Volume 10 Supplement 3
RExPrimer: an integrated primer designing tool increases PCR effectiveness by avoiding 3' SNP-in-primer and mis-priming from structural variation
- Jittima Piriyapongsa†1,
- Chumpol Ngamphiw†1,
- Anunchai Assawamakin†2, 1,
- Pongsakorn Wangkumhang1,
- Payiarat Suwannasri3,
- Uttapong Ruangrit1,
- Gallissara Agavatpanitch1 and
- Sissades Tongsima1Email author
© Piriyapongsa et al; licensee BioMed Central Ltd. 2009
Published: 3 December 2009
Polymerase chain reaction (PCR) is very useful in many areas of molecular biology research. It is commonly observed that PCR success is critically dependent on design of an effective primer pair. Current tools for primer design do not adequately address the problem of PCR failure due to mis-priming on target-related sequences and structural variations in the genome.
We have developed an integrated graphical web-based application for primer design, called RExPrimer, which was written in Python language. The software uses Primer3 as the primer designing core algorithm. Locally stored sequence information and genomic variant information were hosted on MySQLv5.0 and were incorporated into RExPrimer.
RExPrimer provides many functionalities for improved PCR primer design. Several databases, namely annotated human SNP databases, insertion/deletion (indel) polymorphisms database, pseudogene database, and structural genomic variation databases were integrated into RExPrimer, enabling an effective without-leaving-the-website validation of the resulting primers. By incorporating these databases, the primers reported by RExPrimer avoid mis-priming to related sequences (e.g. pseudogene, segmental duplication) as well as possible PCR failure because of structural polymorphisms (SNP, indel, and copy number variation (CNV)). To prevent mismatching caused by unexpected SNPs in the designed primers, in particular the 3' end (SNP-in-Primer), several SNP databases covering the broad range of population-specific SNP information are utilized to report SNPs present in the primer sequences. Population-specific SNP information also helps customize primer design for a specific population. Furthermore, RExPrimer offers a graphical user-friendly interface through the use of scalable vector graphic image that intuitively presents resulting primers along with the corresponding gene structure. In this study, we demonstrated the program effectiveness in successfully generating primers for strong homologous sequences.
The improvements for primer design incorporated into RExPrimer were demonstrated to be effective in designing primers for challenging PCR experiments. Integration of SNP and structural variation databases allows for robust primer design for a variety of PCR applications, irrespective of the sequence complexity in the region of interest. This software is freely available at http://www4a.biotec.or.th/rexprimer.
Polymerase Chain Reaction (PCR) is a common laboratory technique in biological and medical sciences, with a wide range of applications such as DNA cloning, DNA resequencing for single nucleotide polymorphism (SNP) discovery and quantification of gene expression. In the design of any PCR experiment, the first step of designing oligonucleotide primer pairs is crucial for the success of the experiment. Selection of inappropriate primers can result in no amplification (PCR failure) or amplification of non-targeted regions (mis-priming). Therefore, the primer pair is tailored to be specific to the desired target sequence.
The design of target-specific primers for PCR experiments typically requires consideration of different types of genomic information besides the target DNA sequence, such as repetitive DNA elements, intron/exon boundaries, and SNPs, which must be retrieved from various databases. All information is then combined to construct a sequence template for a primer design program. To confirm their specificity, designed primers are usually aligned against the corresponding genome sequence using tools like BLAST , BLAT , and PrimerBLAST in NCBI. If the aligned results return multiple hits, then the primers are regarded as non-specific and have to be redesigned by constructing a new template avoiding previously considered primer-binding regions. The whole process needs to be repeated manually until the desired primers are found. Thus, manually assigning an appropriate primer pair can be a tedious and time-consuming process, especially when high-throughput assays are required.
