How do oncogenes cause cancer? What are proto oncogenes?
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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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