Control of Hematopoietic Stem Cell Function through Epigenetic Regulation of Energy Metabolism and Genome Integrity

Fate determination of hematopoietic stem cells (HSCs), including their maintenance and differentiation, is profoundly influenced by their metabolic state. How HSCs control their metabolism to ensure correct decision making of cell fate remains an outstanding question. Metabolism is regulated by expression and activities of many rate-limiting metabolic enzymes. Histone modifications shape many aspects of DNA-based processes including transcription and DNA damage responses (DDR). H3K4 methylation is best known for its intimate association with active transcription, and is also implicated in DDR, but its role in DDR for stem cell function is unclear. The Set1/Mll complexes comprise one of six different catalytic subunits and several shared core subunits including Dpy30. We have previously shown that Dpy30 directly facilitates genome-wide H3K4 methylation (Jiang et al., Cell 2011), and that Dpy30 knockout (KO) in mouse hematopoietic system disables differentiation and long-term maintenance HSCs (Yang et al., J Exp Med, 2016). While we have identified dysregulation of multiple genes known to be important for HSC maintenance and differentiation, it is unclear what pathways functionally mediate Dpy30's role in HSC fate determination. Our analyses revealed dysregulation of many metabolic genes upon Dpy30 loss in HSCs, prompting us to examine if and how metabolism is affected by Dpy30 loss in HSCs. We found that Dpy30 loss resulted in increased AMPK activation, suggesting a low cellular energy state. Dpy30 loss resulted in significantly decreased mitochondrial membrane potential, while mitochondrial mass was insignificantly reduced, suggesting impaired mitochondrial function in energy production upon Dpy30 loss. Moreover, Dpy30 loss resulted in significant decrease in oxygen consumption in lineage-negative hematopoietic cells. In further support of diminished oxidative phosphorylation, we also found that reactive oxygen species (ROS) was significantly reduced in all hematopoietic lineage cells upon Dpy30 loss. Consistent with the reduced energy production, glucose uptake was found to be significantly reduced in Dpy30 deficient HSCs. Interestingly, we found that the Dpy30 KO HSCs were more quiescent than control HSCs. As HSCs are usually kept quiescent and they increase oxidative phosphorylation and energy production upon activation, our results suggest that Dpy30 plays important role in enabling HSC activation by metabolic reprogramming. In addition to dysregulated energy metabolism, we also found significant increase of γ-H2AX in the Dpy30 KO lineage negative bone marrow cells, suggesting increase in DDR. As the major source of DNA damage, ROS, is decreased in Dpy30 KO HSCs, we examined if the DNA damage repair was affected and thus led to sustained DDR upon Dpy30 loss. We found that Dpy30 KO cells resolved irradiation-induced γ-H2AX foci with significantly lower efficiency, suggesting that Dpy30 and its associated H3K4 methylation is important for efficient DNA damage repair. Importantly, inhibition of DDR by ATM inhibitor partially rescued the colony formation capacity of the Dpy30 KO cells, suggesting that sustained DDR functionally mediates stem cell activities. As we also saw dramatic upregulation of CDK inhibitor p21 upon Dpy30 loss, we reasoned that increased DDR may affect stem cell activity via p21. To test this hypothesis, we have been breeding to get p21 and Dpy30 double KO mice, and will soon (within a month or so) be able to test if loss of p21 can partially rescue the functional defect of Dpy30 KO stem cells, which will demonstrate an important role of CDK inhibitors in stem cell function. Taken together, our results demonstrate that a key chromatin modulator exerts a profound control of stem cell fate determination through regulating energy metabolism and genome integrity. The functional relationship between metabolic dysregulation, DDR, and stem cell function warrants further studies. Moreover, as we previously showed a critical role of Dpy30 in leukemogenesis and Myc-driven lymphomagenesis, it will be of great interest to investigate whether and how loss or inhibition of this key epigenetic modulator affects cellular metabolism and genome integrity as part of cancer-inhibitory mechanisms. Disclosures No relevant conflicts of interest to declare.