A novel in-frame mutation at the boundary between exon 21 and intron 21 of SCN4A in a family with paramyotonia congenita

ABSTRACT

@#Nondystrophic myotonias and periodic paralyses are an important group of genetic skeletal muscle disorders characterized by dysfunction of ion channels that regulate cell membrane excitability. Mutations in the Sodium Voltage-Gated Channel Alpha Subunit 4 (SCN4A) gene are associated with a spectrum of a heterogeneous group of skeletal muscle such as sodium channel myotonia, paramyotonia congenita, hyperkalemic periodic paralysis, congenital myasthenia, and congenital myopathy. Gain of function mutations in SCN4A cause three muscle disorders with overlapping clinical phenotypes myotonia, paramyotonia congenita, and hyperkalemic periodic paralysis. Here, we describe the clinical and genetic features of a new family with paramyotonia. The proband, an eight-year-old girl, began to experience muscle stiffness in her hands and limbs on exposure to exercise exercise at the age of four and presented with myotonia. She was initially misdiagnosed with myotonic dystrophy because of worsening weakness with significant elevation of serum creatinine kinase levels. Two other affected family members had paradoxical myotonia in the face and hands on exposure to cold muscle stiffness in her face, hands, and limbs on exposure to cold and showed grip myotonia on physical examination. A novel heterozygous in-frame insertion, c.3911_3912+1dupAGA, at the boundary between exon 21 and intron 21 of SCN4A was identified using whole exome sequencing. Our finding enhances our understanding of the genotype and phenotype of patients with paramyotonia congenita, caused by mutations in the SCN4A gene.