Background: Mutations in the SLC26A4 gene are responsible for Pendred syndrome and non-syndromic hearing loss (DFNB4). This study analyzed nonsynonymous SLC26A4 mutations newly identified in East Asians, as well as three common mutations in Caucasians, to characterize their molecular pathogenic mechanisms and to explore the possibility of rescuing their processing defects.
Methods: A total of eleven nonsynonymous disease-associated mutations were generated and their effects on protein processing and on ion transporting activities were examined.
Results: Most of the mutations caused retention of the SLC26A4 gene product (pendrin) in the intracellular region, while wild-type pendrin reached the plasma membrane. Accordingly, these mutations abolished complex-glycosylation and Cl-/HCO3- exchange activities of pendrin. However, significant heterogeneity in the processing of mutant pendrin molecules was observed. Each mutant protein exhibited a different cellular localization, a different degree of N-glycosylation, and a different degree of sensitivity to the treatments that rescue processing defects. For example, H723R-pendrin, the most common mutation in East Asians, was mostly expressed in ER, and its defects in protein processing and ion transporting activities were restored considerably by low-temperature incubation. On the other hand, L236P-pendrin, the most common mutation in Caucasians, was mainly in the centrosomal region and temperature-insensitive.
Conclusion: These results indicate that the processing of pendrin mutant protein is determined by mutant-specific mechanisms, and that a mutant-specific method would be required to rescue the conformational defects of each folding mutant.