[Show abstract][Hide abstract] ABSTRACT: Hyperpolarization-activated cyclic nucleotide-gated (HCN) channel-mediated currents, known as I h, are involved in the control of rhythmic activity in neuronal circuits and in determining neuronal properties including the resting membrane potential. Recent studies have shown that HCN channels play a role in seizure susceptibility and in absence and limbic epilepsy including temporal lobe epilepsy following long febrile seizures (FS). This study focused on the potential contributions of abnormalities in the HCN2 isoform and their role in FS. A novel heterozygous missense mutation in HCN2 exon 1 leading to p.S126L was identified in two unrelated patients with FS. The mutation was inherited from the mother who had suffered from FS in a pedigree. To determine the effect of this substitution we conducted whole-cell patch clamp electrophysiology. We found that mutant channels had elevated sensitivity to temperature. More specifically, they displayed faster kinetics at higher temperature. Kinetic shift by change of temperature sensitivity rather than the shift of voltage dependence led to increased availability of I h in conditions promoting FS. Responses to cyclic AMP did not differ between wildtype and mutant channels. Thus, mutant HCN2 channels cause significant cAMP-independent enhanced availability of I h during high temperatures, which may contribute to hyperthermia-induced neuronal hyperexcitability in some individuals with FS.
PLoS ONE 01/2013; 8(12):e80376. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We recently identified a novel missense mutation of the γ(2) subunit at position 40 with serine (N40S) of the GABA(A) receptor from a patient with epilepsy. Here, we report properties of the mutant receptor using the whole cell patch clamp technique. The Hill coefficient for the N40S receptor was greater than for the wild-type (WT) receptor, while the EC50 and kinetics did not differ. Furthermore, the effects of diazepam, Zn(2+), bicuculline, and pH were indistinguishable between WT and N40S receptors. These results suggest that the changes in the steepness of the concentration-response relationship for GABA in the N40S receptor may trigger epilepsy.
Journal of Pharmacological Sciences 12/2012; · 2.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ion channels, specifically voltage-gated sodium channels (Na(v)s), are common culprits in inheritable seizure disorders. Some Na(v) isoforms are particularly susceptible, while others are only weakly associated with neuronal hyperexcitability. Representative of the latter group is Na(v)1.2 (gene name SCN2A): despite its abundance in the brain, Na(v)1.2-related epilepsy is rare and only few studies have been conducted as to the pathophysiological basis of Na(v)1.2 in neuronal hyperexcitability. We here present a detailed functional analysis of Na(v)1.2 mutant, R1312T, which was originally found in a child with Dravet syndrome (formerly known as severe myoclonic epilepsy of infancy or SMEI). Whole-cell voltage clamp analysis revealed clearly compromised function: the mutant channels fast- and slow-inactivated at markedly more negative potentials and recovered from fast inactivation more slowly, which resulted in a use-dependent current reduction to less than 50% of wildtype levels. We also noted a small hyperpolarizing shift in the voltage dependence of activation. Our findings expand the spectrum of abnormal Na(v) channel behavior in epilepsy and raise the question as to how loss-of-function in a sodium channel predominantly expressed in excitatory neurons can lead to hyperexcitability.
Neurobiology of Disease 06/2012; 47(3):378-84. · 5.62 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This study examines whether microdeletions and duplications of the gene encoding α1 subunit of the sodium channel (SCN1A) are underlying causes in Dravet syndrome (DS) with SCN1A missense mutation. Multiple exonic deletions were identified in 8/84 patients without mutation and 0/41 patients with missense mutations. Our findings indicate that while microdeletions are not rare in SCN1A-negative patients, they are not likely to be present simultaneously with other SCN1A mutations.
Brain & development 12/2011; 34(8):617-9. · 1.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To determine the significance of PCDH19 mutations in Japanese females with epilepsy and to delineate their phenotypes.
PCDH19 sequencing analysis was performed in 116 females with various epilepsies, including 97 with Dravet syndrome (83.6%). They were referred for SCN1A analysis, and 52 carried SCN1A mutations.
Seven heterozygous mutations in exon 1 were identified in 7 patients (6.0%): 2 frameshift, 2 nonsense, and 3 missense mutations. One patient was a monozygotic twin, and her sister with mild phenotype carried the same mutation. The main clinical features among these 8 patients included early seizure onset (≤25 months of age), seizure clusters (7/8), fever-associated seizures (7/8), single seizure type (6/8), and late deterioration of intellect (5/8). Seizure durations were generally up to a few minutes, and only one patient developed status epilepticus once. The main seizure types were generalized tonic-clonic (4/8), tonic (3/8) and focal seizures, with (2/8) or without secondary generalization (3/8). Myoclonic, atonic and absence seizures were extremely rare. Two patients had Dravet syndrome (25%), and this proportion was significantly smaller than that in the total subjects (p<0.01).
PCDH19 mutation is a relatively frequent cause of epilepsy in Japanese females. Dravet syndrome was rare in our cohort.
