Expression studies of two novel in CIS-mutations identified in an intermediate case of Hunter syndrome

Genotype-phenotype studies in a large cohort of Brazilian patients with Hunter syndrome

Mucopolysaccharidosis type II (MPS II) is an X-linked inherited disease caused by pathogenic variants in the IDS gene, leading to deficiency of the lysosomal enzyme iduronate-2-sulfatase and consequent widespread storage of glycosaminoglycans, leading to several clinical consequences, with progressive manifestations which most times includes cognitive decline. MPS II has wide allelic and clinical heterogeneity and a complex genotype-phenotype correlation. We evaluated data from 501 Brazilian patients diagnosed with MPS II from 1982 to 2020. We genotyped 280 of these patients (55.9%), which were assigned to 206 different families. Point mutations were present in 70% of our patients, being missense variants the most frequent. We correlated the IDS pathogenic variants identified with the phenotype (neuronophatic or non-neuronopathic). Except for two half-brothers, there was no discordance in the genotype-phenotype correlation among family members, nor among MPS II patients from different families with the same single base-pair substitution variant. Mothers were carriers in 82.0% of the cases. This comprehensive study of the molecular profile of the MPS II cases in Brazil sheds light on the genotype-phenotype correlation and helps the better understanding of the disease and the prediction of its clinical course, enabling the provision of a more refined genetic counseling to the affected families.

Identification and Functional Characterization of IDS Gene Mutations Underlying Taiwanese Hunter Syndrome (Mucopolysaccharidosis Type II)

Hunter syndrome (mucopolysaccharidosis II; MPS II) is caused by a defect of the iduronate-2-sulfatase (IDS) gene. Few studies have reported integrated mutation data of Taiwanese MPS II phenotypes. In this study, we summarized genotype and phenotype correlations of confirmed MPS II patients and asymptomatic MPS II infants in Taiwan. Regular polymerase chain reaction and DNA sequencing were used to identify genetic abnormalities of 191 cases, including 51 unrelated patients with confirmed MPS II and 140 asymptomatic infants. IDS activity was analyzed in individual novel IDS variants using in vitro expression studies. Nineteen novel mutations were identified, in which the percentages of IDS activity of the novel missense mutations c.137A>C, c.311A>T, c.454A>C, c.797C>G, c.817C>T, c.998C>T, c.1106C>G, c.1400C>T, c.1402C>T, and c.1403G>A were significantly decreased (p < 0.001), c.254C>T and c.1025A>G were moderately decreased (p < 0.01), and c.851C>T was slightly decreased (p < 0.05) comparing with normal enzyme activity. The activities of the other six missense mutations were reduced but were insignificant. The results of genomic studies and their phenotypes were highly correlated. A greater understanding of the positive correlations may help to prevent the irreversible manifestations of Hunter syndrome, particularly in infants suspected of having asymptomatic MPS II. In addition, urinary glycosaminoglycan assay is important to diagnose Hunter syndrome since gene mutations are not definitive (could be non-pathogenic).

Mucopolysaccharidosis type II (Hunter syndrome): Clinical and biochemical aspects of the disease and approaches to its diagnosis and treatment

Mucopolysaccharidosis type II (MPS II, Hunter syndrome) is a rare X-linked lysosomal storage disease caused by mutations of the gene encoding the lysosomal enzyme iduronate-2-sulfatase (IDS), the role of which is to hydrolytically remove O-linked sulfates from the two glycosaminoglycans (GAGs) heparan sulfate (HS) and dermatan sulfate (DS). HS and DS are linear, heterogeneous polysaccharides composed of repeating disaccharide subunits of l-iduronic acid (IdoA) or d-glucuronic acid, (1→4)-linked to d-glucosamine (for HS), or (1→3)-linked to 2-acetamido-2-deoxy-d-galactose (N-acetyl-d-galactosamine) (for DS). In healthy cells, IDS cleaves the sulfo group found at the C-2 position of terminal non-reducing end IdoA residues in HS and DS. The loss of IDS enzyme activity leads to progressive lysosomal storage of HS and DS in tissues and organs such as the brain, liver, spleen, heart, bone, joints and airways. Consequently, this leads to the phenotypic features characteristic of the disease. This review provides an overview of the disease profile and clinical manifestation, with a particular focus on the biochemical basis of the disease and chemical approaches to the development of new diagnostics, as well as discussing current treatment options and emerging new therapies.

Molecular analysis of iduronate -2- sulfatase gene in Tunisian patients with mucopolysaccharidosis type II

Mucopolysaccharidosis type II (MPS II, Hunter syndrome) is X-linked recessive lysosomal storage disorder resulting from the defective activity of the enzyme iduronate-2-sulfatase (IDS). Hunter disease can vary from mild to severe, depending on the level of enzyme deficiency. We report the IDS mutation and polymorphisms causing the Hunter syndrome in patients from one family in Tunisia

PATIENTS AND METHODS

A preliminary diagnosis was made by qualitative detection of urinary glycosaminoglycans of the suspected MPS II probands. The IDS mutation and polymorphisms were determined on these probands and their family members by amplifying and sequencing each of the exons and intron-exon junctions of IDS gene.

