IDUA mutational profile and genotype-phenotype relationships in UK patients with Mucopolysaccharidosis Type I

Mucopolysaccharidosis Type I (MPS I) is a lysosomal storage disorder with varying degrees of phenotypic severity caused by mutations in IDUA. Over 200 disease-causing variants in IDUA have been reported. We describe the profile of disease-causing variants in 291 individuals with MPS I for whom IDUA sequencing was performed, focusing on the UK subset of the cohort. A total of 63 variants were identified, of which 20 were novel, and the functional significance of the novel variants is explored. The severe form of MPS I is treated with hematopoietic stem cell transplantation, known to have improved outcomes with earlier age at treatment. Developing genotype-phenotype relationships would therefore have considerable clinical utility, especially in the light of the development of newborn screening programs for MPS I. Associations between genotype and phenotype are examined in this cohort, particularly in the context of the profile of variants identified in UK individuals. Relevant associations can be made for the majority of UK individuals based on the presence of nonsense or truncating variants as well as other associations described in this report.

Expanding the phenotype in argininosuccinic aciduria: need for new therapies

OBJECTIVES

This UK-wide study defines the natural history of argininosuccinic aciduria and compares long-term neurological outcomes in patients presenting clinically or treated prospectively from birth with ammonia-lowering drugs.

METHODS

Retrospective analysis of medical records prior to March 2013, then prospective analysis until December 2015. Blinded review of brain MRIs. ASL genotyping.

RESULTS

Fifty-six patients were defined as early-onset (n = 23) if symptomatic < 28 days of age, late-onset (n = 23) if symptomatic later, or selectively screened perinatally due to a familial proband (n = 10). The median follow-up was 12.4 years (range 0-53). Long-term outcomes in all groups showed a similar neurological phenotype including developmental delay (48/52), epilepsy (24/52), ataxia (9/52), myopathy-like symptoms (6/52) and abnormal neuroimaging (12/21). Neuroimaging findings included parenchymal infarcts (4/21), focal white matter hyperintensity (4/21), cortical or cerebral atrophy (4/21), nodular heterotopia (2/21) and reduced creatine levels in white matter (4/4). 4/21 adult patients went to mainstream school without the need of additional educational support and 1/21 lives independently. Early-onset patients had more severe involvement of visceral organs including liver, kidney and gut. All early-onset and half of late-onset patients presented with hyperammonaemia. Screened patients had normal ammonia at birth and received treatment preventing severe hyperammonaemia. ASL was sequenced (n = 19) and 20 mutations were found. Plasma argininosuccinate was higher in early-onset compared to late-onset patients.

CONCLUSIONS

Our study further defines the natural history of argininosuccinic aciduria and genotype-phenotype correlations. The neurological phenotype does not correlate with the severity of hyperammonaemia and plasma argininosuccinic acid levels. The disturbance in nitric oxide synthesis may be a contributor to the neurological disease. Clinical trials providing nitric oxide to the brain merit consideration.

Discovery of a new biomarker for the mucopolysaccharidoses (MPS), dipeptidyl peptidase IV (DPP-IV; CD26), by SELDI-TOF mass spectrometry

Surface enhanced laser desorption/ionisation time of flight (SELDI-TOF) mass spectrometry has been used to search for new protein biomarkers in the plasma of patients with mucopolysacharidoses (MPS). Differences in the levels of some plasma proteins, particularly the apolipoprotein ApoCI, were observed between MPS patients and normal controls, using the different chromatographic surfaces (ProteinChips). ApoCI was identified by both its mass and by immunological techniques. In plasma, it exists in two forms, ApoCI and a truncated form which lacks two N-terminal amino acids, ApoCI'. In controls, the ratio of ApoCI':ApoCI observed using the cation-exchange surface (CM10) was approximately 1:2 whereas in most MPS patients it varied from 1:1 to 1:0.8. The ratio of ApoCI':ApoCI in plasma is determined by the activity of dipeptidyl peptidase IV, DPP-IV (also known as the leucocyte antigen CD26), which was found to be elevated up to 3-fold in MPS patients. The DPP-IV activity decreased in MPS I patients undergoing enzyme replacement therapy, indicating that it could be a useful biomarker for monitoring the efficacy of treatment in MPS disease. As DPP-IV has an important regulatory role in metabolism, it is possible that its elevation could cause some of the secondary pathology in MPS, and inhibition of DPP-IV might have a role in MPS therapy.

Identification and characterisation of an 8.7 kb deletion and a novel nonsense mutation in two Italian families with Sanfilippo syndrome type D (mucopolysaccharidosis IIID)

Sanfilippo syndrome type D is an autosomal recessive lysosomal storage disease that is caused by a deficiency of N-acetylglucosamine-6-sulphatase, one of the enzymes involved in the catabolism of heparan sulphate. Only 15 patients have been described in the literature and just two mutations have been reported to date. We present the clinical, biochemical and molecular analysis of two Italian Sanfilippo D families. Novel homozygous mutations were identified in the affected patients from each family: a large intragenic deletion of 8723 bp encompassing exons 2 and 3 in family 1 and a nonsense mutation, Q272X, in family 2. The deletion is the first large intragenic deletion to be reported in any of the four Sanfilippo subtypes, including Sanfilippo type C in which the gene has recently been identified.

