polyomavirus

polyomavirus. Polyomaviruses are a family of small, non-enveloped DNA viruses with a circular double-stranded genome that are known to infect a variety of mammals and birds. The family gained its name from the first discovered members, which were noted for their ability to cause multiple tumors in laboratory mice; the prefix "poly-" means many, and "-oma" refers to tumors. While many infections are asymptomatic, certain polyomaviruses are significant human pathogens, particularly in immunocompromised individuals, where they can cause severe diseases affecting the kidney, brain, and skin.

Classification and structure

The family Polyomaviridae is divided into four genera: Alphapolyomavirus, Betapolyomavirus, Gammapolyomavirus, and Deltapolyomavirus, based on genetic sequence analysis and host species. The International Committee on Taxonomy of Viruses oversees this classification. The virion is approximately 40-50 nanometers in diameter and exhibits a T=7d icosahedral symmetry. Its capsid is composed of 72 pentameric capsomers made of the major structural protein VP1, with the minor proteins VP2 and VP3 located internally. The viral genome is a circular, double-stranded DNA molecule of roughly 5,000 base pairs, which is associated with cellular histone proteins H2A, H2B, H3, and H4, forming a minichromosome structure that resembles host chromatin.

Replication cycle

The replication cycle is dependent on the host cell's machinery and begins with attachment to specific sialic acid receptors on the cell surface. For the well-studied SV40, entry occurs via caveolae-mediated endocytosis. The viral genome is transported to the cell nucleus, where early gene transcription is initiated by host RNA polymerase II. The early region encodes the large T antigen and small T antigen, which are crucial for driving the cell into S phase and creating an environment conducive to viral DNA replication. The large T antigen binds to the viral origin of replication and recruits host DNA polymerase for genome amplification. Late gene expression, producing the structural proteins, follows DNA replication. Assembly of new virions occurs in the nucleus, and progeny viruses are released upon cell lysis.

Associated diseases

In immunocompetent hosts, infection is typically lifelong and asymptomatic. However, in the context of immunosuppression, such as in organ transplant recipients, individuals with HIV/AIDS, or those receiving chemotherapy, reactivation can lead to serious illness. The BK virus is a major cause of polyomavirus-associated nephropathy, which can lead to kidney transplant failure. The JC virus is the etiological agent of progressive multifocal leukoencephalopathy, a severe and often fatal demyelinating disease of the central nervous system. The Merkel cell polyomavirus is associated with Merkel cell carcinoma, an aggressive skin cancer. Other members like Trichodysplasia spinulosa-associated polyomavirus cause trichodysplasia spinulosa, a rare skin condition.

Epidemiology and transmission

Primary infection with human polyomaviruses is widespread and usually occurs in childhood. Serological studies indicate that a majority of the adult population has been exposed to viruses like BK virus and JC virus. Transmission is believed to occur via the respiratory tract or oral route, and possibly through contact with contaminated urine. The viruses establish latent, persistent infections primarily in the kidney and lymphoid tissue. Reactivation and viral shedding in urine is common, especially during pregnancy or immunosuppression, facilitating further transmission. The discovery of new human polyomaviruses, such as New Jersey polyomavirus and Lyon IARC polyomavirus, continues to expand understanding of their epidemiology.

Prevention and treatment

There are no licensed vaccines for human polyomaviruses. Prevention in clinical settings focuses on managing immunosuppression. For kidney transplant patients, reducing doses of calcineurin inhibitors like tacrolimus or switching to mTOR inhibitors like sirolimus is a common strategy to control BK virus replication. The antiviral drug cidofovir and its lipid conjugate brincidofovir have been used off-label with limited success, often accompanied by significant nephrotoxicity. For progressive multifocal leukoencephalopathy, restoration of immune function, such as through antiretroviral therapy in HIV patients or plasma exchange in those on natalizumab, is the primary therapeutic approach. Intravenous immunoglobulin and the antidepressant mirtazapine have been attempted with inconsistent results.

Category:DNA viruses Category:Viral diseases