Human papilloma virus

Historical background

Even in ancient times condylomata were well-known in literature and art. The descriptions 'fig' and 'condyloma' have survived till our time (Bäfverstedt 1967). Mucosal changes on the penis described in medieval documents might have been condylomata. At the end of the 18th century condylomata were interpreted as a manifestation of syphilis. The first observation that condylomata were not a sign of syphilis was made by Bell in 1793. This was later confirmed independently by Jonadan (1826) and Ricord (1838). In the early 19th century condyloma was instead interpreted as a manifestation of gonorrhea. However, this standpoint was questioned by Aime Martin, who, in 1872, was able to demonstrate that many patients with genital warts showed no signs of gonorrhea (Oriel 1971).

During the 19th century it was also suggested that condylomata were caused by irritation of the epidermis by dirt, smegma and genital discharge. This theory still had support in the beginning of the 20th century.

A relationship between genital warts and skin warts was suggested by Gemy in 1893 because of the resemblance in histological features. It was also noticed that many patients with genital warts also had common warts, although children with skin warts seldom had genital warts. This hypothesis gained further support through experiments in which extracts from excised penile warts were inoculated to other parts of the body. Plane and common warts were in some cases seen at the site of inoculation (Ciuffo 1907, Serra 1908, Wile & Kingery 1919). Papillomaviruses were identified as viral particles by Strauss in 1949 using the electronmicroscope.

The hypothesis that genital warts could be sexually transmitted was proposed in the early nineteen twenties, but no definite proof could be presented. However, in the middle of the nineteen fifties it was observed that many soldiers returning from the Far East had penile warts and that their wives developed vulvar warts. Barrett, Silbar and McGuinley (1954) suggested that the disease could be sexually transmitted, but they were strongly criticised. In the beginning of the nineteen sixties Teokharov presented epidemiological data confirming the hypothesis that genital warts comprised an independent viral venereal disease, different from non-genital warts (Oriel 1971).

Characteristics of the virus

The papillomaviruses belong to the papovavirus group. The virus particle is 52-55 nm in diameter and has an icosahedral capsid, which is built up of 72 capsomers. There is no lipid envelope. The papillomavirus genome is a circular, double-stranded DNA molecule of about 8000 base-pairs (bp) in size. There is a great variety of HPV types but the organisation of their genomes is almost identical. The viral genomes are functionally divided into two long domains, each containing a series of open reading frames (ORF) that code for viral proteins. The early (E1-7) regions code for proteins associated with genome replication and control, while the late (L1-2) regions code for the structural proteins of the virus capsid. Other regions do not possess any coding function but rather contain viral regulatory elements (von Krogh 1991, zur Hausen & de Villiers 1994).

Today, approximately 70 different types of HPV are known and of these more than 20 affect the genital mucosa (Lorincz et al, 1992, de Roda Husman et al. 1994, de Villiers 1994). Significant heterogenecity at the nucleotide level is found between the different HPV genotype. DNA sequence homology is the basis of HPV genotype classification and a new type of HPV shows less than 90% homology to any known HPV (van Ranst et al, 1993). The genital HPV types have been divided into three groups according to their association with cancer development. Type 16, 45 and 56 are called high-risk types, as they are frequently associated with increasing degree of dysplasia and cervical cancer (de Roda Husman et al, 1994). Type 18 is mainly associated with adenocarcinoma and has earlier been thought to be a high-risk type, but later studies have suggested this type to be an intermediate type (Syrjanen et al, 1990(a)). Other intermediate risk types are 31, 33, 35, 51 and 52. The low-risk types are 6, 11, 42, 43 and 44 (Lorincz et al, 1992).

Clinical manifestations

The classical manifestation of genital HPV infection is condyloma accuminatum, which in Greek means "a pointed rounded tumour". This manifestation is easily diagnosed by the naked eye. Some warts have been called plane or papular and are sometimes difficult to differentiate from lichen ruber planus or verruca seborroica. In these cases a biopsy is often required for differential diagnosis.

One group of manifestations caused by HPV infection is not possible to detect with the naked eye. To visualise them the acetic acid test is useful. They are called macular, flat or microcondylomata and are often said to be subclinical. This terminology is somewhat misleading as these manifestations could be symptomatic and in that sense be clinical. The term subclinical was first applied to HPV lesions of the cervix by Reid and coworkers (1982), but this term has later also been used for lesions on the outer genitals.

Lastly, there is latent genital HPV infection, which harbour HPV DNA, although no lesions can be detected either with acetic acid test or by histology.

Diagnostic methods

Different techniques for detection of viral DNA have been developed during the last decades. The Southern-blot DNA technique was first described by Southern in 1975. It has been considered the golden standard for HPV DNA detection in cell or tissue preparations.

The cellular DNA is extracted from cell material after the digestion of proteins and RNA. The DNA is cleaved using different restriction endonucleases and separated by agarose gel electrophoresis. The DNA is then denatured and transferred ("blotted") onto a hybridisation membrane. Labelled with either radioactive or nonradioactive compounds the cloned DNA is hybridisised to the cellular DNA on a filter at different temperatures below the melting point, which is by definition the point at which 50% of a given DNA exists as single-stranded molecules. By variation of the temperature, salt and formamide concentrations and size of the DNA probe, different levels of stringency can be applied. After 2-4 days of hybridisation, the membrane is washed, air dried and exposed to autoradiography. Non-radioactive markers are visualised by an appropiate reaction. From 0.1 to 0.01 genome equivalents per cell can be detected.

Jason Ladock

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