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Department of Community and Preventive Medicine, Mount Sinai School of Medicine, New York, New York 10029 [J. L. B., S. S. T.]; Department of Applied Medical Sciences, University of Southern Maine, Portland, Maine 04104 [W. D. T.]; Department of Cancer Biology, Cleveland Clinic Foundation, Cleveland, Ohio 44195 [G. C., S. V.]; Department of Cancer Genetics, Roswell Park Cancer Institute, Buffalo, New York 14263 [R. A. D.]; Department of Health Research and Policy, Stanford University School of Medicine, Stanford, California 94305 [A. S. W.]; Department of Preventive Medicine, Keck School of Medicine at the University of Southern California, Los Angeles, California 90033 [A. T. D., S. X., R. W. H.]
| Abstract |
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| Introduction |
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The purpose of this multilaboratory study was to determine the validity of using DNA extracted from archived PETs to detect mutations in relatively rare susceptibility genes, such as BRCA1, and to optimize the screening technique. To accomplish these goals, we carried out an experiment to identify sources of artifacts and errors in mutation screening by simultaneously examining mutation type, age of the PET, amplicon length, and type of polymerase in a series of cases in which BRCA1 gene carrier status had been previously determined from lymphocytes. We also analyzed sequences derived from cloned PCR products to determine the level of sequencing artifacts under these different conditions.
| Materials and Methods |
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T substitution); Laboratory 2 evaluated old blocks of all three types (wild type; frameshift: 185 AG deletion and 2080 A insertion; missense: 300 T
G and 3508 C
G substitutions) and new blocks with frameshift (frameshift: 3053 T deletion) and missense (missense: 1806 C
T substitution) mutations; and Laboratory 3 analyzed old blocks with missense mutations (missense: 300 T
G and 3508 C
G substitutions) and new blocks of all three types (wild type; frameshift: 199 + 1 CATCTG insertion and 3053 T deletion; missense: 1806 and 5622 C
T substitutions). Furthermore, because each laboratory analyzed the eight cases they were assigned, the following two PCR conditions were also altered: amplicon lengths ("short" or 150200 bp versus "long" or 250300 bp) and type of Taq polymerase (native versus HIFI). Thus, each laboratory conducted a total of 32 analyses (eight blocks, each tested under four conditions). The reported mutation type of each sample was confirmed by direct sequencing, ensuring against possible sample mix-up.
Laboratory Analysis
Four 5-32 µm sections were cut from each of the identified PETs using a standard microtome with a fresh disposable blade between cases. Each section was placed on a glass microscope slide and sent to the coordinating center, where it was randomly assigned a study identification number. The tissue sections were removed from the slides, placed in microcentrifuge tubes labeled only with the study identification, and sent to the two nonoriginating laboratories for amplification and analyses. Although the mutation status of the sections was masked, the laboratories were informed of the region to amplify.
Using direct sequencing and TA cloning followed by sequencing of up to five independently isolated fragments to identify the possibility of sequencing artifacts, each laboratory assessed the same set of selected conditions, namely: (a) to PCR-amplify the targeted amplicon within the size range which was defined as either "short" or "long"; (b) to use the specific types of Taq DNA polymerase; (c) to restrict the maximum number of amplification cycles; and (d) to not perform nested PCR. The laboratories used their own protocol for DNA extraction, PCR amplification, and sequencing, but all of them used the same cloning technique (TOPO TA cloning kit, Invitrogen Corp, Carlsbad CA) and the specific methods are described below:
Laboratory Method 1.
Tissue sections were extracted from the PET using the commercially available QiaAmp Tissue Extraction kit (Qiagen, Valencia, CA) in a final volume of 50 µl. Coding regions for assigned PET using this method (listed in Table 1
) were PCR amplified in a final volume of 15 µl containing: 1.5 µl 10x PCR buffer (Invitrogen); 0.75 µl of dNTPs (12.5 mM stock; each); 0.45 µl of MgCl2 (50 mM stock); 0.15 µM each, forward and reverse primers concentration; 0.15 µl of polymerase (5 units/µl; either native or HIFI platinum Taq); and 2 µl of DNA. The PCR product was denatured at 94°C for 5 min, followed by 35 cycles of: denaturation (94°C for 45 s); annealing (for 1 min at 53°C for exons 2, 11, and 5 and 62°C for exon 24); and annealing (72°C for 1 min). This was followed by a final extension at 72°C for 7 min. An aliquot of the PCR products was electrophoresed on an agarose gel or 6% nondenaturing polyacrylamide gel to confirm the presence and the size of a single band. The PCR products were purified using QIA quick PCR purification kit (Qiagen). An aliquot of the sample was submitted for automated sequencing in both the forward and reverse direction using the amplification primers with the ABI 377 sequencer (Perkin-Elmer: Foster City, CA) in the institutional sequencing facility. The remainder of the sample was used for cloning. Individually isolated clones were subsequently isolated, purified, and sequenced.
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Laboratory Method 3.
