Impaired glutamylation of ORF15 presents a unique phenotype in RPGR-related retinal dystrophy

Abstract

Purpose : To determine the genotype-phenotype correlation in patients with RPGR retinal dystrophy, the commonest form of X-linked and recessive retinitis pigmentosa. Post-translational glutamylation of photoreceptor-specific RPGR^ORF15 by TTLL5 enzyme is essential for its function. TTLL5 needs to bind to ORF15 via the C-terminus to glutamylate RPGR. Since TTLL5 loss leads to cone-rod phenotype, in which normal RPGR would not be glutamylated, we hypothesise that the phenotype of distal RPGR mutations would merge with TTLL5 retinal dystrophy.

Methods : We conducted a retrospective study of 102 male patients with RPGR disease at three specialist centres and recorded results of ophthalmic examination, retinal imaging, microperimetry and molecular genetic analysis by next-generation sequencing. In vitro assays were performed to assess the level of RPGR glutamylation in distal mutations and analyse the level of interaction with TTLL5 in these mutant constructs compared to wild-type RPGR.

Results : Of 102 male patients with genetically confirmed RPGR variants, 99 were deep phenotyped. 13 patients had advanced disease with pan-retinal photoreceptor dysfunction. 64 out of remaining 86 patients (74%) had predominant rod-cone phenotype, 18/86 (21%) had cone-rod, 3/86 (3%) had cone only phenotype and 1/86 (1%) patients had mixed cone and cone-rod phenotype. Most mutations associated with cone-rod phenotype were in the distal part of the ORF15 region and those with cone only phenotype were in the very distal part including the basic domain. In-vitro glutamylation assay showed that all tested truncating mutations with a predominant cone phenotype (n=3) had reduced levels of glutamylation, with variable degree of interaction with TTLL5 enzyme.

Conclusions : Mutations found at the proximal N-terminus of RPGR are associated with predominant rod-cone phenotype, with increasing cone involvement towards the distal C-terminus and a predominant cone phenotype associated with the very distal mutations. In-vitro studies demonstrate the importance of glutamylation for normal functioning of RPGR, as the very distal mutations that result in almost full-length normal RPGR, reduce glutamylation and lead to retinal disease with cone phenotype. The application of RPGR with reduced glutamylation in gene therapy clinical trials may convert a rod-cone dystrophy into a cone dystrophy phenotype.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.

 

Distal OFR15 mutation associated with cone dystrophy.