Restoration of photoreceptor ultrastructure and function in retinal degeneration slow mice by gene therapy

Abstract

The gene Prph2 encodes a photoreceptor-specific membrane glycoprotein, peripherin-2 (also known as peripherin/rds), which is inserted into the rims of photoreceptor outer segment discs in a complex with rom-1 (ref. 2). The complex is necessary for the stabilization of the discs, which are renewed constantly throughout life, and which contain the visual pigments necessary for photon capture. Mutations in Prph2 have been shown to result in a variety of photoreceptor dystrophies, including autosomal dominant retinitis pigmentosa and macular dystrophy. A common feature of these diseases is the loss of photoreceptor function, also seen in the retinal degeneration slow (rds or Prph2 Rd2/Rd2) mouse, which is homozygous for a null mutation in Prph2. It is characterized by a complete failure to develop photoreceptor discs and outer segments, downregulation of rhodopsin and apoptotic loss of photoreceptor cells. The electroretinograms (ERGs) of Prph2Rd2/Rd2 mice have greatly diminished a-wave and b-wave amplitudes, which decline to virtually undetectable concentrations by two months. Subretinal injection of recombinant adeno-associated virus (AAV) encoding a Prph2 transgene results in stable generation of outer segment structures and formation of new stacks of discs containing both perpherin-2 and rhodopsin, which in many cases are morphologically similar to normal outer segments. Moreover, the re-establishment of the structural integrity of the photoreceptor layer also results in electrophysiological correction. These studies demonstrate for the first time that a complex ultrastructural cell defect can be corrected both morphologically and functionally by in vivo gene transfer.

Recombinant AAV-mediated gene transfer to the retina: gene therapy perspectives

“…The restoration of vision in RPE65 dogs, 33,34 and the morphological and functional rescue of photoreceptors in the prph2 mouse model 35 and in the RCS rat 36 provides evidence to support rAAV-mediated gene therapy replacement strategies. An alternative to these gene-specific therapies, which target a particular gene defect, is a generic approach using factors promoting cell survival.…”

“…Subretinal injection of rAAV-2 encoding peripherin-2 transgene resulted in the re-establishment of the structural integrity of the photorecetor layer as well as in electrophysiological correction. 35,74,75 Extended analysis of this gene therapy treatment in the same model demonstrated that the potential for ultrastructural improvement is dependent upon the age at which animals are treated and that overexpression of the prph 2 might be harmful to photoreceptor cells. 76 The Royal College of Surgeons (RCS) rat is a widely studied animal model of retinal degeneration in which the inability of the RPE to phagocytose shed photoreceptor outer segments leads to a progressive loss of rod and cone photoreceptors.…”

Gene therapy for optic nerve disease

“…12,[23][24][25] As an example, AAV vectors have recently been used to restore photoreceptor structure and function in retinal degeneration slow (rds) mice. 26 These mice have a mutation in the Prph2 gene and mutations in this gene have also been demonstrated in human retinitis pigmentosa. Rds mice injected subretinally with AAV-Prph2 developed new outer segment structures that contain rhodopsin and look ultrastructurally similar to normal rod photoreceptor outer segments.…”

Gene supplementation therapy for recessive forms of inherited retinal dystrophies

“…33 EARLY PROOF-OF-PRINCIPLE FOR GENE SUPPLEMENTATION THERAPY FOR RETINAL DYSTROPHY Although earlier studies had shown limited efficacy of gene supplementation in the Rd1 murine model of retinal degeneration, which has a defect in the Pde6b gene, [34][35][36][37] the first definitive proof-of-concept for gene supplementation therapy for a photoreceptor cell defect was demonstrated in the Rds mouse, which has a naturally occurring null mutation in the Prph2 gene. 38,39 Subretinal injection of AAV2 carrying the murine Prph2 gene under control of the bovine rhodopsin promoter results in expression of the structural outer segment protein Peripherin 2 and the formation of photoreceptor cell outer segments that are otherwise absent in this mouse model. The generation of outer segments is associated with restoration of the electroretinogram (ERG) 38 to B25% of wild-type levels, and improvement of neuronal responses measured in the superior colliculus.…”

“…38,39 Subretinal injection of AAV2 carrying the murine Prph2 gene under control of the bovine rhodopsin promoter results in expression of the structural outer segment protein Peripherin 2 and the formation of photoreceptor cell outer segments that are otherwise absent in this mouse model. The generation of outer segments is associated with restoration of the electroretinogram (ERG) 38 to B25% of wild-type levels, and improvement of neuronal responses measured in the superior colliculus. 40 However, despite significant improvement of photoreceptor cell function in these mice, the duration of functional benefit is limited by progressive photoreceptor degeneration 39 that may be a consequence of insufficiently rapid onset of AAV2-mediated transgene expression or non-physiological levels of Peripherin 2 production.…”