DNA was extracted from the PET sections using a modification of the protocol of (5)
. Briefly, each 5 µm section was deparaffinized in a 1.5 ml polyethylene microcentrifuge tube by washing twice in xylene, twice in 100% ethanol, and air-dried. The deparaffinized specimen was digested overnight at 50°C in 50 µl buffer containing 200 µg/ml Proteinase K, 50 mM Tris-HCl (pH 8.5), 1 mM EDTA, and 0.5% Tween 20. The specimens were then incubated at 95°C for 8 min to inactivate the Proteinase K. Each PCR reaction used 3 µl of the crude DNA extract in a final volume of 30 µl. The reaction was amplified on an MJ Research programmable thermal controller PTC-100. PCR reactions were performed using a hot start method. The reaction mixture contained 510 pmol each of the forward and reverse primers (Table 1)
, 0.25 mM dNTPs, 1.5 units of either Taq DNA polymerase (Promega, Madison, WI) or HIFI Platinum Taq DNA polymerase (Life Technologies, Inc.), 10 mM Tris-HCL (pH 9.0), 50 mM KCl, 0.1% Triton X-100 (also provided by Promega) and with the appropriate concentration of MgCl2 in the range of 2.03.5 mM, which is optimal for the enzyme performance in each specific PCR reaction. The thermocycling profile was set as 3 min of initial denaturation at 94°C, followed by 35 cycles of denaturation (94°C for 30 s), annealing for 45 s, and elongation (72°C for 1 min). Final extension was 72°C for 5 min. Reaction products and DNA molecular size standards were electrophoresed in a 2% agarose gel containing ethidium bromide (5 mg/ml) to confirm the quality and the size. PCR products were used for both direct sequencing and cloning. Manual direct sequencing using Thermo Sequenase kit (Amersham, Cleveland, OH) was performed in both sense and antisense directions.
Sample Cloning Technique.
Products amplified by either Recombinant or HIFI platinum Taq DNA polymerase were cloned using the TOPO TA cloning kit (Invitrogen). For PCR products using platinum Taq DNA polymerase, the first step required the addition of overhanging "A"s at the 3' end. This PCR product was inserted into the vector. The vectors were then transformed into chemically competent Escherichia coli and propagated in Luria Bertani medium containing ampicillin or kanamycin (50 µg/ml). Vector DNA was isolated as described by Sambrook et al. (6)
or using Qiaprep Spin Miniprep Kit (Qiagen). The clones were linearized by digestion with EcoR1 restriction enzyme at 37°C for 1 h and electrophoresed in a 1% agarose gel. The molecular weight standard was loaded adjacent to the product for reference, and the final product was submitted for sequencing. Each laboratory isolated up to five independent clones per sample.
All of the laboratories used BRCA1 nucleotide sequences in GenBank (accession nos. U14680 and L78833) as well as the standard cDNA numbering system (7 , 8) and mutation nomenclature (9) .
| Results |
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66% of the mutations were correctly identified in all three of the laboratories. All of the missed mutations were from specimens that did not produce DNA or PCR products. Amplification with short DNA regions using HIFI Taq provided the largest percentage of correctly identified mutations (83% overall). Across all Taq types and amplicon lengths, compared with the newer blocks, the older ones were more difficult to characterize. Base substitutions were more often correctly identified in the older samples than were frameshift mutations or wild-type sequences.
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| Discussion |
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Because DNA is extracted from PET (2 , 12 , 19) , its degradation from the aging of the sample (2 , 10 , 12) precludes analysis with techniques requiring high-molecular weight DNA (2) . Our success using the shorter-length primer is consistent with published reports suggesting that the size of the damaged genomic DNA fragment prepared from older archival samples may be smaller than that derived from newer samples (11 , 12 , 15) . Although suboptimal for PCR amplification (13) , using shorter fragments can enhance the sensitivity (10 , 20) . One possibility is that the pH of the original preservative rather than the length of storage per se, impedes successful amplification (10) and that the fixation-induced DNA degradation may be due to the extensive cross-linking of proteins to DNA and acid depurinization of the DNA (13) . As a result, the DNA template can be altered or destroyed, preventing accurate amplification (2 , 18) , which, in turn, yields the relatively short PCR fragments with low recovery (13) . Nevertheless, when the expected length of the PCR product is between 200 and 300 bp, the fixation time used by the diagnostic histology laboratory only slightly decreases the sensitivity of the PCR reaction (21) . The results of amplifying from PET are comparable with that using unfixed tissue (12 , 22) .
The type of Taq or length of amplicon might lead to more false negatives in that certain mutation types might be missed as a result of the physical damage to DNA that can result in misincorporated bases, gaps, and cross-over products, and misincorporation from different Taq polymerases with reduced fidelity. Although we found that all four combinations of techniques (e.g., HIFI and native Taq types and long and short amplicon lengths) detected 100% of the wild type, the rates for detecting frameshift mutations appeared to be dependent on the age of the block; DNA from a 38-year-old block that was known to contain a frameshift mutation was not detected by either of the two testing laboratories, whereas an 18-year-old block with a frameshift mutation amplified as well as the similarly aged PET sample with a missense mutation.
