Colon Cancer and Genetic Testing
How does someone know if he or she may be a member of a family with FAP and may need genetic testing?
An individual is likely to belong to a family with FAP if he or she has more
than 100 adenomatous colon polyps or is a first-degree relative (parent,
sibling, or child) of a person who has more than 100 adenomatous colon polyps.
The number of polyps is less in some families, a condition referred to as
attenuated FAP. Therefore, individuals who have between 20 and 100 adenomatous
colon polyps or are first-degree relatives of individuals with 20 to100
adenomatous colon polyps also may belong to a family with FAP.
How is genetic testing for colon cancer risk done?
There has been much excitement during the past decade because of the
identification of abnormal or defective genes (mutations) associated with
in families where colon cancer is common. When a defective gene can be
identified, it is possible to examine other members of the family to see if they
also carry the defective gene. Those individuals who carry the defective gene
are at a very high risk (75%-100%) for developing colon cancer. The reason for
the excitement is that if an individual is found to have the defective gene, his
or her colon can be carefully monitored and then removed before the cancer occurs.
Only 5% of all colon cancers occur in families with a history of colon cancer
and identifiable genetic defects. Therefore, genetic testing as it exists today
is useful for only a minority of the about 130,000 people each year who are destined
to develop colon cancer. Nevertheless, genetic testing is important because the
risk is so extremely high among individuals who are found to have the genetic
defect. In addition, more defective genes are likely to be found during the next
few years, and this will make genetic testing valuable for an increasing number
of individuals who will develop colon cancer.
At present, there are two types of familial colon cancer in which defective
genes can be identified. One type of cancer is associated with a strong family
history of colon polyps. The other type of colon cancer is not associated with a
family history of colon polyps. The polyp-associated cancerous disease is called
familial adenomatous polyposis (FAP). (Adenomatous polyps are a type of polyp
that have the potential to become cancerous.) The nonpolyp-associated cancerous
disease is called hereditary nonpolyposis colorectal cancer (HNPCC).
Medically Reviewed by a Doctor on 9/11/2017
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