Mucopolysaccharidosis type I: molecular characteristics of two novel alpha-L-iduronidase mutations in Tunisian patients

Mutation Analysis of the IDUA Gene in Iranian Patients with Mucopolysaccharidosis Type 1: Identification of Four Novel Mutations

Background and Purpose: Mucopolysaccharidosis 1 (MPS1) is an autosomal recessive disorder of a lysosomal enzyme called alpha-l-iduronidase caused by mutations in the IDUA gene. This enzyme is responsible for the degradation of the mucopolysaccharides, heparan sulfate, and dermatan sulfate. Based on clinical features and enzyme deficiency, MPS1 is divided into three subtypes, including a severe subtype (Hurler syndrome), an intermediate subtype (Hurler-Scheie syndrome), and an attenuated subtype (Scheie syndrome). The objective of this study was to characterize the mutation profiles of 17 Iranian patients with MPS1 and characterize the clinical features associated with their genotypes. Materials and Methods: Polymerase chain reaction-based sequencing of the IDUA gene was carried out for 10 patients with clinical diagnoses of MPS1 and 50 healthy controls. To estimate the impact of newly identified variants on the structure and function of the encoded alpha-l-iduronidase, in silico analyses was performed. Results: Eight genetic variations were detected, including five missense mutations (p.M1L, p.G51D, p.G134V, p.S157P, p.D301E), two nonsense mutations (p.W402* and p.Y343*), and one deletion (p.GFLNYY197-202), among which p.G134V, p.S157P, p.D301E, and p.GFLNYY197-202 were novel variations that had not been previously reported. Conclusion: After combining the results of the two previous IDUA gene studies performed on Iranian MPS1 patients and the results obtained from the current study, it is inferred that despite the presence of a number of previously known mutations, about half of the detected variations were unique in Iranian patients.

Novel splice site IDUA gene mutation in Tunisian pedigrees with hurler syndrome

BACKGROUND

The mucopolysaccharidosis type I (MPS I) is a lysosomal storage disease resulting from the defective activity of the enzyme α-L-iduronidase (IDUA). The disease has three major clinical subtypes (severe Hurler syndrome, intermediate Hurler-Scheie syndrome and attenuated Scheie syndrome). We aim to identify the genetic variants in MPS I patients and to investigate the effect of the novel splice site mutation on splicing of IDUA- mRNA variability using bioinformatics tools.

METHODS

The IDUA mutations were determined in four MPS I patients from four families from Northern Tunisia, by amplifying and sequencing each of the IDUA exons and intron-exon junctions.

RESULTS

One novel splice site IDUA mutation, c.1650 + 1G > T in intron 11 and two previously reported mutations, p.A75T and p.R555H, were detected. The patients in families 1 and 2 who have the Hurler phenotype were homozygotes for the novel splice site mutation c.1650 + 1G > T. The patient in family 3, who also had the Hurler phenotype, was a compound heterozygote for the novel splice site mutation c.1650 + 1G > T and for the previously reported missense mutation p.A75T. The patient in family 4 who had the Hurler-Scheie phenotype was a compound heterozygote for the novel splice site mutation c.1650 + 1G > T and for the previously reported missense mutation p.R555H. In addition, four known IDUA polymorphisms were identified. Bioinformatics tools allowed us to associate the variant c.1650 + 1G > T with the severe clinical phenotype of MPS I. This variant affects the essential nucleotide + 1 (G to T) of the donor splice site of IDUA intron 11. The G > T in intron 11 leads to wild type donor site broken with minus 19.97% value compared to normal value with 0%, hence the new splice site acceptor has plus 5.59%.

CONCLUSIONS

The present findings indicate that the identified mutations facilitate the accurate carrier detection (genetic counseling of at-risk relatives) and the molecular prenatal diagnosis in Tunisia.

Novel frameshift variant in the IDUA gene underlies Mucopolysaccharidoses type I in a consanguineous Yemeni pedigree

Mucopolysaccharidosis type I (MPS I) is an autosomal recessive storage disorder that result as a consequence of a deficiency in the lysosomal hydrolase, a-L-iduronidase enzyme encoded by IDUA gene. Over a hundred causative variants in IDUA have been identified, which result in a progressive multi-systemic disease with a broad range of severity and disease progression reported across affected individuals. The aim of this study was the detection and interpretation of IDUA mutation in a family with two children affected with lethal MPS I. The IDUA gene was sequenced in the parents of two deceased children who had a clinical diagnosis of MPS I, to assess their carrier status and to help inform on risk in future children. The sequencing analysis was performed by PCR and bidirectional Sanger sequencing of the coding region and exon-intron splice junctions at Labor MVZ Westmecklenburg molecular diagnostics laboratory. A heterozygous c.657delA variant in exon 6 was identified in each parent, which is the most likely explanation for disease in their children. This report represents the first Yemeni family to have a molecular diagnosis for MPS I.

