Abstracts

Rationale: Non-lesional focal epilepsy (NLFE) consists of recurrent seizures arising from an area of the brain with no MRI abnormality. Surgical cure rates in NLFE are far lower than in cases with an identifiable lesion; better understanding of NLFE pathology is critical to improve treatment. Our ongoing study is based on the hypotheses that two types of molecular abnormalities may give rise to seizures in NLFE: 1.somatic mutation (e.g. arising in brain but not in the germline) or 2. the presence of foreign (e.g. viral) nucleic acids that may result in molecular or microstructural abnormalities. We examine brain tissue removed at epilepsy surgery from individuals with NLFE to identify whether somatic mutation or foreign microbial nucleic acids underlie seizure foci, and to determine the range of pathologies in histological samples of individuals with NLFE. Methods: We enrolled surgical candidates with focal onset epilepsy and no causative lesion on MRI at Columbia University and NYU Langone Medical Centers. Medical records were reviewed for etiology and phenotypic characteristics. Seizure protocol MRI (1.5 or 3T), preoperative EEG, and intracranial electrocorticography data were collected. Brain tissue from the seizure focus was examined for pathological changes including histological and immunohistochemical analyses, evidence of alterations in cortical lamination, columnar organization, and cell lineage. Brain tissue and blood were collected for sequencing to identify somatic mutation or foreign nucleic acids in the seizure focus. Results: 299 participants have been screened and 17 enrolled, 15 of whom have had epilepsy surgery (Fig. 1). Pathology is summarized in Table 1. 6/15 had no abnormalities or non-specific reactive changes including microgliosis, astrogliosis, and Chaslin's marginal sclerosis. The remaining 9 revealed subtle focal disorganization in cortical architecture including abnormal radial arrangements (FCD type Ia), abnormal tangential lamination (FCD type Ib), focal blurring of the cortical-white matter junction, and small clusters of neurons with disoriented dendrites, Five participants with NLFE without likely causal germline DEPDC5 mutations were screened for the presence of somatic mutations by sequencing of leukocytes of 281 mTOR-PI3K-AKT3-signaling pathway genes in brain tissue and leukocyte DNA. On average 16 candidate somatic mutations were identified per sample, each present in < 2% of sequencing reads. Confirmation of candidate somatic mutations is on-going. To date, analyses of 10 brain tissue samples have identified no foreign nucleic acids from an infectious pathogen. Deeper sequencing of different sampling regions is planned. Conclusions: These data support our hypotheses that a) some NLFE cases indeed have no structural abnormality, and b) standard neuroimaging studies may miss subtle abnormalities underlying a seizure focus. Preliminary identification of candidate mutations in mTOR pathway genes underscores the importance of a search for a molecular lesion. Identification of somatic mutations in NLFE may target relevant biological pathways and provide novel treatment strategies and biomarkers for epileptogenesis. Funding: NIH/NINDS R01NS089552 EUREKA (Multi-PI: Melodie R. Winawer, Columbia University, Peter Crino Temple University) Discovery of Novel Molecular Abnormalities Underlying Non-Lesional Focal Epilepsy