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Untitled Document
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| Colon
Cancer |
| How Is Colon Cancer Inherited? |
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By
Miriam Komaromy,
MD
Reviewed
by Peggy Conrad,
MS, CGC and Jonathan
Terdiman, MD
Last updated August 4, 2000
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In recent years, scientists have begun to uncover the basis for many types of hereditary cancer. By studying patterns in a family's medical history, researchers have determined which cancers may be inherited. In addition, researchers are now able to identify alterations, or mutations, in specific genes that give rise to certain types of cancer, colon cancer among them.
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Sporadic
Colon Cancer
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we reviewed the family medical histories of 100 people
diagnosed with colon cancer, 75 of them would show little
or no evidence of other family members with the disease.
Because hereditary factors do not appear to have contributed
to these people's risk of cancer, those individuals are
said to have sporadic colon cancer.
Although
nothing in these people's family history puts them at
increased risk for developing colon cancer, we can learn
something about the risk of developing colon cancer
by looking at factors such as age, weight, diet, smoking
history, and exercise. No genetic test currently exists
that can identify those who are at increased risk for
this kind of colon cancer.
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Familial
Colon Cancer
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A
Real Life Example
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| Jake,
who is 55 years old, goes to see his doctor to find
out if he should be tested for colon cancer. He's
worried because his brother was just diagnosed with
colon cancer at the age of 60, and an uncle also
had colon cancer, which was diagnosed at age 70.
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In
continuing to look at those 100 family medical histories,
we would also probably find roughly 20 people who report
that some family members had a history of colon cancer. However, the cases would not necessarily have occurred at an unusually early age, nor would they have necessarily been found on only one side of the family. These people are said to have familial colon cancer. They appear to be somewhat more likely to develop colon cancer than people with no family history of the disease.
Gene mutations that account for familial colon cancer
are only just now being discovered. For example, researchers
have pinpointed one mutation that accounts for some
familial colon cancers among people of Eastern European
Jewish (Ashkenazi) descent. Although DNA testing exists
for this mutation, it is not yet widely used.
Also note that although other genes accounting for familial
colon cancer are being reported in the scientific literature,
there are not yet any commercially available genetic
tests for other types of familial colon cancer. In fact,
most cases of familial colon cancer cannot yet be linked
to a specific abnormal gene.
There
is evidence to suggest that gene mutations involved
in familial colon cancer, although increasing the risk
of the disease, do not automatically carry with them
the same very high likelihood of actually getting it
as the gene mutations that cause hereditary colon cancer
(see below.)
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Hereditary
Colon Cancer
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A
Real Life Example
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| Sally,
a 35-year-old woman, is worried because she has
been having some rectal bleeding. She goes to her
doctor, who asks about her family history. Sally
tells the doctor that her mother died at age 40
of colon cancer. In addition, she reports that her
mother had two brothers, one of whom also died of
colon cancer, at age 52. (The other is alive and
well.) Their father (Sally's grandfather) also died
of colon cancer, at age 50. |
Next,
if we continued with our review of the 100 patients,
we'd find that between 5 and 10 of them had a strong
family history of colon cancer on one side of
their family. In other words, many of their close
relatives on either their mother's or father's
side of the family would have developed colon cancer
at an early age, and many of those relatives would have
died from the disease. People who develop colon cancer
and have this type of family background are said to
have hereditary colon cancer. They have inherited
a mutated
gene from one of their parents, and that mutation has
made it much more likely that they will develop colon
cancer at an early age.
In many cases, a particular gene mutation that has already
been identified as responsible for colon cancer can
be detected using a genetic test.
Individuals who test positive for one of these known
gene mutations are very likely to get colon cancer.
There are two primary types of hereditary colon cancer.
Each has different hallmarks:
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Familial Adenomatous Polyposis (FAP) |
- Colon
cancer develops at an extremely young age (20 to 45)
- Virtually
one hundred percent chance of developing colon cancer
by age 45 (unless the colon has been surgically removed)
- Hundreds
or even thousands of polyps in the colon
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Hereditary Nonpolyposis Colorectal Cancer
(HNPCC) |
- Increased
lifetime risk of developing colon cancer (as high
as 80 percent, compared with 5 percent in the general
population)
- Earlier
cancer development compared to sporadic cancers, though
not as early as in FAP. (In HNPCC patients, cancer
often occurs around age 44, compared with age 64 in
sporadic colon cancer, and age 39 in FAP)
- Increased
risk for cancer of the uterus as well as several other
types of cancer
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References
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Byers,
T. et al. (1997). American Cancer Society guidelines
for screening and surveillance for early detection of
colorectal polyps and cancer: update 1997. CA Cancer
J Clin 47: 154-160.
Petersen, G. et al. (1999). Genetic testing and counseling
for hereditary forms of colorectal cancer. Cancer
86: 1720-30.
Burt, R. et al. (1995). Genetics of colon cancer: Impact
of inheritance on colon cancer risk. Ann Rev Med
46: 371-79.
Kinzler, K. and Vogelstein, B. (1996). Lessons from
hereditary colorectal cancer. Cell 87: 159-170.
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