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| Intro: Genetics/Genetic Testing |
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What do you know about your family tree? Have any of your relatives
had health problems that tend to "run in families"? Which of these
problems affected your parents or grandparents? Which ones affect
you now, or your brothers or sisters? Which problems might you pass
on to your children?
Thanks to advances in genetics, doctors now have the tools to
understand how certain illnesses, or increased risks for certain illnesses,
pass from generation to generation.
According to some health experts, the definition of an inherited or "genetic"
illness should be expanded beyond the classic inherited disorders (like
hemophilia and sickle cell anemia) to include many types of cancer, Alzheimer's
disease, and other illnesses. They look toward a future where genetic test
results are an important part of every healthy person's medical file. Let's
see what this new biotechnology can mean to you and your family.
Genes and Chromosomes
Each of us has a unique set of chemical "blueprints" that determines
how our body looks and functions. These blueprints are contained in
a complex chemical called DNA, a long, spiral-shaped molecule that is
found inside each body cell.
Specific segments of DNA that contain the instructions for making specifi
body proteins are called genes. Right now, scientists believe that human
DNA carries up to 30,000 genes. Some genes direct the formation of
structural proteins that eventually determine how our bodies look (blue
eyes or brown, curly hair or straight), while others code for important
body chemicals called enzymes.
Sometimes, depending on what a specific gene codes for, even a small
genetic error can mean serious problems for the entire body. Sometimes,
an error in just one gene can mean the difference between a normal life
and one that is shortened or physically difficult.
Genes line up along the length of human DNA, neatly packaged in
structures called chromosomes. Human cells have 46 chromosomes,
arranged in 23 pairs, with one member of each pair inherited from
each parent at the time of conception.
After conception, these original 46 chromosomes duplicate again and
again to pass on the same genetic information to each new cell in the
developing child. Human chromosomes are large enough to be seen
with a light microscope, and the 23 pairs can be identified according
to differences in their size, shape, and the way they pick up special
laboratory dyes.
Genetic Problems
Genetic problems can happen for many different reasons. Sometimes
a mistake occurs during cell division, causing an error in chromosome
number either before or shortly after conception. The developing embryo
grows from cells that have either too many chromosomes or too few.
In "trisomy," for example, there are three copies of one particular
chromosome instead of the normal two (one from each parent). Down
syndrome, trisomy 18 (Edwards) syndrome, and trisomy 13 (Patau)
syndrome are all examples of this type of genetic problem. In "monosomy,"
another form of number error, one member of a chromosome pair is
missing. There are too few chromosomes rather than too many.
Sometimes it's not the number of chromosomes that's the problem.
Instead, one or more chromosomes is incomplete or abnormally shaped.
In both deletions and microdeletions, for example, some small part of a
chromosome is missing. In a microdeletion, the missing part of a chromosome
is usually so small that it amounts to only a single gene. Some important
genetic disorders caused by deletions and microdeletions include:
Wolf-Hirschhorn syndrome (affects chromosome 4); Cri-du-chat syndrome
(chromosome 5); DiGeorge syndrome (chromosome 22); and Williams
syndrome (chromosome 7).
In translocations, bits of chromosomes shift from one chromosome to
another, while in inversions, small parts of the DNA code seem to be
snipped out and reinserted backwards.
Very special types of genetic problems also occur when abnormalities
affect the sex chromosomes. Normally, a child will be a male if he inherits
one X chromosome from his mother and one Y chromosome from his father.
A child will be a female if she inherits a double dose of X (one from each
parent),and no Y. Sometimes, however, children are born with only one
sex chromosome (usually a single X), or with an XXY or XYY inheritance.
Sometimes, too, a genetic problem is sex-linked, meaning that it is carried
by either the X or Y chromosome.
Some genetic problems are caused by a single gene that is present but
abnormal. When this is the case, chromosome number and appearance
are often entirely normal. To pinpoint the defective gene, scientists use
sophisticated DNA screening techniques. Some examples of genetic
illnesses caused by a single problem gene include: cystic fibrosis, sickle
cell anemia, Tay-Sachs disease, and achondroplasia (a type of dwarfism).
Although experts originally believed that no more than three percent of
all human diseases were caused by errors in a single gene, new research
suggests that this may be an underestimate. Within the last two years,
scientists have discovered genetic links to many different diseases that
were not originally thought of as "genetic," including several different types
of cancer. They have identified 20 to 30 "cancer susceptibility genes" that
greatly increase a person's odds of getting some form of malignancy.
