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Identification and Characterization of a Novel Mutation in the Carbonic Anhydrase IV Gene that Causes Retinitis Pigmentosa

From ¹Membrane Protein Research Group, Department of Physiology, and the ⁹Department of Biochemistry, University of Alberta, Edmonton Alberta, Canada; the ³Singapore Eye Research Institute, Singapore; the ⁴Department of Ophthalmology and Visual Science and the ⁵Program in Human Molecular Biology and Genetics, Eccles Institute of Human Genetics, University of Utah, Salt Lake City, Utah; the ⁶Sichuan Provincial Medical Academy and ⁷Sichuan Provincial People’s Hospital, Sichuan, People’s Republic of China; the ⁸Singapore National Eye Centre, Singapore; the ¹⁰Genome Institute of Singapore, Singapore; the ¹¹Department of Ophthalmology, National University of Singapore, Singapore.

PURPOSE. The autosomal dominant retinitis pigmentosa (adRP) gene on chromosome 17, region q22 (RP17), was recently identified as a glycosylphosphatidylinositol membrane-anchored zinc metalloenzyme (protein CAIV), highly expressed in the choriocapillaris of the eye and undetectable in the retina. Only two missense mutations have thus far been identified in the gene CA4. Functional analysis of these mutations demonstrated that retinal disease may result from perturbation of pH homeostasis in the outer retina, after disruption of CAIV and sodium bicarbonate cotransporter 1 (NBC1)–mediated bicarbonate transport. CA4 was screened in a panel of patients with RP, to expand the mutation spectrum of this novel adRP gene and understand its pathogenic mechanism.

METHODS. A total of 96 patients with simplex RP and adRP of Chinese ethnicity were screened for mutations in the eight coding exons of the CA4 gene by bidirectional sequencing. Functional consequences of CA4 mutations on the NBC1-mediated bicarbonate transport were studied by measuring bicarbonate fluxes in HEK293 cells cotransfected with NBC1 and CA4 mutant cDNAs.

RESULTS. Thirteen sequence alterations were identified, including a novel mutation within exon 3 of CA4 (R69H) in a patient with simplex RP. R69H was not found in 432 normal chromosomes. R69H CAIV impaired NBC1-mediated pH recovery after acid load.

CONCLUSIONS. A novel mutation has been identified in CA4 that provides further evidence that impaired pH regulation may underlie photoreceptor degeneration in RP17. This study indicates that, as with European patients with RP, mutations in CA4 also account for 1% of Chinese patients with RP.

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