BRAF somatic mutation contributes to intrinsic epileptogenicity in pediatric brain tumors

Abstract

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 BRAF somatic mutation during early brain development to reflect the most frequent mutation, as well as the origin and timing thereof. Therein, the BRAF 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 BRAF-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 BRAF 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.

BRAFV600E Expression in Mouse Neuroglial Progenitors Increase Neuronal Excitability, Cause Appearance of Balloon-like cells, Neuronal Mislocalization, and Inflammatory Immune response

“…However, the intrinsic electrophysiological mechanisms that may lead to seizures at the single cell level in those studies were not interrogated. This may be due to previous studies on FCD and TSC cases that showed no significant increase in intrinsic excitability of malformed components, including cytomegalic neurons, balloon cells and immature misoriented neurons 51,53,54,[74][75][76][77] .…”

“…However, the intrinsic electrophysiological mechanisms that may lead to seizures at the single cell level in those studies were not interrogated. This may be due to previous studies on FCD and TSC cases that showed no significant increase in intrinsic excitability of malformed components, including cytomegalic neurons, balloon cells and immature misoriented neurons 51,53,54,[74][75][76][77] .Here we hypothesized that expression of BRAFV600E mutation associated with LNETs alters gene expression in the affected cortical tissue and increase intrinsic neuronal excitability in BRAFV600E neurons, altering passive and active electrophysiological properties. To this end we used In-utero electroporation that allows to introduce genetic manipulation into radial glia progenitor population affecting a small percentage of cells (5-10%) 78, 79 .…”

“…Reflecting on previous studies in FCD and TSC resected cortical tissue 51,54,[74][75][76][77] that showed increased action potential dependent glutamatergic synaptic events frequencies in TSC compared to FCD tissue and the opposite effect on GABAergic events frequencies. In addition, they showed large amplitude GABAergic pacemaker rhythmic events in immature looking pyramidal neurons and decreased frequencies of action potential dependent mixed glutamatergic and GABAergic events in normal looking neurons from severe FCD cortical areas.…”

Seizures in patients with cancer

“…Tumors with neuronal elements, such as ganglioglioma, provide an exception to this rule. In gangliogliomas and other pediatric brain tumors, the high frequency (on the order of 60%) of BRAF V600E mutations has been linked to epileptogenic activity through increased expression of RE1‐silencing transcription factor (REST), a regulator of gene expression for ion channel genes associated with epilepsy …”

“…In gangliogliomas and other pediatric brain tumors, the high frequency (on the order of 60%) of BRAF V600E mutations has been linked to epileptogenic activity through increased expression of RE1-silencing transcription factor (REST), a regulator of gene expression for ion channel genes associated with epilepsy. 32 In low-grade, diffuse gliomas (diffuse astrocytoma and oligodendroglioma), the higher incidence of seizures is thought to be related to isocitrate dehydrogenase (IDH) gene mutations, 33 which are present in approximately 80% of these tumors. 34 Mutant IDH converts α-ketoglutarate into D-2-hydroxyglutarate (D2-HG), an oncometabolite that, in addition to playing a role in oncogenesis through the alteration of epigenetic states, 35 has a chemical structure similar to that of glutamate and, when secreted extracellularly, is thought to produce a proepileptic state by activating neuronal NMDA receptors, creating excitatory postsynaptic potentials and increasing the likelihood of action potentials.…”