Abstracts

Rationale: Despite efforts to the pathological basis of the epilepsies, genetic and environmental factors underlying epilepsy remain unclear. To date, next-generation sequencing efforts to identify novel epilepsy risk genes have focused on the nuclear genome; this is despite the fact that there are genes associated with a seizure phenotype in the mitochondrial genome. We aim set out to sequence the mitochondrial genome in 280 patients with unexplained epilepsy, for whom sequence data is already available on nuclear-encoded mitochondrial genes associated with epilepsy.  Methods: This is a case-control association study using retrospective data. The patients are all adults (=18 years), in four syndromic groups: 1) comorbid epilepsy and learning disability of unknown cause (n=105); 2) non-lesional focal epilepsy (n=75); 3) juvenile myoclonic epilepsy (n=50); 4) mesial temporal lobe epilepsy (n=50). We compared each of these groups against healthy controls (Irish DNA Atlas study: neurologically-healthy, unrelated individuals, n=200) to identify possible pathogenic mutations. Libraries were prepared for sequencing on an Illumina MiSeq platform using the SeqCap EZ Developer Mitochondrial Genome kit (Roche Nimblegen). Variants were called according to mtDNA-server pipeline. Firstly, we filtered the variants removing synonymous and common variants. We used fourteen algorithms to predict pathogenicity (MitImpact).  We then checked if the variants were associated with described mitochondrial diseases.  Results: To date we have sequenced 229 cases and 150 controls. The average coverage was 1077X. Haplogroups are groups of similar haplotypes (groups of genes) that share several polymorphisms. Among the patients, the distribution of haplogroups was 33%H, 4%HV, 7%I, 11%J, 11%, 1%R, 10%T, 16%U, 4%V, 1%W and in controls was 37%H, 1%HV, 3%I, 13%J, 8%, 1%L3, 9%T, 19%U, 5%V, 3%W, 3%X2. Differences between case and control haplogroup frequencies were insignificant, and reflected previously described haplogroup frequencies in Ireland. We identified 14 high pathogenic score variants across 20 patients. These variants are located in seven genes (MT-ATP6, MT-CO3, MT-CYB, MT-ND1, MT-ND2, MT-ND4, MT-ND5). These genes encode structural subunits of the mitochondrial respiratory chain: complex I, III, IV and V. Mutations in these genes have been reported in several different mitochondrial disorders, including Leber hereditary optic neuropathy (LHON), MELAS syndrome, Leigh syndrome and complex I deficiency, mitochondrial encephalomyopathy, seizures and lactic acidosis and Parkinsonism/MELAS overlap syndrome. Furthermore, we identified two variants that were reported in mitochondrial disease, one is associated with Maternally Inherited Nonsyndromic Deafness (m.3388C>A) and another is associated with LHON (m.4640C>A). Conclusions: Interesting variants have been identified up to date. Additional investigation to confirm the pathogenicity of these variants is necessary. Funding: Irish Institute of Clinical NeuroscienceCAPES