Locked nucleic acid (LNA) single nucleotide polymorphism (SNP) genotype analysis and validation using real-time PCR

With an increased emphasis on genotyping of single nucleotide polymorphisms (SNPs) in disease association studies, the genotyping platform of choice is constantly evolving. In addition, the development of more specific SNP assays and appropriate genotype validation applications is becoming increasingly critical to elucidate ambiguous genotypes. In this study, we have used SNP specific Locked Nucleic Acid (LNA) hybridization probes on a real-time PCR platform to genotype an association cohort and propose three criteria to address ambiguous genotypes. Based on the kinetic properties of PCR amplification, the three criteria address PCR amplification efficiency, the net fluorescent difference between maximal and minimal fluorescent signals and the beginning of the exponential growth phase of the reaction. Initially observed SNP allelic discrimination curves were confirmed by DNA sequencing (n = 50) and application of our three genotype criteria corroborated both sequencing and observed real-time PCR results. In addition, the tested Caucasian association cohort was in Hardy-Weinberg equilibrium and observed allele frequencies were very similar to two independently tested Caucasian association cohorts for the same tested SNP. We present here a novel approach to effectively determine ambiguous genotypes generated from a real-time PCR platform. Application of our three novel criteria provides an easy to use semi-automated genotype confirmation protocol.     

Single nucleotide polymorphism (SNP) allele frequency estimation in DNA pools using Pyrosequencing

Identifying the genetic variation underlying complex disease requires analysis of many single nucleotide polymorphisms (SNPs) in a large number of samples. Several high-throughput SNP genotyping techniques are available; however, their cost promotes the use of association screening with pooled DNA. This protocol describes the estimation of SNP allele frequencies in pools of DNA using the quantitative sequencing method Pyrosequencing (PSQ). PSQ is a relatively recently described high-throughput method for genotyping, allele frequency estimation and DNA methylation analysis based on the detection of real-time pyrophosphate release during synthesis of the complementary strand to a PCR product. The protocol involves the following steps: (i) quantity and quality assessment of individual DNA samples; (ii) DNA pooling, which may be undertaken at the pre- or post-PCR stage; (iii) PCR amplification of PSQ template containing the variable sequence region of interest; and (iv) PSQ to determine the frequency of alleles at a particular SNP site. Once the quantity and quality of individual DNA samples has been assessed, the protocol usually requires a few days for setting up pre-PCR pools, depending on sample number. After PCR amplification, preparation and analysis of PCR amplicon by PSQ takes 1 h per plate.     

A New PCR Method: One Primer Amplification of PCR-CTPP Products

Polymerase chain reaction with confronting two-pair primers (PCR-CTPP) is a convenient method for genotyping single nucleotide polymorphisms, saving time, and costs. It uses four primers for PCR; F1 and R1 for one allele, and F2 and R2 for the other allele, by which three different sizes of DNA are amplified; between F1 and R1, between F2 and R2, and between F1 and R2. To date, we have applied PCR-CTPP successfully for genotyping more than 60 polymorphisms. However, it is not rare that PCR does not produce balanced amplification of allele specific bands. Accordingly, the method was modified by attaching a common sequence at the 5' end of two-pair primers and adding another primer with the common sequence in PCR, in total five different primers in a tube for PCR. The modification allowed one primer amplification for the products of initial PCR with confronting two-pair primers, named as one primer amplification of PCR-CTPP products (OPA-CTPP). This article demonstrates an example for an A/G polymorphism of paraoxonase 1 (PON1) Gln192Arg (rs662). PCR-CTPP failed clear genotyping for the polymorphism, while OPA-CTPP successfully produced PCR products corresponding to the allele. The present example indicated that the OPA-CTPP would be useful in the case that PCR-CTPP failed to produce balanced PCR products specific to each allele.     

