Abstract
Abstract: :
Purpose: Despite the fundamental role of the Rb gene in retinoblastoma, the effect of Rb loss in different retinal cell types has never been addressed. Rb–deficient mice die at embryonic day (E) 13–15, thus to study retinal–specific effects we generated a conditional knockout. Methods: We crossed mice that have a floxed Rb allele with another strain carrying a Cre recombinase transgene under the control of the Pax6 α–enhancer (α–Cre), which is active in peripheral retinal progenitors at E10. Rb deletion in progenitors mimics human retinoblastoma where, by definition, lesions must occur in dividing cells. These mice were also crossed with p107–deficient animals. p107 is an RB–related protein that blocks retinoblastoma in mice. Finally, because RB and its relatives mediate their effects through E2F proteins, we determined whether defects observed in RB– or RB/p107–deficient mice could be rescued by inactivating individual E2Fs. Results: Rb loss did not affect division in progenitors, but caused ectopic division in differentiating precursors. Unexpectedly, we observed highly cell–specific downstream effects. Apoptosis eliminated bipolar, ganglion and many rod neurons. Combined with p107 loss, all rods and cones were also eliminated. Previously, cell death was believed to be the response of all retinal cells to RB loss. Remarkably, however, RB– or RB/p107–deficient amacrine, horizontal and Müller precursors survived, differentiated and eventually stopped dividing. Tumors arose from occasional amacrine/horizontal precursors that escaped growth arrest. These mice represent the first inheritable model of retinoblastoma induced by Rb gene inactivation. All the ectopic division and apoptosis could be rescued by inactivating a single member of the E2F family. Conclusions: The role for RB in blocking precursor cell division is unique as loss of other cell cycle inhibitors(e.g. p27Kip1, p57Kip2, Prox1) does not have this effect. This finding helps explain why defects in Rb, rather than other cell cycle regulators, is critical in retinoblastoma. Current dogma posits that Rb–deficient retinal cells undergo apoptosis, thus the retinoblastoma cell–of–origin must acquire an additional mutation to evade death. Our data show that the cell–of–origin is naturally death resistant, and rather than apoptosis, must evade growth–arrest associated with terminal differentiation. Thus, tumors exploit a natural property of the cell–of–origin that is a hallmark of cancer cells. This finding helps explain why retinoblastoma arises in few steps. Finally, our data link a specific E2F to the deleterious effects of Rb loss in the retina.
Keywords: retinoblastoma • proliferation • apoptosis/cell death