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

Lawrence HJ, Christensen J, Fong S, Hu YL, Weissman I, Sauvageau G, Humphries RK, Largman C

Division of Hematology and Medical Oncology, Department of Medicine, University of California School of Medicine, Hematology Research (151H), Veterans Affairs (VA) Medical Center, 4150 Clement St, San Francisco, CA 94121, USA.

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.

Blood 2005;106(12):3988-94.

Pubmed ID: 16091451

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