第二军医大学学报  2017, Vol. 38 Issue (11): 1444-1448   PDF    
全基因组关联研究中NanoDrop检测D260/D230值在DNA质检中的意义
郭倩, 王赓, 殷健, 李梦梦, 黄少兰, 姜磊, 徐沪济     
第二军医大学长征医院风湿免疫科, 上海 200003
摘要: 目的 探讨NanoDrop检测D260/D230值对全基因组关联研究(GWAS)中DNA标本质检的意义。方法 收集1 494例强直性脊柱炎(AS)患者的血液DNA样本,分别用NanoDrop和PicoGreen检测样本浓度。第一阶段,对24例NanoDrop和PicoGreen浓度>50 ng/μL的DNA样本行Omni中华8芯片检测,比较16例芯片成功样本与8例失败样本的D260/D280值及D260/D230值。第二阶段,选取1 122例NanoDrop和PicoGreen检测浓度均大于50 ng/μL DNA样本行管家基因GAPDH的PCR检测,并对PCR反应成功的样本行Omni中华8芯片检测。采用Mann-Whitney U检验比较PCR反应成功与失败DNA样本的D260/D230值,采用受试者工作特征(ROC)曲线评价D260/D230值对PCR结果的鉴别效率。结果 第一阶段中,芯片成功与失败样本的D260/D280值差异无统计学意义(P=0.444),而D260/D230值差异有统计学意义(Z=-3.920,P < 0.001);第二阶段中,PCR成功的DNA样本基因分型检测成功率为100%,PCR成功与失败组的D260/D230值比较差异有统计学意义(Z=-5.983,P < 0.01)。D260/D230值预测PCR结果的曲线下面积(AUC)为0.727;最佳诊断点的D260/D230值为0.89;特异度为0.95时的D260/D230值为2.305。结论 在进行GWAS时,浓度及D260/D280值均较好而D260/D230值较低的DNA样本可能含有较多杂质,需联用PCR检测以确保样本质量;D260/D230值≥ 2.305时,样本纯度满足GWAS芯片检测的要求,可省略PCR检测。
关键词: 全基因组关联研究     D260/D230     NanoDrop     PicoGreen     质量控制     聚合酶链反应     管家基因    
Significance of D260/D230 ratio of NanoDrop detection in quality assay of DNA in genome wide association study
GUO Qian, WANG Geng, YIN Jian, LI Meng-meng, HUANG Shao-lan, JIANG Lei, XU Hu-ji     
Department of Rheumatology, Changzheng Hospital, Second Military Medical University, Shanghai 200003, China
ΔCo-first authors
Abstract: Objective To investigate the significance of D260/D230 ratio of NanoDrop detection in quality assay of DNA in genome wide association study (GWAS). Methods Blood samples from 1 494 patients with ankylosing spondylitis (AS) were collected and the DNA was extracted. The concentrations of DNA samples were detected by NanoDrop and PicoGreen methods. In first stage, 24 DNA samples with concentrations>50 ng/μL were detected by Omni ZhongHua-8 microarray. Among the samples, 16 cases of chip reaction were successful and 8 were failure, and then the ratios of D260/D280 and D260/D230 were compared between the two groups. In second stage, 1 122 DNA samples with a concentration greater than 50 ng/μL according to NanoDrop and PicoGreen tests were selected for PCR detection of house-keeping genes. Samples with successful PCR reaction were detected by Human Omni ZhongHua-8 microarray. The 1 122 samples are divided into two groups according to the results of PCR (successful and failure groups). Mann-Whitney U test was used to compare the ratio of D260/D230 between the two groups, and receiver operating characteristic (ROC) curve was used to evaluate the predictive efficiency of D260/D230 value in PCR results. Results In the first stage, there were no significant differences in D260/D230 values between DNA samples with successful chip reaction and failure chip reaction (P=0.444), while the D260/D230 value of the fomer samples was significantly lower than that of the latter (Z=-3.920, P < 0.001). In the second stage, the success rate of genotyping of DNA samples with positive PCR results was 100%. There were significant differences in D260/D230 values between the DNA samples with positive and negative PCR results (Z=-5.983, P < 0.01. The area under curve of D260/D230value predicting the PCR results was 0.727; the D260/D230 values of the best diagnostic point and the point of specificity 95% were 0.89 and 2.305, respectively. Conclusion In GWAS, when DNA sample has better concentration and D260/D280 value and has lower D260/D230 value, PCR test should be performed to ensure the quality of the samples; when D260/D230 value is higher than 2.305, the samples are pure enough for microarray detection and the PCR detection can be omitted.
Key words: genome wide association study     D260/D230value     NanoDrop     PicoGreen     quality control     polymerase chain reaction     housekeeping gene    

