Prediction of Sanfilippo phenotype severity from immunoquantification of heparan-N-sulfamidase in cultured fibroblasts from mucopolysaccharidosis type IIIA patients

Abstract

Mucopolysaccharidosis type IIIA (MPS-IIIA) is an autosomal recessive lysosomal storage disorder caused by the deficiency of heparan-N-sulfamidase (NS; EC 3.10.1.1), resulting in defective degradation and subsequent storage of heparan sulfate and leading to a clinical phenotype known as Sanfilippo syndrome. A sensitive and specific monoclonal/polyclonal-based immunoquantification assay has enabled the determination of NS protein, down to approximately 3 pg NS protein, in cultured fibroblasts from control and MPS-IIIA patients. Cultured skin fibroblasts from 15 normal controls contained 11.9 to 105 ng of NS protein/mg extracted cell protein, whereas NS protein ranged from "none detected" to 11 ng/mg in fibroblasts from 35 MPS-IIIA patients. A relationship between genotype/phenotype and amount of NS protein present in these MPS-IIIA fibroblasts was established. Immunoquantification, in combination with a specific and highly sensitive tetrasaccharide-based assay of NS activity, enabled the determination of residual specific NS activity in these fibroblasts. Specific NS activity ranged from 28 to 1289 nmol/min/mg NS protein for MPS-IIIA patients, compared to 870 nmol/min/mg of recombinant human NS. It is proposed that this immunoquantification method, in conjunction with the specific NS activity assay, may be used to predict clinical severity in MPS-IIIA patients, allowing for the selection of individuals best suited for gene- and enzyme-replacement therapy when these methods become available. Also proposed is that an enzyme-replacement therapy achieving a correction of approximately 10% of normal NS activity is required to avoid the onset of a Sanfilippo clinical phenotype.