Abstracts

Rationale: Somatic genetic variation in the brain is known to cause epileptogenic malformations of cortical development (MCD).  We hypothesized that somatic variation in brain might underlie a wider range of intractable neocortical epilepsies, including non-lesional focal epilepsy (NLFE). Methods: We collected brain tissue from individuals undergoing epilepsy surgery for intractable neocortical epilepsy: NLFE and MCD. Using high-depth exome sequencing and targeted deep sequencing in paired DNA from resected brain and leukocytes, we identified somatic brain-only variants from 18 individuals with NLFE and 38 with focal MCD seen on MRI (magnetic resonance imaging).  Collection of tissue samples and analysis of somatic variation is ongoing. Results: We found five out of 56 individuals with intractable neocortical epilepsy had somatic  nonsynonymous variants in SLC35A2, a UDP galactose transporter responsible for entry of galactose into the Golgi apparatus for glycosylation, present in resected brain tissue and absent from matched leukocyte DNA. Three of the five individuals with somatic SLC35A2 variants had no cortical lesion on MRI responsible for the seizures, however pathological examination in two of these three revealed focal cortical dysplasia 1a (FCD1a). Two individuals with SLC35A2 variants were identified in the MCD cohort. Both had intractable focal epilepsy and developmental delay with MRIs suggesting FCD, however FCD was not observed on pathological examination in either. In the NLFE cases the variant allele represented 2-14% of  total alleles in resected brain tissue; in the MCD cases variant allele frequency was higher (10-53%).  Conclusions: Somatic variants in SLC35A2 underlie a significant proportion of NLFE, a largely unexplained condition, as well as focal MCD, previously shown to result from somatic mutation but until now only in PI3K-AKT-mTOR pathway genes. Germline SLC35A2 variants were previously associated with glycosylation defects and rare X-linked encephalopathies. Our findings suggest variants in SLC35A2 also underlie refractory focal epilepsy when acquired post-zygotically and localized to a subset of cells in the brain. This discovery reveals new avenues for understanding pathophysiology of NLFE and MCD, and for developing novel therapies.  Funding: NINDS R01-NS089552; R01-NS094596; NS053998, NS077274, NS077303,  NS077276. BioRepository Core for Neurological Disorders, Dept of Neurology, Boston Children’s Hospital, and the IDDRC (NIH P30HD018655). Translational Research Program, Boston Children’s Hospital, and Boston Children’s Hospital Dept of Neurology Core for Neurological Diseases.Biogen, Inc.; Gilead Sciences, Inc.; UCB; Bryan ADRC NIA P30AG028377; B57 SAIC-Fredrick Inc M11-074; NINDS (RC2NS070344; RC2MH089915; U01NS077303; U01NS053998, U54NS078059, P01HD080642); NHGRI (UM1HG006504, U01HG007672); NIMH (K01MH098126, R01MH097971, R01MH099216, RC2MH089915); NIDDK (R01DK080099); NIAID (1R56AI098588-01A1); UM1AI100645, U19AI067854); NCATS (UL1TR000040); Eunice Kennedy Shriver NICHHD nt(R01HD048805); Ellison Medical Foundation AG-NS-0441-08; the Duke Chancellor’s Fund 2014; Neil Molberger Brain Research Fund; Endocrine Fellows Foundation Grant; Bill and Melinda Gates Foundation; Murdock Study Community Registry & Biorepository; Stanley Institute for Cognitive Genomics at Cold Spring Harbor; Duke Genome Sequencing Clinic; NYPH;  CU P&S; CUMC; J. Willard and Alice S. Marriott Foundation; MDA; The Nicholas Nunno Foundation; JDM Fund for Mitochondrial Research; Arturo Estopinan TK2  Fund; Endocrine Fellows Foundation; Helaine B Allen, Emily Allen Wolff.WHICAP, PO1AG07232, R01AG037212, RF1AG054023) (NIA) NCATS UL1TR001873.