自1953年DNA双螺旋结构被发现以来,遗传学领域取得了突飞猛进的发展。近年来,随着全基因组关联研究(genome wide association study,GWAS)、二代测序、三代测序等技术的问世,遗传学研究成为探索复杂性疾病致病机制的有力工具[1-3]。而高品质的DNA样本是遗传学实验成功与否的重要影响因素,进行有效的DNA样本质检更是控制实验成本的关键措施。以GWAS为例,各研究平台均需要200 ng的DNA,且要求DNA最低浓度为50 ng/μL[4]。NanoDrop和PicoGreen是检测DNA样本质量较为合适的方法,NanoDrop(如ND-1000)是一种全波长紫外/可见光扫描分光光度计,可检测DNA、RNA、蛋白质和染料等的光密度(D)值,自动识别的光谱范围为220~750 nm[5];PicoGreen为一种选择性结合双链DNA的荧光染料,作用与SYBR-GreenⅠ相似[6]。然而,研究获得的DNA等样本中存在杂质时,通过上述方法很难被检测出来,因此需要一种更加可靠的方法评估DNA样品的质量。本研究通过分析24例GWAS芯片失败样本在质量控制过程中的详细情况,发现失败样本的D260/D230值偏低,进而在更大样本中明确D260/D230值与DNA质量的关系,旨在寻找更可靠的DNA质检手段,降低芯片检测失败风险。

1 材料和方法 1.1 研究对象

收集1 494例第二军医大学长征医院收治的强直性脊柱炎(ankylosing spondylitis, AS)患者的全血样本,AS诊断均符合1984年修订的AS美国纽约的诊断标准[7]。本研究通过第二军医大学长征医院医学伦理委员会审批,所有入组患者均签署知情同意书。使用AxyPrep血基因组DNA小量试剂盒提取所有患者血液样本DNA,用于后续检测。

1.2 NanoDrop检测

检测样本前先对ND-1000(NanoDrop, USA)进行校正,取Lambda DNA原液(310 ng/μL,日本TaKaRa公司),用TE缓冲液稀释成3.1、38、78、155、310 ng/μL 5个浓度梯度,使用ND-1000检测各浓度样品在260 nm下的D值并绘制曲线,根据曲线拟合度判断仪器精确性。校正后取待测DNA样品1 μL,上机检测样品浓度及D280D260D230等值。

1.3 PicoGreen检测

(1) DNA待测样品的准备:取1 μL DNA样品加入到349 μL的TE缓冲液中振荡混匀,另取3个空移液管,每管加入100 μL样品混匀液;(2)绘制标准曲线:取λDNA原液(310 ng/μL,日本TaKaRa公司)用TE缓冲液稀释至2 ng/μL,然后再稀释成0(不含DNA)、0.125、0.25、0.5、1 ng/μL 5个浓度梯度,检测并绘制标准曲线。(3)DNA样品检测:取DNA待测样品室温避光孵育5 min,应用FlexStation® 3酶标仪(Molecular Devices, USA)以480 nm为激发波长测定各样品在520 nm下的D值,并使用SoftMax® Pro软件进行分析。每组设3个复孔。

1.4 PCR检测管家基因

选取NanoDrop和PicoGreen检测浓度均大于50 ng/μL的DNA样品行管家基因(GAPDH)的PCR检测。GAPDH引物(452 bp)序列:上游5′-ACC ACA GTC CAT GCC ATC AC-3′,下游5′-ATG TCG TTG TCC CAC CAC CT-3′。20 μL PCR体系:5 μL DNA模板(浓度10 ng/μL),10 μL Premix TAQ酶,上、下游引物各2 μL,1 μL去离子水。反应条件:95 ℃预变性10 min;94 ℃变性1 min、60 ℃退火1 min、72 ℃延伸1 min 25 s,共33个循环;循环结束后72 ℃延伸7 min。取10 μL PCR反应产物行带有GoldView染料(上海赛百盛基因技术有限公司)的琼脂糖凝胶电泳,验证是否出现452 bp的扩增条带。