高保真酶特异性检测大鼠SNP基因型新方法

目的应用高保真酶（Pfu）和3’末端修饰引物在单管双向等位基因特异性扩增（SB-ASA）中区分SNP基因型,建立高保真酶特异性检测SNP基因型的新方法。方法选取近交系大鼠SNP位点,以RS8149053为例,设计两个外部引物和两个等位基因特异性引物,四引物3’末端进行硫代磷酸化修饰,应用高保真聚合酶（Pfu）进行特异性扩增,扩增结果测序验证其可靠性。结果在RS8149053 SNP位点（C/T）上,等位基因型CC扩增出179 bp目的片段,基因型TT扩增出597 bp目的片段,基因型不同则扩增出分子量不同的片段,目的条带测序结果与Rat Genome Database数据库基因型结果一致,高保真酶扩增结果稳定且特异性强。结论高保真酶等位基因特异性扩增技术能有效降低假阳性率,是一种快速、特异的SNP基因分型新方法。     

Non-invasive fetal RHD and RHCE genotyping using real-time PCR testing of maternal plasma in RhD-negative pregnancies.

We assessed the feasibility of fetal RHD and RHCE genotyping by analysis of DNA extracted from plasma samples of RhD-negative pregnant women using real-time PCR and primers and probes targeted toward RHD and RHCE genes. We analyzed 45 pregnant women in the 11th to 40th weeks of pregnancy and correlated the results with serological analysis of cord blood after delivery. Non-invasive prenatal fetal RHD exon 7, RHD exon 10, RHCE exon 2 (C allele), and RHCE exon 5 (E allele) genotyping analysis of maternal plasma samples was correctly performed in 45 out of 45 RhD-negative pregnant women delivering 24 RhD-, 17 RhC-, and 7 RhE-positive newborns. Detection of fetal RHD and the C and E alleles of RHCE gene from maternal plasma is highly accurate and enables implementation into clinical routine. We recommend performing fetal RHD and RHCE genotyping together with fetal sex determination in alloimmunized D-negative pregnancies at risk of hemolytic disease of the newborn. In case of D-negative fetus, amplification of another paternally inherited allele (SRY and/or RhC and/or RhE positivity) proves the presence of fetal DNA in maternal circulation.     

Accessing single nucleotide polymorphisms in genomic DNA by direct multiplex polymerase chain reaction amplification on oligonucleotide microarrays

This study introduces a DNA microarray-based genotyping system for accessing single nucleotide polymorphisms (SNPs) directly from a genomic DNA sample. The described one-step approach combines multiplex amplification and allele-specific solid-phase PCR into an on-chip reaction platform. The multiplex amplification of genomic DNA and the genotyping reaction are both performed directly on the microarray in a single reaction. Oligonucleotides that interrogate single nucleotide positions within multiple genomic regions of interest are covalently tethered to a glass chip, allowing quick analysis of reaction products by fluorescence scanning. Due to a fourfold SNP detection approach employing simultaneous probing of sense and antisense strand information, genotypes can be automatically assigned and validated using a simple computer algorithm. We used the described procedure for parallel genotyping of 10 different polymorphisms in a single reaction and successfully analyzed more than 100 human DNA samples. More than 99% of genotype data were in agreement with data obtained in control experiments with allele-specific oligonucleotide hybridization and capillary sequencing. Our results suggest that this approach might constitute a powerful tool for the analysis of genetic variation.     

四引物扩增受阻突变体系PCR快速测定绵羊 BMPR-IB基因型方法的建立

四引物ARMS PCR是检测SNP有效、快速、简便的方法.绵羊BMPR-lB基因是控制Booroola绵羊多胎性状的主效基因,此研究目的在于建立一种对BMPR-IB基因四引物ARMS PCR检测方法.根据四引物ARMS PCR技术原理,在绵羊BMPR-IB基因突变位点(A746G)设计一对特异性引物,并在突变点两侧设计一对参照引物,用来扩增含有突变点的DNA片段,可在一步PCR反应中根据电泳图谱准确判断绵羊个体的BMPR-IB基因型,对比PCR-RFLP检测结果表明,所建立的方法简单,操作简便,大大提高了检测效率.     

