Restoration of Runx1 Expression in the Tie2 Cell Compartment Rescues Definitive Hematopoietic Stem Cells and Extends Life of Runx1 Knockout Animals Until Birth

Abstract

Mice deficient in the runt homology domain transcription factor Runx1/AML1 fail to generate functional clonogenic hematopoietic cells and die in utero by embryonic day 12.5. We previously generated Runx1 reversible knockout mice, in which the Runx1 locus can be restored by Cremediated recombination. We show here that selective restoration of the Runx1 locus in the Tie2 cell compartment rescues clonogenic hematopoietic progenitors in early Runx1-null embryos and rescues lymphoid and myeloid lineages during fetal development. Furthermore, fetal liver cells isolated from reactivated Runx1 embryos are capable of long-term multilineage lymphomyeloid reconstitution of adult irradiated recipients, demonstrating the rescue of definitive hematopoietic stem cells. However, this rescue of the definitive hematopoietic hierarchy is not sufficient to rescue the viability of animals beyond birth, pointing to an essential role for Runx1 in other vital developmental processes.

Early ontogenic origin of the hematopoietic stem cell lineage

“…The blood-rescued Runx1-null pups were born apparently normal but were unable to feed and died within 1 d postpartum. Reactivation of the same Runx1 LacZ allele in the Tie2 + anatomic compartments led to similar postnatal mortality (21), which may result from abnormal skeletal or neural development (22,23).…”

Embryonic origin of the adult hematopoietic system: advances and questions

“…The mouse umbilical vessels develop Runx1-positive cell clusters similar to those observed in the dorsal aorta (Liakhovitskaia et al, 2009;North et al, 1999), and at E10.5 contain rare dHSC at frequencies comparable with the AGM region at this stage (~1 dHSC per 30 embryos) (de Bruijn et al, 2000b).…”

Signaling axis involving Hedgehog, Notch, and Scl promotes the embryonic endothelial-to-hematopoietic transition

“…Recently, it has been recognized that embryonic hemogenic endothelial intermediates give rise to all adult hematopoietic cells (8,11). The identification of markers of hemogenic endothelium, such as VE-cadherin or Tie2, from imaging and lineage tracing studies has facilitated the study of this phenomenon (5,6), and much of the current focus has been on the role of transcription factor Runx1.…”

“…VE-cadherin is expressed on vascular endothelial cells and is required for vascular lumen formation (9,10). The transcription factor Runx1 is required for the endothelial-to-hematopoietic transition (8,11). However, other transcription factors that can convert such endothelium to hematopoietic cells and the signaling pathways involved in the generation of the hemogenic endothelium remain incompletely described.…”