A Retinoblastoma Allele That Is Mutated at Its Common E2F Interaction Site Inhibits Cell Proliferation in Gene-Targeted Mice

The retinoblastoma protein (pRB) is best known for regulating cell proliferation through E2F transcription factors. In this report, we investigate the properties of a targeted mutation that disrupts pRB interactions with the transactivation domain of E2Fs. Mice that carry this mutation endogenously (Rb1 ΔG ) are defective for pRB-dependent repression of E2F target genes. Except for an accelerated entry into S phase in response to serum stimulation, cell cycle regulation inRb1 ΔG/ΔG mouse embryonic fibroblasts (MEFs) strongly resembles that of the wild type. In a serum deprivation-induced cell cycle exit,Rb1 ΔG/ΔG MEFs display a magnitude of E2F target gene derepression similar to that ofRb1 −/− cells, even thoughRb1 ΔG/ΔG cells exit the cell cycle normally. Interestingly, cell cycle arrest inRb1 ΔG/ΔG MEFs is responsive to p16 expression and gamma irradiation, indicating that alternate mechanisms can be activated in G1 to arrest proliferation. SomeRb1 ΔG/ΔG mice die neonatally with a muscle degeneration phenotype, while the others live a normal life span with no evidence of spontaneous tumor formation. Most tissues appear histologically normal while being accompanied by derepression of pRB-regulated E2F targets. This suggests that non-E2F-, pRB-dependent pathways may have a more relevant role in proliferative control than previously identified.