Contributed by Guest Blogger: Maggie Rasnake ’11
When a virus is not known to be associated with any disease, it is called an orphan virus. Human anelloviruses, like torque teno virus (TTV) and torque teno mini virus (TTMV), are orphan viruses because they do not have known symptoms. TTV was first discovered in a patient with liver disease. However, no definite link between liver disease and the virus has been shown. Anelloviruses are genetically similar to an avian virus called chicken anemia virus (or CAV). CAV has had a large, economic impact on the poultry industry. Unlike TTV, it is known to have symptoms, but it can have a long lag-time between infection and the development of disease.
Both CAV and TTV have similar, single-stranded, circular DNA and have highly variable sections of the genome. It is believed that they evolved from a plant virus. Researchers realized that much of what they learned about CAV could be applied to TTV and vice versa. For example, when they realized that TTV had more than just three proteins encoded by its three open reading frames, they found that the same was true for CAV. When CAV was found to replicate in the bone marrow, it was discovered that a great deal of TTV replication occurs in the bone marrow as well.
CAV is associated with developmental problems for fetuses and young chickens. The virus is less understood in adult chickens, but when chickens have CAV, they are much more likely to suffer from other diseases and have higher mortality rates. Similarly, in infected humans, the viral load of TTV is higher when the individual has other infections. In addition to liver disease, levels of TTV tend to be higher in those with respiratory infections, kidney disease, HPV, and certain cancers, among others. TTV may enhance the pathogenic effects of other pathogens. High levels of TTV are found in individuals with HIV, but it is not known if TTV simply reflects the immune system’s status or if it contributes to the damage. An effective medium for studying TTV has not yet been established. The authors suggest that the virus might be better studied in a novel primate cell line transformed by an oncogenic virus.
