Genetic disease prevention and treatment
Related Terms
Autosomal dominant, autosomal recessive, faulty genes, gene therapy, genetic disease, genetic disorder, genetic screening, genetic testing, inherited disease prevention and treatment, inherited disorder, mutation, newborn screening, prenatal testing.
Background
A genetic disease occurs when a person has one or more abnormal genes, missing genes, extra genes, inactivated genes, or overly active genes that lead to a medical condition. If the defective gene is detected before symptoms of the disease appear, it may be possible to prevent or slow disease progression. Although it is currently not possible to cure a genetic disease, in many cases treatments are available that allow an affected person to live a normal life.
Genes are found within the cells of all organisms. An individual's genes are present in a large molecule called deoxyribonucleic acid (DNA). DNA is made up of different combinations of four nucleic acids (adenine, thymine, cytosine, guanine), which are arranged in different lengths. The sequence of these molecules provides the "code," or instructions, for constructing each of the proteins involved in the development, growth, and function of all the cells in the body. A genetic disorder may occur when the nucleic acid sequences that make up individual genes are incorrect, or mutated. When a gene is mutated, the protein that it codes for no longer functions properly and a medical disorder may result.
Genetic conditions are usually inherited, meaning that they are passed down from parents to their children. However, some genetic disorders occur in individuals with no family history of the disorder and are instead caused by genetic mutations that occur during the development of the egg, sperm, or embryo.
A person's DNA is contained inside two sets of chromosomes. Each parent provides one set of 23 chromosomes to his or her offspring. Therefore, each person has 23 pairs of chromosomes. The X and Y chromosomes are called the sex chromosomes because they distinguish males from females. Females have a pair of X chromosomes, while males have one X and one Y chromosome. Certain genetic diseases occur when there are genetic mutations in sex chromosomes, and these can affect males and females differently. Sex-linked diseases, such as color blindness, are much more prevalent in males because of missing genetic material on the Y chromosome.
To inherit an autosomal recessive trait, a person must inherit two copies of a mutated gene (one from each parent). Individuals who inherit only one copy of the mutated gene do not generally experience disease symptoms but are called "carriers" because they can still pass the trait on to their children. In some cases, carriers have a milder form of the disease than those individuals who have two copies of the mutated gene.
To inherit an autosomal dominant trait, on the other hand, an individual need inherit only a single copy of a mutated gene from one parent. Other types of genetic disorders may be caused by a combination of environmental factors and mutations in several genes. For instance, a person may have a defective gene that does not cause problems until he or she comes into contact with an environmental factor, such as sunlight.
There are thousands of known genetic diseases. Most are rare and affect only one person out of several thousand or more. Cystic fibrosis is one of the most common genetic diseases, with about five percent of Americans carrying at least one copy of the defective gene.
Although many genetic diseases are apparent at birth, some do not show signs until later in life. These are called "adult-onset" diseases and include neurodegenerative disorders such as Huntington's disease, some hereditary cancers, and some hereditary heart conditions. A person known to carry a mutation that causes a genetic disease may benefit from consultation with a genetic counselor. Genetic counselors and doctors work together to analyze the interaction of the mutation that causes the genetic disease with a person's lifestyle and work environment. Based on this information, an individualized plan can be formulated for the treatment and prevention of certain genetic diseases. In addition, genetic counselors can educate patients on health-promoting steps that might reduce or prevent the development of adult-onset conditions.
Methods
General: Although many genetic diseases are apparent at birth and cannot be reversed, treatment may aim to reduce or manage symptoms and prevent or manage complications. For diseases that do not develop until later in life, it may be possible to prevent or delay their progression. The following methods are employed to determine whether an individual has a mutated gene that causes a particular genetic disease.
Genetic testing:
General: A genetic test will either look at the products of a specific gene or test for whether the gene itself is faulty. A sample of a person's blood or saliva is taken and sent to a laboratory to look for the defective gene. The results may not be available for days or even weeks. The test is usually performed only if there is a family history of a disorder. This test may also be performed if the parents are of a certain ethnic or cultural background that is associated with a particular genetic disease.
