BIOLOGY Volume 3 - Guide to General Biology - 2004

25. APPLIED GENETICS

25.7. Human Genetics

25.7.9. Genetic Screening and Prenatal Diagnosis

As noted in Section 25.7.1, every human being is potentially a carrier of several genetic defects. The search for mutant genes in patients is called genetic screening. With modern genetic techniques, this Procedure has become significantly easier than it was in the recent past. Genetic screening is particularly important in three situations discussed below: prenatal Diagnosis, carrier testing, and predictive diagnosis.

PRENATAL DIAGNOSIS. This term encompasses the full range of modern medical techniques used to detect any abnormalities in unborn infants, including the identification of Genetic Disorders. If such a disorder is detected, parents are counseled regarding the future child's quality of life and other potential challenges. Typically, parents are then given the option to consider the Termination of the Pregnancy.

CARRIER TESTING. This method involves identifying individuals who carry a single copy of a defective Gene, meaning they are heterozygous carriers. It is likely that all of us are heterozygous for several genetic disease genes. However, if young couples planning to marry have family histories of the same hereditary disease (such as Sickle-Cell Anemia), they should be advised to undergo testing to determine if they are both carriers. If heterozygosity is confirmed, they can be informed that There is a 1-in-4 probability that their child will be born with the condition. A genetic counselor can address all of the couple's concerns and discuss available options (Section 25.7.10). The range of conditions for which carrier testing is possible is constantly expanding; however, mass screening programs require substantial resources. Will healthcare services be able to implement a mass screening program for even a single disorder, such as cystic fibrosis?

PREDICTIVE DIAGNOSIS. This term refers to the diagnosis of a disease that is not yet clinically manifest, but which, based on the identified genotype, is expected to develop in the future. A classic example of such a "genetic time bomb" is Huntington's disease, in which symptoms typically become noticeable in adulthood. A single dominant gene is responsible for this condition, and anyone who lives long enough will eventually develop the disease; however, such fatality does not apply to many other conditions. For instance, people may be genetically predisposed to Heart disease and Lung Cancer, but they can often avoid these illnesses through exercise, proper diet, and refraining from smoking. Individuals who wish to do so can already test whether they carry the gene for Huntington's disease, and tests to assess heart disease risk will soon become widely available. Society must proactively address A number of related ethical and legal questions. For example, do insurance companies have the right to require clients to undergo genetic testing before issuing a life insurance policy? Will such companies be permitted to penalize those with an increased risk of disease? Furthermore, as testing advances, the demand for Genetic Counseling and its various Applications will inevitably grow. This issue is discussed at the end of this section.

How is genetic screening performed? The four most important Methods are chorionic villus sampling, amniocentesis, pre-implantation genetic diagnosis, and DNA analysis using gene probes.

Chorionic Villus Sampling (CVS)

The chorion is the outermost extraembryonic membrane, forming a thin layer of Cells surrounding the embryo. Chorionic villi (Hair-like projections on its surface) form part of the Placenta and are bathed in maternal Blood. This allows oxygen and nutrients to pass into the embryonic blood, while waste products are removed. The technique was developed in China in the 1970s (Fig. 25.30). The abdominal approach (transabdominal sampling) is most commonly used (Fig. 25.30, A), which involves inserting a hollow needle through the abdominal and uterine walls under local anesthesia. In the cervical approach (Fig. 25.30, B), tissue is sampled using a thin, flexible tube called a catheter, and no anesthesia is required. A syringe is attached to the tube or needle to gently aspirate small pieces of tissue. The procedure is performed under ultrasound guidance in both cases.

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Fig. 25.30. Obtaining a chorionic villus sample. A. Transabdominal sampling. B. Transcervical sampling. The sample length is several millimeters.

All chorionic cells, like embryonic cells, originate from the zygote and are therefore genetically identical to the embryo. Because chorionic cells divide rapidly during early development, they can be used immediately for chromosome analysis (karyotyping). This allows the diagnosis of chromosomal abnormalities such as Down, Klinefelter, or Turner syndromes. It is also possible to determine the sex of the fetus, which is critically important if an X-linked condition is suspected. The harvested cells can be cultured in a suitable medium and used for DNA analysis to detect conditions such as cystic fibrosis, Huntington's disease, or thalassemia.

Cells grow rapidly, and results can be obtained within 5–12 days. A major advantage of this method is that it is performed during early pregnancy, between 8 and 14 weeks. However, the risk of fetal injury is higher with this method than with amniocentesis. It is estimated that the risk of injury is approximately 2% higher than the baseline rate of natural Miscarriage. If testing leads to the decision to terminate the pregnancy, the procedure is physically and emotionally less taxing and risky than later abortions performed following amniocentesis.