RESULTS

The studied probands were homoallelic for p.R88P mutation. In addition, three known polymorphisms/sequence variants: IVS3-16 (c.419-16 delT), T214M (c.641C > T), T146T (c.438 C > T), IVS5-87(c.709-87G > A) and one previously unknown: IVS7+38(c.1006+38T > C were identified in the MPS II patients. These are the first Tunisian MPS II patients to be genotyped.

CONCLUSION

The identification of these mutation and polymorphisms and their genotype-phenotype correlation should facilitate prenatal diagnosis and counseling for MPS II in Tunisia, where a very high rate of consanguinity exists.

Special airway concerns in patients with mucopolysaccharidoses

The mucopolysaccharidoses are comprised of hereditary disorders joined by errant degradation of mucopolysaccharides. The relatively infrequent opportunity to care for these patients is evidenced by a fairly small number of case reports and anecdotal information. Though lifespan is increasing, onset of respiratory pathology or involvement remains portentous. We present two cases that punctuate the need for insightful decision making while managing the airway for these patients.

Mucopolysaccharidosis type II: an update on mutation spectrum

Mucopolysaccharidosis type II (MPS II; Hunter disease) is caused by deficiency of the enzyme iduronate-2-sulphatase (IDS) and patients present with a wide range of clinical signs and symptoms. The level of activity of IDS, however, does not allow prediction of phenotype. In our study of unrelated individuals with MPS II, alterations in the IDS gene could be identified in all 155 patients. Investigations in families in which the occurrence of MPS II was sporadic revealed mosaicism in the mothers of a small number of patients and a high frequency of de novo mutations occurring preferentially during male meiosis. Mutations identified in our patients include 27 large alterations and 128 small gene alterations (96 different alterations). These data further confirm the extreme heterogeneity of IDS gene alterations, as more than 330 have been reported to date. This genetic heterogeneity may explain the high degree of clinical heterogeneity in MPS II. Therefore, attempts have been made to establish genotype-phenotype correlations in order to provide an indication of the likely prognosis and a basis on which to evaluate treatment. To date, some progress has been made in predicting the clinical phenotype from the genotype although it remains difficult in a few individual cases. However, as the crystallographic 3D structure of IDS is yet to be determined, evaluation of the impact of mutations on IDS activity is often time consuming. Furthermore, if a given mutation is recurrent, some patients carrying the same change may present with different phenotypes, suggesting that factors other than the IDS gene (e.g. other genes, environmental factors) can modulate the clinical phenotype.

CONCLUSION

Although genotype-phenotype correlations may be difficult to establish, they will be of increasing importance for choosing the most appropriate therapy for an individual patient, as new therapeutic strategies may be targeted according to phenotype.

Effect of Hunter disease (mucopolysaccharidosis type II) mutations on molecular phenotypes of iduronate-2-sulfatase: enzymatic activity, protein processing and structural analysis

Mucopolysaccharidosis II (Hunter disease), a lysosomal storage disorder caused by a deficiency of iduronate-2-sulfatase (IDS), has variable clinical phenotypes. Nearly 300 different mutations have been identified in the IDS gene from patients with Hunter disease, but the correlation between the genotype and phenotype has remained unclear. We studied the characteristics of 11 missense mutations, which were detected in the patients or artificially introduced, using stable expression experiments and structural analysis. The mutants found in the attenuated phenotype showed considerable residual activity (0.2-2.4% of the wild-type IDS activity) and those in the severe phenotype had no activity. In immunoblot analysis, both the 73-75 kDa precursor and processed forms were detected in the expression of 'attenuated' mutants (R48P, A85T and W337R) and the artificial active site mutants (C84S, C84T). The 73-75 kDa initial precursor was detected in the 'severe' mutants (P86L, S333L, S349I, R468Q, R468L). The truncated 68 kDa precursor form was synthesized in the Q531X mutant. The results of immunoblotting indicated rapid degradation and/or insufficiency in processing as a result of structural alteration of the IDS protein. A combination of analyses of genotype and molecular phenotypes, including enzyme activity, protein processing and structural analysis with an engineered reference protein, could provide an avenue to understanding the molecular mechanism of the disease and could give a useful tool for the evaluation of possible therapeutic chemical compounds.