Clinical and Molecular Characterization of Patients with Mucopolysaccharidosis Type I in an Algerian Series

Mucopolysaccharidoses (MPS’s) represent a subgroup of lysosomal storage diseases related to a deficiency of enzymes that catalyze glycosaminoglycans degradation. Mucopolysaccharidosis type I (MPS I) is a rare autosomal recessive disorder caused by a deficiency of α-l-iduronidase encoded by the IDUA gene. Partially degraded heparan sulfate and dermatan sulfate accumulate progressively and lead to multiorgan dysfunction and damage. The aim of this study is to describe the clinical, biochemical, and molecular characteristics of 13 Algerian patients from 11 distinct families. MPS I diagnosis was confirmed by molecular study of the patients’ IDUA gene. Clinical features at the diagnosis and during the follow-up are reported. Eighty-four percent of the studied patients presented with a mild clinical phenotype. Molecular study of the IDUA gene allowed the characterization of four pathological variations at the homozygous or compound heterozygote status: IDUA NM_00203.4:c.1598C>G-p.(Pro533Arg) in 21/26 alleles, IDUA NM_00203.4:c.532G>A-p.(Glu178Lys) in 2/26 alleles, IDUA NM_00203.4:c.501C>G-p.(Tyr167*) in 2/26 alleles, and IDUA NM_00203. 4: c.1743C>G-p.(Tyr581*) in 1/26 alleles. This molecular study unveils the predominance of p.(Pro533Arg) variation in our MPS I patients. In this series, the occurrence of some clinical features linked to the Scheie syndrome is consistent with the literature, such as systematic valvulopathies, corneal opacity, and umbilical hernia; however, storage signs, facial dysmorphic features, and hepatomegaly were more frequent in our series. Screening measures for these debilitating diseases in highly consanguineous at-risk populations must be considered a priority health problem.

Factors and processes modulating phenotypes in neuronopathic lysosomal storage diseases

Lysosomal storage diseases are inherited metabolic disorders caused by genetic defects causing deficiency of various lysosomal proteins, and resultant accumulation of non-degraded compounds. They are multisystemic diseases, and in most of them (>70%) severe brain dysfunctions are evident. However, expression of various phenotypes in particular diseases is extremely variable, from non-neuronopathic to severely neurodegenerative in the deficiency of the same enzyme. Although all lysosomal storage diseases are monogenic, clear genotype-phenotype correlations occur only in some cases. In this article, we present an overview on various factors and processes, both general and specific for certain disorders, that can significantly modulate expression of phenotypes in these diseases. On the basis of recent reports describing studies on both animal models and clinical data, we propose a hypothesis that efficiency of production of compounds that cannot be degraded due to enzyme deficiency might be especially important in modulation of phenotypes of patients suffering from lysosomal storage diseases.

Structural and clinical implications of amino acid substitutions in α-L-iduronidase: insight into the basis of mucopolysaccharidosis type I

Allelic mutations, predominantly missense ones, of the α-l-iduronidase (IDUA) gene cause mucopolysaccharidosis type I (MPS I), which exhibits heterogeneous phenotypes. These phenotypes are basically classified into severe, intermediate, and attenuated types. We previously examined the structural changes in IDUA due to MPS I by homology modeling, but the reliability was limited because of the low sequence identity. In this study, we built new structural models of mutant IDUAs due to 57 amino acid substitutions that had been identified in 27 severe, 1 severe-intermediate, 13 intermediate, 1 attenuated-intermediate and 15 attenuated type MPS I patients based on the crystal structure of human IDUA, which was recently determined by us. The structural changes were examined by calculating the root-mean-square distances (RMSD) and the number of atoms influenced by the amino acid replacements. The results revealed that the structural changes of the enzyme protein tended to be correlated with the severity of the disease. Then we focused on the structural changes resulting from amino acid replacements in the immunoglobulin-like domain and adjacent region, of which the structure had been missing in the IDUA model previously built. Coloring of atoms influenced by an amino acid substitution was performed in each case and the results revealed that the structural changes occurred in a region far from the active site of IDUA, suggesting that they affected protein folding. Structural analysis is thus useful for elucidation of the basis of MPS I.

An algorithm to predict phenotypic severity in mucopolysaccharidosis type I in the first month of life

Introduction

Mucopolysaccharidosis type I (MPS I) is a progressive multisystem lysosomal storage disease caused by deficiency of the enzyme α-L-iduronidase (IDUA). Patients present with a continuous spectrum of disease severity, and the most severely affected patients (Hurler phenotype; MPS I-H) develop progressive cognitive impairment. The treatment of choice for MPS I-H patients is haematopoietic stem cell transplantation, while patients with the more attenuated phenotypes benefit from enzyme replacement therapy.

