Metastatic uveal melanoma is a deadly disease with no proven standard of care. Here we present a metastatic uveal melanoma patient with an exceptional high sensitivity to a PD-1 inhibitor associated with outlier CpG>TpG mutation burden, MBD4 germline deleterious mutation, and somatic MBD4 inactivation in the tumor. We identify additional tumors in The Cancer Genome Atlas (TCGA) cohorts with similar hypermutator profiles in patients carrying germline deleterious MBD4 mutations and somatic loss of heterozygosity. This MBD4-related hypermutator phenotype may explain unexpected responses to immune checkpoint inhibitors.

Analysis of the TCRα repertoire is particularly appropriate in a prospective way to identify patients with partial immune defects caused by suboptimal V(D)J recombination activity, a DNA repair defect, or both. It also constitutes a valuable tool for the retrospective in vivo functional validation of variants identified through exome or panel sequencing. Its broader implementation might be of interest to assist early diagnosis of patients presenting with hypomorphic DNA repair defects inclined to experience acute toxicity during prehematopoietic stem cell transplantation conditioning.