Genetic testing and cancer

ALSO KNOWN AS:Deoxyribonucleic acid (DNA) testing, gene testing, genetic screening, molecular genetic testing

DEFINITION: Genetic testing is a medical test identifying genetic composition. DNA for genetic testing can be extracted from the cells of many different body fluids or tissues. The majority of genetic tests are completed by using DNA from blood cells. Other sources include cells obtained from the cheek lining using swabs or a mouthwash, hair root cells, or the cells in the fluid surrounding a fetus in the womb (amniotic fluid). Gene analysis is accomplished by examining chromosomes, DNA codes, or the proteins produced by genes.

Why performed: Typically, genetic testing is used to find inherited genetic disorders. Genetic testing can determine an individual’s genetic composition and can help determine if the tested person has an inherited disorder. Genetic testing can also help establish whether a person can develop genetic diseases or pass them on to subsequent generations.

Sometimes genetic tests are used to assess disease risk. A genetic test can help assess genetic predisposition to colon and breast cancer through certain genetic markers. Such a test gives an idea of an individual’s risk of having the disease but does not determine with absolute certainty whether a person will get breast or colon cancer. A person with a negative test (lacking the genetic marker) could still get breast or colon cancer. Conversely, a person with a positive test (possessing the genetic marker) may not get breast or colon cancer. A person with a positive test will, however, have an increased chance of getting breast or colon cancer compared with a person with a negative test.

If a person has a history of a certain type of cancer in their family, it may be helpful for them to complete genetic testing in order to know if they have certain genetic markers and to understand their potential predisposition to develop cancer. However, these tests can also cause people anxiety, as a positive marker does not mean cancer is present.

More than two thousand genetic tests are available, and descriptions of most of them are provided by the National Library of Medicine and the National Genome Research Institute. Some of these genetic tests determine chromosomal abnormalities, such as those resulting in Down syndrome, and some identify DNA genetic code changes, such as those occurring with sickle cell anemia. Some of the more common types of tests are described here.

Newborn screening: Infant screening after birth identifies treatable genetic disorders. This type of genetic screening has been done for decades in the United States and is routinely completed on millions of newborns yearly in all fifty states. For example, infants are screened for phenylketonuria, a disorder that causes intellectual disability if unrecognized and untreated, and other genetic disorders such as congenital hypothyroidism.

Diagnostic testing: Genetic testing to determine if a particular disease is present is called diagnostic testing. Usually, these types of tests are used to diagnose a disease suspected on the basis of the patient’s family health history, personal health history, physical examination, or symptoms.

Carrier testing: Normally, people have two copies of almost all the genes in the body; one copy comes from the father and the other from the mother. Some diseases, known as recessive disorders, will express themselves (cause a disorder) only if both copies of the gene have the disease trait. An example is sickle cell anemia. Carrier testing can determine if a person has one or two copies of the recessive gene. The person who has two copies of the gene with the disease trait will have the disease. The person who has one copy of the gene with the trait and one normal copy of the gene is known as a carrier of the disease. This person does not have the disease itself but carries the disease trait and has a 50 percent chance of passing the trait (gene) to any offspring. This type of testing is useful for individuals with a family history of a genetic disorder. Testing couples who plan to have a child for recessive genetic disorders helps establish their risk of having a child with a genetic disease.

Prenatal testing: Some birth defects and inherited disorders, such as spina bifida and Down syndrome, can be detected with prenatal testing. This type of information can help parents make important decisions regarding the care of a disabled child or the progress of a pregnancy.

Predictive testing: This type of genetic test can determine potential risk for some diseases such as colon cancer, breast cancer, ovarian cancer, or hemochromatosis (an iron metabolism disorder resulting in too much iron in the body). For example, the presence of BRCA1 and BRCA2 genes indicate a higher risk for breast and ovarian cancer.

Forensic testing: Forensic testing, such as DNA fingerprinting or paternity testing, is used for criminal investigations or to establish biological parenthood. DNA fingerprinting is accomplished by breaking down DNA into smaller segments. The technique involves using compounds that attack DNA sequences at particular points, splitting the DNA into several fragments. These break points are different in each person because everyone has a unique DNA sequence (unless the person is one of a pair of identical twins). The fragments are lined up and compared with DNA left at crime scenes (and usually with that of people who are not suspects and serve as controls). Matches are very evident visually.

Pharmacogenomics: Genetic testing to determine response to therapy is known as pharmacogenomics. Many tests to predict how effective a medicine will be in a particular person are being developed, and some of these types of genetic tests are available. For example, a medication called trastuzumab treats breast cancer but is effective only when the breast tumors have estrogen receptors. A genetic test determines if a woman’s breast tumors have estrogen receptors and, therefore, trastuzumab will help treat them. More tests that predict how a specific person may respond to medications for cancer, asthma, heart disease, and other diseases have been developed.