Identification of nine new IDS alleles in mucopolysaccharidosis II. Quantitative evaluation by real-time RT-PCR of mRNAs sensitive to nonsense-mediated and nonstop decay mechanisms

The present study aimed to characterize mutant alleles in Mucopolysaccharidosis II and evaluate possible reduction of mRNA amount consequent to nonsense-mediated or nonstop mRNA decay pathways. A combination of different approaches, including real-time RT-PCR, were used to molecularly characterize seventeen patients. Fifteen alleles were identified and nine of them were new. The novel alleles consisted of three missense mutations (p.S71R, p.P197R, p.C432R), two nonsense (p.Q66X, p.L359X), two frameshifts (p.V136fs75X, p.C432fs8X), one allele carrying two in-cis mutations [p.D252N;p.S369X], and a large deletion (p.G394_X551). Analysing these results it emerged that most of the alterations resulted in mutants leading to mRNAs with premature termination codons, and therefore, potentially sensitive to mRNA surveillance pathway. By using real-time RT-PCR, the mRNAs resulting (i) from substitutions that changed one amino acid to a stop codon (L359X, and S369X), or caused the shifted reading frame with premature introduction of a stop codon (C432fs8X), (ii) from large deletion (p.G394_X551) that included the termination codon, seemed to be subject to degradation by nonsense-mediated (i) or nonstop decay (ii) mechanisms, as mRNA was strongly underexpressed. On the contrary, two mutations (Q66X and V136fs75X) produced transcripts evading mRNA surveillance pathway despite both of them fulfilled the known criteria. These results confirm the wide variability of the mRNA expression levels previously reported and represent a further exception to the rules governing susceptibility to nonsense-mediated decay. A close examination of the molecular basis of the disease is becoming increasingly important for optimising the choices of available or forthcoming therapies such as, enzyme replacement therapy or enzyme enhancement therapy.

Detection of Hunter syndrome (mucopolysaccharidosis type II) in Taiwanese: biochemical and linkage studies of the iduronate-2-sulfatase gene defects in MPS II patients and carriers

BACKGROUND

Hunter syndrome (mucopolysaccharidosis type II) is an X-linked recessive lysosomal storage disease caused by a defect of the iduronate-2-sulfatase (IDS) gene. The result is impaired IDS enzyme function.

METHODS

To characterize the biochemical and molecular defects in IDS-deficient patients and their families, we measured IDS enzyme activity by fluorimetric enzyme assay and identified the IDS gene mutations in 14 unrelated Taiwanese patients with varying clinical phenotypes. In addition, haplotype analysis was also performed.

RESULTS

Three novel (IVS2+1G>C, 1055del12, and G489D) and 7 previously reported (N63K, P228L, K347E, R468Q, R468W, I485R, and 1241delAG) mutations were found. Together R468Q and R468W account for 42.8% mutations found in our patients. Haplotype analysis using IDS flanking markers DXS1113 and DXS1123 revealed that the unrelated R468Q alleles were independent in origin whereas the unrelated R468W alleles are probably of the same origin. The R468Q mutation in patient 1150 and I485R mutation in patient 710 occurred de novo in male meioses. Once the mutation in a family was identified, restriction analysis was also performed for rapid diagnosis of female carriers in 8 families. Leukocyte IDS measurement revealed significantly wide range of IDS activity in normal controls and MPS II carriers (19.2 - 70.6 vs. 8.4 - 26.6 nmol/h/mg cell protein). The average leukocyte IDS activity of normal controls (n=43) was 43.9+/-13.3 nmol/h/mg protein, whereas patients with MPS II (n=14) had <5% of mean normal IDS activity (0.9+/-0.6 nmol/h/mg protein), and carriers (n=13) had a mean activity of 17.5 (+/-5.7) nmol/h/mg protein. The mean leukocyte IDS activity in female carriers was less than a half of the normal level.

CONCLUSION

Due to a small overlapping range of normal and carriers, the level of enzyme activity cannot be used alone for carrier detection.

Expression studies of mutations underlying Taiwanese Hunter syndrome (mucopolysaccharidosis type II)

Nearly 300 different mutations underlying mucopolysaccharidosis type II (MPS II) have been identified worldwide. To investigate the molecular lesions underlying Taiwanese MPS II, probands and families were identified and screened for iduronate-2-sulfatase (IDS) mutation by single-strand conformation polymorphism and DNA sequencing. Five novel and five previously reported mutations were found. Together with those previously reported, a total of 17 identified missense, small deletion, and nonsense mutations were further characterized by transient expression studies. Transfection of COS-7 cells by the mutated cDNA did not yield active enzyme, demonstrating the deleterious nature of the mutations. A 57% decrease in IDS mRNA level was seen with the 231del6 mutation. Among the 11 missense mutations examined, K347E substitution showed apparent normal maturation and targeting on immunoblot and confocal fluorescence microscopy examination. The other 10 missense mutations showed apparent normal precursor with little or reduced mature forms, indicating normal maturation but incorrect targeting of the mutant enzymes. Among the six deletion and nonsense mutations examined, 1055del12 and E521X showed abnormal maturation. The staining pattern of the truncated W267X and 1184delG proteins suggested retention within early vacuolar compartments. The mutated 231del6 and 1421delAG proteins were unstable and largely degraded. Molecular analysis of the IDS gene will clearly identify the cause of the disease within patients and allow antenatal and family studies. The further characterization of gene mutations may delineate their functional consequences on IDS activity and processing and may enable future studies of genotype-phenotype correlation to estimate a prognosis and to lead to possible therapeutic interventions.