The history of retinoblastoma (RB)is particularly interesting for its place in cancer genetics. There are two main types of RB: Bilateral RB results in a tumor appearing, eventually, in both eyes. Unilateral RB shows as a tumor in only one eye.

People with RB tumors of either type are more susceptible to cancer than the general populace. Retinoblastoma is also a heritable trait, so children of people with RB also tend to have RB tumors. These characteristics lead a researcher to propose that, like many other forms of cancer, RB is the result of a genetic defect of some kind.

Unlike other genetically linked cancers, whose genes normally function as promoters of cell growth, the RB gene’s role is to inhibit growth. More precisely, RB regulates the timing of cell division and ensures that cells do not grow and divide until they have completed all of the preparative stages first. It’s rather like a building inspector who has to sign off that the plumbing and electrical work is up to code before the construction of a home can be completed.

Without a functioning RB gene, cells will have a tendency to divide inappropriately, which is the hallmark of cancer.

With this inhibitory function, RB was the first “anti-oncogene” described in science. Learning of the nature of retinoblastoma lead to the discovery of many other growth regulatory genes involved in cancer. Most notable of these is the gene for “p53″ [NOTE: The protein encoded by the RB gene is called "pRB", with the lowercase "p" designating a "protein". The protein called "p53" is a protein with a molecular weight of 53,000 Daltons.].

Do humans have anti-cancer genes to help us prevent cancer? Well, yes and no. The more famous p53 protein does prevent cells from inappropriately dividing, which blocks some forms of cancer. Its function also seems to monitor for the activity of certain viral infections. A cell that is infected with a virus will produce lots of viral DNA. The p53 protein causes cells with an excess of small DNA strands (like viral DNA) to commit suicide. So some anti-oncogenes have antiviral properties as one of their functions. Predictably, viruses have countermeasures that specifically inactivate the p53 protein’s surveillance functions.

This antiviral/antioncogenic dual role for p53 is being exploited by a radically new type of cancer treatment called “Onyx-015″.

This treatment is a genetically modified virus that is lacking the p53 countermeasures (a viral protein called E1A is modified so it does not mess with p53). This virus is unable to infect and grow in normal human cells because their p53 protein will prevent it. Cancer cells that lack p53 also lose this antiviral defense, and so are susceptible to the modified virus. Thus, a tumor that is infected with the mutant Onyx-015 virus will be killed by the virus, while surrounding normal cells will not be affected.

Ha! Quite a long Rathole!