HoxA cluster is haploinsufficient for activity of hematopoietic stem and progenitor cells.
OBJECTIVE
Functional compensation between homeodomain 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.


METHODS
Expression levels of HoxA cluster genes have been determined in primitive hematopoietic populations derived from adult mice using quantitative reverse transcriptase 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.


CONCLUSIONS
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.
Lebert-Ghali
Charles-Etienne
CE
Centre de Recherche de Hôpital Maisonneuve-Rosemont (HMR), HMR, Montréal, Canada.
Fournier
Marilaine
M
Dickson
Glenda J
GJ
Thompson
Alexander
A
Sauvageau
Guy
G
Bijl
Janet J
JJ
D000818:Animals; D001854:Bone Marrow Cells / Q000166:cytology / Q000378:metabolism; D002478:Cells, Cultured; D005260:Female; D005434:Flow Cytometry; D020869:Gene Expression Profiling; D006410:Hematopoiesis / Q000235:genetics; D018380:Hematopoietic Stem Cell Transplantation; D006412:Hematopoietic Stem Cells / Q000378:metabolism*; D018398:Homeodomain Proteins / Q000235:genetics* / Q000378:metabolism; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008815:Mice, Inbred Strains; D018345:Mice, Knockout; D005810:Multigene Family; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013154:Spleen / Q000166:cytology / Q000378:metabolism; D013234:Stem Cells / Q000378:metabolism*; D013950:Thymus Gland / Q000166:cytology / Q000378:metabolism
D018398:Homeodomain Proteins; C088144:HoxA protein