Mosaic variants in KRAS and STAT5B associated with a mixed phenotype of 2 acquired errors of immunity

Structured Abstract Background Mosaic genetic variation has been implicated in the pathogenesis of both malignant and non-malignant immunological disease. Objective Here, we investigated a unique case of postnatal acquisition of a gain-of-function (GoF) KRAS variant, with an additional GoF STAT5B variant, in an eighteen year-old woman with inflammatory bowel disease, splenomegaly, thrombocytopenia, bronchiectasis, monocytosis, and eosinophilia. Methods Diagnostic panel, whole genome, and targeted amplicon sequencing were performed on longitudinal blood and tissue samples to reveal germline and mosaic variants, and their allele frequencies across different cell populations. Short and long read single cell RNA sequencing, and flow cytometry were used to measure the effect of variants on RNA, protein and leukocyte cell state. Results Both mosaic variants were present across multiple leukocyte subsets, and historical blood and tissue samples, with the emergence of both variants coinciding with the clinical presentation. Both variants were expressed in a unique population of monocytes, associated with dysregulated cytokine signaling and the presence of a distinct population of highly granular CD24+ leukocytes. Conclusions Taken together with the clinical presentation, these findings led to a diagnosis of combined RAS-associated autoimmune leukoproliferative disorder (RALD) and non-clonal STAT5B GoF disease. To our knowledge, this is the first reported combination of two distinct acquired errors of immunity causing a mixed clinical phenotype, and highlights the importance of considering acquired monogenic diseases within a broader genomic context.