| Introduction
In recent years, much energy has been put into genetic research both through the individual efforts of interested scientists and through the collaboration of international teams in the Human
Genome Project. Through this work, we have learned a great deal about how genes function and how they can cause certain problems. We
now know how to look for mutations (changes in the gene) that can lead to specific
disorders. Genetic testing is possible for some conditions because we can recognize
the difference between a normal gene and a disease gene.
Genetic testing presents us with both opportunity and concern. There
is opportunity for diagnoses and definitive information and, indeed, a hope that cures may ultimately be possible. On the other hand, we
have seen that genetic information can have far-reaching effects on individuals being tested and on their familiesemotionally, socially, ethically.
What are genes?
Genes are specific pieces of information that tell our bodies how to grow, function, and develop. It is estimated that each person has between 50,000-100,000 genes. These genes, which are
contained on our 23 pairs of chromosomes, make up our genetic blueprint. Each gene codes for a particular set of instructions, and a gene's function is determined by its unique DNA code.
DNA consists of four basic building
blocks called bases that are linked in a specific order. When a change occurs
in the ordering or number of bases, a gene may not function properly. A gene
change which can cause a disease is called a mutation.
How can genes influence disease?
Genes come in pairs, with one copy inherited from each parent. A condition is called dominantly inherited when only one copy of a
disease gene is needed to lead to symptoms of that disease. One example of dominant inheritance is Huntington's Disease (HD).
The HD gene can be passed from one
generation to the next and a person who has the HD gene has a 50% chance of passing
that gene on
to each of his or her children. A person affected by a recessively inherited condition inherits a particular disease gene from each parent. One example is cystic fibrosis in which both parents, by chance, have
passed on a CF gene.
Some diseases do not follow simple patterns of inheritance. Many factors influence how a gene works or who will get a disease and when.
Mutations in several different genes can lead to the same disease, as we see
in some forms of Alzheimer's disease. Genes that increase
one's risk of getting a certain disease are called susceptibility genes.
What is genetic testing?
Genetic testing involves analyzing a person's DNA. Usually a blood sample is taken, and a molecular genetics lab performs special tests to look for mutations in a gene that lead to disease. Genetic testing is available for only a fraction of the many genetic conditions in existence. There is no test
that analyzes a person's DNA and gives him or her a clean bill of health.
Who might want genetic testing?
Genetic testing can be done to confirm or rule out a certain diagnosis. Testing might interest a person who knows or suspects that he/she is at
risk for a genetic disease for which treatment options or preventative measures are available. Also, couples considering having children may
wish to know the risk of passing on an inherited disorder (e.g., Huntington's
disease) to offspring.
What testing is available?
Some of the more common genetic diseases for which genetic tests are available include sickle cell disease, myotonic dystrophy, cystic fibrosis,
Duchenne's muscular dystrophy, and Fragile X syndrome. There are also tests available for some inherited adult-onset disorders, including those described below:
Ataxia
Ataxia means a lack of coordination and can be associated with a degenerative disorder. Testing is currently available for spinocerebellar ataxia (SCA) Types 1, 2 and 3. Type 3 is also known as Machado-Joseph disease.
Dementia is not typically seen in SCA Types 1, 2 and 3. They
are inherited in an autosomal dominant manner, meaning that either men or women can be affected and that an affected person has a 50%
chance of passing the gene on to each of his/her children.
The genes for SCA Types 1, 2 and 3,
like the HD gene, have repeated sections of DNA that are larger
than those in the normally functioning gene.
Huntington's Disease (HD)
Huntington's disease is the most common inherited neurological disease. It is characterized by progressive deterioration.
Motor effects progress to uncontrollable
move-ments (chorea) and deterioration of handwriting, speech and swallowing.
Cognitive symptoms can include memory loss and progress to dementia. Emotional problems may include personality change
and depression.
A person with symptoms of Huntington's disease may have a genetic test to confirm that he/she has HD. People at risk for HD (meaning that
one of their parents has HD) may consider presymptomatic testing to learn if they carry the HD gene and therefore will ultimately develop
HD symptoms.
