A novel UPLC-MS/MS based method to determine the activity of N-acetylglutamate synthase in liver tissue.

BACKGROUND: N-acetylglutamate synthase (NAGS) plays a key role in the removal of ammonia via the urea cycle by catalyzing the synthesis of N-acetylglutamate (NAG), the obligatory cofactor in the carbamyl phosphate synthetase 1 reaction. Enzymatic analysis of NAGS in liver homogenates has remained insensitive and inaccurate, which prompted the development of a novel method. METHODS: UPLC-MS/MS was used in conjunction with stable isotope (N-acetylglutamic-2,3,3,4,4-d5 acid) dilution for the quantitative detection of NAG produced by the NAGS enzyme. The assay conditions were optimized using purified human NAGS and the optimized enzyme conditions were used to measure the activity in mouse liver homogenates. RESULTS: A low signal-to-noise ratio in liver tissue samples was observed due to non-enzymatic formation of N-acetylglutamate and low specific activity, which interfered with quantitative analysis. Quenching of acetyl-CoA immediately after the incubation circumvented this analytical difficulty and allowed accurate and sensitive determination of mammalian NAGS activity. The specificity of the assay was validated by demonstrating a complete deficiency of NAGS in liver homogenates from Nags -/- mice. CONCLUSION: The novel NAGS enzyme assay reported herein can be used for the diagnosis of inherited NAGS deficiency and may also be of value in the study of secondary hyperammonemia present in various inborn errors of metabolism as well as drug treatment.

ALG6-CDG: a recognizable phenotype with epilepsy, proximal muscle weakness, ataxia and behavioral and limb anomalies.

INTRODUCTION: Alpha-1,3-glucosyltransferase congenital disorder of glycosylation (ALG6-CDG) is a congenital disorder of glycosylation. The original patients were described with hypotonia, developmental disability, epilepsy, and increased bleeding tendency. METHODS: Based on Euroglycan database registration, we approached referring clinicians and collected comprehensive data on 41 patients. RESULTS: We found hypotonia and developmental delay in all ALG6-CDG patients and epilepsy, ataxia, proximal muscle weakness, and, in the majority of cases, failure to thrive. Nine patients developed intractable seizures. Coagulation anomalies were present in <50 % of cases, without spontaneous bleedings. Facial dysmorphism was rare, but seven patients showed missing phalanges and brachydactyly. Cyclic behavioral change, with autistic features and depressive episodes, was one of the most significant complaints. Eleven children died before the age of 4 years due to protein losing enteropathy (PLE), sepsis, or seizures. The oldest patient was a 40 year-old Dutch woman. The most common pathogenic protein alterations were p.A333V and p.I299Del, without any clear genotype-phenotype correlation. DISCUSSION: ALG6-CDG has been now described in 89 patients, making it the second most common type of CDG. It has a recognizable phenotype and a primary neurologic presentation.

Clinical, biochemical, and genetic spectrum of MADD in a South African cohort: an ICGNMD study.

BACKGROUND: Multiple acyl-CoA dehydrogenase deficiency (MADD) is an autosomal recessive disorder resulting from pathogenic variants in three distinct genes, with most of the variants occurring in the electron transfer flavoprotein-ubiquinone oxidoreductase gene (ETFDH). Recent evidence of potential founder variants for MADD in the South African (SA) population, initiated this extensive investigation. As part of the International Centre for Genomic Medicine in Neuromuscular Diseases study, we recruited a cohort of patients diagnosed with MADD from academic medical centres across SA over a three-year period. The aim was to extensively profile the clinical, biochemical, and genomic characteristics of MADD in this understudied population. METHODS: Clinical evaluations and whole exome sequencing were conducted on each patient. Metabolic profiling was performed before and after treatment, where possible. The recessive inheritance and phase of the variants were established via segregation analyses using Sanger sequencing. Lastly, the haplotype and allele frequencies were determined for the two main variants in the four largest SA populations. RESULTS: Twelve unrelated families (ten of White SA and two of mixed ethnicity) with clinically heterogeneous presentations in 14 affected individuals were observed, and five pathogenic ETFDH variants were identified. Based on disease severity and treatment response, three distinct groups emerged. The most severe and fatal presentations were associated with the homozygous c.[1067G > A];c.[1067G > A] and compound heterozygous c.[976G > C];c.[1067G > A] genotypes, causing MADD types I and I/II, respectively. These, along with three less severe compound heterozygous genotypes (c.[1067G > A];c.[1448C > T], c.[740G > T];c.[1448C > T], and c.[287dupA*];c.[1448C > T]), resulting in MADD types II/III, presented before the age of five years, depending on the time and maintenance of intervention. By contrast, the homozygous c.[1448C > T];c.[1448C > T] genotype, which causes MADD type III, presented later in life. Except for the type I, I/II and II cases, urinary metabolic markers for MADD improved/normalised following treatment with riboflavin and L-carnitine. Furthermore, genetic analyses of the most frequent variants (c.[1067G > A] and c.[1448C > T]) revealed a shared haplotype in the region of ETFDH, with SA population-specific allele frequencies of < 0.00067-0.00084%. CONCLUSIONS: This study reveals the first extensive genotype-phenotype profile of a MADD patient cohort from the diverse and understudied SA population. The pathogenic variants and associated variable phenotypes were characterised, which will enable early screening, genetic counselling, and patient-specific treatment of MADD in this population.

The glycine N-acyltransferases, GLYAT and GLYATL1, contribute to the detoxification of isovaleryl-CoA - an in-silico and in vitro validation.

