The Yale University School of Medicine
Patient's Guide to Medical Tests
Detailed Descriptions Of The Most Diagnostic Procedures















 24
Maurice J. Mahoney, MD, JD










Genetic Diseases




















Tests Covered in This Chapter








Chromosome Analysis
Chromosome Karyotype, cytogenetics.


506






Gene Analysis
DNA analysis, direct gene analysis


508






Linkage Analysis


510






The case of Donna M., 28, and her 5-month-old son, Billy:
Because I'm relatively young and neither my family nor my husband's had any history of genetic disease, I had no reason to have prenatal testing. So I was really distressed when Billy was born with a cleft palate, a defect of the eye called coloboma, and poor muscle tone. The doctors did some tests and diagnosed a chromosomal disorder.

Billy spent three weeks in the hospital. His oxygen level needed constant monitoring and he required tube feeding. But then we took him home and gradually he grew stronger and healthier until we were able to take him off the monitor and begin bottle-feeding him.
The Tests Doctors analyzed the chromosomes in samples of blood from Billy and his parents. Chromosomes are collections of genes-- strings of chemicals that program every trait and function in the body. Once in a while a chromosome breaks. While machinery in the cell usually repairs the break, in rare instances two broken chromosomes exchange pieces before the repair is made and form what is called a translocation. The tests showed that Billy and Donna both have translocations. Donna's is called a balanced translocation because the exchanged chromosomes have not lost or damaged any genes crucial to health, body form, or structure. Because its overall effect is benign, she was never aware of it. Billy, however, has what is called an unbalanced translocation, because genetic material is either missing, damaged, or overabundant. This usually occurs when one parent, in handing down half of his or her 46 chromosomes, passes on only one of the two or more rearranged chromosomes. The health effects caused by such an imbalance depend on which genes are involved.

The Outcome Although it's too early to predict, Billy may suffer profound retardation and his organs may not develop normally. Or his problems could be comparatively minor: learning disabilities, slight behavioral disorders, and mild physical deformities that would not be considered handicaps. The appearance of his ears, eyes, or the creases around his mouth or eyes may be a little abnormal, or his fingers may be longer or shorter than normal. So far, the muscle tone in his upper body is still weak, and his development is a little slow, but doctors won't be able to measure his mental function until he is about 2 years old. They will also monitor him for any physical defects that can be corrected surgically.




INTRODUCTION


  • Every cell in the human body contains about 100,000 genes, the units of hereditary material that determine the regulation of body processes and a person's physical characteristics, such as hair color or height (see figure 24.1) Except for eggs and sperm, each cell possesses an identical gene set. These genes are arranged in 23 pairs of tiny strings, called chromosomes. A person's entire set is inherited: In every pair, one chromosome comes from the mother and one from the father.
  • FIGURE 24.1
    Blueprints of Life




    The tightly coiled strands of DNA that make up each of the body's millions of genes carry that individual's unique genetic code.Genes are responsible for individual characteri stics; myriad hereditary or genetic diseases also may be carried by these genes.




  • Of these 23 sets, 22 are arranged in matching pairs, called autosomal chromosomes. The 23rd pair, the sex chromosomes, are matching in women but not in men. The female sex chromosomes consist of two strings, known as XX, while the male chromosomes are known as XY. Since the mother can only contribute an X chromosome, it is always the father who determines the sex of a child, depending on whether he contributes an X or a Y.


  • GENETIC DISORDERS


    Genetic disorders may be influenced by several factors. When a disease runs in a family, faulty genes are passed from parent to child. Or a change in the genetic material—a mutation— may occur during formation of an egg or sperm cell, as happened in the cases of Donna M. and her son, Billy. Mutations may also appear during fetal development. In these cases, children with genetic abnormalities may be born to parents without genetic disorders.

    The major types of genetic disorders are described below.

    Chromosomal disorders are abnormalities in the number or structure of the chromosomes. These disorders are often severe and include mental retardation and a variety of physical deformities. Examples include Down syndrome, Turner syndrome, and Klinefelter syndrome.

