Reduced Proliferative Capacity of Hematopoietic Stem Cells Deficient in Hoxb3 and Hoxb4

Abstract

Several homeobox transcription factors, such as HOXB3 and HOXB4, have been implicated in regulation of hematopoiesis. In support of this, studies show that overexpression of HOXB4 strongly enhances hematopoietic stem cell regeneration. Here we find that mice deficient in both Hoxb3 and Hoxb4 have defects in endogenous hematopoiesis with reduced cellularity in hematopoietic organs and diminished number of hematopoietic progenitors without perturbing lineage commitment. Analysis of embryonic day 14.5 fetal livers revealed a significant reduction in the hematopoietic stem cell pool, suggesting that the reduction in cellularity observed postnatally is due to insufficient expansion during fetal development. Primitive Lin⁻ ScaI⁺ c-kit⁺ hematopoietic progenitors lacking Hoxb3 and Hoxb4 displayed impaired proliferative capacity in vitro. Similarly, in vivo repopulating studies of Hoxb3/Hoxb4-deficient hematopoietic cells resulted in lower repopulating capability compared to normal littermates. Since no defects in homing were observed, these results suggest a slower regeneration of mutant HSC. Furthermore, treatment with cytostatic drugs demonstrated slower cell cycle kinetics of hematopoietic stem cells deficient in Hoxb3 and Hoxb4, resulting in increased tolerance to antimitotic drugs. Collectively, these data suggest a direct physiological role of Hoxb4 and Hoxb3 in regulating stem cell regeneration and that these genes are required for maximal proliferative response.

ACKNOWLEDGMENTS

These studies were generously supported by grants from Cancerfonden, Sweden, and Barncancerfonden, Sweden; Astra Draco AB (now AstraZeneca); and Donation Funds, Lund University Hospital, to S.K.; and by a clinical research award (ALF) from Lund University Hospital to S.K. and N.L. J.M.B. was supported by a Graduate Student Award from the Medical Faculty, Lund University.

We thank C. Largman for providing the Hoxb4 probe, H. Gu for the pl2neo plasmid, R. Jaenish for the pNT, and R. Fässler for providing the RI ES cells. We also thank Per Levéen and Mary-Ann Sällstrüm for help with blastocyst injections, Kristina Sundgren and Eva Gynnstam for expert animal care, Lilian Wittman for help with animal experiments, and Sten Eirik Jacobsen and members of The Department of Stem Cell Biology, Lund University, for helpful advice and discussions.

J.M.B. and N.L. contributed equally to this work.