Genetic carrier testing: It is possible to test whether a person is a carrier of a certain genetic disorder, carrying only one copy of a mutant recessive gene. Because these individuals also have a normal copy of the gene on the other chromosome, they tend to not have symptoms as severe as those associated with the full disease. Genetic carrier tests are conducted in the same manner as general genetic tests. In both genetic testing and genetic carrier testing, the reliability of the test is dependent on the specific condition being tested for and the technical ability of the lab.
Diagnostic testing: Currently, genetic testing is available for only certain genetic diseases. These tests are generally done to confirm a diagnosis of a genetic disorder. Before diagnostic testing is done for a suspected genetic condition, the parents, sometimes with the child, discuss the procedure with the physician or the genetic counselor to determine whether the test is necessary.
Prenatal testing: Prenatal testing may be used to check for a genetic disease in a developing fetus. Prenatal screening tests may be able to detect whether a fetus is at increased risk of developing a genetic disease, has the disorder, or is a carrier.
In addition, an ultrasound may be used to detect physical abnormalities associated with the disorder in the fetus. An ultrasound uses high-frequency sound waves to produce an image of the fetus. The procedure is safe but it may not be as accurate or as specific as other tests. An ultrasound does not look at the actual DNA or chromosomes of the fetus. It is therefore most useful for detecting genetic defects associated with physical abnormalities, such as defective heart chambers or brain structures.
Amniocentesis involves inserting a needle into the amniotic cavity, the fluid-filled sac around the developing fetus, and extracting a fluid sample. The DNA of the fetus is then examined for genetic abnormalities. Amniocentesis bears a small risk of miscarriage, but is generally regarded as having less risk than chorionic villus sampling or cordocentesis.
Chorionic villus sampling (CVS) looks for genetic defects by removing a small piece of the placental tissue, the chorionic villi. This is done either transcervically (through the opening of the womb) or transabdominally (through the mother's abdominal wall). The transcervical procedure involves passing an instrument through the cervix. The transabdominal procedure involves making an incision in the uterus and inserting a small plastic tube. Both procedures are done under the guidance of ultrasound imaging. The tissue sample is used to analyze the DNA, chromosomes, and enzymes of the fetus. It is therefore a more specific and often more reliable test than an ultrasound. There is a 2% risk of miscarriage with this procedure. Other risks include bleeding, infection, and rupture of membranes. However, CVS does have the advantage of confirming an earlier diagnosis than amniocentesis.
Cordocentesis involves drawing blood from the fetus' umbilical cord under the guidance of ultrasound. The blood sample is analyzed in a similar manner to amniocentesis. The risk of miscarriage ranges from 1% to 2% for this procedure. Other risks include bleeding, infection, and rupture of membranes.
Preimplantation genetic diagnosis (PGD): Preimplantation genetic diagnosis (PGD) is a test used for embryos during in vitro fertilization (IVF). When the embryo comprises fewer than 10 cells, one or two cells are removed and analyzed. Polymerase chain reaction (PCR) is used to produce copies of, or multiply, the DNA and test for genetic disorders before implantation. IVF is a costly procedure, and PGD is only a small portion of that cost.
Genetic counseling: Before and after genetic testing, genetic counseling is recommended. A genetic counselor can explain the different types of genetic tests, including their potential risks and benefits. In addition, a genetic counselor can advise patients on the appropriateness of the test and help them understand and interpret test results. Psychological counseling is commonly combined with genetic counseling.
Newborn screening:
General: Newborn screening is the practice of testing every newborn for certain harmful or potentially fatal genetic diseases that are not apparent at birth but that develop later in infancy or childhood. Many of these are metabolic disorders, which interfere with the body's use of nutrients to maintain healthy tissues and normal functioning. Metabolic and other inherited disorders can limit an infant's normal physical and mental development. Early detection and treatment can help prevent intellectual impairment, physical disabilities, or life-threatening illnesses.
Newborn screening usually begins with a blood test 24-48 hours after birth. The test is performed by pricking the baby's heel to collect a few drops of blood for analysis. Because screening indicates only the possibility that an infant may have a particular disorder, additional testing is required to confirm abnormal screening results. Some states require newborn screening for fewer than 10 disorders, while others test for 30 or more. The most common genetic diseases that are screened for are listed below. Other conditions that are candidates for newborn screening include Duchenne muscular dystrophy (blood test), HIV disease (blood test), and neuroblastoma (urine test).
