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 families emotionally,
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 spino-cerebellar 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.
