How Does Papovavirus Affect Humans? Key Facts, Symptoms, and Solutions
Papovavirus is a virus that belongs to the Papillomaviridae and Polyomaviridae families. Papovaviruses cause various abnormal growths in animals, including warts (papillomas) in humans, dogs, and other animals, cervical cancer in women, tumours (polyomas) in mice, and vacuoles (open areas) in monkey cells.
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The virus particle (virion) has a diameter of 45 nm (1 nm=10−9 metre). The capsid's outer surface (the protein shell that surrounds the viral nucleic acids) is icosahedral and made up of 72 capsomeres (capsid subunits). Double-stranded deoxyribonucleic acid is found in the capsid (DNA). Papovaviruses grow in the nuclei of cells, where they appear as crystalline arrangements.
Structure of Papovavirus
Polyomaviruses are icosahedral particles with a 45-nm diameter and three capsid proteins but no envelope. They have a circular, double-stranded DNA genome of 5 kbp. All polyomaviruses have the same genome structure, which consists of two or three replicative genes (tumour antigens) encoded on one strand and three structural genes (capsid antigens) encoded on the other.
Classification and Antigenic Types of Papovavirus
The structure of the viral particle is used to classify it. Human and animal polyomaviruses are antigenically distinct, with each virus having only one serotype. The prototype is a monkey-borne simian virus 40 (SV40).
Multiplication of Papovavirus
In the nuclei of infected cells, viral DNA uncoils. Early viral genes are expressed, and host cells are stimulated to enter the S phase, resulting in the production of cellular enzymes that are used to synthesise viral DNA. Progeny virions are assembled in nuclei, and late viral genes are expressed. Cell lysis takes place later. Virus particles are frequently found in cell debris. The papovaviruses have the potential to cause cancer (the papillomaviruses in their natural hosts and the polyomaviruses under experimental conditions).
Host Defenses of Papovavirus
Infections last for a long time and produce humoral antibodies as well as cytotoxic T cells. In immunocompromised people, viral reactivation occurs. The background for the development of progressive multifocal leukoencephalopathy is impaired cell-mediated immunity. Papovaviruses weakly induce interferon, and their sensitivity to interferon's antiviral action varies. Interferon can stop polyomaviruses from transforming cells.
Epidemiology of Papovavirus
Polyomaviruses of the BK and JC types are common. Childhood infections are common, and 70 to 80 percent of adults have antibodies. The transmission route is unknown, but it could be respiratory. Animal reservoirs do not exist for human viruses. Antibodies to SV40, a simian virus, are found in a small percentage of humans. The exact mechanism of SV40 exposure is unknown.
What is Human Papovavirus?
Papovaviruses are non-enveloped DNA viruses with similar morphological characteristics and replication processes. The acronym "papova" is made up of the first two letters of "human papillomavirus," "mouse polyomavirus," and "simian vacuolating virus" (simian virus 40, SV40) (Melnick, 1962). The papovaviruses are naturally divided into two subgroups: the larger papilloma (wart) viruses, which cause benign skin or mucous membrane tumours, and the smaller SV40-polyoma viruses, which mostly cause urinary tract infections and are oncogenic in laboratory animals.
It has been proposed that the papovaviruses be classified as a family, Papovaviridae, with two genera, Papovavirus A (papillomaviruses) and Papovavirus B (viruses of the SV40-polyoma subgroup), and that virus species be assigned numerical designations (Melnick et al., 1974).
Papovavirus Treatment
The human papovaviruses BK and JC are members of the human papovavirus family. After transplantation, virus reactivation has been linked to hemorrhagic cystitis, progressive multifocal leukoencephalopathy, and interstitial nephritis. Papovavirus can be detected in body fluids and tissues using specific in situ hybridization and PCR techniques. In the clinical setting, viral culture is rarely used.
Despite the fact that cidofovir has been used to treat papovavirus infections, an effective treatment has yet to be discovered. In stem cell transplant recipients, CMX001, an oral lipid conjugate of cidofovir with enhanced in vitro activity for BK virus, was found to reduce BK virus-associated cystitis.
