Déborah Mathis

LC-MS/MS based assay and reference intervals in children and adolescents for oxysterols elevated in Niemann-Pick diseases.

BACKGROUND Niemann-Pick type C (NP-C) is a rare progressive neurodegenerative lipid storage disorder with heterogeneous clinical presentation and challenging diagnostic procedures. Recently oxysterols have been reported to be specific biomarkers for NP-C but knowledge on the intra-individual variation and on reference intervals in children and adolescents are lacking. METHODS We established a LC-MS/MS assay to measure Cholestane-3β, 5α, 6β-triol (C-triol) and 7-Ketocholesterol (7-KC) following Steglich esterification. To assess reference intervals and intra-individual variation we determined oxysterols in 148 children and adolescents from 0 to 18 years and repeat measurements in 19 of them. RESULTS The reported method is linear (r>0.99), sensitive (detection limit of 0.03 ng/mL [0.07 nM] for C-triol, and 0.54 ng/mL [1.35 nM] for 7-KC) and precise, with an intra-day imprecision of 4.8% and 4.1%, and an inter-day imprecision of 7.0% and 11.0% for C-triol (28 ng/ml, 67 nM) and 7-KC (32 ng/ml, 80 nM), respectively. Recoveries for 7-KC and C-triol range between 93% and 107%. The upper reference limit obtained for C-triol is 40.4 ng/mL (95% CI: 26.4-61.7 ng/mL, 96.0 nM, 95% CI: 62.8-146.7 nM) and 75.0 ng/mL for 7-KC (95% CI: 55.5-102.5 ng/mL, 187.2 nM, 95% CI: 138.53-255.8 nM), with no age or gender dependency. Both oxysterols have a broad intra-individual variation of 46%±23% for C-triol and 52%±29% for 7-KC. Nevertheless, all Niemann-Pick patients showed increased C-triol levels including Niemann-Pick type A and B patients. CONCLUSIONS The LC-MS/MS assay is a robust assay to quantify C-triol and 7-KC in plasma with well documented reference intervals in children and adolescents to screen for NP-C in the pediatric population. In addition our results suggest that especially the C-triol is a biomarker for all three Niemann-Pick diseases.

Confirmation of mutations in PROSC as a novel cause of vitamin B6-dependent epilepsy

Vitamin-B6-dependent epilepsies are a heterogenous group of treatable disorders due to mutations in several genes (ALDH7A1, PNPO, ALPL or ALDH4A1). In neonatal seizures, defects in ALDH7A1 and PNPO explain a major fraction of cases. Very recently biallelic mutations in PROSC were shown to be a novel cause in five families. We identified four further unrelated patients harbouring a total of six different mutations, including four novel disease mutations. Vitamin B6 plasma profiles on pyridoxine did not enable the differentiation of patients with PROSC mutations. All four patients were normocephalic and had normal cranial imaging. Pyridoxine monotherapy allowed complete seizure control in one, while two patients had occasional febrile or afebrile seizures and one needed additional valproate therapy for photosensitive seizures. Two patients underwent a controlled pyridoxine withdrawal with signs of encephalopathy within a couple of days. Three had favourable outcome with normal intellectual properties at age 12.5, 15.5 and 30 years, respectively, while one child had marked developmental delay at age 27 months. The clinical and electroencephalographic phenotype in patients with PROSC mutations was indistinguishable from ALDH7A1 and PNPO deficiency. We therefore confirm PROSC as a novel gene for vitamin-B6-dependent epilepsy and delineate a non-specific plasma vitamin B6 profile under pyridoxine treatment.

N 8 -acetylspermidine as a potential plasma biomarker for Snyder-Robinson syndrome identified by clinical metabolomics

Clinical metabolomics has emerged as a powerful tool to study human metabolism in health and disease. Comparative statistical analysis of untargeted metabolic profiles can reveal perturbations of metabolite levels in diseases and thus has the potential to identify novel biomarkers. Here we have applied a simultaneous genetic-metabolomic approach in twin boys with epileptic encephalopathy of unclear etiology. Clinical exome sequencing identified a novel missense mutation in the spermine synthase gene (SMS) that causes Snyder-Robinson syndrome (SRS). Untargeted plasma metabolome analysis revealed significantly elevated levels of N8-acetylspermidine, a precursor derivative of spermine biosynthesis, as a potential novel plasma biomarker for SRS. This result was verified in a third patient with genetically confirmed SRS. This study illustrates the potential of metabolomics as a translational technique to support exome data on a functional and clinical level.

