Folate Status, Genomic Uracil, and the Balance of Base Excision Repair Activity

叶酸状态、基因组尿嘧啶和碱基切除修复活性的平衡

• 批准号: 7788586
• 负责人: MICHAEL D. WYATT
• 金额: $ 15.14万
• 依托单位: UNIVERSITY OF SOUTH CAROLINA AT COLUMBIA
• 依托单位国家: 美国
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-12-01 至 2011-11-30
• 项目状态: 已结题
• 来源: https://reporter.nih.gov/project-details/7788586
• 关键词: 5' -deoxyribose phosphate lyase; Adenocarcinoma Cell; Animal Model; Apoptosis; Attention; Base Excision Repairs; Biochemical; Breast; Cancerous; Carbon; Cell Line; Cells; Chemoprevention; Chemopreventive Agent; Chromosomal Instability; Clinical; Colon; CpG Islands; Cytosine; DNA; DNA Damage; DNA-Directed DNA Polymerase; Deamination; Diet; Enzymes; Epidemiology; Epigenetic Process; Epithelial Cells; Equilibrium; Event; Folate; Folic Acid; Folic Acid Deficiency; Gene Silencing; Genes; Genetic; Genomic Instability; Genomics; Genotype; Growth; Guanine; Health; Human; Link; Malignant Neoplasms; Measures; Methylation; Molecular; Mutagenesis; Nutrient; Pathway interactions; Polymerase; Prevention; Process; Source; Staging; Supplementation; Testing; Tetrahydrofolates; Thymidylate Synthase; Thymine; Tumor Suppressor Proteins; Uracil; Variant; anticancer activity; base; cancer prevention; carbene; carcinogenesis; clastogenesis; deprivation; design; folic acid metabolism; nucleotide metabolism; prevent; promoter; public health relevance; repaired; treatment strategy; tumor; tumorigenic; uracil-DNA glycosylase

DESCRIPTION (provided by applicant): Dietary folic acid is an important nutrient for human health and has received much attention as a means of preventing cancer. Studies have shown that folate supplementation suppresses early stages of carcinogenesis. However, concern is growing that folate supplementation may exacerbate the growth of pre- existing tumors. These observations underscore the need for a better understanding of the molecular mechanisms of pro- and anticancer activities of dietary folate. At a biochemical level, folate derivatives are important sources of one-carbon units for de novo synthesis of nucleotides and are necessary for enzymatic methylation of CpG sequences for gene silencing. A significant mechanism by which folate deficiency is thought to be procarcinogenic is by increasing genome instability. In particular, N5,N10 methylene tetrahydrofolate is required by thymidylate synthase to convert dUMP to TMP, which is the only de novo source of TMP. Thus, folate deficiency decreases TTP, increases dUTP, and increases uracil incorporation into DNA. Uracil in DNA is removed by uracil DNA glycosylases (UDGs) of the Base Excision Repair (BER) pathway. BER removes an array of mutagenic DNA base damage. Unfortunately, BER becomes problematic under folate-deficient conditions. Because BER requires a DNA resynthesis step, elevated dUTP presumptively causes the reintroduction of genomic uracil. BER strand break intermediates are themselves clastogenic, and if not repaired can induce apoptosis. In other words, BER initiated by UDGs appear to unwittingly contribute to genome instability during folate deficiency. There is an additional link that is yet to be explored. MBD4 (MED1) specifically removes uracil or thymine when paired opposite guanine in CpG sequences. In other words, MBD4 is thought to act as a tumor suppressor by preventing CpG to TpG mutagenesis caused by deamination of cytosine or 5-methylcytosine. Thus, several questions arise. During folate deprivation, what happens when BER is initiated at CpG islands when BER cannot be completed accurately? What is the balance between mutagenic consequences if BER is not initiated and clastogenic consequences if BER is initiated but cannot be accurately completed? Furthermore, at what point during tumorigenic transformation would folate deficiency cause BER to exert antiproliferative activity? The balance of these folate-dependent procarcinogenic and anticarcinogenic contributions of BER initiation have not been directly tested. The hypothesis to be tested is that BER initiated by uracil DNA glycosylases during folate depletion causes chromosomal instability, which is procarcinogenic to normal epithelial cells and anticarcinogenic to established adenocarcinoma cells. The studies will define the mechanism of an extremely important interaction between genes, epigenetic events, and diet in cancer prevention. It is well known that there are variations in BER capacity among humans. Knowing the cumulative status of the pathways of folate metabolism and BER should provide a better means devising better prevention and treatment strategies based on genotype. PUBLIC HEALTH RELEVANCE: Dietary folic acid is an important nutrient for human health, but there is concern that folic acid may also contribute to growth of a tumor once it has formed. Base excision repair (BER) corrects DNA damage and is generally considered to be valuable, but when folate deficiency occurs, BER may unwittingly contribute to DNA damage and cause cells to become cancerous. We will investigate the links between BER and folic acid that define the seemingly contradictory pro- and anticancer effects of each.

