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Genetic
Testing
Background
Genetics
and Alzheimer's disease
Genetics
is the study of how a specific characteristic, such as
a disease, is passed from one generation to another.
The role of genetics in Alzheimer's disease is a subject
of intense study for scientists and a major concern for
people with Alzheimer's disease and their families.
Alzheimer's disease can be grouped into two types: Familial Autosomal
Dominant Alzheimer's Disease (FAD) and Sporadic Alzheimer's Disease.
- Familial
Autosomal Dominant Alzheimer's Disease (FAD)
This
type of Alzheimer's disease is rare and accounts for
approximately five to 10 per cent of all cases of
Alzheimer's disease. It is known to be inherited. The
disease will occur if the disease gene is present.
In certain families, FAD is passed from one generation
to another through a dominant inheritance pattern.
This means that if a parent is affected, each child
has a 50 per cent chance of inheriting the disease
gene and developing Alzheimer's disease in adulthood.
This form of Alzheimer's disease is usually associated
with early onset of the disease, before age 65.
- Sporadic
Alzheimer's Disease
This
is the more common form of Alzheimer's disease and
accounts for 90 to 95 per cent of all cases. It usually
occurs after the age of 65. Many studies indicate
that people with a relative (parent, brother or sister)
with Alzheimer's disease have a greater chance of developing
the disease than those with no family history. However,
the role of heredity in Sporadic Alzheimer's Disease
is unclear and continues to be the subject of research.
Genetic
testing
There
are two types of genetic investigation of a disease.
Each has its limitations.
- Predictive
genetic testing
This
is a genetic test that can identify whether an unaffected
person has a very high (approaching 100 per cent)
or very low (approaching zero per cent) chance of
developing a certain disease.
With
Alzheimer's disease, predictive genetic testing can
be used only in families that exhibit the very specific
inheritance pattern associated with FAD. For the
vast majority of families with Alzheimer's disease,
predictive testing is not an option.
- Genetic
risk assessment
This
is the identification of genetic risk factors that
could potentially increase a person's chance of getting
an illness. The presence or absence of a genetic
risk factor does not positively predict who will
or will not get a disease. Therefore, a genetic risk
assessment cannot be used to determine whether or
not a person will get a disease in the future.
With
Alzheimer's disease, one known genetic risk factor
is the presence of the apolipoprotein E4 (APOE4)
gene in a person's genotype (genetic makeup). The
presence of APOE4 is associated with an increased
risk of Alzheimer's disease. However, the gene, by
itself, cannot predict that a person will develop
Alzheimer's disease.
The
meaning and accuracy of APOE testing is the subject
of intense study. Advisory groups worldwide have
stated that knowing a person's APOE genotype cannot
be used to predict that the person will develop Alzheimer's disease in the future.
The
issues
For
people with Alzheimer's disease and their families:
Decision
to be tested or not: Predictive genetic testing
for Alzheimer's disease is an option only for people
in families with a history of FAD. The decision whether
or not to undergo genetic testing is difficult for
many people. Will the information help people make
decisions about personal relationships, having children,
financial and lifestyle choices? Also, do people want
to know their chances of developing a disease for which
there may be treatment for symptoms but currently no
cure and no prevention? What effect will this information
have on the individual and other family members? For
example, will it affect their access to insurance and
employment benefits?
Right
to know versus the right not to know: Within a
family, one person may want to know whether or not
he carries the gene or genetic risk factors for Alzheimer's disease; another may not. However, the assessment of
one family member may reveal genetic information about
other family members. Should informed consent be obtained
from all family members before any type of genetic
testing can occur?
Right
to privacy and confidentiality: Who should be notified
of the results of genetic testing? Should family members,
insurance companies and employers be able to obtain
the results? Should the information be kept confidential?
Should the person who has been tested be allowed to
prevent others from getting the results?
Potential
exploitation of a person's desire to know: Some
people are willing to buy any test that promises to
tell the future, whether or not the test is sound or
is accompanied by the essential components of consent,
counselling and confidentiality. How should the potential
exploitation of people concerned about developing Alzheimer's disease be addressed?
What
does this mean for families concerned about Alzheimer's disease?
- Predictive
testing
For
the small group of families where the specific gene
changes (on chromosome 21, 14 or 1) that cause FAD
have been identified, predictive testing can be sought
from clinical genetics units throughout Canada. Any
predictive testing should be done with individual
consent, counselling and confidentiality.
Consent: The
person who is being tested or assessed must be aware
of the type of test that is being done and the potential
results, and must agree to undergo the test or assessment.
Counselling: Test
results may have major psychological, legal and ethical
implications for individuals and families. Counselling
by trained professionals must be an integral part
of the testing process.
Confidentiality: There
must be guarantees that all test results remain confidential.
- Genetic
risk assessment
At
present, given the recommendations of several international
committees, genetic risk assessment for Alzheimer's disease, based on APOE genotyping, is not performed
as a clinical service in Canada.
In
closing...
The
field of genetics is a growing one. Each day, scientists
are learning more about our genetic makeup. The challenge
remains that, with every new discovery, consideration
must be given to the effects on individuals, their families
and society in general.
For
the vast majority of people concerned about developing
Alzheimer's disease, there is, at present, no test to determine
if a specific unaffected person will or will not develop
Alzheimer's disease.
For
the small number of people for whom predictive testing
is possible, the decision to know or not to know is a
personal one. However, it should only be made in a setting
that allows for informed consent, counselling and confidentiality.
Concerned
individuals should contact the Alzheimer
Society or their local clinical genetics unit for
more information.
Resources:
From
the Alzheimer Society of Canada:
- Alzheimer's Disease and Heredity
- Alzheimer's Disease and Risk Factors
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