Genetic carrier screening involves a test of a person's blood to determine whether he or she is a carrier of one of many inherited disorders. It is used to identify a person who has inherited an abnormal gene from one of his or her parents, but does not have the disorder. The carrier may pass the abnormal gene to his or her children.
This screening can be performed on newborns and adults, depending on a particular genetic condition. It also may be done prenatally by testing a sample of amniotic fluid, tissue from the womb or chorionic villi (cells that eventually form placenta) obtained through chorionic villus sampling (CVS). CVS is usually done between 11 weeks and 13 weeks plus six days gestation as part of prenatal care.
What does it entail?
There are two types of genetic testing: amniocentesis and chorionic villus sampling, which can be done from about 10-12 weeks gestation through the end of the first trimester. These procedures involve checking for chromosomal abnormalities such as Down syndrome, spina bifida, and cystic fibrosis. More recently, however, there have been tests developed that allow doctors to screen parents' blood samples for their specific mutations regarding diseases such as sickle cell anemia and Tay-Sachs disease. Only a small number of hospitals offer these tests.
What is involved in such a test?
Since the beginning of life is determined by the moment of fertilization, sperm and egg cells contain genetic information about both parents. To determine if a person has inherited an abnormal gene from one of his or her parents, his or her blood sample will be tested for any disease genes they might carry. This is done using DNA (deoxyribonucleic acid), which carries instructions for most human characteristics and functions. DNA is arranged in long strands called chromosomes, and each chromosome contains thousands of genes that determine everything from what color eyes we have to what illnesses we may get. An individual has 46 chromosomes — 23 from mom and 23 from dad — including two copies (one from each parent) of each of the 22 autosomal chromosomes, as well as two sex chromosomes (X and Y). If a disease-causing variant is found in either parent, it can be passed to their children. The child will then be at risk for developing that disorder.
What are some examples of diseases that might be screened?
Some examples include Huntington's Disease, Tay-Sachs, sickle cell anemia, and cystic fibrosis. Other conditions that appear later in life may also be tested for by direct DNA analysis if a person has a family history of a certain condition. About 25 percent of inherited deafness is caused by specific genes so testing can have important implications for families who have this special need within them. Cystic fibrosis results from malfunctions in a particular gene. A person who inherits two copies of this mutated gene has the disease, but people who inherit only one copy are carriers for cystic fibrosis. They can pass the altered gene to their children.
The risk for two carriers to produce a child who is afflicted with the disease is 25 percent; the risk increases if both parents carry the mutation.
What should I expect from screening?
Those who have had carrier screening done commonly experience no problems, however some women who have had amniocentesis report cramping during the procedure and light bleeding afterward. There is a risk of miscarriage associated with any prenatal test, and in most patients, this possibility has been reduced to less than 1 percent.
There also is a very small chance for complications such as infection or leakage of amniotic fluid from the uterus after CVS. The risks increase with gestational age; thus, CVS should not be performed when there are signs that the fetus may be viable (able to survive outside the womb).
How is genetic carrier screening done?
Both amniocentesis and chorionic villus sampling (CVS) are done early in pregnancy, between the 10th and 13th week of gestation. Both involve drawing a small amount of fluid from the sac surrounding the fetus; CVS is usually performed with ultrasound guidance (using sound waves to produce an image on a video monitor).
Amniocentesis is usually done after 14 weeks. It involves inserting a long needle through the abdomen into the uterus and withdrawing some fluid, which is then screened for chromosomal abnormalities plus several hundred individual genes that can cause inherited disorders. The procedure takes about 20 to 30 minutes. Results take about one week.
Chorionic villus sampling (CVS) provides information on the chromosomal status and some specific inherited disorders. CVS is usually performed between the 10th and 13th weeks of gestation using ultrasound guidance. After injection into the vagina to numb it, a long needle is inserted through the abdominal wall into the uterus. Samples are taken from cells in the placenta called chorionic villi (hence the name chorionic villus sampling) for chromosomal analysis or for DNA testing of about 100 single-gene defects that cause serious inherited disorders. Results take about one week.
What if I'm a carrier?
