杂交探针技术结合熔解曲线快速检测载脂蛋白E基因型

目的 利用杂交探针结合熔解曲线分析技术,建立一种适用于临床的快速、可靠的实时PCR检测载脂蛋白E(apoE)基因型的方法.方法 应用LightCycler技术,优化主要反应条件,设计在1个PCR中可同时分析2个密码子的多态性方法.选取60岁以上中心性肥胖症133例和体检正常老年者108名为研究对象.首先以LC-Red640和LC-Red750标记条针对突变位点为112和158的检测探针的5'端,相应的锚定探针在3'端被荧光标记,再从研究对象基因组DNA中快速循环PCR特异引物扩增apoE基因265 bp片段,将其与特异探针杂交后,引发荧光共振能量转移,通过异源杂交的出现和错配碱基类型的熔解参数来完成和评价野生型和突变基因分型的特征性.结果 峰值对应着序列特异的熔解温度点,各基因型的模板均呈现良好的峰形.中心性肥胖症组突变型以E2/3、E3/4基因型频率最高,与正常组比较差异有统计学意义(x2=4.210,P＜0.004;x2=6.328,P＜0.012),中心性肥胖症组和对照组分布频率分别为E2/3型37例(27.8%)和18例(16.7%)、E3/4型33例(24.8%)和13例(12.0%)、E3/3型56例(42.1%)和74例(68.5%)、E2/4型3例(2.3%)和2例(1.9%)、E4/4型3例(2.2%)和0例(0)、E2/2型1例(0.8%)和1例(0.9%);其结果与测序结果一致;中心性肥胖症组apoE基因总浓度(101.2±73.6)表达水平也较对照组(50.6±27.1)显著升高(t=3.359,P＜0.001).结论 该方法操作简便省时,不易交叉污染;获得的各种基因类型特征性强,适用于临床和流行病学调查的快速诊断分型.apoE基因多态性与老年中心性肥胖症存在相关性,对携带E3/4型、E2/3型或E4/4型的个体,可提示决定个体老年中心性肥胖遗传易感性方面起到重要作用.     

Progress toward ultrafast DNA sequencing using solid-state nanopores

BACKGROUND: Measurements of the ionic current flowing through nanometer-scale pores (nanopores) have been used to analyze single DNA and RNA molecules, with the ultimate goal of achieving ultrafast DNA sequencing. However, attempts at purely electronic measurements have not achieved the signal contrast required for single nucleotide differentiation. In this report we propose a novel method of optical detection of DNA sequence translocating through a nanopore. METHODS: Each base of the target DNA sequence is 1st mapped onto a 2-unit code, 2 10-bp nucleotide sequence, by biochemical conversion into Designed DNA Polymers. These 2-unit codes are then hybridized to complementary, fluorescently labeled, and self-quenching molecular beacons. As the molecular beacons are sequentially unzipped during translocation through a <2-nm-wide nanopore, their fluorescent tags are unquenched and are detected by a custom-built dual-color total internal reflection fluorescence (TIRF) microscope. The 2-color optical signal is then correlated to the target DNA sequence. RESULTS: A dual-color TIRFM microscope with single-molecule resolution was constructed, and controlled fabrication of 1-dimensional and 2-dimensional arrays of solid-state nanopores was performed. A nanofluidic cell assembly was constructed for TIRF-based optical detection of voltage-driven DNA translocation through a nanopore. CONCLUSIONS: We present a novel nanopore-based DNA sequencing technique that uses an optical readout of DNA translocating unzipping through a nanopore. Our technique offers better single nucleotide differentiation in sequence readout, as well as the possibility of large-scale parallelism using nanopore arrays.     

