Quantitative correlation between the residual activity of β-hexosaminidase A and arylsulfatase A and the severity of the resulting lysosomal storage disease

Abstract

A previously suggested model for the correlation between residual activity of a lysosomal enzyme and the turnover rate of its substrate(s) has been extended to a discussion of substrate accumulation rates in individual cells and whole organs. With these considerations, much of the observed variability in age of onset and clinical phenotype, as well as the phenomenon of pseudo-deficiency, can be understood as the consequences of small differences in the residual activity of the affected enzyme. In order to experimentally verify the basic assumptions on which this model rests, studies were performed in cell culture. The radiolabeled substrates ganglioside GM2 and sulfatide were added to cultures of skin fibroblasts with different activities of beta-hexosaminidase A or arylsulfatase A, respectively, and their uptake and turnover measured. In both series of experiments, the correlation between residual enzyme activity and the turnover rate of the substrate was essentially as predicted: degradation increased steeply with residual activity, to reach the control level at a residual activity of approximately 10-15% of normal. All cells with an activity above this critical threshold had a normal turnover. Comparison of the results of these feeding studies with the clinical status of the donor of each cell line basically confirmed our notions but also revealed the limitations of the cell culture approach.

Bone marrow stem cell-based gene transfer in a mouse model for metachromatic leukodystrophy: effects on visceral and nervous system disease manifestations

“…In humans, a residual ASA activity of 5-10% of normal prevents the manifestation of MLD, and thus it was supposed that similar levels might be therapeutically effective in mice. 15,16 The finding that unexpectedly high amounts of ASA have to be substituted for correction of sulfatide storage may be due to several reasons. Low amounts of residual enzyme activity are sufficient to prevent storage in normal equilibrium conditions, in which sulfatide does not accumulate.…”

“…This value exceeds the enzyme activity levels which are sufficient to prevent the manifestation of MLD. 15,16 The enzyme levels in serum correlated with the amount of enzyme found in the kidney and liver, suggesting that endocytosis determines the amount of ASA delivered to these tissues. There was no correlation of serum ASA levels with the amount of enzyme found in the brain, which demonstrated that delivery to the brain is not due to endocytosis, but possibly due to bone marrow-derived cells migrating into the brain, which depending on the proviral integration site may express more or less ASA after microglial differentiation.…”

Prediction of phenotypic severity in mucopolysaccharidosis type IIIA

“…The increase in SGSH activity in the fibroblasts of SP patients was up to 43% of the lower limit of normal activity. Previous studies of other lysosomal enzymes, including glucosylceramide β‐glucosidase and β‐hexosaminidase A, did show that 10 to 15% of the residual lysosomal enzyme activity prevented the accumulation of metabolites 29, 30. This, in combination with the observation that HS levels even normalized in some fibroblast cell lines at 30 °C, suggests that this effect might be of clinical relevance if the same effect were induced, for example, by pharmacological chaperones.…”

Morphological, Biochemical and Molecular Biology Approaches for the Diagnosis of Lysosomal Storage Diseases

“…Accumulation of undegraded substrate occurs only when the residual activity is below the critical threshold of 10-15% of normal enzyme activity. 60 Depending on the nature of the defective enzyme and the type and site of the mutation, there may be great differences in the age of onset, severity of the manifestations, and organ involvement for each lysosomal storage disease. Furthermore, since the genetic defect, whether it be deficient enzyme activity, abnormal en- zyme targeting, or impaired product efflux, etc., occurs in all cell types of affected individuals, the amount of abnormal storage in specific cells is determined by the type and amount of available substrates that are recycled in these cells.…”