    Single-gene (or Mendelian) disorders are caused by a defect in a single gene or pair of genes. Most of these are rare, but together they cause significant illness. They may vary greatly in severity and may not produce symptoms until late in life. These conditions include the following:

    Autosomal dominant disorders such as Marfan syndrome and Huntington disease, which occur when one gene is abnormal and the corresponding one is normal.

    Autosomal recessive disorders such as cystic fibrosis, sickle-cell anemia, and Tay-Sachs disease, which usually occur only when both genes in a pair are defective. While children in these cases may suffer genetic disease, both parents will be healthy because their defective genes are paired with normal copies.

    X-linked disorders such as hemophilia, color blindness, and some muscular dystrophies, which are caused by genes located on the X chromosome. Women are less prone to X-linked disorders because if they have an abnormal X chromosome, they usually have a normal one to compensate for it. However, a man who inherits an abnormal X chromosome with a defective gene will develop the disease even if the gene is recessive, since his Y chromosome can't compensate for it.

    Multifactorial (or polygenic) genetic diseases are the most common but the least understood of all genetic diseases. Although a person inheriting one or more of these genes has an increased risk of a particular disorder, environmental factors will determine whether the disease manifests itself. This category includes such common chronic disorders as coronary heart disease, diabetes mellitus, schizophrenia, and cleft palate.

    Somatic gene disorders are conditions in which gene abnormalities develop only in certain cells. Genetic defects responsible for these disorders are not inherited; rather, they occur in the developing fetus or at some time after birth. For example, cancer is usually a somatic gene disorder in which only genes in the diseased tissue are aberrant.



    HOW YOUR DOCTOR DIAGNOSES GENETIC DISORDERS


    • Now that many more diseases are recognized as having a genetic component, genetic testing is playing an increasing role in diagnosing diseases of all the organ systems. Disease detection, however, is only one reason for genetic testing (see the accompanying box).
    For most disorders, the doctor first orders tests to evaluate the diseased organ and only later to establish whether the disorder has


    Why Genetic Tests Are Done
    The main uses of genetic testing are as follows:

    • To determine whether apparently healthy individuals carry genes that will cause disease later in life or predispose them to deve loping a particular disorder. The results may help determine if there is a need for periodic monitoring or measures to help prevent severe complications. For example, people who carry the genes for familial polyposis, a hereditary disorder that increases the risk of colon cancer, may undergo regular screening for the early detection of this malignancy.

    • To screen fetuses for genetic disorders. While some disorders discovered this way may be treated in the womb, in most cases their detection offers the parents the option of terminating the pregnancy or being prepared for the birth of an affected child (see chapter 25).

    • To screen newborns for about a dozen genetic diseases that can be treated or even prevented if treatment is begun early. These include sickle-cell anemia, galactosemia, hypothyroidism, and phenylketonuria (PKU).

    • To screen people who may carry defective genes and are planning to have children, in order to evaluate their risk of passing on a genetic disease.

    • To diagnose certain genetic conditions. Even when no treatment is available, a diagnosis may help predict the outcome of the disorder or put an end to unnecessary testing.

    • To increase the success of organ transplantation. The closer the match between the genetic types of donor and recipient, the lower the risk of the donor organ being rejected by the recipient's immune system.


    • genetic origins. For example, if you have had a heart attack at a relatively young age, initial tests will evaluate the heart while further tests may be used to detect familial hypercholesterolemia (an inherited tendency to extremely high cholesterol levels), which may be an underlying cause.

    There are cases, however, when doctors believe that disorders are largely or primarily due to a genetic defect. For example, if a newborn baby has recurrent pneumonia, cystic fibrosis may be suspected. In this case, tests aimed at clarifying the genetic origin of the disease are ordered immediately.

