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).

Twenty-two novel mutations in the lysosomal alpha-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.