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

描述(由申请人提供)： 膳食叶酸是人类健康的重要营养素，作为一种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，人们越来越担心补充叶酸可能会加剧先前存在的肿瘤的生长。这些观察结果强调了更好地了解膳食叶酸促癌和抗癌活性的分子机制的必要性。在生化水平上，叶酸衍生物是核苷酸从头合成的一碳单位的重要来源，也是CpG序列的酶甲基化所必需的，从而实现基因沉默。叶酸缺乏被认为是致癌的一个重要机制是通过增加基因组的不稳定性。特别是，胸苷合成酶需要N5，N10亚甲基四氢叶酸将DUMP转化为TMP，TMP是TMP的唯一从头来源。因此，叶酸缺乏降低了TTP，增加了dUTP，并增加了尿嘧啶对DNA的掺入。DNA中的尿嘧啶是由碱基切除修复(BER)途径中的尿嘧啶DNA糖基酶(UDGs)去除的。BER去除了一系列致突变的DNA碱基损伤。不幸的是，在叶酸缺乏的情况下，BER变得有问题。由于BER需要DNA再合成步骤，dUTP升高可能会导致基因组尿嘧啶的重新引入。BER链断裂的中间产物本身就是裂解的产物，如果不加以修复，可以诱导细胞凋亡。换句话说，在叶酸缺乏期间，由UDG启动的BER似乎无意中导致了基因组的不稳定。还有一个额外的联系还有待探索。当在CpG序列中配对相反的鸟嘌呤时，MBD4(MED1)特异性地去除尿嘧啶或胸腺嘧啶。换句话说，MBD4被认为是通过阻止胞嘧啶或5-甲基胞嘧啶脱氨基引起的CpG到TPG的突变而发挥肿瘤抑制作用的。因此，出现了几个问题。在叶酸剥夺期间，当在CpG岛启动BER时，当BER不能准确完成时会发生什么？如果没有启动误码率则会产生突变后果，而如果误码率开始但无法准确完成，则会产生破坏性后果，两者之间的平衡是什么？此外，在肿瘤转化过程中，叶酸缺乏在什么时候会导致BER发挥抗增殖活性？BER启动的这些叶酸依赖的致癌和抗癌作用之间的平衡尚未得到直接测试。需要检验的假设是，叶酸耗竭时，尿嘧啶DNA糖基酶启动的BER会导致染色体不稳定，这对正常的上皮细胞是促癌的，对已建立的腺癌细胞是抗癌的。这些研究将确定基因、表观遗传事件和饮食之间在癌症预防中极其重要的相互作用的机制。众所周知，人类之间的误码率容量存在差异。了解叶酸代谢途径和误码率的累积状态将提供更好的手段，根据基因型别制定更好的预防和治疗策略。 公共卫生相关性： 饮食中的叶酸是人类健康的重要营养素，但人们担心，一旦肿瘤形成，叶酸也可能会促进肿瘤的生长。碱基切除修复(BER)可以纠正DNA损伤，通常被认为是有价值的，但当叶酸缺乏时，BER可能在不知不觉中导致DNA损伤并导致细胞癌变。我们将调查BER和叶酸之间的联系，这些联系定义了它们似乎相互矛盾的促癌和抗癌作用。

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.