    Family History

    If you suffer what seems to be a genetic disorder, your doctor will want to know about your first-degree relatives (parents, siblings, and children), with whom you share certain genes. The doctor will ask about their age and health and, if they are deceased, what the cause of death was. Certain racial and ethnic groups are more susceptible to particular genetic diseases, so identifying your heritage is also crucial.

    Genetic disorders may be diagnosed indirectly, by examining your signs and symptoms, or directly, by examining your genetic tissue (see table 24.1) In the indirect approach, the doctor studies your family history and orders tests that are not specifically genetic. For example, if you are diagnosed with a certain type of kidney disease, the doctor may establish its hereditary origin by simply drawing your family tree and indicating the disorder's distribution among family members.


    TABLE 24.1 Some Genetic Disorders and Their Diagnosis


    Disorder
    Diagnostic Tests
    Basal-cell nevus syndrome
    X-rays of skull and skeleton.


    Cystic fibrosis


    Sweat test, direct gene analysis.


    Familial hypercholesterolemia


    Cholesterol in blood.


    Familial hyperparathyroidism


    Calcium in blood, parathyroid hormone.


    Familial polyposis of the colon


    X-ray of colon, colon, colonoscopy, gene linkage.


    Fragile X syndrome


    Chromosome analysis, direct gene analysis.


    Gilbert disease


    Bilirubin in blood.


    Hemophilia


    Clotting factors in blood, linkage, direct gene analysis.


    Hereditary hemorrhagic telangiectasia


    X-ray of lungs.


    Huntington disease


    Direct gene analysis.


    Muscular dystrophies


    Electromyography (EMG), muscle biopsy, creatine phosphokinase (CPK), electrocardiogram (ECG), direct gene analysis when available.


    Osteogenesis imperfecta


    X-rays of bones. Collagen analysis using cultured skin cells.


    Polycystic kidney disease


    Urinalysis, ultrasound, renal arteriogram, blood pressure measurement, direct gene analysis.


    Tay-Sachs disease


    Enzyme analysis (hexosaminidase A), direct gene analysis.




    Genetic Tests

    When doctors know what function the problematic gene performs in the cell and what proteins it makes, the disorder can be diagnosed by measuring these proteins or enzymes, known as gene products, with standard tests used to analyze blood, urine, or other body fluids. If the genetic material contains an error, the gene product may be abnormal or present in unusual amounts. For example, Tay-Sachs disease is


    DID YOU KNOW?
    Gene analysis has become more widely available thanks to a method of testing called polymerase chain reaction, or PCR. This swift, automated technique helps expand minute amounts of DNA thousands to hundreds of thousands of times. The technique is particularly useful when material available for analysis is scarce, because it allows gene analysis to be performed on genetic material extracted from a single cell. Some forensic medicine specialists now use it to analyze bits of evidence found at crime scenes.


    linked to decreased levels of the enzyme hexosaminidase A. In children and adults, the disease can be diagnosed by measuring the concentration of the enzyme in blood, and in fetuses, by measuring levels in amniotic fluid.

    The tests described in this chapter examine either chromosomes or individual genes. But while chromosome analysis is performed in numerous medical centers, direct gene analysis is expensive and only beginning to reach the public. The advantage of gene analysis is that it can sometimes lead to a more definitive diagnosis. For example, chromosomal abnormalities are detected in only a small percentage of all mentally retarded people; in a larger percentage, the retardation is probably caused by a single gene. (Other significant factors are environmental--fetal alcohol syndrome, lead poisoning, birth trauma, and so on.)


    • The cost of gene analysis is expected to drop soon as the tests become routinely available in hospitals and commercial laboratories throughout the country.
    CHROMOSOME ANALYSIS
    General information
    Where It's Done
    Who Does It
    How Long It Takes
    Discomfort/Pain
    Doctor's office, hospital, or commercial laboratory.


    Doctor, nurse, or technician.
    Usually less than 5 minutes.
    Slight (from drawing blood or obtaining tissue sample).
    Results Ready When
    Special Equipment
    Risks/Complications
    Average Cost
    3 days to 2 weeks.
    Supplies for drawing blood, possibly a biopsy tool, possibly an aspiration needle (for amniocentesis).


