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Genetics of Breast Cancer
Can
breast cancer run in families?
Yes. This has been well established.
In some families, both the men and women are susceptible to
breast cancer. When breast cancer runs in a family, it can
run alone, or as part of a broader illness that includes other
cancers. For example, in some families, breast cancer and
ovarian cancer are both common. Some women will even have
both tumors. In such families, breast cancer is just one problem
in this broader illness. When breast cancer runs in families,
is the reason genes or environment? As in most human disease,
the answer is "both." Breast cancer has strong genetic
components as well as environmental components. So, it's a
matter of nature and nurture, not nature versus nurture.
What genes are involved in breast
cancer?
Researchers have discovered several genes contributing to
a woman's susceptibility to breast cancer. We divide them
into three groups:
- Strong
influence, rare occurrence --
Women
with certain rare genetic disorders have a high risk of
developing breast cancer. In other words, the genes for
these disorders have a strong influence on breast cancer
risk. All told, however, fewer than 1% of breast cancer
cases fall in this category.
- Strong
influence, medium occurrence -- This category
includes two known genes: the BRCA1 gene on chromosome 17
and the BRCA2 gene on chromosome 13 . Over 200 variations
(alleles) for BRCA1 have been discovered and more than 100
for BRCA2. Only some variants of these genes increase the
risk of breast cancer. A woman with one of the high-risk
variants has an 80% chance of developing breast cancer before
age 70. In certain populations, high-risk variants are common.
They are found in 2.5% of all Ashkenazi Jews, accounting
for over 25% of breast cancer diagnosed before age 40 in
that group. In the general population, however, high-risk
variants are not common. They are found in less than 0.5%
of the population and account for perhaps 5-10% of all breast
cancer. Scientists believe at least one more gene will be
discovered in this category.
- Weaker
influence, common occurrence -- Scientists
knows little about this group of genes. Because of their
weak effect, they are difficult to identify. Statistically,
they are responsible for most of the population's genetic
susceptibility to breast cancer, and are fertile targets
for research. The ATM gene on chromosome 11 is one candidate.
High-risk
variants of the BRCA1 and BRCA2 genes are not common in the
general population. But, if you have blood relatives with
breast cancer (male or female) or ovarian cancer, your chances
of having a high-risk variant are increased.
A blood
test can identify whether a person has a known high-risk
variant of the BRCA1 or BRCA2 gene. (Scientists believe more
high-risk variants will be discovered.) Whether or not to
be tested is a complicated decision. Talk to your physician
or a genetic counselor. Some university hospitals have specialized
clinics for members of cancer families.
What
environmental factors are involved in breast cancer?
Scientists
have found several environmental factors in breast cancer.
Some, like radiation, can clearly contribute to breast cancer,
but are rare in the population.
Sex hormones
and reproductive history are important environmental influences.
Women at higher risk include those whose menstrual periods
begin early in life (before age 12), have a late menopause
(age 55 or later), or don't carry a full-term pregnancy by
age 30. Most studies show that birth control pills raise the
risk of breast cancer slightly. Hormone replacement after
the menopause raises risk.
Other
environmental factors have also been examined, but the results
are uncertain. These factors include diet, body weight, alcohol
consumption, exposure to pesticides, use of birth control
pills, breast feeding, and abortion.
One
more point: The environmental factors mentioned
here may not be completely environmental. There are genetic
components to the age at which menses start and stop, as well
as for body weight and alcohol consumption. There are even
genes for sensitivity to radiation.
When breast cancer runs in families,
why don't all family members have it?
It helps to frame this question a little differently.... All
members of a family do not have the same height, weight, and
face. So, it makes sense that they don't all have the same
conditions and diseases -- or the same susceptibility to various
diseases.
Here again, it's genetic and environmental differences that
explain differences in our appearance and health. Some family
members will inherit genes that predispose to breast cancer,
and others will not. Some family members will be exposed to
environmental agents that trigger disease, and others will
not.
There is no breast cancer in my
family. Does this mean I will never get it?
No. Anyone
can get breast cancer (rare exceptions). Most women with breast
cancer do not have a close female relative with the disease.
But if someone in your family does have breast cancer, you
are at greater risk.
How will discoveries about DNA help people and families with
breast cancer?[Top] Further discoveries about breast cancer
genes will lead to more individualized medicine. Prevention,
diagnosis, treatment, and prognosis will be personalized,
based largely on the strengths and weaknesses found in a person's
genes.
-
Treatment -- better use of existing treatments
Medical science already knows that genes in a malignant
breast tumor determine how the tumor responds to treatment.
Learning about these treatment-response genes will allow
oncologists to pick the best treatment to use in each individual
person. (This is a complicated problem because genes in
the tumor can be different from genes in the rest of the
body.)
- Treatment -- discovery of new
treatments
Whenever
scientists discover a gene involved in breast cancer, it's
a doorway to designing new treatments. If the gene is over-active,
then scientists can look for ways to turn it off or interfere
with its activity. If the gene is under-active or broken,
then scientists can look for ways to turn it on or increase
its activity.
- Prevention, Diagnosis, Prognosis
To individualize
prevention, diagnosis, and prognosis, we must be able to
calculate risk accurately. For example, mammography can
detect breast cancer early, when it is more likely to be
cured. But at what age should a woman begin mammography?
To answer that question for an individual, we must know
all the genes contributing to the risk of developing breast
cancer. The stakes become even higher for those women who
are considering prophylactic mastectomy as a prevention.
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