Pathologic TPO-induced TPOR-CSF2RB signaling mediates Down syndrome abnormal myelopoiesis due to GATA1 mutant-dependent CSF2RB overexpression

Transient abnormal myelopoiesis (TAM) is a human myeloid leukemia that occurs in newborns with Trisomy 21, who acquire mutations in the transcription factor GATA1. TAM originates in the fetal liver but spontaneously regresses as hemopoiesis transitions from fetal liver to postnatal bone marrow, unless clonal evolution enables transformation of the TAM clone to Myeloid Leukemia of Down Syndrome. We sought to understand the developmental tissue restriction of TAM. We show that TAM cells are marked by high expression of the cytokine receptor CSF2RB, a direct target of GATA2. Oncogenic GATA1 fails to supress CSF2RB expression. Highly expressed CSF2RB directly interacts with the thrombopoietin receptor (TPOR). Specifically at high TPO levels, activation of JAK-STAT signalling by TPOR-CSF2RB complexes is amplified and distinct from that triggered by TPOR homodimers and drives TAM cell proliferation. CSF2RB-A455D, a previously identified pathogenic gain-of function variant seen in ML-DS, requires interaction with TPOR to induce cytokine-independent cell proliferation. Together, TPOR and CSF2RB jointly exploit TPO produced by the fetal liver to sustain developmentally restricted TAM cell proliferation