Mutations in TMEM76* cause mucopolysaccharidosis IIIC (Sanfilippo C syndrome)

Mucopolysaccharidosis IIIC (MPS IIIC, or Sanfilippo C syndrome) is a lysosomal storage disorder caused by the inherited deficiency of the lysosomal membrane enzyme acetyl-coenzyme A: alpha -glucosaminide N-acetyltransferase (N-acetyltransferase), which leads to impaired degradation of heparan sulfate. We report the narrowing of the candidate region to a 2.6-cM interval between D8S1051 and D8S1831 and the identification of the transmembrane protein 76 gene (TMEM76), which encodes a 73-kDa protein with predicted multiple transmembrane domains and glycosylation sites, as the gene that causes MPS IIIC when it is mutated. Four nonsense mutations, 3 frameshift mutations due to deletions or a duplication, 6 splice-site mutations, and 14 missense mutations were identified among 30 probands with MPS IIIC. Functional expression of human TMEM76 and the mouse ortholog demonstrates that it is the gene that encodes the lysosomal N-acetyltransferase and suggests that this enzyme belongs to a new structural class of proteins that transport the activated acetyl residues across the cell membrane.

Case of X-linked myopathy with excessive autophagy

We report a 12-year-old boy with a vacuolar myopathy with clinical and histologic features of X-linked myopathy with excessive autophagy. This is a rare and slowly progressive disease of skeletal muscle without cardiac, nervous system, or other organ involvement. The differential diagnosis of vacuolar myopathy includes acid maltase deficiency, Danon disease, and X-linked myopathy with excessive autophagy.

Neonatal epileptic encephalopathy caused by mutations in the PNPO gene encoding pyridox(am)ine 5′-phosphate oxidase

In the mouse, neurotransmitter metabolism can be regulated by modulation of the synthesis of pyridoxal 5'-phosphate and failure to maintain pyridoxal phosphate (PLP) levels results in epilepsy. This study of five patients with neonatal epileptic encephalopathy suggests that the same is true in man. Cerebrospinal fluid and urine analyses indicated reduced activity of aromatic L-amino acid decarboxylase and other PLP-dependent enzymes. Seizures ceased with the administration of PLP, having been resistant to treatment with pyridoxine, suggesting a defect of pyridox(am)ine 5'-phosphate oxidase (PNPO). Sequencing of the PNPO gene identified homozygous missense, splice site and stop codon mutations. Expression studies in Chinese hamster ovary cells showed that the splice site (IVS3-1g>a) and stop codon (X262Q) mutations were null activity mutations and that the missense mutation (R229W) markedly reduced pyridox(am)ine phosphate oxidase activity. Maintenance of optimal PLP levels in the brain may be important in many neurological disorders in which neurotransmitter metabolism is disturbed (either as a primary or as a secondary phenomenon).

Molecular defects in Sanfilippo syndrome type B (mucopolysaccharidosis IIIB)

Sanfilippo syndrome type B (mucopolysaccharidosis IIIB) is an autosomal recessive disease that is caused by the deficiency of the lysosomal enzyme alpha-N-acetylglucosaminidase (NAGLU). NAGLU is involved in the degradation of the glycosaminoglycan (GAG) heparan sulphate, and a deficiency results in the accumulation of partially degraded GAGs inside lysosomes. Early clinical symptoms include hyperactivity, aggressiveness and delayed development, followed by progressive mental deterioration, although there are a small number of late-onset attenuated cases. The gene for NAGLU has been fully characterized and we report the molecular analysis of 18 Sanfilippo B families. In total, 34 of the 36 mutant alleles were characterized in this study and 20 different mutations were identified including 8 novel changes (R38W, V77G, 407-410del4, 703delT, A246P, Y335C, 1487delT, E639X). The four novel missense mutations were transiently expressed in Chinese hamster ovary cells and all were shown to decrease the NAGLU activity markedly, although A246P did produce 12.7% residual enzyme activity.

Sanfilippo syndrome type D: identification of the first mutation in the N-acetylglucosamine-6-sulphatase gene

Mucopolysaccharidosis type IIID is the least common of the four subtypes of Sanfilippo syndrome. It is caused by a deficiency of N-acetylglucosamine-6-sulphatase, which is one of the enzymes involved in the catabolism of heparan sulphate. We present the clinical, biochemical, and, for the first time, the molecular diagnosis of a patient with Sanfilippo D disease. The patient was found to be homozygous for a single base pair deletion (c1169delA), which will cause a frameshift and premature termination of the protein. Accurate carrier detection is now available for other members of this consanguineous family.

