1006 - KEY ROLES OF HOXA1 IN HSCS AND MYELODYSPLASTIC SYNDROMES

Abstract

Homeobox a1 (Hoxa1) has 2 different isoforms: a full-length homeodomain-containing Hoxa1 (Hoxa1-FL) and a truncated Hoxa1 (Hoxa1-T), which is spliced within exon 1 of Hoxa1-FL and lacks the homeodomain. Hoxa1-FL and Hoxa1-T are differentially expressed in distinct populations of hematopoietic cells. To examine their roles in hematopoiesis, we retrovirally overexpressed WT-Hoxa1 (which generates both isoforms) or Hoxa1-T in murine bone marrow (BM). We also generated a mutant Hoxa1 (MUT-Hoxa1), which expresses Hoxa1-FL but not Hoxa1-T, by site-directed mutagenesis. We transplanted mice with BM overexpressing MUT-Hoxa1, WT-Hoxa1, Hoxa1-T or empty vector control. The recipients of Hoxa1-T BM showed poor peripheral blood (PB) donor cell reconstitution, accompanied by enhanced differentiation of their HSCs. In contrast, recipients of WT-Hoxa1 or MUT-Hoxa1 BM developed features resembling myelodysplastic syndromes (MDS), including PB macrocytic anemia and thrombocytopenia, dysplastic features in BM erythrocytes and megakaryocytes and increased apoptosis in BM erythrocytes. Recipients of MUT-Hoxa1 BM developed a more severe MDS phenotype that spontaneously progressed to secondary acute myeloid leukemia (sAML). HOXA1-FL expression was significantly increased in CD34+ cells from 50% of MDS patients. Similar to human MDS, DNA damage repair pathways were downregulated in WT-Hoxa1 and MUT-Hoxa1 BM cells. We generated conditional knock-in (KI) mice and crossed them to inducible hematopoietic-specific Cre mice to express the KI in a heterozygous (KI/+) or homozygous (KI/KI) manner. Recipients of the KI BM developed MDS, with KI/KI mice developing a more severe MDS phenotype. In summary, Hoxa1 has key roles in regulating HSCs. Deregulated expression of Hoxa1 in BM caused MDS, with Hoxa1-FL dose influencing the MDS severity. Our mouse models are clinically tractable for MDS research.