Circovirus Molecular Biology
 

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 Circoviridae Classification

The family's name, "circo," is derived from the Greek root "gyro," which means ring or cirular.  This refers to the characteristic circular shape of the circovirus genome.  The family contains the circovirus genus, the gyrovirus genus, and the anellovirus genus.  The type species of the circovirus genus is porcine circovirus, type 1.  Genus classification is made based on phylogenetic differences in the highly conserved CP genomic coding sequence, with viral species with complete genomic sequences with less than 75% similarity, and CP sequences of less than 70% similarity, classified in a different genus. 


Virion Morphology

Viruses in the circoviridae family are small, non-enveloped viruses with icosahedral nucleocapsids.  The nucleocapsid displays icosahedral symmetry of T = 13, and is 17-22 nm in diameter.  It consists of 32 capsomers. 

  

The image above shows a diagram of the T = 13 symmetry of the viral nucleocapsid, as well as circoviridae's scanning electron micrograph appearance.   

 

Genomic Characteristics

The genome of circoviruses is non-segmented and single-stranded.  Of particular note is the fact that the single-strand of genomic DNA is ambisense, and is present in the virion in the shape of a covalently-closed circle.  The genomic DNA is made up of 1800-2000 nucleotides. 

The human circoviruses show greatest similarity to chicken anemia virus (CAV), another circovirus, and seem to have a worldwide distribution in human populations.  TTV and TLMV show great similarity in their genomic organization.  Their genomic DNA each encodes at least 2 overlapping open reading frames and at least 1 non-coding region.  


Host Range

While much of the circoviridae replication strategy is still unknown, members of this family are known to infect a variety of hosts.  Circoviruses are known to infect pigs and other porcine species, geese, canaries, ducks, finches, chickens and gulls.  The first human circovirus was discovered by Nishizawa et al. in 1997.  The human circoviruses include Torque Teno virus (TTV) and Torque Teno-like mini virus (TTMV), both members of the anellovirus genus. 


Replication

While the replication of human circoviruses remains largely unknown, TTV and TLMV have been found to be hepatotropic.  They have also been isolated in leukocytes.  Despite theories that infection with human circoviruses cause liver disease, no definitive links have identified TTV or TLMV as an etiologic agent of liver cirrhosis or disease.