Overexpression of KDM6B, an Epigenetic and Innate Immune Regulator, Results in Hematopoietic Alterations of Mice Including Changes in Hematopoietic Stem Cells

Myelodysplastic syndromes (MDS) and chronic myelomonocytic leukemia (CMML)aremyeloid neoplasms characterized by abnormal bone marrow hematopoiesis and increased risk of transformation to acute myelogenous leukemia (AML). Epigenetic dysregulation and inflammatory hyper-activation have been recognized as key molecular abnormalities in the bone marrow (BM) hematopoietic stem and progenitor cells (HSPC) of MDS and CMML, which implies that key modulators of epigenetic and inflammatory regulation play an important role in the pathophysiology of these diseases, which could also serve as effective therapeutic targets. We recently identified such a candidate molecule: the histone demethylase KDM6B (JMJD3). We demonstrated that KDM6B is significantly overexpressed in the BM HSPCs of patients with MDS and CMML, and the overexpression of KDM6B mediates aberrant epigenetic activation of innate immune/inflammatory signals and consequent differentiation skewing in BM HSPCs of MDS, which can be reversed by targeting KDM6B. Importantly, systematic analysis of the global transcriptomic and genomic data of patients indicates that, although KDM6B is overexpressed in MDS and CMML, genetic lesions in this gene are very rare, and higher KDM6B expression correlates with TET2 mutation. These results imply that constitutive expression of KDM6B potentially interacts with more common genetic lesions during the development of MDS and CMML.

To further investigate the effects of KDM6B overexpression on hematopoiesis and its role in myeloid disorders, we developed a novel hematopoietic KDM6B transgenic (Tg) mouse model that overexpresses KDM6B under the control of the murine hematopoietic specific Vav promoter (Vav-KDM6B). Long-term monitoring of the peripheral blood counts of the mice indicates that, although younger Vav-KDM6B mice display only minor changes in whole white blood cells (WBC), monocytes, and platelets, aged KDM6B mice (>1 year old) have significant increases of WBC (by 22%, p<0.05) and monocyte counts (by 34%, p<0.01). In BM biopsies, the megakaryocytic progenitors of Vav-KDM6B mice specifically possess dysplastic morphology. Analysis of BM HSPCs revealed a tendency of increased numbers of long-term hematopoietic stem cells (LT-HSC) in Vav-KDM6B mice. Because MDS and CMML are associated with a pro-inflammatory BM microenvironment, we applied low-dose lipopolysaccharide (LPS) treatment (6 µg/mouse) for 6 weeks to both Vav-KDM6B and control mice. After chronic immune stimulation, more significant decreases of peripheral red cell (RBC) count and hemoglobin were observed in LPS-treated Vav-KDM6B mice compared to others. In BM biopsies, we noted more dramatic increases of megakaryoblasts in LPS-treated Vav-KDM6B mice than treated wild-type mice. Increases of BM HSPCs were also detected in LPS-treated Vav-KDM6B mice, including LSK cells and LT-HSC populations. Furthermore, the LSK cells isolated from LPS-treated Vav-KDM6B mice consistently demonstrated increased serial plating capacity in methocult-supported colony formation assays. Taken together, these hematopoietic phenotypes observed in the LPS-treated Vav-KDM6B Tg mice indicate that KDM6B overexpression in combination with pro-inflammatory stimulation can accelerate the occurrence of the MDS- and CMML-like abnormalities in BM HSPCs. To gain more insight into the cellular and molecular impacts of KDM6B overexpression on BM HSPCs, detailed analysis, including in vivo repopulating capacity assays as well as gene expression profiling, are being performed.