TET2:a Potential Epigenetic Biomarker andTherapeutic Target for Atherosclerosis

LI Xiaohong,YANG Qin,WEI Dangheng

(The Institute of Cardiovascular Disease,Key Laboratory for Arteriosclerology of Hunan Province,University of South China,Hengyang,421001,China)

·專家筆談·

TET2:aPotentialEpigeneticBiomarkerandTherapeuticTargetforAtherosclerosis

LI Xiaohong,YANG Qin,WEI Dangheng*

(The Institute of Cardiovascular Disease,Key Laboratory for Arteriosclerology of Hunan Province,University of South China,Hengyang,421001,China)

Atherosclerosis,a slow and progressive pathological change,is the usual cause of cardiovascular diseases.Finding new biomarkers focusing on the nature of atherosclerosis is necessary to effectively diagnose and prevent clinical events.Recent studies find the functions of TET2 are deficient or aberrant in many neoplastic disorders.Ten-eleven-translocation 2 (TET2) is a member of the TET family that functions as a regulator of DNA methylation by oxidizing 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC),which contributes to DNA demethylation,repair,and genomic stability.DNA methylation pattern alterations occur in the development of atherosclerosis.Recent studies demonstrated that TET2 has a significant function in the self-renewal and differentiation of hematopoietic stem cells.Its mutation and dysfunction contribute to numerous specific diseases,such as hematopoiesis and hematopoietic diseases.Based on its biochemical,genetic,and functional implications,TET2 is a potential “ideal” epigenetic biomarker and therapeutic target for atherosclerosis.

TET2; atherosclerosis; epigenetic; biomarker

1 Introduction

Atherosclerosis,a pathological change that occurs in large- and medium-sized arteries,is the main pathological basis of cardiovascular diseases.Atherosclerosis can lead to ischemic stroke,myocardial infarction,kidney disease,intermittent claudication,and other serious complications.In the last two decades,a substantial number of molecules have been identified to be involved in the atherosclerotic process.As of these writings,some molecules have been detected and used as biomarkers to assess future cardiovascular events.However,their clinical effects have not yet been established.Although atherosclerotic plaque imaging is a precise predictor of atherosclerotic lesions,it is costly and impractical for the diagnosis of the early stages of atherosclerosis.Therefore,atherosclerotic biomarkers with good sensitivity,specificity,predictive value,and cost-effectiveness for the prediction,diagnosis,and treatment of the disease need to be identified.

DNA methylation is a major epigenetic modification of a genome that can be inherited,and contributes to gene silencing without changing the DNA sequences[1].Both global and gene-specific alterations in DNA methylation are associated with abnormal phenotypes.Epigenetic changes,the heritable nature of these changes,lock in the early stages of pathology,and induce the development of the disease.In human carcinogenesis,DNA methylation pattern alterations are characterized by global genomic DNA hypomethylation and specific gene hypermethylation.Genomic hypomethylation contributes to transformation,tumor progression,and oncogene expression,whereas regional DNA hypermethylation is related to the inactivation of tumor suppressor genes and enhances cell proliferation[2].Similar DNA methylation pattern alterations occur in early atherosclerosis[3-5].Aberrant genomic DNA methylation patterns occur earlier than the pathological and morphological changes of atherosclerosis[6-10].Therefore,the identification of specific changes in DNA methylation and its regulatory mechanism were valued to justify the expectations for novel diagnostic and therapeutic techniques for atherosclerosis.

The DNA methylation landscape of a genome includes two patterns of methylation and demethylation,and is established by methylation and demethylation enzymes.DNA methylation occurs at carbon 5 of cytosine in CpG dinucleotides,and dramatically suppresses transcription in the gene promoter regions[11].Thus,DNA methylation is frequently described as a “silencing” epigenetic mark,whereas DNA demethylation is described as an “activating” mark.Ten-eleven translocation (TET) proteins (TET1-3),a family of DNA demethylases,can catalyze the conversion of 5mC to 5hmC,5-formylcytosine (5fC),and 5-carboxycytosine (5caC),which is a well-characterized epigenetic modification that has crucial functions in regulating gene expression and maintaining cellular identity[12,13].

