Reovirus (Respiratory Enteric Orphan Virus) is a common virus that most people (70% to 100%) have been exposed to in their lifetime. The virus is considered asymptomatic, meaning that there are no particular symptoms associated with it. Unlike other viruses that continue to reside in the body after infection, the body will eliminate reovirus within two weeks. Because it is considered such a "safe" virus, it has been used for decades by research institutions and individuals studying viral replication structure. In 1998, graduate students working in a laboratory at the University of Calgary discovered that this particular virus seemed to be able to replicate itself in cancer cells that have what is called an activated Ras pathway, one of the most common family of genetic defects leading to cancer. Up to two-thirds of all human cancers, including many brain cancers, are Ras-activated, and are therefore a target for reovirus therapy.
Here's how it works: Viruses on their own cannot replicate. They need to borrow a host cell's manufacturing equipment. A virus particle will enter a cell, borrow the manufacturing equipment, and replicate itself within the cell until the cell dies, or the body's natural defenses kill the virus particles. If the virus is successful in killing the cell, the progeny virus are then free to infect and kill surrounding cells.
When the reovirus enters a normal cell and attempts to borrow the cell's manufacturing equipment to replicate itself, an anti-viral protein called PKR is able to quickly neutralize the virus. In a Ras-activated cancer cell, however, this anti-viral response is turned off. The reovirus is able to replicate itself within the cancer cell, resulting in that cancer cell's death. The cycle of infection, replication and cell death will be repeated until there are no longer any cancer cells available.
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