ER stress and apoptosis‐inducing mutations in carbonic anhydrase IV associated with retinitis pigmentosa (RP17) respond to chemical chaperones

Abstract

We examined two human mutations in the carbonic anhydrase IV (CA IV) gene, which are associated with RP17, an autosomal dominant form of retinitis pigmentosa. CA IV is highly expressed in the choriocapillaris, but not expressed in the retina. The R14W CA IV mutation (Rebello et al., Proc. Natl. Acad. Sci. USA 101:6617–6622, 2004) is located in the signal sequence. The subsequently discovered R219S CA IV mutation is located in the sequence of the mature protein. The biological effects of these mutations were studied in transfected COS cells. Although the signal sequence mutation had only a modest effect on total enzyme production (70% of normal), the R219S mutant protein was completely inactive. Both mutations induced upregulation of BiP, PERK, and CHOP, markers of ER stress. Both suppressed levels of production of other secretory proteins. Both dramatically induced apoptosis as measured by Annexin V binding and TUNEL staining. The markers of ER stress and the number of cells undergoing apoptosis were both greatly reduced by growth in 4-phenylbutyric acid or CA inhibitors, which appeared to act as nonspecific and specific chemical chaperones, respectively. We propose that autosomal dominant RP17 associated with both CA IV mutations is a protein folding disease produced by apoptosis in capillary endothelial cells that nourish the overlying retina. These results suggest that patients with RP17 may respond clinically to treatment with chemical chaperones. This research is supported by NIH grant DK40163 to W.S.S.