CUX1 Levels Control Hematopoietic Stem Cell Fate and Plasticity

Hematopoietic stem cells (HSCs) sustain lifelong, multipotent output, however, the molecular mechanisms governing HSC homeostasis remain unclear. Elucidating this process is key to understanding the imbalanced differentiation and self-renewal evident in myeloid malignancies. CUX1 is a transcription factor that is monoallelically mutated or deleted in high-risk myeloid malignancies. We previously reported that CUX1 has critical, dose-dependent roles in hematopoiesis and myeloid neoplasia. The haploinsufficient nature of CUX1 in myeloid neoplasia and the broad dose-dependent requirement of CUX1 in hematopoiesis led us investigate the role of CUX1 in earliest stages of hematopoiesis. We generated a CUX1 reporter mouse in which mCherry is fused in-frame with the C-terminus of CUX1 to study how the level of CUX1 in HSCs affects their behavior. Overall, we find that CUX1 protein levels within the HSC compartment delineates stem cell activity. Stringently defined immunophenotypic HSCs with low CUX1 levels (CUX1^Dim) are phenotypically dormant, as assessed by transcriptional profiling and proliferation assays, as opposed to CUX1^Bright HSCs which have greater cell cycle participation. In transplantation assays, CUX1^Dim HSCs are initially platelet-biased but give rise to long-term, multilineage reconstitution. CUX1^Bright HSCs are initially erythroid-biased but lack long-term HSC activity and the capacity for self-renewal. In other words, despite immunophenotypic HSC markers, CUX1^Bright HSCs are not, by definition, HSCs. By most metrics, CUX1^Intermediate HSCs exhibit a distinct, albeit intermediate, phenotype. Therefore, along a continuous gradient of CUX1 expression, HSCs gradually lose the ability to self-renew as CUX1 expression increases. Remarkably, knockdown of CUX1 in CUX1^Bright HSCs re-endows these cells with the capacity for long-term multilineage chimerism, suggesting low levels of CUX1 are required for stem cell activity in HSCs. Inversely, overexpression of CUX1 in HSCs promotes their expansion and ultimate exhaustion, further demonstrating that the level of CUX1 controls HSC fate. Altogether, we report that low CUX1 levels are required for HSC self-renewal and implicate CUX1 as a molecular rheostat for HSC behaviors such as cell division and differentiation. Further, we describe an unprecedented capacity for cells without demonstrated HSC activity to acquire HSC capacity and a novel role for CUX1 in preventing this plasticity.