A simple dried blood spot-method for in vivo measurement of ureagenesis by gas chromatography–mass spectrometry using stable isotopes

BACKGROUND Clinical management of inherited or acquired hyperammonemia depends mainly on the plasma ammonia level which is not a reliable indicator of urea cycle function as its concentrations largely fluctuate. The gold standard to assess ureagenesis in vivo is the use of stable isotopes. METHODS Here we developed and validated a simplified in vivo method with [15N]ammonium chloride ([15N]H4Cl) as a tracer. Non-labeled and [15N]urea were quantified by GC-MS after extraction and silylation. RESULTS Different matrices were evaluated for suitability of analysis. Ureagenesis was assessed in ornithine transcarbamylase (OTC)-deficient spfash mice with compromised urea cycle function during fasted and non-fasted feeding states, and after rAAV2/8-vector delivery expressing the murine OTC-cDNA in liver. Blood (5μL) was collected through tail vein puncture before and after [15N]H4Cl intraperitoneal injections over a two hour period. The tested matrices, blood, plasma and dried blood spots, can be used to quantify ureagenesis. Upon [15N]H4Cl challenge, urea production in spfash mice was reduced compared to wild-type and normalized following rAAV2/8-mediated gene therapeutic correction. The most significant difference in ureagenesis was at 30min after injection in untreated spfash mice under fasting conditions (19% of wild-type). Five consecutive injections over a period of five weeks had no effect on body weight or ureagenesis. CONCLUSION This method is simple, robust and with no apparent risk, offering a sensitive, minimal-invasive, and fast measurement of ureagenesis capacity using dried blood spots. The stable isotope-based quantification of ureagenesis can be applied for the efficacy-testing of novel molecular therapies.

The value of plasma vitamin B6 profiles in early onset epileptic encephalopathies

BackgroundRecent decades have unravelled the molecular background of a number of inborn errors of metabolism (IEM) causing vitamin B6-dependent epilepsy. As these defects interfere with vitamin B6 metabolism by different mechanisms, the plasma vitamin B6 profile can give important clues for further molecular work-up. This has so far been investigated in only a small number of patients.MethodsWe evaluated the vitamin B6 vitamers pyridoxal 5’-phosphate (PLP), pyridoxal (PL), pyridoxamine (PM), pyridoxine (PN) and the catabolite pyridoxic acid (PA) in the so far largest patient cohort: reference (n = 50); pyridox(am)ine 5’-phosphate oxidase (PNPO) deficiency (n = 6); antiquitin (ATQ) deficiency (n = 21); tissue non-specific alkaline phosphatase (TNSALP) deficiency (n = 2) and epileptic encephalopathy (EE) of unknown etiology tested negative for ATQ and PNPO deficiency (n = 64).ResultsHigh plasma PM concentration was found in all patients with PNPO deficiency irrespective of vitamin B6 supplementation. Their PM concentration and the PM/PA ratio was significantly higher (p < 0.0001), compared to any other patients analysed. One patient with TNSALP deficiency and sampling prior to PN supplementation had markedly elevated plasma PLP concentration. On PN supplementation, patients with TNSALP deficiency, ATQ deficiency and patients of the EE cohort had similar plasma vitamin B6 profiles that merely reflect the intake of supra-physiological doses of vitamin B6. The interval of sampling to the last PN intake strongly affected the plasma concentrations of PN, PL and PA.ConclusionsPM concentrations and the PM/PA ratio clearly separated PNPO-deficient patients from the other cohorts. The plasma PM/PA ratio thus represents a robust biomarker for the selective screening of PNPO deficiency.

Novel Mouse Models of Methylmalonic Aciduria Recapitulate Phenotypic Traits with a Genetic Dosage Effect.

