Gtpase Immunity-Associated Protein Family Member 4 Contributes to the Enhanced Expansion of HSPCs in RUNX1 Deficient Mice

RUNX1 is the transcription factor described as the master regulator of hematopoiesis. Due to its central role during blood development, numerous RUNX1 mutations have been reported in hematologic abnormalities. Mice null for Runx1 die during embryogenesis, lacking definitive HSCs. Conditional Runx1^Δ/Δ mice are viable, but exhibit a variety of blood abnormalities. The most salient defect in these Runx1^Δ/Δ mice is expansion of the hematopoietic stem and progenitor cell (HSPC) population, measured as an increase in number of lineage negative, Sca1 positive, cKit positive (LSK) cells. A shortened form of RUNX1 (RUNX1SF) lacking the C-terminal and part of the N-terminal domain (41-214) acts as a dominant negative regulator of RUNX1 and hence also models RUNX1 loss-of-function.

A differential gene expression analysis of HSPCs derived from Runx1^Δ/Δ compared to wild type mice uncovered GTPase immunity-associated protein family member 4 (GIMAP4) as one of the genes most highly upregulated. Previous studies have focused almost exclusively on the role of GIMAP4 as a pro-apoptotic protein during T-cell development. This study illuminates a novel non-apoptotic role of GIMAP4 in a formerly unstudied HSPC context. Runx1^Δ/Δ mice were crossed with Gimap4^-/- mice to generate a double knockout (dKO) mouse line. These dKO mice exhibited attenuated HSPC proliferation in comparison to Runx1^Δ/Δ mice, suggesting that GIMAP4 functions in this HSPC expansion phenotype. BMT experiments using lethally irradiated C57 mice and RUNX1SF transduced wild type versus Gimap4^-/-bone marrow confirmed this result.

GIMAP4 also worked independently and coordinately with RUNX1 to influence individual progenitor populations. Common lymphoid progenitors (CLP) were affected only by GIMAP4. Gimap4^-/- mice exhibited an expansion of the CLP population, consistent with its pro-apoptotic role in lymphoid populations. Conversely, both RUNX1 and GIMAP4 coordinately exerted an effect on myeloid progenitor populations. Runx1^Δ/Δ mice harbored expanded granulocyte-macrophage progenitor (GMP) and common myeloid progenitor (CMP) populations. This expansion was not observed when GIMAP4 was also ablated. This suggests a pro-proliferative role of GIMAP4 specifically in myeloid populations. These opposing roles of GIMAP4 in lymphoid versus myeloid cells suggest a more contextual, cell-specific role of this GTPase protein. Ultimately, this study provides insight into how RUNX1 and GIMAP4 may coordinate to maintain HSPC homeostasis.