Defective stem cell factor expression in c-myb null fetal liver stroma

High levels of c-Myb are observed in immature precursor myeloid and lymphoi d cells, while downregulation of c-myb accompanies terminal differentiation to a mature phenotype, This has established c-Myb as a crucial transcripti on factor for hematopoiesis, Further evidence for this is the embryonic dea th of the c-myb homozygous mutant mouse at ED15 due to defective fetal live r erythropoiesis, Cells from fetal liver of wild-type and c-myb-/- embryos were examined in detail for their hematopoietic potential and the capacity of the stroma to support wild-type hematopoiesis. The c-myb-/- fetal liver was shown to harbor sevenfold fewer spleen focus-forming cells and a simila rly lower number of cells with long-term repopulating capacity (high prolif erative potential cells), However, shorter term repopulating cells were not substantially reduced, c-myb-/- stromal cells were unable to support the p roliferation of wild-type bone marrow lineage-negative cells. This was foun d to be partly due to a decrease in stem cell factor (SCF) expression while partial rescue of the stromal cell cultures was achieved through the addit ion of exogenous SCF. DNA binding studies for two sites within the SCF prom oter demonstrated an in vitro interaction between the SCF promoter and c-My b and transient transfection studies demonstrated that c-Myb could substant ially transactivate the SCF promoter in HEK293 cells. These data explain wh y the c-myb-/- embryos are so impaired in their ability to establish hemato poiesis. (C) 2001 Academic Press.