1.5 基因分型

采用Illumina公司的Human Omini ZhongHua-8 BeadChip芯片行基因分型检测,基因组DNA经过质控检测后,取合格样品稀释至浓度为50 ng/μL。第一阶段:选取24例NanoDrop和PicoGreen检测浓度>50 ng/μL的DNA样品行芯片检测,有8例芯片反应失败,比较16例成功样本与8例失败样本的NanoDrop检测的D260/D280值及D260/D230值。第二阶段:对NanoDrop和PicoGreen检测浓度>50 ng/μL且PCR-管家基因扩增成功的DNA样品行基因分型检测,并使用HD超微阵序列和iScan系统(Illumina公司)进行扫描。

1.6 统计学处理

采用SPSS 13.0软件行统计分析。呈正态分布的计量资料以x±s表示,呈非正态分布的计量资料以中位数(最小值~最大值)表示。D260/D230值两组间比较采用Mann-Whitney U检验,采用受试者工作特征(receiver operating characteristic,ROC)曲线评价D260/D230值对PCR结果的鉴别效率,并确定最佳诊断点(敏感度和特异度最大)。为达到最严格的芯片质量控制,选取特异度为0.95的点为参考点;同时选取PicoGreen检测浓度>50 ng/μL的样本再计算ROC曲线以排除浓度对PCR结果的影响。使用曲线下面积(area under curve,AUC)评价D260/D230值对PCR结果的预测能力。检验水准(α)为0.05。

2 结果 2.1 NanoDrop与PicoGreen检测结果

1 494例DNA样品的NanoDrop检测浓度为(131.3±116.2) ng/μL,D260/D280值为1.815±0.868,D260/D230值为1.37(0.08~18.61);PicoGreen检测浓度为(123.0±97.5) ng/μL。NanoDrop和PicoGreen检测浓度均大于50 ng/μL的样本有1 122例。

2.2 24例DNA样品芯片检测结果

第一阶段24例NanoDrop和PicoGreen检测浓度>50 ng/μL的DNA样品检测结果见表 1。24例样本中16例芯片检测成功,8例失败。芯片成功样本与失败样本的D260/D280值分别为1.860±0.063与1.841±0.825,差异无统计学意义(P=0.444);而两组的D260/D230值分别为2.37(0.91~4.95)、0.23(0.15~0.45),差异有统计学意义(Z=-3.920,P < 0.001)。

表 1 24例DNA样本浓度及芯片检测结果 Tab 1 Concentrations and microarray detection results of 24 DNA samples

2.3 PCR及基因分型结果

第二阶段1 122例NanoDrop及PicoGreen检测浓度均大于50 ng/μL的DNA样品中979例PCR成功,143例失败。对979例PCR成功的DNA样本行基因分型检测,结果显示其Call rate值达标率为100%。将1 122例样品按照PCR结果分为两组,非参数检验结果显示,PCR成功与失败两组DNA样本的D260/D230值分别为1.45(0.09~18.61)和0.54(0.08~15.17),差异有统计学意义(Z=-5.983, P < 0.01)。

2.4 D260/D230值对PCR结果的鉴别能力

通过绘制ROC曲线评估D260/D230对PCR结果的鉴别能力,结果(图 1)显示,ROC曲线的AUC为0.727;其中最佳诊断点(敏感度和特异度最大)的D260/D230值为89%(敏感度为82%,特异度为61%)。ROC曲线中特异度为95%(敏感度16%)时对应的D260/D230值为2.305。

图 1 D260/D230预测PCR结果的ROC曲线 Fig 1 ROC curve of PCR results predicted by D260/D230 Point A: Best diagnostic point; Point B: Point of specificity 95%. PCR: Polymerase chain reaction; ROC: Receiver operating characteristic

3 讨论

GWAS及二代测序实验的影响因素有多种,如诊断明确的病例和对照组、多项检测的控制、人群分层的控制[8]、有明确表型的研究人群的特点[9]以及DNA样品的数量和质量等[10]。以GWAS为例,由于芯片非常昂贵,所以在进行芯片检测之前确保DNA样本的质量非常重要。

