Missense mutations in the carbonic anhydrase IV (CA IV) 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 CA IV 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 CA IV to the cell surface, resulting in their abnormal intracellular retention. Expression of both CA IV 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 CA IV, 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 CA IV 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.