Residual N‐acetyl‐α‐glucosaminidase activity in fibroblasts correlates with disease severity in patients with mucopolysaccharidosis type IIIB

Abstract

BackgroundMucopolysaccharidosis type IIIB (MPS IIIB) is a rare genetic disorder in which the deficiency of the lysosomal enzyme N-acetyl-α-glucosaminidase (NAGLU) results in the accumulation of heparan sulfate (HS), leading to progressive neurocognitive deterioration. In MPS IIIB a wide spectrum of disease severity is seen. Due to a large allelic heterogeneity, establishing genotype-phenotype correlations is difficult. However, reliable prediction of the natural course of the disease is needed, in particular for the assessment of the efficacy of potential therapies.MethodsTo identify markers that correlate with disease severity, all Dutch patients diagnosed with MPS IIIB were characterised as either rapid (RP; classical, severe phenotype) or slow progressors (SP; non-classical, less severe phenotype), based on clinical data. NAGLU activity and HS levels were measured in patients’ fibroblasts after culturing at different temperatures.ResultsA small, though significant difference in NAGLU activity was measured between RP and SP patients after culturing at 37 °C (p < 0.01). Culturing at 30 °C resulted in more pronounced and significantly higher NAGLU activity levels in SP patients (p < 0.001) with a NAGLU activity of 0.58 nmol.mg-1.hr-1 calculated to be the optimal cut-off value to distinguish between the groups (sensitivity and specificity 100 %). A lower capacity of patients’ fibroblasts to increase NAGLU activity at 30 °C could significantly predict for the loss of several disease specific functions.ConclusionNAGLU activity in fibroblasts cultured at 30 °C can be used to discriminate between RP and SP MPS IIIB patients and the capacity of cells to increase NAGLU activity at lower temperatures correlates with disease symptoms.

Prediction of phenotypic severity in mucopolysaccharidosis type IIIA

“…Because these findings are based on a Dutch patient population containing a homogenous set of mutations, it would be expedient to repeat our studies in populations with different genotypes to confirm our findings. The potential to increase residual enzyme activity in fibroblasts cultured at a low temperature has been reported previously in the context of other genetic diseases, including MPS IIIB 16, 27, 28. The increase in SGSH activity in the fibroblasts of SP patients was up to 43% of the lower limit of normal activity.…”

“…Based on previous studies of MPS IIIB,16 we aimed to determine a method for predicting phenotypic severity in MPS IIIA patients using temperature sensitivity of residual SGSH activity in cultured fibroblasts 16…”

Observational Prospective Natural History of Patients with Sanfilippo Syndrome Type B

“…[13][14][15] Definitive laboratory confirmation of a suspected clinical phenotype depends on a comprehensive assessment of urine GAG substrate, deficient NAGLU enzyme activity, and the identification of pathologic mutations. 10,16 There is no proven effective therapy for MPS IIIB, but encouraging phase 1 results have been reported with enzyme replacement therapy 17 and gene therapy. 18 Management currently is symptomatic.…”

Utilizing ExAC to Assess the Hidden Contribution of Variants of Unknown Significance to Sanfilippo Type B Incidence

“…41 out of 164 assayed VUS displayed enzymatic activities that were less 67 than 15% of wild type (wt) NAGLU activity. 68 We chose 15% wt as a threshold below which mutations are considered deleterious 69 based on the fact that all but three out of 35 tested HGMD annotated pathogenic 70 mutations reduced activity below this level, including three variants associated with 71 attenuated disease (p.S612G, p.E634K, and p.L497V) [14][15][16]. The alleles associated 72 with attenuated phenotype showed activity values in the range of 13% to 14% of wt.…”