Ing4-Deficiency Enhances Hematopoietic Stem Cell Quiescence and Confers Resistance to Inflammatory Stress

In a screen for epigenetic regulators of hematopoiesis in zebrafish, we identified a requirement of the tumor suppressor protein, Ing4, in hematopoietic stem and progenitor cell (HSPC) specification. Though the Ing4 mechanism of action remains poorly characterized, loss of Ing4 has been shown to promote stem cell-like characteristics in malignant cells and it is a frequent target of inactivation in various types of cancer. Mutations in Ing4 cause deregulation of both NF-kB and c-Myc target gene expression. We have also identified a requirement for Ing4 in murine hematopoiesis. Ing4-/- mice have aberrant hematopoiesis and elevated cytokine expression in bone marrow cells. Using RNA-sequencing, we found that Ing4-deficient HSPCs express high levels of c-Myc target genes and genes associated with oxidative phosphorylation and ribosomal biogenesis. Yet, Ing4 deficiency induces G ₀ arrest in HSPCs and they have low levels of reactive oxygen species. This places Ing4-deficient HSPCs in a poised state, where they are quiescent, but express elevated levels of genes associated with differentiation. Under stress hematopoiesis following low-dose irradiation, Ing4-deficient long-term hematopoietic stem cells (LT-HSCs) do not expand, but short-term hematopoietic stem cells (ST-HSCs) function comparably to wild-type. Similarly, under transplantation stress, LT-HSCs fail to contribute to multilineage chimerism, while ST-HSCs contribute at levels equal to wild-type cells. These results are striking, particularly when compared to other models of enhanced NF-kB activity, where HSPCs cannot contribute to multilineage chimerism in transplantation. We sought to target the misregulated pathways in Ing4-deficient HSCs to rescue to effects of Ing4 deficiency. To this end, we chose to target the c-Myc pathway for several reasons: c-Myc target genes are over-represented in our RNA-seq data, c-Myc lies upstream of several of the misregulated pathways observed in Ing4-/- HSCs, and Ing4 has previously been reported to negatively regulate c-Myc activity directly. When treated with the c-Myc inhibitor, 10058-F4, both LT-HSCs and ST-HSCs are pushed into cycling, but this treatment also resulted in fewer cells overall. These results suggest that dampening of the c-Myc pathway can partially rescue Ing4 loss of function. Overall, our findings suggest that Ing4 plays a crucial role in the regulation of hematopoiesis and provides key tools for further identification and characterization of Ing4 pathways and functions. Given the role of Ing4 in both normal hematopoiesis and cancer, this gene likely has a critical role in regulation of stem cell self-renewal and maintenance.