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Genetic Testing #2

GENETIC TESTING                                                    

What is genetic testing?

Genetic testing is the use of laboratory tests to determine the genetic status of individuals already suspected to be at high risk for a particular genetic disorder based on family history or a positive screening test.

What is genetic screening?

Genetic screening is tile use of simple diagnostic tests performed on a large number of individuals to identify those who are at a high risk of having or passing on a specific genetic disorder.

What is the difference between genetic testing and genetic screening?

 Genetic testing and screening are similar in that both involve the use of laboratory tests to reveal the presence of disease-causing genes. Although genetic screening and testing are essentially the same since they both involve the same medical procedures, the major difference between them can be explained in examining WHY an individual undergoes laboratory testing.

  • Genetic screening – If someone desires to be tested due to the possibility that he or she may have a disease gene because a large percentage of people in the same age group or ethnic group are at high risk for having the gene, this individual would need to undergo genetic screening. Screening is often called “population-based” screening since it is used to test those individuals in the population who are at a higher risk for having a disease-gene.
  • Genetic testing – If the individual suspects he or she may have a disease gene as a result of a family member having the gene, this person would need to undergo genetic testing. Genetic testing is used not to screen a population already at risk, but to “test” individuals for the presence of a specific gene.

 What is done in a gene test?

In gene tests, scientists scan a patient’s DNA sample for mutated sequences. A DNA sample can be obtained from any tissue, including blood. For some types of gene tests, researchers design short pieces of DNA called probes, whose sequences are complementary to the mutated sequences. These probes will seek their complement among the three billion base pairs of an individual’s genome. If the mutated sequence is present in the patient’s genome, the probe will bind to it and flag the mutation. Another type of DNA testing involves comparing the sequence of DNA bases in a patient’s gene to a normal version of the gene.

Who can benefit from genetic testing and screening?

  • Those who are concerned that they may have a genetic or chromosomal disorder because of a specific condition in their families
  • Couples who already have a child with a genetic disorder, unexplained mental retardation or a birth defect
  • Women who have had two or more miscarriages or whose baby dies in infancy or deliver a child after 35yrs
  • Couples who would like testing or information about genetic disorders that occur frequently in ethnic group
  • Pregnant women concerned about the effects of exposure to medication, chemicals or radiation
  • Couples who are first cousins or other blood relatives.

Key Points on Genetic Testing

  • Everyone may not need genetic testing, but those with a history of genetic disorders within the family should consider testing.
  • Genetic screening and testing has both positive and negative consequences.
  • It’s important to weigh the advantages and disadvantages when considering genetic screening or testing.
  • The value of genetic testing depends on several factors: the accuracy of the test, the reliability of the interpretation of the results, the ability to treat the condition and the quality of genetic counselling available.
  • Genetic tests can provide medical information that affects the entire family.
  • Testing has implications for future reproductive decisions, usually providing a probability of disease, rather than predicting future “diagnoses.”
  • Reliable genetic tests can predict the chance of having a disease gene with high accuracy. However, not everyone with a disease gene may go on to develop a disease or condition due to possible effects of other genes or environmental factors.
  • Informed consent, the communication of information between a patient and health care provider about a genetic test, can be an important component of genetic testing, yet there are no national standards to guide health care providers on how this information should be given.
  • Although physicians have not agreed on standards for the testing of children, some experts think that testing should be conducted only when there is clear benefit to the child. Others feel that testing should also be considered if it can benefit other family members.
  • The results of a genetic test can confirm or rule out a suspected genetic condition or help determine a person’s chance of developing or passing on a genetic disorder.

What screening tests are available?