Methylmalonic aciduria (MMAuria), caused by deficiency of methylmalonyl-CoA mutase (MUT), usually presents in the newborn period with failure to thrive and metabolic crisis leading to coma or even death. Survivors remain at risk of metabolic decompensations and severe long term complications, notably renal failure and neurological impairment. We generated clinically relevant mouse models of MMAuria using a constitutive Mut knock-in (KI) allele based on the p.Met700Lys patient mutation, used homozygously (KI/KI) or combined with a knockout allele (KO/KI), to study biochemical and clinical MMAuria disease aspects. Transgenic Mutki/ki and Mutko/ki mice survive post-weaning, show failure to thrive, and show increased methylmalonic acid, propionylcarnitine, odd chain fatty acids, and sphingoid bases, a new potential biomarker of MMAuria. Consistent with genetic dosage, Mutko/ki mice have lower Mut activity, are smaller, and show higher metabolite levels than Mutki/ki mice. Further, Mutko/ki mice exhibit manifestations of kidney and brain damage, including increased plasma urea, impaired diuresis, elevated biomarkers, and changes in brain weight. On a high protein diet, mutant mice display disease exacerbation, including elevated blood ammonia, and catastrophic weight loss, which, in Mutki/ki mice, is rescued by hydroxocobalamin treatment. This study expands knowledge of MMAuria, introduces the discovery of new biomarkers, and constitutes the first in vivo proof of principle of cobalamin treatment in mut-type MMAuria.

Antiquitin Deficiency with Adolescent Onset Epilepsy: Molecular Diagnosis in a Mother of Affected Offsprings

Antiquitin deficiency is the most prevalent form of pyridoxine-dependent epilepsy. While most patients present with neonatal onset of therapy-resistant seizures, a few cases with late-onset during infancy have been described. Here, we describe the juvenile onset of epilepsy at the age of 17 years due to antiquitin deficiency in an Indian female with homozygosity for the most prevalent ALDH7A1 missense mutation, c.1279G > C; p.Glu427Gln in exon 14. The diagnosis was established along familial cosegregation analysis for an affected offspring, that had neonatal pyridoxine responsive seizures and had been found to be compound heterozygous for c.1279G > C; p.Glu427Gln in exon 14 and a nonsense mutation c.796C > T; p.Arg266* in exon 9. While seizures in the mother had been incompletely controlled by levetiracetam, she remained seizure-free on pyridoxine monotherapy, 200 mg/day. Her fourth pregnancy resulted in a female affected offspring, who was treated prospectively and never developed seizures with a normal outcome at age 2 years while on pyridoxine. This report illustrates that the phenotypic spectrum of antiquitin deficiency is still underestimated and that this treatable inborn error of metabolism has to be considered in case of therapy-resistant seizures even at older age. It furthermore supports prospective in utero treatment with pyridoxine in forthcoming pregnancies at risk.

PP03.15 – 2755: Clinical metabolomics reveals a novel plasma biomarker for Snyder Robinson syndrome (X-linked spermine synthase deficiency)

Objective Clinical metabolomics has emerged as a powerful tool to study metabolic perturbations in various diseases and to unravel novel biomarkers. In a monocenter study on metabolic-genetic research into epileptic encephalopathies, we applied untargeted metabolomics in parallel with exome sequencing in twin brothers with epileptic encephalopathy of unknown etiology. Genetic analysis confirmed Snyder-Robinson syndrome (SRS), a rare X-linked mental retardation syndrome. The underlying spermine synthase (SMS) deficiency affects polyamine metabolism and reduces intracellular spermine. So far, the diagnosis of SRS relied on a reduced spermine/spermidine ratio in lymphoblasts and/or sequencing of the SMS gene. Case The male monozygotic twins presented with developmental delay, progressive microcephaly and discrete dysmorphic features. From the age of 12 months, they developed therapy-resistant seizures with recurrent irregular myoclonic jerks and serial spasms in twin A. The EEG showed an abnormal background activity and multifocal and generalized spike waves. Additional atonic and tonic seizures and atypical absences occurred in both. At the age of 15 months, they developed a severe encephalopathy with frequent seizures, loss of milestones, impairment of visual interaction and a transient choreo-athetotic movement disorder. Both twins showed signs of osteopenia, twin B suffered a non-traumatic clavicular fraction at 23 months. Methods Clinical exome sequencing was applied in both twins and their parents. Sanger sequencing was applied to confirm the candidate variant. An untargeted plasma metabolomics analysis was performed by liquid chromatography–high-resolution mass spectrometry (LC-MS). Results Exome sequencing revealed a novel SMS missense mutation. Discriminant statistical analysis of the LC-MS data showed a distinct plasma metabolomics profile while comparative analysis versus age-matched controls revealed significantly elevated levels of N-acetylspermidine, a metabolite involved in spermine metabolism. Conclusion A combined clinical “omics” approach revealed N-acetylspermidine as a novel plasma biomarker for spermine synthase deficiency and identified a novel missense mutation in twin boys with SRS.