Aberrant neuronal differentiation is common in glioma but is associated neither with epileptic seizures nor with better survival.

The mechanisms of glioma-associated seizures (GAS) have yet to be fully elucidated. Proneural subtype, isocitrate dehydrogenase 1 (IDH1) mutations, and epileptic seizures are closely associated suggesting that aberrant neuronal differentiation contributes to glioma-associated seizures. In a population-based cohort (n = 236), lack of stem cell marker expression (nestin, musashi) was significantly associated with IDH1 mutations and GAS at diagnosis. In vitro data suggested an association of IDH1 mutations and a more differentiated phenotype. Out of eight glioma stem cell (GSC) lines, seven revealed positivity for the synaptic marker protein synaptophysin. Three had synapse-like structures identified by electron microscopy and were either vGlut1 (glutamatergic) or GAD67 (GABAergic) positive. In vivo, >10% synaptophysin-positive tumour cells were present in >90% of all gliomas. Synaptophysin expression was associated with proneural subtype and vGlut1 expression, suggesting that most synapse-like structures in glioma are glutamatergic. However, we found null associations between vGlut1 protein/mRNA expression and survival, GAS at onset, development of GAS after resection, and refractory GAS. Synapse-like structures were neither functional nor activated by spontaneous action potentials or cellular networks. Thus, aberrant neuronal differentiation including glutamatergic synapse-like structures is detectable in glioma but is associated neither with epileptic seizures nor with better survival.

High expression of cystine-glutamate antiporter xCT (SLC7A11) is an independent biomarker for epileptic seizures at diagnosis in glioma.

Epileptic seizures are an important cause of morbidity in glioma patients. Substantial lines of evidence support the concept of the excitatory neurotransmitter glutamate being a crucial mediator of glioma-associated seizures. In gliomas, non-vesicular secretion of glutamate via the cystine-glutamate exchanger (SLC7A11, xCT) constitutes the main mechanism contributing to high extracellular glutamate concentrations. However, a convincing "proof-of-relevance" of this mechanism in patient material is lacking. A cohort of 229 consecutive patients with newly diagnosed glioma was analyzed with respect to presence, time course, and severity of epileptic seizures. 14 patients were excluded due to previous epileptic seizures, insufficient clinical data or insufficient tumor material. The maximal immunohistochemical expression of xCT was determined in 1-3 independent samples from central tumor areas of each tumor using tissue microarrays. In addition to histological grading of the tumors, isocitrate dehydrogenase 1 (IDH1) R132H mutational status was determined by immunohistochemistry. 215 consecutive glioma patients were included in the study (7.4% grade II, 7.0% grade III, 85.6% grade IV). High xCT expression was significantly associated with seizures at onset (p = 0.05) but not with development of seizures or with refractory seizures. Low-grade gliomas (WHO II/III) had lower xCT expression than glioblastoma (p = 0.001), and tumors without IDH1 R132H mutation tended to have higher xCT levels (p = 0.07). In a multivariate analysis, high xCT expression and WHO tumor grade but not IDH1 R132H mutation, were significantly associated with epileptic seizures at diagnosis (odds ratio 2.2, p = 0.02). Further, xCT expression did not correlate with survival (p = 0.27, log-rank test). Thus, high xCT expression is an independent marker for glioma-associated seizures at diagnosis especially in high-grade glioma, but is not associated with worse survival in our cohort.