Functional severity of CLCNKB mutations correlates with phenotypes in patients with classic Bartter's syndrome

• Published in: The Journal of physiology 2017-08, Vol.595 (16), p.5573-5586
• Main Authors: Cheng, Chih‐Jen, Lo, Yi‐Fen, Chen, Jen‐Chi, Huang, Chou‐Long, Lin, Shih‐Hua
• Format: Article
• Language: English
• Subjects: Adolescent; Adult; Age; Bartter Syndrome - genetics; Bartter's syndrome; C-Terminus; Calcium chloride; Calciuria; Channel gating; Child; Chloride; Chloride Channels - genetics; Chloride Channels - physiology; Chloride conductance; classic Bartter''s syndrome; CLCNKB gene; ClC‐Kb chloride channel; Excretion; Fusion protein; Genetic variability; Genotype & phenotype; Green fluorescent protein; HEK293 Cells; Humans; Infant; Mammalian cells; Membrane proteins; Membrane trafficking; Middle Aged; Missense mutation; Mutation; Phenotype; Proteasomes; Protein transport; Proteins; Proteolysis; Renal; Research Paper; Selectivity; Urine; Young Adult
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1113/JP274344

Key points The highly variable phenotypes observed in patients with classic Bartter's syndrome (BS) remain unsatisfactorily explained. The wide spectrum of functional severity of CLCNKB mutations may contribute to the phenotypic variability, and the genotype–phenotype association has not been established. Low‐level expression of the human ClC‐Kb channel in mammalian cells impedes the functional study of CLCNKB mutations, and the underlying cause is still unclear. The human ClC‐Kb channel is highly degraded by proteasome in human embryonic kidney cells. The C‐terminal in‐frame green fluorescent protein fusion may slow down the proteasome‐mediated proteolysis. Barttin co‐expression necessarily improves the stability, membrane trafficking and gating of ClC‐Kb. CLCNKB mutations in barttin‐binding sites, dimer interface or selectivity filter often have severe functional consequences. The remaining chloride conductance of the ClC‐Kb mutant channel significantly correlates with the phenotypes, such as age at diagnosis, plasma chloride concentration, and the degree of calciuria in patients with classic BS. Mutations in the CLCNKB gene encoding the human voltage‐gated chloride ClC‐Kb (hClC‐Kb) channel cause classic Bartter's syndrome (BS). In contrast to antenatal BS, classic BS manifests with highly variable phenotypes. The functional severity of the mutant channel has been proposed to explain this phenomenon. Due to difficulties in the expression of hClC‐Kb in heterologous expression systems, the functional consequences of mutant channels have not been thoroughly examined, and the genotype–phenotype association has not been established. In this study, we found that hClC‐Kb, when expressed in human embryonic kidney (HEK) cells, was unstable due to degradation by proteasome. In‐frame fusion of green fluorescent protein (GFP) to the C‐terminus of the channel may ameliorate proteasome degradation. Co‐expression of barttin increased protein abundance and membrane trafficking of hClC‐Kb and markedly increased functional chloride current. We then functionally characterized 18 missense mutations identified in our classic BS cohort and others using HEK cells expressing hClC‐Kb‐GFP. Most CLCNKB mutations resulted in marked reduction in protein abundance and chloride current, especially those residing at barttin binding sites, dimer interface and selectivity filter. We enrolled classic BS patients carrying homozygous missense mutations with well‐described functional consequ