For example, a gene has been identified on chromosome number 9 that
may be linked to a common skin cancer called basal cell carcinoma. This
gene, labeled PTC or "patched," may someday be important in screening
for the cancer. Another gene, called HNPCC, is carried by one out of every
300 Americans and may greatly increase an individual's chance of getting
colon cancer. And the doubly dangerous gene called BRCA-1 seems to give
women an 85% chance of developing breast cancer, as well as a 50%
chance of ovarian tumors.
Altered genes may also play a role in the development of many other
devastating illnesses besides cancer. Parkinson's disease, for example, may
be linked to a gene on chromosome number 4, and multiple sclerosis may
be linked to alterations in a gene on chromosome number 6. Alzheimer's
disease, linked to the ApoE4 gene on chromosome 19, can already be
diagnosed (in some cases) by screening for that altered gene.
Although heart disease and diabetes appear to be related to simultaneous
changes in many different genes, the first of these, called LpL PvuII, may
already have been identified. According to American Heart Association reports,
this gene may be an "artery-clogging gene" that almost doubles the risk of
fatty deposits blocking the coronary arteries. Having the LpL PvuII gene may
also triple someone's changes of getting adult-onset diabetes. Like heart
disease, depression and other mental illnesses may be the result of alterations
in several genes at once. Although no specific genes have yet been found,
doctors estimate that 48-75% of depression is inherited, and they believe that
they will find the exact genetic mechanism very soon.
It is important to note that much of the newest information from genetic research
has not yet been translated into useful screening tests for your doctor's office.
However, experts predict that this will soon change, and they estimate that the
number of available genetic tests will increase tenfold in the next decade.
Genetic Testing
At one time doctors could only diagnose genetic problems by karyotyping - checking
the number, shape, and dye stains in chromosomes. Now genetic testing has
developed far enough to allow doctors to pinpoint missing or defective genes. The
type of genetic test needed to make a specific diagnosis depends on the particular
genetic illness that your doctor suspects. If he or she suspects a trisomy, for example,
a simple chromosome check will probably be enough. If the problem is a single
problem gene, however, more high-tech DNA screening will usually be necessary.
For genetic testing in children before birth, chorionic villus sampling or amniocentesis
is usually performed.
Many different types of body fluids and tissues can be used in genetic testing. For
DNA screening, only a very tiny bit of blood, skin, bone, or other tissue is needed.
Even the small amount of tissue at the bottom end of a human hair is usually enough.
When Do Doctors Recommend Genetic Testing?
A doctor may recommend genetic testing for any of the following reasons:
A Word of Caution Although advances in genetic testing have created a revolution in the way
that doctors diagnose and treat certain illnesses, there are still some limits
that parents need to recognize. First, although genetic tests can identify a
particular problem gene, they cannot always predict how severely that gene
will affect the person who carries it. In cystic fibrosis, for example, finding a
problem gene on chromosome number 7 cannot necessarily predict whether
a child will have serious lung problems or only mild respiratory symptoms.
Second, since many illnesses develop from a deadly mix of high-risk genes and
unhealthy lifestyle (for example, a smoker with a family history of heart disease),
simply having problem genes is only half the story. Knowing that you carry high
-risk genes may actually be an advantage, if it gives you the chance to modify
your lifestyle to avoid becoming sick.
A Great Potential Every day researchers are finding new evidence that people who have specific
genes are at a greater risk for illnesses like cancer, heart disease, psychiatric
disorders, and many other medical problems. The next step is to discover just
how these problem genes work. Once we learn this, it may be possible to develop
specific types of gene therapy to prevent some of humanity's major killer diseases.
Right now, gene therapy is already being used successfully to treat cystic fibrosis
and ADA deficiency (an immune deficiency). Sickle cell anemia, the thalassemias,
and other blood disorders may be the next targets for a genetic cure. Although
genetic treatments for major killers, like cancer, may be a long way off, there is
still great hope that many more genetic cures will be found. In less than ten years,
scientists working on the Human Genome Project will have finished identifying and
mapping out all of the genes (up to 100,000) carried in our human chromosomes.
The map will be just a beginning, but it's a very hopeful beginning.
More Information DNA is deoxyribonucleic acid, the molecule that carries the codes for genetic
information. DNA is made of linked subunits called nucleotides. Each nucleotide
contains a phosphate molecule, a sugar molecule (deoxyribose), and one
of four coding molecules called bases (adenine, guanine, cytosine, or thymine).
