Mutant IDH1 and seizures in patients with glioma

Abstract

Objective: Because the d-2-hydroxyglutarate (D2HG) product of mutant isocitrate dehydrogenase 1 (IDH1^mut) is released by tumor cells into the microenvironment and is structurally similar to the excitatory neurotransmitter glutamate, we sought to determine whether IDH1^mut increases the risk of seizures in patients with glioma, and whether D2HG increases the electrical activity of neurons.

Methods: Three WHO grade II-IV glioma cohorts from separate institutions (total N = 712) were retrospectively assessed for the presence of preoperative seizures and tumor location, WHO grade, 1p/19q codeletion, and IDH1^mut status. Rat cortical neurons were grown on microelectrode arrays, and their electrical activity was measured before and after treatment with exogenous D2HG, in the presence or absence of the selective NMDA antagonist, AP5.

Results: Preoperative seizures were observed in 18%-34% of IDH1 wild-type (IDH1^wt) patients and in 59%-74% of IDH1^mut patients (p < 0.001). Multivariable analysis, including WHO grade, 1p/19q codeletion, and temporal lobe location, showed that IDH1^mut was an independent correlate with seizures (odds ratio 2.5, 95% confidence interval 1.6-3.9, p < 0.001). Exogenous D2HG increased the firing rate of cultured rat cortical neurons 4- to 6-fold, but was completely blocked by AP5.

Conclusions: The D2HG product of IDH1^mut may increase neuronal activity by mimicking the activity of glutamate on the NMDA receptor, and IDH1^mut gliomas are more likely to cause seizures in patients. This has rapid translational implications for the personalized management of tumor-associated epilepsy, as targeted IDH1^mut inhibitors may improve antiepileptic therapy in patients with IDH1^mut gliomas.

Figure 1. Chemical structure of d-2-hydroxyglutarate (D2HG) and the close relationship of mutations in isocitrate dehydrogenase 1 (IDH1^mut) glioma cells to neurons

(A) Hematoxylin & eosin section of the left frontal cortex from a 34-year-old man who presented with seizures shows scattered infiltrative oligodendroglioma cells that were highlighted by R132H IDH1 immunohistochemistry (B). Many tumor cells were closely apposed to neurons, even wrapping their processes around the neurons (B, inset). (C) The chemical structure of D2HG, compared with the structure of the excitatory neurotransmitter, glutamate.

Figure 2. Seizures and mutations in isocitrate dehydrogenase 1 (IDH1^mut) in patients with glioma

(A) Patients with newly diagnosed infiltrative gliomas from 3 cohorts were assessed according to whether seizures were a part of their presenting symptoms, segregated into the 3 main molecular subgroups of gliomas: wild-type isocitrate dehydrogenase 1 (IDH1^wt) (yellow), IDH1^mut (white), and IDH1^mut with 1p/19q codeletion (blue). (B, C) Overall survival in cohorts 1 (B) and 3 (B), according to IDH1^mut and seizures on clinical presentation. p Values were calculated based on log-rank tests between IDH1^wt or IDH1^mut curves. (Cohort 2 lacked sufficient follow-up data for survival analyses.)

Figure 3. Effects of d-2-hydroxyglutarate (D2HG) on neuronal activity

(A, B) Raster plots of cultured rat cortical neurons, before (A) and after (B) treatment with 10 mM D2HG. (C, D) Network burst duration (C) and frequency (D), before and after treatment with 10 mM D2HG. (E) Neuronal activity, expressed as mean firing rate relative to baseline, in the presence of increasing concentrations of D2HG. p = 0.003 Via one-way analysis of variance, *p < 0.05 vs 0 and 1 mM via post hoc Tukey test. (F) Mean firing rate of cultured neurons with control vehicle (0 mM D2HG), 10 mM D2HG, or 10 mM D2HG + 100 µM AP5. ***p < 0.001. (C, D, F) Seven wells per condition from 2 biological replicates; E shows 5 wells per condition from 2 biological replicates.

Figure 4. Proposed proseizure effect of isocitrate dehydrogenase 1 (IDH1^mut) glioma cells

IDH1^mut glioma cells infiltrate gray matter and track along neurons. The d-2-hydroxyglutarate (D2HG) product of the mutant IDH1 enzyme is released by tumor cells and activates neuronal NMDA receptors, thereby creating excitatory postsynaptic potentials and increasing the likelihood of action potentials.