If you are diagnosed as a carrier, your partner should be tested to determine his or her risk. If the result is normal, there is no increased risk for having children with these disorders when two carriers reproduce. However, if one partner is not found to be a carrier and the other has the mutated gene responsible for the disease, there is at least a 50 percent chance in each pregnancy that their child will inherit both abnormal genes and develop the disorder; in addition, each pregnancy would have even greater risks than would normally exist.
When both parents are carriers of an autosomal recessive genetic disorder (same as above) then their children have one chance in four of being born with that disorder.
What if I or my partner is found to be a carrier?
If you are already pregnant, the most important thing for both of you to do at this time is to attend genetic counseling. The counselor can design a plan specifically for your situation. Some couples choose not to have any children; others want all their children to be tested before birth so that they know which ones are carriers and which aren't; still others prefer periodic screening of their infants after they're born. A great deal depends on the type of disease involved, how common it is in the population, and what special problems exist (such as risks related to pregnancy) that might justify continuing pregnancies despite the potential for genetic disease.
If you anticipate becoming pregnant, discuss with your doctor ways to maximize the chances of a normal outcome. You may want to undergo carrier screening with your partner in order to improve your information about the risks of having a child with an inherited disorder.
What are our options if we find out we're carriers?
If you and your partner are carriers of the same condition, such as cystic fibrosis or Tay-Sachs disease, it is possible that both of your children will develop the disorder. When this happens in a couple who planned carefully, they may feel very badly about what has happened and may even experience guilt or grief.
This can be one of the most difficult situations in which to make decisions: There is no way to predict with certainty how serious the effects on an individual child will be; there is no cure for these diseases; treatment consists mainly of supportive care.
Couples who discover they are both carriers often choose to have prenatal testing during pregnancy so they can learn whether their fetus has inherited two copies of the abnormal gene.
If the fetus has inherited two copies of the abnormal gene, pregnancy termination may be considered. Couples may choose not to have any children; others might continue pregnancies which would result in affected children. If they do, they will typically need genetic counseling for other family members contemplating having children. Prenatal screening is available for several conditions, including hemoglobinopathies (sickle-cell disease and thalassemia), Tay-Sachs disease, cystic fibrosis, fragile X syndrome, spinal muscular atrophy (SMA) type I, familial dysautonomia (Riley Day Syndrome), hereditary neuropathy with liability to pressure palsies (HNPP), spinal muscular atrophy type II, congenital myasthenic syndrome (CMS), a congenital disorder of glycosylation type Ia, and retinitis pigmentosa (RP).
Cystic fibrosis screening
is widely available, but, as stated above, carrier screening is not the same as prenatal testing. Carrier screening for cystic fibrosis can be done with a simple blood test that analyzes DNA from the patient's blood cells. Prenatally, there are two types of tests available to detect cystic fibrosis: invasive and noninvasive. Invasive testing includes chorionic villus sampling (CVS) at 10-12 weeks gestation or amniocentesis at 15-20 weeks gestation. These procedures carry a small risk of causing a spontaneous miscarriage
Noninvasive prenatal testing requires only a simple blood draw from the mother as early as 10 weeks gestation and analysis of cell-free fetal DNA in her plasma. This approach has much higher sensitivity and much lower specificity than invasive testing. The approach is not without risk, however, as the false positive rate can be as high as 1 in 25 pregnancies.
Carrier screening for cystic fibrosis is available to certain populations including Ashkenazi Jews, people of Northern European descent, Caucasians, African Americans, Latinos, Italians, French-Canadians, South Asians, North Africans. Carrier testing for cystic fibrosis should also be considered if there are affected individuals in your family or if you have a family history of unexplained early deaths due to respiratory disease.
Huntington disease carrier screening
If you have a family history of Huntington disease (HD), it may be possible to detect the abnormal gene that causes HD in your DNA. That way, you could find out if you're at risk of HD before symptoms develop. If you know that you carry the gene for HD, then your relatives can get tested to see if they are also carriers.
Treatments are available to manage some of the symptoms of Huntington disease (HD), but currently, there is no cure for this devastating disorder. Early diagnosis is important because treatments may be available to help slow or prevent symptoms from getting worse.
Huntington disease carrier screening is recommended for people with a family history of HD who do not yet show any signs or symptoms of the disease. People interested in genetic testing should meet with a genetic counselor and work closely with their physician to discuss their personal and family health history.
