Tay Sachs Testing

TAY-SACHS DISORDER
(TSD) is a progressive, neurodegenerative disorder, caused by a deficiency of the enzyme Hexosaminidase A. A baby who has TSD is normal for the first few months of life and then loses mental and physical abilities. Common signs include blindness, inability to interact and paralysis. Death usually occurs between age 3-5 years.

What is the Tay-Sachs enzyme test?
A white blood cell test identifies TSD gene carriers by measuring Hexosaminidase A (Hex-A) enzyme activity. This test should only be performed by an internationally certified lab. If the test shows you are a carrier, DNA testing is recommended to see whether this test result is caused by a TSD gene or by a change in the TSD gene that does not increase your risk for having a baby with TSD.

Who should have TSD carrier testing?
■ Individuals of Eastern European Jewish, Quebec French Canadian, and Cajun ancestries.

■ Individuals with a family history of TSD, regardless of ancestry.

■ Both mother and father during pregnancy if one or both have Jewish, Quebec French Canadian, and Cajun ancestry and carrier testing has not been performed by 13 weeks LMP. (ACOG No.93, March 1991).

Which other disorders occur more often with Jewish ancestry?
■  BLOOM SYNDROME (BS) BS is a rare disorder characterized by growth retardation, decreased immune system function, sun-sensitivity erythema and a high rate of cancer. About half of affected individuals die of cancer before age 30.

■  CANAVAN DISORDER (CD) CD is a progressive, neurodegenerative disorder caused by lack of the enzyme, aspartoacylase. This deficiency results in a progressive mental retardation and poor muscle control. Severity and life expectancy vary with some children dying in the first year of life and others surviving into their teens.

■  CYSTIC FIBROSIS (CF) In CF the outward secreting (exocrine) glands produce mucous in the lungs which interferes with breathing and triggers chronic infections which often results in irreparable lung damage. The mucous may also affect the pancreas, causing poor digestion. The current life expectancy for individuals with CF is about 30 years, but this may increase with improved treatment.

■  FANCONI ANEMIA (FA) FA (Group C) is usually diagnosed in early childhood by finding anemia, growth retardation, learning disabilities and sometimes mental retardation. Heart, kidney, and skeletal abnormalities may also be present. Affected individuals have a high rate of leukemia.

■ GAUCHER DISORDER (GD) GD is caused by a deficiency in glucocerebrosidase, the enzyme responsible for breaking down the fatty substance glucocerebroside. GD signs result when this fatty substance accumulates in specific cells of the body.

Type I, which mostly affects those of Jewish descent, is the most common, mildest, and most variable subtype. It is also the only type without neurological involvement. Signs may be seen as early as childhood or as late as the 5th to 6th decade of life, if ever. Signs may include enlargement of the spleen and liver, resulting in blood abnormalities such as anemia and impaired blood clotting. Accumulation of this fatty substance in the bone marrow may also cause bone weakening and fracturing. Treatment is limited and costly. Enzyme replacement therapy varies in individual effectiveness and is not a cure. Bone marrow transplantation can provide a lifelong cure if successful.

■ NIEMANN-PICK DISORDER (NP) Type A & B Niemann-Pick Disorders are caused by a deficiency in the enzyme, acid sphingomyelinase. 

Type A, a serious neurodegenerative disorder, is the most common and the most severe. A baby who has Type A experiences a progressive loss of motor and mental function beginning at a few months of age, and ending in death, usually by age 3.

Type B does not have neurological involvement and is characterized by a progressive enlargement of the liver and spleen in childhood and pulmonary involvement in adolescence into adulthood. Individuals with type B typically survive into adulthood.

How are all these disorders inherited?
Each disorder is caused by a gene that has a mutation and is not working properly. These genes are inherited in an autosomal recessive manner. Autosomal means the abnormal gene is not on a sex chromosome. Recessive means the effect of the gene would only be seen when a baby inherits an abnormal gene from both parents. Both normal parents of an affected child have one abnormal recessive gene and one normal dominant gene and are, therefore, carriers. When both parents are carriers of the same disorder, they have a 1 in 4 risk (25%) of having an affected baby, a 1 in 2 chance (50%) of having a carrier baby, and a 1 in 4 chance (25%) of having an unaffected, non-carrier baby.

The chance a normal relative of an affected individual will carry the abnormal gene is:

Affected Individual:

Carrier Chance:

Brother or sister

2 in 3 (67%)

Niece or nephew

1 in 2 (50%)

Aunt or uncle

1 in 3 (33%)

First Cousin

1 in 4 (25%)

Who is more likely to be a carrier?
About 1 in 25 individuals of Eastern European Jewish, or Quebec French Canadian, and Cajun ancestry are TSD carriers. The carrier frequency in the Jewish population is about 1 in 101 for Bloom, 1 in 29 for CF, 1 in 37 for Canavan disorder, 1 in 89 for Fanconi Anemia ,1 in 14 for Gaucher, and 1 in 90 for Niemann-Pick disorder.

How is testing done?
You may discuss testing with your healthcare provider and consider having genetic counseling. Healthcare providers may contact our Center for genetic consultations. Blood specimens may then be sent by the health care provider or drawn at GeneCare.

Who should consider carrier screening, genetic counseling and prenatal testing?
Because carriers are normal, only accurate carrier testing can tell whether someone is a carrier. If both parents are found to be carriers for a disorder (or if one parent is a TS carrier and the other parent has an inconclusive TS test), prenatal diagnosis can tell whether the unborn fetus is affected. Genetic counseling can help families understand their risk of having an affected child and testing.

How long does it take to complete the test(s)?  
Most carrier and affected individual tests can be completed within 7-10 working days. Prenatal testing takes approximately 2-3 weeks.

Specimen Collection:
■ Call (800) 277-4363 to discuss clinical indications, genetic counseling, current testing, informed consent, fees and payment method.

■ Complete test Requisition.

■ Draw blood Monday through Friday.

■ For Tay-Sachs enzyme testing, Collect one 10ml green top sodium heparin blood tube.

■ For Tay-Sachs, Canavan, Cystic Fibrosis, Niemann-Pick, Gaucher, Bloom and/or Fanconi Anemia DNA testing, collect two 5-7 ml lavender EDTA blood tubes.

■ Label each specimen tube with patient name, birth and collection date.

 Specimen Transport:
■ Enclose both Requisition and Consent

■ Specimen must be transported with a COOL PACK, (NOT FROZEN) on the day of blood draw to:

GeneCare Medical Genetics Center 120 Conner Drive, Suite 201, Chapel Hill, NC 27514-7085
(919) 942-0021 • 1-(800) 277-4363 • Fax (919) 967-9519

OR

GeneCare Wilson Genetics Laboratory 4701 Randolph Road, Suite G9, Rockville, MD 20852
(301) 881-2592 • 1-(866) 881-2592 • Fax (301) 881-2596

 OR

Tay-Sachs Prevention Program, Jefferson Medical College, Medical Office Building
1100 Walnut Street, Suite 400, Room 422,Philadelphia, PA 19107
(215) 955-8320 • Fax (215) 955-7560

 

■ *If specimen cannot be delivered to GeneCare the same day, REFRIGERATE and ship next day.

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Last modified: May 25, 2004 04:26 PM

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