“PCR
has become the cornerstone of modern molecular biology”
—Dr Carl Wittwer,
Professor of Pathology, University of Utah Medical School, Salt Lake
City, Utah, USA
Polymerase Chain
Reaction (PCR) is a method that allows exponential amplification of
short DNA sequences (usually 100 to 600 bases) within a longer double
stranded DNA molecule. First discovered in 1983, PCR has now become a
common and indispensable method that researchers and scientists use in
research labs for a variety of different applications. From its initial
application in DNA amplification and genotyping, PCR is now applied to
fields such as forensics, archaeology and even in food and beverage
industries.
A decade old, the Roche
LightCycler 480 system is one that has been widely used in research
labs worldwide. Since its invention, the LightCycler system has evolved
into a sophisticated system that comprises five excitation channels and
six detection channels, where researchers can observe and track the
progress of experiments in real time to ensure that they get an
improved PCR result.
In an email interview
with BioSpectrum, Dr Carl T Wittwer, the inventor of the LightCycler
system, shares his views about the evolution of the system and where he
sees PCR progressing in the next decade. Professor of Pathology at the
University of Utah Medical School, Dr Wittwer developed rapid-cycle PCR
techniques for DNA amplification in the early 1990s and adapted flow
cytometry optics to thermal cycling for the read-time monitoring of PCR
in the mid 1990s.
What are the major
breakthroughs achieved with PCR over the last decade?
PCR has enabled many breakthroughs in research and clinical medicine.
The ability to amplify and duplicate genetic material within a very
short period has made it possible for researchers to unravel the human
genome and understand infectious and hereditary diseases. For example,
with PCR we have been able to understand genetic contributions to
Parkinson’s disease better over the last decade. Previously
unsuspected genes have also been discovered that contribute to acute
myeloid leukemia, a particularly deadly form of leukemia through PCR.
Over the years, many variants of PCR have been developed. One of them
is quantitative real-time PCR, more commonly know as qPCR. With qPCR,
the exponential phase of PCR is monitored, allowing determination of
the initial amount of target. The contributions of real-time PCR
systems are now indispensable in the research lab. PCR technology today
is being applied across different industries and fields of study,
including forensic science, virology, food microbiology and even
environmental monitoring.
Roche provides research laboratories all over the world with a series
of different real-time PCR systems to suit their needs. With the
LightCycler 2.0 and LightCycler 480 real-time PCR systems, researchers
have access to both rapid carousel-based and high-throughput systems.
The LightCycler introduced many “firsts” to
real-time PCR, including rapid cycling, SYBR Green I, dual
hybridization probes, single hybridization probes, and melting
analysis. As a rapid, open system, it remains the most user-friendly
real-time instrument for assay and technique development. The first
genetic tests to receive FDA approval in the US (F5 and F2) were on the
carousel LightCycler in 2002, setting the standard for other companies
to follow.
The extreme sensitivity of real-time PCR, with the ability to detect
even a single nucleic acid molecule, makes it possible to detect fetal
DNA in maternal serum for noninvasive prenatal diagnosis. Digital PCR
is a recent modification where the sample volume or concentration is
reduced so much so that each reaction only contains one starting
template. Emulsion PCR creates millions of nanoliter PCRs within oil, a
process that is central to most next-generation sequencing methods.
Who are the major
players in real time PCR systems? What is the current market size of
real time PCRs (number of PCR systems sold per year?)
Roche introduced the LightCycler in 1998, the first system to automate
absolute quantification and display PCR in real-time as the reaction
progresses. To date, over 7,000 LightCycler systems have been sold. The
market size for real-time PCR continues to expand in research and
medical diagnostics. Based on rapid cycle PCR, carousel LightCyclers
can complete PCR in 15-30 minutes, providing rapid turn around for
critical clinical settings and sequential research. In situations where
high throughput is more important than turn around time, the LC480
allows amplification on 96 or 384-well plates. A 1536-well LightCycler
(again a first in real-time PCR) will be introduced in 2009.
What are the factors
that drive PCR evolution?
The need for speed, accuracy and cost containment are factors that
drive PCR evolution. Initially, finding a polymerase that withstood the
high temperatures involved in the duplication of DNA was critical. The
discovery and usage of the heat-resistant Taq polymerase in 1988
greatly revolutionized the PCR technique by allowing automated
temperature cycling without reagent addition. The process of PCR
suddenly became much simpler and more accessible. However, quantitative
data was still difficult to obtain, and the process remained
qualitative until the introduction of real-time PCR instruments in the
mid 1990s. By monitoring fluorescence each cycle of PCR, the precise
amount of starting template could be determined, allowing true
quantification. Quantification was critical for mRNA quantification in
research and viral load determination in clinical applications. Time
consuming and expensive microbial diagnostics began to be replaced by
faster, more accurate real-time PCR assays.
The next advance was melting analysis, a process that goes beyond
conventional real time PCR. In addition to monitoring fluorescence once
each cycle, fluorescence is monitored continuously as the temperature
changes so that hybridization can be followed. Both PCR
product hybridization with SYBR Green I and probe hybridization for
genotyping can be monitored at the end of PCR. As melting technology
became better and better, high resolution melting analysis was
introduced as the latest method for product analysis. Labeled probes
were no longer necessary for genotyping and entire PCR products could
be scanned for single base changes in only one copy of diploid DNA.
The LightCycler real-time PCR systems are a good example of how the PCR
technique has evolved. The LightCycler 1.5 real time PCR system was the
first to monitor the hybridization process through melting curve
analysis and automate absolute quantification. The LightCycler 2.0
system advanced multicolor analysis to provide researches with up to
six colours of analysis. As real-time PCR evolved and high throughput
analysis became more important, Roche developed the LightCycler 480
system to accommodate both 96 and 384-well microtiter plates. The
variable plate formats available on the LightCycler 480 meant that
scientists could exchange thermal block cyclers in a few minutes
without having to recalibrate the instrument.
What will be the
future for PCR in the next decade?
PCR will become faster, more precise, and more affordable with lower
sample volumes. For most applications, 5-10 min PCR is feasible.
Unlabeled probes will replace labeled probes. High resolution melting
will expand into additional applications such as sequence matching for
transplantation testing. The flexibility and wide application base of
PCR are difficult to beat. For specific applications, alternative
technologies will excel. For example, arrays for copy number variations
are very powerful, and provide data that is difficult to obtain by PCR
alone. However, PCR has become the cornerstone of modern molecular
biology. It will be with us forever.
Narayan Kulkarni in
Singapore