Pathogenic myeloid phenotypes drive disease pathology in a novel human neurohistiocytosis model

Innate immunity is increasingly recognized as a driver of neurodegeneration, though pathogenic mechanisms are incompletely understood. Langerhans cell histiocytosis (LCH) is an inflammatory myeloid neoplastic disorder caused by activating somatic mutations in mitogen-activated protein kinase (MAPK) pathway genes, most commonly BRAFV600E, in myeloid precursors. A subset of LCH patients develop progressive neurodegeneration (LCH-ND). We generated a human induced pluripotent stem cell (iPSC) model from patients with somatic hematologic mosaicism for BRAFV600E. Brain macrophages/microglia from LCH iPSCs exhibit unique disease specific pathogenic features. Stepwise differentiation identified hematopoietic progenitors as hyperproliferative, whereas brain macrophages were apoptosis resistant. Through application of cerebral organoids and a humanized murine xenotransplantation model we identify marked heterogeneity of differentiation potential within clonal BRAFV600E lines in vivo. This model phenocopied human specific phenotypes including dense basal ganglia foci of abnormal macrophages, marked neurodegeneration with astrogliosis, and progressive ataxia. This approach will allow for preclinical testing of therapeutics for LCH-ND.