Dissection of brain somatic mutations affecting neural circuits and behaviors

Abstract

Part1. BRAF somatic mutation contributes to intrinsic epileptogenicity in pediatric brain tumors. Pediatric brain tumors are highly associated with epileptic seizures. However, their epileptogenic mechanisms remain unclear. Here, we show that the oncogenic BRAF somatic mutation p.Val600Glu (V600E) in developing neurons underlies intrinsic epileptogenicity in ganglioglioma, one of the leading causes of intractable epilepsy. To do so, we developed a mouse model harboring the BRAFV600E somatic mutation during early brain development to reflect the most frequent mutation, as well as the origin and timing thereof. Therein, the BRAFV600E mutation arising in progenitor cells during brain development led to the acquisition of intrinsic epileptogenic properties in neuronal lineage cells, whereas tumorigenic properties were attributed to high proliferation of glial lineage cells. RNA sequencing analysis of patient brain tissues with the mutation revealed that BRAFV600E-induced epileptogenesis is mediated by RE1-silencing transcription factor (REST), which is a regulator of ion channels and neurotransmitter receptors associated with epilepsy. Moreover, we found that seizures in mice were significantly alleviated by an FDA-approved BRAFV600E inhibitor, vemurafenib, as well as various genetic inhibitions of Rest. Accordingly, this study provides direct evidence of a BRAF somatic mutation contributing to the intrinsic epileptogenicity in pediatric brain tumors and suggests that BRAF and REST could be treatment targets for intractable epilepsy.

Part2. Identification of mechanism of changes in brain network and behavior derived from somatic mutations in focal cortical dysplasia (FCD). Coexistence of a neuron with somatic mutation in normal cells has not been understood in the network level. Here, we found that seizure triggering hyperexcitability was originated from non-mutated neurons around rather than in cells carrying the mechanistic target of rapamycin complex (MTOR) somatic mutation. These neurons had no change in level of excitatory to inhibitory synapse compared to wildtype one, but their activity was decreased by treating inhibitor of adenosine kinase (ADK), which could release an extrinsic stimulant from MTOR mutant cell. Thus, brain MTOR somatic mutation underlies epileptic network by non-cell autonomous mechanism. Further, the level of brain somatic mutation leading phenotypic change is required to explain how they can change brain network and in vivo behavior. These results showed the possibility that a small number of somatic genetic mutations may alter brain networks.