Cabelof Diane C, Raffoul Julian J, Nakamura Jun, Kapoor Diksha, Abdalla Hala, Heydari Ahmad R
Department of Nutrition and Food Science, Wayne State University, Detroit, MI 48202, USA.
J Biol Chem. 2004 Aug 27;279(35):36504-13. doi: 10.1074/jbc.M405185200. Epub 2004 Jun 24.
The mechanism by which folate deficiency influences carcinogenesis is not well established, but a phenotype of DNA strand breaks, mutations, and chromosomal instability suggests an inability to repair DNA damage. To elucidate the mechanism by which folate deficiency influences carcinogenicity, we have analyzed the effect of folate deficiency on base excision repair (BER), the pathway responsible for repairing uracil in DNA. We observe an up-regulation in initiation of BER in liver of the folate-deficient mice, as evidenced by an increase in uracil DNA glycosylase protein (30%, p < 0.01) and activity (31%, p < 0.05). However, no up-regulation in either BER or its rate-determining enzyme, DNA polymerase beta (beta-pol) is observed in response to folate deficiency. Accordingly, an accumulation of repair intermediates in the form of DNA single strand breaks (37% increase, p < 0.03) is observed. These data indicate that folate deficiency alters the balance and coordination of BER by stimulating initiation without subsequently stimulating the completion of repair, resulting in a functional BER deficiency. In directly establishing that the inability to induce beta-pol and mount a BER response when folate is deficient is causative in the accumulation of toxic repair intermediates, beta-pol-haploinsufficient mice subjected to folate deficiency displayed additional increases in DNA single strand breaks (52% increase, p < 0.05) as well as accumulation in aldehydic DNA lesions (38% increase, p < 0.01). Since young beta-polhaploinsufficient mice do not spontaneously exhibit increased levels of these repair intermediates, these data demonstrate that folate deficiency and beta-pol haploinsufficiency interact to increase the accumulation of DNA damage. In addition to establishing a direct role for beta-pol in the phenotype expressed by folate deficiency, these data are also consistent with the concept that repair of uracil and abasic sites is more efficient than repair of oxidized bases.
叶酸缺乏影响致癌作用的机制尚未完全明确，但DNA链断裂、突变和染色体不稳定的表型提示其缺乏修复DNA损伤的能力。为阐明叶酸缺乏影响致癌性的机制，我们分析了叶酸缺乏对碱基切除修复（BER）的影响，BER是负责修复DNA中尿嘧啶的途径。我们观察到叶酸缺乏小鼠肝脏中BER起始上调，尿嘧啶DNA糖基化酶蛋白增加（30%，p<0.01）及活性增加（31%，p<0.05）证明了这一点。然而，未观察到叶酸缺乏导致BER或其限速酶DNA聚合酶β（β-pol）上调。因此，观察到以DNA单链断裂形式存在的修复中间体积累（增加37%，p<0.03）。这些数据表明，叶酸缺乏通过刺激起始而不随后刺激修复完成来改变BER的平衡和协调，导致功能性BER缺陷。为直接确定叶酸缺乏时无法诱导β-pol及引发BER反应是有毒修复中间体积累的原因，叶酸缺乏的β-pol单倍体不足小鼠DNA单链断裂进一步增加（增加52%，p<0.05），醛基化DNA损伤也积累（增加38%，p<0.01）。由于年轻的β-pol单倍体不足小鼠不会自发表现出这些修复中间体水平升高，这些数据表明叶酸缺乏和β-pol单倍体不足相互作用会增加DNA损伤的积累。这些数据除了确立β-pol在叶酸缺乏所表达表型中的直接作用外，还与尿嘧啶和无碱基位点的修复比氧化碱基的修复更有效的概念一致。
