A single, abbreviated RPGR-ORF15 variant reconstitutes RPGR function in vivo.

PURPOSE: The retinitis pigmentosa GTPase regulator (RPGR) is essential for the maintenance of photoreceptor viability. RPGR is expressed as constitutive and ORF15 variants because of alternative splicing. This study was designed to examine whether the retina-specific ORF15 variant alone could substantially substitute for RPGR function. A further objective was to test whether the highly repetitive purine-rich region of ORF15 could be abbreviated without ablating the function, so as to accommodate RPGR replacement genes in adenoassociated virus (AAV) vectors.
METHODS: A cDNA representing RPGR-ORF15 but shortened by 654 bp in the repetitive region was placed under the control of a chicken beta-actin (CBA) hybrid promoter. The resultant construct was transfected into mouse embryonic stem cells. Clones expressing the transgene were selected and injected into mouse blastocysts. Transgenic chimeras were crossed with RPGR knockout (KO) mice. Mice expressing the transgene but null for endogenous RPGR (Tg/KO) were studied from 1 month to 18 months of age by light and electron microscopy, immunofluorescence, and electroretinography (ERG). The results were compared with those of wild-type (WT) and RPGR-null control mice.
RESULTS: Transgenic RPGR-ORF15 was found in the connecting cilia of rod and cone photoreceptors, at approximately 20% of the WT level. Photoreceptor morphology, cone opsin localization, expression of GFAP (a marker for retinal degeneration) and ERGs were consistent with the transgene exerting substantial rescue of retinal degeneration due to loss of endogenous RPGR.
CONCLUSIONS: RPGR-ORF15 is the functionally significant variant in photoreceptors. The length of its repetitive region can be reduced while preserving its function. The current findings should facilitate the design of gene replacement therapy for RPGR-null mutations.