KCNC3^R420H, a K⁺ channel mutation causative in spinocerebellar ataxia 13 displays aberrant intracellular trafficking

Spinocerebellar ataxia 13 (SCA13) is an autosomal dominant disease resulting from mutations in KCNC3 (Kv3.3), a voltage-gated potassium channel. The KCNC3^R420H mutation was first identified as causative for SCA13 in a four-generation Filipino kindred with over 20 affected individuals. Electrophysiological analyses in oocytes previously showed that this mutation did not lead to a functional channel and displayed a dominant negative phenotype. In an effort to identify the molecular basis of this allelic form of SCA13, we first determined that human KCNC3^WT and KCNC3^R420H display disparate post-translational modifications, and the mutant protein has reduced complex glycan adducts. Immunohistochemical analyses demonstrated that KCNC3^R420H was not properly trafficking to the plasma membrane and surface biotinylation demonstrated that KCNC3^R420H exhibited only 24% as much surface expression as KCNC3^WT. KCNC3^R420H trafficked through the ER but was retained in the Golgi. KCNC3^R420H expression results in altered Golgi and cellular morphology. Electron microscopy of KCNC3^R420H localization further supports retention in the Golgi. These results are specific to the KCNC3^R420H allele and provide new insight into the molecular basis of disease manifestation in SCA13.