In the last decade,numerous studies have suggested a significant function for these enzymes in the epigenetic transcriptional regulation of eukaryotes primarily by hydroxylation reactions[14].TET2 is one of the most frequently mutated genes in myelodysplastic syndromes[15,16],and the loss of TET2 and 5hmC is an early key epigenetic event in aggressive melanoma[17].Studies have shown that the deletion of TET2 alone is sufficient to initiate myeloid transformation.TET2-null mouse models mainly exhibit chronic myelomonocytic leukemia-like disease.Patients with TET2 mutations show low levels of genomic 5hmC and global hypomethylation in the marrow compared with those of wild-type TET2,which is similar to the DNA methylation pattern in atherosclerotic lesions[18,19].TET-mediated oxidative demethylation was recently established to have a key function in reprogramming fibroblasts to pluripotency[20].The depletion of TET2 in mouse hematopoietic progenitors results in monocyte/macrophage differentiation dysfunction,leading to an impaired upregulation of macrophage markers as well as phagocytic capacity[16].Therefore,measurements of TET2 may carry important prognostic information and subsequent clinical complications,which are independent of traditional risk factors.

2 Proposed hypothesis

We propose that pro-atherosclerotic risk factors down-regulate TET2 expression,which leads to anti-atherosclerosis genes promoter DNA hypermethylation and atherosclerosis (Figure 1).Therefore,TET2 has the potential to be utilized as an ideal epigenetic biomarker for atherosclerosis,and contributes to the prediction and diagnosis of the disease.Notably,TET2 may be an ideal therapeutic target for the prevention or therapy of atherosclerosis because the TET2-mediated dynamic changes in DNA methylation and gene expression are reversible.

3 The value of the hypothesis

Atherosclerosis,as well as the resulting coronary heart disease and cerebral stroke,is still the leading cause of death and disability worldwide[21].Currently,atherosclerotic lesions cannot be effectively predicted and prevented.Thus,better biomarkers and therapeutic targets need to be identified.

DNA methylation regulates fundamental biological processes,such as gene expression,genomic stability,mutation rate,genomic imprinting,and X-chromosome inactivation.Both global and gene-specific alterations in DNA methylation are associated with abnormal phenotypes.DNA methylation has been described as an early diagnostic marker,and is a valuable molecular treatment strategy for cancer because of its dynamic nature[22].In atherosclerosis-prone apolipoprotein E knockout mouse,aortas,and peripheral blood leukocytes,specific changes in DNA methylation precede the formation of aortic lesions and are detectable in as early as four weeks[10].Low DNA methylation content has also been observed in peripheral blood leukocytes of patients with atherosclerotic cardiovascular disease[3].Alterations in DNA methylation affect the transcription of critical regulatory genes that induce a pro-atherogenic cellular phenotype[23].DNA methylation in leukocytes is associated with the expression of soluble mediators and surface molecules,which contribute to margination,adhesion,and trans-intimal migration.In particular,DNA methylation modification has been demonstrated as the bridge that links pro-atherosclerotic risk factors to atherosclerosis.Studies showed that homocysteine,a prevalent risk factor for cardiovascular events,increases the DNA methylation level of the ATP-binding cassette transporter A 1 (ABCA1) gene and decreases acetyl-CoA acetyltransferase 1 (ACAT1) DNA methylation to promote the accumulation of cholesterol in monocyte-derived foam cells[24,25].Low-density lipoprotein,a pro-atherosclerotic risk factor,represses endothelial KLF2 expression via DNA methylation,and downregulates several target genes,namely,endothelial NO synthase,plasminogen activator inhibitor-1,and thrombomodulin,to promote the development and progression of atherosclerosis[26,27].Notably,a study showed that pro-atherosclerotic risk factor disturbed flow resulted in mechanosensitive genes promoter hypermethylation and that the DNA methyltransferases inhibitor 5Aza treatment restored normal methylation patterns and antiatherogenic gene expression.Those confirmed evidences showed that DNA methylation dysfunction is a critic event in process of atherosclerotic.Further studies for the mechanism might improve the clinic outcome of atherosclerotic patients.

Figure 1 Schematic view of the hypothesis of TET2 as a potential idea biomarker of atherosclerosis

Recent studies discovered that TET2 has a critical function in regulating the expansion and function of hematopoietic stem cells[28]by controlling the 5hmC levels,and erythrocyte development by regulating lineage-specific genes via DNA oxidative demethylation[29].In hematopoietic systems,the deletion of TET2 is sufficient to cause a significant loss of 5hmC in genomic DNA.TET2 mutation is a plausible cause for aberrant epigenetic regulation of gene expression.

