RRH, Encoding the RPE-Expressed Opsin-Like Peropsin, Is Not Mutated in Retinitis Pigmentosa and Allied Diseases

The Gluopsins: Opsins without the Retinal Binding Lysine

Opsins allow us to see. They are G-protein-coupled receptors and bind as ligand retinal, which is bound covalently to a lysine in the seventh transmembrane domain. This makes opsins light-sensitive. The lysine is so conserved that it is used to define a sequence as an opsin and thus phylogenetic opsin reconstructions discard any sequence without it. However, recently, opsins were found that function not only as photoreceptors but also as chemoreceptors. For chemoreception, the lysine is not needed. Therefore, we wondered: Do opsins exists that have lost this lysine during evolution? To find such opsins, we built an automatic pipeline for reconstructing a large-scale opsin phylogeny. The pipeline compiles and aligns sequences from public sources, reconstructs the phylogeny, prunes rogue sequences, and visualizes the resulting tree. Our final opsin phylogeny is the largest to date with 4956 opsins. Among them is a clade of 33 opsins that have the lysine replaced by glutamic acid. Thus, we call them gluopsins. The gluopsins are mainly dragonfly and butterfly opsins, closely related to the RGR-opsins and the retinochromes. Like those, they have a derived NPxxY motif. However, what their particular function is, remains to be seen.

Gene discovery and prevalence in inherited retinal dystrophies

Inherited retinal dystrophies are Mendelian neurodegenerative conditions classified as pigmentary retinopathies, macular dystrophies and others. Over a 21-year period, from 1990 to 2011, we have screened in Montpellier 107 genes in 609 families and have identified a causal mutation in 68.5% of them. Following a gene candidate approach, we established that RPE65, the isomerohydrolase of the visual cycle, is responsible for severe childhood blindness (Leber congenital amaurosis or early onset retinal dystrophy). In an ongoing study, we screened the genes in a series of 283 families with dominant retinitis pigmentosa and we have estimated that 80% of the families have a mutation in a known gene. A similar study is currently undergoing for autosomal recessive retinitis pigmentosa. Finally, we have identified IMPG1 as a responsible gene for rare cases of macular vitelliform dystrophy with a dominant or recessive inheritance.