Comprehensive Analysis of Somatic Reversion Mutations in Homologous Recombination Repair (HRR) Genes in A Large Cohort of Chinese Pan-cancer Patients

Purpose: Mutations leading to homologous recombination deficiency (HRD) increase the tumor sensitivity to platinum-based chemotherapy and PARP inhibitors. However, reversion mutations often develop conferring acquired drug resistance. There is still a lack of comprehensive investigation on HRR reversion mutations in large pan-cancer cohorts, especially in the Eastern Asian population. This study aims to characterize reversion mutations in homologous recombination repair (HRR)-related genes in a large cohort of Chinese pan-cancer patients. Methods: Sequencing data from 23,375 patients across over 17 cancer types were retrospectively analyzed for pathogenic/likely pathogenic (P/LP) germline mutations in 15 HRR genes. Somatic mutations detected in tumor or circulating cell-free DNA predicted to restore the open reading frame of the deleterious allele were subsequently identified as reversion mutations. Results: 654 cases out of 23,375 (2.8%) unselected pan-cancer patients were identified with HRR germline mutations. Secondary somatic mutations were further analyzed in their matched tumor/plasma samples. The overall frequency of reversion mutation was 1.7% (11/654). The reversion mutations occurred only in 3 out of the 15 HRR genes: BRCA1 (3.8%), BRCA2 (3.5%) and PALB2 (2.0%) from 11 patients (6 breast cancers, 1 ovarian cancer, 1 pancreatic cancer, 1 lung cancer and 2 breast and ovarian dual cancers). We identified total 25 reversion events (BRCA1, n=9; BRCA2, n=8; PALB2, n=8), including 12 pure deletions, 10 missense single nucleotide variants, 2 insertions and 1 splice site mutation. Besides, we detected microhomology length >1bp in seven out of the reversion deletions (58.3%), suggestive of microhomology-mediated end-joining (MMEJ) repair signature. Intriguingly, a positive correlation (r=0.85, p=0.001) between the length of deletion and the microhomology length was also observed. We obtained disease courses from 6/11 patients with reversion events. Four acquired reversions after the failure of the PARP inhibitor treatment. Two patients had somatic reversion mutations identified after progressing on platinum-based treatment. Conclusion: This study comprehensively depicts the prevalence and characteristics of HRR reversion mutation of germline mutations in an unselected Chinese pan-cancer cohort. The reversion mutations predominantly occurred in BRCA1, BRCA2 and PALB2. The results revealed that reversion mutations frequently occurred after resistance to platinum-based chemotherapy and/or PARP inhibitor. Our study provides insight into the underlying mechanism of drug resistance in HRD tumors and suggests that monitoring HRR mutation status along the disease course could be beneficial especially for informing resistance mechanisms and guiding subsequent therapies.