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Testing for Human Papillomavirus

Helen Londe, M.D.
Contributing Editor

Human papillomaviruses (HPVs) are small, closed circle, double-stranded DNA viruses, which are classified by nucleic acid sequence homology rather than serologic reactivity. Genital HPVs are the most prevalent sexually transmitted pathogens. Of the approximately 50 anogenital HPVs, there are a limited number of high and intermediate risk types detected in anogenital cancer biopsy specimens. Worldwide, HPV has been associated with 99.7% of squamous cell cancers of the cervix.

Most genital HPV infections in women produce transient squamous cell abnormalities of the cervix that are resolved completely, and the probability of any one HPV infection progressing to cervical cancer is quite small. Nevertheless, cervical cancer represents the second most common cancer in women, with 465,000 new cases and 200,000 deaths worldwide every year. Men infected with oncogenic HPVs are almost always clinically asymptomatic, and the risk for HPV-associated penile cancer is low. Anal HPV infection in men who have sex with men has a high risk for anal neoplasia, however, especially for those with HIV infections.

The recent promising results of the multi-country Gardasil vaccine Phase 3 clinical study showed that the vaccine, which targets HPVs 16 and 18, was 100% effective in preventing HPV 16/18-related moderate and high grade cervical intraepithelial neoplasia (CIN) and adenocarcinoma in situ. HPV 16 and 18 account for about 70% of cervical cancers. It is uncertain how long the vaccine is effective, although data suggest 5 to 10 years. Key questions regarding the vaccine target population and religious and cultural biases against giving a vaccine that prevents sexually transmitted disease to children need to be resolved. Although the public health success of HPV vaccination shows tremendous potential, oncogenic HPVs not included in the vaccine will continue to contribute to risk for development of CIN and cancer . Therefore cervical cancer screening will have an ongoing major role in secondary prevention.

The development of methods that directly detect HPV DNA has greatly changed the field of cervical cancer testing. Current guidelines include standard Papanicolaou (Pap) test cytology in conjunction with newer HPV DNA diagnostic tests. The two indications for HPV DNA testing are Pap equivocal results (ASC-US triage) and primary cervical cancer screening in women older than 30 years of age. Primary HPV screening of women older than 30 is 20% more sensitive than Pap testing, and though less specific, HPV-positive women with negative or borderline cytology can be monitored by repeat testing, thereby reducing total colposcopy referrals while maintaining improved detection of precancerous lesions. Because of the low cancer risk and unknown natural history of HPV infection in men, HPV testing of men in general is not recommended; however, screening by anal cytology and anoscopy may be beneficial among the high-risk group of men who may become infected with anal HPV.

Testing for HPV relies exclusively on molecular biology techniques using nucleic acid probes, because HPVs cannot be cultured in vitro. There is a single diagnostic assay that is approved by the US FDA for the specific detection of high-risk HPV infection. The hc2 High-Risk HPV DNA Test (Digene Diagnostics, Gaithersburg) is designed to collectively detect one or more of 13 HPV genotypes (16/18/31/33/35/39/45/51/52/56/58/59/68) having confirmed association with cervical cancer development. Digene also offers the hc2 HPV DNA Test, which differentiates between low risk types 6/11/42/43/44 and high risk types. The HC2 assays utilize cocktails of specific RNA probes that are directed toward target HPV DNA sequences. An antibody to DNA-RNA hybrids is used for both capture and detection, which uses labeled antibody for signal amplification in a microplate chemiluminescence detection system. Cervical specimens that can be tested with the Digene tests include specimens collected with the Digene Cervical Sampler/DNA Collection Device, biopsies collected in Specimen Transport Medium, and specimens collected using a broom type collection device and placed in Cytyc ThinPrep Pap Test PreservCyt Solution. This assay is thought to be best suited for medium-sized to large laboratories with substantial liquid-based Papanicolaou test volume (LBPT). Digene offers The Rapid Capture system for high throughput testing, which handles up to 352 specimens in eight hours.

Until recently, Digenes exclusive rights to many of the high-risk HPV sequences have limited commercialization of diagnostic HPV tests, but the availability of HPV sequences has broadened. Ventana Medical Systems (Tucson) offers Inform HPV, an analyte specific reagent assay that can be performed on both tissue specimens and LBPTs. The assay is an in situ hybridization test performed on histology or LBPT slides that are incubated with fluorescein-tagged DNA probes and counterstained. Microscopic observation of specifically stained target cells indicates HPV is present. Staining is performed on an automated instrument, the Ventana BenchMark. There are separate probe kits for oncogenic and nononcogenic HPV types.

HPV DNA technology with European CE marking includes Amplicor HPV and Linear Array HPV Genotyping Tests (Roche Diagnostics, Basel) and PapilloCheck (Greiner Bio-One GmbH, Frickenhausen). Both the Amplicor and Linear Array tests are PCR-based. Amplicor simultaneously identifies all 13 high-risk HPV genotypes, and the Linear Array test identifies 37 high- and low-risk HPV genotypes. PapilloCheck is a biochip that can differentiate a total of 18 high risk and 6 low risk HPV types. PapilloCheck is a ready-to-use complete test system with all the necessary buffers, solutions, and on chip controls.

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