Complex Autoinflammatory Syndrome Unveils Fundamental Principles of JAK1 Transcriptional and Biochemical Function
Autoinflammatory disease can result from monogenic errors of immunity. We describe herein the first example of a patient with early-onset widespread autoinflammation resulting from a mosaic, heterozygous, gain-of-function mutation (S703I) in JAK1, encoding a kinase essential for signaling downstream of over twenty-five cytokines. By first-of-its-kind custom single-cell RNA sequencing, we examine mosaicism with single cell resolution. We uncover that JAK1 transcription is predominantly restricted to a single allele across different immune cells, introducing the concept of a mutational \"transcriptotype\" that differs from the genotype. Functionally, the S703I mutation not only increased JAK1 kinase activity, but also resulted in transactivation of partnering JAKs, independently of its catalytic domain. Further, S703I JAK1 was not solely hypermorphic for cytokine signaling, but neomorphic as well, as it enabled downstream signaling cascades not canonically mediated by JAK1. Given these results, the patient was treated with tofacitinib, a JAK inhibitor, which led to rapid resolution of her clinical disease. Together, these findings represent an unprecedented degree of personalized medicine with the concurrent discovery of fundamental biological principles.