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Xun Sun, Oleg V Bulgakov, Michael Adamian, Zhijian Wu, Tiansen Li; Isoform Switch of RPGR During Photoreceptor Development. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2972. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Mutations in the Retinitis Pigmentosa GTPase Regulator (RPGR) gene leads to one of the most severe forms of retinal degenerations and accounts for up to 80% cases of XLRP and 20% of all RP. RPGR undergoes complex splicing and the ORF15 isoform, considered to be photoreceptor specific, contains a glutamic acid-rich domain and is a mutational hotspot. There is a current discrepancy in subcellular localization of RPGR between rodent and primate photoreceptors, which raises questions about their differential functions among species and the use of rodent models to test gene replacement therapies. This study aims to determine subcellular localization of RPGR in human retinas and to explore a cellular and biochemical basis for the genetic finding that all disease-causing mutations in RP3 exclusively affect ORF15 transcripts only.
We generated and validated a battery of isoform-specific antibodies against both human and mouse RPGR. We performed Western blot, fluorescence microscopy, immuno-EM, and AAV-mediated gene delivery in RPGR knockout mice.
We found that native human RPGR predominantly localized at the connecting cilia of human rod and cone photoreceptors. Recombinant human RPGR transgene products, upon transduction with AAV vectors, were able to localize at the connecting cilia of mouse photoreceptors and interact with mouse RPGRIP1. The ORF15 isoform of the protein is the predominant form in mature photoreceptors of both humans and mice, of which the most abundant version was found to lack the c-terminus. RPGR expression undergoes an isoform switch during postnatal photoreceptor development. The default RPGR isoform is detected during retinal progenitor development, which is replaced by the ORF15 isoform as the predominant variant beginning at postnatal day 7 and plateaus when photoreceptors mature.
RPGR localize at the connecting cilia regardless of species of origin. The RPGR default isoform may play a role in early stages of ciliogenesis common to all ciliated cell types, whereas the ORF15 isoform functions specifically in mature photoreceptor cells where it is presumed to be involved in ciliary trafficking. This explains why ORF15 is the only transcript clinically known to cause retinal degeneration. Finally our unexpected findings that the bulk of ORF15 in vivo variably lacks the C-terminal sequence may have implications for understanding the normal functions of RPGR and future therapeutic studies.
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