Estimation of allele frequency in pooled DNA by using PCR–RFLP combined with microchip electrophoresis

Biallelic marker, most commonly single nucleotide polymorphism (SNP), is widely utilized in genetic association analysis, which can be speeded up by estimating allele frequency in pooled DNA instead of individual genotyping. Several methods have shown high accuracy and precision for allele frequency estimation in pools. Here, we explored PCR restriction fragment length polymorphism (PCR–RFLP) combined with microchip electrophoresis as a possible strategy for allele frequency estimation in DNA pools. We have used the commercial available Agilent 2100 microchip electrophoresis analysis system for quantifying the enzymatically digested DNA fragments and the fluorescence intensities to estimate the allele frequencies in the DNA pools. In this study, we have estimated the allele frequencies of five SNPs in a DNA pool composed of 141 previously genotyped health controls and a DNA pool composed of 96 previously genotyped gastric cancer patients with a frequency representation of 10–90% for the variant allele. Our studies show that accurate, quantitative data on allele frequencies, suitable for investigating the association of SNPs with complex disorders, can be estimated from pooled DNA samples by using this assay. This approach, being independent of the number of samples, promises to drastically reduce the labor and cost of genotyping in the initial association analysis.     

High fidelity of whole-genome amplified DNA on high-density single nucleotide polymorphism arrays

Current microarray technology allows researchers to genotype a large number of SNPs with relatively small amounts of DNA. Nevertheless, researchers and clinicians still frequently face the problem of acquiring enough high-quality DNA for analysis. Whole-genome amplification (WGA) methods offer a solution for this problem, and earlier studies have shown that WGA samples perform reasonably well in small-scale genetic analyses (e.g. Affymetrix 10K array). To determine the performance of WGA products on a large-scale genotyping array, we compared the Affymetrix 250K array genotyping results of genomic DNA and their WGA products from four individuals. Our results indicate that WGA product performs well on the 250K array compared to genomic DNA, especially when using the BRLMM calling algorithm. WGA samples have high call rates (97.5% on average, compared to 99.4% for genomic DNA) and excellent concordance rates with their corresponding genomic DNA samples (98.7% on average). In addition, no apparent systematic genomic amplification bias can be detected. This study demonstrates that, although there is a slight decrease in the total call rates, WGA methods provide a reliable approach for increasing the amount of DNA samples for use with a common SNP genotyping array.     

Testing the reliability of noninvasive genetic sampling by comparing analyses of blood and fecal samples in Barbary macaques (Macaca sylvanus).

Genetic studies of wild animal populations are often hindered by difficulties in obtaining blood samples. Recent advances in molecular biology have allowed the use of noninvasive samples as sources of DNA (e.g., hair or feces), but such samples may provide low-quality DNA and prevent the determination of true genotypes in subsequent DNA analysis. We present a preliminary study aimed at assessing the reliability of using fecal samples for genotyping in Barbary macaques (Macaca sylvanus). The test was performed on samples of blood and feces from 11 captive animals, using three dinucleotide microsatellites. The CTAB DNA extraction method was found to be the most relevant for Barbary macaque feces, yielding successful amplification at all loci for 70% of PCRs. All the fecal samples tested gave correct genotypes at least once for each locus when referenced against blood-derived genotypes. An average of 18.3% of PCRs displayed spurious genotypes (false homozygous or false allele). The minimum theoretical probability required to obtain a 100% accurate genotype is 0.74, based on the criterion that a correct genotype is assessed only if it was observed at least twice. The observed probability of obtaining a correct genotype from three PCRs, based on our genotyping results, was greater (0.81 on average) than the minimum threshold. In conclusion, our comparison of blood and fecal samples showed that fecal sampling is a reliable tool for the further study of wild Barbary macaque populations.     

Identification of N-acetyltransferase 2 genotypes by continuous monitoring of fluorogenic hybridization probes.

