Mutational studies in a patient with the hydrops fetalis form of mucopolysaccharidosis type VII.

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.