Comparative genomic profiling of circulating tumor DNA and paired formalin-fixed paraffin-embedded (FFPE) DNA from Indian patients with breast cancer.

Abstract

e14554 Background: Breast cancer (BC) is the most frequent malignancy and a leading cause of mortality in women worldwide. Although most BCs are treated with curative intent, 30-40 % patients experience disease progression or metastatic relapse. Considering high heterogeneity of BC, mutational profiling of solid biopsy may not be adequate for informed treatment decisions in all cases. Liquid biopsy based on circulating tumor DNA (ctDNA) analysis is emerging as an equivalent and alternative to tissue biopsy in clinical settings. Though extremely critical for management of BC, liquid biopsy is not a standard of care in India. Here we compared comprehensive genomic profiles of ctDNA and paired biopsy samples to determine the efficacy of ctDNA assay in detecting actionable mutational targets. Methods: Retrospectively, genomic DNA and matched ctDNA was isolated from both FFPE and blood samples of14 BC patients. The patients were confirmative with pathological findings, having metastatic infiltrating ductal carcinoma, or triple negative breast cancers. Genome-wide HRD, TMB and MSI scores based on the OncoIndx comprehensive gene panel (CGP) was determined. OncoIndx CGP targets 1080 cancer related genes along with 47 HRR genes and 1600 MSI signature sites. Libraries were prepared using a hybridization-capture method based on custom-designed OncoIndx 1080 CGP and sequenced (depth 5500 X) on Illumina Nextseq 2000 in a pair-end mode (150 x2). Variant calling was performed using a proprietary bioinformatics pipeline iCare. Results: All patients were diagnosed with metastatic infiltrating ductal carcinoma, or triple negative breast cancers. From the patient cohort, microsatellite instability (MSI) from ctDNA showed 100% concordance with FFPE DNA while tumor mutation burden (TMB) and homologous recombination deficiency (HRD) scores showed 71.4 % and 64.2 % concordance with FFPE DNA respectively. At the individual gene mutation level, the concordance TP53, BRCA1/2, PI3K family of genes, ESR1, PTEN, ERBB2, PALB2, and BRAF genes between ctDNA and FFPE DNA was 54.7%, 92.8%, 54.7%, 78.5%, 92.8%, 100%, 100%, and 100% respectively. Many important genomic alterations including PIK3CA:p.L334H, PIK3CA:p.E81K, ESR1:p.Q375L, BRCA2:p.X3040_splice, PTEN:p.D19H were exclusive to ctDNA thereby denoting potential tumor heterogeneities which could be missed from FFPE DNA. Conclusions: Our results suggest a high correlation between ctDNA and FFPE DNA detected by OncoIndx test in metastatic or triple negative breast cancer patients of Indian ethnicity. Although not a part of standard of care, liquid biopsy could be of significant potential for management of BC in the standard clinical settings. However, a thorough investigation of cancer stage, and clinical presentations are necessary to carefully evaluate the selection of ctDNA or FFPE DNA-based genomic profiling.