Biotinidase deficiency: Infants with biotinidase deficiency are missing biotinidase, which is the enzyme that recycles the B vitamin biotin in the body. The deficiency may cause seizures, poor muscle control, immune system impairment, hearing loss, mental retardation, coma, and even death. If the deficiency is detected early, the disease can be prevented by giving the baby extra biotin. About one in 90,000 infants tests positive.
Congenital adrenal hyperplasia: Congenital adrenal hyperplasia is a group of rare disorders caused by a deficiency of certain hormones produced by the adrenal gland. These diseases can affect the development of the genitals and may cause death due to the loss of sodium from the kidneys. Treatment involves hormone replacement therapy. About one in 12,000 individuals are affected.
Congenital hypothyroidism: Babies affected by congenital hypothyroidism do not have enough thyroid hormone and suffer from below average growth and brain development. The thyroid, a gland at the front of the throat, releases chemical substances that control metabolism and growth. If this disorder is detected early, a baby can be treated with supplemental thyroid hormone. This genetic disease affects about one in 4,000 infants.
Cystic fibrosis: Cystic fibrosis is an inherited disorder that affects the mucus glands of the lungs, liver, pancreas, and intestine. These glands secrete thick mucus that results in chronic respiratory disease, digestion problems, and poor growth. Treatment involves therapy to clear lung secretions, prompt administration of antibiotics when lung infections occur, digestive enzymes, and nutritional supplements. About one in 2,000 Caucasians are affected. Cystic fibrosis is less common in African-Americans, Hispanics, and Asians.
Galactosemia: Babies with galactosemia are missing the enzyme that converts the sugar galactose into glucose. Milk and other dairy products must therefore be eliminated from the diet to prevent the buildup of galactose in the body and damage to the body's cells and organs, which can result in blindness, mental impairment, growth deficiency, and even death. About one in 70,000 people test positive for galactosemia.
Homocysteinuria: Homocysteinuria is caused by a deficiency of one of several enzymes. If untreated, it can lead to dislocated eye lenses, mental retardation, skeletal abnormalities, and abnormal blood clotting. Treatment includes a restricted diet combined with dietary supplements. About one in 75,000 infants test positive.
Maple syrup urine disease (MSUD): Infants with MSUD are missing an enzyme needed to process three amino acids essential for the body's normal growth. These amino acids then build up in the body and give the urine the odor of maple syrup. These infants usually have poor appetites and are extremely irritable. Untreated, MSUD can cause intellectual impairment, physical disability, and even death. Treatment involves a special diet that eliminates certain high-protein foods. MSUD affects about one in 250,000 individuals.
PKU: Phenylketonuria (PKU) is a genetic disorder characterized by an inability of the body to utilize the amino acid phenylalanine. When PKU is detected early, feeding an infant a special formula low in phenylalanine can help prevent intellectual impairment. A low-phenylalanine diet must be followed throughout childhood and adolescence and perhaps into adult life. Approximately one in 15,000 people test positive for PKU.
Sickle cell anemia: Sickle cell anemia, or sickle cell disease, is an inherited blood disease in which red blood cells are abnormally shaped. This causes anemia (a low number of red blood cells), pain, damage to the lungs and kidneys, and even death. Young children with sickle cell anemia are especially prone to certain dangerous bacterial infections, such as pneumonia and meningitis. This disease is most common among African-Americans (about one in 500) and Hispanic-Americans (about one in every 1,200). Sickle cell anemia is inherited in an autosomal recessive fashion. Carriers of the mutation are said to have the "sickle cell trait." These individuals do not develop the disease but can pass it on to their children.
Tyrosinemia: Babies with this disorder cannot break down the amino acid tyrosine. When tyrosine builds up in the body, it can cause learning disabilities and liver disease. Treatment includes a restricted diet and sometimes a liver transplant.
Research
General: Currently, a significant amount of research is being conducted on the diagnosis, prevention, and treatment of genetic diseases. About 1,000 genetic tests are now available. Ongoing research will increase the number of genetic disorders that can be diagnosed.
