QRT-PCR

Quantitative reverse transcriptase PCR (QRT-PCR or qRT-PCR) is a PCR technique used to determine the amount of cDNA in a sample. It is the most commonly used form of quantitative PCR (qPCR). This technique is also called real-time reverse transcriptase PCR.

Comparison of normalisation methods

There is an ongoing debate what is the best way to normalise qPCR data. Reference genes are the most common method, although single unverified reference genes invalidate the qPCR data generated. Total RNA, ribosomal RNA, and genomic DNA have been suggested as alternative methods.

Reference genes

Most common method. Best practise is a panel, e.g. [1] not just a single reference gene and including data on suitability as reference genes. Often housekeeping gene  is used here instead of reference gene but the term is poorly defined and can be misleading.

RNA

Total rRNA [2] [3], or total RNA. Drawback: rapidly dividing cells will have more rRNA and different rRNA/mRNA ratio which will complicate comparison; difference in cDNA synthesis not taken into account.

Genomic DNA

Genomic DNA or cell number. Drawbacks: RNA degrades faster than RNA which can distort the data; sample cannot be DNase treated; efficiency of cDNA synthesis not taken into account.

Reference mRNAs

Main article: Choosing reference genes for qPCR normalisation

Picking reference genes will make or break your quantification via qPCR (real time PCR). If you pick only one reference gene and your pick is not constant across different conditions or samples, your results will be skewed. Choose several reference genes and check whether they satisfy the criteria for a good reference gene. Some commonly used reference genes, like 18S and GAPDH, are known to be problematic but continue to be used.

• Ajeffs 06:55, 21 April 2007 (EDT): Screen a handful of ref genes, select the most stable using genorm, bestkeeper etc, use at least 2 reference genes for subsequent reactions and normalisation. Inlcude your genorm M values when publishing qPCR data.

Primer selection

Main article: Choosing primers for qPCR

Choosing suitable primers is an early crucial step in your qPCR experiment. Reusing a tested primer pair from a repository or publication can save you some time. Otherwise primer selection from scratch is similar to that for a standard qualitative PCR experiment but the product size is typically much smaller (below 200nt) and the amplification characteristics of the primer have to be rigorously tested.

Quantification methods

There are 3 common quantification methods. The standard curve method is the only one that gives you are absolute concentration. Both the Pfaffl method and the ΔΔC_t method produce relative data with the Pfaffl method being superior.

Standard curve method

• requires template at known concentration (e.g. cDNA or TA cloned PCR product)
• requires dilution series of known template for standard curve (more wells)
• yields absolute concentrations by comparing unknown samples to known

Pfaffl method

• requires that primer efficiency be known but needs to be determined only once with a standard curve
• produces relative amount (e.g. treated is 2x untreated)

(named after the inventor; see Pfaffl 2001 PMID 11328886)

ΔΔC_t (delta delta Ct)

• easiest, oldest, least reliable
• assumes that primers for unknown and reference gene have very similar efficiency

or that v little correction is necessary (i.e. reference gene almost same level)

• yields relative amounts

(Ct = cycle threshold; point when fluorescence reading surpasses a set baseline)

Sources of variability

Operator

Due to the small amount of liquid handled and the sensitivity of the technique, operator variability is high. Bustin reports that the same qPCR experiment repeated by 3 people using the same reagents lead to very different copy number estimations [Bustin 2002 PMID 12200227, figure 3]:

• person A: 8·7 × 10⁵
• person B: 2·8 × 10⁵ different by a factor of 3!
• person C: 2·7 × 10³ different by a factor of 300!!

Reagent lots/age

Different lots of reagents can lead to different results. Experiment repeated by same operator 5 times, same RNA sample, different kits; values are copies/μg total RNA:

• kit 1: 13±32 × 10⁷
• kit 2: 5.4±1.6 × 10⁷ - different by a factor of 2.4

Similar experiment with old (9 months 4°C) and new probe (3 months 4°C), values are copies/μg total RNA:

• old: (5.6 ± 1.3) x 10³
• new: (3.8 ± 0.6) x 10⁸ - different by a factor of 100'000!!

both experiments above from [Bustin 2002 PMID 12200227, figure 4]

Notes

• The most commonly used specialist reverse transcriptase enzyme for cDNA production is AMV reverse transcriptase. It has RNase H activity (so that RNA molecules are only transcribed once) and has a high temperature stability (to reduce RNA secondary structure and nonspecific primer annealing) [1].
• Since RNA can degrade with repeated freeze-thaw steps, experimental variability is often seen during successive reverse transcription reactions of the same RNA sample [1].
• Reverse transcriptase enzymes are notorious for their thermal instability. Repeated removals from the freezer can degrade the efficiency of the enzyme [1].
• Producing total cDNA from total RNA can be advantageous because
  1. cDNA is more stable than RNA so making total cDNA allows you to make multiple sequence-specific RNA measurements [1].
  2. This approach could reduce experimental variability stemming from RNA degradation [1].
• To make total cDNA
  1. Use a polyT primer (most but not all eukaryotic mRNA) or random decamers (prokaryotic mRNA) [1].
  2. Random decamers give longer cDNAs on average than random hexamer primers [1].
  3. Use longer reverse transcription reaction times [1].
  4. Ensure that the concentration of deoxynucleotides doesn't run out [1].

Appendix

Specific protocols

• Endy:Real-time RT-PCR (in progress)
• qRT-PCR/Two tubes (in progress)
• qRT-PCR/Single tube

See also

• Q-PCR and Real-time PCR
• PCR techniques

External links

• venerable qpcrlistserv. Anyone doing qPCR should be subscribed to this list.

• excellent, detailed qPCR tutorial with many figures by Margaret and Richard Hunt, University of South Carolina
• detailed qPCR tutorial by East Tennessee State University facility
• gene-quantification.info - a site dedicated to qPCR edited by M W Pfaffl, Technical University Munich (very busy design, lots of advertisement but a real trove of qPCR info)
• qPCR (real-time PCR) backgrounder without figures but with many links by Tevfik Dorak
• 15 page comparison of PCR vs qPCR with several figures by Applied Biosystems
• short qRT-PCR summary by Ambion

References

1. ISBN:0415374723