Targeted disruption of the three Rb-related genes leads to loss of G(1) control and immortalization

The retinoblastoma protein, pRB, and the closely related proteins p107 and p130 are important regulators of the mammalian cell cycle. Biochemical and genetic studies have demonstrated overlapping as well as distinct functions for the three proteins in cell cycle control and mouse development. Howeve r, the role of the pRB family as a whole in the regulation of cell prolifer ation, cell death, or cell differentiation is not known. We generated embry onic stem (ES) cells and other cell types mutant for all three genes. Tripl e knock-out mouse embryonic fibroblasts (TKO MEFs) had a shorter cell cycle than wild-type, single, or double knock-out control cells. TKO cells were resistant to G(1) arrest following DNA damage, despite retaining functional p53 activity. They were also insensitive to G(1) arrest signals following contact inhibition or serum starvation. Finally, TKO MEFs did not undergo s enescence in culture and do possess some characteristics of transformed cel ls. Our results confirm the essential role of the Rb family in the control of the G(1)/S transition, place the three Rb family members downstream of m ultiple cell cycle control pathways, and further the link between loss of c ell cycle control and tumorigenesis.