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1
ARUP Institute for Clinical and Experimental Pathology, 500 Chipeta Way, Salt Lake City, UT 84108.
2
Department of Pathology, University of Utah Medical
Center, 50 N. Medical Dr., Salt Lake City, UT 84132. Fax 801-581-4517;
e-mail Carl_Wittwer{at}hlthsci.med.utah.edu
a Author for correspondence.
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The usual method for detecting factor V Leiden is to amplify by PCR a portion of the gene containing the mutation and then digest the products with a restriction enzyme that cuts only the wild-type sequence. Size separation of digested products by gel electrophoresis distinguishes wild-type from homozygous and heterozygous mutant genotypes. This process, not including DNA isolation, requires at least 4–6 h and involves several postamplification handling steps, with the associated risks of end-product contamination, sample tracking errors, and incomplete enzyme digestion.
Fluorescent probes can be used to detect and monitor DNA amplification. When glass capillaries are used in rapid-cycle DNA amplification (3)(4), the optically clear sample vessel serves as a cuvette for fluorescence analysis (3)(5)(6)(7). Instruments that combine rapid-cycle PCR with fluorescence analysis for continuous monitoring during amplification are commercially available (8). Various sequence-specific probes have been described that utilize fluorescence resonance energy transfer between two fluorophores to monitor product production (5)(9)(10). Resonance energy transfer between adjacent fluorescein- and Cy5 (indodicarbocyanine)-labeled hybridization probes has been used as a measure of the amount of specific product generated during rapid-cycle DNA amplification (5).
If PCR is carried out with a Cy5-labeled primer, a single fluorescein-labeled probe can also be used to monitor amplification (6). When the probe is annealed to the extension product of the Cy5-labeled primer, the fluorophores are brought into close enough contact for resonance energy transfer to occur, increasing the fluorescence of the Cy5. If the fluorescein-labeled probe is placed across the mutation site, the presence of the mutation can be evaluated by monitoring fluorescence while the sample is being heated through the melting temperature of the probe. A single base change will cause the probe to melt at a lower temperature than if the probe is completely complementary. With asymmetric amplification, the strand formed from the labeled primer can be produced in excess, allowing probe hybridization without competition from the annealing of full-length strands.
As an alternative to sequential amplification, restriction enzyme digestion, and electrophoresis, we were able to genotype the factor V mutation by fluorescence melting-curve analysis during 45 cycles of asymmetric amplification.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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primer and probe synthesis
Oligonucleotides were synthesized by phosphoramidite chemistry on
a Gene Assembler PlusTM (Pharmacia Biotech). The
3'-fluorescein probe was synthesized on fluorescein controlled-pore
glass (BioGenix, San Ramon, CA). With the final trityl group attached,
failure sequences were removed with reversed-phase HPLC on a 4 x
250 mm C18 column (Hypersil ODS; Hewlett-Packard) in a 0.1
mol/L triethylammonium acetate mobile phase with a linear gradient with
acetonitrile. At a flow rate of 1 mL/min, the desired fraction eluted
at an acetonitrile content of 25% (250 mL/L). Detritylation was
performed on a PolyPak column (Glen Research) and the oligonucleotide
was eluted with an equivolume solution of acetonitrile in water. Two
additional 3'-fluorescein probes of the same size were also
synthesized, one with a 3-bp overlap with the primer (a shift of 6
bases from that shown in Fig. 1
), and one complementary to the mutation
instead of to the wild-type.
Synthesis of the Cy5-labeled primer required both automated and manual steps. First, automated synthesis of the oligonucleotide incorporated an amino-modifier C6dT (Glen Research) in place of the most 3' thymidine residue. Next, the Cy5 moiety was attached as the monovalent N-hydroxysuccinimide ester of Cy5 (Cy5.29–OSu, Amersham; absorbance maximum 650 nm, absorptivity >200 000 L mol-1cm-1; see (5) for additional references), according to the manufacturer's instructions. The Cy5-labeled primer was purified by reversed-phase HPLC as described above and eluted at a mobile-phase acetonitrile content of ~22%. Primer and probe purity were assessed by analytical HPLC with tandem absorbance and fluorescence detectors (Waters Models 486 and 474, respectively). The ratio of the concentration of fluorescent label to that of oligonucleotide, as judged by absorbance measurements (11), was 1.0 for the Cy5-labeled primer and 0.8–0.9 for the fluorescein-labeled probes.