The potential of newborn screening (NBS) for MPS I is currently studied in many countries. NBS for MPS I, however, necessitates early assessment of the phenotype, in order to decide on the appropriate treatment. In this study, we developed an algorithm to predict phenotypic severity in newborn MPS I patients.

Methods

Thirty patients were included in this study. Genotypes were collected from all patients and all patients were phenotypically categorized at an age of > 18 months based on the clinical course of the disease. In 18 patients, IDUA activity in fibroblast cultures was measured using an optimized IDUA assay. Clinical characteristics from the first month of life were collected from 23 patients.

Results

Homozygosity or compound heterozygosity for specific mutations which are associated with MPS I-H, discriminated a subset of patients with MPS I-H from patients with more attenuated phenotypes (specificity 100%, sensitivity 82%). Next, we found that enzymatic analysis of IDUA activity in fibroblasts allowed identification of patients affected by MPS I-H. Therefore, residual IDUA activity in fibroblasts was introduced as second step in the algorithm. Patients with an IDUA activity of < 0.32 nmol x mg^-1 × hr^-1 invariably were MPS I-H patients, while an IDUA activity of > 0.66 nmol × mg^-1 × hr^-1 was only observed in more attenuated patients. Patients with an intermediate IDUA activity could be further classified by the presence of differentiating clinical characteristics, resulting in a model with 100% sensitivity and specificity for this cohort of patients.

Conclusion

Using genetic, biochemical and clinical characteristics, all potentially available in the newborn period, an algorithm was developed to predict the MPS I phenotype, allowing timely initiation of the optimal treatment strategy after introduction of NBS.

Molecular confirmation of founder mutation c.-167A>G in Tunisian patients with PMLD disease

Pelizaeus Merzbacher disease and Pelizaeus Merzbacher like disease (PMLD) are hypomyelinating leucodystrophies of the central nervous system (CNS) with a very similar phenotype. PMD is an X-linked recessive condition caused by mutations, deletion duplication or triplication of the proteolipid protein 1 gene (PLP1). However, PMLD is a recessive autosomal hypomyelinating leukodystrophy caused by mutations of the GJC2 gene. In this study, we analyzed 5 patients belonging to 4 Tunisian families. Direct sequencing of GJC2 gene in all probands showed the same homozygous founder mutation c.-167A>G localized in the promoter region. We also generated two microsatellite markers GJC2 195GT and GJC2 76AC closed to the GJC2 gene to confirm the presence of a founder effect for this mutation. Haplotype study showed that the c.-167A>G promoter mutation occurred in a specific founder haplotype in Tunisian population. The identification of this founder mutation has important implications towards genetic counseling in relatives of these families and the antenatal diagnosis.

Hurler disease (mucopolysaccharidosis type IH): clinical features and consanguinity in Tunisian population

Mucopolysaccharidosis type I (MPS I) was a group of rare autosomal recessive disorder caused by the deficiency of the lysosomal enzyme, alpha -L -iduronidase, and the resulting accumulation of undergraded dematan sulfate and heparan sulfate. MPS I patients have a wide range of clinical presentations, that makes it difficult to predict patient phenotype which is needed for genetic counseling and also impedes the selection and evaluation of patients undergoing therapy bone marrow transplantation.

AIM OF THE STUDY

consanguinity rates have been determined among 14 families with mucopolysaccharidosis type I, seen in the pediatric departments of different geographic areas of Tunisia (Central and Southern areas) for the period August 2004 - August 2011 in order to investigate the relation between consanguinity and this disorder.

PATIENTS AND METHODS

Clinical and molecular analyses confirmed the diagnosis for MPS type I in the studied families.

RESULTS

Most of the Tunisian MPS I patients have been identified at the homozygous status: p.P533R mutation (7 homozygous and one double heterozygous p.L578Q/p.P533R patients; 41.66% of all the investigated MPSI patients), p.F177S (1 homozygous patient; 5.55%), p.L530fs (1 patient; 5.55%), p.Y581X (2 patients; 11.11%), p.F602X (3 patients; 16.66%), p.R628X (1 patient; 5.55%). Another mutation: p.L578Q has been identified at the heterozygous status in the only double heterozygous p.L578Q/p.P533R case. Part of the mutations was the result of a founder effect. These described points are the consequences of the high rate of consanguinity.

CONCLUSION

The high frequency of p.P533R mutation could be explained by the high degree of inbreeding. This is due to the richness of the genetic background of the studied population.A multidisciplinary approach is essential to develop adequate preventive program adapted to the social, cultural, and economic context.