Oncology is the study of cancer, and lysis describes the breakdown of cells. An oncolytic virus is one that can kill cancer cells. Although the field of oncolytic viral therapy is relatively new, as of 2011, studies do show that some viruses are capable of having some effect on some cancers. Genetically engineered viruses that can infect tumor cells but leave healthy cells unaffected are under study.
Viruses are tiny organisms, that require another organism's cell to replicate in. In fact, viruses are so simple, containing a strand or two of nucleic material, and perhaps a protective envelope, that some scientists do not regard them as living creatures. The simplicity of viruses is useful in that little energy expenditure is necessary to stay intact or make new viral particles, but it also means that the viruses need to enter other organism's cells to use their cell machinery to reproduce.
To do this, viruses devote some of the space on the relatively small strands of genetic material to gene products that help them to invade cells and hijack the machinery. When viruses are successful at getting inside the cell and forcing it to make copies of the virus, the new viral particles then get out into the environment by breaking the cell open. This process is known as lysis, which derives from the Greek word for loosening, which is luein.
Normally, viruses do not attack tumor cells instead of healthy cells. With the advent of molecular biology and genetic engineering in the later years of the 20th century, however, scientists figured out ways to make viruses target tumor cells preferentially. If the virus under study was able to destroy some tumor cells, it was called an oncolytic virus.
Cells in the body generally all contain a full complement of genes, and the cell reads the information present in these genes to produce necessary cell products. As the body contains many different types of cell, however, from nerve cells to skin cells, the products necessary for that cell is typically different from other types of cell. Tumor cells have a different profile to healthy cells around them, even if the original tumor cell was once the same as the neighboring cells. This is because the tumor cells replicate abnormally, and so need more building molecules than non-dividing cells.
It is this difference in profile of cells that scientists exploit to tweak a virus to target cancer cells specifically. For example, a virus that lacks a normal gene for a particular product, which it needs for replicating, cannot replicate inside a cell that does not produce that product. When the oncolytic virus gets into a tumor cell that does make this product as part of the cancer process, then the virus is able to take this cell product and replicate. The replication then leads to cell lysis, and the tumor can shrink as a result.
Examples of the natural virus species that have potential applications in cancer treatment include herpes simplex virus-1, adenovirus and reovirus. Typically, as they are able to infect humans naturally, scientists choose human pathogens to test for ability to kill human tumor cells. Sometimes the viruses have been engineered to remove genes that are not useful for therapeutic effects, or which may be dangerous to the patient. As of 2011, oncolytic viral therapy is not commonplace in cancer treatment, but clinical studies are under progress for a variety of strains. If found to be useful, an oncolytic virus product may be used in conjunction with other anticancer treatments like chemotherapy or radiation, or if it is very successful, it may be used alone.