Three polymorphic sites in the N-acetyltransferase 2 (NAT2) gene were detected using rapid cycle DNA amplification with allele-specific fluorescent probes and melting curve analysis. Two fluorogenic adjacent hybridization probes were designed to NAT2*5A (C(481)T), NAT2*6A (G(590)A), and NAT2*7A (G(857)A). During amplification, probe hybridization is observed as fluorescence resonance energy transfer. The fluorescence increases every cycle as the product accumulates during amplification. A single base mismatch resulted in a melting temperature shift (T(m)) of 5 to 6 degrees C, allowing for the easy distinction of a wild-type allele from the mutant allele. The protocol is rapid, requiring 40 min for the completion of 45 cycles including the melting curves. It is also a simple and flexible method, since DNA templates prepared from different sources, including DNA from serum and paraffin-embedded tissue sections, could be used without adverse effects. Fluorescence genotyping of all three polymorphisms in a total of 155 DNA samples correlated perfectly with our previously validated genotyping by restriction enzyme digestion (PCR-RFLP). This new facile approach allows for the easy detection of NAT2 polymorphisms in hundreds of samples in only a day.     

Allele-specific amplification in cancer revealed by SNP array analysis

Amplification, deletion, and loss of heterozygosity of genomic DNA are hallmarks of cancer. In recent years a variety of studies have emerged measuring total chromosomal copy number at increasingly high resolution. Similarly, loss-of-heterozygosity events have been finely mapped using high-throughput genotyping technologies. We have developed a probe-level allele-specific quantitation procedure that extracts both copy number and allelotype information from single nucleotide polymorphism (SNP) array data to arrive at allele-specific copy number across the genome. Our approach applies an expectation-maximization algorithm to a model derived from a novel classification of SNP array probes. This method is the first to our knowledge that is able to (a) determine the generalized genotype of aberrant samples at each SNP site (e.g., CCCCT at an amplified site), and (b) infer the copy number of each parental chromosome across the genome. With this method, we are able to determine not just where amplifications and deletions occur, but also the haplotype of the region being amplified or deleted. The merit of our model and general approach is demonstrated by very precise genotyping of normal samples, and our allele-specific copy number inferences are validated using PCR experiments. Applying our method to a collection of lung cancer samples, we are able to conclude that amplification is essentially monoallelic, as would be expected under the mechanisms currently believed responsible for gene amplification. This suggests that a specific parental chromosome may be targeted for amplification, whether because of germ line or somatic variation. An R software package containing the methods described in this paper is freely available at http://genome.dfci.harvard.edu/~tlaframb/PLASQ.     

Quantitative determination of allele frequency in pooled DNA by using sequencing method

Quantitative determination of the allele frequency of single-nucleotide polymorphism (SNP) in pooled DNA samples is a promising approach to clarify the relationships between SNPs and diseases. Here, we present such a simple, accurate, and inexpensive method for quantitative determining the allele frequency in pooled DNA samples. Three steps of DNA pooling, PCR amplification and sequencing are involved in this assay. Although direct determination of the allele frequency from the two allele-specific fluorescence intensities is possible, correction for differential response of alleles is important. We explored the effect of differential response of alleles on test statistics and provide a solution to this problem based on heterozygous fluorescence intensities. We demonstrate the accuracy and reliability of this assay on pooled DNA samples with pre-determined allele frequencies from 7.1% to 53.9%. The accuracy of allele frequency measurements is high, with a correlation coefficient of r2 = 0.997 between measured and known frequencies. We believe that by providing a means for SNP genotyping up to hundreds of samples simultaneously, inexpensively, and reproducibly, this method is a powerful strategy for detecting meaningful polymorphic differences in candidate gene association studies.     