Amniocentesis

Amniocentesis (sampling Amniotic Fluid for subsequent analysis) is an older and more widely available method than chorionic villus sampling, having been first used in 1967. It is typically performed at 15–16 weeks of gestation, though it can be done earlier or later—up to 18 weeks. It cannot be performed before there is a sufficient volume of amniotic fluid and cells for analysis. The method is safer than chorionic villus sampling, increasing the risk of spontaneous miscarriage by only about 0.5% (roughly 1 in 200). A hollow needle is inserted through the abdominal wall into the uterine cavity under ultrasound guidance using local anesthesia (Fig. 25.31). Approximately 20 cm³ of amniotic fluid is aspirated from the amniotic sac. The amniotic sac wall is elastic, and the puncture site seals itself. The fluid is normally clear and yellow, resembling urine. The fetus continuously swallows amniotic fluid, which then passes through its digestive tract, meaning viable fetal cells are shed into the fluid. Additionally, the fluid contains amniotic cells which, like the chorion, originate from the zygote and are genetically identical to the fetus. These cells are pelleted in a centrifuge and cultured, yielding A large number of dividing cells suitable for chromosomal and DNA analysis. The procedure takes 3–4 weeks, which is a major drawback if the test results indicate a need for termination. Amniotic fluid is also analyzed for the presence of defective gene products, such as α-fetoprotein, elevated levels of which indicate an increased risk of certain birth defects, such as cleft palate.

Fig. 25.31. A physician collects an amniotic fluid sample from a pregnant woman. In the other hand, the physician holds an ultrasound transducer used to determine THE POSITION OF the fetus and placenta within the Uterus. The fetal image is visible on the display screen in the Background.

Pre-implantation Genetic Diagnosis (PGD)

This is a form of prenatal diagnosis performed on an embryo prior to its implantation in the uterus. PGD is an integral step in In vitro Fertilization (IVF) programs (Section 21.7.4). Following extracorporeal fertilization, at approximately the 8-cell stage, a single cell can be removed from the developing embryo without compromising its capacity for continued normal development. Based on the test results, clinicians determine whether the specific embryo can be implanted in the mother's body, thereby eliminating The Need for subsequent termination of pregnancy. However, this expensive procedure is not always successful and is unlikely to become a routine clinical practice.

Gene Probes and DNA Analysis

Genetic screening that involves examining an individual's DNA is referred to as DNA analysis. It consists of the following steps (Fig. 25.32).

1. First, DNA is extracted from cells. Cells are obtained either from the fetus using the methods described above, or from a child or adult, for example, from blood or saliva samples.

2. Next, the DNA is cleaved into fragments of varying lengths using Restriction Endonucleases (restriction Enzymes).

3. The fragments are separated by size using agarose gel Electrophoresis, as described previously (Fig. 25.4). Smaller fragments move much more rapidly toward the end of the gel than larger ones.

4. The aim of this stage is to determine which fragment contains the DNA sequence of interest. This is achieved using a gene probe. Because the probe does not move easily through the gel, a technique known as Southern blotting is employed (developed by Edward Southern in 1975). The procedure is illustrated in Fig. 25.32 (see also the section on genetic fingerprinting, 25.7.12).

The DNA in the gel is first rendered single-stranded (denatured) by heating. It is then transferred onto a nitrocellulose or nylon filter via blotting, yielding an exact replica of the gel. A stack of paper towels acts as a wick that draws the buffer through the sponge, the gel, and the filter, ensuring the DNA becomes firmly bound to the filter.

5. Next, a gene probe is applied (Section 25.1.6). This is a short, single-stranded DNA fragment whose base sequence is complementary to part of the gene of interest. Probes can be obtained in large quantities through cloning. They are radiolabeled with ³²P and then added to a solution to hybridize (bind complementarily) with the DNA on the nitrocellulose or nylon filter.

6. Finally, autoradiography is performed by placing the filter against X-ray film. Radiation from any band to which the probe has bound will expose the film (Fig. 25.32). Many Mutations result in chromosomal deletions, which in turn reduce the length of the restriction fragment. Such a fragment is more mobile in the gel and produces a distinct band on the autoradiogram. This method can be used to identify the gene for sickle-cell anaemia.

Fig. 25.32. Several steps of DNA analysis in genetic screening. During Southern blotting, denatured (single-stranded) DNA is transferred from an agarose gel to a nitrocellulose or nylon filter. Nowadays, paper towels are rarely used; instead, electroblotting or vacuum Blotting techniques are applied.

Other Methods

Ultrasound scanning can be used to detect severe developmental abnormalities, many of which are hereditary, such as Spina bifida.

Maternal blood analysis during pregnancy makes it possible to measure chemical substances (such as α-fetoprotein) associated with specific inherited fetal disorders.

Advantages of Genetic Screening

As noted earlier, despite the obvious benefits of genetic screening, it also raises a number of issues. Below we examine both the Advantages and disadvantages of this method. All of the points listed warrant careful thought and Structure/133.html">Discussion.

