment. The methylation-specific PCR was done with 2 different methods after DNA bisulfite conversion: a digital droplet PCR, and a PCR followed by capillary electrophoresis, to score the methylated / non methylated ratios in tumor samples. Methods were validated for specificity and sensibility using 100, 20, 10, 5 and 0% methylated commercial DNA for fragment analysis with a detection cutoff of 5-10%. Limit of blank was defined as 5 dropplets/20µL for RAD51C and 1 dropplet/20µL for BRCA1 for ddPCR. Samples were reviewed by a pathologist, macrodissected before DNA extraction to obtain 50-60% of tumoral cells. DNAs were treated for bisulfite conversion and analyzed using both methods in parallel to known positive and negative controls in each run.

Results and conclusion: No methylation at BRCA1 or RAD51C was found in this series of PA suggesting that HRD gene promoter methylation is a rare event in European patients.