Single-cell transcriptome analysis profiles the enlarged subsets of myeloid-biased HSPCs with preleukemic characters in disuse osteoporosis mice.

BACKGROUND: Osteoporosis (OP) could lead to the alteration of bone marrow microenvironment and non-homeostasis of hematopoiesis, which could increase the incidence of hematologic malignancies. However, whether myeloid-biased hematopoiesis occurred and contributed to the leukemogenesis under the condition of OP remains unclear. RESULTS: This study successfully induced a mouse model for OP by hindlimb unloading, which shows increased myeloid cells and decreased B cells in the peripheral blood (PB). Furthermore, our study demonstrates that the myeloid-biased subset of HSPCs (hematopoietic stem and progenitor cells) with reduced differentiation and apoptosis, including multipotent progenitor 3 (MPP3) and granulocyte-monocyte progenitors (GMPs), were expanded in the OP mice. The expansion of myeloid-biased HSPCs contributes to the accumulation of HSPCs in the bone marrow and increased myeloid cells in the PB of OP mice. In the expanded pool of HSPCs, OP mice specifically enriched subsets were identified and profiled by single cell RNA-seq, including subHSCs from primitive HSCs, MPP3-1 from MPP3, GMP5 from GMPs, MkP2 from megakaryocyte progenitors and EryP1 from erythrocyte progenitors. Meanwhile, those OP-HU mice enriched subsets shared significantly up- and down-regulated genes enriched in chromatin modification and cell differentiation and apoptosis such as Bromodomain-containing protein 4 (Brd4), encoding an important chromatin remodeling protein, and Proteinase 3 (Prtn3). Moreover, the specific transcription factors corresponding to the expansion of subHSCs, MPP3-1, GMP5 and EryP1 in OP-HU mice were identified as Zfp951, Nfic, Maz and Ezh2. Finally, inhibition of BRD4 in vivo could partially restore the phenotype of OP-HU mice and the expression of genes regulating HSPC expansion, differentiation and apoptosis. CONCLUSIONS: First of all, our study shows that OP could induce the unbalanced hematopoiesis and enhances the myeloid-biased hematopoiesis. Secondly, OP mice enriched subsets of HSPCs were identified and characterized with enhanced chromatin remodeling, reduced differentiation and resistance to apoptosis. Finally, this study demonstrate that Brd4 regulated gene programs endow the myeloid-biased subsets of HSPCs with tumor cell-like characters in OP mice, which may increase the incidence of the leukemic evolution. This study sheds light on the importance for the prevention of myeloid leukemogenesis in human with OP.