The clinical phenotype of two patients with a complete deletion of the iduronate-2-sulphatase gene (mucopolysaccharidosis II--Hunter syndrome)

Mapping an X-linked locus that influences heat-induced febrile seizures in mice

PURPOSE

Febrile seizures (FS) are the most common seizure type in children between the age of 6 months and 5 years. Although FS are largely benign, recurrent FS are a major risk factor for developing temporal lobe epilepsy (TLE) later in life. The mechanisms underlying FS are largely unknown; however, family and twin studies indicate that FS susceptibility is under complex genetic control. We have recently developed a phenotypic screen to study the genetics of FS susceptibility in mice. Using this screen in a phenotype-driven genetic strategy we analyzed the C57BL/6J-Chr #(A)/NaJ chromosome substitution strain (CSS) panel. In each CSS line one chromosome of the A/J strain is substituted in a genetically homogeneous C57BL/6J background. The analysis of the CSS panel revealed that A/J chromosomes 1, 2, 6, 10, 13, and X carry at least one quantitative trait locus (QTL) for heat-induced FS susceptibility. The fact that many X-linked genes are highly expressed in the brain and have been implicated in human developmental disorders often presenting with seizures (like fragile X mental retardation) prompted us to map the chromosome X QTL.

METHODS

C57BL/6J mice were mated with C57BL/6J-Chr X(A) /NaJ (CSSX) to generate F(2)-generations-CXBL6 and BL6CX-originating from CSSX or C57BL/6J mothers, respectively. Heat-induced FS were elicited on postnatal day 14 by exposure to a controlled warm airstream of 50°C. The latency to heat-induced FS is our phenotype. This phenotype has previously been validated by video-electroencephalography (EEG) monitoring. After phenotyping and genotyping the F(2)-population, QTL analysis was performed using R/QTL software.

KEY FINDINGS

QTL analysis revealed a significant peak with an LOD-score of 3.25. The 1-LOD confidence interval (149,886,866-158,836,462 bp) comprises 52 protein coding genes, of which 34 are known to be brain expressed. Two of these brain-expressed genes have previously been linked to X-linked epilepsies, namely Cdkl5 and Pdha1.

SIGNIFICANCE

Our results show that the mouse genetics of X-linked FS susceptibility is complex, and that our heat-induced FS-driven genetic approach is a powerful tool for use in unraveling the complexities of this trait in mice. Fine-mapping and functional studies will be required to further identify the X-linked FS susceptibility genes.

A novel mutation of IDS gene in a Chinese patient with mucopolysaccharidosis II by next-generation sequencing

BACKGROUND

Mucopolysaccharidosis (MPS) is induced by the absence or malfunctioning of lysosomal enzymes. MPS I and MPS II are similar in phenotypes but they are different in genotypes, which are caused by the deficiencies of alpha-L-iduronidase gene (IDUA) and iduronate 2-sulfatase gene (IDS) respectively. In this work, a 5-year-old Chinese young male with manifestations of MPS in a family with unaffected parents was described.

METHODS

12 kb of all the targeted exon sequences plus flanking sequences chromosomal DNA of IDS and IDUA genes from the proband and 20 other case-unrelated controls were captured and sequenced by using next-generation sequencing technology.

RESULTS

One single-nucleotide deletion variant (c.1270delG) resulting in frameshift and premature truncation of I2S enzyme was detected, out of 20 controls, only in the proband, and which was further verified by Sanger sequencing. The proband's mother was also proved carrying c.1270delG by Sanger method but not for his father.

CONCLUSIONS

The novel variant (c.1270delG) is a candidate disease-causing mutation predicted to affect the normal structure and function of the enzyme. Target sequence capture and next-generation sequencing technology can be effective for the gene testing of MPS II disorder.

