Colon Cancer, The Genetic Factor
Medical Author: Dennis Lee, MD
Medical Editor: Jay W. Marks, MD, and
Barbara
K. Hecht, Ph.D.
Introduction
Colon
cancer is the fourth most common cancer in the
United States. In the US an estimated 130,000 men and women will develop colon
cancer and more than 50,000 will die from it each year. The life-time risk for
individuals to develop colon cancer is approximately 6%, but the risk increases
to 18% among individuals who have a first degree relative (parent, sibling or child) with
colon cancer.
What causes colon cancer?
Mutant genes cause colon cancer. The human body is composed of trillions of
cells. Inside each cell are two sets of 23 chromosomes, one set
from each parent. Each chromosome contains long strands of DNA. The DNA encodes thousands
of genes. The genes carry the genetic information that is passed down from both
parents. Different genes are responsible for different structures and functions
in the body. For example, some genes influence eye color, hair color, height,
and other physical characteristics. Other genes regulate cell growth and
division. Still other genes prevent cells from invading neighboring tissues or
spreading to distant organs.
When genes that normally control cell multiplication and
growth mutate (change), the cells may multiply and grow without restraint. When
cells grow without restraint inside the colon, a colon polyp develops. If
additional genetic changes occur in that polyp, the polyp can turn cancerous and
invade adjacent tissue and spread to distant sites. The
sequence of events by which polyps form, become cancerous, invade nearby tissue
and spread can and often does take years.
How does one acquire the mutations that cause colon cancer?
Mutations
(changes within individual genes) can be inherited from either or both parents
or can be acquired any time after conception. Environmental factors such as
radiation, chemicals, oxidants, and viruses may trigger these types of
mutations.
Inherited mutations will end up in every cell in the
body, whereas acquired mutations will be present only in the original cell in
which it occurred and in all the cells descended from that original cell. For
example, the mutation may be restricted to the cells of the polyp or cancer.
Generally, acquired mutations tend to cause only one or
a few colon polyps that can
be removed by colonoscopy. (For further information, please see the article on
colonoscopy.) Removing these
polyps effectively prevents the progression from colon polyp to colon cancer.
On the other hand, inherited gene mutations present
in every cell have a tendency to cause numerous (sometimes thousands) of colon
polyps. These polyps may be too numerous or too large to be removed by
colonoscopy. In addition, the progression from polyp to invasive cancer may be
so rapid that even frequent colonoscopy is not adequate to remove the polyps and
prevent colon cancer.
Finally, some cancers may develop directly from cells of
the colon lining, without the development of polyps that can be recognized and
removed. For this reason, surgical removal of the colon may be necessary to
prevent colon cancer in patients with inherited forms of colon cancer.
What are hereditary colon cancer syndromes?
Hereditary colon cancer syndromes
are caused by specific inherited mutations that are sufficient in themselves to
cause colon polyps, colon cancers, and non-colonic cancers. Hereditary colon
cancer syndrome can affect multiple members of a family. Approximately 5% of all
colon cancers in the US are due to hereditary colon cancer syndromes. Patients
who have inherited one of these syndromes have an extremely high risk for
developing colon cancer, approaching 90%-100%. Fortunately, blood tests are now available to test for these
hereditary colon cancer syndromes, once a syndrome has been suspected within a
family.
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