Understanding the role of the Glycine Cleavage System in Neural Tube Defects

神经管缺陷(NTD)是一种常见的出生缺陷，发生在怀孕早期，由神经管的不完全形成引起，后来会发展到大脑和脊髓。结果，胎儿的大脑和/或脊髓受到不可逆转的损害，导致出生前后死亡，或幸存儿童长期残疾。NTDS最常见的形式是影响大脑的无脑畸形和影响下段脊髓的脊柱裂。这种疾病的发病率约为每1000例孕妇中就有1-2例，全球每年至少新增病例17万例。NTDS的风险既取决于遗传因素，也取决于环境影响，如母亲的饮食、糖尿病或接触某些化学物质。由于许多可能的促成因素，任何受影响个体的NTDS的确切原因通常是未知的。然而，如果母亲在怀孕前和怀孕早期服用叶酸补充剂，受影响怀孕的风险可以大大降低。不幸的是，并不是所有的NTDS都能被叶酸预防--可能高达50%的病例没有反应--因此需要额外的治疗。为了在预防所有非肿瘤疾病方面取得进一步进展，我们需要更好地了解其原因，特别是增加个人患非肿瘤疾病风险的基因。此外，重要的是确定新的NTDS预防疗法，这些疗法可以单独使用，也可以与叶酸联合使用。在已确定遗传风险因素的家庭中，这也意味着可以提供针对家庭的治疗。所有细胞都需要有效地处理与叶酸相关的小分子叶酸，以实现许多不同的功能。一些NTDS似乎是由胚胎细胞处理叶酸的方式遗传异常引起的。我们研究了一组被称为“甘氨酸裂解系统”(GCS)的蛋白质，它们与叶酸的处理有关。一些NTDS患者在这些蛋白上存在缺陷，而未受影响的人则没有。这一发现表明，GCS的问题可能直接导致NTDS。为了支持这一想法，具有GCS缺陷的小鼠胚胎也会发育成NTD。该项目将利用GCS中缺乏甘氨酸脱羧酶(GLDC)功能的小鼠模型。小鼠模型提供了一个机会来研究这些叶酸处理蛋白在胚胎中的作用以及如何预防相关的NTD。我们已经发现，在缺乏GLDC的胚胎中，叶酸的处理方式发生了变化，现在我们将使用详细的代谢研究来准确地找出这些变化是如何发生的。我们将使用遗传方法关闭GLDC功能或仅在神经管中恢复功能。这将告诉我们，发育中的胚胎中哪些组织需要GLDC功能，以确保正常发育。下一步是了解GLDC缺陷胚胎中叶酸代谢的哪些输出受到干扰，并测试哪些变化导致NTDS。这将通过使用特定细胞变化的标记进行详细的生化分析和胚胎成像，然后使用补充剂来纠正这些缺陷来实现。叶酸代谢对于细胞分裂所需的DNA的合成以及改变基因表达和其他功能的甲基化反应是必不可少的。可能还需要叶酸代谢来调节细胞内可能造成损害的活性氧物种(自由基)的水平。这些分子的产生和移除的不平衡被称为氧化应激，在包括糖尿病在内的许多疾病中都是重要的。我们将研究GLDC缺乏是否会在发育中的胚胎中引起氧化应激，这可能会导致NTDS。了解GLDC缺陷胚胎中NTDS的分子和细胞原因是开发新的治疗方法以预防人类NTDS的重要一步，而不是目前仅使用叶酸。

Neural tube defects (NTDs) are common birth defects that arise in early pregnancy caused by incomplete formation of the neural tube, which will later develop into the brain and spinal cord. As a result, the brain and/or spinal cord of the fetus become irreversibly damaged, resulting in death around birth, or long term disability in surviving children. The most common forms of NTDs are anencephaly, affecting the brain, and spina bifida, which affects the lower spinal cord. They occur in approximately 1-2 per 1,000 pregnancies and total at least 170,000 new cases per year worldwide.The risk of NTDs depends on both inherited factors and environmental influences such as maternal diet, diabetes or exposure to certain chemicals. Because of the many possible contributory factors, the exact causes of NTDs in any affected individual are usually unknown. However, the risk of an affected pregnancy can be substantially reduced if the mother takes folic acid supplements before and during early pregnancy. Unfortunately, not all NTDs are prevented by folic acid - perhaps up to 50% of all cases fail to respond - and so additional therapies are needed. In order to make further progress towards prevention of all NTDs we need a better understanding of their causes, in particular the genes that increase a person's risk of NTD. Moreover, it will be important to identify new preventive therapies for NTDs which may be used individually or in combination with folic acid. In families where genetic risk factors have been identified this also means that family-specific therapies may be offered.All cells require efficient handling of small molecules called folates, which are related to folic acid, for many different functions. It appears that some NTDs are caused by an inherited abnormality in the way cells in the embryo handle folates. We studied a group of proteins called the "glycine cleavage system" (GCS), that are involved in folate handling. Some patients with NTDs had defects in these proteins, whereas unaffected people did not. This finding suggests that problems with the GCS may directly cause NTDs. In support of this idea, mouse embryos that have GCS defects also develop NTDs.This project will make use of mouse models lacking function of glycine decarboxylase (Gldc), part of the GCS. The mouse models provide an opportunity to study the role of these folate handling proteins in the embryo and how the associated NTDs may be prevented. We have found that handling of folates is altered in Gldc-deficient embryos and we will now use detailed metabolic studies to work out exactly how these changes come about. We will use genetic approaches to turn off Gldc function or to restore function only in the neural tube. This will tell us which tissues in the developing embryo need Gldc function, to ensure normal development.The next step is to understand which of the outputs of folate metabolism are disrupted in Gldc-deficient embryos and to test which of the changes are responsible for NTDs. This will be achieved by detailed biochemical analysis and embryo imaging using markers of particular cellular changes, followed by use of supplements to correct these defects. Folate metabolism is essential for synthesis of DNA, which is needed for cells to divide, and methylation reactions that modify gene expression and other functions. Folate metabolism may also be needed for regulation of the level of reactive oxygen species (free radicals) which can be damaging within cells. An imbalance in production and removal of these molecules, termed oxidative stress, is important in a number of diseases, including diabetes. We will examine whether Gldc-deficiency causes oxidative stress in developing embryos, that may contribute to NTDs.Understanding the molecular and cellular causes of NTDs in Gldc-deficient embryos is an important step towards developing new therapies to prevent more NTDs in humans than is currently possible using folic acid alone.