Papovaviruses: Properties & Classification PDF

Papillomas or warts have been recognized in animals for centuries; a stable master for the Caliph of Baghdad described equine warts in the 9th century. That papillo- mas have a viral etiology was recognized as long ago as 1907, but it was not until 1978 that it was realized that bovine papillom...

Papillomas or warts have been recognized in animals for centuries; a stable master for the Caliph of Baghdad described equine warts in the 9th century. That papillo- mas have a viral etiology was recognized as long ago as 1907, but it was not until 1978 that it was realized that bovine papillomas and papillomas in other species are caused by several different viruses. In 1935, Peyton Rou observed that benign rabbit papillomas occasionally pro- gressed to carcinomas; this was one of the earliest associ- ations of viruses with cancer. Today, bovine papillo- matosis, canine oral papillomatosis, and equine sarcoid may present significant clinical problems. With few exceptions, papillomaviruses cannot be grown in cell culture; nevertheless, DNAs of many of the viruses that infect animals and about half of the many viruses that infect humans have been purified, cloned, and sequenced completely. This work was stimu- lated by the discovery in the 1980s that certain papillo- maviruses cause cervical, anogenital, and laryngeal carci- nomas in humans. This discovery, in turn, has prompted much research on the nature and mechanisms of papillo- mavirus oncogenesis, which is now advancing our under- standing of papillomas in animals. Polyomaviruses are also highly species specific. Ex- cept for rare neurologic and urologic diseases in immu- nologically incompetent humans and a disease in budger- igars, these viruses are of little concern as pathogens in nature. Properties of Papovaviruses Classification The family Papovaviridae comprises two rather dispa- rate genera: (1) the genus Papillomavirus, containing the many papillomaviruses of mammals and birds, (2) the genus Polyomavirus, containing a few pathogens of ani- mals and humans. The first two syllables of the name Papovaviridae refer to the genera Papillomavirus and Polyomavirus; \"va\" alludes to \"vacuolating agent,\" an old name for the prototype polyomavirus, simian virus 40 (SV40). Papillomaviruses are distinguished on the basis of host range and DNA sequence relatedness. Types are designated by numbers following the chronological or- der of their identification. By convention, a new virus type must have less than 50% overall DNA sequence homology with other viruses from the same species (greater than 50% but less than 100% homology defines new subtypes, which are designated by serial letters). Using this system, 6 types of bovine, 2 types of equine, and more than 77 types of human papillomaviruses have been identified (Table 20.1). Papillomaviruses have also been found in chimpanzees, colobus and rhesus mon- keys, deer, dogs, elephant, elk, opossum, mice, turtles, chaffinches, and parrots. There is little sequence homol- ogy between DNAs of papillomaviruses from different species. TABLE 20.1 Diseases Caused by Papillomaviruses Bovine papillomaviruses have been divided further into two \"groups\": (1) bovine papillomaviruses 1, 2, and 5 are related immunologically, have the same ge- nome size, and share nucleotide sequence homologies and (2) bovine papillomaviruses 3, 4, and 6 have smaller genomes and share DNA sequence homologies. The two groups are related only distantly. Papillomaviruses can also be categorized according to their tissue tropism and the histologic character of the lesions they cause: group I (bovine papillomaviruses types 3 and 6 and cottontail rabbit papillomavirus) in- duce cutaneous neoplasia; group II (bovine papillomavi- rus type 4) induce hyperplasia of nonstratified squamous epithelium; group III (bovine papillomaviruses types 1, 2, and 5) induce subcutaneous fibromas in addition to cutaneous papillomas; and group IV (deer papillomavi- rus) induce primarily fibromas with minimal cutane- ous hyperplasia. Virion Properties Papovavirus virions are nonenveloped, spherical in out- line, with icosahedral symmetry. Virions are 55 (genus Papillomavirus) or 45 (genus Polyomavirus) nm in dia- meter. Virions are constructed from 72 hexavalent (six- sided) capsomers arranged most unusually in pentameric (five-sided) arrays (Figures 20.1 and 20.2). Both \"empty\" and \"full\" virus particles and tubular and other aberrant forms are seen by electron microscopy. The genome con- sists of a single molecule of circular double-stranded DNA, 8 (genus Papillomavirus) or 5 (genus Polyomavi- rus) kbp in size. The DNA has covalently closed ends, is supercoiled, and is infectious. Six polypeptides have been identified, two forming the capsid (Figure 20.3). The viruses are resistant to diverse environmental insults: infectivity survives lipid solvents and detergents, low pH, and high temperatures. Viral Replication Papillomavirus replication is linked tightly to the growth and differentiation of cells in stratified squamous and mucosal epithelium from their origin in basal layers to their shedding at the epidermal surface of the skin or mucous membranes. Actively dividing basal cells in the stratum germinativum are infected initially and are be- lieved to maintain the virus in a proviral, possibly latent, state throughout cellular differentiation. Virus-induced hyperplasia, induced by early viral gene products, leads to increased basal cell division and delayed maturation of cells in the stratum spinosum and stratum granulosum. These cells become massed into nascent papillomas. Late viral genes encoding capsid proteins are expressed, first in cells of the stratum spinosum. Virions are first seen at this stage of cellular differentiation. The accumulation of large numbers of virions and associated cytopathology are most pronounced in the stratum granulosum. Virions are shed with exfoliated cells of the stratum corneum of the skin or nonkeratinized cells of mucosal surfaces (Figure 20.4). Virions attach to cellular receptors, enter via recep- tor-mediated endocytosis, and are transported to the nucleus where they are uncoated, releasing their DNA. During productive infection, transcription of the viral genome is divided into early and late stages. Transcrip- tion of early and late coding regions is controlled by separate promoters and occurs on opposite DNA strands in the case of polyomaviruses and on the same strand with papillomaviruses. First, the half of the genome that contains the early genes is transcribed, forming mRNAs that direct the synthesis of enzymes involved in viral replication. Late mRNAs that direct the synthesis of virion structural proteins are transcribed from the other half of the viral genome after DNA synthesis has begun. Progeny DNA molecules serve as additional template, amplifying the production of structural proteins greatly. Several different translational strategies are employed to enhance the limited coding capacity of the viral genomes. Papovavirus DNA replication begins at a single unique origin of replication and proceeds bidirectionally, terminating about 180 \~ away on the circular DNA. An initiation complex binds to the origin and unwinds a region (the replication bubble and fork); nascent DNA chains are formed; one strand synthesized continuously in the direction of unwinding, the other synthesized dis- continuously in the opposite direction. As replication proceeds the torsional strain created by the unwinding of the parental strands of DNA is released by the action of a specific viral enzyme. Bidirectional replication pro- ceeds around the full genomic DNA circle, at which point the progeny DNA circles separate. Virions are assembled in the nucleus and are re- leased on cell death, often just as a consequence of cellu- lar replacement in epithelia. Some cells exhibit a charac- teristic cytopathic effect, marked by cytoplasmic vacuolization. An infected cell may produce 10,000 to 100,000 virions. Bovine papillomaviruses I and 2 and DNA isolated from them can transform cells in vitro. In contrast to other transforming DNA viruses, papillomavirus DNA remains episomal and is rarely integrated into the cellular genome. The genes expressed in such cells encode pro- teins concerned with replication and regulation of viral transcription as well as proteins that affect cellular trans- formation directly. In association with cofactors, bovine, rabbit, and human papillomaviruses may produce carci- nomas in vivo (Table 20.2). Properties of Papovaviruses Two genera, Polyomavirus and Papillomavirus Virions are nonenveioped, spherical in outline, with icosahe- dral symmetry. Virions are 55 (genus Papillomavirus) or 45 (genus Polyomavirus) nm in diameter The genome consists of a single molecule of circular double- stranded DNA, 8 (genus Papillomavirus)or 5 (genus Poly- omavirus) kbp in size. The DNA has covalently closed ends, is circular and supercoiled, and is infectious Members of both genera replicate in nucleus; members of the genus Polyomavirus grow in cultured cells; most mem- bers of the genus Papillomavirus have not been grown in culture, but will transform cultured cells; infectious virions produced only in terminally differentiated epithelial cells During replication, polyomavirus DNA is transcribed from both strands, whereas papillomavirus DNA is transcribed from one strand Integrated (genus Polyomavirus) or episomal (genus Papillo- mavirus) DNA may be oncogenic and 6, equine, and cotton- tail rabbit papillomaviruses \~ DISEASES CAUSED BY MEMBERS OF THE GENUS PAPlLLOMAVIRUS Bovine Papillomatosis Papillomas or warts are seen more commonly in cattle than in any other domestic animal. All ages are affected, but the incidence is highest in calves and yearlings. Bovine papillomaviruses 1 and 2 exhibit a some- what broader host range and tissue tropism than other types, causing fibropapillomas in cattle and sarcoids in horses. Bovine papiUomaviruses 3 to 6 have been shown by both natural and experimental infection to be re- stricted to cattle. Transmission from cattle to humans was suspected from the high incidence of cutaneous warts in butchers; however, the virus isolated from these people does not appear to be related to any known bovine virus. Clinical Features The various papillomaviruses recognized in cattle are associated with distinct lesions (Table 20.3); bovine pap- illomaviruses 1, 2, and 5 cause \"teat frond\" warts, com- mon cutaneous warts, and \"rice grain\" fibropapillomas, respectively. These papillomas have a fibrous core cov- ered to a variable depth with stratified squamous epithe- lium, the outer layers of which are hyperkeratinized. The lesions vary from small firm nodules to large cauliflower- like growths; they are grayish to black in color and rough and spiny to the touch. Large fibropapillomas are subject to abrasion and may bleed. FibropapiUomas are common on the udder and teats and on the head, neck, and shoul- ders; they may also occur in the omasum, vagina, vulva, penis, and anus (Figure 20.5A). In contrast, bovine papillomaviruses 3, 4, and 6 induce epithelial and cutaneous lesions without fibro- blast proliferation. Lesions caused by bovine papillo- mavirus 3 have a tendency to persist and are usually flat with a broad base in contrast to the more usual fibropapillomas that protrude and are often peduncu- lated. In upland areas of Scotland and northern England, papillomas due to bovine papillomavirus 4 occur only in the alimentary tract and in the urinary bladder and may progress to squamous cells carcinomas (Figure 20-5B) (see Chapter 11 ). Ingestion of bracken fern ( Pteri- dium aquilinum) is a major contributing factor (both cocarcinogen and immunosuppressive agent) in the tran- sition from benign papillomas to invasive carcinoma of the alimentary tract (type 4) or bladder (type 2), the latter leading to so-called chronic endemic hematuria. Pathogenesis, Pathology, and Immunity Papillomas develop after the introduction of virus through abrasions of the skin. Infection of epithelial cells results in hyperplasia with subsequent degenera- tion and hyperkeratinization. These changes begin usu- ally 4 to 6 weeks after exposure. In general, fibropa- pillomas persist for 4 to 6 months before spontaneous regression; multiple warts usually regress simulta- neously. Stages in the pattern of papilloma develop- ment can be discerned. Thus, stage 1 papillomas appear as slightly raised plaques, starting at about 4 weeks after exposure. Stage 2 papillomas are characterized by cytopathology, virus replication, and crystalline aggregates of virions in lesions, starting at about 8 weeks. Stage 3 papillomas are characterized by fibrotic, pedunculated bases and rough, lobate, or fungiform surfaces, starting after about 12 weeks. The level of neutralizing antibody appears to be correlated with the regression of lesions and with protection against re- infection. Laboratory Diagnosis The clinical appearance of papillomas is characteristic, and laboratory diagnosis is seldom