Mayo Clinic Guide to a Healthy Pregnancy: From Doctors Who Are Parents, Too!

CHAPTER 20. Genetic screening

Genetic screening is one of those topics that parents often don’t like to discuss because it can be frightening and complicated. No parent wants to read or hear about things that could go wrong with a child. Don’t be frightened, though, if this topic comes up. Many care providers will address the issue of genetic screening during an early prenatal appointment. Keep in mind that most babies are born healthy with no problems.

WHAT IS IT?

Tests are available that offer expectant and new parents an opportunity to explore whether their future or newborn child may be at risk of a genetic disorder. These tests are called genetic screening tests.

Genetic screening allows a couple to test for inherited genetic diseases, as well as genetic problems that occur without any family history. Genetic screening can be done before conception, during pregnancy or immediately after an infant is born.

DIFFERENT TYPES OF GENETIC TESTS

The process is often the same, but genetic testing is done for different reasons.

Diagnostic testing. If you have symptoms of a disease that may be caused by a genetic abnormality, diagnostic testing can confirm that you have the suspected disorder. Genetic testing is available for an ever-increasing number of diseases. Some disorders for which genetic testing may be used to confirm a diagnosis include polycystic kidney disease, Marfan syndrome and neurofibromatosis.

Presymptomatic screening for adult disorders. If you have a family history of a genetic condition, undergoing genetic testing before you develop symptoms may show if you’re at risk of developing that condition. Examples include testing for inherited breast and ovarian cancers (BRCA 1 and 2) and Huntington disease.

Carrier testing. If you or your partner has a family history of a genetic disorder, such as cystic fibrosis or spinal muscular atrophy, you may choose to have genetic testing before you have children. Carrier testing can determine if you carry a copy of an altered gene that would put a child at risk of developing the disorder.

Prenatal testing. If you’re pregnant, tests are available that can detect abnormalities in your fetus. Spina bifida and Down syndrome are two congenital abnormalities that are common enough that many mothers consider such testing. Prenatal tests for fetal abnormalities are discussed in Chapter 21.

Newborn screening. This is the most common type of genetic screening. Newborn screening is performed in all 50 states and the District of Columbia. The tests screen for specific conditions, such as congenital hypothyroidism and phenylketonuria. Newborn testing is important because if a disorder is found, treatments that may prevent symptoms can begin immediately. These tests are discussed in Chapter 16.

There are different types of genetic screening tests, including family-history-based screening, ethnic-based screening, prenatal screening and newborn screening. Screenings based on family history and ethnicity are done before conception or early in pregnancy. Prenatal screening, discussed in Chapter 21, is done in the first trimester or early second trimester. Newborn screening, discussed in Chapter 16, is typically done when an infant is a day old.

Family-history-based and ethnic-based genetic screening tests are designed to identify people who don’t have signs and symptoms of a specific disease, but who carry a copy of an altered gene or chromosome that they could pass on to their children, possibly leading to disease.

Prospective parents who are worried about possible genetic disorders in their children can undergo genetic screening before or shortly after a baby is conceived. With this information, potential parents can consider what the risk is of having a child with a genetic disease and make decisions accordingly.

If you’re planning to become pregnant and you or your partner has a family history of a specific genetic disorder, you might consider family-history-based screening. You might consider ethnic-based screening if you’re part of a racial or ethnic group in which a genetic disease is known to be more common.

ISSUES TO CONSIDER

The decision to pursue genetic screening is a personal one. Consider these questions as you make your decision.

8 key questions

1.            Is there a family history of a certain disorder? Some families have a history of a known genetic disease.

2.            Are you in a racial or ethnic group that puts you at higher risk of being a carrier for a certain disorder?

3.            How will you use the information if you find that you and your partner are carriers of the same genetic condition?

4.            Do your religious or spiritual beliefs affect your view of genetic testing?

5.            How might you respond emotionally to the testing process?

6.            Will the test be covered by your insurance? If not, will you be able to afford the testing? Many genetic tests are relatively expensive. Be aware that tests used for purposes other than diagnosing a condition may not be covered by insurance.

7.            Will the time it takes to complete the test give you adequate time to make decisions regarding beginning or continuing a pregnancy?

8.            Would discussing the options and issues related to genetic carrier screening with a genetic counselor, geneticist or other care provider benefit you? These individuals are specially trained in human genetics and genetic counseling.

GENETIC DISEASES

Here are some diseases that result from a gene abnormality and that can be identified with genetic testing.

Alpha-thalassemias result in a deficiency of red blood cells (anemia). The most severe form results in fetal or newborn death. Most cases are much less severe.

Beta-thalassemias also result in anemia. In its most severe form (thalassemia major), children require regular blood transfusions. With proper treatment, most people with this condition live into adulthood. Less severe forms cause varying degrees of complications related to the need for more red blood cells.

Sickle cell disease prevents blood cells from moving smoothly through the body. Affected infants have an increased susceptibility to infection and slow growth rates. The disease can cause bouts of severe pain and damage to vital organs. With early and consistent medical treatment, complications can be minimized.

Cystic fibrosis affects the respiratory and digestive systems, causing severe chronic respiratory disease, diarrhea, malnutrition and exercise limitation. Recent treatments enable most affected people to live into adulthood.

