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Genetic Testing - Pregnancy, Childbirth, Andgenetic Testing

dna cells blood specific

There are thousands of genetic diseases, such as sickle-cell anemia, cystic fibrosis, and Tay-Sachs disease, that may be passed from one generation to the next. Numerous tests have been developed to help screen parents at risk of passing on genetic disease to their children as well as to identify embryos, fetuses, and newborns that suffer from genetic diseases.

Carrier Identification

Carrier identification is the term for genetic testing to determine whether a healthy individual has a gene that may cause disease if passed on to his or her offspring. It is usually performed on people considered to be at higher than average risk, such as those of Ashkenazi Jewish descent, who have a one in thirty chance of being Tay-Sachs carriers (in other populations the risk is about one in three hundred). Testing is necessary because many carriers have just one copy of a gene for an autosomal recessive trait and are unaffected by the trait or disorder. Only someone with two copies of the gene will actually have the disorder. So while it is widely assumed that everyone is an unaffected carrier of at least one autosomal recessive gene, it only presents a problem in terms of inheritance when two parents have the same recessive disorder gene (or both are carriers). In this instance the offspring would each have a one in four chance of receiving a defective copy of the gene from each parent and developing the disorder.

Another example of carrier identification is the test for a deletion in the dystrophin gene, which results in Duchenne muscular dystrophy. Carriers may avoid having an affected child by preventing pregnancy or by undergoing prenatal testing for Duchenne muscular dystrophy, with the option of ending the pregnancy if the fetus is found to be affected.

Using genetic testing to detect carriers poses some challenges. Typically, a carrier has inherited a mutant gene from one parent and a normal gene from another parent. If, however, the carrier harbors a mutation that is only found in germ cells (the sperm or eggs), and only in some of these germ cells, then conventional genetic testing, which is performed on white blood cells, will miss the mutation.

Preimplantation Genetic Diagnosis

Preimplantation genetic diagnosis (PGD) is a newer genetic test that enables parents undergoing in vitro fertilization (fertilization that takes place outside the body) to screen an embryo for specific genetic mutations when it is no larger than six or eight cells and before it is implanted in the uterus to grow and develop. Successful PGD was reported by Yury Verlinsky et al. in "Preimplantation Diagnosis for Sonic Hedgehog Mutation Causing Familial Holoprosencephaly" (New England Journal of Medicine, vol. 348, no. 15, April 10, 2003). The article reports on a couple at risk of transmitting holoprosencephaly, a disorder characterized by face and brain malformations that occurs in about one in every 16,000 live births and is responsible for one in 250 miscarriages. The disorder may cause facial disfigurement, such as a single eye or a malformed nose located above an eye, brain defects, or death. People with the disorder carry a mutation in a gene known as sonic hedgehog (shh), which also plays a role in some cancers.

The couple discovered they were at risk of passing the disease to their offspring when they gave birth to a son and daughter with the disorder. The daughter, who had a severe form of holoprosencephaly, died shortly after birth. The researchers tested the man's sperm cells and found that only some sperm cells carried the shh mutation. Since not all sperm cells were affected, the researchers speculated that the father had not inherited the mutation from his parents; rather, the mutation had arisen in his body after conception. Performing PGD on the couple's potential embryos allowed the researchers to select only those at low risk of disease and enabled the couple to give birth to a healthy girl. The success of PGD prompted the authors to conclude, "PGD can be recommended for any congenital disorder for which the causative gene is known, and when the couples would like to avoid traditional prenatal diagnosis and termination of pregnancy."

Prenatal Testing

Prenatal genetic testing enables physicians to diagnose diseases in the fetus. Most genetic tests examine blood or other tissue to detect abnormalities. An example of a blood test is the triple marker screen. This test measures levels of alpha fetoprotein (AFP), human chorionic gonadotropin (hCG), and unconjugated estriol, and can identify some birth defects such as Down's syndrome and neural tube defects. (Two of the most common neural tube defects are anencephaly—absence of the majority of the brain—and spina bifida—incomplete development of the back and spine.)

The fetal yolk sac and the fetal liver make AFP, which is continuously processed by the fetus and excreted into the amniotic fluid. A small amount crosses the placenta and can be found in maternal blood. Maternal screening for AFP levels is based on maternal age, fetal gestation, and the number of fetuses the mother is carrying. Elevated levels of AFP are associated with conditions such as spina bifida and low levels are found with Down's syndrome. Since AFP levels alone may not always detect disorders adequately, two other blood serum tests have been developed. Human chorionic gonadotropin (hCG) is a glycoprotein produced by the placenta. Normally, hCG is elevated at the time of implantation, but decreases at about eight weeks of gestation, and then drops again at approximately twelve weeks of gestation. Elevated levels of hCG are found with Down's syndrome. The placenta also produces unconjugated estriol. As with AFP, lower unconjugated estriol maternal serum levels also are found with Down's syndrome. Triple marker screen results are usually available within several days and women with abnormal results are often advised to undergo additional diagnostic testing such as chorionic villus sampling (CVS), amniocentesis, or percutaneous umbilical blood sampling (withdrawing blood from the umbilical cord).

