Lnk Controls Hematopoietic Stem Cell Self-Renewal through Direct Interactions with JAK2 and Contributes to Oncogenic JAK2-Induced Myeloproliferative Diseases in Mice

Hematopoietic stem cell (HSC) homeostasis and self-renewal are regulated by intrinsic cytokine signaling pathways. One important signaling axis for HSC is the cell surface receptor, Mpl, and its ligand, thrombopoietin (Tpo). Upon Tpo stimulation, Mpl activates Janus Kinase (JAK2) that triggers a cascade of downstream signal transduction pathways that regulates many aspects of cell development. Under steady-state conditions, mice lacking the inhibitory adaptor protein Lnk harbor an expanded HSC pool with enhanced self-renewal. Surprisingly, we found that Lnk−/− HSCs have an increased quiescent fraction, decelerated cell cycle kinetics, and enhanced resistance to repeated 5-Florouracil (5-FU) treatments in vivo compared to wild type HSCs. We further provided genetic evidence demonstrating that Lnk controls HSC quiescence and self-renewal predominantly through Mpl. Consistent with this observation, Lnk deficiency in HSCs potentiates JAK2 activation in response to TPO. Biochemical experiments reveal that Lnk directly binds to phosphorylated tyrosine residues in JAK2 following TPO stimulation. Dysregulation of cytokine receptor signaling pathways leads to hematological malignancies. Abnormal activation of JAK2 by a chromosomal translocation between the transcription factor Tel and JAK2 (Tel/JAK2) was shown to cause atypical Chronic Myelogenous Leukemia (aCML). Recently, the JAK2 V617F mutation has been observed at high frequencies in several myeloproliferative diseases (MPDs). The JAK2V617F mutant retains Lnk binding ability, suggesting Lnk status could modify MPD development. Indeed, we found that loss of Lnk accelerates oncogenic JAK2- induced CML/MPD in the mouse transplant models. Therefore, we identified Lnk as a physiological negative regulator of JAK2 in stem cells that may contribute to leukemic transformation conferred by oncogenic JAK2.