Abstract 4144: The role of GABA metabolism in medulloblastoma spread
Authors: Martirosian Vahan , Lin Michelle , Chen Thomas C. , Neman Josh 
Published: August 2018

Central nervous system (CNS) tumors remain the leading cause of cancer-related childhood mortality. Amongst these, medulloblastoma (MB), a highly malignant cancer originating in the cerebellum, remains the most common pediatric CNS tumor. Patients are clinically stratified by age at diagnosis, extent of resection, and metastatic status. Patients with incomplete tumor resection or leptomeningeal spread at the time of diagnosis are considered high-risk. Standard of care for all patients consists of surgical resection followed by radiation and chemotherapy. MB tumors have recently been molecularly stratified into 4 groups, with each group associated with a particular effected pathway. Group 1 and 2 have deregulated WNT and SHH pathways, respectively. Group 3 and 4, however, are less characterized, but have been shown to be associated with a GABAergic and glutamanergic phenotype, respectively. While Group 1 and 2 MBs have an overall 5-year survival rate of 75%, Group 3 and 4 have a worse prognosis, with survival rates closer to 30-40%. This is correlated with the latter 2 groups having an increased rate of metastasis at diagnosis, as metastatic lesions are more advanced and therapeutically difficult to treat. Therefore, efforts must be made to elucidate mechanisms and treatments of metastasis, as well as finding subgroup specific therapeutic targets.

Although canonical cancer therapy counters the tumors' effects on the surrounding environment, manipulating the effects of the tumor microenvironment on the tumor has also shown promise as a treatment regimen. Given that the cerebellum, the initiation site for MB, has high concentrations of GABA, and Group 3 tumors have upregulation of GABAergic properties, we explored GABA's role in this context. For these studies, we interrogated the GABA shunt pathway - the principal pathway for GABA metabolism - in two Group 3 MB cell lines. We utilized lentivirus mediated gene knockdown of GABA Transaminase (ABAT), the initial protein in the GABA shunt, to deduce phenotypic and genotypic changes in these cells. Cells supplemented with GABA had an increased ability to detach and form spheres. These spheres thrived in GABA supplemented floating conditions, while no similar phenomenon was seen in cells cultured in control medium or GABA supplemented ABAT knockdown cells. Genotypic analysis of these cells using RT-qPCR revealed an increase in the epithelial-to-mesenchymal transition profile of GABA supplemented spheres as compared to control conditions. To determine if knocking down ABAT changes tumor seeding capabilities in vivo, we transplanted MB cells into the lateral ventricle of athymic nude mice. Control MBL cells regardless of prior exposure to GABA or floating conditions, were able to seed and form tumors; While ABAT KD cells were unable to do so. These findings provide evidence that GABA metabolism in MB metastatic cells may be imperative for survivability in the CSF and leptomeningeal spread.

Citation Format: Vahan Martirosian, Michelle Lin, Thomas C. Chen, Josh Neman. The role of GABA metabolism in medulloblastoma spread [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4144.