Genetic Counseling and Genetic Testing

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Our knowledge of human genetic diseases and disorders has expanded rapidly in the past 20 years. Victor McKusick's Mendelian Inheritance in Man now lists more than 13,000 human genetic diseases, disorders, and traits that have a simple genetic basis. Research has provided a great deal of information about the inheritance, chromosomal location, biochemical basis, and symptoms of many of these genetic traits. This information is often useful to people who have a genetic condition.

Genetic Counseling

Genetic counseling is a new field that provides information to patients and others who are concerned about hereditary conditions. It is also an educational process that helps patients and family members deal with many aspects of a genetic condition. Genetic counseling often includes interpreting a diagnosis of the condition; providing information about symptoms, treatment, and prognosis; helping the patient and family understand the mode of inheritance; and calculating probabilities that family members might transmit the condition to future generations. Good genetic counseling also provides information about the reproductive options that are available to those at risk for the disease. Finally, genetic counseling tries to help the patient and family cope with the psychological and physical stress that may be associated with their disorder. Clearly, all of these considerations cannot be handled by a single person; so most genetic counseling is done by a team that can include counselors, physicians, medical geneticists, and laboratory personnel. Table 6.4 lists some common reasons for seeking genetic counseling.

Genetic counseling usually begins with a diagnosis of the condition. On the bases of a physical examination, biochemical tests, chromosome analysis, family history, and other information, a physician determines the cause of the condition. An accurate diagnosis is critical, because treatment and the probability of passing on the condition may vary, depending on the diagnosis. For example, there are a number of different types of dwarfism, which may be caused by chromosome abnormalities, single-gene mutations, hormonal imbalances, or environmental factors. People who have dwarfism resulting from an autosomal dominant gene have a 50% chance of passing the condition to their children, whereas people with dwarfism caused by a rare recessive gene have a low likelihood of passing the trait to their children.

When the nature of the condition is known, a genetic counselor sits down with the patient and other family members and explains the diagnosis. A family pedigree may be constructed, and the probability of transmitting the condition to future generations can be calculated for different family members. The counselor helps the family interpret the genetic risks and explains various reproductive options that are available, including prenatal diagnosis, artificial insemination, and in vitro fertilization. A family's decision about future pregnancies frequently depends on the magnitude of the genetic risk, the severity and effects of the condition, the importance of having children, and religious and cultural views. The genetic counselor helps the family sort through these factors and facilitates their decision making. Throughout the process, a good genetic counselor uses nondirected counseling, which means that he or she provides information and facilitates discussion but does not bring his or her own opinion and values into the discussion. The goal of nondirected counseling is for the family to reach its own decision on the basis of the best available information.

Genetic conditions are often perceived differently from other diseases and medical problems, because genetic conditions are intrinsic to the individual person and can be passed on to children. Such perceptions may produce feelings of guilt about past reproductive choices and intense personal dilemmas about future choices. Genetic counselors are trained to help patients and family members recognize and cope with these feelings. __

Concepts B

Genetic counseling is an educational process that provides patients and their families with information about a genetic condition, its medical implications, the probabilities that other family members may have the disease, and reproductive options. It also helps patients and their families cope with the psychological and physical stress associated with a genetic condition.

[Table 6.3 Common reasons for seeking genetic counseling

1. A person knows of a genetic disease in the family.

2. A couple has given birth to a child with a genetic disease, birth defect, or chromosomal abnormality.

3. A couple has a child who is mentally retarded or a close relative is mentally retarded.

4. An older woman becomes pregnant or wants to become pregnant. There is disagreement about the age at which a prospective mother who has no other risk factor should seek genetic counseling; many experts suggest that any prospective mother age 35 or older should seek genetic counseling.

5. Husband and wife are closely related (e.g., first cousins).

6. A couple experiences difficulties achieving a successful pregnancy.

7. A pregnant woman is concerned about exposure to an environmental substance (drug, chemical, or virus) that causes birth defects.

8. A couple needs assistance in interpreting the results of a prenatal or other test.

9. Both parents are known carriers for a regressive genetic disease.

www.whfreeman.com/pierce Information on genetic counseling and human genetic diseases, as well as a list of genetic counseling training programs accredited by the American Board of Genetic Counseling

Genetic Testing

Improvements in our understanding of human heredity and the identification of numerous disease-causing genes have led to the development of hundreds of tests for genetic conditions. The ultimate goal of genetic testing is to recognize the potential for a genetic condition at an early stage. In some cases, genetic testing allows early intervention that may lessen or even prevent the development of the condition. In other cases, genetic testing allows people to make informed choices about reproduction. For those who know that they are at risk for a genetic condition, genetic testing may help alleviate anxiety associated with the uncertainty of their situation. Genetic testing includes newborn screening, heterozygote screening, presymptomatic diagnosis, and prenatal testing.

