| Colon Cancer |
| Considering Genetic Testing
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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
order to diagnose the two most common hereditary colon
cancer syndromes — hereditary nonpolyposis colorectal cancer (HNPCC), and familial adenomatous polyposis (FAP) — doctors
mainly depend on the results of your physical exam and
information about your family medical history. In general,
they do not rely on genetic testing to make an initial diagnosis. However, under some circumstances genetic testing is also an appropriate tool for diagnosis. (For recent news about genetic testing in people at risk for colon cancer, see Related News below.)
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When
Is Testing Appropriate?
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| If
you are uncertain about your level of risk for inherited
colon cancer, you should use Genetic Health's TreeBuilder
tool which will help you assess the risk in your
family. |
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Genetic
testing for colon cancer is often appropriate if you have
a family medical history that's strongly suggestive of
familial or hereditary colon cancer. The results of a
genetic test can indicate which family members have inherited
a mutated gene —
and
thus an increased risk of cancer. This is important information,
since it will affect your screening and prevention options,
and indicate whether your children or siblings are at
risk.
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Who Should Get A Genetic Test?
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If
your family medical history shows that you are at increased
risk for hereditary colon cancer, you may want to talk
to your doctor or genetic counselor about genetic testing
for a specific colon cancer-related mutation.
If you are a healthy person with a strong family history
of cancer, you may be surprised to find that your health
care provider does not want to perform a genetic test
on you. Instead, he or she will recommend that one of
your family members who actually has cancer or multiple
polyps be tested. This
person is often referred to as an affected family member.
If
an affected family member tests positive for a known
mutation, then other people in the family can be tested
for that same mutation. If no mutation can be found
in an affected person, it means that the mutation responsible
for that family's high incidence of colon cancer has
not yet been discovered and it would be pointless for
other family members to undergo genetic testing.
The likelihood that an affected family member will have
a mutation is much higher than it is for an unaffected
family member. However, if no affected family members
are available (or willing) to take a genetic test, sometimes
a doctor or genetic counselor will agree to test an
unaffected family member. If a mutation is found in
an unaffected family member, then you have useful medical
information. Unfortunately, if no mutation is found,
this doesn't definitively rule out the possibility that
this individual has inherited a genetic syndrome. The
unaffected person may have not inherited a mutation
that causes colon cancer syndromes, but they also could
still have inherited a mutation that has not yet been
discovered and thus does not show up in genetic tests.
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Getting
a Genetic Test
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A
medical geneticist or genetic counselor can order a genetic
test as part of a consultation. In addition, your regular
doctor can also order such a test if he or she considers
it appropriate. Whichever route you take, it's important
that you obtain your genetic test through a health care
provider who is knowledgeable about inherited colon cancer.
In this way, you ensure that you get the correct
genetic test and that the results — as well as the
issues involved — are clearly explained.
The genetic test itself is a simple process that usually
involves drawing a small amount of blood from your arm
for analysis by a testing lab. (The test is usually performed
on DNA taken from white blood cells.) The analysis, however,
can take weeks or even months to complete because the
DNA must first be isolated and then time-consuming genetic
tests performed. Then the tests have to be repeated to
confirm any positive result. Both the time it takes to
receive your results and the cost of testing will vary,
depending on the type of test being performed and the
particular lab running the test. |
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Genetic
Tests Are Often Misinterpreted by Doctors From
A Wide Variety of Specialties
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A 1997 study looked at use
of genetic testing for FAP and found that:
- Less
than 20 percent of patients received genetic
counseling or signed written informed consent
before undergoing genetic testing.
- Doctors
misinterpreted test results in more than 30
percent of cases, providing patients with false
assurance that they did not have FAP when in
fact their results were inconclusive.
Patients
undergoing genetic testing need to be well informed
about the tests and need to find health care providers
with training and experience in genetic testing
and test interpretation.
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After
Testing, What Next?
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Once a harmful mutation has been identified in a family,
other family members can be tested for the same mutation.
This type of subsequent testing is usually faster and
less expensive than the initial test because the lab will
be looking for one particular mutation rather than all
possible mutations in colon cancer-related genes. Such
tests are also highly accurate and reliable. By undergoing
this type of testing, families can sort out who has and
who hasn't inherited the mutation. Those family members
who have the mutation will need to undergo more intensive
screening and prevention efforts. Those who are free of
the mutation will be able to follow the simpler cancer
screening and prevention recommendations for the general
public.
If no mutation is found in an affected family member,
there's usually no reason to test other family members.
In effect, the genetic cause of this particular family's
cancer syndrome has not been determined. However, because
the disease evidently "runs in the family" —
even though the cause has not be identified — all
family members should act as though they are at high risk
for cancer and follow the screening guidelines recommended
for people with inherited genetic risk for cancer.
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Informed
Consent
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remember, you have a right to be fully informed about
the risks, benefits, and limitations of any test you are
considering before you take it. Among other things, you
need to understand why this test is appropriate for you,
how your test results can predict your risk of cancer
or other illness, and how these results will impact your
screening and prevention options. You also need to understand
your chances of getting an uninformative test result,
as well as the anxiety or distress that can be caused
by such tests. Finally, you must be aware of the possible
loss of confidentiality and discrimination that can result
from such tests.
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 top
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More
on Informed Consent (Coming Soon)
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A
Genetic Testing Case Study
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To
better understand the specifics of genetic testing, let's
consider the case of 35-year-old Mary, whose mother died
of colon cancer at age 50. She also had a cousin who died
of colon cancer at age 45, and an uncle diagnosed with
the disease a year ago at age 52 who is currently undergoing
treatment.
Under some circumstances, Mary's doctor might test her
for alterations in the genes associated with HNPCC. However,
if the results are negative — no mutations are found
— the test result yields more questions than answers.
Because no mutation has been identified as the culprit
for the cancers in Mary's family, no one can be certain
whether there is a detectable mutation in the family that
Mary simply did not inherit, or whether there is no detectable
mutation in the family at all. The doctor can't do much
to reassure Mary because he has no information to explain
the clustering of her family's early-onset colon cancer.
Something is causing it, although it is not necessarily
a mutation of this particular gene. Thus Mary may still
be at increased risk for the disease, just not for this
reason.
If, on the other hand, Mary's doctor had recommended that
testing begin with her uncle (who has cancer and is thus
an affected family member), the interpretation of the
test results would be quite different. If tests showed
that Mary's uncle had a mutation known to cause HNPCC,
Mary could be tested for the same mutation — a much
less expensive process than being tested for all known
mutations. If Mary's test results indicated that she did
not have the same mutation as her uncle, she could
feel greatly reassured: her uncle's test results reveal
the cause of the cancer cluster in her family — an
identifiable mutation — and Mary did not inherit
it. This means that Mary is no more likely to develop
colon cancer than members of the general population, despite
her worrisome family history.
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References
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Petersen,
G. et al. (1999). Genetic testing and counseling for
hereditary forms of colorectal cancer. Cancer
86: 1720-30.
Giardello, F. et al. (1997). The use and interpretation
of commercial APC gene testing for familial adenomatous
polyposis. N Engl J Med 336: 823-7.
Geller, G. et al. (1997). Genetic testing for susceptibility
to adult-onset cancer: the process and content of informed
consent. JAMA 277:1467-1474.
Terdiman, J. et al. (1999). Genetic testing in hereditary
colorectal cancer: Indications and procedures. The
American Journal of Gastroenterology 94(9): 2344-2356.
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