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J. Neidhardt, E. Glaus, D. Barthelmes, C. Zeitz, J. Fleischhauer, W. Berger; Identification and Characterization of a Novel Isoform of the Retinitis Pigmentosa GTPase Regulator (RPGR). Invest. Ophthalmol. Vis. Sci. 2007;48(13):3784.
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© ARVO (1962-2015); The Authors (2016-present)
To characterize the functional relevance of a novel RPGR isoform in the human retina.
The novel RPGR isoform was analyzed by RT-PCR, Western blots, immunohistochemistry, co-immunoprecipitations and mutation screening of 78 RP patients. Patients and family members were characterized clinically.
A novel exon, designated 9a, was identified in RPGR transcripts. Western blot anaylses with polyclonal antibodies against an exon 9a derived peptide confirmed its protein expression in human retina. Immunohistochemistry localized the novel isoform predominantly to inner segments of cone photoreceptors where it co-localizes with RPGRIP1. Our findings describe the first RPGR isoform that is predominantly expressed in inner segments of cones.The novel exon contains a stop codon that truncates the RCC1 homologous protein domain (RCC1h) of RPGR. This domain is known to mediate the assembly of RPGR and the RPGR interacting protein 1 (RPGRIP1). Homology modeling of the novel isoform revealed a lack of one characteristic subdomain in the RCC1h structure. We performed co-immunoprecipitation assays to analyze the impact of this structural alteration on the interaction complex of RPGR and RPGRIP1. The results showed that the novel RPGR isoform recruits distinct protein variants of RPGRIP1 in the human retina. This suggests the formation of isoform-specific RPGR/RPGRIP1 complexes.Mutation screening in patients identified a nucleotide substitution in a splicing regulator upstream of exon 9a, which leads to a 3.5 times higher level of exon 9a transcripts in the patient compared to non-affected family members. Clinical findings of the patient are in accordance with a mild RP phenotype.
Together, these results suggest that the tight regulation of specific isoforms in RPGR is a prerequisite for proper function of photoreceptors and improve the understanding of the complex disease of Retinitis Pigmentosa.
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