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Journal of Clinical Microbiology, March 2007, p. 928-934, Vol. 45, No. 3
0095-1137/07/$08.00+0 doi:10.1128/JCM.02098-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Laboratory of Molecular Virology, ISP/Institut Pasteur, rue Engeland 642, 1180 Brussels, Belgium,1 Department of Pathology, CHU Charleroi, Boulevard Zoé Drion 1, 6000 Charleroi, Belgium,2 Department of Biostatistics and Medical Informatics, Brussels Free University/VUB, Brussels 1090, Belgium3
Received 12 October 2006/ Returned for modification 19 December 2006/ Accepted 5 January 2007
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Multiple concurrent and sequential infections with different oncogenic types of HPV are common, usually transient, and disappear spontaneously without clinical lesion. Only persistent high-risk HPV infection induces higher risk for the development of a high-grade precancerous lesion or cervical cancer (14, 41, 43). Since the absence of HPV infection means that the risk of cervical cancer is negligible, testing for HPV has now been incorporated into screening programs that previously relied only on cytology (6). HPV DNA analysis has shown encouraging results when used in conjunction with cytological analysis in primary screening for cervical cancer in women 30 years of age or older (5, 23). As a result of large randomized clinical trials, testing for HPV DNA is now recommended for most women with equivocal findings on cervical cytological analysis (atypical squamous cells of undetermined significance, also called ASCUS) (1, 28). Furthermore, HPV DNA testing provides a unique advantage for early detection of treatment failure (3, 9, 33, 42). Recently, encouraging results with prophylactic HPV vaccines have been reported. These vaccines are believed to be effective against four different HPV types, including the most frequently encountered type, HPV-16. This further emphasizes the need for HPV DNA testing, which will be useful to guide vaccination and to monitor the frequency and the severity of infection by unaffected HPV types (15, 20, 21).
Highly sensitive HPV DNA tests have been developed that rely on molecular biology techniques. For example, the FDA approved the HPV detection kit of Digene/Hybrid Capture 2 (HC2) manufactured by DiGene (MD). Within this test, genotype-specific RNA probes are mixed in a high-risk or a low-risk cocktail, and RNA-DNA hybrids are recognized by an antibody, used both for the capture step and for a signal amplification detection method. This technique fails to discriminate among different genotypes (6). A higher degree of resolution in the identification of genotypes can be achieved using multiple consensus primers and type-specific primers (6, 7, 22, 24, 25). Furthermore, the use of PCR followed by a reverse hybridization line probe assay (LiPA), such as the HPV INNO-LiPA (Innogenetics NV, Ghent, Belgium), has been described as a highly sensitive method to detect and identify multiple infections (25). Here, we have developed and optimized a strategy for HPV detection and genotyping by combining general primer-mediated PCR sequencing and type-specific PCR. This approach was compared against commercially available assays.
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Cytological diagnosis. Cytological analyses were performed as previously described (16). We used the Bethesda System 2001 to classify our observations: within the normal limits, ASCUS, and low- and high-grade squamous intraepithelial lesions (Lg-SIL and Hg-SIL, respectively) (32). The HPV detection and genotyping assays were performed blindly without previous knowledge of the cytological analysis results. The Hg-SIL were determined by histopathologic analysis.
HC2 assay. The HC2 assays were performed on 5 ml of the residual liquid-based samples. Samples were pelleted at 2,000 rpm for 5 min, and the supernatant was discarded. The cellular pellet was washed once with 1 ml of phosphate-buffered saline, resuspended in 100 µl of cervical sample (Digene Corp., Beltsville, MD), and denatured in an alkaline solution. Classical hybridization, detection, and calibration were made according to the HC2 kit's instructions (27). Probes were used only against the high-risk HPV genotypes: 16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, and 68. The HC2 results were expressed as positive or negative depending on the relative light unit of 1 pg/ml of HPV DNA. The HC2 tests were blindly executed by an independent laboratory (Molecular Diagnostic Center, Loverval, Belgium).
