Quantitative reverse transcription PCR (RT-qPCR) is used when the starting material is RNA. In this method, RNA is first transcribed into complementary DNA (cDNA) by reverse transcriptase from total RNA or messenger RNA (mRNA). The cDNA is then used as the template for the qPCR reaction. RT-qPCR is used in a variety of applications including gene expression analysis, RNAi validation, microarray validation, pathogen detection, genetic testing, and disease research.
RT-qPCR can be performed in a one-step or a two-step assay (Figure 1, Table 1). One-step assays combine reverse transcription and PCR in a single tube and buffer, using a reverse transcriptase along with a DNA polymerase. One-step RT-qPCR only utilizes sequence-specific primers. In two-step assays, the reverse transcription and PCR steps are performed in separate tubes, with different optimized buffers, reaction conditions, and priming strategies.
Figure 1. One-Step vs. Two-Step RT-qPCR.
When designing a RT-qPCR assay it is important to decide whether to use total RNA or purified mRNA as the template for reverse transcription. mRNA may provide slightly more sensitivity, but total RNA is often used because it has important advantages over mRNA as a starting material. First, fewer purification steps are required, which ensures a more quantitative recovery of the template and a better ability to normalize the results to the starting number of cells. Second, by avoiding any mRNA enrichment steps, one avoids the possibility of skewed results due to different recovery yields for different mRNAs. Taken together, total RNA is more suitable to use in most cases since relative quantification of the targets is more important for most applications than the absolute sensitivity of detection1.
Three different approaches can be used for priming cDNA reactions in two-step assays: oligo(dT) primers, random primers, or sequence specific primers (Figure 2, Table 2). Often, a mixture of oligo(dT)s and random primers is used. These primers anneal to the template mRNA strand and provide reverse transcriptase enzymes a starting point for synthesis.
|Primer Options||Structure and Function||Advantages||Disadvantages|
|Oligo(dT)s (or anchored oligo(dT)s)||Stretch of thymine residues that anneal to poly(A) tail of mRNA; anchored oligo(dT)s contain one G, C, or A (the anchor) residue at the 3′ end|
|Random Primers||Six to nine bases long, they anneal at multiple points along RNA transcript|
|Sequence Specific Primers||Custom made primers that target specific mRNA sequence|
Reverse Transcriptase is the enzyme that makes DNA from RNA. Some enzymes have RNase activity to degrade the RNA strand in the RNA-DNA hybrid after transcription. If an enzyme does not possess RNase activity, an RNaseH may be added for better qPCR efficiency. Commonly used enzymes include Moloney murine leukemia virus reverse transcriptase and Avian myeloblastosis virus reverse transcriptase. For RT-qPCR, it is ideal to choose a reverse transcriptase with high thermal stability, because this allows cDNA synthesis to be performed at higher temperatures, ensuring successful transcription of RNA with high levels of secondary structure, while maintaining their full activity throughout the reaction producing higher cDNA yields.
RNase H activity degrades RNA from RNA-DNA duplexes to allow efficient synthesis of double-stranded DNA. However, with long mRNA templates, RNA may be degraded prematurely resulting in truncated cDNA. Hence, it is generally beneficial to minimize RNase H activity when aiming to produce long transcripts for cDNA cloning. In contrast, reverse transcriptases with intrinsic RNase H activity are often favored in qPCR applications because they enhance the melting of RNA-DNA duplex during the first cycles of PCR (Figure 3).
PCR primers for the qPCR step of RT-qPCR should ideally be designed to span an exon-exon junction, with one of the amplification primers potentially spanning the actual exon-intron boundary (Figure 4). This design reduces the risk of false positives from amplification of any contaminating genomic DNA, since the intron-containing genomic DNA sequence would not be amplified.
If primers cannot be designed to separate exons or exon-exon boundaries, it is necessary to treat the RNA sample with RNase-free DNase I or dsDNase in order to remove contaminating genomic DNA.
A minus Reverse Transcription control (-RT control) should be included in all RT-qPCR experiments to test for contaminating DNA (such as genomic DNA or PCR product from a previous run). Such a control contains all the reaction components except for the reverse transcriptase. Reverse transcription should not occur in this control, so if PCR amplification is seen, it is most likely derived from contaminating DNA.
Figure 4. Primer Design for the qPCR step of RT-qPCR. 1) If one primer is designed to span an exon-intron boundary, the possible contaminating genomic DNA is not amplified, because the primer cannot anneal to the template. In contrast, cDNA does not contain any introns, and is efficiently primed and amplified. 2) When primers flank a long (e.g. 1 kb) intron, the amplification cannot occur because the short extension time is sufficient for the short cDNA sequence but not for the longer genomic target.
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