Developmentally regulated promoter-switch transcriptionally controls Runx1 function during embryonic hematopoiesis.
BACKGROUND
Alternative promoters usage is an important paradigm in transcriptional control of mammalian gene expression. However, despite the growing interest in alternative promoters and their role in genome diversification, very little is known about how and on what occasions those promoters are differentially regulated. Runx1 transcription factor is a key regulator of early hematopoiesis and a frequent target of chromosomal translocations in acute leukemias. Mice deficient in Runx1 lack definitive hematopoiesis and die in mid-gestation. Expression of Runx1 is regulated by two functionally distinct promoters designated P1 and P2. Differential usage of these two promoters creates diversity in distribution and protein-coding potential of the mRNA transcripts. While the alternative usage of P1 and P2 likely plays an important role in Runx1 biology, very little is known about the function of the P1/P2 switch in mediating tissue and stage specific expression of Runx1 during development.


RESULTS
We employed mice bearing a hypomorphic Runx1 allele, with a largely diminished P2 activity, to investigate the biological role of alternative P1/P2 usage. Mice homozygous for the hypomorphic allele developed to term, but died within a few days after birth. During embryogenesis the P1/P2 activity is spatially and temporally modulated. P2 activity is required in early hematopoiesis and when attenuated, development of liver hematopoietic progenitor cells (HPC) was impaired. Early thymus development and thymopoiesis were also abrogated as reflected by thymic hypocellularity and loss of corticomedullary demarcation. Differentiation of CD4/CD8 thymocytes was impaired and their apoptosis was enhanced due to altered expression of T-cell receptors.


CONCLUSIONS
The data delineate the activity of P1 and P2 in embryogenesis and describe previously unknown functions of Runx1. The findings show unequivocally that the role of P1/P2 during development is non redundant and underscore the significance of alternative promoter usage in Runx1 biology.
Pozner
Amir
A
Department of Molecular Genetics, The Weizmann Institute of Science, Rehovot, Israel. apozner@genetics.utah.edu <apozner@genetics.utah.edu>
Lotem
Joseph
J
Xiao
Cuiying
C
Goldenberg
Dalia
D
Brenner
Ori
O
Negreanu
Varda
V
Levanon
Ditsa
D
Groner
Yoram
Y
D000483:Alleles; D000818:Animals; D017209:Apoptosis; D002454:Cell Differentiation; D003114:Colony-Forming Units Assay; D050676:Core Binding Factor Alpha 2 Subunit / Q000235:genetics*; D017931:DNA Primers; D004622:Embryo, Mammalian; D053595:Embryonic Stem Cells / Q000166:cytology; D005434:Flow Cytometry; D018507:Gene Expression Regulation, Developmental; D005804:Genes, Lethal; D006411:Hematopoiesis, Extramedullary / Q000235:genetics*; D006412:Hematopoietic Stem Cells / Q000166:cytology; D017403:In Situ Hybridization; D051379:Mice; D018345:Mice, Knockout; D010641:Phenotype; D011401:Promoter Regions, Genetic; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013950:Thymus Gland / Q000166:cytology / Q000196:embryology*
D050676:Core Binding Factor Alpha 2 Subunit; D017931:DNA Primers; C493726:Runx1 protein, mouse