Background: Mucopolysaccharidosis 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. Methods: To 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. Results: A 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. Conclusion: NAGLU 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.

dx.doi.org/10.1007/s10545-016-9916-2, hdl.handle.net/1765/87169
Journal of Inherited Metabolic Disease
Department of Clinical Genetics

Meijer, O.L.M, Welling, L, Valstar, M.J, Hoefsloot, E.H, Brüggenwirth, H.T, van der Ploeg, A.T, … van Vlies, N. (2016). Residual N-acetyl-α-glucosaminidase activity in fibroblasts correlates with disease severity in patients with mucopolysaccharidosis type IIIB. Journal of Inherited Metabolic Disease, 39(3), 437–445. doi:10.1007/s10545-016-9916-2