necessary. Virions \~an be seen by electron microscopic examination. Hy- bridization assays and the polymerase chain reaction can be used to detect papillomavirus DNA, but these methods are seldom used for routine diagnosis in veteri- nary medicine. Epidemiology, Prevention, and Control Virus is transmitted between animals by contaminated halters, nose leads, grooming and earmarking equip- ment, rubbing posts, and other articles contaminated by contact with diseased cattle. Cattle that have been groomed for show may have extensive lesions. Sexual transmission of warts in cattle is likely as such lesions are rare in animals that are artificially inseminated. The disease is more common in housed cattle than in cattle on pasture. Natural bovine papillomavirus infection of horses generally occurs after housing animals in stalls that previously held cattle. Prevention and treatment of papillomas are diffi- cult to evaluate because the disease is self-limiting and its duration varies. Bovine interferon \~ has been used to treat cattle, but rarely seems indicated. Psoralen-based photodynamic therapy has been used, but again appro- priate clinical circumstances are rarely encountered. In- oculation with homogenized, autologous wart tissue, treated with formalin, has been used for many years, but its efficacy has always been evaluated anecdotally. Vaccination with viral capsid proteins produced by re- combinant DNA technology has been encouraging, but vaccines must contain multiple virus types because there is no cross-protection. Equine Papillomatosis and Sarcoids Lesions caused by equine papillomavirus appear occa- sionally as small, elevated, keratinized papillomas around the lips and noses of horses (Figure 20.5C). They generally regress after 1 to 9 months. Warts that interfere with the bit or bridle can be removed surgically. Congeni- tal equine papillomas have been recorded on several oc- casions. Sarcoids are naturally occurring skin tumors of horses that have the histological appearance of fibrosar- comas. Although they do not metastasize, they persist for life and are locally invasive, often recurring after surgical removal or treatment with radioactive implants. Transmission trials with sarcoid material have usually been unsuccessful. Sarcoids have not been observed to spread from affected horses to other horses by direct contact. On the basis of their appearance, sarcoids have been classified into type 1 (verrucous type, usually hair- less and slowly growing), type 2 (fibroblastic, \"proud flesh\" type, comprising an intradermal fibroblastic pro- liferative response, often growing rapidly and ulcerat- ing), type 3 (mixed type, showing features of types 1 and 2), and type 4 (occult type, flat with rough thickened skin and a surrounding area of alopecia). Types 1 to 3 may be either sessile or pedunculated. Subcutaneous fibrous nodules beneath apparently normal skin have been observed in association with sarcoids, particularly in the periorbital region, and may represent a fifth cat- egory. Horses are susceptible to experimental infection with bovine papillomaviruses I and 2, and the tumors produced are similar to sarcoids. Bovine papillomavirus DNA sequences have been detected in high copy number by hybridization in both experimental and natural le- sions. Also, bovine papillomaviruses have been shown to be able to transform equine fibroblasts in vitro. These data, together with the observation that sarcoids can occur in epidemic form, suggest that bovine papillomavi- ruses may be the cause of equine sarcoids. However, unlike their natural counterparts, the induced tumors regress spontaneously, and horses infected experimen- tally develop antibodies against bovine papillomavi- ruses, which are absent in horses with naturally oc- curring sarcoids. The variable success and hazards of several at- tempted therapies (surgery, laser surgery, radiation, topi- cal drugs) have led to an interest in immunotherapy. Stimulation of cell-mediated responses by the injection of immunopotentiators has been somewhat successful. Ocular sarcoids have regressed following the injection of viable Bacillus Calmette-Guerin (BCG) mycobacteria. Canine Papillomatosis Warts in dogs usually begin on the lips and can spread to the buccal mucosa, tongue, palate, and pharynx before regressing spontaneously (Figure 20-5D). The lesions