RPGRIP1 mutant dogs show variable cone ERG functions

Abstract

Purpose : Retinitis pigmentosa GTPase regulator interacting protein 1 (RPGRIP1) is a core component of the ciliary axoneme of photoreceptors, and anchors Retinitis pigmentosa GTPase regulator (RPGR) at the connecting cilium. A 44 bp insertion (^ins/ins) in exon 3 of RPGRIP1 is thought to be causal for canine cone-rod dystrophy 1 (cord1) along with an age-of-onset modifying locus that has been identified more recently. Observation of a canine research colony segregating RPGRIP1^ins/ins has revealed that none of the RPGRIP1^ins/ins dogs, to date (7.5 years), develop retinal degeneration/clinical blindness. Still, their photopic cone-derived ERG functions vary from normal, reduced to absent in dogs that are homozygous for the modifier. To explore the basis of the cone-specific phenotypic variation, genes involved in cone phototransduction were studied.

Methods : Retinal morphology was assessed by H&E staining in paraformaldehyde-fixed and optimal cutting temperature compound-embedded tissues. A series of antibodies was used for immunohistochemistry to examine the cone phototransduction cascade, retinal stress, and synaptic layers. Further, qRT-PCR was performed to study the expression of selected retinal genes.

Results : Cone ERG-absent (RPGRIP1^ins/ins) retina showed no defect in retinal morphology, synaptic layers or any sign of inner retinal stress. However, their photoreceptor inner segment appeared short and stubby while the outer segment was narrow and elongated at certain locations. L/M-opsin was mislocalized to the inner segment in the cone-ERG absent (RPGRIP1^ins/ins) retinas, but the numbers of L/M- and S-opsins were comparable to the wild type and cone ERG-normal (RPGRIP1^ins/ins) retinas. None of the genes studied by qRT-PCR showed any differential expression.

Conclusions : Considerable variation in cone ERG function was observed among RPGRIP1^ins/ins dogs. L/M-opsin-specific subcellular changes in the cone-ERG absent dogs that are clinically normal indicate the role of S-cone in clinically sufficient vision but insufficient for eliciting recordable cone ERG. The lack of phenotype in a subset of dogs suggests that RPGRIP1^ins/ins by itself does not cause cord1, indicating that another locus, other than the known modifier, is responsible for the variable cone ERG in RPGRIP1^ins/ins dogs.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.