samples and controls
Human genomic DNA was obtained by phenol/chloroform extraction and
ethanol precipitation (12) from 103 samples of
EDTA-anticoagulated blood submitted to Associated Regional and
University Pathologists reference laboratory for factor V Leiden
testing. The DNA was resuspended to a concentration of 50 µg/mL in 10
mmol/L Tris, pH 8.0, containing 0.1 mmol/L EDTA. Three of these
samples, with normal, decreased, and highly decreased activated protein
C sensitivity ratios (1), were used as controls and
confirmed by direct DNA sequencing to be respectively wild-type,
heterozygous, and homozygous for factor V Leiden.
restriction digest protocol
PCR was performed by rapid-cycling techniques
(3)(4) in a reaction volume of 10 µL with
each dNTP at 200 µmol/L, each primer at 0.5 µmol/L, 3.0 mmol/L
MgCl2, 50 mmol/L Tris, pH 8.3, 500 mg/L bovine serum
albumin, 20 g/L sucrose, 0.1 mmol/L cresol red, 50 ng of DNA, and 0.4 U
of Taq polymerase. The samples were loaded into glass capillaries (no.
1705; Idaho Technology, Idaho Falls, ID) and sealed with a butane
torch. The 250-bp product was amplified for 45 cycles in a rapid air
thermal cycler (RapidcyclerTM; Idaho Technology) with
denaturation at 94 °C for 0 s, annealing at 55 °C for 2
s, and extension at 74 °C for 5 s. The amplification was
completed in <20 min. The amplified products were dispensed into
microcentrifuge tubes with 1 µL of Mnl1 (5000 U/mL) and 1
µL of buffer: 500 mmol/L NaCl, 100 mmol/L Tris-HCl, pH 7.9, 100
mmol/L MgCl2, and 10 mmol/L dithiothreitol (NE Buffer 2;
New England Biolabs.). After incubation for 2 h at 37 °C,
samples were electrophoresed on 1.5% agarose gels at 5 V/cm for 60 min
in the presence of 0.5 mg/L ethidium bromide. The DNA fragments were
visualized with UV light, and the samples were genotyped by restriction
fragment length polymorphism as previously described (1).
Samples homozygous for factor V Leiden were characterized by a band at
200 bp, wild-type samples by a band at 163 bp, and heterozygous samples
by bands at both 200 and 163 bp.
fluorescence protocol
PCR was performed as above except that 0.5 µmol/L Cy5-labeled
primer (Fig. 1
), 0.2 µmol/L intron 10 primer (for a 187-bp product),
and 0.1 µmol/L fluorescein-labeled probe (Fig. 1
) were used without
sucrose or cresol red. The samples were transferred to disposable
capillary cuvettes (no. 1720; Idaho Technology) and centrifuged to
place the sample at the capillary tip before capping. Amplification was
performed for 45 cycles of denaturation (94 °C for 0 s),
annealing (50 °C for 10 s), and extension (75 °C for 0
s) on a rapid-temperature cycler with integrated fluorescence
monitoring (LightcyclerTM; Idaho Technology). The ramp
rates were programmed at 20 °C/s from denaturation to annealing,
1 °C/s from annealing to extension, and 20 °C/s from extension to
denaturation. The epi-illumination fluorometer in the LightCycler uses
a blue light-emitting diode to excite the capillary tips at 450–490
nm. Total internal reflection at the glass/air interface along the
capillary axis increases the observed fluorescence after spectral
filtering and focusing on photodiodes. The optical design is similar to
that used in flow cytometers (8).
