Accelerated DNA replication in E2F1- and E2F2-deficient macrophages leads to induction of the DNA damage response and p21CIP1-dependent senescence

E2F1-3 proteins appear to have distinct roles in progenitor cells and in differentiating cells undergoing cell cycle exit. However, the function of these proteins in paradigms of terminal differentiation that involve continued cell division has not been examined. Using compound E2F1/E2F2-deficient mice, we have examined the effects of E2F1 and E2F2 loss on the differentiation and simultaneous proliferation of bone-marrow-derived cells toward the macrophage lineage. We show that E2F1/E2F2 deficiency results in accelerated DNA replication and cellular division during the initial cell division cycles of bone-marrow-derived cells, arguing that E2F1/E2F2 are required to restrain proliferation of pro-monocyte progenitors during their differentiation into macrophages, without promoting their cell cycle exit. Accelerated proliferation is accompanied by early expression of DNA replication and cell cycle regulators. Remarkably, rapid proliferation of E2F1/E2F2 compound mutant cultures is temporally followed by induction of a DNA damage response and the implementation of a p21(CIP1)-dependent senescence. We further show that differentiating E2F1/E2F2-knockout macrophages do not trigger a DNA damage response pathway in the absence of DNA replication. These findings underscore the relevance of E2F1 and E2F2 as suppressors of hematopoietic progenitor expansion. Our data indicate that their absence in differentiating macrophages initiates a senescence program that results from enforcement of a DNA damage response triggered by DNA hyper-replication.