To resolve this situation, a number of automated primer designing tools have been developed based on Primer3  as web applications. These programs include SNPbox , ELXR , ExPrimer , MutScreener , EasyExonPrimer , PrimerZ , and others. Most existing tools are limited to specific regions of the human genome and hence they are not flexible enough for users to choose desired target genomic regions (e.g. promoter, intron/exon, SNP) to be amplified. After primers have been picked, most of the tools use UCSC In-Silico PCR  to verify the uniqueness of desired primer pair; however, when the selected primers perform poorly, the information from these unsuccessful primer pairs is not considered by these tools for redesigning primers.
While these programs provide some solutions related to the aforementioned primer design process, they are not always able to effectively design primers for two main problems, namely 1) no amplification due to severe mismatching, or lack of target and 2) mis-priming (non-specific binding besides the target sequence). These two problems lead to increased PCR failure rate . The first problem may arise because of unexpected SNPs in the primer 3' end (SNP-in-Primer). Alternatively, insertion/deletion (indel) polymorphisms may exist which either alter the length of the desired target, or prevent primer binding to the desired target. In some cases, the target sequence may be entirely absent, e.g. copy number variation (CNV) covering large stretches of DNA. Three prominent primer-designing tools that attempt to avoid SNP-in-Primer include ExonPrimer , EasyExonPrimer  and VariantSEQr . However, it is becoming increasingly clear that CNVs are also common and population-specific . Length and copy number polymorphisms can also cause both mismatching and non-primer binding to the desired target, yet these genetic variants are not considered by current primer designing tools. The second problem of mis-priming arises from the structural complexity of the genome. The human genome has many layers of repetition ranging from widespread chromosome segmental duplications, to gene families and pseudogenes to numerous repetitive elements (e.g. SINES, LINES, satellite sequences, etc.), which can all contribute to mis-priming . To our knowledge, there are no primer designing tools that can simultaneously address both of these issues.
To address these issues, we present a graphical web-based tool, named RExPrimer, which allows users to automatically design PCR primer pairs for amplifying human genomic sequence without leaving the website. RExPrimer uses Primer3 as the design core, since this open source software has been continuously adopted by research communities as the de facto standard [4–9]. The novel modules that address the aforementioned problems were created on top of the Primer3 core by locally incorporating annotated human genomic sequences. RExPrimer assesses primer candidates for SNP-in-Primer, indel polymorphisms, CNV, and related target sequences (e.g. pseudogenes) by crosschecking with local databases. Large integrated SNP and indel polymorphism databases can notify SNP-in-Primer effects, while information from structural variation databases identifies possible mis-priming. RExPrimer uniquely offers a redesign module for assisting users to correct the notified problems.
RExPrimer offers three modules of primer design: 1) for resequencing genomic DNA (promoter, exon/intron boundary, any genomic region), 2) for SNP genotyping (gene based and region based), and 3) for oligonucleotide checking. The following types of identifiers are supported as input: HUGO gene name, NCBI Gene ID, and chromosomal locations. Since there are also other interesting non-gene regions, such as regulatory regions and intergenic regions, our program also supports arbitrary genomic regions as input based on chromosome location. For the SNP genotyping module, the program allows one additional input format as a SNP ID (rs-number) or set of SNP IDs. Because this application supports batch design for SNP genotyping primers, it can enhance productivity of high-throughput SNP sequencing projects.
After the input is received, the local human genome database module is interrogated and the corresponding target sequence information, including the genomic sequence, annotations (e.g., promoters, introns, exons) and polymorphisms is subsequently identified and retrieved using query language. When a gene name is used as an input, more than one associated sequence, such as splicing variants may be found in the database. In this case, users have an option to choose the desired isoform. Then, the program supplies the option of selecting the gene regions to be amplified: whole gene (only exons or all gene regions), region of interest (specified by intron/exon number e.g. from intron/exon x to intron/exon y, list of non-contiguous intron/exon).
