E2f4 regulates fetal erythropoiesis through the promotion of cellular proliferation

The E2F proteins are major regulators of the transcriptional program required to coordinate cell cycle progression and exit. In particular, E2f4 has been proposed to be the principal family member responsible for the regulation of cell cycle exit chiefly through its transcriptional repressive properties. We have previously shown that UN-/- mice display a marked macrocytic anemia implicating E2f4 in the regulation of erythropoiesis. However, these studies could not distinguish whether E2f4 was required for differentia-tion, survival, or proliferation control. Here, we describe a novel function for E2f4 in the promotion of erythroid proliferation. We show that loss of E2f4 results in an impaired expansion of the fetal erythroid compartment in vivo that is associated with impaired cell cycle progression and decreased erythroid proliferation. Consistent with these observations, cDNA microarray analysis reveals cell cycle control genes as one of the major class of genes down-regulated in EV4-1- FLs, and we provide evidence that E2f4 may directly regulate the transcriptional expression of a number of these genes. We conclude that the macrocytic anemia of EV4(-/-) mice results primarily from impaired cellular proliferation and that the major role of E2f4 in fetal erythropoiesis is to promote cell cycle progression and cellular