目前遗传学研究大多采用NanoDrop和PicoGreen检测DNA样品的质量。NanoDrop检测方法简单、经济。DNA在波长260 nm左右有最大吸收峰,而蛋白质的吸收峰在280 nm左右[5],因此可通过D260/D280值及D260/D230值检测DNA浓度;但RNA、单链DNA (ssDNA)和双链DNA (dsDNA)均在260 nm处有最大吸收峰,导致整体的D260值可能偏高,影响检测结果。PicoGreen是一种选择性结合dsDNA的荧光染料,与dsDNA结合时其荧光急剧增强,而未结合染料几乎不发出荧光,因此被用来检测DNA浓度;且PicoGreen稳定不易脱色,允许暴露较长的时间[6]。PCR-扩增管家基因(GAPDH)的方法常被用于确定样本基因组DNA的完整性及纯度[8, 11],因此本研究利用PCR-扩增管家基因以提高DNA芯片检测的成功率。

NanoDrop和PicoGreen测定DNA浓度时,纯度高的核酸其D260/D280值一般为1.8~2.0。但该比值受pH和离子浓度影响,酸性溶液一般会使其降低0.2~0.3,而碱性溶液则可能使其上升0.2~0.3;异常的D260/D280值表示样品中可能含有蛋白质、苯或其他在280 nm附近有吸收的污染物。D260/D230值是另一用于检测DNA纯度的指标,纯度高的核酸其D260/D230值一般为1.8~2.2,此值明显偏小时表明样品可能被碳水化合物(糖类)、盐类或有机溶剂污染,需要纯化样品或进一步优化核酸提取方法[12]。大多数研究中,一般使用D260/D280值评价DNA的纯度,而D260/D230值则被认为是次要的[13]。但近年来,涉及到DNA质控的遗传学研究越来越强调D260/D230值的价值。Solomon等[14]提取海水微生物DNA时发现,采用新的提取方法后,样本的D260/D230值可改善2.3%~45%,而D260/D280值未见明显改变,抽提样本可降低22.5%~34.5%的蛋白质污染,DNA合格率及16S RNA的PCR成功率均显著提高。本研究第一阶段中PCR结果阳性与阴性两组的D260/D280值差异无统计学意义,而D260/D230值差异有统计学意义(P < 0.001)。造成这种差异的原因可能是抽提的部分DNA样本中含有较多的碳水化合物(糖类)、盐类等污染物,这些污染物对230 nm的波段吸收较好而对280 nm波段吸收不佳,即表现为D260/D280值无明显变化而D260/D230值偏低。表明在衡量样本纯度时D260/D230值可能有重要意义,在检测样本时忽视该值也可能无法确保样本的质量。

本研究中,ROC曲线的AUC为0.727,表明所建立的D260/D230值对PCR结果模型有较好的预测效能。本研究得到的最佳诊断点D260/D230值为0.89,低于正常的D260/D230值范围(1.8~2.2),推测原因可能为当DNA混有一定程度的杂质时PCR仍可成功。但芯片对DNA纯度要求较高,故在芯片基因分型前的质检阶段,特异度(即D260/D230值预测PCR成功的可靠性)较敏感度更为重要,它可以避免因样本质量问题而导致的芯片浪费。因此本研究选择并推荐特异度为95%的诊断点(D260/D230值为2.305)为衡量标准,D260/D230值≥2.305且浓度合格的样本,其PCR-扩增管家基因成功的可能性超过95%,样本纯度较高,基本满足芯片检测要求。

在本研究第一阶段,即使样本经过NanoDrop及PicoGreen检测显示浓度已达到50 ng/μL,但仍有部分芯片检测失败。而第二阶段中经过NanoDrop、PicoGreen及PCR质检的样本芯片检测成功率为100%,表明与单用NanoDrop或PicoGreen的质检方法相比,其联合应用PCR检测能更大程度地确保GWAS的成功。

综上所述,在GWAS的DNA质检过程中,尽管NanoDrop检测中D260/D230值的正常范围为1.8~2.2,但当D260/D230 < 2.305时,DNA样本仍可能有污染,有可能导致后续遗传学研究失败,此时应进行PCR-扩增管家基因质检以确保GWAS芯片检测成功;当D260/D230≥2.305时则可认为PCR成功率超过95%,样本纯度基本满足芯片检测要求,故不需再进行PCR质检,可提高样本质检效率。本研究不足之处在于样本量有限,且PCR成功组与失败组的数量差异较大,仍需更大的多中心数据予以支持。

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