After all of the target DNA sequence information is retrieved from the local database module, it is passed as input to Primer3 for the automatic generation of primer pairs. The RExPrimer user interface allows the users to specify parameters for Primer3 primer selection, such as product size, melting temperature (Tm), GC content. If desired, each designed primer could be screened against a repeat database to reduce nonspecific priming by choosing this available option. The amplicon size is user-defined according to the constraints of the experiment, e.g. accurate sequencing limits amplicons to a few hundred base pairs. If the input sequence is larger than a user-specified product size limit, the program automatically subdivides the template sequence into smaller segments with user-defined segment overlap size in order to ensure that the overlapping sequences can give high quality sequencing data. Primers are then designed separately for each overlapping fragment.
When strong primer-destabilizing effects such as SNP-in-Primer are reported, users can immediately redesign primers without repeating the same procedure again. Optionally, the users can directly specify the approximate range of primer or target region, e.g., to avoid regions of high SNP density. The redesign of primer pairs can then be forced to a confined user-specified region.
Case study: CYP2D6
To validate whether RExPrimer is effective in designing primers in challenging PCR applications, RExPrimer was used to design primers for the Cytochrome P450 2D6 locus (CYP2D6), which is thought as one of the most important enzymes for the metabolism of many clinically used drugs. CYP2D6 is highly polymorphic, which can be expressed as variation in CYP2D6-related drug metabolism among individuals. Besides a growing number of SNPs, numerous CNVs have been reported worldwide for this locus. Therefore, extensive population-wide study of genetic variations at CYP2D6 has medical importance. There are several problems that could interfere with the accuracy of genotyping, including:
Pseudogenes (CYP2D7P and CYP2D8P which contain almost 98% of sequence homology to CYP2D6)
CNVs from unequal crossing-over
SNPs reported to influence the enzyme activity
Existing primer design tools could have failed to consider the effect from CYP2D6 pseudogenes, SNPs, and CNVs. Mis-priming to pseudogenes can lead to artifactual PCR. SNPs and CNVs can lead to mismatching and non-primer binding to the desired region. The number of potential primers that are specific to CYP2D6 are thus limited.
The designed primers for CYP2D6 analysis obtained from RExPrimer
Sequence 5' → 3'
whole gene amplification
one-copy internal standard in penta-plex PCR
two-copy internal standard in penta-plex PCR
In this study, we have developed a comprehensive tool for PCR primer design which covers a broad range of functionalities including resequencing target genes and SNP genotyping. The primer design pipeline is composed of three key components: 1) pseudogene detection, 2) primer design core, and 3) SNP-in-Primer/genomic variation notification. Additional file 1 presents the feature comparison among existing primer designing software.
RExPrimer utilizes the publicly available sequence information including the human genome and annotation database, SNP databases, and the pseudogene database. By incorporating these databases and the Primer3 program, reliable and accurate primer designs can be achieved. The current pseudogene database module comprises of approximately sixteen thousand pseudogenes  while the genomic variation module (indels, CNVs, inversions) consists a total of thirty-eight thousand entries . SNP database hosts more than nineteen million common and population specific SNPs from various populations, which is larger and more comprehensive than existing primer design programs [8, 12, 13].
Most primer design tools verify the uniqueness of the PCR target sequence by using UCSC In-Silico PCR  after the primer candidates are picked. However, if the desired target has no unique segments, the primer specificity search would run indefinitely, thus slowing down the primer generation procedure. RExPrimer avoids this problem by excluding target regions shared with pseudogenes and other related sequences before primers are generated. The program also takes care of other genomic variation issues. This was demonstrated in the CYP2D6 case study provided in the result section.
RExPrimer appends several key features before and after the primer design process using Primer3, enabling the selection of unique primer sequences and reliably amplifiable targets, which other currently available software cannot match. However, with the caveat that the primers designed are unique and the targets are amplifiable (no CNV or SNP-in-Primer), no extra claims for the actual performance of the primers beyond what is predicted by Primer3 are made. Hence, we have not attempted to determine the success rate of RExPrimer for designing primers, since the cost of performing multiple PCR experiments is not justified. Finally, RExPrimer has a major strength on its graphical web interface, especially the gene structure visualization, which makes RExPrimer intuitive and user friendly.