After many years of intense research, the HD gene was identified in 1993. It was discovered that a three base pair section of the DNA of the HD gene is repeated many times in individuals who have HD.
The normal functional gene does not
have this enlargement. Current testing analyzes the HD
gene to look for the presence or absence of this enlargement (or expanded repeat). At this time, the function of the HD gene and how it causes HD is not known. |
Implications of Testing
Benefits
Although there are no cures for these adult-onset disorders, genetic testing for actual gene mutations can provide an accurate diagnosis
or rule out a specific condition.
Having a clear diagnosis can allow a
person and his/her family to anticipate disease progression and make informed
decisions about the future. In some cases, treatment options may be available
to slow the progression of symptoms.
Persons at risk (e.g., a person with a parent with Huntington's disease) might feel uncertain about their own future and that of their children. A negative test (indicating that a person does not have the gene) can give a tremendous sense of relief. A positive test result can relieve uncertainty
and let the person plan for the future.
Limitations
There are not tests available for every adult-onset disorder. One important limitation for gene testing is that diagnostic information often is not matched by effective treatment strategies or therapies.
Risks
Since most genetic tests involve only a blood sample, there is no significant physical risk. Any potential risks have more to do with the way the results of the test might change a person's life.
There can be a major psychological impact on people considering and undergoing genetic testing. The knowledge that one does or does not carry a disease gene can provoke many emotions.
Many people with a family history
of certain diseases have already seen relatives become affected by the disorder.
The news that they have
the disease gene can lead to depression or anger. These emotions can impact the person and reverberate throughout the family. A person
who finds he/she does not carry a disease gene may feel guilty.
There is also concern about confidentiality. People have expressed concern that testing information could someday be used against
them.
Genetic discrimination
As knowledge about the genetic basis of common disorders grows, so does the potential for discrimination in obtaining health or life
insurance. People also have concerns about discrimination in employment.
At the state and federal levels, legislation is being pursued to help ensure that genetic information is not used against people. The Americans with Disabilities Act (ADA) provides employment anti-
discrimination protection for people with dis-abilities and neurological disorders.
In addition, as an example of state law, the State of California prohibits insurers, to varying degrees, from requiring or requesting genetic tests or their results, from denying coverage on the basis of genetic tests, and from using tests to determine rates and benefits. California law has
provisions to protect the privacy of genetic information.
However, in this time of flux and
changing health care systems, it is not clear to what extent consumers are protected.
People considering genetic testing need to consider potential risks for discrimination.
Whom to contact
Your primary care physician may be able to make a referral to a specialist such as a neurologist and genetic counselor as appropriate.
The National Society of Genetic Counselors may also be a helpful source of referrals. A trained professional can help evaluate family history, document diagnosis and discuss whether testing options are available.
How can genetic counseling help?
Genetic counselors are specially trained health professionals who help families learn about and cope with genetic conditions. If a person is
considering testing, a genetic counselor would discuss risks, benefits, and limitations and provide balanced information for the individual to
make an informed decision about whether to proceed with testing.
There are many issues to consider
including psychological impact, family issues, and privacy. Genetic counseling
can be helpful in addressing these issues. Genetic counselors support families
and individuals in making decisions about genetic testing and in adjusting to
test results.
How do people decide about genetic
testing?
The decision about whether to have testing is a very personal one. It should also be voluntary; people should have the test only if they want the information and should not be pressured into testing by relatives or health care providers.
Because the issues are so complex and the consequences so profound, the decision to have a genetic test deserves careful preparation and thought.
As a final note, it is also important to understand that the available information
is changing rapidly as genetic research continues. It is likely that more information and genetic tests will be available in the future. Please use the Resource listings below to help stay informed and up to date.
Resource
Family Caregiver Alliance
425
Bush Street, Suite 500
San Francisco, CA 94108
(415) 434-3388
(800) 445-8106 (in CA)
Website: http://www.caregiver.org
DelMar Caregiver Resource Library
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