Moyer, D.L.

Homeobox (HOX) transcription factors are encoded within highly organized loci expressed along an anterior–posterior axis through embryogenesis and in a pleiotropic manner in hematopoiesis. HOX expression has been exhaustively studied in the context of oncogenesis and malignancy, but the compensatory substitution of HOX paralogs makes mechanistic annotation in steady-state hematopoiesis challenging. Despite this, HOX genes reflect numerous non-redundant roles in healthy hematopoiesis including HSC self-renewal, development, lymphopoiesis, myelopoiesis, and erythropoiesis. Here, we review historical and current insights into HOX functions in steady-state hematopoiesis and highlight unexplored avenues in their biology that could further elucidate their significance to hematopoietic homeostasis.

Abstract Description Hematopoiesis declines with age due to intrinsic and extrinsic dysregulation resulting in diminished lymphopoiesis. Hoxa9 is a pleiotropic hematopoietic regulator necessary for stem/progenitor cell (HSPC) maintenance and lymphoid differentiation. Interestingly, bone marrow (BM) HSPC and B lineage compartments of young hoxa9-/- mice mimic that which accompanies aging. Hoxa9 is a negative regulator of p16ink4a, a cyclin-dependent kinase inhibitor upregulated in hoxa9-/- HSPCs and a gatekeeper of the G1 to S cell cycle checkpoint. We hypothesized that increased expression of p16ink4a contributes to the HSPC and/or B progenitor alterations due to hoxa9-deficiency. Interestingly, BM B lymphopoiesis was normalized in hoxa9-/-p16ink4a-/- (DKO) mice, but alterations were less obvious in HSPCs. Flow analysis of the HSPC compartment proliferation showed contraction of unbiased but not myeloid-biased progenitors prior to lineage fate commitment. hoxa9-/- CFU-PreB were blunted then rescued by p16ink4a co-ablation. Primary BM transplant demonstrates DKO like hoxa9-/- has reduced reconstitution efficiency but differs with superior B lineage reconstitution similar to controls. Bioinformatic analysis of recent open access CUT&RUN data in HSPCs confirms Hoxa9 binds promoters in genes relevant to G1 to S cell cycle checkpoint, these findings currently being validated. Our work emphasizes HSPC regulatory factors as crucial targets in combating age-associated maladies of the immune system. Funding Sources Supported by NIH R01AG076163 Topic Categories Hematopoiesis and Immune System Development (HEM)