Rupak Datta, Abdul Waheed, Giuseppe Bonapace, Gul N. Shah, and William S. Sly | March 3, 2009 | Vol. 106, Issue 9 |
Missense mutations in the carbonic anhydrase IV (CA4) gene have been identified in patients with an autosomal dominant form of retinitis pigmentosa (RP17). We used two transient expression systems to investigate the molecular mechanism by which the newly identified CA4 mutations, R69H and R219S, contribute to retinal pathogenesis. Although the R219S mutation drastically reduced the activity of the enzyme, the R69H mutation had a minimal effect, suggesting that loss of CA activity is not the molecular basis for their pathogenesis. Defective processing was apparent for both mutant proteins. Cell surface-labeling techniques showed that the R69H and R219S mutations both impaired the trafficking of CA4 to the cell surface, resulting in their abnormal intracellular retention. Expression of both CA4 mutants induced elevated levels of the endoplasmic reticulum (ER) stress markers, BiP and CHOP, and led to cell death by apoptosis. They also had a dominant-negative effect on the secretory function of the ER. These properties are similar to those of R14W CA4, the signal sequence variant found in the original patients with RP17. These findings suggest that toxic gain of function involving ER stress-induced apoptosis is the common mechanism for pathogenesis of this autosomal-dominant disease. Apoptosis induced by the CA4 mutants could be prevented, at least partially, by treating the cells with dorzolamide, a CA inhibitor. Thus, the use of a CA inhibitor as a chemical chaperone to reduce ER stress may delay or prevent the onset of blindness in RP17.
Retinitis pigmentosa (RP) is a group of progressive eye diseases characterized by the deaths of retinal photoreceptors. Patients suffer from night blindness, gradual constriction of visual fields, and eventual loss of central vision (1). Increasing evidence points to a causative role for mutations in the carbonic anhydrase IV (CA4) gene in pathogenesis of RP17, an autosomal dominant form of RP (2–4).
Three different missense mutations in the coding region of CA IV have been discovered. The first was a signal sequence mutation, changing Arg14 to Trp (R14W) (2). The other 2 mutations change amino acids in the mature portion of the protein, replacing Arg69 and Arg219 with His and Ser, respectively (R69H and R219S) (3, 4). RP associated with the R14W and R219S mutations is inherited in an autosomal-dominant fashion (2, 3). The R69H mutation was found in a sporadic case of RP (4). Because CA IV is not expressed in the retina (5), explaining the pathogenesis of RP17 presented a challenge. Understanding the pathogenesis is essential to design mechanism-based therapies for this disease for which no effective treatment is currently available.
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3.) Z Yang, et al., Mutant carbonic anhydrase 4 impairs pH regulation and causes retinal photoreceptor degeneration. Hum Mol Genet 14, 255–265 (2005).
4.) BV Alvarez, et al., Identification and characterization of a novel mutation in the carbonic anhydrase IV gene that causes retinitis pigmentosa. Invest Ophthalmol Vis Sci 48, 3459–3468 (2007).
5.) GS Hageman, XL Zhu, A Waheed, WS Sly, Localization of carbonic anhydrase IV in a specific capillary bed of the human eye. Proc Natl Acad Sci USA 88, 2716–2720 (1991).