Natural disease history and characterisation of SUMF1 molecular defects in ten unrelated patients with multiple sulfatase deficiency

Abstract

Background: Multiple sulfatase deficiency is a rare inherited metabolic disorder caused by mutations in the SUMF1 gene. The disease remains poorly known, often leading to a late diagnosis. This study aimed to provide improved knowledge of the disease, through complete clinical, biochemical, and molecular descriptions of a cohort of unrelated patients. The main objective was to identify prognostic markers, both phenotypic and genotypic, to accelerate the diagnosis and improve patient care.

Methods: The phenotypes of ten unrelated patients were fully documented at the clinical and biochemical levels. The long-term follow-up of each patient allowed correlations of the phenotypes to the disease outcomes. Each patient's molecular defects were also identified. Site-directed mutagenesis was used to individually express the mutants and assess their stability. Characterisation of the protein mutants was completed by in silico analyses based on sequence comparisons and structural models.

Results: The most severe cases were characterised by the presence of non-neurological symptoms as well as the occurrence of psychomotor regression before 2 years of age. Nine novel SUMF1 mutations were identified. Clinically severe forms were often associated with SUMF1 mutations that strongly affected the protein stability and/or catalytic function as predicted from in silico and western blot analyses.

Conclusions: This detailed clinical description and follow-up of a cohort of patients, together with the molecular characterisation of their underlying defects, contribute to improved knowledge of multiple sulfatase deficiency. Predictors of a bad prognosis were the presence of several non-neurological symptoms and the onset of psychomotor regression before 2 years of age. No strict correlation existed between in vitro residual sulfatase activity and disease severity. Genotype-phenotype correlations related to previously reported mutants were strengthened. These and previous observations allow not only improved prediction of the disease outcome but also provision of appropriate care for patients, in the expectation of specific treatment development.

Figure 1

Western blot analysis of overexpressed wild-type and mutant FGE. Lysates of HEK293T cells transiently transfected with a vector encoding C-terminally flagged wt or mutant FGE (25 μg protein) were assessed for FGE expression by Western-blot. Upper panel: FGE. Lower panel: loading control. NT, not transfected; DUP, p.V174-P318dup.

Figure 2

Representation of FGE and the main mutated residues identified in this and previous studies. Cartoon representation of the FGE structure (PDB code 1Y1E) with helices in blue, beta-strands in violet, and loops in yellow. Represented in ball-and-stick and labelled are (i) essential or important residues for substrate binding and catalytic activity (orange carbon atoms), (ii) residues whose mutations have been previously described (magenta carbon atoms) [6,16,17,27], and (iii) newly identified mutations (cyan carbon atoms). N, O, and S atoms are in blue, red, and green, respectively. The two structural calcium ions are also represented (green spheres).

Figure 3

Structural modifications of FGE due to mutations identified in MSD patients. Details of the conformation of wt (left panel) and mutant (right panel) FGE are shown for three missense mutations: p.R343S (A); p.A298E (B) and p.N259S (C).