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

ALDH1L1 是叶酸代谢中的常见酶，可将 10-甲酰四氢叶酸 (10-甲酰-THF) 转化为四氢叶酸 (THF)。已知该反应调节从头嘌呤生物合成和叶酸依赖性同型半胱氨酸再甲基化循环。它还可以在控制一碳基团向合成代谢途径的通量方面发挥关键作用。为了支持叶酸代谢调节中的作用，我们最近发现，敲除小鼠中 ALDH1L1 基因的缺失会导致功能性叶酸缺乏，即使小鼠摄入了足够的叶酸也是如此。对Aldh1l1敲除小鼠的分析还表明，该酶是肝脏中甘氨酸代谢的主要调节剂：KO小鼠的甘氨酸和甘氨酸结合物水平较低，表明该酶参与叶酸依赖性从丝氨酸合成甘氨酸。我们进一步报道，人类 ALDH1L1 在多态位点 rs3796191、rs2886059、rs9282691、rs2276724、rs1127717 和 rs4646750 处有 6 个常见的非同义外显子 SNP，与这些 SNP 相关的单倍型的出现在种族群体之间存在显着差异。我们对西班牙裔儿童 Viva La Familia 队列的分析表明，携带 rs2276724 和 rs3796191 的儿童血清甘氨酸显着降低，丝氨酸/甘氨酸比率增加，表明丝氨酸向甘氨酸转化的失调，并概括了我们的小鼠研究结果。 ALDH1L1 非同义 SNP 也与该队列中的代谢应激和肥胖标志物相关。根据我们的发现，我们假设非同义 ALDH1L1 SNP 产生的酶变体具有改变的催化活性和/或稳定性，影响其代谢 10-甲酰基-THF 的能力，从而放松甘氨酸代谢。因此，具有特定单倍型的个体具有不同的THF/10-甲酰基-THF和丝氨酸/甘氨酸比例，以及改变的甘氨酸及其缀合物水平，代谢型的扰动代表代谢健康的特征。该提案将通过追求以下具体目标，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，以及如何通过叶酸补充来改变单倍型特异性效应的问题。目标 1. 对常见人类单倍型的 ALDH1L1 酶变体进行功能表征。目标 2. 确定主要 ALDH1L1 单倍型对细胞代谢的影响以及对各种叶酸补充剂的单倍型特异性反应。目标 3. 将 ALDH1L1 单倍型与人类甘氨酸代谢和健康结果的叶酸依赖性调节联系起来。 ALDH1L1 变异在不同人群中非常常见，但它们在叶酸稳态和代谢疾病病因学中的作用很大程度上尚未被探索。预计 ALDH1L1 单倍型会以不同的方式介导对膳食叶酸的代谢反应，这可能需要调整携带某些 ALDH1L1 SNP 的个体的叶酸摄入量。通过填补这一知识空白，拟议的研究将为 ALDH1L1 SNP 的代谢调节提供机制见解，并为评估人群特异性 ALDH1L1 单倍型作为疾病危险因素奠定基础。

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.