Mechanistic and metabolomic underpinnings of ALDH1L1 polymorphisms in the regulation of glycine metabolism

ALDH1L1是叶酸代谢中的一种常见酶，它将10-甲酰基四氢叶酸(10-甲酰-THF)转化为四氢叶酸(THF)。已知该反应调节从头合成嘌呤和叶酸依赖的同型半胱氨酸重新甲基化循环。它还可以在控制一碳基团流向合成代谢途径方面发挥关键作用。为了支持ALDH1L1基因在叶酸代谢调节中的作用，我们最近发现，ALDH1L1基因的缺失会导致功能性叶酸缺乏，即使在小鼠有足够的叶酸摄入的情况下也是如此。对ALDH1L1基因敲除小鼠的分析还表明，该酶是肝脏甘氨酸代谢的主要调节因子：KO小鼠的甘氨酸和甘氨酸结合物水平较低，表明该酶参与了依赖叶酸从丝氨酸合成甘氨酸的过程。我们进一步报道了人类ALDH1L1在rs3796191、rs2886059、rs9282691、rs2276724、rs1127717和rs4646750等6个常见的非同义外显子SNPs，与这些SNPs相关的单倍型在不同种族之间存在显著差异。我们对已建立的西班牙裔儿童队列Viva La Familia的分析显示，携带rs2276724和rs3796191的儿童血清甘氨酸显著降低，丝氨酸/甘氨酸比率增加，这表明丝氨酸到甘氨酸的转化放松了管制，并重新证实了我们的小鼠的研究结果。在该队列中，ALDH1L1非同义SNPs也与代谢应激和肥胖的标志物相关。根据我们的发现，我们假设非同义的ALDH1L1 SNP产生催化活性和/或稳定性改变的酶变体，影响它们代谢10-甲酰-四氢呋喃的能力，从而解除对甘氨酸代谢的调控。因此，具有特定单倍型的个体具有不同的四氢呋喃/10-甲酰-四氢呋喃和丝氨酸/甘氨酸的比率，并改变甘氨酸及其结合物的水平，代谢型的扰动代表代谢健康的标志。这项提案将解决单倍型特异性ALDH1L1变体如何影响叶酸代谢和整体细胞代谢型的问题，以及叶酸补充如何改变单倍型特异性效应的问题，通过追求以下具体目标。目的1.对人类常见单倍型的ALDH1L1酶变异体进行功能鉴定。目的2.确定主要ALDH1L1单倍型对细胞代谢的影响以及对不同叶酸补充的单倍型特异性反应。目的3.将ALDH1L1单倍型与人类甘氨酸代谢和健康结局的叶酸依赖调节联系起来。ALDH1L1变异体在不同的人群中非常常见，但它们在叶酸稳态和代谢性疾病病因学中的作用在很大程度上尚不清楚。预计ALDH1L1单倍型不同地介导了饮食中叶酸的代谢反应，这可能需要携带某些ALDH1L1 SNPs的个体调整叶酸摄入量。通过填补这一知识空白，拟议的研究将提供对ALDH1L1 SNPs代谢调节的机械性洞察，并将为评估群体特有的ALDH1L1单倍型作为疾病风险因素奠定基础。

ALDH1L1, a common enzyme in folate metabolism, converts 10-formyltetrahydrofolate (10-formyl-THF) to tetrahydrofolate (THF). This reaction is known to regulate the de novo purine biosynthesis and folate-dependent homocysteine re-methylation cycle. It could also play a key role in controlling the flux of one-carbon groups to anabolic pathways. In support of the role in the regulation of folate metabolism, we have recently shown that the loss of the ALDH1L1 gene in knockout mice causes functional folate deficiency, even when the mice had sufficient folate intake. Analysis of Aldh1l1 knockout mice also showed that the enzyme is the main regulator of glycine metabolism in the liver: KO mice have lower levels of glycine and glycine conjugates, indicating that the enzyme is involved in the folate-dependent synthesis of glycine from serine. We have further reported that human ALDH1L1 has six common non-synonymous exonic SNPs at the polymorphic loci rs3796191, rs2886059, rs9282691, rs2276724, rs1127717 and rs4646750 with the occurrence of haplotypes associated with these SNPs being remarkably different between ethnic populations. Our analysis of an established cohort of Hispanic children, Viva La Familia, has shown a significant reduction of serum glycine and increase in serine/glycine ratio in children with rs2276724 and rs3796191, indicating deregulation of serine to glycine conversion and re- capitulating our mouse findings. ALDH1L1 non-synonymous SNPs were also associated with markers of metabolic stress and adiposity in this cohort. Based on our findings, we hypothesize that non-synonymous ALDH1L1 SNPs produce enzyme variants with altered catalytic activity and/or stability that affects their ability to metabolize 10-formyl-THF and thus deregulates glycine metabolism. Accordingly, individuals with specific haplotypes have different ratios of THF/10-formyl-THF and serine/glycine, and altered levels of glycine and its conjugates, with perturbations in the metabotype representing a signature of metabolic health. This proposal will address the question of how haplotype-specific ALDH1L1 variants affect folate metabolism and the overall cellular metabotype, and how the haplotype-specific effect is modified by folate supplementation, by pursuing the following specific aims. Aim 1. Functionally characterize the ALDH1L1 enzyme variants from common human haplotypes. Aim 2. Determine the impact of major ALDH1L1 haplotypes on cellular metabolism and haplotype- specific responses to various folate supplementations. Aim 3. Link ALDH1L1 haplotypes to the folate-dependent regulation of glycine metabolism and health outcomes in humans. ALDH1L1 variants are very common in different populations but their role in folate homeostasis and in the etiology of metabolic disease is largely unexplored. It is expected that ALDH1L1 haplotypes differently mediate the metabolic response to dietary folate that might require adjustments of folate intake for individuals bearing certain ALDH1L1 SNPs. By filling this knowledge gap, the proposed research will provide mechanistic insight into the metabolic regulation by ALDH1L1 SNPs and will lay ground for the evaluation of population-specific ALDH1L1 haplotypes as a disease risk factor.