Newborn screening Testing for genetic disorders in newborn infants is called newborn screening. Most states in the United States and many other countries require that newborn infants be tested for phenylketonuria and galac-tosemia. These metabolic diseases are caused by autosomal recessive alleles; if not treated at an early age, they can result in mental retardation, but early intervention—through the administration of a modified diet — prevents retardation (see p. 000 in Chapter 5). Testing is done by analyzing a drop of the infant's blood collected soon after birth. Because of widespread screening, the frequency of mental retardation due to these genetic conditions has dropped tremendously. Screening newborns for additional genetic diseases that benefit from treatment, such as sickle-cell anemia and hypothyroidism, also is common.

Heterozygote screening Testing members of a population to identify heterozygous carriers of recessive disease-causing alleles, who are healthy but have the potential to produce children with the particular disease, is termed heterozygote screening.

Testing for Tay-Sachs disease is a successful example of heterozygote screening. In the general population of North America, the frequency of Tay-Sachs disease is only about 1 person in 360,000. Among Ashkenazi Jews (descendants of Jewish people who settled in eastern and central Europe), the frequency is 100 times as great. A simple blood test is used to detect Ashkenazi Jews who carry the Tay-Sachs allele. If a man and woman are both heterozygotes, approximately one in four of their children is expected to have Tay-Sachs disease. A prenatal test for the Tay Sachs allele also is available. Screening programs have led to a significant decline in the number of children of Ashkenazi ancestry born with Tay-Sachs disease (now fewer than 10 children per year in the United States).

Presymptomatic testing Evaluating healthy people to determine whether they have inherited a disease-causing allele gene is known as presymptomatic genetic testing. For example, presymptomatic testing is available for members of families that have an autosomal dominant form of breast cancer. In this case, early identification of the disease-causing allele allows for closer surveillance and the early detection of tumors. Presymptomatic testing is also available for some genetic diseases for which no treatment is available, such as Huntington disease, an autosomal dominant disease that leads to slow physical and mental deterioration in middle age (see introduction to Chapter 5). Presymptomatic testing for untreatable conditions raises a number of social and ethical questions (Chapter 18).

Several hundred genetic diseases and disorders can now be diagnosed prenatally. The major purpose of prenatal tests is to provide families with the information that they need to make choices during pregnancies and, in some cases, to prepare for the birth of a child with a genetic condition. A number of approaches to prenatal diagnosis are described in the following sections.

Ultrasonography Some genetic conditions can be detected through direct visualization of the fetus. Such visualization is most commonly done with ultrasonography—usually referred to as ultrasound. In this technique, high-frequency sound is beamed into the uterus; when the sound waves encounter dense tissue, they bounce back and are transformed into a picture ( FIGURE 6.15). The size of the fetus can be determined, as can genetic conditions such as neural tube defects (defects in the development of the spinal column and the skull) and skeletal abnormalities.

Amniocentesis Most prenatal testing requires fetal tissue, which can be obtained in several ways. The most widely used method is amniocentesis, a procedure for obtaining a

I 6.15 Ultrasonography can be used to detect some genetic disorders in a fetus and locate the fetus during amniocentesis and chorionic villus sampling. (SIU School of Medicine/Photo Research.)

Genetic Testing

The New Genetics

A couple are seeking help at a clinic that offers preimplantation genetic diagnosis (PGD), which combines in vitro fertilization with molecular analysis of the DNA from a single cell of the developing embryo, and permits the selection and transfer to the uterus of embryos free of a genetic disease. Before PGD, the only alternative for those wishing to prevent the birth of a child with a serious genetic disorder was early chorionic villus sampling or amniocentesis, followed by abortion if the fetus had a disorder.

Consider a couple at risk of having a second child with severe combined immune deficiency (SCID). A child born with this condition has a seriously impaired immune system. As recently as 20 years ago, those affected died early in life, but the use of bone-marrow transplantation, which can provide the child with a supply of healthy blood stem cells, has greatly extended survival. In general, the earlier the transplantation and the closer the tissue match of the marrow donor, the better a recipient child's chances.