Preimplantation genetic diagnosis: In vitro reproduction occurs when egg cells are removed from a woman’s ovaries and fertilized with sperm outside the body. The embryos formed this way are then implanted in a woman’s uterus. Before implantation, cells are taken from the embryos and screened for specific genetic disorders. This ensures that embryos lacking the specific genetic disorders are implanted.

Direct-to-consumer genetic testing: Some genetic testing laboratories sell genetic tests directly to consumers through print, television, and Internet advertisements. A test for the BRCA1 and BRCA2 genes that have been associated with breast and ovarian cancer was marketed directly to consumers, as were tests for markers for hemochromatosis and cystic fibrosis. Although this type of marketing can raise awareness of the tests, concerns have been raised about the accuracy of information presented in advertisement as well as the accuracy of the tests themselves.

One form of direct-to-consumer genetic test arouses particular concern. Some websites sell genetic tests for aging, behavior, and nutrition. The tests provide genetic profiles that are matched to consumer goods such as creams and dietary supplements. Little scientific study supports the commercial pairing of these types of genetic tests with the goods offered.

Regardless of the type of direct-to-consumer genetic test, the lack of genetic counseling and the absence of health care professionals in the process raise concerns about how well these genetic tests are interpreted and applied. Often, genetic tests have great implications for not only the person taking the test but also relatives of the person taking the tests. These types of concerns have caused some to advocate more government oversight and regulation of direct-to-consumer genetic testing.

The case of breast cancer genes: A closer look at BRCA1 and BRCA2, the breast cancer genes, provides some background information and shows how testing relates to the science of genetics.

The human body is constantly replacing cells in a process called mitosis. Mitosis is the orderly duplication of cells that ensures each cell has the exact same genetic information. Over the course of many years, some mutations or changes in the genetic code can appear, and these changes may result in uncontrolled cell division. When mitosis is not carefully regulated, tumors or cancers can develop. The majority of cancers are due to noninherited changes in the genetic code that occur during life.

However, about 3 percent of all breast cancers and 10 percent of ovarian cancers may have a genetic basis. A person can inherit a mutation in a tumor-suppressor gene, which controls cell division and growth and therefore protects against cancer. From birth, the person who inherits the mutated tumor-suppressor gene does not have carefully controlled cell division. Many factors play a role in determining if the person who inherits the mutated gene will actually develop cancer. People have two copies of every gene (one from the mother and one from the father), and the normal copy of that tumor-suppressor gene may prevent cancer from developing. Additional tumor-suppressor genes may also prevent the development of cancer.

A person inheriting a mutated tumor-suppressor gene has an increased risk of developing cancer but not an absolute (100 percent) risk of getting cancer. The cancer itself is not inherited, but a defective gene that does not adequately protect against cancer is inherited. If a person inherits one of these defective genes and develops cancer, the condition is called hereditary cancer.

Ovarian or breast cancer are regulated by many genes, but two of them have been named and can be found through genetic testing. BRCA1 is short for breast cancer 1 gene, and BRCA2 is short for breast cancer 2 gene. BRCA1 and BRCA2 are tumor-suppressor genes, and they keep cell reproduction from getting out of control. BRCA1 and BRCA2 genes are present in both men and women. If a person inherits a faulty or mutated copy of BRCA1 or BRCA2, that person has an increased risk of developing breast or ovarian cancer, along with a slightly higher risk of developing any type of cancer.

Because half of all genes are inherited from the mother and half are inherited from the father, a mutated BRCA1 or BRCA2 gene can be inherited from either parent. If a person has a faulty BRCA1 or BRCA2 gene, each of the person’s children has a 50 percent chance of inheriting the faulty gene. Although inheriting a faulty BRCA1 or BRCA2 gene increases a person’s genetic predisposition for breast or ovarian cancer, environmental factors still play a large role in determining whether the person develops breast or ovarian cancer. More mutations in other tumor-suppressor genes need to occur for the person to develop cancer. The causes of the mutations acquired during a lifetime are largely unknown and subject to much speculation and scientific research. One branch of genetic research involves looking not only at the genetic code present in DNA but also into how ribonucleic acid (RNA), another molecule that carries genetic information, may be contributing to cancers.

This BRCA1 and BRCA2 example shows how one of more than two thousand available genetic tests relates to the science of genetics. It demonstrates the complex relation between genes and disease, and it underscores the necessity of genetic information and competent and caring counseling. As the number of genetic tests grows, it becomes more important to understand the science behind the tests as well as their social implications and applications.

Bibliography

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