Chicago Center for Jewish Genetics Disorders - About Jewish Genetic Disorders

A Technical Overview on Screening and Testing

There are two main types of genetic "testing": screening and diagnostic testing:

Genetic screening surveys a population for persons possessing certain genotypes that are associated with a disease, that predispose to a disease, or may lead to disease in their descendents. These individuals are often asymptomatic. Screening is not considered diagnostic; in other words, someone who has a positive test needs to have the diagnosis confirmed by a diagnostic test. It is also referred to as population-based screening. For example, population-based screening is recommended for the Ashkenazi Jewish population for several disorders known to occur at a higher incidence in this specific population than in the general population.
Diagnostic testing is performed to identify (diagnose) a disease in an individual. Testing would be done on an individual with symptoms or an individual with a suggestive family history of a disorder. For example, consider a family with a child with Gaucher disease. The siblings of this child might be tested to see if they are also affected. If the specific mutation(s) known to cause disease in the family are tested for, a negative test would definitively mean that an individual does NOT have one or both of the mutations and these mutations could not be passed either on to their children.

Before Undergoing Screening or Testing: Some Things to Consider

Before an individual undergoes genetic testing, there are several technical aspects to understand. First, there are two main types of genetic testing:

DNA mutational analysis: This type of testing looks at an individual's DNA to determine if there are mutations, or changes, in a specific gene that will result in a non-functional or poorly functional gene product (protein).
Enzyme assays: This type of testing measures the activity of a particular gene product (enzyme), in other words, how well the enzyme does its job. The level of enzyme activity is then compared to "normal" or "average" levels in individuals who have two normal copies of the gene. A person who is a carrier for a recessive disorder has one normal (non-mutated) copy of the gene and one mutated copy of the gene. Thus, they would be expected to produce about half the amount of functional product that a non-carrier individual would, or have 50% of normal enzyme activity. An individual who has two mutated copies of the gene will have much less gene product than is typically expected. An individual is not usually affected with symptoms of a disorder until his or her enzyme activity level is less than 10% of normal.

Second, genetic testing is a complicated process and results are not always conclusive. A test may not allow for the diagnosis of a disorder and further testing may be needed. Third, the accuracy of a genetic test depends both upon the reliability of the test and the correct interpretation of the results.

For example, in order for a test to be reliable it must find the best balance between detecting as many mutations as possible (sensitivity) and only detecting real mutations, not benign changes (specificity). Sensitivity is a measure of the how likely a test is to detect someone who is affected. Specificity is a measure of how likely a test is to detect ONLY individuals who are affected. If a test is too stringent in its criteria for a positive result (or when a person has either the mutation or the enzyme deficiency), it may miss individuals who are affected (False Negative). But if the criteria is lessened to make sure everyone who is affected will test positive, it may incorrectly indicate that someone is affected who is not (False Positive). A different balance is needed for carrier testing (higher sensitivity than specificity) versus that needed for diagnostic testing (higher specificity than sensitivity). Once the best balance has been found between specificity and sensitivity for the type of testing done, various additional factors need to be considered in order to correctly interpret a positive or negative result.

What does a negative test result mean?

DNA mutational testing: If a negative result is obtained when DNA mutational testing for a Mendelian disorder (a disorder caused by a single gene) is done, there are three reasons for the negative outcome: 1) The person truly does not have any mutation in the gene known to cause the disorder. 2) The person may have a mutation that was not tested for in the disease-causing gene. Oftentimes, only the most common mutations are tested for and a negative result does not completely rule out the possibility of a mutation; a rarer disease causing mutation that was not tested for could still be present. Thus, a negative result would not be a true negative. 3) The person may have a mutation in a different gene that causes a similar disorder. Since these other genes were not analyzed on the testing done, results would not reveal a mutation and would be negative while not truly being negative. This situation is called genetic heterogeneity.

Enzyme assays: For enzyme assays (testing measuring how well an enzyme is working), results are not defined as positive or negative, but are defined by what range they fall into: the normal range, the carrier range, and the affected range. If your results fall into the normal range, you are considered to be unaffected and not a carrier.