1. Genetic screening helps prevent tragic family situations. It can be argued that we have no moral right to bring a child into the world when we know in advance that they will suffer. Genetic screening offers couples a choice, allowing them to decide whether or not to proceed with a pregnancy involving a genetic defect.

2. Genetic screening reduces national economic costs. For example, it has been estimated that the current annual expenditure of the UK National Health Service on caring for a child with cystic fibrosis is around £40,000.

3. While many people wish to be tested, the UK National Health Service is not yet able to offer cystic fibrosis screening on a mass scale. Nevertheless, 80–90% of individuals who are offered the test choose to undergo screening.

4. If individuals know they are genetically predisposed to a particular condition, they can adopt a lifestyle that minimizes their risk of developing the disease. For instance, someone aware of a predisposition to CORONARY HEART DISEASE might choose to quit smoking.

5. Disease screening programs, when understood and embraced by society, have successfully lowered disease incidence. A notable example is thalassemia screening in Cyprus. Thalassemia, which like sickle-cell anaemia is caused by a recessive gene, results in a severe hemoglobinopathy; however, being a carrier of this gene confers increased resistance to malaria. Thalassemia was highly prevalent in Cyprus until a screening program for women was introduced. Women carrying an affected fetus were informed and advised to consider an abortion.

6. The UK National Health Service offers free amniocentesis or chorionic villus sampling to all pregnant women over the age of 35, due to the significantly higher probability of giving birth to a child with Down's syndrome.

Issues Associated with Genetic Screening

1. In time, we will become able to predict who is at high risk of suffering a heart attack or developing rheumatoid Arthritis. Will insurance companies or employers demand access to individual genotype data and potentially discriminate against those predisposed to severe illnesses, even if the disease never actually develops? Will companies that choose not to use testing data be placed at a disadvantage? Should legislation be enacted to guarantee genetic privacy?

2. Genetic defects vary widely in the severity of the syndromes they cause, forming a continuous spectrum. Who should decide where the boundary lies between severe and moderate disorders? In the UK, abortions are permitted beyond 24 weeks of gestation in cases of “severe abnormalities.” Should abortion be permitted—or supported by screening programs—in the case of a treatable condition such as phenylketonuria?

3. Individuals suffering from Hereditary diseases often feel like marginalized members of society. There is a risk that as our success in combating these diseases grows and the number of people with genetic defects decreases, individuals with genetic anomalies will feel increasingly alienated. Their worldview differs significantly from that of healthy people, and their life values are also distinct. Society must learn to respect the Perspectives of patients. Are our own preconceptions about life values perhaps flawed?

4. The eugenics Movements of the early 20th century demonstrated the dangers associated with genetic knowledge. Eugenics is The Study of how to improve the genetic quality of a species, with selective breeding being one of its primary methods. Negative eugenics involves eliminating harmful genes, whereas positive genes involve adding advantageous ones. The Nazis, for instance, justified the extermination of mentally and physically disabled individuals under the noble guise of creating a genetically “pure” race. Instances of racial discrimination based on genetics are well documented. If eliminating harmful genes becomes standard practice, might it not inspire geneticists to develop methods for “custom-designing” children to parental specifications—such as musical, athletic, or mathematical talent?

5. Screening programs aimed at eradicating diseases can increase social pressure on women or couples, compelling them to undergo abortions against their own wishes. This occurred with some women in Cyprus as a consequence of the thalassemia screening program.

6. When a disease is prevalent within a specific ethnic group, screening programs can exacerbate discrimination. This happened during sickle-cell anaemia screening in the United States in the 1970s.

7. What is currently considered a minor deviation from the norm may later be viewed as an undesirable trait, such as a low intelligence quotient (IQ).

8. How accurate must a test be for it to be implemented? Is it ethical, for example, to tell someone there is an 80% probability they will develop a fatal disease?

9. What should a geneticist tell an individual diagnosed with a late-onset hereditary disorder that leads to physical disability and a relatively rapid fatal outcome (such as Huntington's disease)?

10. If a couple decides not to terminate a pregnancy after learning they will bring a child with a hereditary disorder into the world, should they cover the cost of Treatment, or should society bear the expense? In 1963, Francis Crick predicted that a day would come when we would need a license to have children. Under a law enacted in China in 1995, couples at high risk for hereditary defects are not permitted to have a child without special authorization. In the United States, it has been proposed to strip health insurance from couples who, despite information regarding fetal abnormalities, choose to proceed with the pregnancy.

11. Could children sue parents who allowed them to be born knowing they would be ill? In the United States, successful lawsuits brought by children against their parents regarding other medical issues are well documented.

12. Genes that will later exert a detrimental effect on an individual's life may confer survival advantages during early life.

13. Current techniques make it possible to determine a person's sex, enabling parents to eliminate a fetus of an undesired sex.