Genomic analysis of advanced malignant soft tissue tumors to suggest effect of genome-wide loss-of-heterozygosity of germline mutations/variants on anti-PD-1 immunotherapy response and survival of the patients.

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11531 Background: Malignant soft tissue tumor is a rare cancer with few therapeutic options. Although recent genomic characterization of soft tissue sarcoma revealed massive CNA and an excess of polygenic burden of pathological germline variants, their clinical and therapeutic significance remains to be understood. Methods: We recruited 155 patients with malignant soft tissue tumors (135 female and 20 male, mean age 51, 100 LMS, 19 LPS, 4 ESS, 3 UPS, 3 AS, 3 MPT, 3 GIST and others) of confirmed metastasis/recurrence. Whole exome sequencing was performed as reported in 2018ASCO. The MSI status was analyzed by PCR. Tumor immune microenvironment was assessed by immunohistochemistry. Results: Of the 595 COSMIC genes, heterozygous germline mutations/variants of the genome-wide 0-44 genes (av. 9.7/tumor) showed somatic loss-of-heterozygosity (LOH) with allele frequency of more than 70%. Patients with less than 33% LOH (n=53) in the total of somatic and LOH mutations showed improved 5-year survival rate compared with those (n=102) with more LOH (71% vs 52%, p=0.037). LMS (n=100) had higher value of LOH mutations than other tumors (n=55)(av. 55.5 vs 31.2%, p<0.001). Two patients with bone metastasis, one from liver undifferentiated sarcoma (case 1) and the other from uterine LMS (case 2) were identified as MSI-High and resultant higher TMB of 6.48 and 6.60/Mb, respectively than 1.47/Mb in av. Tumors from both cases had de novo mutations of MMR deficiency as EXO I (A153V) and WRN (S1120F) in case 1 and MSH2 (G674D) in case 2. Case 1 with pleural dissemination was treated with 5 cycles of Pembrolizumab (200mg/body, d1 q3weeks) but was progressive disease, while case 2 had no evaluable lesion after surgical removal of bone metastasis. Number of CD8+ T-cell infiltration (TIL), one of the best parameter with response to PD-1 blockade, was much higher in case 2 than in case 1 (av. 907 vs 290/mm2). Case 2 had no LOH mutations while case 1 had 37% LOH with more total mutations in tumor (16.1 vs 85.9/Mb). Higher values of LOH (av. 67 vs 19%) were clearly correlated with decreased density of CD8+TIL in tumor tissues (av. 9.6 vs 429/mm2, n=5, p=0.018). Conclusions: Our results, for the first time, suggest that in malignant soft tissue tumors, accumulation of genome-wide LOH of germline mutations/variants, from which self-antigens could be generated, may influence tumor immune microenvironment, and thus influence immunotherapy response and survival of the patients.