The ratio of fluorescein (520–560 nm) to Cy5 (655–695 nm) fluorescence was acquired during temperature cycling to monitor amplification and probe hybridization. The fluorescence at the end of each annealing step reflects the cumulative amount of product resulting from extension of the Cy5-labeled primer during asymmetric amplification. In some experiments, the dependence of probe hybridization on temperature was monitored continuously within a temperature cycle. After amplification was complete, a final melting curve was usually performed by cooling to 50 °C, holding at 50 °C for 1 min, and then heating slowly at 0.2 °C/s until 75 °C. Fluorescence was collected continuously during this heating to monitor the dissociation of the 3'-fluorescein-labeled probe. The fluorescence ratio (Cy5/fluorescein, FR) was plotted against temperature (T) to give melting curves for each sample.
Melting curves were converted to derivative melting curves in two steps. First, the negative derivative of the fluorescence ratio with respect to temperature (-dFR/dT) was plotted against temperature. The resulting derivative curves (-dFR/dT vs T) had a beginning baseline (before the melting transition) higher than the final baseline (after the melting transition). Each curve was then corrected by reducing the initial baseline to the level of the final baseline. Intermediate points within the melting transition of each curve were reduced in proportion to the fractional area under the final derivative curve.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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, this ratio approximately doubles during 45
cycles of asymmetric amplification (Fig. 2
). A majority of the ratio change (85–90%) comes from
increased Cy5 fluorescence; only a little (10–15%) results from
decreased fluorescein fluorescence (data not shown). No increase in the
fluorescence ratio is observed during temperature cycling in the
absence of template (Fig. 2
). The observed fluorescence with the labels
separated by 5 bases (Fig. 1
) was more than twice that with an
alternative probe in which the fluorescein was directly opposite the
Cy5 with a 3-bp overlap between primer and probe (data not shown).
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The resonance energy transfer signal develops in each cycle as the
fluorescein-labeled probe hybridizes to the Cy5-labeled strand. By
observing fluorescence throughout a cycle of PCR (Fig. 3
), the process of hybridization and melting of the probe to
target can be monitored. As the sample is cooled to 50 °C, the
fluorescence signal increases and continues to increase as the sample
is held at 50 °C for 10 s. When the sample is subsequently
reheated, the probe melts from the target sequence, as evidenced by a
decrease in fluorescence. The melting transitions of homozygous
wild-type, heterozygous mutant, and homozygous mutant sequences at the
factor V Leiden locus can be observed directly during cycling. Fig. 3
shows fluorescence vs temperature tracings during cycle 40 of
amplification with a 1 °C/s temperature transition during melting.
Melting of the amplified homozygous mutant sample shows a rapid
decrease in fluorescence at 57–58 °C, whereas the wild-type
transition occurs at 65–66 °C. The heterozygous mutant sample
exhibits two distinct decreases in fluorescence, corresponding to both
the mutation and the wild-type locuses. Change in the fluorescent
signal between 75 °C and 94 °C is minimal, because all of the
probe is dissociated from its target in that region.
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Melting-curve analysis at the completion of 45 cycles of PCR is shown
in Fig. 4
. Fluorescence data are acquired as the sample is heated at
0.2 °C/s from 50 °C to 75 °C. A plot of the ratio of Cy5 to
fluorescein fluorescence vs temperature (Fig. 4A
) illustrates the
melting properties of the fluorescein-labeled probe with each genotype.
By plotting the negative derivative of the ratio with temperature vs
temperature (Fig. 4B
), the data show peaks for where the maximum
melting occurs. When the wild-type probe melts from a wild-type target,
the peak melting occurs at 66 °C. With a homozygous mutant sample,
the A–C mismatch results in a melting peak 8 °C lower, at 58 °C.
The heterozygous sample contains both types of target sites and
generates both peaks. When a 3'-fluorescein probe was designed to match
the mutation instead of the wild-type, the resulting shift in peak
melting from the G–T mismatch was 4 °C (data not shown).
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Evaluation of 100 human DNA samples for factor V Leiden by both the restriction digest and the fluorescence protocols yielded identical genotyping results by both methods: 60 samples were wild-type, 37 were heterozygous, and 3 were homozygous for the factor V Leiden mutation. These samples had been originally submitted for clinical factor V testing and hence were from patients already biased for thrombotic tendencies.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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When resonance energy transfer between two adjacent hybridization probes is monitored as the temperature changes during PCR, a sharp decrease in fluorescence is observed around the melting temperature of the probes (5). If one fluorophore is on a primer and the other on an internal hybridization probe, the resonance energy transfer depends only on hybridization of the single internal probe (6). This allows the melting characteristics of the internal probe to be monitored during amplification. Sequence alterations in the target at the probe site can be detected because the stability of the duplex and hence the melting temperature of the probe decrease when mismatches are present.