oc- casionally become extensive, requiring veterinary at- tention. Papillomatosis in Other Mammalian Species Classic studies on viral oncogenesis were carried out in the late 1930s with the Shope rabbit papillomavirus. Papillomas caused by this virus often progress to carcino- mas in both their natural host, the cottontail rabbit (Syl- vilagus spp.), and in laboratory rabbits infected experi- mentally. Oral papillomatosis occurs naturally in domestic rabbits (Oryctologus cuniculus); the tumors are small, gray-white, filiform or pedunculated nodules (5 mm in diameter) and are localized mostly on the underside of the tongue. The causative papillomavirus is distinct from the Shope rabbit papilloma virus (cottontail rabbit papil- lomavirus). Experimentally, oral papillomatosis has been reproduced in various rabbit species, and in nature it is widespread among domestic rabbits, particularly in older animals. Virus spread in animal rooms seems not to occur, but transmission from the mother to offspring during the suckling period is common. Oral papillomas of rabbits show no tendency to malignancy and may persist for many months. Papillomatosis in Birds A member of the genus Papillomavirus has been identi- fied in birds: the Fringilla (finch) papillomavirus. Infec- tion with a second papillomavirus-like virus has been described in African green parrots. Both viruses have been demonstrated in papillomatous lesions in their re- spective host species. In finches, papillomas occur exclu- sively on the legs and show stages of development from a slight node on a digit to heavy involvement of the foot and tarsometatarsus, often obscuring the individual digits and resulting in overgrowth and distortion of the claws. In severe cases the tumor may account for up to 5% of the bird\'s total body weight, but affected birds seem to remain in good condition otherwise. DISEASES CAUSED BY MEMBERS OF THE GENUS POLYOMAVIRUS Polyomaviruses cause inapparent infections in most hosts; they have restricted host ranges and their onco- genic potential has often only been revealed when inocu- lated experimentally into heterologous hosts. Several polyomaviruses have been found in rodents and lago- morphs in addition to murine polyoma virus, the virus that gave this genus its name: K virus of mice, latent hamster virus, and rabbit kidney vacuolating virus. Poly- omaviruses of interest in veterinary medicine occur in cattle and birds. Bovine Polyomavirus Infection About 60% of bovine sera, including fetal and neonatal calf sera, contain a bovine polyomavirus, which grows well in monkey kidney cells. A construct containing its early genes transforms rodent cells, and these cells induce tumors in immunocompromised rats. In a survey done in The Netherlands, about 60% of veterinarians were found to have antibodies to this virus. Despite this preva- lence, the possible significance of this virus remains un- known. Budgerigar Fledgling Disease An avian polyomavirus has been identified as the cause of an acute generalized disease in fledgling budgerigars (Melopsittacus undulatus). The same virus may also be responsible for \"French molt,\" which is a milder disease of budgerigars that results in chronic disorders of feather formation, and it is also widespread among chickens as a subclinical infection. Budgerigar fledgling disease has been reported from various aviaries in the United States, with mortalit- ies ranging between 30 and 80%. Affected birds have full crops, die acutely, and exhibit abdominal distention and reddening of the skin. Postmortem examination re- veals visceral changes such as hydropericardium, en- larged heart and liver, and swollen congested kidneys. On histologic examination, cells with enlarged nuclei containing inclusions are seen along with necrotic foci in several organs. By electron microscopy, polyomavirus virions can be visualized in the nuclei of epithelial cells, e.g., in renal tubules. The virus has been isolated in budgerigar embryo fibroblasts inoculated with tissue homogenates from af- fected birds and can be adapted easily to growth in chicken embryo fibroblasts. The virus is similar to poly- omaviruses of mammals, having a similar genome but a smaller large T antigen and a different organization of its origin of replication. Little similarity was observed by physical mapping with restriction endonucleases.