Tay-Sachs and Sandhoff diseases
What are the symptoms of Tay-Sachs and Sandhoff diseases?
Babies with the classic (infantile) forms of Tay-Sachs and Sandhoff diseases appear healthy at birth and seem to develop normally for the first few months of life. Symptoms generally appear by about 6 months of age when the baby gradually stops smiling, crawling, turning over and reaching out. The baby continues to lose skills gradually and eventually becomes blind, paralyzed and unaware of surroundings. Babies with Tay-Sachs disease usually die by age 4; those with Sandhoff disease, by age 3 (National Institute of Neurological Disorders and Stroke (NINDS), 2007; NINDS, 2009; Online Mendelian Inheritance in Man #268800, 2009).
What causes the symptoms of these diseases?
Babies with classic Tay-Sachs and Sandhoff diseases lack an enzyme (protein) called hexosaminidase. There are two versions of this enzyme, hex A and hex B. Babies with Tay-Sachs disease do not make hex A, and babies with Sandhoff disease do not make either hex A or hex B. A small number of babies with Tay-Sachs disease (AB variant) make both versions of the enzyme but lack another protein that is needed for these enzymes to work properly.
Hexosaminidase is necessary for breaking down certain fatty substances (called GM2 gangliosides) in cells of the brain. Without this enzyme, these fatty substances build up and gradually destroy brain cells, until the entire central nervous system stops working.
Are there other forms of Tay-Sachs and Sandhoff diseases besides the classic type that affects babies?
There are late-onset forms of these diseases, with symptoms developing in childhood or adulthood. While babies with the classic forms of these diseases do not produce any enzyme, individuals with the late-onset forms produce very small amounts. This is probably why their symptoms begin later in life and generally are milder than in the classic form.
There are three late-onset forms of Tay-Sachs disease:
- Juvenile (subacute): Symptoms begin between 2 and 10 years of age and resemble those of the classic form (Kaback, 2006; Online Mendelian Inheritance in Man, #272800, 2009). Although the course of the disease is slower, death generally occurs by age 15 (Online Mendelian Inheritance in Man, #272800, 2009; Maegawa, 2006).
- Chronic: Symptoms begin by age 10 and progress slowly (Kaback, 2006). Symptoms vary and may include poor coordination, unsteady gait, muscle cramps, slurred speech and, sometimes, mental illness. Cognitive abilities may not be affected or may be affected late in the course of the disease (Kaback, 2006). Life expectancy varies (Chicago Center for Jewish Genetic Disorders, 2007).
- Adult-onset: This is the mildest form with symptoms developing in adolescence or adulthood. Symptoms vary greatly in severity and can include slurred speech, muscle weakness, muscle cramps, tremors, unsteady gait and, sometimes, mental illness (Kaback, 2006; National Tay-Sachs and Allied Diseases Association, Inc., 2009). Affected individuals usually do not lose vision or hearing. Some individuals may have loss of certain mental abilities, including problems with memory. Life expectancy varies and, in some cases, appears to be unaffected (Kaback, 2006; National Tay-Sachs and Allied Diseases Association, Inc., 2009).
Late-onset forms of Sandhoff disease are rare and appear to share many of these symptoms.
Is there any treatment for these diseases?
There is currently no treatment to prevent these diseases from running their course. Affected individuals can be made as comfortable as possible and given other supportive care.
Researchers are investigating whether stem cell transplants could help babies with classic Tay-Sachs and Sandhoff diseases. Stem cells are immature blood cells that produce all other kinds of blood cells. Stem cells are obtained from umbilical cord blood or from the bone marrow of a donor. Unfortunately, stem cell transplantation has not yet been successful in stopping or reversing brain damage in Tay-Sachs or Sandhoff diseases, and this treatment poses a high risk of death in affected babies (NINDS, 2007; NINDS, 2009).
Researchers also are studying the effectiveness of drug treatments (including a drug called miglustat, which is approved by the Food and Drug Administration to treat a related disorder) to help reduce the build-up of fatty substances in brain cells in individuals with these diseases (National Tay-Sachs and Allied Diseases Association, Inc., 2009).
Who is at risk for Tay-Sachs and Sandhoff diseases?
Tay-Sachs disease occurs most frequently in descendants of Central and Eastern European (Ashkenazi) Jews. About 1 out of every 30 American Jews carries a mutation in the gene that codes for hex A (Kaback, 2006; American College of Obstetricians and Gynecologists (ACOG), 2005). Some non-Jewish individuals of French-Canadian ancestry (from the St. Lawrence River Valley of Quebec) and members of the non-Jewish Cajun population in Louisiana and the Old Order Amish in Pennsylvania also are at increased risk (Kaback, 2006; American College of Obstetricians and Gynecologists (ACOG), 2005). Individuals in other ethnic groups in this country have about a 1 in 300 chance of carrying a mutation in this gene (Kaback, 2006; American College of Obstetricians and Gynecologists (ACOG), 2005).
Sandhoff disease can occur in any ethnic group, though it is uncommon. Individuals not of Jewish ancestry are more likely than those of Jewish ancestry (1 in 600 vs. 1 in 1,000) to carry one of the gene mutations that cause Sandhoff disease (Online Mendelian Inheritance in Man #268800, 2009).
How are the diseases transmitted?
