BACKGROUND: The phenotypic spectrum of many rare disorders is much wider than previously considered. Mucopolysaccharidosis type III (Sanfilippo syndrome, MPS III), is a lysosomal storage disorder traditionally considered to be characterized by childhood onset, progressive neurocognitive deterioration with a rapidly or slowly progressing phenotype. The presented MPS III case series demonstrates adult onset phenotypes with mild cognitive impairment or non-neuronopathic phenotypes. METHODS: In this case series all adult MPS III patients with a mild- or non-neuronopathic phenotype, who attend the outpatient clinic of 3 expert centers for lysosomal storage disorders were included. A mild- or non-neuronopathic phenotype was defined as having completed regular secondary education and attaining a level of independency during adulthood, involving either independent living or a paid job. RESULTS: Twelve patients from six families, with a median age at diagnosis of 43 years (range 3-68) were included (11 MPS IIIA, 1 MPS IIIB). In the four index patients symptoms which led to diagnostic studies (whole exome sequencing and metabolomics) resulting in the diagnosis of MPS III; two patients presented with retinal dystrophy, one with hypertrophic cardiomyopathy and one with neurocognitive decline. The other eight patients were diagnosed by family screening. At a median age of 47 years (range 19-74) 9 out of the 12 patients had normal cognitive functions. Nine patients had retinal dystrophy and 8 patients hypertrophic cardiomyopathy. CONCLUSION: We show the very mild end of the phenotypic spectrum of MPS III, ranging from late-onset stable neurocognitive impairment to a fully non-neuronopathic phenotype. Awareness of this phenotype could lead to timely diagnosis and genetic counseling.

Additional Metadata
Keywords Learning difficulties, Mucopolysaccharidosis type III, Neuropsychology assessment, Phenotypic spectrum, Sanfilippo syndrome
Persistent URL dx.doi.org/10.1186/s13023-019-1232-0, hdl.handle.net/1765/121620
Journal Orphanet journal of rare diseases
Citation
Nijmeijer, S.C.M. (Stephanie C M), van den Born, L.I. (L Ingeborg), Kievit, A.J.A, Stepien, K.M. (Karolina M.), Langendonk, J.G, Marchal, J.P. (Jan Pieter), … Wagenmakers, M.A.E.M. (Margreet A E M). (2019). The attenuated end of the phenotypic spectrum in MPS III: from late-onset stable cognitive impairment to a non-neuronopathic phenotype. Orphanet journal of rare diseases, 14(1). doi:10.1186/s13023-019-1232-0