Genetic Testing Overview
Genetic Testing and Cancer
Medically reviewed by Dr. C.H. Weaver M.D. Medical editor
The expanding field of genetics and growing research linking mutations in specific genes to increased risk of cancer (cancer susceptibility genes) have led to an interest in predictive genetic testing. This testing may help identify people who are at an increased risk for developing certain types of cancer. While predictive genetic testing may provide information and benefits for some people, it also carries many limitations and risks. People considering undergoing genetic testing need to fully understand the process and its implications.
Genetics and Cancer
A gene is a hereditary unit of DNA that occupies a specific location on a chromosome. Genes carry directions to cells and tell them to make specific proteins that perform and regulate all body functions. Genes are capable of replicating themselves at each cell division. A mutation is a change in the usual DNA sequence of a particular gene. Mutations can be beneficial, harmful, or neutral. Many diseases, including cancer, begin in the genes. The genetic mutation that causes cancer can be inherited from a parent or it can be a random mutation that occurs as a result of a mistake during cell division or in response to environmental factors.
Current research suggests that only 5-10% of cancers are inherited. This hereditary influence begins with the genes that are passed from parent to child. Genes come in pairs, with one copy inherited from each parent. Parents can pass on a normal copy or, if they have one, an abnormal or mutated copy of a gene. Determining the probability of inheriting a gene mutation and/or of developing cancer as a result of a gene mutation is a complicated process that requires an understanding of heredity, genetics and the role of genes.
Predictive Genetic Testing
Modern technology has enabled us to identify relationships between specific genetic mutations and some cancers. As we continue to learn more about genetic mutations and identify additional mutations, the role of genetic testing will continue to grow.
Predictive genetic testing is used to determine if an individual has a genetic that may predispose him/her to developing cancer. An accurate test will reveal a genetic mutation, but cannot guarantee that a person will develop cancer. Likewise, a genetic test that does not find a specific mutation cannot guarantee that an individual will not develop cancer. These tests only suggest that a person may or may not be at some level of increased risk.
Genetic Counseling: Genetic counseling is crucial to the entire process of genetic testing. Individuals considering undergoing genetic testing should first meet with a genetic counselor. The genetic counselor has a multi-faceted role. Prior to testing, the genetic counselor can address individuals’ needs and concerns and educate people about what to expect from genetic testing. The genetic counselor also can help people to understand their family history and their genetic risks. In addition, the genetic counselor informs people of the risks, limitations and benefits of undergoing testing, so that they can make informed choices about whether genetic testing is appropriate for them. Should an individual choose to undergo the testing, genetic counselors then help him/her evaluate and understand the results and make informed choices about future health care.
Family History: Prior to undergoing genetic testing, it is important to develop a complete family history. The family history should include information from both the biologic mother and father and all of their close relatives. In addition, geographical heritage and ethnicity may prove to be key factors influencing genetic risk. The family history needs to include information about cancer, as well as any other significant health problems in the family. Once a complete family history is developed, a genetic counselor can develop a pedigree, which is a graphic representation of family relationships that shows patterns of disease. The genetic counselor can then analyze the pedigree to determine whether a cancer susceptibility syndrome is present in the family and to determine the most likely pattern of inheritance. The pedigree can also provide clues regarding the risk of cancer.
Testing: If a pedigree indicates that a hereditary genetic mutation could exist in a family, a patient may choose to undergo genetic testing. Many experts recommend undergoing genetic testing only when a pedigree analysis suggests the presence of an inherited cancer syndrome for which a specific mutation has been identified. Other guidelines suggest that genetic testing should be pursued only when the test will impact future medical care and decisions. Predictive genetic tests provide the most useful information when a living family member who is affected with the cancer is tested first. If a mutation is found, then other family members may wish to be tested for the presence or absence of this mutation. However, if no mutation is found in the affected family member, there is no reason to test unaffected family members because the test will be considered uninformative. There are many different types of genetic tests that are used to test for different mutations; therefore, it is important that the genetic counselor carefully examines the pedigree and selects the appropriate genetic test.