Semiautomated DNA mutation analysis using a robotic workstation and molecular beacons

BACKGROUND: Our increasing knowledge of the genetic basis of inheritable diseases requires the development of automated reliable methods for high-throughput analyses. METHODS: We investigated the combination of semiautomated DNA extraction from blood using a robotic workstation, followed by automated mutation detection using highly specific fluorescent DNA probes, so-called molecular beacons, which can discriminate between alleles with as little as one single-base mutation. We designed two molecular beacons, one recognizing the wild-type allele and the other the mutant allele, to determine genotypes in a single reaction. To evaluate this procedure, we examined the C677T mutation in the methylenetetrahydrofolate reductase (MTHFR) gene, which is associated with an increased risk for cardiovascular disease and neural tube defects. DNA was isolated from 10 microL of fresh EDTA-blood samples by use of a robotic workstation. The DNA samples were analyzed using molecular beacons as well as conventional methods. RESULTS: Both methods were compared, and no differences were found between outcomes of genotyping. CONCLUSIONS: The described assay enables robust and automated extraction of DNA and analysis of up to 96 samples (10 microL of blood per sample) within 5 h. This is superior to conventional methods and makes it suitable for high-throughput analyses.     

Enhanced discrimination of single nucleotide polymorphisms using 3′ nucleotide differences in ligase detection reaction probes

The ligase detection reaction (LDR) is a highly specific genotyping method for single nucleotide variations. Although LDR typically discriminates single nucleotide polymorphism (SNP) alleles at the 3′ end of so-called LDR discriminating probes, we designed probes in which the position of nucleotide differences for discrimination was shifted to the second and third nucleotides from the 3′ end. Using the 3′-modified probes, we targeted SNPs of the human ABO group and investigated the specificity and efficiency of ligation by a universal LDR assay. We demonstrated that one or two nucleotide shifts of differences in discriminating probes improve the allele balance in detecting both base substitutions and short deletions. In regard to short deletions, moreover, the shifts of nucleotide differences in discriminating probes form the perfect-machted or multiple-mismatched structures (the bulge structures) in the discriminating probe-target DNA duplex and may contribute to enhance ligation efficiency.     

Ligation-mediated rolling-circle amplification-based approaches to single nucleotide polymorphism detection

Ligation-mediated single nucleotide polymorphism detection coupled with an efficient method of signal enhancement, such as rolling-circle amplification, hyperbranched rolling-circle amplification or PCR, has provided the foundation for the development of variable single nucleotide polymorphism genotyping and analyzing methods for different applications. Several methods based on the above approaches have been developed, enabling rapid genotyping of a large number of single nucleotide polymorphisms directly from a small amount of genomic DNA and large-scale multiplex single nucleotide polymorphism (>1000 single nucleotide polymorphisms per assay) analysis on microarrays. This review categorizes different approaches and describes the principles of each approach for single nucleotide polymorphism detection. Possible future research directions including the development of optimized methods for analysis of cytologic samples and other applications are also discussed.     

A method for homogeneous color-compensated genotyping of factor V (G1691A) and methylenetetrahydrofolate reductase (C677T) mutations using real-time multiplex fluorescence PCR

OBJECTIVES: To set up an optimized multiplex polymerase chain reaction for real-time genotyping of the prothrombotic risk factors methylenetetrahydrofolate reductase C677T and factor V Leiden on the LightCycler. DESIGN AND METHODS: Novel primer and probe sets were designed on the basis of thermodynamic double-strand DNA stability calculations. Detection probes were labeled with LC-Red640 or Cy5.5 dye. RESULTS: The polymerase chain reaction efficiency was reduced in multiplex polymerase chain reaction but this could be overcome by the design of novel amplification primers. The selection of detection probes with a lower melting temperature (T(m)) and high Delta T(m) improved the discrimination of heterozygous samples. Color compensation was not compromised by either the use of the Cy5.5 dye or different fluorescein linker chemistries. CONCLUSIONS: Probes with a Delta T(m) of 5 degrees C or more between the matched and mismatched state are desirably for genotyping. Such probes can be selected by using a priori calculations based on the thermodynamic nearest neighbor model.     

