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A pseudodeficiency allele (D152N) of the human beta-glucuronidase gene
1Department of Medical Genetics, University Hospital, Vrije Universiteit Brussel, Belgium.
Abstract:
We present evidence that a 480G-->A transition in the coding region of the beta-glucuronidase gene, which results in an aspartic-acid-to-asparagine substitution at amino acid position 152 (D152N), produces a pseudodeficiency allele (GUSBp) that leads to greatly reduced levels of beta-glucuronidase activity without apparent deleterious consequences. The 480G-->A mutation was found initially in the pseudodeficient mother of a child with mucopolysaccharidosis VII (MPSVII), but it was not on her disease-causing allele, which carried the L176F mutation. The 480G-->A change was also present in an unrelated individual with another MPSVII allele who had unusually low beta-glucuronidase activity, but whose clinical symptoms were probably unrelated to beta-glucuronidase deficiency. This individual also had an R357X mutation, probably on his second allele. We screened 100 unrelated normal individuals for the 480G-->A mutation with a PCR method and detected one carrier. Reduced beta-glucuronidase activity following transfection of COS cells with the D152N cDNA supported the causal relationship between the D152N allele and pseudodeficiency. The mutation reduced the fraction of expressed enzyme that was secreted. Pulse-chase experiments indicated that the reduced activity in COS cells was due to accelerated intracellular turnover of the D152N enzyme. They also suggested that a potential glycosylation site created by the mutation is utilized in approximately 50% of the enzyme expressed.
Insights
A specific gene mutation (480G-->A) creates a pseudodeficiency allele for beta-glucuronidase, significantly lowering enzyme activity without causing harm. This finding aids in understanding genetic variations and their clinical implications.
Area of Science:
- Biochemistry
- Genetics
- Molecular Biology
Background:
- Beta-glucuronidase deficiency causes mucopolysaccharidosis VII (MPSVII).
- Pseudodeficiency alleles reduce enzyme activity without severe clinical symptoms.
- Identifying specific mutations is crucial for genetic diagnostics.
Purpose of the Study:
- To investigate a novel mutation (480G-->A) causing beta-glucuronidase pseudodeficiency.
- To understand the molecular mechanism of this pseudodeficiency allele.
- To determine the prevalence of this allele in the general population.
Main Methods:
- Genetic sequencing to identify the 480G-->A mutation.
- Functional assays using transfected COS cells to assess enzyme activity.
- Pulse-chase experiments to analyze enzyme stability and secretion.
Main Results:
- The 480G-->A transition results in an aspartic-acid-to-asparagine substitution (D152N), creating a pseudodeficiency allele (GUSBp).
- Transfected cells expressing the D152N allele showed significantly reduced beta-glucuronidase activity.
- The mutation led to accelerated intracellular turnover and reduced secretion of the enzyme.
- One carrier for the 480G-->A mutation was detected in 100 unrelated individuals.
Conclusions:
- The D152N mutation is a pseudodeficiency allele for beta-glucuronidase, leading to reduced enzyme activity through increased intracellular degradation.
- This finding contributes to the understanding of genetic variability in enzyme function.
- The identified mutation and its carrier frequency provide valuable information for genetic counseling and MPSVII diagnosis.