Abstracts

Rationale: Lissencephaly is caused by disruption of neuronal migration and doublecortin protein (DCX) and tubulin are known to do critical role in migration of neurons. DCX mutations classically cause anterior dominant agyria or pachygyria and infantile-onset seizures in males and subcortical band heterotopia in females. Some studies questioned whether the location of DCX mutations can predict malformation severity or not, although genotype-phenotype correlation is still vague. We aimed to describe the clinical characteristics of a large family presenting with familial late childhood-onset focal seizures and anterior pachygyria in addition to discover a major causative gene and modifiers for the different phenotypes. Methods: Clinical, electrophysiological and brain image findings were characterized in affected patients. Whole exome sequencing was performed in two affected male cousins and one unaffected sibling, using Agilemt SureSelect Human All Exon Capture Kit and 100bp paired-end sequencing on Illumina HiSeq2000 plaform. Frequent variants were filtered out referring data from 1000 Genome Project, dbSNP129 and Korean Personal Genome Project. Effect, impact, function and conservation of variants were predicted in silico using annotation programs. The final candidate variants were genotyped in 200 neurologically normal control subjects by High Resolution Melting technique and were directly sequenced in all available blood samples from this family. Results: In affected members, seizures started after 5 years of age in all male patients but began after adolescents in females. Seizures were mainly focal dyscognitive ones and were rarely generalized ones. In males, development was delayed from infantile period and was markedly deteriorated after seizure-onset. In electroencephalography, focal epilepti-form discharges were observed and generalized discharges were rare. Brain MRI of patients showed anterior pachygyria, which was asymmetric and milder in affected females than in males. In patients and asymptomatic female carriers, a novel DCX mutation in exon 2 was identified (c.269A>G, NM_000555.3; p.D90G, NP_000546.2). The other important novel mutation was found from the gene encoding tubulin, beta-1 (TUBB1) in exon 3 (c.184C>T, NM_030773.3; p.R62fsX, NP_110400.1). It showed ‘high' impact and effect scores, ‘deleterious' on likelihood ratio test, and ‘disease causing automatic' on MutationTaster prediction. TheTUBB1 mutation was identified only in female patients with cognitive deficit and male patients with severe mental retardation. It was not found in normal individuals without a DCX mutation, asymptomatic female carriers with a DCX mutation and male patients with milder cognitive deficit. Conclusions: The novel DCX mutation in the N-terminal is a major causative gene for familial late childhood-onset focal seizures and milder form of anterior dominant pachygyria in this family and another novel mutation of TUBB1 might be a modifier for cognitive deficit in family members with a DCX mutation.