Currently, RExPrimer can offer the oligonucleotide primer design for human genomic sequence in which SNP and pseudogene data are largely available. In the future, the program and locally built databases will be expanded to support primer generation for different organisms once their pseudogene and polymorphism data are readily available. To enhance the quality of primer design, the local SNP databases will be automatically updated to make use of the latest information, including as wide a range of population diversity as available.
RExPrimer is a one-stop tool for PCR primer design, which can support high-throughput resequencing and mutation screening research. The incorporation of large SNP and genomic variation databases make it possible to efficiently detect sequence variation in the designed primers that might cause PCR failure. The notification system of target pseudogenes before the primer design step helps users to select the appropriate target regions, which can significantly shorten the design process. RExPrimer is shown in this study to be indeed effective for designing primers for CYP2D6. We expect that RExPrimer is able to fill the gap and accomplish current needs for automated primer design procedure.
Users provide input parameters required to design primer pairs such as gene name, SNP ID, genomic location.
The input parameters are used to query the targeted sequence from the local human genome sequence database, which was retrieved from NCBI and stored locally on our server.
RExPrimer prepares the input file for Primer3 from the primer conditions and target sequence. If the target sequence is greater than the target size parameter set in Primer3, the system will construct a set of overlapping fragments that covers the entire target sequence.
Primer3 processes the input files prepared in step 4 and generates the primer pair results in Primer3 format.
The primer pair results are then crosschecked against different locally constructed SNP and genomic variation databases covering a wide range of human SNP variation, CNV, and indel polymorphisms.
Primer pairs are visualized on the locus-specific region using scalable vector graphics (SVG). The software also provides primer information in HTML format (see Figure 3) as a link on the resulting graphic page (see Figure 2).
Each resulting primer pair can be validated for uniqueness using local In-Silico PCR (see Figure 3). All of the designed primer pair results presented on the graphic and HTML page can be linked-out to a RExPrimer redesign module (see Figure 2, 3).
RExPrimer stores public sequence information and genomic variant information in order to accelerate the speed of processing and allow the user to seamlessly unify the required information used in RExPrimer. These databases are hosted on MySQLv5.0, which offers several important services to RExPrimer. The human genome sequence database module was downloaded from NCBI build 36.3 to be used as template sequence as well as providing gene organization information. The SNP database module comprises of common and population-specific SNPs from various databases, namely NCBI dbSNP  build 129, HapMap  public release 27, JSNP  release 35, and ThaiSNP  release 2, which can notify SNP-in-Primers. The genomic variation module for human genome build 36 (hg18) consists of indels, inversions, CNVs data obtained from Database of Genomic Variant . Furthermore, the pseudogene database  for human genome build 36 was incorporated into the system to assist in detecting potential mis-priming.
Other papers from the meeting have been published as part of BMC Bioinformatics Volume 10 Supplement 15, 2009: Eighth International Conference on Bioinformatics (InCoB2009): Bioinformatics, available online at http://www.biomedcentral.com/1471-2105/10?issue=S15.
The authors would like to thank Dr. Chanin Limwongse for supporting with CYP2D6 experimental analysis and Philip J. Shaw for helpful remarks on the manuscript. We thank Kridsadakorn Chaichoompu for designing the RExPrimer web logo. We acknowledge the Cluster and Program Management Office, National Science and Technology Development Agency (Grant No. BT-B-02-IM-GI-5101). JP, CN, PW, UR, and ST were supported by BIOTEC. AA was partially supported by the Royal Golden Jubilee Ph.D. Program (Grant No. PHD/4.I.MU.45/C.1). PS was supported by the 90th anniversary of Chulalongkorn University fund, Chulalongkorn University graduate scholarship to commemorate the 72nd anniversary of His Majesty King Bhumibol Adulyadej.
This article has been published as part of BMC Genomics Volume 10 Supplement 3, 2009: Eighth International Conference on Bioinformatics (InCoB2009): Computational Biology. The full contents of the supplement are available online at http://www.biomedcentral.com/1471-2164/10?issue=S3.
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