First experience of enzyme replacement therapy with idursulfase in Spanish patients with Hunter syndrome under 5 years of age: case observations from the Hunter Outcome Survey (HOS)

Hunter syndrome (mucopolysaccharidosis type II [MPS II], OMIM309900) is a rare X-linked lysosomal storage disorder caused by deficiency of the enzyme iduronate-2-sulphatase, resulting in accumulation of glycosaminoglycans (GAGs), multisystem organ failure and early death. Enzyme replacement therapy (ERT) with idursulfase is commercially available since 2007. Early access programs were established since 2005. However, limited information on the effects of ERT in young children is available to date. The aim of this analysis was therefore to determine the effects of ERT on patients younger than 5 years of age. We report data from six Spanish patients with confirmed Hunter syndrome who were younger than 5 years at the start of ERT, and had been treated with weekly intravenous infusions of idursulfase between 6 and 14 months. Baseline and treatment data were obtained from the Hunter Outcome Survey (HOS). HOS is an international database of MPS II patients on ERT or candidates to be treated, that collects data in a registry manner. HOS is supported by Shire Human Genetic Therapies, Inc. (Cambridge, MA, USA). At baseline, all patients showed neurological abnormalities, including ventriculomegaly, hydrocephaly, cerebral atrophy, perivascular changes and white matter lesions. Other signs and symptoms included thoracic deformity, otitis media, joint stiffness and hepatosplenomegaly, demonstrating that children under 5 years old can also be severely affected. ERT reduced urinary GAG levels, and reduced spleen (n = 2) and liver size (n = 1) after only 8 months. Height growth was maintained within the normal range during ERT. Joint mobility either stabilized or improved during ERT. In conclusion, this case series confirms the early onset of signs and symptoms of Hunter syndrome and provides the first evidence of ERT beneficial effects in patients less than 5 years of age. Similar efficacy and safety profiles to those seen in older children can be suggested, although further studies including a direct comparison with older patients would still be required.

Neurological findings in Hunter disease: pathology and possible therapeutic effects reviewed

Hunter disease (mucopolysaccharidosis type II, MPS II) is an X-linked lysosomal storage disease caused by deficiency of iduronate-2-sulfatase. Accumulation of chondroitin sulfate B and heparan sulfate in various tissues is the biochemical consequence of MPS II. Children with Hunter disease are normal at birth, and symptoms occur between 2 and 10 years of age. Typical symptoms include coarse facies with enlarged tongue and prominent forehead as well as a short, stocky built stature with short neck. The cardiovascular, respiratory and gastrointestinal systems may be affected, and oral, dermatological and psychiatric as well as neurological complications are described. Life expectancy is markedly reduced and may be limited to 12 years for severely affected patients. The most common causes of death are airway obstruction and cardiac failure. The most severe symptoms may result from neurological symptoms or complications including hydrocephalus, spinal cord compression, cervical myelopathy, optic nerve compression, and hearing impairment. Patients may also develop carpal tunnel syndrome, sleep apnoea, seizures or mental retardation. This review describes characteristic neurological manifestations in MPS II and its underlying pathophysiology. In addition, an appraisal is given whether or not enzyme replacement therapy may be able to improve in particular the neurological symptoms of Hunter disease.

Mucopolysaccharidosis type II (Hunter syndrome): a clinical review and recommendations for treatment in the era of enzyme replacement therapy

Mucopolysaccharidosis type II (MPS II; Hunter syndrome) is a rare X-linked recessive disease caused by deficiency of the lysosomal enzyme iduronate-2-sulphatase, leading to progressive accumulation of glycosaminoglycans in nearly all cell types, tissues and organs. Clinical manifestations include severe airway obstruction, skeletal deformities, cardiomyopathy and, in most patients, neurological decline. Death usually occurs in the second decade of life, although some patients with less severe disease have survived into their fifth or sixth decade. Until recently, there has been no effective therapy for MPS II, and care has been palliative. Enzyme replacement therapy (ERT) with recombinant human iduronate-2-sulphatase (idursulfase), however, has now been introduced. Weekly intravenous infusions of idursulfase have been shown to improve many of the signs and symptoms and overall wellbeing in patients with MPS II. This paper provides an overview of the clinical manifestations, diagnosis and symptomatic management of patients with MPS II and provides recommendations for the use of ERT. The issue of treating very young patients and those with CNS involvement is also discussed. ERT with idursulfase has the potential to benefit many patients with MPS II, especially if started early in the course of the disease.

