Genetic research is leading to the development of more genetic tests that can be used for the diagnosis of genetic conditions. Genetic testing is available for infants, children, and adults. Genetic tests can be used to diagnose a disease in an individual with symptoms and to help measure risk of developing a disease. Adults can undergo preconception testing before deciding to become pregnant, and prenatal testing can be performed during a pregnancy. Results of genetic tests can help physicians select appropriate treatments for their patients.
Genetic tests look for alterations in a person's genes or changes in the level or structure of key proteins coded for by specific genes. Genetic tests can also be used to look at levels of RNA that play a role in certain conditions. Abnormal results on these tests could mean that someone has a genetic disorder.
Types of genetic tests include:
Gene tests (individual genes or relatively short lengths of DNA or RNA are tested)
Chromosomal tests (whole chromosomes or very long lengths of DNA are tested)
Biochemical tests (protein levels or enzyme activities are tested)
What is a gene test?
Gene tests look for signs of a disease or disorder in DNA or RNA taken from a person's blood, other body fluids like saliva, or tissues. These tests can look for large changes, such as a gene that has a section missing or added, or small changes, such as a missing, added, or altered chemical base (subunit) within the DNA strand. Gene tests may also detect genes with too many copies, individual genes that are too active, genes that are turned off, or genes that are lost entirely.
Gene tests examine a person's DNA in a variety of ways. Some tests use DNA probes. A probe is a short string of DNA with base sequence complementary to (able to bind with) the sequence of an altered gene. These probes usually have fluorescent tags attached to them. During the test, a probe looks for its complement within a person's genome. If the altered gene is found, the complementary probe binds to it, and the fluorescent label can be used to identify the presence of the alteration.
Another type of gene test relies on DNA or RNA sequencing. This test directly compares the base-by-base sequence of DNA or RNA in a patient's sample to a normal version of the DNA or RNA sequence.
What is a chromosomal test?
Chromosomes are the large DNA-containing structures in the nucleus of a cell. Humans normally have 23 pairs of chromosomes: 22 pairs of autosomes (numbered 1 through 22) and 1 pair of sex chromosomes (either XX for females or XY for males). Chromosomal tests look at features of a person's chromosomes, including their structure, number and arrangement. These tests look for changes, such as pieces of a chromosome being deleted, expanded, or being switched to a different chromosomal location.
Types of chromosomal tests include:
Karyotype - This test gives a picture of all of a person's chromosomes from the largest to the smallest. This type of testing can identify changes in chromosome number and large changes in DNA structure. A karyotype would, for instance, identify Down syndrome caused by the presence of an extra copy of chromosome 21
FISH analysis (fluorescent in situ hybridization) - This test identifies certain regions on chromosomes using fluorescent DNA probes. FISH analysis can find small pieces of chromosomes that are missing or have extra copies. These small changes can be missed by the overall karyotype test. FISH analysis can, for instance, be used to reveal the missing fragments of DNA on chromosome 22 that are characteristic of velocardiofacial syndrome.
What is a biochemical test?
Biochemical tests look at the amounts or activities of key proteins. Since genes contain the DNA code for making proteins, abnormal amounts or activities of proteins can signal genes that are not working normally. These types of tests are often used for newborn screening. For example, biochemical screening can detect infants who have metabolic conditions such as phenylketonuria (PKU). Because of a genetic defect, people with PKU lack the enzyme that breaks down a particular amino acid (protein building block) called phenylalanine. Consequently, phenylalanine builds up to higher than normal levels in the body, leading to a variety of health problems. If diagnosed early, PKU can be treated with a strict diet that is low in phenylalanine, avoiding foods that are high in protein or that contain certain artificial sweeteners.
Show whether a person is a carrier for a genetic disease. Carriers have an altered gene, but will not get the disease. However, they can pass the altered gene on to their children.
Help expectant parents know whether an unborn child will have a genetic condition. This is called prenatal testing.
Screen newborn infants for abnormal or missing proteins that can cause disease. This is called newborn screening.