DNA preparation. Total cellular DNA was extracted by a freeze-thaw method as previously described (30, 38). Briefly, 1 ml of cell suspension was centrifuged (for 10 min at 4°C at 3,000 x g), washed with 1 ml of phosphate-buffered saline, further pelleted, and resuspended in 100 µl of 10 mM Tris-HCl, pH 7.5. The suspension was incubated for at least 3 h at 70°C and defrosted for 15 min at 100°C. Cell debris was removed by centrifugation at 14,000 rpm for 30 s. The quality of the extracted DNA was evaluated by PCR using ß-globin-specific primers, PC04 (5'-CAACTTCATCCACGTTCACC-3') and GH20 (5'-GAAGAGCCAAGGACAGGTAC-3'), as described previously (34). Further DNA purification was carried out using a QIAamp blood minikit (QIAGEN) only when no ß-globin PCR product could be obtained from the freeze-thaw extraction. Only DNA preparations giving positive amplification after PCR using ß-globin primers were further analyzed for HPV detection and genotyping.
PCR. Each amplification reaction was carried out in a total volume of 50 µl containing 1.25 units of HotStarTaq DNA polymerase (QIAGEN), 1x PCR buffer provided with the enzyme, and a 200 µM concentration of each deoxynucleoside triphosphate. The concentrations of MgCl2 and primers were optimized for each set of primers. Positive and multiple negative controls were included in all runs. Strict procedures were followed to avoid a false-positive reaction due to contamination. Reagent preparation, DNA extraction, and the addition of positive sample DNA were carried out in separate rooms. Gel electrophoresis and PCR amplification and sequencing were also geographically remote from the pre-PCR preparations. The PCR products were analyzed by electrophoresis on a 2.5% agarose gel stained with ethidium bromide.
General HPV PCR. For the PCR using the PGMY primers, the reactions were performed as previously described (18). Briefly, the PCR mixture was complemented with 4 mM MgCl2, 10 pmol of each primer, and 10 µl of DNA preparation. The consensus PCR amplifications were performed in the DNA thermal iCycler (Bio-Rad) because of its ramp modulating capacities. DNA amplification was optimized for the use of the HotStarTaq DNA polymerase in the iCycler. The activation of the enzyme was carried out at 95°C for 9 min, followed by 40 amplification cycles of 60 s at 95°C, 60 s at 55°C, and 60 s at 72°C, with the ramps to 95°C at 2.2°C/s, to 55°C at 2.7°C/s, and to 72°C at 2.2°C/s and a final extension of 7 min at 72°C. The sensitivity assays were performed by amplification of HPV-positive cells (HeLa cells, 10 to 50 copies of HPV-18/cell; SiHa cells, 1 to 2 copies of HPV-16/cell) (29). The nested PCR (nPCR) was performed using GP5+/6+ primers (12): 2 µl of the PGMY PCR product was amplified with 50 pmol of GP5+/6+ primers in a reaction mixture containing 3.5 mM MgCl2; the amplification program was 15 min of activation at 95°C, followed by 40 cycles of 30 s at 94°C, 10 s at 38°C, 30 s at 40°C, and 60 s at 72°C with the ramps to 94°C at 2.2°C/s, to 38°C at 2.7°C/s, to 40°C at 0.2°C/s, and to 72°C at 2.2°C/s and a final extension of 7 min at 72°C.
Type-specific PCR. Type-specific detection of HPV-16, -18, -31, and -52 was performed with type-specific PCR primers for each genotype (Table 1). PCR was carried out in the DNA Perkin-Elmer 2400 thermal cycler or in the iCycler (Bio-Rad) using 50 pmol of each specific primer and 10 µl of the isolated DNA. For HPV-16 detection, the reactions were performed using the new primers HPV-16 V1 and HPV-16 V2 and 4 mM MgCl2. The PCR conditions were as follows: activation of the HotStarTaq for 15 min at 95°C, followed by 35 cycles of denaturation at 94°C for 45 s, annealing at 54°C for 45 s, and primer extension at 72°C for 60 s, with a final extension of 7 min at 72°C. For HPV-18 detection, the reactions were performed using the primers HPV-18/1 and HPV-18/2 as described earlier (2) and 2.5 mM MgCl2. The PCR conditions were identical to the HPV-16 PCR except that the annealing step was performed at 51°C for 30 s. For HPV-31 detection, the reactions were performed using the new primers HPV-31F and HPV-31R and 2.5 mM MgCl2. The PCR conditions were activation for 15 min at 95°C, followed by 35 cycles of denaturation at 94°C for 60 s, annealing at 53°C for 60 s, and primer extension at 72°C for 60 s, with a final extension of 7 min at 72°C. For HPV-52 detection, the reactions were performed using the new primers HPV-52F and HPV-52R and 3.5 mM MgCl2. The PCR conditions were identical to HPV-18 PCR except that the annealing temperature was 53°C.
