Knock-out ofHopxdisrupts stemness and quiescence of hematopoietic stem cells in mice

HOPX is a stem cell marker in hair follicles and intestines. It was shown critical for primitive hematopoiesis. We previously showed an association between higherHOPXexpression and clinical characteristics related to stemness and quiescence of leukemic cells in acute myeloid leukemia (AML) patients. To further explore its physiologic functions in hematopoietic system, we generated a mouse model with hematopoietic cell-specific knockout ofHopx(Hopx(-/-)). In youngHopx(-/-)mice, the hematopoietic stem cells (HSC) showed decreased reconstitution ability after serial transplantation. Further transcriptomic study revealed decreased HSC signatures in long-term HSCs from theHopx(-/-)mice. At 18 months of age, half of theHopx(-/-)mice developed cytopenia and splenomegaly. Bone marrow (BM) from the sick mice showed myeloid hyperplasia with predominant mature neutrophils, and decreased progenitor cells and lymphocytes. These phenotypes suggested critical functions of Hopx in maintaining HSC quiescence. Transcriptomic study of theHopx(-/-)marrow cells showed significant downregulation of theCxcl12-Cxcr4axis, which is critical for maintenance of HSC quiescence. We next examined the role of Hopx in AML by using theMN1overexpression murine leukemia model. Mice transplanted withMN1-overexpressedHopx(-/-)BM cells developed AML with more aggressive phenotypes compared with those transplanted withMN1-overexpressedHopx-wild cells.Hopx(-/-)MN1-overexpressed leukemia cells showed higher proliferation rate and downregulation ofCxcl12andCxcr4. Furthermore, in human AML, BM plasma CXCL12 levels were lower in patients with lowerHOPXexpression. In conclusion, our study highlights the roles of Hopx in maintenance of quiescence of the hematopoietic stem cells through CXCL12 pathway in vivo and provides implication of this protein in normal and malignant hematopoiesis.