In: Biology
Explain how qPCR works- discuss SYBR green, 2-step cycles, how quantitation is achieved with an explanation of threshold fluorescence, Ct or Cq, melting curve protocol and its significance
With the development of thermal cyclers incorporating
fluorescent detection, PCR has a new, innovative application. In
routine PCR, the critical result is the final quantity of amplicon
generated after the process. Real-time or Quantitative PCR and
RT-PCR use the linearity of DNA amplification to determine absolute
or relative amounts of a known sequence in a sample. By using a
fluorescent reporter in the reaction, it is possible to measure DNA
generation.
In quantitative PCR, DNA amplification is monitored at each cycle of PCR. When the DNA is in the log linear phase of amplification, the amount of fluorescence increases above the background. The point at which the fluorescence becomes measurable is called the Quantification Cycle (Cq) or crossing point. By using multiple dilutions of a known amount of standard DNA, a standard curve can be generated of log concentration against Cq. The amount of DNA or cDNA in an unknown sample can then be calculated from its Cq value.
A) The different phases of the reaction:
Baseline: The initial concentration of template is
low; therefore, the fluorescence intensity is too low to be
detected and only the background signal is evident.
Exponential: After the target yield has reached
the detection threshold, shown as the red threshold line, the
course of the reaction can be followed through the exponential
phase.
Linear: As the concentration of template
increases, the available DNA polymerase concentration reduces and
the reaction rate decreases.
Plateau: There is insufficient free enzyme to
continue amplification and so after this point, the reaction is at
the maximum yield, or the plateau phase.
B) Individual reactions are characterized by the
cycle at which fluorescence first rises above the threshold, which
is referred to as the Quantification Cycle (Cq). If the starting
material is abundant, amplification is observed in earlier cycles,
and the Cq is lower. If the starting material is scarce,
amplification is observed in later cycles, and the Cq is higher.
This correlation between fluorescence, Cq, and amount of amplified
product enables quantification of the template over a wide dynamic
range.
Real time PCR also lends itself to relative studies. A reaction may be performed using primers unique to each region to be amplified and tagged with different fluorescent dyes. Several commercially available quantitative thermal cyclers include multiple detection channels. In this multiplex system, the amount of target DNA/cDNA can be compared to the amount of a housekeeping sequence e.g. GAPDH or β-actin
SYBR Green qPCR Applications | |
Mass Screening | XX |
Microarray Validation | XX |
Multiple target genes/few samples | X |
SNP detection | NR |
Allelic discrimination | NR |
Pathogen detection | X |
Multiplexing | NR |
Viral load quantification | NR |
Gene expression analysis | X |
Gene copy determination | NR |
End point genotyping | NR |
in vitro quantification | NR |
NR= not recommended X= recommended XX= preferred method |
Assay Considerations
DNA Preparation
The single most important step in assuring success with PCR is high
quality DNA preparation. Integrity and purity of DNA template is
essential. Quantitative PCR involves multiple rounds of enzymatic
reactions and is therefore more sensitive to impurities such as
proteins, phenol/chloroform, salts, EDTA, and other chemical
solvents. Contaminants can also interfere with fluorescence
detection. The ratio of absorbance values at 260 nm and 280 nm
gives an estimate of DNA purity. Pure DNA has an A260/A280 ratio of
1.8-2.0. Lower ratios indicate the presence of contaminants such as
proteins.
Template
Very few copies of target nucleic acid (equivalent to about 100 pg
of gDNA or cDNA) are needed to initiate qPCR. To minimize
contamination with reaction inhibitors, the starting template
amount should be kept to the minimum required to achieve accurate
quantification. When the starting material is RNA, primer design
and DNase I treatment will reduce signals that may be generated
from gDNA contamination.