Quantitative technologies for allele frequency estimation of SNPs in DNA pools

We have compared several genotyping methods to assess their applicability to single nucleotide polymorphism (SNP) allele frequency estimation in DNA pools. The accuracy of these methods (restriction fragment length polymorphism, real-time pyrophosphate DNA sequencing, single base extension with fluorescently labeled ddNTPs, homogeneous 5'-nuclease assay, and MALDI-TOF mass spectrometry) was tested by calculating the standard deviation among heterozygous individuals (which are natural DNA pools with 50% representation of each allele) and by estimating allele frequency in artificial pools. We show that although the methods differ in their accuracy, they can all serve for quantification of allele frequency in DNA pools with reasonable accuracy. We found that the influence of the error variance attributed to pool construction on quantification accuracy is insignificant and is SNP dependent.     

LightTyper platform for high-throughput clinical genotyping

DNA sequence variations due to single nucleotide changes or polymorphisms (SNPs) have demonstrated an association with certain diseases as causative agents or surrogate biomarkers. Identification and genotyping of SNPs requires reliable and robust technologies. Multiple genotyping platforms are available to detect SNPs. Although many of these platforms meet the requirements of the research environment, technologies have also emerged for high-throughput clinical genotyping as well. The LightTyper is one such platform, providing SNP identification by employing melting curve analysis of fluorescently labeled probes. The LightTyper has been used to identify SNPs associated with myocardial infarction, developing and validating assays for approximately 100 SNPs in 30 candidate genes. The LightTyper is also amenable to the use of assays already developed for the LightCycler, which is widely used in clinical laboratories. The initial experience presented here suggests the potential use of the LightTyper for high-throughput clinical genotyping.     

Rapid combined genotyping of factor V, prothrombin and methylenetetrahydrofolate reductase single nucleotide polymorphisms using minor groove binding DNA oligonucleotides (MGB probes) and real-time polymerase chain reaction.

Risk factors for cardiovascular diseases and venous thromboembolism involve both acquired and hereditary conditions. Among the latter, mutations in genes coding for coagulation factors (factor V Leiden [Arg506Gly], G20210A in the 3'-untranslated region of factor II ) and variant C677T of the methylenetetrahydrofolate reductase (MTHFR ) are often involved and co-inherited. These three factors were genotyped simultaneously in the same 96-well plate, using a real-time polymerase chain reaction (PCR) Taqman assay and minor groove binding DNA oligonucleotides (MGB probes). While primers and MGB probes matched their corresponding single nucleotide polymorphism (SNP), the real-time MGB program was identical for each target gene. Homozygous wild-type (WT; -/-), heterozygous (+/-) or homozygous (+/+) variants (n = 362) were selected for factor V (n = 115, with -/-, 40; +/-, 40; +/+, 35), factor II (n = 122, with -/-, 60; +/-, 60; +/+, 2), and MTHFR (n = 120, with -/-, 40; +/-, 40; +/+, 40), according to the results of conventional PCR-restriction fragment length polymorphism (PCR-RFLP), but the allelic discrimination was performed blind. Results of the real-time MGB and PCR-RFLP assays were identical. This new assay was easy and fast with high throughput, without risk of molecular carryover, and cost-effective for laboratories utilizing the Taqman or related fluorescence reading methods. These advantages make it particularly suitable for large-scale combined genotyping of several polymorphisms in the routine setting.     

Molecular beacons as diagnostic tools: technology and applications.