Evaluating the results: After the test, an individual may still choose not to receive the results because with a greater understanding of the implications of the test, they may have decided that they would prefer not to know the results. The genetic counselor plays an important role in this decision process and should ensure that the individual knows the limitations of the test and the implications of the results before committing to seeing the results. If an individual does decide to view the results, the genetic counselor can help to explain the results and what they mean.
If a result is positive, the genetic counselor can help the person to understand the risk of developing cancer. In addition, the counselor can help the person develop a plan of action for notifying family members of their potential risk for carrying an inherited mutation. At this point, the counselor can also discuss potential preventive measures and screening procedures that the person can undergo in order to prevent or detect the cancer early, should it develop.
It is important to understand that if an individual does test positive for a mutation that is not present in an affected family member, it is difficult to interpret the risk posed by this mutation. In such cases, it is unlikely that the mutation was inherited. Rather, it was probably the result of mistakes during cell division in their lifetime. While such results would be of interest to the individual, they do not indicate risk for other family members.
If the result is negative, the genetic counselor can help the patient interpret what this means. A negative test result is not a guarantee that a person will not develop cancer. In fact, a genetic counselor should discuss the difference between a false negative and a true negative. A false negative means that the person does indeed carry a genetic mutation, but the test missed it. In addition, there is always the chance that the individual has a different genetic mutation that cannot be identified by the specific test that was used.
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Implications of Predictive Genetic Testing
There are not only benefits, but also limitations and risks involved with undergoing predictive genetic testing. People considering these tests need to understand the limitations before they commit to undergoing the procedure.
Limitations: Perhaps the greatest limitation of predictive genetic testing is that it is predictive, not definitive. The test results provide few black and white answers. A negative test result does not mean that a person will not develop cancer, just as a positive test result does not mean that a person will develop the disease. In addition, the results are not modifiable, so if a person is found to be at an increased risk for developing cancer and pursues preventive strategies, there is no way to measure the impact of these strategies. Despite technological advances, no tests are 100% accurate. A test may fail to identify an existing cancer-causing mutation (false negative) or it may incorrectly identify a gene as mutated (false positive.) Testing techniques vary, therefore, it is important to know which method is being used and what the chances are of finding an existing mutation.
Benefits: Predictive genetic testing can identify the cause for cancer in a family and, as a result, could help to identify family members who are at a high risk for developing cancer. This would allow people to take preventive measures and to undergo more frequent screening procedures to detect cancers at early stages when they are most treatable. In addition, genetic testing could identify that a person is not at an increased risk for developing cancer and, as a result, eliminate uncertainty or anxiety. This would also eliminate the need for more frequent screening procedures and would prevent unnecessary preventive measures.
Risks: The potential that the results of these tests could be placed in medical files poses risks for discrimination. People identified as high-risk for developing cancer could be discriminated against in terms of obtaining health, life and disability insurance and employment. On the other hand, if people identified as high-risk manage to withhold the results from their insurance company, they may not be able to justify their need for frequent screening procedures. There are also psychological risks associated with genetic testing. Some people may experience increased anxiety regarding their chance of developing cancer. Others may experience guilt as a result of learning that they did not inherit a mutation while other family members did. These situations can cause tension within family relationships as well.
It is important for people to understand all of the issues surrounding genetic testing before committing to undergoing the procedure. Genetic testing can be valuable if people can use the information to make medical and lifestyle decisions that could help to decrease their risk of developing cancer, or at least assist them in detecting the cancer early when it is most treatable. Anyone considering genetic testing should first determine if there is a test designed to identify a mutation for the specific cancer in which they are interested. If so, it will be important to study the information about the tests and the groups in which it has been used. A genetic counselor can play a vital role in advising people and helping them through this process.