Smooth muscle cell (SMC) dedifferentiation significantly contributes to atherosclerosis[30].Studies showed that SMCs in advanced atherosclerotic plaques proliferate and are characterized by hypomethylation.Hypomethylation of collagen,type XV,and alpha 1 occurs during SMC proliferation,and the increase in gene expression contributes to the SMC phenotype and atherosclerosis formation[31].TET2 was recently demonstrated to be a novel and necessary master epigenetic regulator of SMC dedifferentiation[32].TET2 mutation promotes phenotypic alterations in vascular SMCs (VSMCs) from the “contractile” phenotype to the active “synthetic” phenotype.Consequently,these active VSMCs migrate from the media to the intima,proliferating and producing excessive amounts of extracellular matrix[32].

Therefore,the loss of TET2 may be an essential manifestation in atherosclerosis development.Alterations in TET2-mediated DNA demethylation precede and parallel the development of atherosclerosis,and are reversible.TET2 may be an “epigenetic biomarker” for risk stratification and “epigenetic therapeutic target” for the prevention of atherosclerosis.

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（四）充分發(fā)揮脫貧攻堅“助推器”功能，有效引領了貧困群眾脫貧奔小康。在解決新型農業(yè)經營主體融資難、融資貴問題的基礎上，通過創(chuàng)新貼息、貼擔保費的機制，鼓勵融資主體結對幫扶貧困戶，并采取提供就業(yè)、代耕代養(yǎng)、定點收購農產品和流轉土地等方式進行聯(lián)結，既順應國家提倡的產業(yè)扶貧思路，解決貧困戶長效增收問題，又讓貧困戶在聯(lián)結主體的指導下參與相關項目建設，力所能及提供必要勞動，在思想上堅定自力更生、勤勞致富的信念，根除了“等、靠、要”的思想，走出了內生性脫貧的新路子。

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TET2：潛在的動脈粥樣硬化表觀遺傳生物標記物和治療靶點

李小紅，楊 沁，危當恒

(南華大學心血管疾病研究所，動脈硬化學湖南省重點實驗室，湖南 衡陽 421001)

危當恒，博士、教授、碩士生導師、留學歸國人員。國際動脈粥樣硬化學會會員、中國病理生理學會會員，湖南省病理生理學會理事，湖南省“225”工程高層次衛(wèi)生人才人選。主要研究方向為動脈粥樣硬化的發(fā)病機制及其防治，主持和完成國家自然科學基金2項。在《Annals of Biomedical Engineering》、《Lipids》、《DNA and cell biology》等雜志發(fā)表科研論文50余篇。

動脈粥樣硬化是一種慢性炎癥性的病理改變，是心血管疾病最主要的病因，尋找新的生物標記物對有效診斷和治療動脈粥樣硬化有非常重要的作用。最新研究發(fā)現(xiàn)在很多腫瘤疾病中出現(xiàn)了TET2的功能缺失或異常。甲基雙加氧酶TET2為TET蛋白家族成員，主要功能為催化5甲基胞嘧啶(5mC) 生成5羥甲基胞嘧啶(5hmC)，參與DNA的去甲基化和修復及基因組的穩(wěn)定。DNA 甲基化模式的改變發(fā)生于動脈粥樣硬化的發(fā)生發(fā)展過程中。最近的研究表明，TET2在造血干細胞的自我更新及分化過程中起著非常重要的作用，TET2突變及功能失調與多種疾病，特別是血液性疾病的發(fā)生、發(fā)展密切相關?；赥ET2的生物學作用，我們推測TET2可能是動脈粥樣硬化的理想的表觀遺傳學生物標記物以及防治的靶點。

甲基雙加氧酶TET2； 動脈粥樣硬化； 表觀遺傳； 生物標記物

10.15972/j.cnki.43-1509/r.2015.01.002

date：2014-05-29；

date2014-09-23

SupportedFundingNational Natural Science Foundation of China (81370378) and the construct program of the key discipline in Hunan province.

*CorrespondingauthorE-mail:weizhonghua99@126.com.

R541.4DocumentCodeA

(此文編輯：秦旭平)