Although this study focused on detecting BRCA1 mutations in PET, our findings should not be mutation specific, because the factors that most impeded our ability to successfully detect mutations (i.e., false negatives) and amplify the PCR product likely reflect the different laboratory methods. For example, differences in the type of sequencing (manual versus automated) and the sequencing enzymes used by each of the laboratories may have had a significant effect on the processivity and nucleotide incorporation capabilities in the sequencing reaction. Laboratory method 3 may have been advantaged by using ThermoSequenase DNA polymerase for manually sequencing because the impure DNA templates from the PET samples can result in uneven peak patterns making it difficult to call single-base substitutions. Although convenient for high throughput work, automated sequencing is prone to misclassifying the small peaks of a real base as background, while identifying high background signals as real bases (23) . In contrast, manual sequencing provides more uniform band intensities, allowing heterogeneity in the DNA template to be reproducibly identified without ambiguous interpretation of sequences using software that discerns signal strength patterns indicative of mixed-base positions. In our study, missed frameshift mutations accounted for a majority of the discrepant laboratory results reported by the laboratories (data not shown), possibly caused by the compression arising from the template secondary structure. An important caveat to this discussion is that every laboratory did not evaluate every sample included in the study because the originating laboratory could not test its own block. Laboratory 3 donated the oldest sample and, therefore, did not test it. Thus, in the absence of data, we can conjecture that this methodology may account for Laboratory 3s 100% success in amplifying samples, but only using younger blocks. Furthermore, primer position, length, and composition are known to affect the ability to amplify a DNA fragment as also do the method of fixation, the length of time between tumor excision and fixing, and the condition (e.g., humidity etc.) of storage of the tissue blocks. Therefore, differences among these in the various laboratories may have contributed to differences in the frequency of successful amplification and mutation detection, in addition to the age of the block.
A prior study by Wong et al. (3)
indicated that the vast majority of artifacts detected in fixed tissues were base substitutions, not insertion/deletion. So, in this study, we considered frameshift as well as base substitutions as two different types of mutations, even though sequencing is equally sensitive in detecting both types of mutations. The clone data indicate that the errors may be attributable to the specific nature of the mispairs (A:C or T:G) generated by the Taq polymerase because all of the additional variants were single-base substitutions, and almost all of those observed were transitions, with AT
GC being the most predominant (34 of 50; data not shown). Katagiri et al. (24)
and Tindall et al. (25)
found that the PCR error rate when using native Taq was estimable from the template product ratio of the PCR assay. However for PET, this may not be feasible because the amplification often involves the use of crude DNA extracts to avoid loss of DNA during the purification process. Additionally, repeating all of the base-substitute variants for quality control is costly. An alternative would be to reanalyze the samples with the mutation using a technique that can differentiate true positives from false positives. For example, using retroviral or nonretroviral primers, which have a built-in signature sequence, would assure the detection of extremely low amounts of carryover as a form of false positives (26)
. Regardless, as these artifacts were observed only in the cloned samples, amplification using native Taq might be sufficient for use in large-scale epidemiological studies if the screening method is carefully selected and quality control maintained.
In summary, the primary goal of this study was to compare the accuracy of different techniques used for mutation detection in BRCA1 to identify optimal conditions for analyzing PETs. We also examined whether artifacts would result from the routine mutation detection methods as performed in this study and, if so, whether the methods could be easily modified to minimize the number of artifacts without requiring a labor-intensive cloning technique to be used as quality control. Although our study is limited by the relatively small sample size within each analytic stratum, our factorial study design enabled us to simultaneously examine the effects of block age, mutation type, amplicon size, and polymerase type, on the accuracy of BRCA1 mutation screening, and our results provide insight as to techniques that can be used most successfully. Specifically, we observed that mutation detection by direct sequencing of DNA isolated from PET was more accurate using younger blocks. For any aged block, the shorter amplicon length, regardless of the type of Taq, was more predictive of success, although HIFI Taq was optimal. Successful amplification greatly depended on the laboratory method used and on the age of the block. These results demonstrate that DNA derived from PET archived for up to 18 years can be used successfully to detect BRCA1 gene mutations in a multicenter laboratory study if quality control is strictly maintained.
| Acknowledgments |
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| Footnotes |
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1 Supported by NIH Grant CA69631 and Core Grant 16056 [Roswell Park Cancer Institute (RPCI)]. The latter supported the sectioning and sequencing performed at RPCI. ![]()
2 To whom requests for reprints should be addressed, at Mount Sinai School of Medicine, One Gustave L. Levy Place, Box 1043, New York, NY 10029. Phone: (212) 241-8495; Fax: (212) 360-6965; E-mail: jonine.bernstein{at}mssm.edu ![]()
3 The abbreviations used are: PET, paraffin-embedded tissue; HIFI, high fidelity. ![]()
Received 12/14/01; revised 4/30/02; accepted 5/14/02.
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