Loss of expression of the Hoxa-9 homeobox gene impairs the proliferation and repopulating ability of hematopoietic stem cells

HJ Lawrence, J Christensen, S Fong, YL Hu… - Blood, 2005 - ashpublications.org
HJ Lawrence, J Christensen, S Fong, YL Hu, I Weissman, G Sauvageau, RK Humphries…
Blood, 2005ashpublications.org
The homeobox gene Hoxa-9 is normally expressed in primitive bone marrow cells, and
overexpression of Hoxa-9 markedly expands hematopoietic stem cells, suggesting a
function in early hematopoiesis. We present evidence for major functional defects in Hoxa-9-
/-hematopoietic stem cells. Hoxa-9-/-marrow cells have normal numbers of
immunophenotypic stem cells (Lin-c-kit+ flk-2-Sca-1+[KLFS] cells). However, sublethally
irradiated Hoxa-9-/-mice develop persistent pancytopenia, indicating unusual sensitivity to …
The homeobox gene Hoxa-9 is normally expressed in primitive bone marrow cells, and overexpression of Hoxa-9 markedly expands hematopoietic stem cells, suggesting a function in early hematopoiesis. We present evidence for major functional defects in Hoxa-9-/- hematopoietic stem cells. Hoxa-9-/- marrow cells have normal numbers of immunophenotypic stem cells (Lin-c-kit+flk-2-Sca-1+ [KLFS] cells). However, sublethally irradiated Hoxa-9-/- mice develop persistent pancytopenia, indicating unusual sensitivity to ionizing irradiation. In competitive transplantation assays, Hoxa-9-/- cells showed an 8-fold reduction in multilineage long-term repopulating ability, a defect not seen in marrow cells deficient for the adjacent Hoxa-10 gene. Single-cell cultures of KLFS cells showed a 4-fold reduction in large high-proliferation potential colonies. In liquid cultures, Hoxa-9-deficient Lin-Sca-1+ cells showed slowed proliferation (a 5-fold reduction in cell numbers at day 8) and delayed emergence of committed progenitors (a 5-fold decrease in colony-forming cells). Slowing of proliferation was accompanied by a delay in myeloid maturation, with a decrease in Gr-1hiMac-1hi cells at the end of the culture. Retroviral transduction with a Hoxa-9 expression vector dramatically enhanced the cytokine-driven proliferation and in vivo engraftment of Hoxa-9-/- marrow cells. Hoxa-9 appears to be specifically required for normal hematopoietic stem cell function both in vitro and in vivo.
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