Duchenne muscular dystrophy affects the muscles of the pelvis, upper arms and upper legs. In later stages of the disease, it may also affect the muscles of the diaphragm and heart. Because it’s an X-linked disease, it occurs in young boys and is the most common form of muscular dystrophy affecting children. Females may have less severe symptoms of the disease. In severe cases, the muscle weakness can lead to death in the late teenage years or early adulthood.

Fragile X syndrome is the most common genetically inherited cause of mental retardation. It results from alterations on the X chromosome. Carriers may have mild symptoms including infertility and movement problems (ataxia). The genetics of Fragile X is complicated and should be discussed with someone who’s very familiar with the disorder.

Canavan disease is a severe condition of the nervous system that’s usually diagnosed soon after a child’s birth. Death usually occurs in early childhood.

Tay-Sachs disease is a condition in which the enzyme needed to break down certain fats (lipids) is absent. These substances build up and gradually destroy brain and nerve cells until the central nervous system stops working. Death usually occurs in early childhood.

FAMILY HISTORY SCREENING

If a certain genetic disorder runs in your family, you may wish to be tested to determine whether you carry the genetic abnormality that may put your future children at risk of inheriting the disorder.

An inherited genetic disorder may result from a problem with a single gene. This includes autosomal recessive diseases, X-linked diseases, some adult-onset genetic diseases and diseases that may be mild in a parent but more severe in a future child. Inherited disorders may also result from changes in the structure of a chromosome or changes in the mitochondrial DNA, a small piece of DNA inherited only from the mother. Some of these problems are discussed in more detail in the remainder of this chapter.

Carrier testing for autosomal recessive diseases. Autosomal recessive diseases occur when both parents are carriers of an abnormal gene. Carriers have one normal and one abnormal gene. The normal gene is able to compensate for the abnormal gene, so carriers generally have no symptoms. However, if both you and your partner contribute an abnormal gene for the same disease to the fetus, the baby will be affected with the disease. Testing for autosomal recessive diseases may be done for specific ethnic groups in which certain recessive diseases are more common or if there’s a family history of an autosomal recessive disease. Examples include cystic fibrosis and Tay-Sachs disease.

AUTOSOMAL RECESSIVE DISORDER can result when both parents carry a single altered gene for the same disease. If both parents pass the altered gene to the child, the child is affected by the disease.

Presymptomatic testing for adult-onset autosomal dominant diseases. These disorders occur when a person has one abnormal copy of a gene. Some autosomal dominant diseases don’t cause symptoms until adulthood, and occasionally not until after the person carrying such a gene has had children. If a person is found to carry the disease, each of his or her children has a 50 percent risk of inheriting the disease. Examples include Huntington disease and some inherited cancers.

X-LINKED RECESSIVE DISORDERS result from an altered gene carried on one of the mother’s X chromosomes.

X-linked disorders. These disorders result from an altered gene residing on the X chromosome. With rare exceptions, a woman carries two X chromosomes and a man an X and a Y chromosome. A woman may be a carrier of an X-linked disorder, but have no symptoms or mild symptoms because the normal gene on her other X chromosome provides most of the needed gene function. However, the woman has a 50 percent chance of passing the altered chromosome on to her child. If the child is a boy, he will develop the disease because boys have only one X chromosome. If the child is a girl, she, like her mother, will be a carrier. Examples of X-linked disorders include Duchenne muscular dystrophy, hemophilia A and Fragile X syndrome.

Chromosome structural abnormalities. In some families, babies are born with birth defects caused by missing or duplicated pieces of a chromosome. This can happen if a parent carries a structural chromosome arrangement that’s different from normal. If the parent passes on an abnormal arrangement in which the genetic structure is unbalanced, it could result in severe problems. A chromosome rearrangement can also lead to more miscarriages than expected in a family. If you have a family member who’s experienced a high number of miscarriages, mention this to your care provider.

Mitochondrial disorders. Some disorders are caused by errors in a separate set of genes found in mitochondria, the energy-producing organs of the cell. Since mitochondria are inherited only from the mother, most of these diseases are passed from mother to child. Mitochondrial disorders can have many different signs and symptoms, such as low blood sugar, muscular problems, blindness and seizures.

HOW IT’S DONE

Often, genetic screening requires only a blood sample. A needle is inserted into a vein in your arm and blood is drawn. In newborns, blood is collected via a heel prick. Occasionally, a swab sample from the inside of your cheek may be needed. Rarely, a skin or muscle biopsy is needed. The sample is sent to a lab for analysis.

With some genetic tests — those in which both parents need to be carriers for the disorder to occur — testing may be done in just one partner to begin with. If the results are normal, the second partner isn’t tested. If testing in the first partner finds an abnormality, then it’s recommended the other partner be tested.

UNDERSTANDING THE RESULTS

If test results indicate that you’re not a carrier, no special precautions are necessary. However, don’t take the results as a guarantee that your child will be healthy. Genetic carrier screening doesn’t identify all carriers. Some diseases have many mutations, and the test may focus only on the most common mutations.

Just the opposite, the test may tell you you’re a carrier of an altered gene, but not reveal how severe the disease may be if one of your children develops it. If an increased risk is indicated, your care provider, a geneticist or a genetic counselor can help you understand the implications of the disease and assess your options.