CVS enables obstetricians and perinatologists (physicians specializing in evaluation and care of high-risk expectant mothers and infants) to assess the progress of pregnancy during the first trimester (the first three months). A physician passes a small, flexible tube called a catheter through the cervix to extract chorionic villi tissue—cells that will become the placenta and are genetically identical to the baby's cells. The chorion develops from trophoblasts, or the same cells as the fetus, and contains the same DNA and chromosomes. At the end of the eighth week of gestation, the chorionic sac is covered completely by trophoblastic villi, which degenerate by about the eleventh week of pregnancy. During this period, it is considered safe to remove a sample of chorionic villi for testing. The optimal time for CVS is between ten and eleven and a half weeks of gestation. The cells obtained via CVS are examined in the laboratory for indications of genetic disorders such as cystic fibrosis, Down's syndrome, Tay-Sachs, and thalassemia, and the results of testing are available within seven to fourteen days. CVS provides the same diagnostic information as amniocentesis; however, the risks (miscarriage, infection, vaginal bleeding, and birth defects) associated with CVS are slightly higher. Approximately one in every one hundred pregnancies is miscarried as a result of CVS.

Amniocentesis involves taking a sample of the fluid that surrounds the fetus in the uterus for chromosome analysis. An amniocentesis is usually performed at fifteen to eighteen weeks of gestation, although it can be done as early as twelve weeks. A pregnant woman has an abdominal ultrasound (an imaging study) to locate a pocket of amniotic fluid away from the fetus and placenta. About twenty milliliters of amniotic fluid is obtained when the physician inserts a hollow needle through the abdominal wall and the wall of the uterus. Fetal karyotyping, DNA analysis, and biochemical testing may be performed on the isolated fetal cells. Like CVS, amniocentesis samples and analyzes cells derived from the baby to enable parents to learn of chromosomal abnormalities, as well as the gender of the unborn child, about two weeks after the test is performed. The risk of miscarriage—about one in every two hundred pregnancies—resulting from amniocentesis is lower than the risk associated with CVS.

Using samples of genetic material obtained from amniocentesis or CVS, physicians can detect disease in an unborn child. Down's syndrome (also known as trisomy 21, because it is caused by an extra copy of chromosome 21) is the genetic disease most often identified using this technique. Down's syndrome is rarely inherited; most cases result from an error in the formation of the ovum (egg) or sperm, leading to the inclusion of an extra chromosome 21 at conception. As with prenatal diagnosis for most inherited genetic diseases, this use of genetic testing is focused on reproductive decision making.

The most invasive prenatal procedure for genetic testing is percutaneous umbilical blood sampling. Under high-resolution ultrasound, a sample of fetal blood is removed from the umbilical cord. This test can be performed from approximately sixteen weeks gestation to term. Percutaneous umbilical blood sampling poses the greatest risk to the unborn child—one in fifty miscarriages occur as a result of this procedure. It is used when a diagnosis must be made quickly. For example, when an expectant mother is exposed to an infectious agent with the potential to produce birth defects, it may be used to examine fetal blood for the presence of infection.

Genetic Testing of Newborns

The most common form of genetic testing is the screening of blood taken from newborn infants for genetic abnormalities. In the United States about four million newborns are screened every year for specific genetic disorders such as phenylketonuria (PKU) as well as other medical conditions that are only indirectly genetically linked, such as congenital hypothyroidism (underactive thyroid gland). PKU is an inherited error of metabolism resulting from a deficiency of an enzyme called phenylalanine hydroxylase. The lack of this enzyme can produce mental retardation, organ damage, and postural problems. Children born with PKU must pay close attention to their diets in order to lead healthy, normal lives.

TESTING GUIDELINES FOR INFANTS KINDLES DEBATE.

In "Panel to Advise Testing Babies for 29 Diseases" (New York Times, February 21, 2005), Gina Kolata reported that a federal advisory group recommendation advocating screening infants for twenty-nine rare medical conditions generated debate among consumers and health care professionals. The advisory group not only sought to expand newborn screening but also to standardize it from state to state. In 2005 some states screened for just four conditions while others tested newborns for as many as thirty-five diseases.