Fragile X syndrome carrier screening
Carrier screening for fragile X syndrome (both premutation carriers and full mutation carriers) is recommended for couples in which one partner is a premutation or full mutation carrier. Young adults of any age diagnosed with mental retardation, developmental delay, autism spectrum disorder, learning disabilities, attention-deficit/hyperactivity disorder, school problems, or any other neurodevelopmental conditions should be tested for fragile X syndrome.
Sickle cell screening
A blood test looks for a single change in the DNA sequence that causes sickle cell disease. It can be used to screen an adult who does not have any symptoms of sickle cell disease but is at risk due to family history. The American Congress of Obstetricians and Gynecologists (ACOG) recommends prenatal carrier screening for sickle cell disease when there's a family or personal history of the disorder.
Thalassemia carrier screening is recommended for couples who are of Southeast Asian, African, Middle Eastern, or Mediterranean descent. Thalassemia carrier screening can also be considered if one partner has sickle cell disease or beta-thalassemia trait.
Genetic counseling and genetic testing are used together to help people make informed decisions about their health. Genetic counselors are experts in helping patients understand the link between genes, diseases, and how a disease might be inherited. They use this information to counsel individuals who may be at risk due to family history or other factors such as ethnicity. The ACOG recommends that all women of childbearing age receive genetic counseling before pregnancy. If you choose to have prenatal testing, it's important that you receive genetic counseling too.
Many people are carriers of Tay-Sachs disease without being aware of it. Testing can help you know if you're a carrier, allowing you to make informed choices about your future. If one parent is a carrier, each child has a 50% chance of inheriting the disease gene. Prenatal testing before or during pregnancy may be available in some areas for couples who are both carriers of an affected child. It is important to note that not all states test newborn babies for Tay-Sachs disease so it's very important for parents to discuss this possibility with their doctor and have genetic counseling before deciding whether or not to have a prenatal diagnosis.
Preimplant Genetic Diagnosis
In Preimplant genetic diagnosis (PGD) and Preimplantation genetic screening (PGS) testing, a cell from an embryo is tested for specific inherited diseases before the embryo is transferred to the uterus. This type of genetic testing can be performed on eggs, sperm, or embryos.
FAQ (or What are the Risk Factors?)
What are autosomal recessive disorders?
A disorder is caused by an abnormality in one of the individual's genes. One copy of the gene, inherited from each parent, must be altered for a person to develop the disorder; this is called autosomal recessive inheritance.
An autosomal disorder may occur if both parents carry a copy of an altered gene for that condition, but show no signs or symptoms themselves. The risk of passing on the abnormal gene to their children is 25 percent for each pregnancy. Each child born to these parents has a 25 percent chance of inheriting both copies of the mutated gene and developing the disease; a 50 percent chance of inheriting one altered gene and being a carrier like the parents; or a 25 percent chance of inheriting neither copy of the gene and not being affected by the disorder.
What are X-linked disorders?
When a gene abnormality is located on the X chromosome, it is called an X-linked disorder. Because females have two copies of the X chromosome but males have only one, they are more frequently affected by X-linked disorders. The abnormal gene often occurs spontaneously rather than being inherited from carrier parents. Males get only one copy of the gene so there is no chance for them to inherit another copy from their parents. Females who inherit an abnormal gene are carriers because they usually do not show symptoms unless they also get a defective gene from their other parent.
What are the symptoms of a genetic disorder?
Symptoms vary greatly depending on the type and severity of the condition, as well as other factors such as your age and overall health. Some common symptoms include joint problems; organ disorders; mental retardation; seizures...
Many couples choose to increase their risk awareness by screening for known family-specific diseases before pregnancy. Genetic testing enables carriers to confirm that they will not pass on a mutated copy of a specific disease-causing gene to their children. Most doctors recommend that both prospective parents undergo carrier screenings and discuss the results with an experienced physician or genetics counselor. At this time, prenatal tests can be performed if needed, such as preimplantation genetic diagnosis (PGD).
Where can I get more information?
For further information on this condition or gene testing, you should consult a genetic counselor, who is trained in evaluating family histories and providing advice on how to reduce health risks associated with genetic diseases. Your physician can provide a list of specialists in your area.