Comparison of three PCR-based methods to detect a Piedmontese cattle point mutation in the Myostatin gene

Despite many recent advances in single nucleotide polymorphism (SNP) genotyping technologies, there is still the need for methodologies with a reasonable throughput. In this study, we compared three PCR-based methods for SNP detection: (1) a conventional PCR-based allele detection system with fluorescent genotyping technology, (2) a SNaPshot methodology by single nucleotide primer extension and, (3) a real-time PCR-based method by allele-specific minor groove-binder probes. These three methodologies were used to analyze 104 meat samples of a particular Italian cattle breed known for producing excellent quality meat and for a characteristic increased development of muscle mass, caused by a point mutation (C313Y) in the Myostatin gene. The analysis revealed 98 samples to be homozygous (mh/mh) and five to be heterozygous (mh/+) for the mutation whereas one sample resulted to be homozygous for the wild type (+/+). The results obtained with the three different assays were consistent. Overall, all three methodologies proved to be efficient for allelic discrimination studies; however, real-time PCR was faster and allowed to genotype up to 96 samples in a single step, minimizing the number of steps required for samples manipulation.     

QuantiLyse: reliable DNA amplification from single cells

Amplification of DNA sequencesfrom single cells via PCR is increasingly used in basic research and clinical diagnostics but remains technically difficult. We have developed a cell lysis protocol that uses an optimized proteinase K solution, named QuantiLyse and permits reliable amplification from individual cells. This protocol was compared to other published methods by means of real-time PCR with molecular beacons. The results demonstrate that QuantiLyse treatment of single lymphocytes renders gene targets more availablefor amplification than other published proteinase K methods or lysis in water. QuantiLyse and an optimized alkaline lysis were equally effective in terms of target availability, although QuantiLyse offers greaterflexibility, as it does not require neutralization and can comprise a higher percentage of the final PCR volume. Maximum gene target availability is also obtained following QuantiLyse treatment of samples containing up to 10000 cells (the largest number tested). Thus, QuantiLyse maximizes the chances that targeted DNA sequences will be available for amplification during the first cycle of PCR, thereby reducing the variability among replicate reactions as well as the likelihood of amplification failure or allele drop-out. QuantiLyse will be useful in a range of investigations aimed at gene detection in small numbers of cells.     

Genomic organization of Tropomodulins 2 and 4 and unusual intergenic and intraexonic splicing of YL-1 and Tropomodulin 4

Background

Single nucleotide polymorphisms (SNPs) are the foundation of powerful complex trait and pharmacogenomic analyses. The availability of large SNP databases, however, has emphasized a need for inexpensive SNP genotyping methods of commensurate simpliCity, robustness, and scalability. We describe a solution-based, microtiter plate method for SNP genotyping of human genomic DNA. The method is based upon allele discrimination by ligation of open circle probes followed by rolling circle amplification of the signal using fluorescent primers. Only the probe with a 3' base complementary to the SNP is circularized by ligation.

Results

SNP scoring by ligation was optimized to a 100,000 fold discrimination against probe mismatched to the SNP. The assay was used to genotype 10 SNPs from a set of 192 genomic DNA samples in a high-throughput format. Assay directly from genomic DNA eliminates the need to preamplify the target as done for many other genotyping methods. The sensitivity of the assay was demonstrated by genotyping from 1 ng of genomic DNA. We demonstrate that the assay can detect a single molecule of the circularized probe.

Conclusions

Compatibility with homogeneous formats and the ability to assay small amounts of genomic DNA meets the exacting requirements of automated, high-throughput SNP scoring.     

基于荧光定量PCR扩增反应的SNP测定法

建立一种利用荧光定量PCR扩增反应进行单核苷酸多态性(SNP)快速测定的方法.以人β肾上腺素受体2基因中的Arg16Gly为研究对象,利用荧光染料SYBRGreenⅠ标记定量PCR产物,通过PCR生长曲线和融解曲线分析结果进行SNP分型.为提高SNP测定的特异性,分别在野生型和突变型等位基因的特异性引物3′端倒数第3个碱基位置,引入了一个人为错配碱基,使引物的错误延伸率显著降低,大大提高了SNP分析的准确性.通过DNA测序验证荧光定量PCR对β肾上腺素受体2基因中Arg16Gly分型结果的准确率.实验结果表明,所建立的方法操作简便,结果准确,适合进行大规模样品的SNP检测工作.     