Twenty-two novel mutations in the lysosomal ?-glucosidase gene (GAA) underscore the genotype-phenotype correlation in glycogen storage disease type II

Patients with glycogen storage disease type II (GSDII, Pompe disease) suffer from progressive muscle weakness due to acid alpha-glucosidase deficiency. The disease is inherited as an autosomal recessive trait with a spectrum of clinical phenotypes. We have investigated 29 cases of GSDII and thereby identified 55 pathogenic mutations of the acid alpha-glucosidase gene (GAA) encoding acid maltase. There were 34 different mutations identified, 22 of which were novel. All of the missense mutations and two other mutations with an unpredictable effect on acid alpha-glucosidase synthesis and function were transiently expressed in COS cells. The effect of a novel splice-site mutation was investigated by real-time PCR analysis. The outcome of our analysis underscores the notion that the clinical phenotype of GSDII is largely dictated by the nature of the mutations in the GAA alleles. This genotype-phenotype correlation makes DNA analysis a valuable tool to help predict the clinical course of the disease.

Mutational analysis of 85 mucopolysaccharidosis type I families: frequency of known mutations, identification of 17 novel mutations and in vitro expression of missense mutations

The lysosomal storage disorder, mucopolysaccharidosis type I (MPS I), is caused by a deficiency of the enzyme alpha-L-iduronidase, which is involved in the breakdown of dermatan and heparan sulphates. There are three clinical phenotypes, ranging from the Hurler form characterised by skeletal abnormalities, hepatosplenomegaly and severe mental retardation, to the milder Scheie phenotype where there is aortic valve disease, corneal clouding, limited skeletal problems, but no mental retardation. In this study, 85 MPS I families (73 Hurler, 5 Hurler/Scheie, 7 Scheie) were screened for 9 known mutations (Q70X, A75T, 474-2a>g, L218P, A327P, W402X, P533R, R89Q, 678-7g>a). W402X was the most frequent mutation in our population (45.3%) and Q70X was the second most frequent (15.9%). In 30 families, either one or both of the mutations were not identified, which accounted for 25.9% of the total alleles. Therefore, all 14 exons of the alpha-L-iduronidase gene were screened in these patients and 23 different sequence changes were found, 17 of which were previously unknown. The novel sequence changes include 4 deletions (153delC, 628del5, 740delC, 747delG), 5 nonsense mutations (Q60X, Y167X, Q400X, R619X, R628X), 6 missense mutations (C205Y, G208V, H240R, A319V, P496R, S633L), a splice site mutation (IVS12+5g>a), and a rare polymorphism (A591T). The polymorphism and novel missense mutations were transiently expressed in COS-7 cells and all of them except the polymorphism showed complete loss of enzyme activity. In total, 165 of the 170 mutant alleles were identified in this study and despite the high frequency of W402X and Q70X, the identification of many novel mutations unique to individual families further highlights the genetic heterogeneity of MPS I.

Identification of 12 novel mutations in the alpha-N-acetylglucosaminidase gene in 14 patients with Sanfilippo syndrome type B (mucopolysaccharidosis type IIIB)

Sanfilippo syndrome type B or mucopolysaccharidosis type IIIB (MPS IIIB) is one of a group of lysosomal storage disorders that are characterised by the inability to breakdown heparan sulphate. In MPS IIIB, there is a deficiency in the enzyme alpha-N-acetylglucosaminidase (NAGLU) and early clinical symptoms include aggressive behaviour and hyperactivity followed by progressive mental retardation. The disease is autosomal recessive and the gene for NAGLU, which is situated on chromosome 17q21, is approximately 8.5 kb in length and contains six exons. Primers were designed to amplify the entire coding region and intron/exon boundaries of the NAGLU gene in 10 fragments. The PCR products were analysed for sequence changes using SSCP analysis and fluorescent DNA sequencing technology. Sixteen different putative mutations were detected in DNA from 14 MPS IIIB patients, 12 of which have not been found previously. The mutations include four deletions (219-237del19, 334-358del25, 1335delC, 2099delA), two insertions (1447-1448insT, 1932-1933insGCTAC), two nonsense mutations (R297X, R626X), and eight missense mutations (F48C, Y140C, R234C, W268R, P521L, R565W, L591P, E705K). In this study, the Y140C, R297X, and R626X mutations were all found in more than one patient and together accounted for 25% of mutant alleles.

Cloning and nucleotide sequence of the gene encoding dinitrogenase reductase (nifH) from the cyanobacterium Nostoc 6720

The nucleotide sequence of the 3' end of the nifU coding sequence, the complete coding sequence of nifH and a substantial part of the 5' end of nifD coding sequence from Nostoc 6720 is presented. The coding sequences are highly conserved with those of Anabaena 7120 and Anabaena sp. L31. However the intergenic region between nifU and nifH contains two segments of short tandemly repetitive repeat sequences (STRRs) that differ from the STRR that is common to both Anabaena7120 and Anabaenasp. L31. Various sequence structures that are common to Nostoc 6720, the Anabaena strains and Plectonema boryanum are discussed.