There is a balance that is carefully maintained between growth and growth restriction genes. The genes that code for proteins whose job is to stop replication are called tumor suppressor genes. If the function of these genes and therefore the protein are altered or lost, tumors can form. A fitting analogy is the brake and gas pedals in a car. The tumor suppressor genes are like the brake pedal for cellular replication. If the brake is deactivated, the car will not stop. This is also true for tumor suppressor genes, once deactivated the signal to tell the cell to stop replicating is lost. This can lead to tumor formation, as the cell will continue to divide unchecked.
There are numerous mutations that can cause a tumor suppressor to be deactivated. It is relatively easy to change the code of a gene in a way that will cause the protein to change shape and therefore lose function. This is termed loss of function. It is much harder to change the genetic code such that the protein will gain a function. The cell will still respond to the stop signals from a tumor suppressor if only one copy of the gene is mutated. However, when both copies have been mutated such that the protein is no longer functional, tumorigenesis can occur.
Tumor suppressor genes are often indicated by a lower-case “p” in front of the gene name. Well known tumor suppressors include p53, pRb, and pVHL.
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