Isovaleric acidemia (IVA), due to isovaleryl-CoA dehydrogenase (IVD) deficiency, results in the accumulation of isovaleryl-CoA, isovaleric acid and secondary metabolites. The increase in these metabolites decreases mitochondrial energy production and increases oxidative stress. This contributes to the neuropathological features of IVA. A general assumption in the literature exists that glycine N-acyltransferase (GLYAT) plays a role in alleviating the symptoms experienced by IVA patients through the formation of N-isovalerylglycine. GLYAT forms part of the phase II glycine conjugation pathway in the liver and detoxifies excess acyl-CoA's namely benzoyl-CoA. However, very few studies support GLYAT as the enzyme that conjugates isovaleryl-CoA to glycine. Furthermore, GLYATL1, a paralogue of GLYAT, conjugates phenylacetyl-CoA to glutamine. Therefore, GLYATL1 might also be a candidate for the formation of N-isovalerylglycine. Based on the findings from the literature review, we proposed that GLYAT or GLYATL1 can form N-isovalerylglycine in IVA patients. To test this hypothesis, we performed an in-silico analysis to determine which enzyme is more likely to conjugate isovaleryl-CoA with glycine using AutoDock Vina. Thereafter, we performed in vitro validation using purified enzyme preparations. The in-silico and in vitro findings suggested that both enzymes could form N-isovaleryglycine albeit at lower affinities than their preferred substrates. Furthermore, an increase in glycine concentration does not result in an increase in N-isovalerylglycine formation. The results from the critical literature appraisal, in-silico, and in vitro validation, suggest the importance of further investigating the reaction kinetics and binding behaviors between these substrates and enzymes in understanding the pathophysiology of IVA.

An Overview of Benefits and Challenges of Rare Disease Biobanking in Africa, Focusing on South Africa.

The North-West University's Centre for Human Metabolomics (CHM) is in the process of establishing the first rare disease (RD) biobank in South Africa and Africa. The CHM Biobank's main focus is on the collection of samples and information for rare congenital disorders. Approximately 72% of all RDs have a genetic origin, of which 70% have an exclusive pediatric onset. The need for such a biobank was identified by the CHM diagnostic laboratory. Feedback toward this initiative was overwhelmingly positive at the first stakeholder meeting in August 2019. However, gaining support from the public sector and recruiting of participants have proven to be challenging. Problems experienced to date include lack of support from government and clinicians; lack of knowledge on RDs (patients and clinicians); public health care focus not directed toward RDs; patients not returning for follow-up visits; and unwillingness to participate due to fear of exploitation. The CHM Biobank's vision and goals are aligned to address a national and international research need: it will provide a valuable resource for scientists to improve what is known about these diseases; to better understand the natural history and pathophysiology; to optimize diagnostic methods; and to potentially develop treatments. The genetic variability of the South African population provides added value to the RD biobank. This review provides a brief overview of the literature on the challenges and benefits of an RD biobank and how this relates to low- and middle-income countries (LMIC) like South Africa. The aim of the review is to draw attention to the potential benefits of such an undertaking and to create awareness, at both local and global level, toward some of the unique collective considerations that an RD biobank in LMIC (also unique South African challenges) faces on an operational, collaborate, and sustainability level.

Disorders of flavin adenine dinucleotide metabolism: MADD and related deficiencies.

Multiple acyl-coenzyme A dehydrogenase deficiency (MADD), or glutaric aciduria type II (GAII), is a group of clinically heterogeneous disorders caused by mutations in electron transfer flavoprotein (ETF) and ETF-ubiquinone oxidoreductase (ETFQO) - the two enzymes responsible for the re-oxidation of enzyme-bound flavin adenine dinucleotide (FADH2) via electron transfer to the respiratory chain at the level of coenzyme Q10. Over the past decade, an increasing body of evidence has further coupled mutations in FAD metabolism (including intercellular riboflavin transport, FAD biosynthesis and FAD transport) to MADD-like phenotypes. In this review we provide a detailed description of the overarching and specific metabolic pathways involved in MADD. We examine the eight associated genes (ETFA, ETFB, ETFDH, FLAD1, SLC25A32 and SLC52A1-3) and clinical phenotypes, and report ∼436 causative mutations following a systematic literature review. Finally, we focus attention on the value and shortcomings of current diagnostic approaches, as well as current and future therapeutic options for MADD and its phenotypic disorders.

A novel mutation in ETFDH manifesting as severe neonatal-onset multiple acyl-CoA dehydrogenase deficiency.

Neonatal-onset multiple acyl-CoA dehydrogenase deficiency (MADD type I) is an autosomal recessive disorder of the electron transfer flavoprotein function characterized by a severe clinical and biochemical phenotype, including congenital abnormalities with unresponsiveness to riboflavin treatment as distinguishing features. From a retrospective study, relying mainly on metabolic data, we have identified a novel mutation, c.1067G>A (p.Gly356Glu) in exon 8 of ETFDH, in three South African Caucasian MADD patients with the index patient presenting the hallmark features of type I MADD and two patients with compound heterozygous (c.1067G>A+c.1448C>T) mutations presenting with MADD type III. SDS-PAGE western blot confirmed the significant effect of this mutation on ETFDH structural instability. The identification of this novel mutation in three families originating from the South African Afrikaner population is significant to direct screening and strategies for this disease, which amongst the organic acidemias routinely screened for, is relatively frequently observed in this population group.