Molecular beacons are single-stranded, fluorophore-labeled nucleic acid probes that are capable of generating a fluorescent signal in the presence of target, but are dark in the absence of target. Molecular beacons allow multiplex detection of PCR products in real time in a homogeneous assay format. Real time detection is inherently quantitative and affords a greater dynamic range than end-point detection methods. Reactions in a homogeneous assay format are sealed before amplification takes place, providing improved contamination control. A single cycler/reader instrument, coupled with automated sample preparation, results in higher throughput and greater ease of use. A multiplex qualitative assay that detects Chlamydia trachomatis and Neisseria gonorrhoeae, along with an internal control, has been developed. High specificity is achieved through careful selection of primers, probes and assay conditions. Quantitative HIV, HCV, and HBV viral load assays, with sensitivities of 50 copies/ml, 20 IU/ml, and 50 copies/ml, respectively, are achievable. The viral load assays are designed to quantitate all subtype and genotype specimens equivalently. A molecular beacon assay has been designed to detect a single nucleotide polymorphism in the beta2 adrenergic receptor gene.     

ABO blood group genotyping by quenching probe method

We developed a multiplex ABO genotyping method with quenching probes (Q-probe). In this method, it is possible to discriminate the mutations, not only frequently used positions 261 and 796 but also position 703 in a single PCR. Each probe was designed to have cytosine residue at 5' or 3' end and labeled with three different fluorescence dyes, enabling the triplex detections of these polymorphisms. All polymorphisms were successfully detected by using fluorescence labeled Q-probe in a specifically amplified PCR product. Each Q-probe showed unique dissociation patterns depending on the polymorphism types. All of the results obtained with Q-probe were compared with standard serotyping and TaqMan PCR method and resulted in complete match with each other. Consequently, these results indicated that multiplex ABO genotyping method is quite accurate and convenient method for the determination of ABO genotype.     

Chimeric RNA-DNA molecular beacon assay for ribonuclease H activity

Current methods to detect and assay ribonuclease H (RNase H) activity are indirect and time-consuming. Here we introduce a direct and sensitive method, based on the fluorescence quenching mechanism of molecular beacons, to assay RNA cleavage in RNA:DNA hybrids. An RNA-DNA chimeric beacon assay for RNase H enzymatic activity was developed. The substrate is a single-stranded RNA-DNA chimeric oligonucleotide labeled with a 5'-fluorescein and a 3'-DABCYL. The fluorophore (fluorescein) of the probe is held in close proximity to the quencher (DABCYL) by the RNA:DNA stem-loop structure. When the RNA sequence of the RNA:DNA hybrid stem is cleaved, the fluorophore is separated from the quencher and fluorescence can be detected as a function of time. Chimeric beacons with different stem lengths and sequences have been surveyed for this assay with E. coli RNase H. We found that the beacon kinetic parameters are in qualitative agreement with previously reported values using more cumbersome assays. This method permits real-time detection of RNase H activity and a convenient approach to RNase H kinetic and mechanistic study.     

A new single nucleotide polymorphism genotyping method based on gap ligase chain reaction and a microsphere detection assay.

BACKGROUND: The establishment of new methods to detect human genetic variations, such as single nucleotide polymorphisms, is of importance in hereditary disease diagnosis and pharmacogenomics. Several single nucleotide polymorphism genotyping technologies have been presented in recent years. However, techniques which would allow accurate, fast and cheap allelic determination and multiple single nucleotide polymorphism detection in parallel are still in great need. METHODS: Here, we present a new genotyping technique based on gap ligase chain reaction and a fluorescent polystyrene microsphere measurement platform. We chose the human polymorphism rs3730386 as our candidate to establish this method. Probes for gap ligase chain reaction were designed to recognize the target alleles sensitively and specifically and to produce templates for amplifying the correspondent target fragments used in the following hybridization. The genotypes were finally determined by a hybridization process based on the Luminex fluorescent polystyrene microspheres measurement platform. RESULTS: This method was successfully applied for the detection of selected single nucleotide polymorphisms with high sensitivity and specificity. The genotypes were validated by DNA sequencing. CONCLUSIONS: The new genotyping method benefited from the high sensitivity and specificity of gap ligase chain reaction and the detection platform of Luminex. The method allows multiplex analysis of single nucleotide polymorphism, because 100 types of microspheres are available from Luminex.     

