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.
ABSTRACT: 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 consequences and clinical presentations for genotype-phenotype analysis. We found significant correlations of mutant
chloride current with the age at diagnosis, plasma
chloride concentration and urine
calcium excretion rate. In conclusion, hClC-Kb expression in HEK cells is susceptible to
proteasome degradation, and fusion of GFP to the C-terminus of hClC-Kb improves
protein expression. The functional severity of the CLCNKB mutation is an important determinant of the phenotype in classic BS.