Single-base mismatches were first noted to lower the melting temperature of oligonucleotide probes by ~10 °C many years ago (13). One study demonstrated that a C–A mismatch in the center of a 15-mer probe lowered the melting temperature by 8 °C (14), the same amount observed here for the factor V Leiden C–A mismatch. The extent of destabilization depends on the specific mismatch, the nearest neighbor environment, and the position of the mutation site relative to the probe. For example, the mismatch between the probe homologous to factor V Leiden and wild-type sequence (G–T) lowered the melting temperature by only 4 °C.
Melting-curve resolution is affected by the temperature transition
rate. Conventional absorbance-based melting curves are usually obtained
at very slow rates, ranging from 1 °C/min to 1 °C/h, to ensure
equilibrium (15). However, fluorescence melting curves of
PCR products with SYBRTM Green I can be obtained at
0.1–1 °C/s (7). As the rate is increased from
0.1 °C/s to 1 °C/s, the transition broadens and shifts ~1 °C
to higher temperatures. When probe melting is monitored, genotyping is
easily discerned at transition rates of 1 °C/s during amplification
(Fig. 3
). Higher-quality melting curves can be obtained by slowing the
transition rate to 0.2 °C/s (Fig. 4
). The time required for
amplification and analysis depends on the type of monitoring desired
and the melting-curve resolution needed. For example, 45 cycles of
rapid-cycle amplification (94 °C for 0 s, 55 °C for 2
s, 74 °C for 5 s) that is not monitored at each cycle but is
followed by a 0.2 °C/s melting curve can complete the genotyping in
<20 min. Alternatively, monitoring the hybridization and melting for
each cycle requires slower cycling and may take as long as 45 min. An
interesting option for future instrumentation might use fluorescence
feedback control to automatically terminate temperature cycling once a
given amount of hybridization fluorescence is obtained and then
initiate a melting-curve cycle for genotyping.
Whenever hybridization or restriction enzyme digestion is used to identify a particular sequence change, there is always some risk of other sequence alterations occurring at the recognition site. A silent A1692C transversion, resulting in false-positive genotyping of factor V Leiden by the restriction digest protocol, has been reported (16). Although the frequency of this change is unknown, the correlation between functional and restriction digest assays suggests it is low (17)(18). Most other genotyping methods for factor V Leiden would also be affected by this adjacent sequence change, including allele-specific amplification (19), single-stranded conformational polymorphism (20), oligonucleotide ligation (21), and heteroduplex analysis (22). In the fluorescence hybridization assay reported here, the resulting C–T mismatch of the A1692C transversion would lower the melting temperature of the 23-bp probe, although not necessarily to the same extent as the C–A mismatch arising from the factor V Leiden mutation. Different mismatches destabilize to different extents (23) and can often be distinguished. Sequencing could be used to rule out any chance of a false positive. As an alternative, a fluorescence probe designed to match the mutant sequence could be used. An apparently positive G1691A genotype with the wild-type probe could be reamplified with the mutant probe. The fully complementary G1691A allele would be easily distinguished from the A1692C allele that is mismatched to the mutant probe at 2 bases. Indeed, any base change in the region of the probe at positions other than 1691 would be mismatched at 2 bases with the mutant probe and should be easily distinguishable. A sequence change at position 1691 other than G1691A would result in single-base mismatches with both wild-type and mutant probes. Whether or not testing for potential false positives should be performed will depend on the frequency of A1692C and similar alleles. The extent of polymorphism in this area is not currently known. However, the persistence of a unique haplotype in individuals with factor V Leiden, as well as its focal Indo-European distribution, suggests that the mutation occurred only once and that the region is very conserved (24).
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Acknowledgments |
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References |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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