Accumulation of genome-wide somatic loss of heterozygosity (LOH) as a prominent feature of advanced malignant soft tissue tumors and association with the BRCAness status, suppression of immune responses, and lower survival rates.

11533 Background: Malignant soft tissue tumor is a rare cancer with few therapeutic options. Recent genomic analysis revealed widespread CNA and the cumulative burden of cancer-related pathogenic germline mutations/variants, their clinical and therapeutic significance were unknown. Methods: We recruited 155 patients with advanced malignant soft tissue tumors (135 female and 20 male, mean age 51 at analysis, 100 LMS, 19 LPS, 4 ESS, 3 UPS, 3 AS, 3 MPT, 3 GIST and others) with confirmed metastasis/recurrence and information on familial cancer burden. Whole exome sequencing was performed in both blood and tumor samples as described in 2018ASCO. The copy number of BRCA2 gene was determined by the MLPA method. Tumor immune microenvironment was assessed by immunohistochemistry. The MSI status was analysed by PCR. Results: We analyzed the LOH status in 595 COSMIC genes and found that genome-wide accumulation of somatic LOH of polygenic germline mutations/variants. Patients with more than 33% LOH genes (n=102) in the total of somatic and LOH mutations showed significantly lower OS rates compared with those (n=53) with less LOH genes (5-year survival rates; 49 vs 75%, p=0.010), which constitute 78% of LMS (n=78/100) and 26% of LPS (n=5/19). Total of 41 patients (26%, n=41/155) including 33 LMS (33%, n=33/100) showed LOH in the BRCA2 locus with hemizygous VUS. Those patients with BRCA2 LOH (n=41) showed significantly lower OS rates compared with those without BRCA2 LOH (n=114) (5-year survival rates; 43 vs. 64%, p=0.019). Neither TMB nor the MSI status was associated with LOH. In contrast, accumulation of somatic LOH (mean LOH values of 71.7 vs. 15.7%) was clearly and negatively associated with CD8+T-cell immune infiltrates (T-cells; 44±23 vs 555±180/mm2, n=7, p=0.016), CD20+B-cell accumulation in tertiary lymphoid structures (TLS) (TLS; 0.57±0.43 vs. 20.1±6.1/tumor, n=7, p=0.008) and low levels of neutrophil-to-lymphocyte ratio (NLR) (NLR; 3.63±0.45 vs. 1.71±0.17, n=7, p=0.002), hallmarks of the immunological response to tumors. Conclusions: This study suggests that in advanced malignant soft tissue tumors, accumulation of genome-wide LOH of germline mutations/variants is associated with the BRCAness status and suppression of the immune responses to tumors, and thus influences therapeutic response and survival of the patients.