Hyposulfatemia, growth retardation, reduced fertility, and seizures in mice lacking a functional NaSi-1 gene

Inorganic sulfate is required for numerous functions in mammalian physiology, and its circulating levels are proposed to be maintained by the Na+-SO42- cotransporter, (NaSi-1). To determine the role of NaSi-1 in sulfate homeostasis and the physiological consequences in its absence, we have generated a mouse lacking a functional NaSi-1 gene, Nas1. Serum sulfate concentration was reduced by >75% in Nas1-/- mice when compared with Nas1+/+ mice. Nas1-/- mice exhibit increased urinary sulfate excretion, reduced renal and intestinal Na+-SO42- cotransport, and a general growth retardation. Nas1-/- mouse body weight was reduced by >20% when compared with Nas1+/+ and Nas1+/- littermates at 2 weeks of age and remained so throughout adulthood. Nas1-/- females had a lowered fertility, with a 60% reduction in litter size. Spontaneous clonic seizures were observed in Nas1-/- mice from 8 months of age. These data demonstrate NaSi-1 is essential for maintaining sulfate homeostasis, and its expression is necessary for a wide range of physiological functions.

Mutation analysis in 57 unrelated patients with MPS II (Hunter's disease)

Genomic DNA from 57 unrelated MPS II (Hunter's disease) patients was analysed for mutations of the iduronate sulphatase (IDS) gene. The aim of the study was threefold: to identify the primary genetic lesion in patients, to investigate the correlation between genotype and phenotype, and most importantly, to provide reliable carrier testing for female members once the family mutation was identified. In 42 patients, point mutations were identified involving single base substitutions, deletions, or insertions. These included four new nonsense mutations (R8X, C84X, E245X, Y466X), six new missense mutations (D45N, N115Y, P228L, P266R, E434K, I485K, W502C), three new insertions (c70C71ins, c652C654ins, c709G710ins), six new deletions (c500delC, c705delC, c1023delA, c1049delA, c1141delC, c1576delG), and five new mutations involving splice sites (IVS1-2 a-->g, IVS2-10 t-->g, IVS5 + 2 t-->g L236L, IVS7 + 2 t-->c). One patient had a new seven base deletion in exon 9 (c1482-1488del). Four patients were shown to have complete deletions of the IDS gene and two deletions involved one or more exons. Previously described mutations present in these patients were Q80X, P86L, R172X, G374G, S333L, R443X, and R468Q. In eight patients, no mutation was detected throughout the entire coding region. Most mutations that result in MPS II appear to be unique. Absence of the probands' mutations in eight of nine maternal grandmothers suggests many mutations have arisen recently. Prediction of the clinical phenotype from the identified genotype was difficult in some families, and further studies using reverse transcription polymerase chain reaction are needed to confirm the predicted effects on the IDS mRNA suggested by genomic analysis.

Molecular and phenotypic variation in patients with severe Hunter syndrome

Severe Hunter syndrome is a fatal X-linked lysosomal storage disorder caused by iduronate-2-sulphatase (IDS) deficiency. Patients with complete deletion of the IDS locus often have atypical phenotypes including ptosis, obstructive sleep apnoea, and the occurrence of seizures. We have used genomic DNA sequencing to identify several new genes in the IDS region. DNA deletion patients with atypical symptoms have been analysed to determine whether these atypical symptoms could be due to involvement of these other loci. The occurrence of seizures in two individuals correlated with a deletion extending proximal of IDS, up to and including part of the FMR2 locus. Other (non-seizure) symptoms were associated with distal deletions. In addition, a group of patients with no variant symptoms, and a characteristic rearrangement involving a recombination between the IDS gene and an adjacent IDS pseudogene (IDS psi), showed normal expression of loci distal to IDS. Together, these results identify FMR2 as a candidate gene for seizures, when mutated along with IDS.

Bone marrow transplantation in Hunter syndrome

Hunter syndrome (mucopolysaccharidosis II) is a rare X-linked disorder of mucopolysaccharide metabolism that typically progresses to severe mental retardation and death by 18 years of age. A child with Hunter syndrome received an allogeneic bone marrow transplantation from an unaffected human leukocyte antigen-identical sibling at the age of 29 months without complications. Despite full and sustained engraftment now at 70 months after transplantation, the patient's neurocognitive abilities have continued to deteriorate. In this case, replacement of defective marrow-derived macrophages by bone marrow transplantation was not effective in preventing the neurologic progression of the disease in a child with the severe phenotype of Hunter syndrome.

