E2F1 mediates ectopic proliferation and stage-specific p53-dependent apoptosis but not aberrant differentiation in the ocular lens of Rb deficient fetuses.

The retinoblastoma tumor suppressor, Rb, is a transcription cofactor that controls cell proliferation, survival and differentiation. Mutant mouse embryos lacking Rb exhibit ectopic proliferation and apoptosis that are mediated in some tissues by E2F1, a major partner of Rb, and by the p53 tumor suppressor. Whether E2F1 and p53 also mediate the differentiation defects in Rb mutant embryos is, however, not clear. Here we show that partially rescued mgRb:Rb-/- mutant fetuses exhibit ectopic lens epithelial cell proliferation, apoptosis and severe cataract. The abnormal cell proliferation and apoptosis were significantly suppressed in the lens of compound mutant fetuses lacking both Rb and E2F1 at embryonic day (E) E15.5. Interestingly however, at E18.5, only ectopic proliferation, not apoptosis, was dramatically reduced in mgRb:Rb-/-:E2F1-/- lenses. In contrast, p53 did not exert such a stage-specific effect and apoptosis was invariably suppressed in mgRb:Rb-/-:p53-/- composite mutant lenses throughout embryogenesis. Using RT-PCR and in situ hybridization analyses, we identified a subset of lens specific genes, most notably the late differentiation marker filensin, which were not properly induced during lens development in mgRb:Rb-/-fetuses. Remarkably, despite the inhibition of cell proliferation and apoptosis, the degeneration of lens fibers and aberrant expression of filensin were only marginally corrected in mgRb:Rb-/-:E2F1-/- fetuses at E15.5 but not at all at E18.5 or in mgRb:Rb-/-:p53-/mutant fetuses. Thus, inactivation of E2F1 reduces ectopic cell proliferation and stage-specific p53-dependent apoptosis but does not rescue the differentiation defects associated with loss of Rb during lens development.