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TABLE 1. Primers used for type-specific HPV detection by PCR
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SPF10 PCR-LiPA. The SPF10 PCR-LiPA (Innogenetics, Ghent, Belgium) assay was performed as previously described (25, 26). The current version of the SPF10-LiPA contains probes for high-risk HPV genotypes 16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, and 68; undetermined-risk HPV genotypes 53, 66, and 70; and low-risk HPV genotypes 6, 11, 40, 42 to 44, 54, and 74. The SPF10 HPV PCR and hybridization steps were carried out following the instructions included with the INNO-LiPA kit. The PCR was optimized for the use of the HotStarTaq DNA polymerase (QIAGEN) in the iCycler as follows: activation of the enzyme was carried out by a 15-min incubation at 95°C, and the amplification was carried out during 40 cycles that included denaturation at 95°C for 30 s, annealing at 52°C for 45 s, and primer extension at 72°C for 45 s, with the ramps to 95°C at 2.7°C/s, to 52°C at 1.5°C/s, and to 72°C at 1.5°C/s; a final extension was carried out for 5 min at 72°C. Ten microliters of the PCR product, containing biotin at the 5' end of the primers, was denatured in an alkaline solution and incubated under stringent conditions with oligonucleotide probes containing a poly(dT) tail immobilized in parallel lines on nitrocellulose membrane strips. After stringent washing, the hybrids were detected by alkaline phosphatase-streptavidin conjugate and the substrates 55-bromo-4-chloro-3-indolylphosphate and nitroblue tetrazolium, resulting in a purple precipitate at the positive probe lines.
Statistics. Statistical analyses were performed using SPSS, version 11.0, for Windows. Statistical tests were performed two sided at the 5% level of significance. The kappa statistic was calculated to evaluate the agreement between HPV detection methods (13). In general, a kappa value above 0.75 indicates excellent agreement, between 0.4 and 0.75 indicates fair to good agreement, and below 0.4 represents poor agreement. Significance testing for the unequal distribution of discordant results was performed by McNemar's chi-square test for matched pairs when dichotomous outcomes are compared (4). The number of types detected per sample by each genotyping strategy was compared using the Wilcoxon signed-rank test.
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FIG. 1. Strategy for HPV detection and genotyping in cervical cells using PCR sequencing techniques. DNA isolated from cervical scrapes by a freeze-thaw method was subjected to ß-globin PCR. Negative samples were further purified on a mini-column, and ß-globin-positive samples were analyzed in PGMY PCR. If the PGMY PCR yielded a negative result, the sample was considered HPV negative. Samples with borderline results were retested in nPCR using PGMY as external primers and GP5+/6+ as internal primers. Typing of all the positive samples was carried out by sequencing the amplimers using the PGMY or the GP5+/6+ primers. In parallel, all HPV-positive samples were also tested in HPV-16, -18, -31, and -52 type-specific PCRs.
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The results obtained using different techniques, the PCR sequencing strategy, hybridization (HC2 from Digene), or PCR hybridization (INNO-LiPA HPV genotyping from Innogenetics), are summarized in Tables 2, 3, and 4.
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TABLE 2. Comparison of HPV detection results obtained by the HC2 assay and the PCR sequencing strategya
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TABLE 3. Comparison of HPV detection results obtained by the INNO-LiPA and the HC2 assaya
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TABLE 4. Comparison of HPV detection results obtained by the INNO-LiPA and the PCR sequencing strategya
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= 0.729). However, a higher percentage of samples was scored HPV positive with the PCR sequencing strategy than with the HC2 assay (McNemar's test, P = 0.041). Among the PCR sequencing-positive, HC2-negative samples (n = 15), two corresponded to cells from the Hg-SIL profile by cytological analysis (Table 5), suggesting a lower sensitivity of the HC2 assay. |
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TABLE 5. Distribution of the HPV detection results obtained by the INNO-LiPA, the PCR sequencing strategy, and the HC2 assay and correlation with cytological analysis
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= 0.552) (Table 3). The HPV-positive score was statistically much higher in the INNO-LiPA than in the HC2 assay (McNemar's test, P < 0.001).
Interestingly, the percentage of agreement was the highest (92.6%;
= 0.843) when the results obtained by the INNO-LiPA and the PCR sequencing strategy were compared (Table 4). Although the INNO-LiPA was slightly more sensitive than the PCR sequencing strategy as it detected seven more positive samples (6%), the difference was not statistically significant (McNemar's test, P = 0.070).