Summary
Papillomavirus and Polyomavirus are two genera in the Papovaviridae family. Papillomaviruses are naturally tumorigenic viruses that cause papillomas (warts) in a variety of species. Only those that affect cattle, horses, and dogs are likely to necessitate veterinary care. They appear in young animals as areas of simple hyperplasia or benign neoplasms, and they usually go away on their own. Bovine, human, and rabbit papillomaviruses, on the other hand, produce carcinomas when they interact with certain cofactors. Papillomaviruses have never been grown in cultured cells, which has limited research to the pathology of lesions, virion ultrastructure, and experiments on t cells.
However, cloning restriction fragments of Papillomavirus DNAs obtained from virions purified from papillomas became possible a few years ago. Molecular hybridization, heteroduplex mapping, and DNA sequencing have all been made possible as a result of this. This has greatly increased our understanding of papillomaviruses and sparked renewed interest in their tumorigenesis mechanisms. Although polyomavirus infections do not cause disease in their natural hosts, some of these viruses can cause tumours when inoculated into newborn rodents.
FAQs on Papovavirus: Structure, Types, and Importance
1. What is a papovavirus?
A papovavirus refers to a member of a formerly recognized virus family, known for being small, non-enveloped DNA viruses. The name 'Papova' is a historical acronym from the first viruses in this group: Papilloma virus, Polyoma virus, and Simian Vacuolating agent (SV40). Although the term is still used informally, modern virology has officially split this group into two distinct families: Papillomaviridae and Polyomaviridae, based on genetic differences.
2. What are the main examples of viruses once classified as papovaviruses?
The viruses originally grouped as papovaviruses are now classified into two main families. Key examples include:
- Papillomaviridae: The most famous member is the Human Papillomavirus (HPV), which causes warts and is a leading cause of cervical cancer.
- Polyomaviridae: This family includes viruses like the BK virus and JC virus, which typically cause significant disease only in immunocompromised individuals. Simian Virus 40 (SV40), originally found in monkeys, is another well-studied example.
3. What are the primary diseases associated with the papovavirus group?
These viruses are responsible for a wide spectrum of diseases, from benign growths to life-threatening conditions. Notable diseases include:
- Benign skin growths like common warts and plantar warts.
- Anogenital warts and laryngeal papillomas (growths in the throat).
- Progressive Multifocal Leukoencephalopathy (PML), a rare brain disease caused by the JC virus in individuals with weakened immune systems.
- Certain high-risk strains of HPV are oncogenic, meaning they can cause cancers, most notably cervical cancer, as well as cancers of the anus, throat, and penis.
4. What is the basic structure of a virus from the papovavirus group?
Viruses from the former papovavirus group are characterised by a simple yet robust structure. They are non-enveloped, meaning they lack an outer fatty membrane. Their genetic material consists of a single molecule of circular, double-stranded DNA. This DNA is protected within a protein shell known as a capsid, which has a distinct icosahedral (20-sided) shape.
5. Why is the term 'Papovavirus' no longer used in formal scientific classification?
The term 'Papovavirus' is considered obsolete in official virology because advanced genetic analysis revealed major differences between its two subgroups. While they look similar, their genome size, genetic organisation, and replication methods are quite distinct. To reflect their correct evolutionary relationship, the International Committee on Taxonomy of Viruses (ICTV) split the single family into two: Papillomaviridae (like HPV) and Polyomaviridae (like BK and JC viruses).
6. How do certain papovaviruses like high-risk HPV cause cancer?
High-risk types of HPV are considered oncogenic viruses. They cause cancer by inserting their viral DNA into a host cell's chromosome. This integrated DNA produces viral proteins, specifically E6 and E7. These oncoproteins target and inactivate the host cell's own tumour suppressor proteins (like p53 and pRb), which normally control cell division. By disabling these cellular 'brakes', the virus leads to uncontrolled cell growth, which can ultimately result in cancer.
7. Are Papovavirus and Parvovirus related?
No, Papovavirus and Parvovirus are entirely different and unrelated groups of viruses, a common point of confusion for students. The main differences lie in their genetic material and the diseases they cause. Papovaviruses have double-stranded DNA (dsDNA) and cause warts and sometimes cancer. In contrast, Parvoviruses have single-stranded DNA (ssDNA) and are known for causing diseases like 'fifth disease' in humans and severe illness in canines.