MARA: a novel approach for highly multiplexed locus-specific SNP genotyping using high-density DNA oligonucleotide arrays

We have developed a locus-specific DNA target preparation method for highly multiplexed single nucleotide polymorphism (SNP) genotyping called MARA (Multiplexed Anchored Runoff Amplification). The approach uses a single primer per SNP in conjunction with restriction enzyme digested, adapter-ligated human genomic DNA. Each primer is composed of common sequence at the 5′ end followed by locus-specific sequence at the 3′ end. Following a primary reaction in which locus-specific products are generated, a secondary universal amplification is carried out using a generic primer pair corresponding to the oligonucleotide and genomic DNA adapter sequences. Allele discrimination is achieved by hybridization to high-density DNA oligonucleotide arrays. Initial multiplex reactions containing either 250 primers or 750 primers across nine DNA samples demonstrated an average sample call rate of ~95% for 250- and 750-plex MARA. We have also evaluated >1000- and 4000-primer plex MARA to genotype SNPs from human chromosome 21. We have identified a subset of SNPs corresponding to a primer conversion rate of ~75%, which show an average call rate over 95% and concordance >99% across seven DNA samples. Thus, MARA may potentially improve the throughput of SNP genotyping when coupled with allele discrimination on high-density arrays by allowing levels of multiplexing during target generation that far exceed the capacity of traditional multiplex PCR.     

Whole genome amplification of buccal cytobrush DNA collected for molecular epidemiology studies.

When cytobrush buccal cell samples have been collected as a genomic DNA (gDNA) source for an epidemiological study, whole genome amplification (WGA) can be critical to maintain sufficient DNA for genotyping. We evaluated REPLI-g WGA using gDNA from two paired cytobrushes (cytobush 'A' kept in a cell lysis buffer, and 'B' dried and kept at room temperature for 3 days, and frozen until DNA extraction) in a pilot study (n=21), and from 144 samples collected by mail in a breast cancer study. WGA success was assessed as the per cent completion/concordance of STR/SNP genotypes. Locus amplification bias was assessed using quantitative PCR of 23 human loci. The pilot study showed > 98% completion but low genotype concordance between cytobrush wgaDNA and paired blood gDNA (82% and 84% for cytobrushes A and B, respectively). Substantial amplification bias was observed with significantly lower human gDNA amplification from cytobrush B than A. Using cytobrush gDNA samples from the breast cancer study (n =20), an independent laboratory demonstrated that increasing template gDNA to the REPLI-g reaction improved genotype performance for 49 SNPs; however, average completion and concordance remained below 90%. To reduce genotype misclassification when cytobrush wgaDNA is used, inclusion of paired gDNA/wgaDNA and/or duplicate wgaDNA samples is critical to monitor data quality.     

寡核苷酸芯片法、实时PCR 和测序法进行乙肝病毒 基因分型的比较

目的:比较寡核苷酸芯片法、实时荧光PCR和测序法在对慢性乙肝患者病毒基因分型的比较和方法学评价。方法:对126例不同基因型的慢性乙肝患者的血清样本分别用寡核苷酸芯片法、实时荧光PCR法和测序法进行基因分型,并评价各种方法的临床表现、所需时间和检测成本。结果:寡核苷酸芯片法、实时荧光PCR分别能检测到1%和0.1%比例的基因型。在126例慢性乙肝患者的临床样本中,寡核苷酸芯片法、实时荧光PCR和测序法分别检测出41(33%)、41(33%)和45(36%)例为B型,76(60%)、76(60%)、81(64%)例为C型。寡核苷酸芯片法、实时荧光PCR均检出9例B、C混合基因型。在三种检测方法中实时荧光PCR是最快速和廉价的。结论:寡核苷酸芯片法、实时荧光PCR能检出B、C混合基因型,而测序法只能检测出样本的主导基因型。