Carrier Testing

Play it safe - Find out if you are a carrier

You could be a carrier for Canavan, Tay-Sachs or any of the allied diseases—
even if your family has no history of them.

Here’s why:

Everyone has two copies of their genes, one from each parent. A carrier of a genetic disorder has one defective copy and one normal copy of a gene, which is enough to prevent the disease. However, if two carriers with the same gene defect have a baby, there is a 25 percent chance that their child will receive two defective genes and develop the disease. Conversely, there is a 75 percent chance that the child will be healthy, which explains why a defective gene can be transmitted through multiple generations before it’s expressed.

Thanks to medical and technological advances, a simple blood test can identify carriers before they have children. Laboratories analyze the blood with either a biochemical test called an enzyme assay, which measures levels of enzymes linked to specific diseases, or through DNA studies, which look for specific mutations in genes that “code” for the enzymes.

For example, because Tay-Sachs is caused by the absence of an enzyme called Hex-A, the enzyme assay measures the level of Hex-A in the blood. Carriers have less Hex- A than non-carriers. DNA-based carrier testing looks for mutations in the gene that controls Hex-A production.

Carriers of Sandhoff disease not only have less Hex-A than normal, but also less Hex-B, another vital enzyme. As a result, screening methods for Sandhoff mimic those for Tay-Sachs.

Carriers of Niemann-Pick Types A and B, both caused by the lack of an enzyme called acid sphingomyelinase, can be spotted by biochemical testing or by looking for gene mutations. For Type C, which prevents cholesterol from moving out of cells properly, DNA-based testing is more reliable.

Although Canavan disease is caused by the complete lack of an enzyme called aspartoacylase, biochemical tests are not sensitive enough to detect carriers. DNA testing of Ashkenazi Jews, the ethnic group with the highest known carrier rate, can tell with more than 95 percent certainty whether someone is a carrier. Other populations are not immune to Canavan, but DNA testing for them may be less helpful because many of the genetic mutations that cause Canavan are unique to individual families.

For people at increased risk due to ethnic background or family history, DNA mutation analysis can identify carriers of other diseases, including Gaucher, Familial Dysautonomia, Fanconi anemia C and Mucolipidosis type IV.

Certified genetic counselors play a key role in screening, regardless of the disease or the testing method. They not only explain the tests and when they should be done, but also the options carrier couples have in planning a family. These include:

• diagnosis early in pregnancy so that a couple may choose to continue or terminate the pregnancy
• artificial insemination with sperm from a non-carrier
• preimplantation genetic diagnosis (PGD) following in-vitro fertilization
• adoption. Genetic counselors can also help carriers inform family members and encourage
  them to get tested.

By knowing if you are a carrier and understanding the possible implications, you can protect yourself and your family from these and other devastating genetic diseases.