Detection of homologous recombination deficiency (HRD) in cell-free DNA (cfDNA) using an amplicon-based next-generation sequencing (NGS) assay.

Abstract

3042 Background: Homologous recombination (HR) deficiency is characterized by tumor genomic instability, often due to alterations in BRCA1/2 and other HR-related genes. HRD predicts sensitivity to PARP inhibitors (PARPi) in prostate, ovarian and breast cancers. For the subset of cancers that have genomic instability without detectable alterations in HR genes, profiling biomarkers of HRD such as loss of heterozygosity (LOH) could identify additional HRD positive (HRD+) patients that may benefit from PARPi. Here, we present a novel cfDNA NGS assay that can detect tumor LOH non-invasively to improve assessment of HRD+ status. Methods: An amplicon-based HRD NGS assay covering > 1000 SNPs was developed to detect LOH both globally and on the gene level in cfDNA. Analytical validation was done using LOH+ BRCA-mutant cell lines in limiting dilution admixtures. Clinical performance was assessed by benchmarking findings from 46 tumor tissue DNA samples against results from an orthogonal NGS-based genomic scarring assay. Clinical utility of the HRD assay was evaluated in 75 cfDNA samples, including 72% (54/75) from BRCA-associated cancers (36 breast, 12 prostate, 4 ovarian, 2 pancreatic) and 28% from other cancer types. All cfDNA samples were previously clinically tested by an NGS assay that included BRCA1/2 (LiquidHALLMARK). A subset of cfDNA samples identified as HRD+ were further assessed for alterations in 26 key HR genes ( ARID1A, ATM, ATR, ATRX, BAP1, BARD1, BRCA1, BRCA2, BRIP1, CDK12, CHEK1, CHEK2, FANCA, FANCC, FANCG, FANCL, MRE11, NBN, PALB2, PTEN, RAD51, RAD51B, RAD51C, RAD51D, RAD54L, and XRCC2). Results: In analytical validation, the assay could detect HRD in admixtures with as low as 18% tumor fraction. In tissue samples, overall concordance of HRD status with the orthogonal test was 91.3% (42/46), with a positive percent agreement of 94.4% (34/36) and negative percent agreement of 80.0% (8/10). In cfDNA, 26.7% (20/75) of samples were HRD+, including 29.6% of BRCA-associated cancers (16/54; 12 breast, 2 prostate, 1 ovarian, and 1 pancreatic) and 19.1% (4/21) of other cancer types. In the three HRD+ breast cancers with pathogenic BRCA mutations, the assay also identified BRCA-specific LOH. Nine HRD+ breast cancers (two BRCA+ and seven BRCA-) were further analyzed for alterations in 26 HR genes. Of the two BRCA+ HRD+ breast cancers, one harbored an additional PTEN loss-of-function (LOF) mutation. Of the seven BRCA- HRD+ breast cancers, one harbored biallelic PALB2 and CDK12 LOF mutations, while another harbored a CDK12 LOF mutation. Conclusions: LOH detection in cfDNA provides additional diagnostic yield of HRD+ status in multiple cancer types, even in the absence of pathogenic HR gene alterations. Further clinical studies to evaluate the utility of HRD detection in cfDNA using LOH and to determine concordance with tumor tissue are ongoing.