A novel N440K sodium channel mutation causes myotonia with exercise-induced weakness - exclusion of CLCN1 exon deletion/duplication by MLPA

We report a 4-generation Turkish family with 10 affected members presenting with myotonia and potassium- and exerciseinduced paralytic attacks. The clinical presentation was neither typical for the chloride channel myotonias Thomsen and Becker nor for the separate sodium channel myotonia entities potassiumaggravated myotonia, paramyotonia congenita, and hyperkalemic periodic paralysis. It is best described by a combination of potassium-aggravated myotonia and hyperkalemic periodic paralysis. We excluded exonic chloride channel mutations including CLCN1 exon deletion/duplication by MLPA. Instead we identified a novel p.N440K sodium channel mutation that is located at the inner end of segment S6 of repeat I. We discuss the genotype phenotype relation.