Colon Polyps (cont.)
In this Article
Are there situations in which polyps carry a very high risk of
becoming cancerous?
Defective genes cause colon polyps and colon cancers. Individuals who have
inherited colon cancer causing gene defects are at high risk of developing colon
polyps and caners early in life. The human body is composed of trillions of
cells. Inside each cell are two sets of 23 chromosomes; one set is inherited
from each parent. Each chromosome contains long strands of DNA that are
comprised of thousands of genes strung together. The genes carry the genetic
information that is passed down from both parents. Different segments of the DNA
strands contain genes responsible for different structures and functions in the
body. For example, certain segments of DNA carry genes that determine one's eye
color, hair color, height, and other physical characteristics. Other DNA
segments of DNA carry genes that regulate the duplication and the rate of growth
of cells. Still other genes prevent cells from organs within the body from
invading neighboring organs or spreading to distant organs.
When chromosomes are damaged, genes become defective. When genes that normally
prevent cells from multiplying and growing are defective, the cells containing
the defective genes can multiply and grow without restraint. When cells grow
without restraint inside the colon, a colon polyp develops. When additional
genetic defects occur in that polyp, the polyp can turn cancerous and invade
adjacent tissue and /or spread to distant organs.
Genetic damage (damage to individual genes) can be inherited from either
parent or can be acquired after birth when normal genes in normal cells become
damaged by environmental factors such as radiation, chemicals, oxidants,
viruses, etc. Inherited genetic defects are present in every cell in the body,
whereas acquired defects are present only in the cells that are damaged and the
cells they produce, for example, in the cells of the polyp or cancer.
Generally, acquired genetic defects tend to cause only one or a few colon
polyps that can be removed by colonoscopy. (For further information, please see
the articles on colonoscopy.) Removing these polyps effectively prevents the
progression from colon polyp to colon cancer. On the other hand, inherited
genetic defects that are present in every cell have a tendency to cause numerous
(sometimes thousands) of colon polyps. These polyps may be too large or too
numerous 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 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 all of these reasons, among
patients with inherited genetic defects that cause colon cancer, surgical
removal of the colon is necessary to prevent colon cancer.
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.
Familial adenomatous polyposis (FAP) syndrome is an example of a hereditary
colon cancer syndrome, characterized by thousands of adenomatous polyps forming
in the colon, with colorectal cancer an inevitable consequence. This usually
occurs 10 to 15 years after the onset of polyps, which most often begins at
puberty. The average age of diagnosis of familial polyposis is 25 years of age,
with cancers developing at age 20 to 30. In this syndrome, polyps also can be
present in the stomach, duodenum and the terminal ileum. The basis of this
disease seems to be a genetic abnormality on chromosome number five.
A variant of the familial adenomatous polyposis syndrome is Gardner's
syndrome. These patients also develop thousands of adenomatous colonic polyps.
What distinguishes these two syndromes from one another is that Gardner's
syndrome is associated with extra- colonic manifestations, including bony tumors
of the mandible, skull, and long bones, as well as fatty, fibrous, and mixed
tumors, which may occur anywhere in the body.
Another familial polyposis syndrome is Turcot's syndrome, a variant of familial
adenomatous polyposis in which there are fewer colon adenomas.
The Lynch syndrome also is known as hereditary non-polyposis colorectal cancer
(HNPCC). In the Lynch syndrome individuals do not form multiple colon polyps as
other individuals with the other hereditary colon polyp syndromes. Nevertheless,
they are at high risk for developing colon cancer. The syndrome is inherited,
and because it is inherited in an autosomal dominant manner, cancer is very
common within families with this syndrome. Lynch syndrome has been broken down
into Lynch syndrome I, and Lynch syndrome II, with Lynch syndrome II involving
the same risk of colon cancer as Lynch I, but in addition, cancers outside the
colon as well, particularly in the uterus, ovaries, and breast. In both Lynch
syndromes, colon cancer usually develops at a young age (40-50 years) and more
often occurs on the right side of the colon where it cannot be found by
sigmoidoscopic examination. Surveillance for all family members should include
twice-yearly fecal occult blood testing and yearly colonoscopy, beginning at an
age 10 years younger than the age at which the youngest family member has
developed cancer or at age 20 if this information is not available.
Next: What about genetic counseling and testing? »
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