Abstract
Purpose:
To quantify morphological alterations in the outer retina of a mouse model for X linked Retinitis pigmentosa due to a mutation in the RPGR gene and to correlate them with functional data and the photoreceptor cell death status.
Methods:
The mouse model B6J.Sv129-Rpgrtm1Sti was analysed at time points 1, 3, 6, 9, 12, 15, 18, and 21 months of age (M). Animals were examined in vivo by OCT and funduscopy (MICRON III System, Phoenix Research Inc., US) in the peripapillary area and in the periphery. ERG data were generated using the Espion E3 mouse stand (Diagnosys LLC, US). Post mortem morphological analysis included immunofluorescence staining (PKCa, CtBP2). The neuronal cell death status was studied by gene expression analysis of 13 genes (aif, bcl2l1, alpain1 and 2, caspase 12, parp, bcl10, caspase3, dr5, fas, c-fos, c-jun, trp63) and TUNEL staining.
Results:
Peripapillary retinal thickness on OCT was reduced at 1M in affected animals (mean=207+/-7μm) compared to wild type (mean=225+/-4μm) and heterozygous animals (mean=221+/-6μm). At the same time, ONL thickness was reduced in affected (mean=54+/-4μm) and heterozygous (mean=61+/-0.5μm) animals, compared to wild type (mean=71+/-7μm). This difference was seen similarly in the periphery and throughout the time course. Reduced visibility of the inner segment ellipsoid (Ise) was seen at 1M in affected as well as carrier animals. Post mortem analysis showed sprouting of bipolar dendrites as early as 3-6M in affected animals. Number of TUNEL positive cells was increased early in heterozygous (3-6M) and late in affected animals (15-18M). Gene expression profiles of the 13 genes revealed no clear involvement of a specific pathway at any time point.
Conclusions:
The mouse model shows onset of morphological alterations in the outer retina within the first weeks of life. Similar to the situation in humans, the question of whether photoreceptors develop correctly and degenerate over time or whether they were not developed correctly is subject of on-going investigations. Since the model was designed specifically for studying new therapeutic approaches, the answer to this question will help to define the optimal treatment strategy.