Loss-of-function of the nucleoside transporter SLC29A3 drives ERK activation, establishing an alternative pathway for histiocytosis development

Histiocytoses are inflammatory myeloid neoplasms, driven by somatic activating mutations in mitogen-activated protein kinase (MAPK) genes. H syndrome is a genetic disorder caused by germline loss-of-function mutations in SLC29A3, encoding the lysosomal equilibrative nucleoside transporter 3 (ENT3). Patients with H syndrome are predisposed to develop histiocytosis, yet the mechanism is unclear. Here, we demonstrate that loss-of-function of ENT3 leads to MAPK signaling activation, via the nucleoside sensor Toll-like receptor (TLR). We show that H syndrome monocytes exhibit an inflammatory expression pattern, display elevated levels of phospho-ERK, and secrete enhanced amounts of inflammatory cytokines in a TLR8/MEK-dependent manner. Finally, MEK inhibitor therapy successfully eliminated a histiocytic tumor of a patient with H syndrome. These results demonstrate a yet-unrecognized link between a defect in a lysosomal transporter and pathological activation of MAPK signaling, a novel alternative pathway to histiocytosis development, and establish MEK inhibition as a promising new treatment for H syndrome. Overall design: PBMCs were isolated from fresh whole blood and then monocytes were enriched using a pan-monocyte negative selection method. Cells were analyzed using 10X scRNAseq.