A CACNB4 mutation shows that altered Ca v 2.1 function may be a genetic modifier of severe myoclonic epilepsy in infancy Iori Ohmori a, ⁎ , 1 , Mamoru Ouchida b, 1 , Takafumi Miki c , Nobuyoshi Mimaki d , Shigeki Kiyonaka c , Teiichi Nishiki a , Kazuhito Tomizawa a , Yasuo Mori c , Hideki Matsui a a Department of Physiology, Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University, 5-1Shikata-cho, 2-chome, Okayama 700-8558, Japan b Department of Molecular Genetics, Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University, Okayama 700-8558, Japan c Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan d Pediatrics, Kurashiki Medical Center, 250 Bakuro-cho, Kurashiki, Okayama 710-8522, Japan abstract article info Article history: Received 27 May 2008 Revised 23 July 2008 Accepted 25 July 2008 Available online 3 August 2008 Keywords: Severe myoclonic epilepsy in infancy Dravet syndrome SCN1A CACNB4 Genetic modifier Mutations of SCN1A, encoding the voltage-gated sodium channel α1 subunit, represent the most frequent genetic cause of severe myoclonic epilepsy in infancy (SMEI). The purpose of this study was to determine if mutations in other seizure susceptibility genes are also present and could modify the disease severity. All coding exons of SCN1B, GABRG2, and CACNB4 genes were screened for mutations in 38 SCN1A-mutation- positive SMEI probands. We identified one proband who was heterozygous for a de novo SCN1A nonsense mutation (R568X) and another missense mutation (R468Q) of the CACNB4 gene. The latter mutation was inherited from his father who had a history of febrile seizures. An electrophysiological analysis of heterologous expression system exhibited that R468Q-CACNB4 showed greater Ba 2+ current density compared with the wild-type CACNB4. The greater Ca v 2.1 currents caused by the R468Q-CACNB4 mutation may increase the neurotransmitter release in the excitatory neurons under the condition of insufficient inhibitory neurons caused primarily by the SCN1A mutation. © 2008 Elsevier Inc. All rights reserved. Introduction Severe myoclonic epilepsy in infancy (SMEI or Dravet syndrome; MIM# 607208) is a malignant epileptic syndrome beginning in the first year of life (Dravet et al., 2005). Prolonged, generalized, or unilateral clonic seizures are typically triggered by fever. The seizures often evolve into life-threatening status epilepticus. In the second year of life, other types of afebrile seizures appear, including myoclonic, absence, generalized tonic–clonic, and partial seizures. Development is normal in the first year of life followed by developmental slowing and regression. A family history of epilepsy or febrile convulsions is often observed (Commission on classification and terminology of the international league against epilepsy, 1989). Mutations of SCN1A (MIM# 182389), encoding the voltage-gated sodium channel α1 subunit (Na v 1.1), represent the most frequent genetic cause of SMEI (Claes et al., 2001; Sugawara et al., 2002; Ohmori et al., 2002). Although the percentage of family history of convulsive disorders ranges from 25% to 71% in several studies (Dravet et al., 2005), approximately 90% of SCN1A mutations in SMEI probands arise de novo (Harkin et al., 2007). Epilepsy in relatives of probands with SMEI had the characteristics of febrile seizures or idiopathic generalized epilepsy (Benlounis et al., 2001; Singh et al., 2001). SMEI probands could therefore possibly have a genetic predisposition to convulsive disorders in addition to SCN1A mutations. We hypothesized that modifier genes or seizure susceptibility genes that are carried from parents may contribute to the malignant phenotype of SMEI. We decided to screen the genes related to idiopathic epilepsy or febrile seizures plus for candidates of genetic modifiers. We screened the SCN1B, GABRG2, and CACNB4 genes in 38 SCN1A mutation-positive probands. We identified an SMEI proband who had a de novo R568X mutation of the SCN1A gene and another R468Q mutation of the CACNB4 gene that was inherited from his father who had a history of febrile seizures. Materials and methods Subjects A total of 38 previously reported SMEI probands with SCN1A mutations were recruited for this study (Hattori et al., 2008). The probands with SMEI fulfilled the following criteria: normal develop- ment before seizure onset, the occurrence of either generalized, unilateral, or partial seizures during the first year of life, seizures that Neurobiology of Disease 32 (2008) 349–354 ⁎ Corresponding author. Fax: +81 86 235 7111. E-mail address: iori@md.okayama-u.ac.jp (I. Ohmori). 1 Authors contributed equally to this work. Available online on ScienceDirect (www.sciencedirect.com). 0969-9961/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.nbd.2008.07.017 Contents lists available at ScienceDirect Neurobiology of Disease journal homepage: www.elsevier.com/locate/ynbdi