The couple tell the medical geneticist that they are seeking his help in identifying and transferring only embryos free of the SCID mutation so that they can begin their pregnancy knowing that it will be healthy. Some weeks later they reveal another reason for their interest in this technology: the health of their six-year-old daughter, who is affected with SCID, is on a downward course despite one partly matched bone-marrow transplant earlier in her life. Their child's best hope of survival is another bone-marrow transplant, using tissue from a compatible donor, preferably a sibling. Is it possible, they ask, to test the healthy embryos for tissue compatibility and transfer only those that match their daughter's type?

The geneticist responds that it is indeed technically possible to do so but he wonders whether helping the couple in this way is ethically appro priate. Is it right to conceive a child for this purpose? In addition, because tissue compatibility is not a disease, would responding to this request constitute an unwise step into a world of positive, or "enhancement," genetics, where parents' desires, not medical judgment, dictate the use of genetic knowledge?

PGD offers significant new reproductive opportunities for families or persons affected by genetic disease. However, the very power of this technology raises new ethical issues that will grow in importance as PGD and related embryo manipulation procedures become more widely available. PGD offers a technology that is medically, psychologically and, in the view of many, morally superior to the existing use of abortion for genetic selection.

The case described here is not entirely novel. Even before the advent of PGD, couples who had sought to insure the birth of a child whose HLA (human leukocyte antigen) status could be compatible with that of an existing sibling would establish a pregnancy, undergo testing, and then abort all fetuses that did not have the appropriate HLA type. Because a woman in the United States has a right to abortion for any reason through the second trimester, this option is legal. However, it is certainly not a desirable one from a medical or psychological point of view.

Because pregnancy is never begun, PGD avoids the emotional trauma of abortion. Although some would object to the discarding of human embryos even at this early stage, the selection of viable embryos and the discarding of others is a routine part of in vitro fertilization procedures today the and raises few moral questions in the minds of most people. So, from the narrow perspective of parental decision making, the alternative described in this case is a significant medical and moral step

by Ron Green forward. We should not lose sight of this fact as we consider other ethically troubling aspects of the case.

This case of parental selection raises at least two distinct questions. First, even if the means of selection is relatively innocuous from a moral standpoint, is it appropriate for parents to bring a child into being at least partly for the purpose of saving the life of a sibling? A second question is whether genetic professionals should cooperate with a selection process that entails a nondisease trait.

With regard to the first question, some people believe that the parents' wishes in this situation violate the ethical principle not to use a person merely as a means to an end, as well as the modern principle of responsible parenthood, which judges each child to be of inestimable value. They also worry about future psychological harm to a child conceived in this way. Others argue that children are usually born for a specific purpose, whether it's to gratify the parents' need for a family, to cement the relationship of a couple, or whatever else. As a result, they argue that the important question is not whether the purpose for which the child was conceived is eithical but whether the parents will be able to accept the child in its own unique identity once it is born.

In response to the second question, some ethicists see any involvement in nondisease testing as a dangerous diversion of genetic testing down paths long since rejected for good reason. They see that a consensus has emerged in popular opinion and among geneticists and ethics advisory boards that nondisease characteristics should not be subject to prenatal testing. HLA testing, however well intentioned, runs counter to thus concensus. Departures from these views about genetic tests could raise very difficult questions about eugenics in the future. __

| Under the guidance of ultrasound, a sterile needle is inserted through the abdominal wall into the amniotic sac.

| A small amount of amniotic fluid is withdrawn through the needle.

^ The amniotic fluid contains fetal cells, which are separated from the amniotic fluid.

.and cultured.

| Under the guidance of ultrasound, a sterile needle is inserted through the abdominal wall into the amniotic sac.

| A small amount of amniotic fluid is withdrawn through the needle.

^ The amniotic fluid contains fetal cells, which are separated from the amniotic fluid.

.and cultured.

Chromosomal analysis

|5| Tests are then performed on the cultured cells to detect errors of metabolism, analyze DNA,...

6.16 Amniocentesis is a procedure for obtaining fetal cells for genetic testing.

|5| Tests are then performed on the cultured cells to detect errors of metabolism, analyze DNA,...

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A Beginner's Guide to Healthy Pregnancy. If you suspect, or know, that you are pregnant, we ho pe you have already visited your doctor. Presuming that you have confirmed your suspicions and that this is your first child, or that you wish to take better care of yourself d uring pregnancy than you did during your other pregnancies; you have come to the right place.

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