Primer Design
Whether using a dsDNA-binding dye or a probe-based detection
chemistry, designing high-quality primers is one of the most
crucial pre-experimental steps in qPCR. Specific primers for PCR
should be designed with the aid of primer design software to
eliminate the complications introduced with primer-dimers and
secondary structures. Lower primer concentrations decrease the
accumulation of primer-dimer formation and nonspecific product
formation, which is critical in using SYBR Green I dye in
quantitative PCR.
dNTPs
Standard PCR/qPCR mastermixes contain dATP, dCTP, dGTP, and dTTP.
However, some mixes are available that replace dTTP with dUTP.
Products from previous reactions run with dUTP will contain uracil
instead of thymine. These are then susceptible to cleavage by
Uracil-DNA-Glycosylase (UNG). Therefore, prior incubation of
subsequent reactions with UNG prevents carry-over contamination
between reactions. To be effective, all reactions in the laboratory
must use dUTP.
Magnesium Concentration
Magnesium chloride (MgCl2) is necessary for reverse transcriptase,
Taq DNA polymerase, and Taq DNA 5’ to 3’ exonuclease activity.
Optimum Mg2+ concentrations for reactions containing DLP are
usually between 3 – 6 mM. Lower magnesium chloride concentrations
usually result in the formation of fewer nonspecific products. Some
ReadyMix solutions are provided at a 2X concentration of 7 mM
magnesium chloride (final concentration 3.5 mM). In some cases, a
vial of a 25 mM magnesium chloride solution is provided for further
optimization of the final magnesium chloride concentration if
necessary. A reaction mix that does not contain MgCl2 may sometimes
be required so that a low concentration can be used, e.g. when
using Scorpion Probe detection.
Reverse Transcriptase
A reverse transcriptase enzyme that provides high yields of cDNA,
while retaining activity at high temperature, is critical to the
success of RT-qPCR. Performance at high temperatures helps to
ensure that regions of RNA with significant secondary structure are
destabilized and accessible for hybridization and subsequent
amplification. When performing one-step RT-qPCR, high-temperature
performance allows the use of gene-specific primers with high
melting temperatures (Tm), which increases reaction specificity.
When performing two step protocols, it is important to ensure that
the enzyme results in a linear and proportional yield of cDNA from
RNA. Minimizing pipetting can decrease variability. Some ReadyMixes
contain primers and other reagents needed to perform RT, for
example, ReadyScript® cDNA Synthesis Mix (RDRT).
Taq DNA Polymerase
As with selecting the most appropriate reverse transcriptase for
the RT, selection of the appropriate enzyme is vital. A fundamental
problem with natural Taq DNA polymerase is that the enzyme has
residual activity at low temperature. Non-specific primer binding
leads to non-specific product formation as a result of this
residual polymerase activity. Antibody-blocked or
chemically-blocked Taq DNA polymerases (‘hot-start’) help to
rectify this situation by preventing enzyme activity until the
high-temperature, denaturation step begins. Refer to the PCR Mix
Selection Guide to define the best hot-start polymerase for your
application.
Internal Reference Dye by Instrument Type
Some real-time PCR thermal cyclers require a loading dye such as
ROX to control for variability in the optical system and to
normalize differences in signal intensity. Likewise, some thermal
cyclers require fluorescein to create a virtual background when
working with SYBR Green I dye assays (which have very low
background). These may be supplied in the ReadyMix or as separate
components so the appropriate concentration can be used. In some
cases, a vial of internal reference dye is included for reaction
normalization. Maximum excitation of this dye is 586 nm and maximum
emission is 605 nm. Standard instrument settings for ROX reference
dye are satisfactory for the measurement of the internal reference
dye. This internal reference dye is necessary for ABI Sequence
Detection Systems.
Instruments
Reagents compatible with instruments will need to be selected.
Platforms use different normalization dyes, so reagents with
compatible normalization dyes will need to be selected (refer to
Appendix 1).