Advocates of expanded newborn screening asserted that while the twenty-nine conditions occur infrequently, there are effective, potentially lifesaving medical treatments for each condition. Opponents claimed that the efficacy of treatment had not been adequately evaluated or substantiated. They further cautioned that screening detects even those infants with the mildest forms of the disorders and essentially forces parents and health care professionals to consider treatment that may be more harmful than the mild disorder. Critics also wondered about the advisability of labeling infants with diagnoses of diseases from which they will never suffer.

Laboratory Techniques for Genetic Prenatal Testing

Genetic testing is performed on chromosomes, genes, or gene products to determine whether a mutation is causing or may cause a specific condition. Direct testing examines the DNA or the RNA that makes up a gene. Linkage testing looks for disease-causing gene markers in family members from at least two generations. Biochemical testing assays certain enzymes or proteins, which are the products of genes. Cytogenetic testing examines the chromosomes.

Generally, a blood sample or buccal smear (cells from the mouth) is used for genetic tests. Other tissues used include skin cells from a biopsy, fetal cells, or stored tissue samples. Testing requires highly trained, certified technicians and laboratories because the procedures are complex and varied, the technology is new and evolving, and hereditary conditions are often rare, so many testing techniques require special expertise. In the United States laboratories performing clinical genetic tests must be approved under the Clinical Laboratory Improvement Amendments (CLIA), passed by Congress in 1988 to establish the standards with which all laboratories that test human specimens must comply in order to receive certification. CLIA standards determine the qualifications of laboratory personnel, categorize the complexity of various tests, and oversee quality improvement and assurance. By 2005 more than 200 laboratories in the United States were performing genetic testing to detect and diagnose more than 300 conditions.

Fetal nucleated red blood cells are used for prenatal genetic testing because the DNA in these cells can be visualized directly via cell surface markers. The magnetic-activated cell-sorting technique and other separation methods use monoclonal antibodies that attach to specific cells. The most common antibodies studied are the transferrin receptor (CD71), the thrombospondin receptor (CD36), and glycophorin A (GPA). These antibodies attach to fetal cell surface antigens, allowing them to be separated from the maternal blood. The CD71 antibody is the most commonly used for fetal cell selection.

Chromosomes are identified by their size, centromere location, and banding position. Historically, the banding technique most widely used to identify chromosomes was giemsa-trypsin (GTG) banding. This technique arrests cells early in mitosis, which enables laboratory workers to more readily identify chromosomal aberrations. The GTG-banding technique has limitations in that it can achieve a resolution only at the single band level.

The polymerase chain reaction technique permits rapid cloning and DNA analysis and allows selective amplification of specific DNA sequences. A polymerase chain reaction can be performed in hours and is a sensitive test that may be used to screen for altered genes, but it is limited by the size and length of the DNA sequences that can be cloned.

Current DNA technology has made it possible to create fluorescent-labeled DNA probes that hybridize to specific gene loci. A fluorescent label is attached to the DNA probe that will bind to the complementary DNA strands. Fluorescence in situ hybridization (FISH) provides a unique opportunity to view specific genetic codes. The FISH technique may be used on cells and fluid obtained by CVS and amniocentesis as well as maternal blood. With this technique, a single strand of DNA is used to create a probe that attaches at the specific gene location. To separate the double-stranded DNA, heat or chemicals are used to break the chemical bonds of the DNA and obtain a single strand. (See Figure 6.1.)

There are three kinds of chromosome-specific probes: repetitive probes, painting probes, and locus-specific probes. Repetitive probes are the most commonly used. They produce intense signals by creating tandem repeats of base pairs. Painting probes are collections of specific DNA sequences that may extend along either part or all of an individual chromosome. These probe labels are most useful for identifying complex rearrangements of genetic material in structurally abnormal chromosomes. Probes that can hybridize to a single gene locus are called locus-specific probes. They can be used to identify a gene in a particular region of a chromosome. Locus-specific probes are used to identify a deletion or duplication of genetic material. The FISH procedure allows a signal to be visualized that indicates the presence or absence of DNA. The entire process can be completed in fewer than eight hours using five to seven probes. For this reason, FISH is frequently used as a rapid screen for trisomies and genetic disorders.

Although FISH is the most commonly used and most readily available prenatal diagnostic cytogenetic technique, it has limited ability to detect translocations, deletions, and inversions. Another method (known as microdissection FISH), also used for prenatal diagnosis, is more sensitive to these alterations. Microdissection FISH constructs probes to define specific regions of the human chromosome. Figure 6.2 shows how the microdissection technique is used to identify structurally abnormal chromosomes.

