The Role of GABA Transaminase ABAT in Pediatric Brain Tumor Medulloblastoma Development and Spread

髓母细胞瘤（MB）是最常见的小儿脑肿瘤起源于富含GABA的小脑。40% MB 患者表现为小脑和脑脊髓液（CSF）软脑膜转移，导致令人沮丧的 成果。该项目的长期目标是确定MB如何成为威胁生命的软脑膜转移， 从神经发育和癌症神经科学的角度研究肿瘤和大脑微环境。虽然 在第3 MB组中已经描述了异常的GABA能受体激活，直到我们最近的研究还没有 阐明受体非依赖性GABA代谢对MB发病和转移的作用。总的来说， 我们首次确定GABA代谢分流酶GABA转氨酶（ABAT）的表达与 MB利用它在脑脊液微环境中生存，并促进小脑发育。 软脑膜播散具体地说，我们发现在原发性小脑部位的增殖性MB细胞具有低ABAT。 然而，ABAT阳性MB细胞的亚群在原发性小脑部位显示神经元样特征 （包括代谢）并且是MB转移的种子。除了作为一种GABA代谢酶，我们还确定 ABAT的表达通过显著降低组蛋白第4位赖氨酸的乙酰化而诱导表观遗传修饰， 组蛋白H3蛋白（H3K4ac）的残基，并显着增加组蛋白去乙酰化酶活性。此外，ABAT 表达波动取决于肿瘤微环境中代谢物的变化， 上调ABAT表达。我们发现转移性MB细胞需要ABAT来维持在代谢物缺乏的环境中的生存能力。 CSF通过使用GABA通过GABA代谢分流作为能源替代品，从而促进软脑膜 转移形成因此，我们假设ABAT具有双重生物学作用：在正常情况下缺乏表达 小脑的发育有助于增殖，导致第3组髓母细胞瘤的形成; MB发展后的过度表达导致肿瘤休眠，导致播散细胞的存活益处， 软脑膜转移利用神经生物学的基础，使用：a）人类和小鼠来源的神经干细胞， 神经元和神经胶质细胞，B）患者异种移植物的独特强度，c）BarTeL转基因小鼠，d）新型小分子ABAT 作为一种跨血脑屏障的抑制剂，我们将首先探讨ABAT在小脑神经发育中的作用 和组3 MB起始，核ABAT通过组蛋白乙酰化修饰对转录的第二个贡献， 第三，研究ABAT作为MB转移的潜在治疗靶点。目前的提案将进一步减少 了解转移性MB细胞如何发育和适应其神经生态位，为新的 对患有这种毁灭性疾病的儿童进行治疗干预。

Medulloblastoma (MB), is the most common pediatric brain tumor originating in the GABA-rich cerebellum. 40% of MB patients present as both cerebellar and cerebral spinal fluid (CSF) leptomeningeal metastases which results in having dismal outcomes. The project’s long-term goal is to determine how MB become life-threatening leptomeningeal metastases by studying the tumor and brain microenvironment from a neurodevelopment and cancer neuroscience perspective. Although abnormal GABAergic receptor activation has been described in group 3 MB, no studies until ours recently had yet to elucidate the contribution of receptor-independent GABA metabolism to MB pathogenesis and metastasis. Overall, we were the first to identify GABA metabolic shunt enzyme GABA Transaminase (ABAT) expression is correlated with cerebellar development and is used by MB to survive in the cerebrospinal fluid microenvironment and promote leptomeningeal dissemination. Specifically, we show proliferative MB cells at the primary cerebellar site have low ABAT. However, a sub-population of ABAT positive MB cells at the primary cerebellar site display neuron-like characteristics (including metabolism) and are the seeds of MB metastasis. In addition to being a GABA metabolic enzyme, we determined that expression of ABAT induces epigenetic modification by significantly reducing histone acetylation at the 4th lysine residue of the histone H3 protein (H3K4ac) and significantly increase in histone deacetylase activity. Furthermore, ABAT expression fluctuates depending on metabolite changes in the tumor microenvironment, with nutrient-poor conditions upregulating ABAT expression. We found metastatic MB cells require ABAT to maintain viability in the metabolite-scarce CSF by using GABA through the GABA metabolic shunt as an energy source substitute, thereby facilitating leptomeningeal metastasis formation. Therefore, we hypothesize ABAT has a dual-biological role: lack of its expression during normal cerebellar development contributes to proliferation leading to group 3 medulloblastoma formation; While its overexpression after MB development results in tumor dormancy leading to survival benefit in disseminated cells in leptomeningeal metastasis. Utilizing foundations of neurobiology by using: a) human and mouse-derived neural stem cells, neurons and glial cells, b) unique strengths of patient-xenografts, c) BarTeL transgenic mice, d) novel small molecule ABAT inhibitor which crosses the blood-brain barrier, we will: 1st interrogate the role of ABAT in cerebellar neurodevelopment and Group 3 MB initiation, 2nd contribution of nuclear ABAT to transcription through histone acetylation modification in metastases, 3rd investigate ABAT as a potential therapeutic target for MB metastases. The current proposal will shed further light on understanding how metastatic MB cells develop and adapt to their neural niche opening avenues for novel therapeutic interventions for children that have this devastating disease.