A case of mucopolysaccharidosis I with severely increased albuminuria and Graves’ disease, and functional analysis of previously uncharacterized variant

Abstract

Abstract Background Mucopolysaccharidoses (MPS) are a group of inherited lysosomal storage disorders which results in accumulation of glycosaminoglycans throughout the body. MPS I is a subtype characterized by mutations in the α-L-iduronidase (IDUA) gene with subsequent multiorgan dysfunction. Case description Here, we describe an adult patient with MPS I who presented with anasarca, as well as severely increased albuminuria, and was initially diagnosed with hyperthyroidism. Subsequent testing revealed clinical characteristics and deficient IDUA enzyme activity consistent with MPS I. Genetic testing identified two variants of interest in the IDUA gene: IDUA c·265C > T (p.R89W) and IDUA c.1403-2A > G. Methods IDUA enzymatic activity was measured in HEK293T cells transfected with wildtype or p. R89W mutant IDUA expression constructs using the fluorogenic substrate 4-methylumbelliferyl-α-L-iduronide (4MU-iduronide). A minigene splicing assay was performed using IDUA exon 9–11 (including intron 9 and 10) DNA fragments cloned into the pMini-CopGFP vector, with the mutant plasmid containing the IDUA c.1403-2A > G variant generated using site-directed mutagenesis. HEK293T cells were transfected with either the wildtype or mutant plasmids, and splice products were separated by gel electrophoresis with distinct bands excised for Sanger sequencing. Results The IDUA c·265C > T (p.R89W) mutant construct was cloned into HEK293T cells and showed significantly decreased intracellular (10.86 ± 0.36%, p < 0.01) and extracellular (1.18 ± 0.09%, p < 0.01) enzymatic activity compared to wild type. A minigene splicing assay of the previously uncharacterized IDUA c.1403-2A > G splice site variant found retention of intron 9 between exon 9 and 10 leading to a premature termination codon. Conclusion This manuscript describes a unique clinical presentation of MPS I with simultaneous Graves’ disease and severely increased albuminuria. Functional analysis of the IDUA variants identified in our patient adds to the current understanding of genetic variants implicated in the disease.