Tumor mutation burden, DNA mismatch repair status and checkpoint immunotherapy markers in primary and relapsed malignant rhabdoid tumors

Introduction: Malignant rhabdoid tumor (MRT) is a rare, aggressive pediatric tumor of nuclear lineage. It is mainly characterized by germline or somatic SMARCB1 (INI1) driver mutations. To characterize the potential for immunotherapy in untreated and treated MRT, current study investigated tumor mutational burden (TMB) and other biomarkers in MRT. Material and methods: Normal-tumor paired whole exome sequencing (WES) and/or immunohistochemistry (IHC) of DNA mismatch repair (MMR) proteins, PD-L1, PD-1 and CD8 were performed in 16 cases, some with both primary and relapsed tumor. Results: Five cases subjected to WES demonstrated germline SMARCB1 (INI1) mutations. TMB was 0.7–1.07/Mb in 4 of the 5 primary untreated tumors, and 33.81/Mb in one case with pathogenic MMR, POLD, and POLE mutations. Ten cases tested for MMR status by IHC showed retained nuclear expression of the proteins. Eight of the 16 cases (8/16, 50%) showed membranous expression of PD-L1 in 10–70% of tumor cells (tumor proportion score, TPS). Nine cases (9/16, 56.3%) showed high (>2/HPF) tumor infiltrating lymphocytes with PD-1 staining ranging 10–60%, correlating with tumor PD-L1 staining (p < 0.0001). Between post-treatment metastatic tumors and the pre-treatment primary tumors, TMB was similar while PD-L1 TPS was similar or lower. Conclusion: MRT has a low TMB. Nonetheless, because a subset of MRT cases have a PD-L1 TPS greater than the cutoff for checkpoint therapy in other malignancies, the utility of immune checkpoint inhibitors should be studied in this patient population.