Show whether a person has an inherited disposition to a certain disease before symptoms start.
Determine the type or dose of a medicine that is best for a certain person. This is called pharmacogenetics.
People in families at high risk for a genetic disease have to live with uncertainty about their future and their children's future. A genetic test result that can show that a known alteration causing disease is not present in a person can provide a sense of relief.
A genetic test result showing that a person has a disease-causing gene alteration can also provide benefits. Such a test result might lead a person to take steps to lower his/her chance of developing a disease. For example, as the result of such a finding, someone could be screened earlier and more frequently for the disease and/or could make changes to health habits like diet and exercise. Such a genetic test result can lower a person's feelings of uncertainty, and this information can also help people to make informed choices about their future, such as whether to have a baby.
What are reasons to get different types of genetic tests?
Diagnostic testing is used to confirm a diagnosis when a person has signs or symptoms that suggest a genetic disease. The particular genetic test used depends on the disease for which a person is tested. For example, if a patient has physical features that suggest Down syndrome, a chromosomal test is used to see if the patient has an extra copy of chromosome 21. To test for Duchenne muscular dystrophy, a gene test is done to look for missing sections in the dystrophin gene.
Predictive testing can show which people have a higher chance of getting a disease before symptoms appear. For example, one type of predictive test screens for inherited genetic risk factors that make it more likely for someone to develop certain cancers, such as colon or breast cancer, or diseases that usually develop later in life, such as adult onset (Type 2) diabetes. Someone with an inherited genetic risk factor may have an increased chance of getting a disease, although this does not mean that the person will certainly get the disease.
Presymptomatic is a type of predictive testing that can indicate which family members are at risk for a certain genetic condition already known to be present in their family. This type of testing is done with people who do not yet show symptoms of that disease. This can be done for Huntington's disease, for example. For some diseases, this type of testing can lead to prevention or treatment options. For example, when a disease-causing alteration for Graves' disease is found in a family, testing is recommended for all close blood relatives (such as parents and siblings). Graves' disease is an autoimmune disease that leads to over-activity of the thyroid gland (hyperthyroidism). Family members with the genetic alteration can be offered treatment, including surgery to remove the thyroid. With other types of diseases, there are no prevention or treatment options. For example, there is no treatment for family members who have a gene alteration causing Huntington's disease. People with this alteration are certain to get the disease.
Preconception/Carrier testing can tell individuals if they have (carry) a gene alteration for a type of inherited disorder called an autosomal recessive disorder. Autosomal means that the altered gene is on one of the 22 chromosomes other than the sex chromosomes (X or Y chromosomes). Recessive means that the person with only one altered copy of the disease gene will not get the disease, but might pass the alteration to their children. If both parents are carriers, their children might inherit an alteration from each parent and get the disease. Examples of autosomal recessive disorders are cystic fibrosis and Tay-Sachs disease.
Prenatal testing is available to pregnant women during pregnancy. Some reasons to have genetic testing include:
Age of the mother. Women age 35 or older are at a higher risk for having a child with chromosomal abnormalities or other birth defects. However, some tests are recommended for all pregnant women, regardless of age.
Ancestry or ethnic background indicating that the parents might have a higher chance of carrying an inherited disorder such as sickle cell anemia, common in people of African descent; thalassemia, common in people of Italian, Greek, Middle Eastern, Southern Asian, or African descent; or Tay-Sachs disease, common in people of eastern European (Ashkenazi) Jewish descent.
Three diagnostic procedures are common in prenatal testing: ultrasound, amniocentesis, and chorionic villus sampling (CVS). Ultrasound uses the reflection of sound waves to create an overall picture of the developing fetus. Amniocentesis involves testing a sample of amniotic fluid from the womb surrounding the fetus. CVS involves taking a tiny sample of tissue from a region of the placenta that carries fetal cells rather than maternal cells.
