Abstract
Purpose: :
Expression of human T17M Rhodopsin (Rho) in mice leads to Autosomal Dominant Retinitis Pigmentosa (adRP) by a mechanism tightly associated with activation of Unfolded Protein Response (UPR). The goal of this study is to verify the hypothesis that down-regulation of the pro-apoptotic CHOP protein, a downstream UPR marker, is a potential treatment for adRP and validate this therapeutic approach to prevent vision loss in T17M Rho mice.
Methods: :
Five mice of each of the following groups, Chop+/+ (WT Rho), Chop-/- (WT Rho), T17M Rho Chop+/+ and T17M Rho Chop-/- were recruited in this study. All groups of mice were subjected to scotopic electroretinography (ERG) and spectral domain optical coherence tomography (SD-OCT) analysis at 1, 2 and 3 months of age. In addition, 1-month old mice retinas were collected and analyzed by quantitative RT-PCR to detect the expression of Nrl, Crx, mouse and human Rho genes.
Results: :
Surprisingly, at 1 month of age, a-wave and b-wave amplitudes of scotopic ERG were reduced by 76% and 40%, respectively, in T17M Rho Chop-/- mice compared with T17M Rho Chop+/+ mice. For the next two months, the same amplitudes were maintained in T17M Rho Chop-/- mice, while T17M Rho Chop+/+ a-wave and b-wave amplitudes declined with time compared with Chop+/+ and Chop-/-. Functional reduction of T17M Rho Chop-/- photoreceptors correlated with thickness of outer nuclear layer (ONL) as measured by OCT. ONL thickness was found to be reduced by 78% in 1 month old T17M Rho Chop-/- mice as compared with T17M Chop+/+ mice. Interestingly, we did not find any difference between Chop+/+ (WT Rho) and Chop-/- (WT Rho) mice either by ERG or OCT analysis. qRT-PCR data indicated that T17M Rho Chop-/- photoreceptors degenerated because of down-regulation of Nrl and Crx transcriptional factors by 94% and 60%, respectively, leading to reduction of both mouse ( 95 %) and human ( 82 %) Rho compared with T17M Rho Chop+/+ photoreceptors.
Conclusions: :
CHOP protein, a downstream mediator of the UPR, cannot be considered as a therapeutic target for adRP photoreceptors expressing misfolded Rho. The link between down-regulation of Nrl , Crx gene expression and the absence of CHOP protein in T17M Rho CHOP-/-retina has to be examined to determine new therapeutic targets.
Keywords: retinal degenerations: cell biology • gene/expression • transcription factors