    Negligible for drawing blood; other risks depend on which tissue cells are extracted for analysis.
    $$
    Other names
    Chromosome karyotype or cytogenetics


    Purpose
    To look for the cause of birth defects, mental retardation, or retarded growth.

    To detect chromosomal disorders in the fetus.

    To reveal the cause of infertility or repeated miscarriages.

    To evaluate couples with chromosomally abnormal children.

    To evaluate women who aren't menstruating.

    To diagnose certain cancers or evaluate their course and the effectiveness of treatment.


    How it works
    Chromosomes isolated from cells are examined, and their number and structure are evaluated.


    Preparation
    None for analysis of blood, skin, or bone marrow samples. See chapter 25 for pregnancy-related tests.


    Test procedure
    Cells are removed to examine their chromosomes. These cells can be from the blood, skin, or bone marrow or, in a pregnant woman, from the placenta, amniotic fluid, or chorionic villi. The cells are cultured, and a karyotype (a picture of the chromosomes) is created.


    After the test
    The care you receive depends on the procedure used to obtain a tissue sample for chromosome analysis. (See other chapters for individual tests.)

    The slides with chromosomes are preserved for several years until they fade and can no longer be used. Photographs will last much longer.


    Factors affecting results
    Chromosomes that are too condensed or stretched out may be difficult or impossible to evaluate in number or structure.

    Samples may contain insufficient cells or ones that divide infrequently.

    Fetal cells may be mixed with the mother's cells.


    Interpretation

    A doctor or lab technician examines the chromosomes for abnormalities in structure or number.

    Some chromosomes can be analyzed by fluorescentin situ hybridization (FISH), a new procedure that uses a DNA probe and is being rapidly added to the traditional ways of analyzing chromosomes.

    In mosaicism, in which abnormal chromosomes are interspersed with normal ones, the ratio of the two types is used to predict the severity of the genetic disorder. This is difficult in prenatal diagnosis.


    Advantages
    The test is completely accurate for most chromosomal disorders.


    Disadvantages
    It's expensive.

    It detects abnormalities only in chromosomes and not in single genes, so it often fails to produce a diagnosis.

    It requires a high level of technical expertise and specialized equipment.


    The next step
    A positive diagnosis is considered definitive. This may lead to further counseling, treatment, and, if appropriate, reproductive decisions. If the result is negative, further tests (eg, MRI, enzyme tests, and urine biochemistry) may be ordered.


    GENE ANALYSIS
    General information
    Where It's Done
    Who Does It
    How Long It Takes
    Discomfort/Pain
    Doctor's office, hospital, or commercial laboratory.


    Doctor, nurse, or lab technician.
    Usually less than 5 minutes.
    Slight (from drawing blood or obtaining tissue sample).
    Results Ready When
    Special Equipment
    Risks/Complications

    Average Cost

    2-4 weeks.
    Supplies for drawing blood or a cheek brush (for removing cheek cells); rarely, a biopsy tool.


    Almost negligible risk associated with obtaining tissue cells.
    $$$
    Other names
    DNA analysis or direct gene analysis


    Purpose
    To diagnose diseases caused by defective genes (see table 24.1). In some cancers, to predict the course of the disease.

    To detect genes that may cause or increase the risk of disease later in life or be passed on to offspring.

    To detect a severe genetic disorder in a fetus when there is a family history.


    How it works

    Blood or other tissue is analyzed for a known disease-causing gene by using specific chemicals that bind to defective genes.


    Preparation
    None for blood test. For tests on fetal tissue, See chapter 25.


    Test procedure
    Tissue or blood is taken for genetic analysis.


