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 re-port, we investigate the properties of a targeted mutation that disrupts pRB interactions with the transactivation domain of E2Fs. Mice that carry this mutation endogenously (Rb1G) are defective for pRB-dependent repression of E2F target genes. Ex-cept for an accelerated entry into S phase in response to serum stimulation, cell cycle regulation in Rb1G/Gmouse embryonic fibroblasts (MEFs) strongly resembles that of the wild type. In a serum deprivation-induced cell cycle exit, Rb1G/GMEFs dis-play a magnitude of E2F target gene derepression similar to that of Rb1/ cells, even though Rb1G/G cells exit the cell cycle normally. Interestingly, cell cycle arrest in Rb1G/GMEFs is responsive to p16 expression and gamma irradiation, indicating that alternate mechanisms can be activated in G1 to arrest proliferation. Some Rb1 G/Gmice die neonatally with a muscle de-generation 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. The retinoblastoma tumor suppressor protein (pRB) has a cen-tral role in the regulation of the G1-to-S-phase transition. In-activation of its control over cell cycle progression is one of the most common events in cancer (1). The RB protein is thought t