Newer technologies allow for the identification of all human chromosomes, thereby expanding the FISH application to the entire genome. Multiplex FISH applies combinations of probes to color components of fluorescent dye during metaphase to visualize each chromosome. This type of FISH procedure evolved from the whole-chromosome painting probes and uses a combination of fluorochromes to identify each chromosome. Just five fluorophores are needed to decode the entire complement of human chromosomes. Multicolor spectral karyotyping uses computer imaging and Fourier spectroscopy to increase the analysis of genetic markers. (See Figure 6.3.) Although FISH techniques FIGURE 6.1
Fluorescence in situ hybridization
SOURCE: "Fluorescence In Situ Hybridization," in Talking Glossary of Genetic Terms, U.S. Department of Health and Human Services, National Institutes of Health, National Human Genome Research Institute, http://www.genome.gov/Pages/Hyperion//DIR/VIP/Glossary/Illustration/fish.shtml (accessed February 16, 2005)
are considered highly reliable, there are limitations to multiplex and multicolor FISH, in that inversion or subtle deletions may be overlooked. The greatest advantage of multiplex FISH is its application in cases of dysmorphic infants. Spectral karyotyping techniques can detect cryptic unbalanced translocations that cannot be identified by other techniques.

GENETIC TESTING FOR SICKLE-CELL ANEMIA.

Sickle-cell anemia is an autosomal recessive disease that results when hemoglobin S is inherited from both parents. (When hemoglobin S is inherited from only one parent, the individual is a sickle-cell carrier.) Since normal and sickle hemoglobins differ at only one amino acid in the hemoglobin gene, a test called hemoglobin electrophoresis is used to establish the diagnosis.

Genetic testing for sickle-cell anemia involves restriction fragment length polymorphism (DNA sequence variant) that uses specific enzymes to cut the DNA. (See Figure 6.4.) These enzymes cut the DNA at a specific base sequence on the normal gene but not on a gene in which a mutation is present. As a result of this technique, there are longer fragments of sickle hemoglobin. Another technique known as gel electrophoresis sorts the DNA fragments by size. Autoradiography renders the DNA fragments by generating an image after radioactive probes have labeled the DNA fragments that contain the specific gene sequence. The location of the fragments distinguishes carrier status (heterozygous) from sickle-cell anemia (homozygous), or normal blood.

New Techniques Detect Fetal Gene Mutations

In 2005 an international team of investigators reported success with a technique that identifies fetal gene mutations such as beta-thalassemia from samples of maternal blood. The technique relies on the fact that circulatory fetal DNA sequences comprise fewer than 300 base pairs, while maternal DNA exceeds 500 base pairs. The investigators used PCR amplification to select for paternally inherited DNA sequences. Presence of the paternal mutant alleles for beta-thalassemia was then detected by allele-specific PCR.

FIGURE 6.2
How microodissection works
SOURCE: "How Does Microodissection Work?" in "Chromosome Microdissection Fact Sheet," U.S. Department of Health and Human Services, National Institutes of Health, National Human Genome Research Institute, http://www.genome.gov/10000204 (accessed February 16, 2005)

They tested the new technique on maternal blood samples from expectant mothers whose fetuses were at risk for beta-thalassemia because the father was a carrier for one of four beta-globin gene mutations. The results were verified by comparing the findings from chorionic villus sampling. The investigators also reported the cost-effectiveness of the new technique. Since it does not require complex machinery and relies on currently available technology, they estimated that the cost of a single analysis might be just $8.00 (Ying Li et al., "Detection of Paternally Inherited Fetal Point Mutations for β-Thalassemia Using Size-Fractionated Cell-Free DNA in Maternal Plasma," Journal of the American Medical Association, vol. 293, no. 7, February 16, 2005).

Another new technique reported in 2005 was analysis of amniotic fluid using oligonucleotide hybridization and FIGURE 6.3
Spectral karyotyping
SOURCE: "Spectral Karyotyping," in Talking Glossary of Genetic Terms, U.S. Department of Health and Human Services, National Institutes of Health, National Human Genome Research Institute, http://www.genome.gov/Pages/Hyperion//DIR/VIP/Glossary/Illustration/sky.shtml (accessed February 16, 2005)
microarray data analysis. (Figure 6.5 shows how micro-array technology is performed.) The researchers found that gene expression patterns correlated with the gender of the fetus, gestational age, and disease status. They asserted that this technology could assist in advancing human developmental research, as well as identifying new biomarkers for prenatal assessment (Paige Larrabee et al., "Global Gene Expression Analysis of the Living Human Fetus Using Cell-Free Messenger RNA in Amniotic Fluid," Journal of the American Medical Association, vol. 293, no. 7, February 16, 2005).

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