Restoration of Runx1 expression in the Tie2 cell compartment rescues definitive hematopoietic stem cells and extends life of Runx1 knockout animals until birth.

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 Cre-mediated 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.