    After the test
    If blood has been taken, follow the normal procedure for drawing blood (see chapter 4) Otherwise, care depends on the procedure used to obtain a tissue sample


    DID YOU KNOW?
    A relatively new technique, fluorescent in situ hybridization (FISH), allows chromosomes and genes to be analyzed simultaneously. Scientists apply a fluorescent dye, called a genetic probe, that attaches itself only to its exact copy. So a probe containing a colon-cancer gene will attach to and light up another colon-cancer gene. More than 50 different colors can be used to identify genes or sets of chromosomes.
    Factors affecting results
    Samples are sometimes contaminated with someone else's DNA.


    Interpretation
    Genetic material is examined for alterations, rearrangements, or deletions in genes that cannot be seen under a microscope.

    In such cancers as acute lymphoblastic leukemia and non-Hodgkin's lymphoma, identifying specific genetic changes helps establish the diagnosis and, in some cases, prognosis


    Advantages
    It provides a definitive diagnosis of disorders caused by known genetic defects


    Disadvantages
    It's expensive.

    It's limited to disorders for which specific genes have been identified.

    It's not widely available for many genes, although this is changing rapidly.


    The next step
    Both positive (abnormal gene present) and negative (abnormal gene not present) findings are considered definitive. A positive test result may lead to further counseling, treatment, and, if appropriate, reproductive decisions. If the result is negative, further tests may be ordered to investigate the problem. They will vary with the symptoms.


    LINKAGE ANALYSIS
    General information


    Where It's Done
    Who Does It
    How Long It Takes
    Discomfort/Pain
    Doctor's office, hospital, or commercial laboratory.


    Doctor, nurse, or lab technician.
    Less than 5 minutes.
    Slight (from drawing blood or obtaining tissue sample).
    Results Ready When
    Special Equipment


    Risks/Complications
    Average Cost
    2-4 weeks.
    Supplies for drawing blood or a or cheek brush (for removing cheek cells); rarely, a biopsy tool.
    Almost negligible risk associated with obtaining tissue cells.
    $-$$
    Other names
    None


    Purpose
    To detect genes that may cause or increase the risk of disease later in life or be passed on to offspring.

    To detect a severe genetic disorder in a fetus when there is a family history of it.


    How it works
    DNA "markers" linked to the problem genes are identified in your chromosomes and those of your close relatives.


    Preparation
    None.


    Test procedure
    Blood is usually drawn (although other tissue samples may be taken) from the patient and several of his or her relatives.

    The DNA of affected and unaffected relatives is extracted and analyzed to determine which DNA structures are most likely to represent markers of the disease-causing gene.

    The patient's DNA is examined for the presence of the markers.


    After the test
    If blood has been taken, follow the normal procedure for drawing blood (see chapter 4). Otherwise, the care you receive depends on the procedure used to obtain a tissue sample.


    Factors affecting results
    Correctly identifying how people in a particular family are related to each other is crucial to accurate results.

    The test will have limited value if close relatives crucial to establishing linkage relationships are not available.

    Occasionally, DNA segments may have changed places when the subjects' eggs and sperm were formed, causing the markers to move from their usual locations.


    Interpretation
    The presence of DNA markers can help predict whether a fetus or a person with no symptoms carries the defective genes that run in the family.

    If the markers are not found, the presence of abnormal genes cannot be ruled out because DNA structures in the vicinity of the gene may have been rearranged when inherited.

    A small number of disorders have gene markers that are found in most people with the disease (and in few people without the disease), but in most cases the markers are specific to a particular family, and will differ from family to family.

    In an experimental approach with limited availability, results can be used to make cancer treatment decisions or take preventive measures.


    Advantages
    The test makes it possible to diagnose genetic abnormalities when the gene causing the disorder has not yet been identified.


    Disadvantages
    It's expensive.

    It's available in only a few medical centers and for a limited number of disorders.

    Close relatives must be available for testing.

    It's laborious, more time-consuming, and less reliable than direct gene analysis.


    The next step
    Both positive (abnormal gene present) and negative (abnormal gene not present) findings are considered highly informative. Either result may lead to further counseling, treatment, and, if appropriate, reproductive decisions. Direct gene analysis may be suggested in the future if it becomes available for the gene in question.