Mismatch repair–deficient (dMMR) cancers generate a substantial number of immunogenic neoantigens, rendering them sensitive to immunotherapy. Yet, there is considerable variability in responses, and roughly one-half of dMMR cancers are refractory to immunotherapy. Here we study a patient with dMMR lung cancer refractory to immunotherapy. The tumor exhibited typical dMMR molecular features, including exceptionally high frameshift insertions and deletions (indels). Despite the treatment inducing abundant intratumoral T-cell infiltrates, it failed to elicit tumor regression, pointing to the T cells lacking cytotoxic activity. A post-treatment tumor demonstrated compound heterozygous frameshift deletions located upstream of the kinase domain in the gene encoding JAK1 protein, down-regulation of JAK1 and mediators of its signal transduction, and total loss of JAK1 phosphorylation. Importantly, one of the JAK1 mutations, despite not being detected in the pretreatment tumor, was found at low variant allele frequency in the pretreatment circulating tumor DNA, suggesting clonal selection of the mutation. To our knowledge, this report provides the most detailed look yet at defective JAK1 signaling in the context of dMMR and immunotherapy resistance. Together with observations of JAK1 frameshift indels being enriched in dMMR compared with MMR-proficient tumors, our findings demonstrate the critical function of JAK1 in immunological surveillance of dMMR cancer.
