The molecular and cellular basis of rhodopsin retinitis pigmentosa reveals potential strategies for therapy

Abstract

Inherited mutations in the rod visual pigment, rhodopsin, cause the degenerative blinding condition, retinitis pigmentosa (RP). Over 150 different mutations in rhodopsin have been identified and, collectively, they are the most common cause of autosomal dominant RP (adRP). Mutations in rhodopsin are also associated with dominant congenital stationary night blindness (adCSNB) and, less frequently, recessive RP (arRP). Recessive RP is usually associated with loss of rhodopsin function, whereas the dominant conditions are a consequence of gain of function and/or dominant negative activity. The in-depth characterisation of many rhodopsin mutations has revealed that there are distinct consequences on the protein structure and function associated with different mutations. Here we categorise rhodopsin mutations into seven discrete classes; with defects ranging from misfolding and disruption of proteostasis, through mislocalisation and disrupted intracellular traffic to instability and altered function. Rhodopsin adRP offers a unique paradigm to understand how disturbances in photoreceptor homeostasis can lead to neuronal cell death. Furthermore, a wide range of therapies have been tested in rhodopsin RP, from gene therapy and gene editing to pharmacological interventions. The understanding of the disease mechanisms associated with rhodopsin RP and the development of targeted therapies offer the potential of treatment for this currently untreatable neurodegeneration.

Rhodopsin: A Potential Biomarker for Neurodegenerative Diseases

“…Rhodopsin is a G-protein coupled receptor, and is the most abundant protein in the rod cells found in the retina (Figure 1). It functions as the primary photoreceptor molecule of vision, and contains two parts: an opsin molecule linked to a chromophore, 11-cis-retinal (Athanasiou et al, 2018). The opsin molecule is comprised of 348 amino acids, and has seven transmembrane domains (Athanasiou et al, 2018).…”

Characterizing variants of unknown significance in rhodopsin: a functional genomics approach

“…Next, six variants with new or unexpected findings were selected for further validation using immunofluorescence with confocal microscopy. A164V and G109R are class 2 mutants according to Rakoczy and Athanasiou(Rakoczy, 2011;Athanasiou, 2018), but unexpectedly showed intermediate or relatively preserved surface RHO staining in all three assays-standard and pooled flow cytometry (Table 1) as well as confocal microscopy (Figure 7). The following variants were originally of unknown pathogenicity: L47R (literature) and G18D, G101V, P180T…”

“…The analysis was therefore limited to subjects with class 2 mutations (N=69 subjects). (Class 2 mutations also have the best biological rationale and precedent for correlating with disease severity in this assay (Rakoczy, 2011;Athanasiou, 2018).) Standard linear regression was used to predict the logged 30 Hz ERG amplitude based on the logged NGS-based final surface expression ratio.…”

“…Next, the pooled assay results were graphed by the variant class derived from preexisting literature and database annotations ( Figure 6). The reported biochemical classes were obtained from the literature (Mendes, 2005;Rakoczy, 2011;Athanasiou, 2018). (Additional historical categorizations have also been published (Sung, 1993;Krebs, 2010), as well as classifications that integrate pharmacologic response (Behnen, 2018 Labeled variants that were previously considered VUS (L47R, G18D, G101V, and P180T) have now been demonstrated to express at pathogenic levels.…”

“…The rich body of existing data provides a context for interpreting data on new variants, and conversely, provides an opportunity to refine existing models of RHO structure and function (Rakoczy, 2011). A recent review (Athanasiou, 2018) notes that ongoing questions about the pathogenicity of RHO variants "reinforce the need for thorough genetics, such as segregation analyses, and in-depth functional analyses to confirm pathogenicity." (Note that the term "variant" is used in this study to include any sequence change, whether that sequence change is a "mutant" that is known to be pathogenic, a VUS, or a benign change such as a synonymous control.…”