Analysis of DNA from brain tissue on stereo-EEG electrodes reveals mosaic epilepsy-related variants

Somatic mosaic variants contribute to focal epilepsy, but genetic analysis has been limited to patients with drug-resistant epilepsy (DRE) who undergo surgical resection, as the variants are mainly brain-limited. Stereoelectroencephalography (sEEG) has become part of the evaluation for many patients with focal DRE, and sEEG electrodes provide a potential source of small amounts of brain-derived DNA. We aimed to identify, validate, and assess the distribution of potentially clinically relevant mosaic variants in DNA extracted from trace brain tissue on individual sEEG electrodes. We enrolled a prospective cohort of eleven pediatric patients with DRE who had sEEG electrodes implanted for invasive monitoring, one of whom was previously reported. We extracted unamplified DNA from the trace brain tissue on each sEEG electrode and also performed whole-genome amplification for each sample. We extracted DNA from resected brain tissue and blood/saliva samples where available. We performed deep panel and exome sequencing on a subset of samples from each case and analysis for potentially clinically relevant candidate germline and mosaic variants. We validated candidate mosaic variants using amplicon sequencing and assessed the variant allele fraction (VAF) in amplified and unamplified electrode-derived DNA and across electrodes. We extracted DNA from >150 individual electrodes from 11 individuals and obtained higher concentrations of whole-genome amplified vs unamplified DNA. Immunohistochemistry confirmed the presence of neurons in the brain tissue on electrodes. Deep sequencing and analysis demonstrated similar depth of coverage between amplified and unamplified samples but significantly more called mosaic variants in amplified samples. In addition to the mosaic PIK3CA variant detected in a previously reported case from our group, we identified and validated four potentially clinically relevant mosaic variants in electrode-derived DNA in three patients who underwent laser ablation and did not have resected brain tissue samples available. The variants were detected in both amplified and unamplified electrode-derived DNA, with higher VAFs observed in DNA from electrodes in closest proximity to the electrical seizure focus in some cases. This study demonstrates that mosaic variants can be identified and validated from DNA extracted from trace brain tissue on individual sEEG electrodes in patients with drug-resistant focal epilepsy and in both amplified and unamplified electrode-derived DNA samples. Our findings support a relationship between the extent of regional genetic abnormality and electrophysiology, and suggest that with further optimization, this minimally invasive diagnostic approach holds promise for advancing precision medicine for patients with DRE as part of the surgical evaluation.