Why Genetic Testing is Important for Hemophilia

Genetic tests find changes, called mutations, in genes that cause disease. These tests can be a helpful tool in confirming a disease diagnosis. Different types of mutations may cause a single disease. Some drugs are more effective for certain types of mutations and some drugs only work on certain mutation types. Knowing what type of mutation is present may predict how a person will respond to certain drugs. In deciding to have or not have a genetic test, it is helpful to understand a little about how genes work, what types of mutations genes may carry, and how genetic testing is performed.

Background
Cells are specialized small units that make up all living things and the human body contains trillions of cells. There are many different types of human cells, including brain cells, liver cells, and muscle cells. Every cell contains 20,000 to 25,000 genes. Genes, made up of DNA, are the “blueprints” of the cell and tell our body how to make proteins. Proteins form the structure of the cells and perform most important functions in the human body. Proteins produce traits – such as eye color, foot size, and height – and can be passed from parent to child.

Clotting factors are important proteins made in blood cells. These proteins help stop bleeding and repair the damaged blood vessels if a person is cut or injured. When clotting factor proteins are not working properly in the body, excessive bleeding can happen even with a minor injury.

Hemophilia is the result of a change or mutation in the genetic blueprint that leads to a loss of clotting factors or production of ineffective clotting factors. There are two types of hemophilia: Hemophilia A and Hemophilia B. Hemophilia A is caused by mutations in the Factor 8 (F8) gene. Hemophilia B is caused by mutations in the Factor 9 (F9) gene. Hemophilia occurs almost exclusively in males, but females can be carriers. Mutations in the F8 or F9 genes lead to reduced clotting or a complete loss of clotting. This in turn leads to an increased susceptibility to injury. Several types of mutations within the DNA of F8 and F9 genes can result in hemophilia. They include:

• Deletion – a part of the DNA blueprint for hemophilia is missing.
• Duplication – a piece of F8 or F9 DNA is repeated.
• Insertion – a small extra piece of DNA is added to the F8 or F9 gene.
• Missense – a segment of the genetic code is changed, which leads to an error in the F8 or F9 protein.
• Nonsense – a segment of the genetic code is changed and read as a premature “stop signal” in the gene, so that cells do not make the complete F8 or F9 protein, and instead make an incomplete protein. When a nonsense mutation is the cause of hemophilia A or B, the resulting disease is almost always severe.

Potential Uses of Hemophilia Genetic Testing
Genetic testing can confirm whether or not someone has hemophilia and can indicate which type it is and which genetic mutation (deletion, duplication, insertion, missense, or nonsense) is causing it.. This knowledge may make a person eligible to participate in a clinical trial of a drug designed to overcome his or her specific type of mutation.
For example, PTC Therapeutics is developing an investigational new drug called ataluren (formerly referred to as PTC124®). This drug is designed to allow the cells to override the premature stop signal caused by a nonsense mutation in the F8 and F9 genes. Ataluren is now being studied in a clinical trial in patients with hemophilia who have the disease due to a nonsense mutation. If this and further clinical trials in hemophilia patients show positive results, ataluren may be approved by regulatory authorities and be made available to treat patients who have hemophilia due to a nonsense mutation. Knowing the type of mutation may help patients and families choose between clinical trials or treatment options. Carefully choosing a treatment plan not only increases the chance of success, but also avoids exposure to a drug that may have little or no chance for benefit and may expose patients to harmful side effects.
Hemophilia gene testing can be performed on boys who are suspected of having hemophilia, on brothers of boys with hemophilia, and on mothers and sisters who believe they may be carriers of the disease and are concerned they might pass it on to their children.

Having a carrier test is not only important for family planning decisions but may be important as well for the health of women who are identified as carriers; some carrier females are at risk for bleeding episodes and should be monitored by their physician accordingly.

Performing Hemophilia Genetic Testing
To perform the gene test for hemophilia a single routine blood sample is required. In most cases, a primary care doctor can order the test. To arrange for testing, the doctor can contact a gene testing laboratory (see below) to get instructions on the type of blood collection tube to use and the details of where to send the blood sample. Often the doctor’s office staff can also provide information on the cost of the test and whether health insurance will provide full or partial coverage.

Once the sample is sent to the gene-testing laboratory, the laboratory will check to see if a mutation is present in the hemophilia genes. The laboratory will also test for the type of mutation and its location within the hemophilia gene.

Different technologies are necessary to detect the various mutation types and may require a two-step process. The laboratory will test for gene changes including insertions, missense and nonsense mutations first because these are the most common mutations. This test is called a gene sequencing or sequence analysis test. If no mutation is detected in the first test, the healthcare provider will then consider a second test to look for deletions and duplications. Usually laboratories charge separately for each test.

Genetic test results are sent to the doctor who ordered the test. Depending upon the laboratory and whether both tests are required, the results are normally available in 3 to 8 weeks.

Deciding Whether to Have Hemophilia Genetic Testing
As noted above, genetic testing can have advantages. However, in some situations, the knowledge that comes with testing may bring concerns. Family members may differ in their responses to genetic test results. These differing responses may result in changes in relationships among family members. Choices regarding reproductive decision-making and options for prenatal testing are things that every family should consider before having testing. Insurance companies vary on coverage for genetic testing and families should consider the cost of testing. It is important for each family to consider the advantages and disadvantages of the test before it is performed.

Genetic Testing Resources

Physicians who treat hemophilia will generally be aware of laboratories that perform genetic testing. Laboratories that perform testing may also be located through GeneTests (www.genetests.org), a website sponsored by the United States National Institutes of Health. Once on the genetests site, click on the Laboratory Directory link at the top of the page to find a list of genetic testing laboratories. Note that since listing on the GeneTests website is voluntary, not all laboratories will be listed. To find a specific laboratory doing genetic testing for hemophilia, search for the disease name: “hemophilia.”

GeneTests can be searched for the location of a laboratory in the country or region of the person to be tested. Not all countries have genetic testing laboratories. Orphanet (http://www.orpha.net/consor/cgi-bin/index.php) is a similar resource for European laboratories. If the patient’s physician or genetic counselor cannot recommend a nearby laboratory, it may be possible to have blood drawn near the patient’s home and shipped to a laboratory in another country for testing. It is important to contact the laboratory first to see if this is possible and to find out the procedure.

Additional Genetics Resources
In addition to the doctor, speaking with a genetic counselor may benefit the patient and his or her family. Genetic Counselors can help with the decision-making process and explain or coordinate testing. Genetic counselors can help interpret the results of the gene sequence test; provide guidance on the best use of the information gained from the test; and answer questions about genetics in general and the types of tests that are available.

Many online resources offer information on genetics and genetic testing:
• Foundation for Genetic Education and Counseling (www.fgec.org) :Educational material for doctors and patients
• National Human Genome Research Institute (www.genome.gov) : Government website with background information as well as research and resources
• National Society of Genetic Counselors (www.nsgc.org/consumer ) : Find a genetic counselor
• U.S. Department of Energy Department of Science (http://www.ornl.gov/sci/techresources/Human_Genome/medicine/genetest.shtml ) :
Basics of genetic testing and FAQ’s about testing
• Your Genes Your Health (www.ygyh.org) : General information about hemophilia
• Your Genome (www.yourgenome.org) : Genetics background information and research
• Gene Test (www.genetests.org): Genetic background and testing information on hemophilia and many other types of diseases
• The Hemophilia & Thrombosis Research Society (HTRS)
(http://www.htrs.org/Resources/Registry/): hemophilia patient registry