Evidence That High-Mobility Group A2 (Hmga2) Expression Contributes to the Ontogeny-Dependent Properties of Fetal Hematopoietic Stem Cells.

Abstract 2634

Fetal and early neonatal hematopoietic stem cells (HSCs) are distinct from their adult counterparts by their rapid turnover and expansion rates in vivo. However, the mechanisms underlying the regulation of these properties are poorly understood. In previous studies using serial limiting-dilution competitive repopulating transplant assays, our lab has shown that the rapid expansion phenotype of fetal HSCs is at least partially intrinsically determined since significantly more daughter HSCs are produced from fetal as compared to adult HSCs when similar numbers are transplanted into the same type of irradiated adult host. Additionally, we have observed a conversion of fetal HSCs to the adult regeneration phenotype that occurs within six weeks of transplantation in the primary host. To facilitate a comparison of highly-purified subsets of fetal and adult HSCs identified by an identical phenotype, we adopted the use of the CD45⁺EPCR⁺CD150⁺CD48⁻ (E-SLAM) phenotype which we found gave HSC purities of 20–50% for hematopoietic tissues from early fetal to aged adulthood. We then used comparative gene expression analysis to identify candidate regulators of the fetal HSC high self-renewal program. This gave 20 candidate genes whose transcript levels were measured by quantitative real time PCR in E-SLAM cells isolated from E14.5 fetal liver (FL) and adult bone marrow (ABM). Of these genes only Hmga2 and Smarcc1 showed significant differences (p<.05) in expression between fetal and adult HSCs and only Hmga2 maintained this differential expression when the same cells were stimulated to divide for 48 hrs in vitro. To test the hypothesis that high expression of Hmga2 is a necessary and sufficient factor in determining the fetal HSC self-renewal program, purified adult E-SLAM HSCs were transduced with Hmga2-overexpressing or control lentiviruses and the kinetics of transduced vs untransduced hematopoietic cells in a congenic serial-transplantation model were then analyzed. Interestingly, when BM cells from the primary repopulated mice (transplanted 6-weeks earlier) were injected into secondary animals and the peripheral blood was analyzed for donor-type %Y/GFP chimerism, the Hmga2-overexpressing cells were observed to have a competititve advantage and exhibited an ∼6-fold expansion relative to the untransduced cells. In contrast, the control virus-infected BM cells were found to be equally competitive. These findings support the hypothesis that high expression of Hmga2 may be a critical mediator of the high self-renewal phenotype of fetal HSCs.