Ectopic Runx1 Expression Rescues Tal-1-Deficiency in the Generation of Primitive and Definitive Hematopoiesis

The transcription factors SCL/Tal-1 and AML1/Runx1 control the generation of pluripotent hematopoietic stem cells (pHSC) and, thereby, primitive and definitive hematopoiesis, during embryonic development of the mouse from mesoderm. Thus, Runx1-deficient mice generate primitive, but not definitive hematopoiesis, while Tal-1-deficient mice are completely defective. Primitive as well as definitive hematopoiesis can be developed ‘‘in vitro’ ’ from embryonic stem cells (ESC). We show that wild type, as well as Tal-12/2 and Runx12/2 ESCs, induced to differentiation, all expand within 5 days to comparable numbers of Flk1+ mesodermal cells. While wild type ESCs further differentiate to primitive and definitive erythrocytes, to c-fms+Gr1+Mac1+ myeloid cells, and to B220+CD19+ B- and CD4+/CD8+ T-lymphoid cells, Runx12/2 ESCs, as expected, only develop primitive erythrocytes, and Tal-12/2 ESCs do not generate any hematopoietic cells. Retroviral transduction with Runx1 of Runx12/2 ESCs, differentiated for 4 days to mesoderm, rescues definitive erythropoiesis