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Mutational studies in a patient with the hydrops fetalis form of mucopolysaccharidosis type VII
1E. A. Doisy Department of Biochemistry and Molecular Biology, St. Louis University School of Medicine, Missouri 63104.
Abstract:
Four prior mutations have been reported in three patients with beta-glucuronidase deficiency mucopolysaccharidosis (MPS VII), none of whom had the severe, infantile, hydropic form of the disease. We identified two mutations in the first reported case of nonimmune hydropic MPS VII whose cultured fibroblasts had < 1% of residual activity. The first mutation was a C-->T transition at position 1061 of the cDNA in exon 6 that gave rise to an Ala-->Val substitution in codon 354 (A354V). The second was a C-->T transition at position 1831 in exon 12 that produced an Arg-->Trp substitution in codon 611 (R611W). Transient expression in COS-7 cells revealed that both mutant enzymes were synthesized as normal-size precursors in normal quantities, but both exhibited accelerated turnover. The expressed A354V enzyme had a t0.5 (half-life) of 33 hr (wild-type t0.5 > 60 hr) and a specific activity 35% of wild-type enzyme. The R611W enzyme had a t0.5 of 20 hr and no detectable catalytic activity. The t0.5 of enzyme produced on cotransfection with A354V and R611W was nearly identical to that of A354V alone. Mutant enzyme expressed in transfected murine MPS VII cells gave similar residual activities relative to the wild-type enzyme. In COS cells, the A354V monomers formed mixed tetramers with coexpressed rat monomers, but the product of R611W did not. The higher than expected activity, both in COS cells and in murine MPS VII cells expressing A354V, provides further evidence that overexpression can partially correct some beta-glucuronidase mutations, apparently by driving the folding reaction of monomers or the assembly into tetramers by mass action.
Insights
We identified two novel mutations in beta-glucuronidase deficiency mucopolysaccharidosis (MPS VII), revealing insights into the severe infantile hydropic form of the disease. Overexpression partially corrected some mutations by promoting enzyme folding and tetramer assembly.
Area of Science:
- Genetics
- Biochemistry
- Molecular Biology
Background:
- Mucopolysaccharidosis type VII (MPS VII) is a rare genetic disorder caused by beta-glucuronidase deficiency.
- Prior studies reported mutations in MPS VII patients, but none presented the severe, infantile, hydropic form.
- This study investigates the genetic basis of the severe, nonimmune hydropic MPS VII phenotype.
Observation:
- Two novel mutations, A354V and R611W, were identified in the first reported case of nonimmune hydropic MPS VII.
- Fibroblasts from the patient showed less than 1% of residual beta-glucuronidase activity.
- Transient expression studies in COS-7 cells revealed accelerated turnover rates for both mutant enzymes.
Findings:
- The A354V mutation resulted in an enzyme with a 33-hour half-life and 35% residual activity.
- The R611W mutation yielded an enzyme with a 20-hour half-life and no detectable catalytic activity.
- Overexpression of the A354V mutant in MPS VII cells partially corrected enzyme activity, suggesting a mass action effect on folding and tetramer assembly.
Implications:
- These findings expand the mutational spectrum of MPS VII and provide molecular insights into the severe infantile hydropic form.
- The study highlights the potential of gene augmentation therapy strategies by demonstrating partial functional correction through overexpression.
- Understanding these mutations aids in genetic counseling and the development of targeted therapeutic approaches for MPS VII.