No abstract

SUMMARYMutations in a number of genes encoding voltage-gated sodium channels cause a variety of epilepsy syndromes in humans, including genetic (generalized) epilepsy with febrile seizures plus (GEFS+) and Dravet syndrome (DS, severe myoclonic epilepsy of infancy). Most of these mutations are in the SCN1A gene, and all are dominantly inherited. Most of the mutations that cause DS result in loss of function, whereas all of the known mutations that cause GEFS+ are missense, presumably altering channel activity. Family members with the same GEFS+ mutation often display a wide range of seizure types and severities, and at least part of this variability likely results from variation in other genes. Many different biophysical effects of SCN1A-GEFS+ mutations have been observed in heterologous expression systems, consistent with both gain and loss of channel activity. However, results from mouse models suggest that the primary effect of both GEFS+ and DS mutations is to decrease the activity of GABAergic inhibitory neurons. Decreased activity of the inhibitory circuitry is thus likely to be a major factor contributing to seizure generation in patients with GEFS+ and DS, and may be a general consequence of SCN1A mutations.

Voltage-gated sodium channels are required for the initiation and propagation of action potentials. Mutations in the neuronal voltage-gated sodium channel SCN1A are associated with a growing number of disorders including generalized epilepsy with febrile seizures plus (GEFS؉), 7 severe myoclonic epilepsy of infancy, and familial hemiplegic migraine. To gain insight into the effect of SCN1A mutations on neuronal excitability, we introduced the human GEFS؉ mutation SCN1A-R1648H into the orthologous mouse gene. Scn1a RH/RH mice homozygous for the R1648H mutation exhibit spontaneous generalized seizures and premature death between P16 and P26, whereas Scn1a RH/؉ heterozygous mice exhibit infrequent spontaneous generalized seizures, reduced threshold and accelerated propagation of febrile seizures, and decreased threshold to flurothyl-induced seizures. Inhibitory cortical interneurons from P5-P15 Scn1a RH/؉ and Scn1a RH/RH mice demonstrated slower recovery from inactivation, greater use-dependent inactivation, and reduced action potential firing compared with wild-type cells. Excitatory cortical pyramidal neurons were mostly unaffected. These results suggest that this SCN1A mutation predominantly impairs sodium channel activity in interneurons, leading to decreased inhibition. Decreased inhibition may be a common mechanism underlying clinically distinct SCN1A-derived disorders.

Inactivation of the beta4 subunit of the calcium channel in the mouse neurological mutant lethargic results in a complex neurological disorder that includes absence epilepsy and ataxia. To determine the role of the calcium-channel beta4-subunit gene CACNB4 on chromosome 2q22-23 in related human disorders, we screened for mutations in small pedigrees with familial epilepsy and ataxia. The premature-termination mutation R482X was identified in a patient with juvenile myoclonic epilepsy. The R482X protein lacks the 38 C-terminal amino acids containing part of an interaction domain for the alpha1 subunit. The missense mutation C104F was identified both in a German family with generalized epilepsy and praxis-induced seizures and in a French Canadian family with episodic ataxia. These coding mutations were not detected in 255 unaffected control individuals (510 chromosomes), and they may be considered candidate disease mutations. The results of functional tests of the truncated protein R482X in Xenopus laevis oocytes demonstrated a small decrease in the fast time constant for inactivation of the cotransfected alpha1 subunit. Further studies will be required to evaluate the in vivo consequences of these mutations. We also describe eight noncoding single-nucleotide substitutions, two of which are present at polymorphic frequency, and a previously unrecognized first intron of CACNB4 that interrupts exon 1 at codon 21.

Autism is a psychiatric disorder with estimated heritability of 90%. One-third of autistic individuals experience seizures. A susceptibility locus for autism was mapped near a cluster of voltage-gated sodium channel genes on chromosome 2. Mutations in two of these genes, SCN1A and SCN2A, result in the seizure disorder GEFS þ . To evaluate these sodium channel genes as candidates for the autism susceptibility locus, we screened for variation in coding exons and splice sites in 117 multiplex autism families. A total of 27 kb of coding sequence and 3 kb of intron sequence were screened. Only six families carried variants with potential effects on sodium channel function. Five coding variants and one lariat branchpoint mutation were each observed in a single family, but were not present in controls. The variant R1902C in SCN2A is located in the calmodulin binding site and was found to reduce binding affinity for calcium-bound calmodulin. R542Q in SCN1A was observed in one autism family and had previously been identified in a patient with juvenile myoclonic epilepsy. The effect of the lariat branchpoint mutation was tested in cultured lymphoblasts. Additional population studies and functional tests will be required to evaluate pathogenicity of the coding and lariat site variants. SNP density was 1/kb in the genomic sequence screened. We report 38 sodium channel SNPs that will be useful in future association and linkage studies.