Many qPCR instruments have been designed to support a specific range of applications, e.g. contrast the ABI 7900 high throughput capability using automatic loading of 384-well plates with the Illumina Eco instrument that supports a single 48-well plate. The most suitable instrument meets the needs of the research. It is desirable to select an instrument with user friendly software that performs the most desirable functions and has flexibility in terms of data output so that it can easily be manipulated in downstream statistical analysis software packages. This reduces the time required to train personnel and therefore to begin generating results. Additional features that are required include a PCR block that is absolutely uniform (an absolute maximum deviation of 1Cq = 2 fold across 96 wells of replication) and an optical system that excites and detects emission as sensitively and as evenly as possible across a wide range of wavelengths. This allows for a wide choice of fluorophores and enables multiplexing. Other features to consider are the operating costs associated with specific consumables, e.g. if a standard microtiter plate is not used for reactions and also the convenience of loading plates/tubes that are non-standard format.
Controls
A positive control is always helpful to make sure all of the kit
components are working properly. A no template/negative control is
necessary to determine if contamination is present. A signal in the
no template control demonstrates the presence of DNA contamination
or primer dimer formation.
Buffer
Buffers or reaction mastermixes typically contain dNTPs, a Taq DNA
polymerase, MgCl2, and stabilizers. SYBR Green I, ROX™,
fluorescein, and inert loading dyes may also be included, depending
on the detection chemistry, instrument, and reaction requirements.
The PCR buffer components and stabilizers are typically proprietary
to the manufacturer. If purchased separately, maximum flexibility
is possible, since each ingredient can be individually optimized in
the reaction. However, in contrast, while purchasing the
ingredients together as a mastermix reduces flexibility, it
increases batch consistency and convenience while reducing the
number of pipetting steps, and hence, the chances of error and
contamination.
Data Analysis
Follow the recommendations of the real time instrument used to
perform quantitative SYBR Green PCR. The following may help new
instrument users. Generally the number of cycles is plotted against
the fluorescence. Threshold cycles (CTs) or crossing points are
used to determine the template amount in each sample. Threshold
cycle or crossing point is the first cycle that shows a detectable
increase in fluorescence due to the formation of PCR products. The
cycles before the crossing point are the baseline cycles. The
baseline cycles show no detectable increase in fluorescence due to
PCR products. The threshold used to determine when the first
detectable increase in fluorescence occurs may also be adjusted
manually. The threshold should always be done on a logarithmic
amplification plot. In a logarithmic amplification plot the
threshold should be set in the log-linear range and not the plateau
phase.
Melting Curves
Performing a melting curve analysis at the end of the run will help
to analyze only the PCR product of interest. Follow the real time
instrument manufacturer’s instructions for melting curve analysis.
Successive runs with the same primers can be modified to remove the
contribution of primer dimer formation to product signal by
collecting data in an additional cycling step, the temperature of
which must lie between the already determined dimer and product
melting temperatures (TMs).
Methods of Quantification
Standard Curves
Standard curves are necessary for both absolute and relative
quantification. When generating standard curves, different
concentrations of DNA (typically five) should be used to generate a
standard curve that will bracket the concentration of the unknown.
Each concentration should be run in duplicate.
Absolute and Relative Quantification
This SYBR Green PCR kit may be used to quantify target DNA using
either absolute or relative quantification. Absolute quantification
techniques are used to determine the amount of target DNA in the
initial sample, while relative quantification determines the ratio
between the amount of target DNA and a reference amplicon. The
ideal reference amplicon would have invariant, constitutive
expression. In practice, a housekeeping gene is chosen for this
function, but there are other reference choices which better adhere
to the above requirements.1
Absolute quantification uses external standards to determine the absolute amount of target nucleic acid of interest. To remove the differences in quantification due to annealing, the primer binding sites of the external standards must be the same as those in the target sequence. The ideal external standard contains sequences that are the same as the target sequence or which vary only slightly from the target sequence. Equivalent amplification efficiencies between the target and external standard are necessary for absolute quantification. Once a suitable construct or amplicon is identified, a standard curve of external standard dilutions is generated and used to determine the concentrations of unknown target samples.