What does a positive test result mean?

DNA mutational analysis: There are two factors that should be considered when the result of a DNA mutational analysis for a Mendelian disorder (a disorder caused by a single gene) is positive. First, how penetrant is the mutation? In other words, how likely is it for an individual who has a mutation to develop the disorder? For example, Torsion Dystonia (a dominant disorder) is thought to have 30% penetrance. This means that only 30% of all individuals who have a mutation known to cause Torsion Dystonia ever develop symptoms. Second, if a person tests positive for a mutation, this usually gives no indication of the severity of the disorder in an individual. Individuals with the exact same mutation may have very different severity of the disorder, a phenomenon called Variable Expressivity.

Enzyme assays: For enzyme assays, results are not defined as positive, but are defined by what range they fall into. If results fall into the affected range, you are considered to be affected. If results fall into the carrier range, you are considered to be a carrier, and unaffected.

What does an ambiguous test result mean?

DNA mutational analysis: A final consideration when testing for a Mendelian disorder is the possibility of an ambiguous result. With DNA mutational analysis, sometimes an individual may be found with variation in the gene being tested and it is unclear if this change is associated with disease or not. Often, more research needs to be done before a mutation of this type can be determined to be benign or disease causing.

Enzyme assays: With enzyme assays, an ambiguous result can occur when the activity level is between the designated carrier level and non-carrier level, or when it is between the carrier level and the affected level. Either of these results is called an indeterminate result and further testing would be needed to clarify an individual's status.

DNA Mutational Analysis and Enzyme Assay Testing: A Comparison

When is DNA mutational analysis used and when are enzyme assays used? Often, one or both may be used depending upon what type of information is sought from testing.

Enzyme assays are most often used for diagnostic testing and not carrier screening. In other words, enzymatic testing is used to determine if a person is affected with a disorder. This is because an individual's enzymatic activity level often has been very low before they are affected. However, enzymatic assay testing may not be accurate for carrier screening. This is because the carrier (unaffected) range and normal range may overlap. It may also not be technically possible to do enzymatic testing of the protein product. For example, enzyme analysis is not available for Bloom's Syndrome testing.
DNA mutational analysis is often used for carrier screening when very few mutations are known to cause the majority of cases of disease within a specific population. For example, in Canavan disease, three mutations have been seen in 99% of individuals of Ashkenazi Jewish descent who are affected. These same three mutations are only seen in 50 to 60% of all individuals affected who are NOT Ashkenazi Jewish. Therefore, DNA mutational analysis for Canavan disease is highly accurate for an individual of this Ashkenazi Jewish ancestry but not of any another ethnicity.
Both enzyme assay and DNA mutational analysis might be used for the most complete and accurate testing. For example, both types of testing are commonly used for Tay-Sachs disease carrier screening. Enzyme analysis may give a false positive (this occurs in 2% of Ashkenazi Jews with enzyme analysis in the carrier or affected range) or it may give an indeterminate result (overlap between carrier and affected ranges or overlap between carrier and unaffected ranges). DNA mutational analysis detects 94% of mutations in individuals of Ashkenazi Jewish descent. The use of both DNA and enzyme analysis for Tay-Sachs increase the mutation detection rate up to 97 to 98% in both Ashkenazi Jews and non-Ashkenazi Jews.

What else should be considered?

It is important to note that most disorders are NOT Mendelian disorders, but rather are caused by a combination of genes and environmental factors. An individual may never develop the disorder associated with a specific mutation if they are not exposed to certain environmental factors, or if they do not have other genetic mutations that are necessary for the development of the disorder. In colon cancer, for example, it is estimated that 7 specific gene mutations must occur in one cell before that cell becomes cancerous. It is hypothesized that after the first mutation occurs, both genetic and environmental factors lead to mutations in the other 6 genes. All of these additional factors are called "risk modifiers."

It is equally important to note that there are emotional ramifications to genetic screening and testing. These should be considered prior to testing because the genetic information cannot be unlearned once it is obtained. For a more in-depth discussion of this topic, see Non-technical Aspects of Genetic Testing.

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