Twist-1 Increases Hematopoietic Stem Cell Self-Renewal and Causes Myeloid Skewing

Twist-1 protein belongs to the large family of basic Helix-Loop-Helix transcription factors with diverse physiological functions in mesoderm-derived tissues. Growing evidences now link Twist-1 to the acquisition of stem-cell-like properties. However, there is little information available regarding its expression pattern and functional role in the hematopoietic system. We analyzed Twist-1 expression patterns in different hematopoietic cell populations from normal mouse bone marrow, and found that Twist-1 was most highly expressed in long-term hematopoietic stem cells (LT-HSCs) but showed a low abundance in more differentiated descendants. To investigate Twist-1 gene function, retroviral-mediated overexpression or removal experiments were performed. Competitive repopulation studies demonstrated that enforced expression of Twist-1 in HSC-enriched Lin⁻c-Kit⁺Sca-1⁺ (LKS) cells resulted in an increase in the size of the G₀ population, and in their reconstitution ability after the first and a second transplantation. Conversely, removal of Twist-1 in LKS cells impaired their ability to repopulate. In addition, increased Twist-1 expression caused a shift toward production of myeloid cells. Twist-1 transduction in LKS cells activated myeloid lineage-determining factors PU.1 and GATA-1 and down-regulated lymphoid factor GATA-3 in vitro, suggesting that Twist-1-mediated myeloid skewing occurs in hematopoietic stem and progenitor cells (HSPCs). These findings indicate that Twist-1 is not only involved in the maintenance of HSC dormancy and self-renewal capacity but also implicated in the myeloid lineage fate choice of HSPCs. Exploration of the underlying mechanisms revealed that Twist-1 overexpression lead to altered expression of Runx1/c-Mpl/Tie2 regulatory pathway, and quiescence-associated N-cadherin and Hes1 in LKS cells, which may contribute to the phenotypes observed in Twist-1-overexpressing mice. These studies shed additional light on the mechanisms involved in the maintenance of normal HSC and myeloid lineage differentiation, and may also provide clues to the mechanisms controlling pathogenesis and preservation of leukemic stem cells.