Background: Screening with combined cytologic and HPV testing has led to the highest number of excessive colposcopic referrals due to high false positive rates of the current HPV testing in the USA. How best to capitalize on the enhanced sensitivity of HPV DNA testing while minimizing false-positive results from its lower specificity is an important task for the clinical pathologists.
Methods: The HPV L1 gene DNA in liquid-based Pap cytology specimens was initially amplified by the degenerate MY09/MY11 PCR primers and then re-amplified by the nested GP5+/GP6+ primers, or the heminested GP6/MY11, heminested GP5/MY09 primers or their modified equivalent without sample purification or DNA extraction. The nested PCR products were used for direct automated DNA sequencing. A 34- to 50-base sequence including the GP5+ priming site was selected as the signature sequence for routine genotyping by online BLAST sequence alignment algorithms.
Results: Of 3,222 specimens, 352 were found to contain HPV DNA, with 92% of the positive samples infected by only 1 of the 35 HPV genotypes detected and 8% by more than 1 HPV genotype. The most common genotype was HPV-16 (68 isolates), followed by HPV-52 (25 isolates). More than half (53.7%) of the total number of HPV isolates relied on a nested PCR for detection although the majority of HPV-16, -18, -31, -33 -35 and -58 isolates were detected by a single MY09/MY11 PCR. Alignment of a 34-base sequence downstream of the GP5+ site failed to distinguish some isolates of HPV-16, -31 and -33. Novel variants of HPV with less than "100% identities" signature sequence match with those stored in the Genbank database were also detected by signature DNA sequencing in this rural and suburban population of the United States.
Conclusion: Laboratory staff must be familiar with the limitations of the consensus PCR primers, the locations of the signature sequence in the L1 gene for some HPV genotypes, and HPV genotype sequence variants in order to perform accurate HPV genotyping.