Ablation of the X-linked retinitis pigmentosa 2 (Rp2) gene in mice results in opsin mislocalization and photoreceptor degeneration.

Li L, Khan N, Hurd T, Ghosh AK, Cheng C, Molday R, Heckenlively JR, Swaroop A, Khanna H.

Invest Ophthalmol Vis Sci. 2013 ;54(7):45031.

Department of Ophthalmology, University of Massachusetts Medical School, Worcester, Massachusetts, USA.

Abstract

PURPOSE:

Mutations in the RP2 gene are associated with 10% to 15% of X-linked retinitis pigmentosa (XLRP), a debilitating disorder characterized by the degeneration of retinal rod and cone photoreceptors. The molecular mechanism of pathogenesis of photoreceptor degeneration in XLRP-RP2has not been elucidated, and no treatment is currently available. This study was undertaken to investigate the pathogenesis of RP2-associated retinal degeneration.

METHODS:

We introduced loxP sites that flank exon 2, a mutational hotspot in XLRP-RP2, in the mouse Rp2 gene. We then produced Rp2-null allele using transgenic mice that expressed Cre-recombinase under control of the ubiquitous CAG promoter. Electroretinography (ERG), histology, light microscopy, transmission electron microscopy, and immunofluorescence microscopy were performed to ascertain the effect of ablation of Rp2on photoreceptor development, function, and protein trafficking.

RESULTS:

Although no gross abnormalities were detected in the Rp2(null) mice, photopic (cone) and scotopic (rod) function as measured by ERG showed a gradual decline starting as early as 1 month of age. We also detected slow progressive degeneration of the photoreceptor membrane discs in the mutant retina. These defects were associated with mislocalization of cone opsins to the nuclear and synaptic layers and reduced rhodopsin content in the outer segment of mutant retina prior to the onset of photoreceptor degeneration.

CONCLUSIONS:

Our studies suggest that RP2 contributes to the maintenance of photoreceptor function and that cone opsin mislocalizationrepresents an early step in XLRP caused by RP2 mutations. The Rp2(null) mice should serve as a useful preclinical model for testing gene- and cell-based therapies.

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Additional Information:

Hemant Khanna, PhD,  Corresponding author

Mutations in the RP2 gene are associated with X-linked forms of retinitis pigmentosa, which is characterized by progressive degeneration of rod photoreceptors (resulting in night blindness) followed by death of cone photoreceptors (responsible for day vision). Our studies reported in the present article revealed that ablation of Rp2 in mice results in early onset and progressive cone photoreceptor defect and a slow degeneration of rods. This is consistent with recent reports that several RP2 patients exhibit early cone dysfunction and degeneration. As humans depend on cone photoreceptors for our daytime vision, these studies provide an opportunity to understand the mode of dysfunction and degeneration of cone photoreceptors in a rod dominant mammalian retina, which is that case in human retina.

 

The figure shows a cross-section of mouse retina with staining of cone photoreceptors (red). Other photoreceptors are rods, indicating minority of cones in mammalian retina. Sensory cilium of all photoreceptors is labeled green with marker antibody. Note that one cone photoreceptor in this plane extends distally. Nuclei are stained with Hoechst dye (blue).

Ablation of the X-Linked Retin