HoxA cluster is haploinsufficient for activity of hematopoietic stem and progenitor cells

Objective Functional compensation between hemeodomain proteins has hindered the ability to unravel their role in hematopoiesis using single gene knockouts Because HoxB genes are dispensable for hematopoiesis, and most HoxA genes are expressed an order of magnitude higher than other cluster genes in hematopoietic stem cell (HSC) - enriched populations, we hypothesize that maintenance of HoxA cluster expression is important for adult hematopoiesis and that global decrease of HoxA gene expression levels affects steady state hematopoiesis Materials and Methods Expression levels of HoxA cluster genes have been determined in primitive hematopoietic populations derived from adult mice using quantitative reverse tran scriptase polymerase chain reaction Furthermore, the functional effect of single allelic deletion of the entire HoxA cluster on hematopoietic cells was analyzed by competitive repopulation assays using HoxA(+/-) mice Results We show that the HoxA cluster is predominantly expressed in long term HSCs and that expression declines with progression to short term HSCs and early progenitors in a quantifiable manner Monoallelic deletion of the HoxA cluster caused a general increase in primitive hematopoietic cell populations, but a decrease in side populations In addition exhaustion of B cell progenitors with age was observed, resulting in less mature B cells Moreover, bone marrow of HoxA(+/-) mice had a significant larger population of Mac1/Gr1 neutrophils, which might be caused by accelerated maturation of myeloid progenitors Transplantation assays demonstrated that Hoxa(+/-) HSCs were less competitive in long-term repopulation of myeloablated recipients, which appeared intrinsic to HSCs Conclusion These results show for the first time that maintenance of adult HSCs and progenitors is particularly sensitive to HoxA gene levels, suggesting a specific role for the HoxA cluster in primary regulation of definitive hematopoiesis Crown Copyright (C) 2010 Published by Elsevier Inc on behalf of the ISEH - Society for Hematology and Stem Cells All rights reserved