Abstract
Background: Mucopolysaccharidosis type I (MPS I) is a lysosomal storage disease caused by α-L-iduronidase (IDUA) deficiency. MPS I is a severe condition with a heterogeneous clinical spectrum including progressive visceral, bone and, in severe forms, neurological damage. The aim of this study was the detection of a novel mutation and a mutation that has been already described in the IDUAgene from four MPS I patients with various clinical phenotypes (severe; 2 cases, intermediate 1 case and mild; 1 case).
Patients and methods: Biological and molecular studies were carried out on 4 patients from 3 distinct families, each from a consanguineous marriage and originating from different regions of Tunisia: Mahres (Sax), Skhira (Sfax) and Kairouan. Indeed, bioinformatics software were used to predict the potential functional impact of the identified mutations on IDUA protein.
Results: Two IDUA mutations were detected: one is a p.His356_Gln362del mutation, a novel mutation found in two patients with a severe phenotype. The other mutation p.P533R that produces an intermediate and a mild phenotype was found in two patients.
Crystallographic analysis of the IDUA protein revealed that an amino acid sequence spanning from His 356 to Gln 362 forms an essential bend involved in substrate binding. Indeed, the new mutation results in a deletion of seven amino acids (His356_Gln362del) of this elbow, resulting in undectable enzymatic activity. This observation was confirmed in patient P3, who died at the age of 6 years. The p.P533R mutation involves the modification of a proline amino acid with an arginine in the IDUA protein. This substitution results in the introduction of a bulkier amino acid, requiring more space in the contact region between the β-sheet structure and the substrate-bound helix. It is likely that this leads to a decrease in the affinity between the IDUA protein and its substrate.
Conclusion: Our study on the genetic profile of MPSI has provided more information into the disease, particularly through the identification of a novel small deletion (His356_Gln362del) and the identification of the most frequently encountered in Tunisian population p.P533R.