Human demyelinating disease and the polyomavirus JCV

Many human neurological diseases involve demyelination of the central and/or peripheral nervous systems. These include the hereditary leukodystrophies - which have a genetic basis; multiple sclerosis ( MS) - where the underlying cause of demyelination remains unknown; and progressive multifocal leukoencephalopathy (PML) - where the etiology is well-established as being viral. The human neurotropic polyomavirus - JC virus ( JCV) - is the etiologic agent of PML, a fatal demyelinating disease of the central nervous system that occurs mainly in immunosuppressed patients, especially those with HIV/AIDS. JCV belongs to the polyomavirus family of tumor viruses that are characterized by non-enveloped icosahedral capsids containing small, circular, double-stranded DNA genomes. Serological studies have shown that JCV is widespread throughout the human population, but infections are usually restricted by the immune system, particularly cell-mediated immunity, causing the virus to enter a latent phase. An important corollary of this is that situations of severe immunosuppression may permit JCV to replicate and are thus a risk factor for PML.