Relative quantification allows calculation of the ratio between the amount of target template and a reference template in a sample. Since this method measures the amount of target relative to a presumably invariant control, relative qPCR is most often used to measure genetic polymorphism differences, for instance, between tissues or between healthy and diseased samples. The advantage of this technique is that using an internal standard can minimize the variations in sample preparation and handling. When using SYBR systems, the target and internal reference quantification must be run in separate reactions.
The accuracy of relative quantification depends on the appropriate choice of a reference template for standards. Variability of the standard will influence the results and so it is most important that standards be appropriate.1 Some researchers choose not to run a standard curve and report target quantities as a fraction of the reference, a technique termed comparative quantitation. Alternatively, one may assume that the amplification efficiencies of target and reference are negligible and quantify target based solely on the standard curve determined for the reference sequence. Finally, in the most accurate of the relative quantification techniques, the amplification efficiencies of both the reference and target are measured, and a correction factor is determined. This process, termed normalization,1 requires a sample containing known concentrations of both target and reference and the generation of two standard curves.
Determination of PCR Reaction
Efficiencies
The PCR efficiency between a reference sample and a target sample
is determined by preparing a dilution series for each target. The
CT values of the reference are subtracted from the target and this
difference in CT values is plotted against the logarithm of the
template amount. If the resulting slope of the straight line is
less than ± 0.1 the amplification efficiencies are judged to be
similar.
Equipment
Supplies
Protocol
Preparation
Standard SYBR Green I Dye Reaction
Note: We have observed that assays run in KiCqStart ReadyMix are
optimal when using a higher primer concentration than in
conventional PCR. In the protocols below, we use 450 nM final
concentration which we have observed to be the optimal
concentration for several independent assays.
1. Prepare enough master mix to run all samples in duplicate.
a. Be sure to include duplicate No template
Negative Controls (NTC).
b. Select appropriate table below based
upon qPCR reagent selected.
c. Calculate amount of reagents to mix. Add
10% volume to allow for pipetting error
d. Mix well, avoiding bubbles.
Master mix for KiCqStart reagents:
Reactions | Target Final Concentration | Volume per single 20 μL reaction (µL) |
2X qPCR mix | 1X | 10 μL |
Forward primer (10 µM stock) | 0.45 µM | 0.9 μL |
Reverse primer (10 µM stock) | 0.45 µM | 0.9 μL |
PCR grade water | - | 3.2 μL |
2. Setup reactions:
a. For NTC reactions, add 4 μL of water to
the reaction tube.
b. For experimental reactions, add 4 μL of
cDNA solution to the reaction tube.
c. Centrifuge all tubes briefly. Visually
confirm that all tubes or wells contain sample at the bottom at the
correct volume.
d. Carefully aliquot 16 μL of template
master mix into each qPCR tube or plate well.
e. Mix reactions well and spin if
needed.
f. Cap tubes or seal the PCR plate and
label (according to instrument requirements). (Make sure the
labelling does not obscure instrument excitation/detection light
path.)
3. Run samples as per instrument manufacturer recommendations.
Examples of standard and fast cycling have been included
below.
Standard cycling parameters:
Temp | Time | |
Initial denaturation | 94 °C | 2 min |
40 cycles: | ||
Denaturation | 94 °C | 15 sec |
Annealing, extension, and read fluorescence |
60 °C or 5 °C below lowest primer TM |
1 min |
(Optional) Hold | 4 °C only if products will be run out on a gel |
Fast cycling parameters:
Temp (ºC) | Time (s) | |
Initial denaturation | 95 | 30 |
40 cycles: | ||
Step 1 | 95 | 5 |
Step 2 | 58 | 15 |
Step 3 | 72 | 10 |
Note: Use standard dissociation curve protocol (data
collection).
4. Refer to instrument manual for guidance on how to analyze
data.