Overall, the percentages of agreement between the three methods, summarized in Fig. 2, suggested that the results obtained by the HC2 assay are more often discordant (14% instead of 1% for PCR and sequencing and 7% for the INNO-LiPA).
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FIG. 2. Mathematic representation of the distribution of the HPV detection results obtained for 107 samples using three different methods, PCR sequencing strategy, HC2 assay, and HPV INNO-LiPA. Overlapping sections between circles represent identical results (positive and negative) obtained with two or three methods.
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TABLE 6. Comparison of HPV multiple-infection genotyping results obtained by the INNO-LIPA and the PCR sequencing strategya
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FIG. 3. Distribution of high-risk and probable high-risk HPV genotypes according to the PCR sequencing strategy (gray) or to the HPV INNO-LiPA (black) among the samples tested by both methods. The total numbers of positive results by HPV type are plotted. The type-specific results include positive results for HPV types from specimens infected with both single and multiple HPV types.
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The PCR sequencing approach was compared to the commercially available high-risk HPV detection HC2 assay (Digene) and to the HPV genotyping INNO-LiPA (Innogenetics). The concordance between the PCR sequencing strategy, the INNO-LiPA, and the HC2 assay was high, and agreement was very good or nearly very good (
= 0.843 or
= 0.729, respectively). Overall, the INNO-LiPA proved to be more sensitive. In the amplification of SiHa cell DNA, PGMY PCR was positive using 100 pg of genomic DNA, as previously reported (17). This sensitivity is at least 10 times higher when other general HPV primers such as the CPI/IIG or the GP5+/6+ are used (7, 37; V. Fontaine, unpublished data). The SPF10 PCR (HPV INNO-LiPA; Innogenetics) and the nPCR proved to be even more sensitive, as previously reported (17, 22). nPCR was, however, impractical because of its higher risk of contamination and, hence, was utilized only to confirm ambiguous PGMY PCR results. The 60 bp amplified by the SPF10 primers are probably easier to obtain than the large fragments of 450 bp amplified with the PGMY primers (18). The efficiency of the primer pair has, indeed, been inversely correlated to the length of the amplimer (2).
SPF10 PCR had the highest sensitivity and was able to identify 38% or 16% HPV positivity of otherwise HPV-negative samples as determined by the HC2 or the PCR sequencing method, respectively. Most types found were included in the HC2 probe cocktail; only 3 of 18 corresponded to types not found in the cocktails (one HPV-53 and two HPV-66), and all could potentially be detected by the PCR sequencing strategy. Multiple infections were especially more often detected by the INNO-LiPA. Recently, a restriction fragment length polymorphism typing method using the HpyCH4V enzyme has been described as an easy tool to discriminate between single or multiple infections (35). This strategy could be of use to improve the PCR sequencing strategy by orienting type-specific PCR and sequencing analysis. Overall, the HPV detection rate increased with the severity of the cytological diagnosis (Table 5). Of note, two negative results obtained by the HC2 assay for samples corresponded to an Hg-SIL profile, known to be always associated with HPV.
Using our PCR sequencing strategy, some uncommon HPV genotypes could be identified (HPV-62, -69, -82, and -84; hom62 and hom91) in contrast with the HC2 assay and the HPV INNO-LiPA, which are probe-restricted assays. HPV-82 is considered a high-risk HPV while the others are of undetermined risk. All were detected in cells with a normal cytology profile except for HPV-69, closely related to the probable high-risk HPV-26 in phylogenetic classification and identified in cells with a low-grade intraepithelial cell neoplasia (8). Because of the larger number of HPV genotypes identified, the PCR sequencing techniques may be useful in a research setting. This emphasizes that the involvement of certain HPV types in various lesions could be underestimated if the method used for HPV detection and identification does not cover a broad spectrum of types.
In conclusion, our results underline some advantages of the HPV INNO-LiPA over the PCR sequencing strategy. The INNO-LiPA had a higher degree of sensitivity, was more rapid, and was capable of identifying more multiple infections. Nonetheless, the PCR sequencing strategy allowed the identification of a broader range of HPV types due to the wide spectrum of the PGMY PCR combined with the unrestricted capacity of the sequencing method.
We thank Pablo Bifani for critical reading of the manuscript.
Published ahead of print on 17 January 2007. ![]()
Present address: Department of Pathology, AZ-Brussels Free University/VUB, Brussels 1090, Belgium. ![]()
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