Use of molecular beacons to detect an antifolate resistance-associated mutation in Plasmodium falciparum

A PCR-based technique using new fluorescent probes, called molecular beacons, was developed to detect the antifolate resistance-associated S108N point mutation in Plasmodium falciparum. One hundred African clinical isolates were tested by the new method in comparison with the PCR-restriction fragment length polymorphism method. This new molecular technique appears to be a promising tool for epidemiological studies.     

Electronic detection of nucleic acids: a versatile platform for molecular diagnostics

A novel platform for the electronic detection of nucleic acids on microarrays is introduced and shown to perform well as a selective detection system for applications in molecular diagnostics. A gold electrode in a printed circuit board is coated with a self-assembled monolayer (SAM) containing DNA capture probes. Unlabeled nucleic acid targets are immobilized on the surface of the SAM through sequence-specific hybridization with the DNA capture probe. A separate signaling probe, containing ferrocene-modified nucleotides and complementary to the target in the region adjoining the capture probe binding site, is held in close proximity to the SAM in a sandwich complex. The SAM allows electron transfer between the immobilized ferrocenes and the gold, while insulating the electrode from soluble redox species, including unbound signaling probes. Here, we demonstrate sequence-specific detection of amplicons after simple dilution of the reaction product into hybridization buffer. In addition, single nucleotide polymorphism discrimination is shown. A genotyping chip for the C282Y single nucleotide polymorphism associated with hereditary hemochromatosis is used to confirm the genotype of six patients’ DNA. In addition, a gene expression-monitoring chip is described that surveys five genes that are differentially regulated in the cellular apoptosis response. Finally, custom modification of individual electrodes through sequence-specific hybridization demonstrates the potential of this system for infectious disease diagnostics. The versatility of the electronic detection platform makes it suitable for multiple applications in diagnostics and pharmacogenetics.     

Molecularly resolved,label-free nucleic acid sensing at solid–liquid interface using non-ionic DNA analogues

Nucleic acid-based biosensors, where the capture probe is a nucleic acid, e.g., DNA or its synthetic analogue xeno nucleic acid (XNA), offer interesting ways of eliciting clinically relevant information from hybridization/dehybridization signals. In this respect, the application of XNA probes is attractive since the drawbacks of DNA probes might be overcome. Within the XNA probe repertoire, peptide nucleic acid (PNA) and morpholino (MO) are promising since their backbones are non-ionic. Therefore, in the absence of electrostatic charge repulsion between the capture probe and the target nucleic acid, a stable duplex can be formed. In addition, these are nuclease-resistant probes. Herein, we have tested the molecularly resolved nucleic acid sensing capacity of PNA and MO capture probes using a fluorescent label-free single molecule force spectroscopy approach. As far as single nucleobase mismatch discrimination is concerned, both PNA and MO performed better than DNA, while the performance of the MO probe was the best. We propose that the conformationally more rigid backbone of MO, compared to the conformationally flexible PNA, is an advantage for MO, since the probe orientation can be made more upright on the surface and therefore MO can be more effectively accessed by the target sequences. The performance of the XNA probes has been compared to that of the DNA probe, using fixed nucleobase sequences, so that the effect of backbone variation could be investigated. To our knowledge, this is the first report on molecularly resolved nucleic acid sensing by non-ionic capture probes, here, MO and PNA.

Improved nucleic acid sensing in terms of single nucleobase mismatch discrimination, as achieved by the surface-confined non-ionic PNA and MO capture probes, is exemplified by single molecule force spectroscopy.     

