Science Topics – 109

Partial loss of neuronal KCC2 function by biallelic SLC12A5 mutations causes migrating focal seizures and developmental delay

Miho Watanabe¹, Tenpei Akita¹, Hirotomo Saitsu², Atsuo Fukuda¹

Department of Neurophysiology, Hamamatsu University School of Medicine
Department of Biochemistry, Hamamatsu University School of Medicine

Epilepsy of infancy with migrating focal seizures (EIMFS) is one of the early-onset epileptic syndromes characterized by migrating polymorphous focal seizures. We performed whole exome sequencing (WES) in ten sporadic and one familial case of EIMFS and found novel compound heterozygous SLC12A5 (encoding the neuronal K⁺-Cl^– co-transporter KCC2) mutations in two families: c.279+1G>C causing skipping of exon 3 in the transcript (p.E50_Q93del) and c.572C>T (p.A191V) in individuals 1 and 2, and c.967T>C (p.S323P) and c.1243A>G (p.M415V) in individual 3. We further searched for SLC12A5 mutations in WES data from 526 patients and SLC12A5-targeted resequencing data from 141 patients with infantile epilepsy, and found another patient (individual 4) with migrating multifocal seizures and compound heterozygous mutations [c.953G>C (p.W318S) and c.2242_2244del (p.S748del)]. These individuals exhibited severe developmental delays and postnatal microcephaly. KCC2 is a membrane transport protein that extrudes Cl^– from neurons and helps maintain low intracellular Cl^– concentrations and hyperpolarizing GABA/glycinergic synaptic potentials. To determine the extent of disruption of neuronal Cl^– extrusion mediated by KCC2 in the patients, gramicidin-perforated patch-clamp analysis was performed. Results demonstrated strongly suppressed Cl^– extrusion function of E50_Q93del and M415V mutants, with mildly impaired function of A191V and S323P mutants. Cellular distribution and cell surface expression levels of these KCC2 mutants were similar to wildtype KCC2. Heterologous expression of two KCC2 mutants, mimicking the patient status, produced a significantly greater intracellular Cl^– level than with wildtype KCC2, but less than without KCC2. In conclusion, our data demonstrated that individual mutations in EIMFS patients causes variable loss of KCC2 function, and that the combinatory effect of partial loss of KCC2 function in each patient results in focal seizures, severe developmental delays, and postnatal microcephaly.

Saitsu H^†, Watanabe M^†, Akita T^† (^† equal contribution), Ohba C, Sugai K, Ong WP, Shiraishi H, Yuasa S, Matsumoto H, Beng KT, Saitoh S, Miyatake S, Nakashima M, Miyake N, Kato M, Fukuda A^*, Matsumoto N^*: Impaired neuronal KCC2 function by biallelic SLC12A5 mutations in migrating focal seizures and severe developmental delay. Scientific Reports, 6: 30072, 2016.
*corresponding author