Abstracts

Rationale: Ohtahara syndrome, or early-infantile epileptic encephalopathy with suppression-burst, is one of the most severe and earliest forms of epilepsy. Ohtahara syndrome is classified as a symptomatic epileptic syndrome and was believed to be non-inherited in most cases because of its frequent association with hemimegalencephaly or other brain malformations (Epilepsia, 1989; 30:389-399). Recently, two genes, ARX and STXBP1, have been found to be responsible for Ohtahara syndrome (Am J Hum Genet, 2007;81:361-366, Nat Genet, 2008;40:782-788). Longer expansion of the first polyalanine tract of ARX is causative for Ohtahara syndrome without brain malformation, while a premature termination mutation of ARX causes severe brain malformations, such as lissencephaly or hydranencephaly (Hum Mutat, 2004;23:147-159). We investigated the molecular basis of Ohtahara syndrome in two families demonstrating X-linked inheritance. Methods: Blood samples were collected from two patients with Ohtahara syndrome and from their mother, after informed consent from the guardians and approval from IRB had been obtained. Both patients developed West syndrome, and one of these later developed Lennox-Gastaut syndrome. Brain MRI of all patients showed no brain malformations. Genomic DNA was extracted and was amplified for all coding exons and flanking introns of ARX. Bidirectional direct sequencing of the purified PCR products was performed on an automated DNA sequencer. Results: Both families carried two different novel frameshift mutations in exon 5, which is the last exon of ARX. Patient 1 had a hemizygous 5-base pair (bp) duplication (1564_1568dupACGGC), resulting in a truncated protein with 523 N-terminal amino acids (A524fsX534). The mother of Patient 1 had the same mutation heterozygously. Patient 2 had a hemizygous 1-bp insertion (1604_1605insT), which was thought to result in a frameshift from at amino acid 536 and alter the position of a stop codon in the 3’ UTR (E536fsX672). Patient 2 also carried a 9-bp deletion in exon 2 (325_333delGCGGCGGCG), resulting in a contraction of the first polyalanine tract. Contractions of the polyalanine tract are generally considered not to be harmful, and it seems to be a rare polymorphism. The mother of Patient 2 carried both changes heterozygously. None of these mutations identified in our patients were found in the 200 male and 100 female controls (400 X-chromosomes). Conclusions: The etiology of Ohtahara syndrome is heterogeneous; however, the molecular analysis of ARX should be considered in sporadic or familial male patients with Ohtahara syndrome. The mutations existing in the last exon of ARX are considered to cause nonsense-mediated mRNA decay (NMD) avoidance, indicating that NMD is critically involved in the molecular mechanism of ARX-related interneuronopathies (J Child Neurol, 2005;20:392-397). The mutations impair the Aristaless domain and are thought to enhance the original transcription repression activity of ARX (Neuroscience, 2007;146:236-247); i.e. they are gain-of-function mutations.