Rapid detection of codon 460 mutations in the UL97 gene of ganciclovir-resistant cytomegalovirus clinical isolates by real-time PCR using molecular beacons

A rapid real-time polymerase chain reaction (PCR) assay using molecular beacons has been developed for the simultaneous detection of wild-type and mutant strains of cytomegaloviruses (CMV) with respect to codon 460 of the UL97 gene has been developed. The molecular beacons were designed to complement the wild-type codon 460 or the mutant sequence arising from a single base-pair difference (point mutation). Discrimination between wild-type and mutant templates was demonstrated as the beacons did not generate fluorescence with their respective mismatch targets but only with those that they were designed to perfectly anneal with. Samples that harbor mixed populations of CMV could also be readily recognized. Applied to a small number of clinical samples, the retrospective screening by this assay are in general concordance with that obtained by PCR-RFLP. Using molecular beacons strategy, codon 460 mutation was detected in ten out o the total number of 40 samples, whereas the latter method identified nine samples as containing the mutation. The discrepant result arose from the genotyping of one clinical sample as mixed (containing both wild-type and mutant CMV strains) by molecular beacons but as wild-type by PCR-RFLP, suggesting that this real-time strategy is possibly more sensitive for mutation analysis.     

Bioelectronic detection of point mutations using discrimination of the H63D polymorphism of the Hfe gene as a model.

BACKGROUND: A bioelectronic detection platform has recently been developed that facilitates the detection and characterization of nucleic acids. The DNA chip platform is compatible with homogeneous assays because separate labeling and wash steps are not required. A one-step, bioelectronic detection assay was developed to genotype patient samples with respect to the H63D polymorphism of the Hfe gene, associated with hereditary hemochromatosis. METHODS AND RESULTS: Electrode arrays were modified with DNA capture probes that were perfectly matched to the wild-type or mutant allele of H63D. Amplicons containing the polymorphic site were hybridized with the capture probes on the electrode arrays in the presence of electronically labeled reporter (signaling) probes. Voltammetric analysis of the electrode arrays was conducted first at ambient temperature and then at elevated temperature. The electronic signal was preferentially diminished at elevated temperature from electrodes that hybridized with mismatched target amplicons. CONCLUSION: An assay for bioelectronic genotyping of the H63D polymorphism was developed and used with six patient specimens to show the feasibility of this system as a model for point mutation detection.     

应用寡核苷酸芯片进行HLA-DQA1位点基因分型

为了构建HLA-DQA1位点寡核苷酸分型芯片并籍此建立一种应用于HIA系统基因分型的集成化技术平台，在多态性集中的HLA-DQA1第二外显子区域，自行设计一套寡核苷酸分型探针；采用本实验室自建的方法，制成寡核苷酸芯片。提取的基因组DNA以组间特异性引物，单侧引物荧光标记，行不对称扩增。杂交后扫描检测分析杂交信号，确定HLA等位基因型；分型结果以标准DNA和PCR产物测序检测。结果表明：100例样本芯片分型全部成功，其中50例标准DNA与其模板相符，另50例临床样本中随机选取的10例与其测序结果相符，其中2例分型结果不完全；重复杂交实验中，位点重现率95％。结论：研制的HLA-DQA1等位基因分型芯片，其准确性和重现性理想，且操作简便、快捷，有广泛的应用前景。     

The enhancement of homogenous mass extension reaction: comparison of two enzymes

Reliable and efficient PCR and extension reactions using standardized procedures are key elements for successful single nucleotide polymorphism (SNP) genotyping projects. To improve the cost efficiency and overall performance of SNP genotyping we evaluated two commercial thermostable DNA polymerases used for the extension reaction in the homogeneous mass extension MassARRAY genotyping system. The aim was to study whether the quality, accuracy, and expenses of a new TERMIPol DNA polymerase are competitive to the commonly used ThermoSequenase DNA polymerase. We compared the enzymes by testing 96 SNPs genotyped for DNA samples of 31 unrelated individuals and one water control. The success rates, congruence between the genotypes and completeness of extension reactions support the use of TERMIPol, especially when the amplification of the higher mass allele is difficult. Further, using TERMIPol enabled successful genotyping (>93%) of several SNPs that failed (<80% success) when using ThermoSequenase.