描述（由申请人提供）： 膳食叶酸是人体健康的重要营养素，作为预防癌症的手段而备受关注。研究表明，补充叶酸可以抑制癌症发生的早期阶段。然而，人们越来越担心叶酸补充剂可能会加剧预先存在的肿瘤的生长。这些观察结果强调需要更好地了解膳食叶酸的促癌和抗癌活性的分子机制。在生物化学水平上，叶酸衍生物是核苷酸从头合成的一碳单元的重要来源，并且是用于基因沉默的CpG序列的酶促甲基化所必需的。叶酸缺乏被认为是致癌的一个重要机制是增加基因组的不稳定性。特别地，胸苷酸合成酶需要N5，N10亚甲基四氢叶酸将dUMP转化为TMP，这是TMP的唯一从头来源。因此，叶酸缺乏降低TTP，增加dUTP，并增加尿嘧啶掺入DNA。DNA中的尿嘧啶通过碱基切除修复（BER）途径的尿嘧啶DNA糖基化酶（UDG）去除。BER去除了一系列致突变DNA碱基损伤。不幸的是，BER在叶酸缺乏的条件下变得有问题。由于BER需要DNA再合成步骤，推测升高的dUTP会导致基因组尿嘧啶的重新引入。BER链断裂中间体本身是致染色体断裂的，如果不修复，可以诱导细胞凋亡。换句话说，由UDG启动的BER似乎无意中导致叶酸缺乏期间的基因组不稳定性。还有一个额外的联系尚待探索。当与CpG序列中的鸟嘌呤配对时，MBD 4（MED 1）特异性地去除尿嘧啶或胸腺嘧啶。换句话说，MBD 4被认为通过阻止由胞嘧啶或5-甲基胞嘧啶脱氨引起的CpG至TpG突变而发挥肿瘤抑制因子的作用。因此，出现了几个问题。在叶酸剥夺期间，当BER不能准确完成时，当BER在CpG岛启动时会发生什么？如果未启动BER，致突变后果与如果启动BER但无法准确完成，致染色体断裂后果之间的平衡是什么？此外，在肿瘤发生转化过程中，叶酸缺乏会导致BER发挥抗增殖活性？这些叶酸依赖性BER启动的致癌和抗癌贡献的平衡尚未直接测试。待检验的假设是，在叶酸耗竭期间由尿嘧啶DNA糖基化酶引发的BER导致染色体不稳定性，这对正常上皮细胞是致癌的，对已建立的腺癌细胞是抗癌的。这些研究将确定在癌症预防中基因、表观遗传事件和饮食之间极其重要的相互作用机制。众所周知，人类的BER能力存在差异。了解叶酸代谢途径的累积状态和BER应该提供一种更好的方法，根据基因型设计更好的预防和治疗策略。 公共卫生相关性： 膳食叶酸是人体健康的重要营养素，但人们担心叶酸一旦形成也可能有助于肿瘤的生长。碱基切除修复（BER）可以纠正DNA损伤，通常被认为是有价值的，但当发生叶酸缺乏时，BER可能会无意中导致DNA损伤并导致细胞癌变。我们将研究BER和叶酸之间的联系，这些联系定义了每一种看似矛盾的亲和抗癌作用。