Noncoding GATA2 Genetic Variation As a Determinant of Hematopoietic Stem/Progenitor Cell Activity and Mobilization Efficiency

Germline coding and enhancer variants within the hematopoietic regulator GATA2 create a bone marrow failure and leukemia predisposition (Soukup and Bresnick, 2020). We generated a mouse model of GATA2 deficiency syndrome by engineering mice with a single-nucleotide variant in an intronic enhancer (+9.5) from GATA2 deficiency patients combined with deletion of the enhancer from the second allele (compound heterozygous; CH). This system models patients harboring a germline pathogenic allele accompanied by epigenetic silencing of the normal allele. Unlike homozygous Gata2 coding or regulatory deletions that are embryonic lethal, CH represents a unique system to analyze postnatal GATA2 function. We demonstrated that adult germline Gata2 variation alters steady-state HSPCs, blocks HSPC expansion and differentiation upon chemotherapy, attenuates long-term repopulating activity following bone marrow transplantation, leads to bone marrow failure following chronic inflammation by the viral mimetic polyI:C, alters HSPC response to the bacterial cell wall component LPS and attenuates HSPC mobilization in response to granulocyte colony stimulating factor (G-CSF) (Soukup et al., 2019, Soukup et al., 2021).

As short-term G-CSF treatment induces HSPC metabolic gene signatures (Fast et al., 2021) and expansion and cell cycle entry (Schuettpelz et al., 2014), and CH have decreased MPPs (LSKCD150^-CD48^-) and common myeloid progenitors (CMPs) (Lin⁻Sca1⁻Kit⁺FcγR⁻CD34⁺) (Soukup et al., 2021), we asked if mobilization defects reflected decreased functional HSPCs in bone marrow. CH had 2.2-fold fewer colony forming units (CFU) from bone marrow (p = 0.007) than wild type (WT), reflecting a paucity of functional HSPCs. 16 hours following a single G-CSF dose, WT bone marrow immunophenotypic HSCs (LSKCD150⁺CD48^-) and MPPs expanded 1.8-fold (p = 0.001 and 0.01, respectively); CH HSCs and MPPs were unchanged. CH led to decreased HSPCs and failed to induce rapid HSPC expansion post-G-CSF.

We tested whether Gata2 variation impairs mobilization in response to different stimuli, or if select pathways are impacted via disrupting the CXCR4/CXCL12 chemokine axis with the CXCR4 antagonist plerixafor, stromal/neutrophil CXCR2 activation with rhIL-8, or VLA-4/VCAM1 binding with VLA-4 inhibitor BIO5192. IL-8 or BIO5192 resulted in 3.9- and 2.2-fold increased WT CFU, respectively (p < 0.0001 and 0.0014) without altering CH CFU. By contrast, mobilization with plerixafor resulted in a 4.3-fold increase in WT CFU (p < 0.0001) and 4.5-fold increase in CH CFU (p = 0.004). Targeting these pathways collectively with G-CSF was insufficient to restore normal mobilization in CH. Competitive transplantations with peripheral blood revealed G-CSF + plerixafor increased WT donor-derived hematopoiesis 24- and 20-fold at 8 and 12 weeks, respectively, versus vehicle control. Treatment of CH did not increase donor-derived contributions.

Since GATA2 deficiency syndrome and somatic GATA2 variation involve heterozygous alleles, we tested whether monoallelic +9.5 Ets motif variation disrupts mobilization. G-CSF increased CFU 28.5-fold (p < 0.0001) relative to vehicle-treated WT. Mobilization of Gata2 Ets^+/- HSPCs were reduced significantly (p = 0.003). CH variation severely disrupted mobilization, with 7.3-fold fewer CFU than Gata2 Ets^+/- (p = 0.02). Synergistic G-CSF/plerixafor treatment increased CFU 89-fold in WT versus untreated (p < 0.0001), and Gata2 Ets^+/- mobilization was reduced (p = 0.006) and not improved relative to Gata2 Ets^-/- or CH, resembling G-CSF/BIO5192. Thus, Gata2 heterozygosity impaired G-CSF-induced HSPC mobilization with all regimens. Although GATA2 noncoding variation is not commonly evaluated, analysis of human data will be performed to assess relationships between variation and poor mobilization outcomes.

Gata2 variation depletes functional HSPCs in the bone marrow, reducing the available HSPCs to mobilize and rendering most mobilization regimens inefficient. Ongoing studies are elucidating molecular mechanisms underlying the mobilization blockade. Our mobilization-defective system offers unique utility for elucidating fundamental HSPC mechanisms e.g., in the context of additional environmental and/or genetic insults. Future knowledge linking functionality of mobilization regimens in these contexts may unveil insights into mechanisms operational in poor mobilizers.