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C. Punzo, C. Cepko; Rod Toxin and Rod Trophic Factor in Retinitis Pigmentosa. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2039.
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Retinitis pigmentosa (RP) is a retinal degeneration characterized by loss of rod and cone photoreceptors (PR), resulting in blindness. In many cases the disease has a genetic cause, with the disease gene expressed exclusively in rods. Nonetheless, both rods and cones are affected, cone death typically following rod death. This observation could be due to sick or dying rods producing a factor(s), which leads to the death of cones, or could be due to the lack of a rod produced factor that is required for cone survival. In this study, rod and cone death was examined in four mouse models of RP to allow an examination of these hypotheses concerning rod and cone death.
One autosomal dominant and three autosomal recessive mouse models of RP were analyzed to evaluate the rod factor hypotheses. The progression of both rod and cone cell death was monitored over time.
In all 4 mutants, the initiation of cone death overlapped with the end of the major period of rod death, and not with the initiation or peak of rod death. In two of the three autosomal recessive models, cones were present in large numbers even 20 weeks after nearly all rods had died, while in the third autosomal recessive model most cones died within 7 weeks after the period of major rod death. In the autosomal dominant model, the end phase of rod death was very slow, and many rods were still present when cones were dying.
The data suggest that neither a toxic factor produced by dying rods, nor the lack of a rod-produced trophic factor required for cone survival, are the sole underlying cause of cone death in these 4 models of RP. The toxin hypothesis is unlikely because the onset of cone death did not overlap with the onset or peak of rod death. The trophic factor hypothesis is unlikely because many cones could survive months after all rods had died.
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