Distal truncation of KCC3 in non–French Canadian HMSN/ACC families

Abstract

<b>Background: </b> Hereditary motor and sensory neuropathy with agenesis of the corpus callosum (HMSN/ACC) is a severe and progressive autosomal recessive polyneuropathy. Mutations in the potassium-chloride cotransporter 3 gene (<i>KCC3</i>) were identified as responsible for HMSN/ACC in the French Canadian (FC) population. In the present study, the authors were interested in finding new mutations in non-FC populations, assessing the activity of mutant proteins and refining genotype-phenotype correlations. <b>Methods: </b> The authors screened <i>KCC3</i> for mutations using direct sequencing in six non-FC HMSN/ACC families. They then assessed the functionality of the most common mutant protein using a flux assay in <i>Xenopus laevis</i> oocytes. <b>Results: </b> The authors identified mutations in exon 22 of KCC3: a novel mutation (del + 2994-3003; E1015X) in one family, as well as a known mutation (3031C→T; R1011X) found in five unrelated families and associated with two different haplotypes. The function of the cotransporter was abolished, although a limited amount of mutant proteins were correctly localized at the membrane. <b>Conclusions: </b> KCC3 mutations in exon 22 constitute a recurrent mutation site for hereditary motor and sensory neuropathy with agenesis of the corpus callosum (HMSN/ACC), regardless of ethnic origin, and are the most common cause of HMSN/ACC in the non–French Canadian (FC) families analyzed so far. Therefore, for genetic analysis, exon 22 screening should be prioritized in non-FC populations. Finally, the R1011X mutation leads to the abrogation of KCC3’s function in <i>Xenopus laevis</i> oocytes, likely due to impaired transit of the cotransporter. <b>GLOSSARY: </b><b>ANOVA</b> = analysis of variance; <b>CTD</b> = C-terminal domain; <b>FC</b> = French Canadian population; <b>HMSN/ACC</b> = hereditary motor and sensory neuropathy with agenesis of the corpus callosum; <b>KCC3</b> = potassium-chloride cotransporter 3; <b>mut</b> = mutated sequence; <b>NGS</b> = normal goat serum; <b>RVD</b> = regulatory volume decrease; <b>WT</b> = wild type sequence.