Abstracts

Rationale: Dravet syndrome is an infantile-onset epileptic encephalopathy characterized by refractory seizures, cognitive arrest or regression associated with frequent epileptiform activity, and a poor prognosis. Heterozygous mutations in SCN1A, the gene that encodes the voltage-gated sodium channel alpha 1 subunit, are found in 80% of patients with Dravet syndrome. Our lab has recently identified GABRA1 and STXBP1 as novel genetic causes of Dravet syndrome (Carvill et al, 2014), explaining a few additional cases. Despite these advances, in almost 20% of patients with Dravet syndrome, the molecular diagnosis is still unknown. Given the large proportion of cases explained by a small number of genes, we hypothesize that somatic mutations in these known genes (SCN1A, GABRA1, and STXBP1) are causative in a subset of currently unsolved cases. Methods: We are screening for somatic mutations in a cohort of 51 patients with unexplained Dravet syndrome that have already undergone exome sequencing or candidate gene screening. Targeted sequencing using single-molecule Molecular Inversion Probes (smMIPs), a technology that can detect allele frequencies as low as 0.2% (Hiatt et al., 2013), is being used to screen for low-level somatic mosaicism. We are testing DNA from blood in all patients and from a second tissue (buccal swab) when possible. Testing another tissue type, such as buccal cells, may yield additional mutations not detected in blood. Results: We have collected blood samples from 51 patients with unsolved Dravet syndrome; for 10 cases we also have DNA from buccal swabs. We have performed the smMIPs capture protocol and high-throughput DNA sequencing and variant calling is underway. Conclusions: Somatic mutations, well recognized as disease causing in cancer, are implicated in more than 30 monogenic disorders. These studies are likely to discover previously undetected mutations that will identify the cause in some of the remaining patients with unexplained Dravet syndrome.