The sequence of these four bases determines the genetic code.
Enzymes are proteins that act as natural catalysts. They help to direct and
control the chemical reactions that happen within the body.
Down syndrome is a trisomy of chromosome number 21, and it affects one
of every 600 to 800 newborns. Children with Down syndrome have a distinct
facial appearance, with eyes that slant upwards at the corners, a small flat
face, and a large tongue that tends to stick out. Down syndrome causes
varying degrees of mental retardation and is associated with an increased
risk for certain heart problems, intestinal atresia (part of the intestines
shriveled or missing), and acute leukemia.
Trisomy 18 syndrome affects one out of every 8,000 newborns. Children with this
syndrome have a low birth weight and a small head, mouth, and jaw. Their hands
typically form closed fists with abnormal finger positioning. They may also have
malformations involving the hips and feet, heart and kidney problems, and mental
retardation. Only about five percent of these children live longer than one year.
Trisomy 13 syndrome affects one out of every 20,000 newborns. This syndrome
causes cleft lip, flexed fingers with extra digits, hemangiomas (blood vessel
malformations) of the face and neck, and many different structural abnormalities
of the skull and face. It can also cause malformations of the ribs, heart, abdominal
organs, and sex organs.
A deletion happens when part of a chromosome is missing. Several different
syndromes have been linked to deletions of specific parts of chromosomes 4,
5, 9, 13, 18, and 21. For example, Wolf-Hirschhorn syndrome is caused by a
deletion of part of chromosome 4. Children born with this syndrome can have
abnormalities of the head and face, malformations of the heart, and mental
retardation.
Microdeletions happen when a very small piece of a chromosome is missing.
Often the missing piece is so small that it involves only a single gene. Because
microdeletions cannot be seen with a simple light microscope, they must be
detected using sophisticated DNA studies. Syndromes caused by microdeletions
include: DiGeorge syndrome, Prader-Willi syndrome, Angelman syndrome,
and Williams syndrome.
In translocations, small bits of chromosomes shift from one chromosome to another.
Translocations may either be inherited from a parent or arise spontaneously in a
child's own chromosomes. They affect one out of every 500 newborns. Translocations
typically cause no malformations or developmental problems in the children who
have them. However, when these children grow to adulthood and wish to become
parents, they may have an increased risk of miscarriage or chromosome abnormalities
in their own children.
In a chromosome inversion, a small portion of a chromosome breaks off, then
reinserts backwards. Inversions affect about one out of every 100 newborns
and typically cause no malformations or developmental problems in the children
who have them. However, when these children grow to adulthood and wish
to become parents, they may have an increased risk of miscarriage or chromosome
abnormalities in their own children.
In DNA screening, laboratories use molecular probes to check for a specific
coding sequence along the length of a person's DNA molecule. This specific
coding sequence is usually one that has been linked to causing an inherited
disease.
Karyotyping is a laboratory test that shows chromosome number and appearance.
In karyotyping, a photo is taken of a person's chromosomes as they appear under
a microscope.
Chorionic villus sampling (CVS) is a test performed on a pregnant woman, usually
between the tenth and twelfth weeks of pregnancy. In CVS, the doctor removes
a small piece of the placenta to check for genetic problems in a child before birth.
Because chorionic villus sampling is an invasive test, there is a small risk that it
can induce a miscarriage.
Amniocentesis is a test performed on a pregnant woman, usually between her
sixteenth and eighteenth week of pregnancy. The doctor inserts a hollow needle
into the woman's abdomen to remove a small amount of amniotic fluid from
around the developing fetus. This fluid can be tested to check for genetic problems
and to determine the sex of the child. When there is risk of cesarean section or
premature birth, amniocentesis may also be done to see how far the child's lungs
have matured. Like chorionic villus sampling, amniocentesis carries a slight risk
of inducing a miscarriage.
Source: KidsHealth www.KidsHealth.com is a project of The Nemours Foundation which is dedicated to improving the health and spirit of children. Today, as part of its continuing mission, the Foundation supports the operation of a number of renowned children's health facilities throughout the nation, including the Alfred I. duPont Hospital for Children in Wilmington, Delaware, and the Nemours Children's Clinics throughout Florida. Visit The Nemours Foundation to find out more about them and its health facilities for children http://www.nemours.org/no/ |
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