Glioma is the most common and deadly primary brain tumor. For many patients intractable headaches and epileptic seizures are early symptoms. Seizures arise from tumor associated "peritumoral" brain and are particularly common in low grade gliomas where up to 90% of patients present with spontaneous recurring seizures, or "peritumoral epilepsy" that is often refractory to treatment. The central hypothesis developed during the previous funding cycle of this grant posits that seizures are caused by the deliberate release of glutamate from gliomas into the peritumoral brain, causing abnormal neuronal glutamate receptor activation, and, over time, glutamate causes tumor associated brain regions to become hyperexcitable. It is hypothesized that glutamate is released as an obligatory by-product of cystine uptake into tumor cells via the system xc- (SXC) cystine-glutamate exchanger. In addition, the tumor may co-opt adjacent astrocytes or recruit microglial cells to release glutamate. Whether and how glutamate release from either of these cells is mechanistically involved in seizure initiation is the principle question of this proposal. This question will be addressed by comparing the seizure phenotype of tumor bearing mice generated from human xenografts with high or low SXC expression, shRNA silencing or pharmacological inhibition. The time-course of seizure development will be documented by EEG/video monitoring with the expectation that reduced SXC expression delays seizure onset and reduces seizure severity. To study cellular changes underlying the neuronal hyperexcitability, including neurotransmitter receptor and transporter expression and function, acute brain slices from animals with known seizure status will be studied biophysically. These results will be complemented by biochemical and immunohistochemical analysis of biopsies and tissue micro arrays from over 300 human glioma patients to search for changes in proteins involved in glutamate release, reuptake or catabolism. If a role for glutamate in seizure initiatio is confirmed, the availability of two FDA approved drugs, Sulfasalazine and Talampanel that inhibit either glutamate release or glutamate receptors should allow for a rapid translation towards clinical treatment of patients. Previous glioma research has almost exclusively focused on high grade gliomas and hence a novel objective of this study is a focus on a more effective management of peritumoral seizures in low grade gliomas, which present a significant clinical challenge.
Malignant brain tumors often present with epileptic seizures that are resistant to antiepileptic medications. The proposed studies will study the cellular and molecular causes of these seizures with the goal to identify novel drug targets.
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