Epilepsy research 11/2011; 99(1-2):28-37. · 2.48 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Mutations in SCN2A, the gene encoding α2 subunit of the neuronal sodium channel, are associated with a variety of epilepsies: benign familial neonatal-infantile seizures (BFNIS); genetic epilepsy with febrile seizures plus (GEFS+); Dravet syndrome (DS); and some intractable childhood epilepsies. More than 10 new mutations have been identified in BFNIS, all of them are missense. To date, only one nonsense mutation has been found in a patient with intractable childhood epilepsy and severe mental decline. Recently, microduplication of chromosome 2q24.3 (containing eight genes including SCN2A, SCN3A, and the 3' end of SCN1A) was reported in a family with dominantly inherited neonatal seizures and intellectual disability. Functional studies of SCN2A mutations show that they can cause divergent biophysical defects in Na(V)1.2 and impair cell surface expressions. There is no consistent relationship between genotype and phenotype.
Brain & development 10/2011; 34(7):541-5. · 1.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Alterations of the genes encoding α1 and α2 subunits of voltage-gated sodium channels (SCN1A, SCN2A) have been reported as causes of various types of epilepsy, most of which occur during the first year of life; as yet, however, the detailed mechanisms are unclear. We suppose that developmental changes of SCN1A and SCN2A in the human brain, which are unknown yet, may play an important role. So here, we studied the developmental changes of their corresponding proteins (Na(v)1.1 and Na(v)1.2) in the human hippocampus and temporal lobe in 28 autopsy cases, which age from 13weeks of gestation (GW) to 63years of age (Y). Using comparative microscopic immunohistochemical (IHC) analysis, we found that Na(v)1.1 and Na(v)1.2 immunoreactivity first appeared at 19GW, simultaneously in the hippocampus and the white matter of temporal lobe. In nearly all age groups, Na(v)1.1 immunoreactivity was weak and relatively homogeneous. In general, Na(v)1.1 immunoreactive (IR) neurons and neurites increased during the late fetal and postnatal periods, reached their peaks 7-9months after birth (M), then decreased and remained stable at a relatively low level during childhood and adulthood. On the other hand, Na(v)1.2 immunoreactivity was strong and heterogeneous. In the hippocampus, Na(v)1.2 IR neurons increased gradually during the late fetal period, reached their peaks at 7-9M, sustained this high level during childhood, and then decreased slightly at adulthood. In the temporal lobe, Na(v)1.2 IR neurons reached a high level during the late fetal period, and maintained that level during subsequent developmental stages; Na(v)1.2 IR neurites also increased to a relatively high level during the late fetal period and continued to increase up to and during adulthood. Using double-staining IHC, we found that Na(v)1.1 and Na(v)1.2 had a relatively high colocalization rate with parvalbumin and showed distinct developmental changes. These findings extend our previous understanding of sodium channels and may help us discover the pathomechanisms of sodium channel-related age-dependent epilepsy.
Brain research 03/2011; 1389:61-70. · 2.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A few mutations in the gene encoding the gamma 2 subunit of the gamma-aminobutyric acid receptor type A (GABRG2) have been reported in various types of epilepsy. The aim of this study is to investigate the role of GABRG2 in the pathogenesis of childhood epilepsy in a large Japanese cohort. Genetic analysis of GABRG2 was performed on 140 Japanese patients with various childhood epilepsies largely including Dravet syndrome and genetic epilepsy with febrile seizures plus. The mutational analysis identified one novel missense mutation of GABRG2 (c.236A>G: p.N40S) in a patient with generalized tonic-clonic seizures (GTCS). The mutation was heterozygous and replacing a highly conserved Asn residue with a Ser. The affected amino acid was located at residue 40 of the mature GABRG2 protein, which was near the first one of two high-affinity benzodiazepine-binding domains of the gamma2 subunit (Lys-41-Trp-82). This mutation in such an important position may hamper the function of the channel and contribute to the case's pathogenesis of GTCS.
Journal of Human Genetics 06/2010; 55(6):375-8. · 2.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Mutations of the gene encoding the alpha2 subunit of the neuronal sodium channel, SCN2A, have been found in benign familial neonatal-infantile seizures (BFNIS). In Dravet syndrome, only one nonsense mutation of SCN2A was identified, while hundreds of mutations were found in the paralogue gene, SCN1A, which encodes the alpha1 subunit. This study examines whether SCN2A mutations are associated with Dravet syndrome. We screened for mutations of SCN1A, SCN2A and GABRG2 (the gene encoding gamma2 subunit of the GABA(A) receptor) in 59 patients with Dravet syndrome and found 29 SCN1A mutations and three missense SCN2A mutations. Among the three, one de novo SCN2A mutation (c.3935G>C: R1312T) identified in a patient was thought to affect an arginine residue in a voltage sensor of the channel and hence, to be pathogenic. This finding suggests that both nonsense mutations and missense SCN2A mutations cause Dravet syndrome.
Brain & development 09/2009; 31(10):758-62. · 1.74 Impact Factor