Human Genome Project (HGP): The Human Genome Project (HGP), which began in 1990, is an international research program that was designed to map out all of the genes that make up humans--the human genome--and their respective hereditary characteristic controls. The genome was completed in 2003. However, researchers are still studying it in order to improve treatments, prevention strategies, and diagnoses of genetic diseases such as Down syndrome and Huntington's disease.
Gene therapy: Gene therapy is an experimental technique designed to treat or prevent inherited disorders. Several methods are currently being studied. Gene therapy is being evaluated for its ability to either replace or inactivate the defective gene causing a disorder. Other gene therapy studies involve inserting a new gene to help the body fight against a specific disease. Although gene therapy appears to be a promising treatment, further research is needed to determine its safety and effectiveness. Because the safety of gene therapy remains unknown, it is being studied only for the treatment of diseases that have no cure. No gene therapy products have been approved for sale in the United States. Currently, gene therapy is available only through research studies. Gene therapy is a growing area of research and hundreds of animal and human trials are ongoing.
Tandem mass spectrometry: A new screening technique known as tandem mass spectrometry (often abbreviated as MS/MS) is becoming more common, especially in university hospitals. This technology can detect elevated blood components in certain genetic disorders and is capable of screening for more than 20 inherited metabolic disorders with a single test.
Implications
As researchers continue to learn more about genetic disorders, many ethical questions have been raised. For instance, in most cases, healthcare insurers do not cover the cost of genetic tests, which may range from $200 to $3,000. However, those insurance companies that do cover the testing will have access to the test results.
In May 2008, the Genetic Information Nondiscrimination Act (GINA) became law in the United States. The bill protects Americans against discrimination based on their genetic information that might affect health insurance coverage and employment. The long-awaited measure, which has been debated in the US Congress for 13 years, will allow people to have access to personalized medicine without fear of discrimination.
When genetic diseases are detected early and treatment options are discussed with physicians and genetic counselors, steps can be taken to promote health and reduce risk for disease development. Relatives should be informed of predispositions to genetic diseases within a family so they can also be evaluated for their risk of developing the condition if they choose to do so. In many cases, psychological counseling may be necessary to help an individual cope with the knowledge of having a genetic disease.
Limitations
Genetic testing has been controversial because these tests provide only a probability for developing a particular disorder. In some cases, some people who carry a disease-associated mutation may never develop the disease. As a result, patients who test positive for a specific gene may become excessively worried about developing the disorder, which may have implications for their quality of life.
Many people feel that children should not be tested for untreatable adult-onset genetic diseases. Testing for these types of genetic diseases is considered acceptable during early fetal development, when parents may make the difficult decision to terminate a pregnancy. However, it is not clear whether there is any benefit in testing children for diseases that cannot be prevented or treated. Doing so may cause unnecessary emotional distress, although a disease for which there are no current treatments may be treatable before the child reaches maturity.
Future research
General: Treatment for many genetic diseases is often limited. Treatment focuses on the symptoms of the disease rather than the underlying cause. However, researchers are currently studying therapies, such as gene therapy, that may one day cure certain inherited disorders.
Cancer Genome Atlas: A new initiative, called the Cancer Genome Atlas, aims to identify all of the genetic abnormalities associated with 50 of the most common types of cancer.
Drug development: Newer, more effective drugs with fewer side effects are likely to be developed in the future for genetic disorders, such as Huntington's disease, cystic fibrosis, and Parkinson's disease. For instance, a new drug, called PTC124, has shown promising results as a treatment for cystic fibrosis and muscular dystrophy. Although this drug is still undergoing research to determine its safety and efficacy, researchers hope that it can help treat a wide range of inherited disorders.
Scientists are also researching the effects of genetics on a person's ability to break down, absorb, and excrete drugs, herbs, and supplements. Some individuals are highly susceptible to side effects of certain medications. Research in this area may help individuals choose safer and more effective treatment options for inherited disorders.
Genetic screening: Patients may eventually have their individual genomes analyzed for genes that are associated with specific diseases or disorders. As a result, preventive treatments and programs may be designed based on the patient's specific needs. However, preemptive screening may also lead to new ethical dilemmas.
Author information
This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).
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