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Dynamics of Genomic and Immune Responses During Primary Immunotherapy Resistance in Mismatch Repair Deficient Tumors

We study a patient with dMMR lung cancer refractory to immunotherapy. Post-treatment tumor demonstrated compound heterozygous frameshift deletions located upstream of the kinase domain in the gene encoding JAK1 protein, downregulation 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 pre-treatment tumor, was found at low variant allele frequency in the pre-treatment circulating tumor DNA, suggesting clonal selection of the mutation. This report provides the most detailed look yet of 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.