Expression of synaptic vesicle protein 2A in epilepsy-associated brain tumors and in the peritumoral cortex

Abstract

Synaptic vesicle protein 2A (SV2A) has been identified as the binding site for the antiepileptic drug levetiracetam and is thought to decrease neuronal excitability. Since knockout of SV2A in mice leads to seizures, we hypothesized that a reduction in SV2A expression promotes seizure generation in epilepsy-associated brain tumors. We compared the SV2A expression and distribution in surgically removed tumor tissue (n = 63) and peritumoral cortex (n = 31) of patients with glial and glioneuronal tumors to normal control cortex obtained at autopsy in nonbrain tumor patients (n = 6). Additionally, we compared the SV2A expression and distribution in tumor patients with epilepsy (n = 39) with SV2A expression in tumor patients without epilepsy (n = 24). Immunohistochemistry in control cortex demonstrated strong and diffuse SV2A immunoreactivity (IR) throughout all cortical layers. Similar strong SV2A IR (with the same diffuse distribution pattern) was observed in the peritumoral cortical specimens in both patients with and without epilepsy. Modest SV2A IR was observed within the tumor area. The SV2A-positive cells detected within the tumor area were mainly entrapped neurons. Oligodendrogliomas and glioneuronal tumors displayed variable SV2A neuropil staining. In ganglioglioma (GG), strong SV2A IR was present along the dysplastic neuronal cell borders and processes. In both GG and dysembryoplastic neuroepithelial tumors, SV2A IR was occasionally observed within the neuronal perikarya. We found no differences in SV2A expression in the peritumoral cortex between the patients with and without epilepsy, which suggests that the role of SV2A in epileptogenesis in patients with glial tumors is questionable. The distinct pattern of SV2A IR in glioneuronal tumors suggests a redistribution of SV2A.

Fig. 1.

SV2A IR in control, peritumoral cortex, and glial tumors. (A) and (B) Histologically normal cortex (CTX) and peritumoral CTX showing diffuse and strong neuropil SV2A IR. Inserts in (A) show colocalization of synaptophysin (red; a) with SV2A (green; b); c, merged image. (C) and (D) SV2A IR in astrocytomas (astrocytomas WHO grade II and anaplastic astrocytomas, WHO grade III) showing low SV2A IR within the tumor area, with occasional expression in few residual neurons in the infiltration zone of the lesion (arrows in C); insert in (C) shows colocalization (yellow; merged image) of synaptophysin (red) with SV2A (green). (E) and (F) SV2A IR in GBM showing low SV2A IR within the tumor area, with occasionally focal areas of immunostained tumor cells. Insert in (E), merged image, shows GFAP-positive cells (red) without SV2A (green) IR. Insert in (F), merged image, shows occasional colocalization of SV2A (green) with GFAP (red) in a tumor cell. (G) and (H) SV2A IR in oligodendrogliomas showing variable matrix IR for SV2A in two anaplastic oligodendroglioma (Oligo, WHO grade III). Scale bar in (A). (A) and (B) 70 µm; (C) and (E) 80 µm; (F)–(H) 40 µm.

Fig. 2.

Expression of SV2A in glial and glioneuronal tumors (A). Representative immunoblot of SV2A in total homogenates from 3 cortex (Ctx), 3 astrocytomas grade II (Astr), 3 glioblastoma multiforme (GBM), and 3 ganglioglioma (GG). (B) Densitometric analysis of Western blots. Values (Optical density (OD) units) are mean ± SEM of 3 control cortex (Ctx), 3 Astr II, 3 GBM, and 3 GG relative to the OD of β-actin; all tumor specimens exhibited lower SV2A expression compared with control cortex. *P < .05.

Fig. 3.

SV2A IR in glioneuronal tumors GGs, gangliogliomas; DNT, dysembryoplastic neuroepithelial tumors. (A)–(D) SV2A IR in GG showing variable matrix SV2A IR within the tumor area with strong SV2A IR along the dysplastic neuronal cell borders and processes (arrows in [A]–[C]). SV2A IR is occasionally observed within the neuronal perikarya (arrows in [B] and [C]). (D) Coexpression of synaptophysin (a, SYN; red) with SV2A (b; green) in a dysplastic neuronal cell (c, merged image). (E)–(G) SV2A IR in DNT showing low SV2A IR within the tumor area ([E]–[F]), but diffuse and strong neuropil IR in the peritumoral cortex (Peritumoral CTX [E]). SV2A IR is occasionally observed within the neuronal perikarya within the tumor (arrow in [G]); the surrounding oligodendrocyte-like cells are negative. Scale bar: (A)–(C), (F) 40 µm; (D) and (G) 20 µm; (E) 286 µm.