Study population
The centralized cervical cancer screening program (CCSP) of BC has been operational since 1960. It processes every Pap smear done in BC at a single facility; all cytology results are stored in a single database. More than half a million women participate in the CCSP each year and over 70% of eligible women in BC are screened, on average, every 30 months.
Specimen collection and cytological interpretation
A flowchart summarizing sample collection and experiments is shown in Supplemental Online Figure A. The 8,700 samples used in this study were derived from a feasibility study of liquid-based smears collected by 99 high-volume smear-takers from different parts of BC within the CCSP between March and July 2004 [8]. The sample included women aged 13–86; median age was 38. About 98.2% of the smears in this study are from the cervix or endocervix; 1.8% from vaginal samples. Practitioners were instructed to obtain the sample from the transformation zone of the cervix using a Rovers® Cervex Brush. Swabs were placed in SurePath® media. TriPath Imaging Inc. equipment was used to process samples according to the manufacturer’s instructions. Cervical smears were interpreted by Canadian-registered CCSP cytotechnologists and the BC Cancer Agency-based cytopathologists. Cytological interpretation was reported using the British Society of Clinical Cytology terminology currently in use in BC. For this analysis, however, results were reclassified using the Bethesda system. Negative and benign changes were kept as originally categorized. Mild dyskariosis was classified as low-grade intraepithelial lesions (LGIL) of the squamous or glandular type; moderate or severe dyskariosis and suspicious smears were classified as high-grade intraepithelial lesions (HGIL) of the squamous or glandular type. Smears showing squamous (87.7%) and glandular (12.3%) abnormalities were not separated in our main analysis of LGIL or HGIL for simplicity of data presentation. Individual typing data has been separated by glandular or squamous type and is included in a separate table (Table 1). The categories of ASCUS and AGUS were not used.
Table 1 HPV type distribution according to cellular origin of abnormality, 95% CI shown in brackets
This study was approved by the joint Clinical Research Ethics Board of the BC Cancer Agency and the University of British Columbia. Use of specimens for this study was performed according to the ‘Secondary Use of Personal Information in Health Research: Case Studies’ (Canadian Institutes of Health Research, November 2002). Cytology results were recorded in the CCSP database. Each sample was assigned a study number, and the data including the age of the participant, geographic region of the smear taker, cytology result and previous screening history were attached to the study number. Subsequently, the remainder of each sample and the data were stripped of potential patient identifiers. The data and samples left over after cytology were then transferred to the Genome Sciences Centre at the BC Cancer Research Centre for HPV analysis.
Study sample selection
From the total study sample set of 8,700, forty samples were from repeat smears from the same women and were excluded, leaving 8,660 independent samples. PCR analysis was performed on 4,980 samples including all 614 cytologically abnormal samples and a random selection (every second sample by study number) of 4,366 normal and benign cytology smears. This sample showed a representative distribution to that of the remaining samples. Neither normal nor benign smears showed a statistically significant difference in age distribution or geographic location between selected and not selected smears. Age was tested using the t-test, and also using Mantel-Hanzel chi-square analysis with six age categories (<20, 20–29, 30–39, 40–49, 50–59 and 60+). Geographic location was tested using the chi-square test.
DNA extraction, quantification and quality control
The portion of each sample remaining after cytology (1–6 ml) was pelleted by centrifugation, re-suspended in 300 μl of phosphate-buffered saline, and stored at −80°C. DNA was extracted from 150 μl of thawed re-suspended cellular material using the PureGene DNA isolation kit (Gentra Systems, MN, USA). DNA samples were quantified by fluorometry and 10 ng aliquots arrayed in 96-well plates for PCR analysis. Plates were arrayed according to sample number and were not separated according to cytology. The β-globin gene primers were used to confirm the competence each DNA sample to support PCR. The percentage of samples that passed this quality control test is 96.8% (4,821 samples) samples that did not pass this test were not included in HPV testing (see Supplemental Online Figure A).
HPV testing and HPV type determination
Tagged GP5+/GP6+ consensus primers [9, 10] were used to detect HPV by amplifying a 150 bp sequence of the viral L1 gene from virtually any HPV type, and bi-directional sequencing was used to determine HPV type(s) present in each sample. The GP5+/6+ primers [9, 10] were modified by the addition of SeqA2 (GAATTCTCTAGATGATCAGCGGC) or Seq B2 (CGAACTTTATTCGGTCGAAAAGG) tags to their 5′ ends to simplify later sequencing. Testing of known HPV types mixed with genomic DNA demonstrated effectiveness of the tagged primers in detecting various HPV types. PCR analysis was carried out as previously described [9] with minimal changes (95°C 30 s, 40°C 1 min, 68°C 30 s for 40 cycles). An aliquot of each PCR product was separated on a 3% agarose gel for visualization. Samples that showed the expected 150 bp band were designated as HPV positive. Aliquots of PCR products from HPV positive samples were then re-arrayed into 96-well plates and purified by the AMPure magnetic bead system (Agencourt Bioscience Corporation, Beverly, Massachusetts, USA). Purified PCR products were bi-directionally sequenced using BigDye 3.1 at 1/24 chemistry and run on 3730xl capillary sequencers (Applied Biosystems, Foster City, California). Sequence traces that produced apparent multiple overlapping sequences were flagged as possible multiple infections (MI). PCR products from such samples were phosphorylated with polynucleotide kinase (New England BioLabs, MA, USA) and subcloned by blunt end ligation into pUC19. Sixteen clones of each putative MI were bi-directionally sequenced using the -21 M13 Forward (TGTAAAACGACGGCCAGT) and M13 Reverse (CAGGAAACAGCTATGAC) primers. Sequences were aligned to a database of all known HPV L1 sequences using local BLAST alignment, and the best match scored as a specific HPV type if it had greater than 95% similarity over more than 50 bases.
For this study types 16, 18, 26, 31, 33, 35, 39, 45, 51, 52, 53, 56, 58, 59, 66, 68, 73, and 82 were considered high-risk HPV types.
Statistical analyses
Statistical analyses were performed using the SAS package (SAS Institute Inc., Cary, NC, USA). All cytologically abnormal samples were HPV typed, but not all normal or benign samples were typed; it was, therefore, necessary to weight by cytology in the final prevalence analyses. Weighting was performed as follows. Column “Study Sample” in Tables 2 and 3 adjusts prevalence estimated from successful HPV testing to reflect cytology distribution in the study sample. The weight for each normal, benign, LGIL and HGIL is the proportion it constitutes of the study sample, divided by the proportion it constitutes of successful HPV tests. Multiply infected samples were defined as samples for which two or more HPV types were detected. Such samples were counted as a positive for one type of HPV and also included among positives for another or other types of HPV, in calculations of the prevalence of each HPV type.
Table 2 HPV Prevalence and type distribution, shown by cytology group, (95% CI shown in parentheses)
Table 3 Multiple infections, shown by cytology group (95% CI shown in parentheses)
The Cochrane–Armitage trend test was used on HPV prevalence rates by 5-year age groups shown in Figs. 1 and 2a.