Newborn screening is the most widespread type of genetic testing. It is an important public health program that can find disorders in newborns that might have long-term health effects. Newborn screening analyzes infant blood samples for abnormal or missing gene products (proteins). For example, infants are often screened for phenylketonuria (PKU), a metabolic disease in which an enzyme deficiency can cause severe mental retardation if the child is not treated. Metabolic disorders are diseases in which gene alterations lead to an inability to break down (metabolize) food and other substances. In the past, newborn screening focused on only a few disorders that lead to mental retardation. Regulations vary from state to state, but all states are now required by law to test for at least 21 disorders, although some states test for 30 or more disorders. These programs now test for disorders that can lead to increased risk of infectious disease, premature death, hearing loss, and heart problems.
Pharmacogenetic Testing examines a person's genes to understand how drugs may move through the body and be broken down. The goal of pharmacogenetic testing is to help select drug treatments that are best for each person. (Also see: Frequently Asked Questions About Pharmacogenomics). For example, a test used in patients who have chronic myelogenous leukemia (CML) can show which of these patients would benefit from a medicine called imatinib. Another test looks at the liver enzyme cytochrome P450 (CYP450). This enzyme breaks down certain types of drugs, such as warfarin, which is used to prevent blood clots. Alterations in the CYP450 gene can affect how well people's bodies break down certain drugs. By taking a single dose of a drug, people with a less active form of the enzyme might get too much of the drug in their body, while people with a more active form of the enzyme might get too little of the drug. Too much warfarin, for example, can lead to internal bleeding, while too little warfarin may still allow blood clots to form. Pharmacogenetic testing can help make sure that people get the right amount of a medicine based on their particular genetic makeup.
People have many different reasons for being tested or not being tested. For many, it is important to know whether a disease can be prevented if a gene alteration causing a disease is found. For example, those who have inherited predispositions to breast or colon cancer have options such as earlier and more frequent disease screening or early treatment. Pharmacogenetic testing can indicate the best medicine or dose of a medicine for a certain person.
In other cases, there may be no treatment for the disease. For example are no preventive steps or cures for Huntington's disease (http://www.genome.gov/10001215), but test results might help a person make life decisions, such as career choice, family planning, or insurance coverage.
What about direct-to-consumer genetic tests available on the Internet?
A variety of genetic tests are being offered directly to consumers, often over the Internet. Such direct-to-consumer (DTC) genetic testing usually involves the individual scraping a few cells from inside the cheek and mailing the sample to a laboratory that performs the test. If you are considering such genetic tests, it is a good idea to first discuss the issue with your health-care provider or a genetic counselor.
Here are some major types of direct-to-consumer genetic tests:
Also, your test results may show that you are at increased risk for a condition, such as Alzheimer's disease, for which there is currently no effective prevention or cure. Such knowledge may help you plan your life, but it may also make you and your loved ones anxious or depressed.
Finally, ask yourself if you are prepared to make changes in your lifestyle based on the test results. If you are not willing to take actions like stopping smoking or exercising more, such tests may not be of much benefit to you.
Health-related: These tests can detect changes in all or selected parts of your genome for variants that may influence your risk of developing certain diseases. Be aware that current tests provide just a partial picture of your disease risk. For most diseases, many pieces of the genetic puzzle remain to be discovered, along with how those pieces interact with lifestyle and environmental factors. This means that today's tests may falsely reassure people with undiscovered risk factors, or needlessly alarm people with undiscovered protective factors.
Nutrigenetic: These tests are offered by companies that claim they can use information about your genetic makeup to develop an individualized diet plan. A government report published in 2006 found that such tests may be misleading or even harmful because they make claims that cannot be scientifically proven. For more information on this subject, read the General Accounting Office's report, Nutrigenetic Testing: Tests Purchased from Four Web Sites Mislead Consumers
Non-medical/Other: Other non-medical types of tests may scan your genome for genetic variants related to various aspects of your life. These may include genetic markers that might your physical traits, your ancestry, or your personality. These tests often do not claim to provide specific information about disease risk, but some of the results may have implications for your health. As with other DTC tests, there are concerns that the validity and reliability of these tests are not adequate (See: Regulation of Genetic Tests; ASHG Statement on DTC Testing ).