All forms of Tay-Sachs and Sandhoff diseases are inherited. Tay-Sachs disease is caused by mutations in a gene on chromosome 15 that codes for hex A. Sandhoff disease is caused by mutations in a gene on chromosome 5 that codes for hex B. Both diseases are passed on through parents who carry one of these mutations. A carrier does not have the illness. However, when two carriers become parents:
- There is a 25-percent (1 in 4) chance that any child they have will inherit a gene mutation from each parent and have the disease.
- There is a 25-percent chance (1 in 4) that the child will inherit the normal gene from each parent. The child will not have the disease and will not be a carrier.
- There is a 50-percent (2 in 4) chance that the child will inherit one normal and one abnormal gene. The child will not have the disease but will be a carrier like the parents.
If only one parent is a carrier, the couple’s children cannot inherit the disease. However, each child has a 50-percent chance of inheriting the gene mutation and being a carrier.
Carrier screening is commonly performed before or during pregnancy for adults in populations who are at risk for these disorders.
How can people find out if they are carriers?
An individual can take a test that measures the amount of hexosaminidase in the blood. Tay-Sachs carriers have about half as much of hex A as noncarriers, but this is plenty for the carrier’s own needs. Similarly, carriers of Sandhoff disease have reduced but adequate amounts of both hex A and hex B.
A blood sample can be used to perform DNA-based genetic testing for known mutations in the hex A and hex B genes. Genetic testing may be recommended if the results of the carrier screening test are uncertain.
Carrier screening is available from a genetic services center or clinic. A health care provider can provide referrals to local sites where testing is available, as can the National Tay-Sachs and Allied Diseases Association.Trained genetic counselors explain test results so that individuals know whether or not their children are at risk for the disease.
Can these diseases be diagnosed before birth?
Yes. Prenatal tests called chorionic villus sampling (CVS) and amniocentesis can diagnose these diseases before birth. These tests are available when both members of a couple are carriers or when one is a carrier and the other has uncertain or unknown carrier status.
CVS generally is done between 10 and 12 weeks of pregnancy. In CVS, the doctor retrieves a sample of cells from the developing placenta either through a thin tube inserted through the vagina or by inserting a needle through the mother’s abdomen. The placenta contains cells that are genetically identical to those of the fetus, and these cells are examined for the presence of hex A (when testing for Tay-Sachs) or hex A and hex B (when testing for Sandhoff). The lab can test for gene mutations in addition to the enzyme.
Amniocentesis usually is done between 15 and 20 weeks of pregnancy. In this test, the doctor inserts a needle into the mother’s abdomen to take a sample of fluid that surrounds the fetus. The fluid contains fetal cells, which are tested for the presence of the enzyme and/or gene mutations.
Some medical centers offer genetic testing to carrier couples who undergo in vitro fertilization (a process in which eggs are removed from a woman’s ovaries and fertilized in the laboratory with her partner’s sperm). The embryos are tested for a genetic disease, and only healthy ones are implanted in the mother. This is called preimplantation genetic testing.
Couples who are carriers of a Tay-Sachs or Sandhoff gene or those who may be at increased risk due to ethnic background or family history may want to consult a genetic counselor. These health professionals help families understand what is known about the causes of a birth defect and the chances of the birth defect occurring in a pregnancy. They also help guide families through the testing process. Genetic counselors can provide referrals to medical experts and appropriate support groups in the community. Genetic counseling is available at most large medical centers and teaching hospitals. To find a genetic counselor in their area, individuals can ask their health care provider or contact the National Society of Genetic Counselors.
Does the March of Dimes support research on Tay-Sachs and Sandhoff diseases?
Yes. March of Dimes grantees helped pinpoint mutations in the hex A gene that are responsible for late-onset forms of Tay-Sachs disease. Information about specific mutations leads to improved diagnosis and carrier screening for all forms of Tay-Sachs disease.
Current grantees are attempting to develop drug treatments that may prevent the production of certain fatty substances that build up and impair brain cells in individuals with Tay-Sachs and Sandhoff diseases. This approach eventually may help prevent the loss of central nervous system function and early deaths associated with these diseases.
For more information
- American College of Obstetricians and Gynecologists (ACOG). (2005). Screening for Tay-Sachs disease. (ACOG Committee Opinion, volume 318, reaffirmed 2007).
- Chicago Center for Jewish Genetic Disorders. (2007). Tay-Sachs disease. Retrieved September 7, 2007 from: jewishgenetics.org/?q=content/tay-sachs-disease.
- Kaback, M.M. (2006). Hexosaminidase A deficiency. GeneReviews. Retrieved June 23, 2009 from: ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=gene&part=tay-sachs.
- Maegawa, G.H.B. (2006). The natural history of juvenile or subacute GM2 gangliosidosis: 21 new cases and literature review of 134 previously reported. Pediatrics, 118, e1550-e1562.
- National Institute of Neurological Disorders and Stroke (NINDS). (2009). NINDS Sandhoff disease information page. Retrieved April 19, 2009 from: ninds.nih.gov/disorders/sandhoff/sandhoff.htm.
- National Institute of Neurological Disorders and Stroke (NINDS). (2007). NINDS Tay-Sachs disease information page. Retrieved December 2, 2009 from: ninds.nih.gov/disorders/taysachs/taysachs.htm.
- National Tay-Sachs and Allied Diseases Association, Inc.(2009). What is Tay-Sachs disease? Retrieved June 23, 2009 from: ntsad.org.
- Online Mendelian Inheritance in Man. (2009). Sandhoff disease #268800. Retrieved April 10, 2009 from: ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=268800.
- Online Mendelian Inheritance in Man. (2009). Tay-Sachs disease #272800. Retrieved March 5, 2009 from: ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=272800/.