| MicrobiologyBytes: Virology: TTV | Updated: January 29, 2008 | Search |
In 1997, a novel DNA virus was identified in serum of a Japanese patient (TT) with post-transfusion hepatitis. The virus had little sequence similarity to any known virus, thus it was not clear to which family it belonged, and it is likely that it represents a new family. These viruses were initially known as TTV but are now called Torque teno virus. They are currently classified as a freestanding virus genus (not yet assigned to any family of viruses), the Anelloviruses.
No suitable cell systems which support the growth of these viruses have been discovered, so much of the biology of this virus is still obscure. These viruses are prevalent in non-human primates and human TTV isolates can cross-infect chimpanzees. TTV-like sequences have also been detected in chickens, pigs, cows and sheep. In humans, TTV can be transmitted by mother-to-child infection. However, within a year after birth, the prevalence reaches the same level for children born to both TTV-positive and TTV-negative mothers even without breast-feeding.
These particles are dense and 40nm in diameter.
The TTV genome contains two large overlapping open reading frames (ORF-1 and ORF-2) encoding 770 and 202 amino acids and several small ORFs (22-105 amino acids). The non-coding region surrounding a short 113 nt GC-rich stretch and occupying approximately one-third of the genome is considered likely to contain the putative replication origin. Three species of mRNAs are expressed by TTV, 3.0 and 1.2 and 1.0 kb. A protein translated from the 3.0 kb mRNA is considered to be the major capsid protein as well as replicase. The nature of the proteins translated by the other two mRNAs are still unknown. Individual genome sequences vary by up to 40%.
Since its discovery, it has become clear that TTV infection is present worldwide among blood donors and is common in patients with liver disease, including cryptogenic cirrhosis and fulminant hepatic failure. Alternatively, TTV appears to be present in up to 70% of healthy people in the Gambia and Ecuador, and around 10% of blood donors in the UK and USA. In some studies, TTV DNA has been found more frequently in patients with liver cirrhosis and hepatocellular carcinoma than in those with chronic hepatitis. However, virus DNA is not integrated in tumour cells, which may suggest that the virus is a passenger rather that causative of the tumour. Further studies are required to determine the role of TTV in the pathogenicity of acute and/or chronic liver disease. Therefore, the significance of TTV infection in liver disease is, at present, analagous to that of HGV. Although it has been suggested that TTV may be associated with various syndromes, the possible involvement of these viruses in human disease remains obscure. |
Torque teno virus (TTV): current status. Rev Med Virol. 2007 17: 45-57.
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