Tumor suppressors function in many key cellular processes including the regulation of transcription, DNA repair and cell:cell communication. The loss of function of these genes leads to abnormal cellular behavior.
Continuing with the analogy from the previous page, tumor suppressors can be likened to the brake system in a car. If you think of each copy of any tumor suppressorgene as contributing some 'braking power' to the cell, then the analogy is reasonably good. When both copies of a tumor suppressor gene are functioning (represented below by the highlighed genes and stop signs) the cell can stop dividing (the car can stop moving).
A single defective tumor suppressor will still leave the cell with one functioning copy. It would be like stopping a car with only the rear or front brakes instead of both. It may not work quite as well, but it still works! The cells with a single defective version of a tumor suppressor can still control their cell division. When the second copy in the cell is lost, the cell loses the ability to prevent division.
A very good example of this principle will be discussed in the section on the retinoblastoma (Rb) tumor suppressor in the following pages.
All cancers demonstrate alterations in one or more tumor suppressors and oncogenes. In normal cells, these two groups of proteins work together to regulate cell division but in cancer cells the controls are no longer functioning properly.
Because these genes are so important to the development of cancer, the next few pages will examine some specific tumor suppressors and the cancers with which they are associated. The number of genes involved in these processes is increasing almost daily but the ones presented here are some of the best studied to date and should give a good introduction to some of the cellular functions that are disturbed in cancer.