  • Chronic villus sampling (CVS) – usually done at 10 to 12 weeks of pregnancy to obtain a sample of the placenta by passing a plastic tube through the vagina and into the uterus or by passing a needle through the abdomen and into the uterus. This allows doctors to diagnose many of the same conditions as amniocentesis, but earlier in the pregnancy.
  • Amniocentesis may be done at 13 to 18 weeks of pregnancy and is a widely-used procedure of obtaining amniotic fluid from the uterus by using a needle to pass through the abdomen.
  • Ultrasound – usually performed as early as possible in pregnancy, and is a non-invasive procedure that provides a visual image of the foetus.
  • Embryo biopsy can be used on an embryo conceived by in vitro fertilization to determine if the embryo is free of certain genetic disorders before it is implanted in the uterus. Increased risk for disease. Because genetic conditions often run in families, information about your genetic makeup might be useful to other family members. If family members are aware that a genetic condition runs in the family, it might prevent them from being misdiagnosed. This information might also be of use to them when they are planning children.

TYPES OF GENETIC TESTING

  • Newborn screening is used just after birth to identify genetic disorders that can be treated early in life. Newborn screening identifies biochemical or other inherited conditions in newborns that may result in mental retardation or other complications. Newborn screening is an effective measure in preventing mental retardation. However, only a handful of disorders are screened for since screening all newborns is expensive.
  • Diagnostic testing is used to identify or rule out a specific genetic or chromosomal condition. In many cases, genetic testing is used to confirm a diagnosis when a particular condition is suspected based on physical signs and symptoms. Diagnostic testing can be performed before birth or at any time during a person’s life, but is not available for all genes or all genetic conditions. The results of a diagnostic test can influence a person’s choices about health care and the management of the disorder.
  • Carrier testing is used to identify people who carry one copy of a gene mutation that, when present in two copies, causes a genetic disorder. This type of testing is offered to individuals who have a family history of a genetic disorder and to people in certain ethnic groups with an increased risk of specific genetic conditions. If both parents are tested, the test can provide information about a couple’s risk of having a child with a genetic condition. Carrier screening is often considered by couples who want to have children, but who are concerned that they may “carry” a gene for a certain disorder that has the potential to affect their children.
  • Prenatal testing is used to detect changes in a fetus’s genes or chromosomes before birth. This type of testing is offered during pregnancy if there is an increased risk that the baby will have a genetic or chromosomal disorder. In some cases, prenatal testing can lessen a couple’s uncertainty or help them make decisions about a pregnancy. It cannot identify all possible inherited disorders and birth defects, however. Prenatal screening is available to those individuals who have a higher risk for passing a disease gene on to the child. It is used to determine the genetic make-up of an unborn child.
  • Pre-implantation Genetic Diagnosis (PGD) is a specialized technique that can reduce the risk of having a child with a particular genetic or chromosomal disorder. It is used to detect genetic changes in embryos that were created using assisted reproductive techniques such as in-vitro fertilization. In-vitro fertilization involves removing egg cells from a woman’s ovaries and fertilizing them with sperm cells outside the body. To perform preimplantation testing, a small number of cells are taken from these embryos and tested for certain genetic changes. Only embryos without these changes are implanted in the uterus to initiate a pregnancy.
  • Predictive and presymptomatic types of testing are used to detect gene mutations associated with disorders that appear after birth, often later in life. These tests can be helpful to people who have a family member with a genetic disorder, but who have no features of the disorder themselves at the time of testing.
  • Predictive testing can identify mutations that increase a person’s risk of developing disorders with a genetic basis, such as certain types of cancer.
  • Presymptomatic testing is used for predicting adult-onset disorders. It can determine whether a person will develop a genetic disorder, before any signs or symptoms appear. It is also for estimating the risk of developing adult-onset genetic disorders (e.g. Huntington’s, Alzheimers)
  • Forensic testing uses DNA sequences to identify an individual for legal purposes. Unlike the tests described above, forensic testing is not used to detect gene mutations associated with disease. This type of testing can identify crime or catastrophe victims, rule out or implicate a crime suspect, or establish biological relationships between people (for example, paternity).

PROS & CONS of Genetic Testing