DUSP1 is a Key Driver of Disease Persistence and Potential Therapeutic Target in Hairy Cell Leukemia

Classic hairy cell leukemia (HCL) is a rare indolent B cell lymphoproliferative disorder characterized by the driver mutation BRAF V600E. Standard treatment with purine analogs (i.e. cladribine) induces long-term remissions, but up to 25% of patients relapse early. Even when targeting BRAF V600E, residual HCL cells frequently persist in the bone marrow (BM). To unravel additional biologic alterations contributing to HCL disease persistence, we performed single-cell RNA sequencing in sorted primary HCL cells from long-term versus short-term cladribine responders (LT-R versus ST-R: >10 versus ≤3 years PFS) at diagnosis and in ST-R at diagnosis versus relapses. We identified a distinct HCL subcluster characterized by elevated DUSP1, FOS, and JUND expression, which was detected in all patients and persisted or even expanded at relapses. Cancer pathway analysis suggested enhanced tumor microenvironment dependency as reflected by suppression of the p38-MAP kinase pathway. In the absence of a suitable BRAF V600E-mutated HCL cell line, we validated HAIR-M cells (BRAF D594E) as a bona fide experimental HCL model. The activating BRAF D594E mutation mimics V600E-induced downstream signals that can efficiently be targeted by BRAF inhibitors (BRAFi). HAIR-M co-culture with BM stromal cells (BMSC) strongly induced DUSP1 that was accompanied by protection from BRAFi-induced HAIR-M apoptosis. The functional importance of DUSP1 was corroborated by demonstrating that BMSC-induced protection from cell death could be overcome through DUSP1 inhibition. Our results might set the stage for future clinical testing of DUSP1 inhibition to eliminate minimal residual disease (MRD) and prevent disease relapse in HCL.