130 kb of DNA sequence reveals two new genes and a regional duplication distal to the human iduronate-2-sulfate sulfatase locus

Deficiency of IDs activity results in Hunter Syndrome (mucopolysaccharidosis type II), a fatal X-linked recessive disorder. We report characterization of 28 cosmids around the IDS locus in Xq28. Four overlapping cosmids have been sequenced in their entirety generating a 130-kb contig. These studies show the fine structure of the IDS gene and identify an IDS pseudogene-like structure located 20 kb distal to the active gene. Two novel genes have also been identified in this sequence, and one of these genes is also locally duplicated. Both homologs are expressed, and a number of alternative transcript products have been characterized. The presence of a highly conserved pseudogene-like structure within a larger duplicated region close to the IDS gene has significant implications for the study of mutations at this locus.

Mucopolysaccharidosis type II (Hunter disease): identification and characterization of eight point mutations in the iduronate-2-sulfatase gene in Japanese patients

Mucopolysaccharidosis type II (Hunter disease) is a lysosomal storage disorder caused by a deficiency of the enzyme iduronate-2-sulfatase. Varied clinical phenotypes of this disease have been described. To identify mutations in individual patients and to examine possible correlations between mutations and clinical phenotypes, we analyzed the iduronate-2-sulfatase gene in Japanese patients with different clinical phenotypes. Five missense mutations, S333L (severe), R468Q (severe), R468L (severe), W337R (intermediate), R48P (mild), and three nonsense mutations, W345X (severe), R443X (intermediate), Q531X (mild), were identified by the RT-PCR method. Transient expression in the enzyme-deficient fibroblasts revealed that all five missense mutant enzymes were synthesized as the normal-size precursor (73 kD), and the nonsense mutant enzymes were synthesized as truncated ones (W345X:54 kD, R443X:59 kD, and Q531X:69 kD), although stable mature enzymes (45-56 kD) were not detected by Western blot analysis. Furthermore, expression of the eight mutant cDNAs resulted in severe reductions of iduronate-2-sulfatase enzyme activity in comparison with a normal cDNA.

Molecular basis of mucopolysaccharidosis type II: mutations in the iduronate-2-sulphatase gene

A number of mutations in the X-chromosomal human iduronate-2-sulphatase gene have now been identified as the primary genetic defect leading to the clinical condition known as Hunter syndrome or mucopolysaccharidosis type II. The mutations that are tabulated include different deletions, splice-site and point mutations. From the group of 319 patients thus far studied by Southern analysis, 14 have a full deletion of the gene and 48 have a partial deletion or other gross rearrangements. All patients with full deletions or gross rearrangements have severe clinical presentations. Twenty-nine different "small" mutations have so far been characterised in a total of 32 patients. These include 4 nonsense and 13 missense mutations, 7 different small deletions from 1 to 3 bp, with most leading to a frameshift and premature chain termination, and 5 different splice-site mutations also leading to small insertions or deletions in the mRNA. A 60 bp deletion, that results from a new donor splice-site, has been observed in five unrelated patients with relatively mild clinical phenotypes. This information will not only be useful for MPS II patient and carrier diagnosis, but also will aid in the understanding of the structure and function of iduronate-2-sulphatase, and possibly in correlating genotype with phenotype.

Deletion of the hunter gene and both DXS466 and DXS304 in a patient with mucopolysaccharidosis type II

Hunter syndrome is an X-linked mucopolysaccharidosis due to deficiency of the lysosomal enzyme iduronate-2-sulfatase (IDS). A cDNA clone containing the entire coding region of the human IDS gene, mapped in Xq28, has been used as molecular probe to study a patient with Hunter syndrome. A submicroscopic deletion has been detected that spans the IDS gene as well as DXS466 and DXS304, 2 loci mapped probably not more than 900 kb from the IDS locus. A detailed clinical description of the patient is provided and his phenotype is compared to that of other patients with IDS deletion described recently. By following the segregation of a restriction fragment length polymorphism at the IDS locus in the patient's family, our data suggest that the deletion occurred in the germ cells of the patient's grandfather.