How do oncogenes cause cancer? What are proto oncogenes?

What are oncogenes?

Oncogenes, which are genes that can cause cancer, have become a hot topic in cancer research. The word comes from the Greek term onco, meaning tumor. Most cancer researchers believe that somehow, usually over a period of years, cancer causing agents (carcinogens) repeatedly brought into the body, finally damage a critical piece of a cell's genetic code. The damage causes the cell to send out abnormal messages related to some aspect of cell growth. As new cells spring from old, the misled cell leads an onslaught of others that result in runaway growth. The altered ones oncogenes take charge. More than twenty cancer genes have been identified in experiments in animals or in normal cells grown in the laboratory.

How do oncogenes work? 

The oncogenes start dominating the way the cells behave. They disrupt the usual schedule and direct the cell to continue to grow. Most researchers believe that at least two cancer genes must be created, by random error, before the process starts. Although the role of each cancer gene is not yet understood, scientists believe that the cancerous cell growth is set off through a series of steps within the cell. Some cancer genes apparently instruct the cell to overproduce a growth factor protein or mistakenly produce an abnormal growth factor. Others may tell the cell to ignore signals to stop growing, perhaps by leaving growth factor receptor switches on at various points along the cell's surface.

Where have oncogenes been found? 

Scientists have found versions of oncogenes in several kinds of human cancer, including those of the breast, lung, bladder, and bowel. In addition, DNA sequences nearly identical to oncogenes have been discovered in normal tissue cells throughout the animal kingdom, including those of man. Researchers think that such "proto-oncogenes" have existed throughout evolution and play a useful role in normal cell division. What the researchers are investigating is how these seemingly harmless genes become altered and turned into cancer genes. They have found that by replacing the genetic "on-off" switch of one gene, called c-myc, they are able to control when and in what tissues it causes tumors in mice. This is a critical breakthrough in learning precisely how this gene and perhaps other cancer genes function. The gene c-myc plays a role in the human cancer known as Burkitt's lymphoma. Other researchers working with another strain of oncogene, the ras family of oncogenes, have found a single changed amino acid in the protein product as the site for an altered gene product which they feel is somehow critical in the development of cancer.

Oncogenes and proto oncogenes

Can viruses cause cancer? 

Researchers believe there is some relationship between viruses and cancers of the liver and cervix, Burkitt's lymphoma, nasopharyngeal cancer, and adult T-cell leukemia. However, although it is believed that these cancers may be stimulated by viruses, there is no evidence that they spread like typical viruses. Some other factor appears to be necessary to cause the disease to spread.


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