Mucopolysaccharidosis type II in Tunisian families: IDS gene Variations Disrupting Substrate Binding and A Novel deep Intronic Deletion Reducing IDS expression

Abstract: Hunter syndrome is an X linked recessive lysosomal storage disease. This syndrome is caused by the deficiency of iduronate-2-sulfatase enzyme (IDS, EC3.1.6.13) that is involved in the degradation of macromolecules glycosaminoglycans, dermatan sulfate and heparan sulfate. Materials and Methods: This study involved three MPS II patients (WB, HMR, and BN) from three unrelated families (F1, F2, and F3) originating respectively from Kef, Ksar Saïd, and Tbolba. Genetic alterations were identified using DNA sequencing. To characterize the functional impact of a large intronic deletion and to assess the expression level of the IDS gene, the total mRNA was extracted from peripheral blood. Bioinformatics tools, including the SWISS-MODEL server and DynaMut, were used for structural modeling and to predict the impact of the mutations on protein stability and mechanism catalytic. Results: Two missense mutations, p.R88P and p.H138Y in the IDS gene were identified. A novel large intronic deletion in intron 3 was discovered in patient with severe MPS II phenotypes, while a previously reported missense mutation p.R88P and p.H138Y was found in two patients with mild phenotypes. Moreover, we identified a large number of single nucleotides sequences’ variants in hemizygous status. The real- time PCR expression analysis demonstrated a marked reduction in IDS mRNA levels, suggesting a deleterious effect of the large intronic deletion on transcript stability and IDS gene expression level. Structural analysis revealed that the two missense mutations cause structural deformation of IDS protein, and disrupts the protein’s substrate-binding site resulting in a complete loss of enzymatic activity. Conclusion: This study reports a novel deep intronic deletion in the IDS gene in Tunisian MPS II patients, alongside the previously described mutations. The findings enhance understanding of the molecular basis of MPS II.