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R. Bremner, M. Pacal, D. Chen; Mutant Cell Competition, Normal Rods, and E2F1-Driven Differentiation Defects in Rb/p107-Null Retinal Clones. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1929.
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© ARVO (1962-2015); The Authors (2016-present)
There is controversy over retinoblastoma (Rb) family function in retinal development, whether Rb has direct effects on differentiation, and which targets mediate this function. We exploited a sporadic knockout (KO) approach to define cell-autonomous defects in retinal clones lacking Rb and its relative p107, and to identify downstream effectors.
Sporadic KO of the Rb gene was induced by delivering a Cre retrovirus to the subretinal space of newborn floxed Rb mice. Clones were identified by GFP fluorescence. Division and death were assessed by BrdU and TUNEL labeling. Cell types were identified with specific markers.
Newborn retinal transition cells (RTCs) are normally post-mitotic, but RbKO or Rb/p107 double KO (DKO) RTCs proliferated ectopically. Different division and apoptosis rates in mutant clones depleted weaker bipolar RTCs but increased fitter amacrine cells, yet remarkably, mature clones were normal sizes. "Cell competition" selects the fitter of normal versus mutant cells, but in our case competition was mutant-specific. Strikingly, while mutants did not affect normal cells, the latter may support mutant rods, since sporadic mutant rods showed improved survival over blanket mutant rods. Others suggested Rb controls rod differentiation, but all RbKO and most DKO cells, including rods, differentiated normally. Novel defects were discovered in DKO Müller and bipolar cells. Classically, Rb promotes differentiation by potentiating tissue-specific factors, not repressing E2F, but inactivating E2F1 suppressed every defect.
Stem cells or progenitors are favored as the origins of many tumors, but retinoblastoma could begin from an aberrantly dividing differentiating cell. The fittest dividing cells are likely to be the most tumor-prone. Improved survival of sporadic Rb-deficient rods may explain why sporadic RB loss in humans generates photoreceptor-like retinoblastoma. Unexpectedly, E2F1 orchestrates all mutant RTC defects, including aberrant differentiation and the novel phenomenon of mutant cell competition. E2Fs may have been overlooked as Rb differentiation targets in other scenarios.
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