Background: Tay-Sachs Disease (TSD) is an autosomal recessive inborn error of metabolism whose basic biochemical defect is a deficiency of a lysosomal enzyme known as hexosaminidase A (hex A), which normally catalyzes a step in the degradation of a membrane glycolipid called ganglioside GM2. In the absence of hex A activity, GM2 accumulates in central nervous system cells, eventually compromising their function. In the classical form of TSD, infantile TSD, clinical symptoms typically appear at three to six months of age and progress rapidly to blindness, deafness, uncontrollable seizures and death before age five years. The disease occurs with increased frequency in the Ashkenazi Jewish population, with frequencies of heterozygotes ranging from 1 in 25 to 1 in 45.
An adult form of TSD, resulting from a partial deficiency of hex A activity, is associated with an age of onset in the twenties or thirties and is characterized by an unsteady gait followed by progressive central nervous system deterioration. There are no effective therapies currently available for either form of TSD. Characterization of the enzyme defect in infantile TSD in the 1960’s resulted in development of a test for hex A activity that allowed for identification of heterozygotes and prenatal diagnosis of affected fetuses through amniocentesis. The availability of these tests combined with the relatively high frequency of heterozygotes in a well defined population led to TSD carrier screening programs being instituted in most major cities in the United States. The Tay-Sachs gene has now been identified on chromosome 15 and three mutations that result in TSD have been characterized, allowing for more accurate diagnosis. Studies of TSD carriers have shown that 78% have a four-nucleotide insertion mutation in exon 11.
Procedure: To analyze these cases, use PCR with the TSD primers to amplify a portion of the hexosaminidase A (hex A) gene from blood DNA samples. (NOTE: Use the PCR function on the Data Screen, rather than the 96-well PCR.) Then, using the dot blot, load the probes into the spots and add the PCR-amplified DNA samples into the corresponding wells.
Case A: When Megan and Greg announce their plans to get married, Megan’s mother, Rachel, finally explains why Megan never got the baby brother or sister that she always asked for when she was younger. Shortly after Megan was born, her parents learned that a Tay-Sachs Disease carrier screening program was being organized in their area. Since they were planning to have more children, they decided to be tested. The news they received was not what they had hoped for; they both tested positive for carrier status. Because they did not want to risk having a child with TSD and their religious beliefs did not permit aborting an affected fetus, they chose not to have any more children. When Megan tells Greg this news, he questions his parents and learns that they had chosen not to be tested because of fear of stigmatization and discrimination. Greg and Megan decide that they must be tested before they get married.
DNA samples:
- Megan
- Greg
- Control DNA with the TSD mutation
- Control normal DNA without the TSD mutation
Results of PCR/dot blot analysis for Case A
- What is the chance that Megan carries the Tay-Sachs gene, based on her parents’ test results?
- What conclusions can you draw from the results of the DNA analysis?
- How would you counsel Megan and Greg based on the results of their tests?
- What issues are raised by large-scale genetic screening?
Case B: Lisa had always wondered about the results of her first Tay-Sachs Disease carrier test. She had been tested at age 18 when large-scale screening was done in her hometown of Minneapolis. The test used then measured levels of the Tay-Sachs enzyme and Lisa’s test results were in a range that made the diagnosis uncertain. Even repeat testing could not resolve the question. Since she was not planning to have children right away, Lisa had put her concern aside and gone on with her life. With a busy career and an active social life, she had never married. Now, at age 40, she suddenly found herself with an unplanned pregnancy and facing some difficult decisions. Although the thought of being a single parent is daunting, Lisa decides that she wants to have a child. The father is not interested in being involved in the child’s future and also refuses to undergo genetic testing. Lisa decides to have DNA testing done to resolve her carrier status and to determine the genotype of the fetus.
DNA samples:
- Lisa
- Fetus
- Control DNA with the TSD mutation
- Control normal DNA without the TSD mutation
Results of PCR/dot blot analysis for Case B
- What conclusions can you draw from the results of the DNA analysis?
- How would you counsel Lisa based on the results of her tests?
- What issues are raised by this case?
Case C: When Lisa (see Case B) tells her younger sister, Rose, about her decision to be tested for Tay-Sachs, Rose informs Lisa that she and her husband, Frank, are also expecting a child. Rose is now concerned about her own Tay-Sachs status and decides that she and Frank should be tested. They also decide to test their unborn child, while she is in the early stages of the pregnancy.
DNA samples:
- Rose
- Frank
- Fetus
- Control DNA with the TSD mutation
- Control normal DNA without the TSD mutation
Results of PCR/dot blot analysis for Case C
- What conclusions can you draw from the results of the DNA analysis?
- How would you counsel Rose based on the results of her tests?
- What issues are raised by this case?