Biochemical and molecular characterization of mutant hexosaminidase A in a Turkish family.

BACKGROUND

Tay-Sachs disease is a form of monosialoganglioside triaose (GM2) gangliosidosis that results from the mutations in the alpha-subunit gene of hexosaminidase A. In the B1 variant, the active site of the alpha-subunit of the enzyme is thought to be affected. In the present study, a patient who had previously been diagnosed as a B1 variant is further analyzed. The patient's parents and brother were also analyzed.

METHODS

Single-stranded conformational polymorphism (SSCP) and DNA sequencing analysis were conducted in all cases. In addition, hexosaminidase A (Hex A) was isolated from leukocyte homogenates of the patient's parents and brother using DE 52 ion-exchange chromatography, and thermostability analyses of the isolated enzymes were performed.

RESULTS

Hexosaminidase A of the parents was found to be more thermostable than normal Hex A. DNA sequencing analysis revealed a 12-bp deletion mutation in exon 10 of the Hex A gene. The patient was a homozygote and the parents were heterozygotes for the mutation, which could also be observed at the DNA double strands by SSCP analysis. These deleted bases are located within the catalytic domain of the alpha-subunit.

CONCLUSIONS

The 12-bp deletion mutation in exon 10 of Hex A is responsible for the increased thermostability of the enzyme. Considering this mutation has previously been found in a Turkish Tay-Sachs patient, the patient in the present study may have another mutation on the Hex B gene that causes decreased thermostability of the enzyme. Thermal inactivation assay may not be sufficient for a correct diagnosis in such unusual cases.