Mutations in PPCS, Encoding Phosphopantothenoylcysteine Synthetase, Cause Autosomal-Recessive Dilated Cardiomyopathy.

Coenzyme A (CoA) is an essential metabolic cofactor used by around 4% of cellular enzymes. Its role is to carry and transfer acetyl and acyl groups to other molecules. Cells can synthesize CoA de novo from vitamin B5 (pantothenate) through five consecutive enzymatic steps. Phosphopantothenoylcysteine synthetase (PPCS) catalyzes the second step of the pathway during which phosphopantothenate reacts with ATP and cysteine to form phosphopantothenoylcysteine. Inborn errors of CoA biosynthesis have been implicated in neurodegeneration with brain iron accumulation (NBIA), a group of rare neurological disorders characterized by accumulation of iron in the basal ganglia and progressive neurodegeneration. Exome sequencing in five individuals from two unrelated families presenting with dilated cardiomyopathy revealed biallelic mutations in PPCS, linking CoA synthesis with a cardiac phenotype. Studies in yeast and fruit flies confirmed the pathogenicity of identified mutations. Biochemical analysis revealed a decrease in CoA levels in fibroblasts of all affected individuals. CoA biosynthesis can occur with pantethine as a source independent from PPCS, suggesting pantethine as targeted treatment for the affected individuals still alive.

Riboflavin-Responsive and -Non-responsive Mutations in FAD Synthase Cause Multiple Acyl-CoA Dehydrogenase and Combined Respiratory-Chain Deficiency.

Multiple acyl-CoA dehydrogenase deficiencies (MADDs) are a heterogeneous group of metabolic disorders with combined respiratory-chain deficiency and a neuromuscular phenotype. Despite recent advances in understanding the genetic basis of MADD, a number of cases remain unexplained. Here, we report clinically relevant variants in FLAD1, which encodes FAD synthase (FADS), as the cause of MADD and respiratory-chain dysfunction in nine individuals recruited from metabolic centers in six countries. In most individuals, we identified biallelic frameshift variants in the molybdopterin binding (MPTb) domain, located upstream of the FADS domain. Inasmuch as FADS is essential for cellular supply of FAD cofactors, the finding of biallelic frameshift variants was unexpected. Using RNA sequencing analysis combined with protein mass spectrometry, we discovered FLAD1 isoforms, which only encode the FADS domain. The existence of these isoforms might explain why affected individuals with biallelic FLAD1 frameshift variants still harbor substantial FADS activity. Another group of individuals with a milder phenotype responsive to riboflavin were shown to have single amino acid changes in the FADS domain. When produced in E. coli, these mutant FADS proteins resulted in impaired but detectable FADS activity; for one of the variant proteins, the addition of FAD significantly improved protein stability, arguing for a chaperone-like action similar to what has been reported in other riboflavin-responsive inborn errors of metabolism. In conclusion, our studies identify FLAD1 variants as a cause of potentially treatable inborn errors of metabolism manifesting with MADD and shed light on the mechanisms by which FADS ensures cellular FAD homeostasis.

Biallelic Mutations in NBAS Cause Recurrent Acute Liver Failure with Onset in Infancy.

Acute liver failure (ALF) in infancy and childhood is a life-threatening emergency. Few conditions are known to cause recurrent acute liver failure (RALF), and in about 50% of cases, the underlying molecular cause remains unresolved. Exome sequencing in five unrelated individuals with fever-dependent RALF revealed biallelic mutations in NBAS. Subsequent Sanger sequencing of NBAS in 15 additional unrelated individuals with RALF or ALF identified compound heterozygous mutations in an additional six individuals from five families. Immunoblot analysis of mutant fibroblasts showed reduced protein levels of NBAS and its proposed interaction partner p31, both involved in retrograde transport between endoplasmic reticulum and Golgi. We recommend NBAS analysis in individuals with acute infantile liver failure, especially if triggered by fever.

Human thioredoxin 2 deficiency impairs mitochondrial redox homeostasis and causes early-onset neurodegeneration.

Thioredoxin 2 (TXN2; also known as Trx2) is a small mitochondrial redox protein essential for the control of mitochondrial reactive oxygen species homeostasis, apoptosis regulation and cell viability. Exome sequencing in a 16-year-old adolescent suffering from an infantile-onset neurodegenerative disorder with severe cerebellar atrophy, epilepsy, dystonia, optic atrophy, and peripheral neuropathy, uncovered a homozygous stop mutation in TXN2. Analysis of patient-derived fibroblasts demonstrated absence of TXN2 protein, increased reactive oxygen species levels, impaired oxidative stress defence and oxidative phosphorylation dysfunction. Reconstitution of TXN2 expression restored all these parameters, indicating the causal role of TXN2 mutation in disease development. Supplementation with antioxidants effectively suppressed cellular reactive oxygen species production, improved cell viability and mitigated clinical symptoms during short-term follow-up. In conclusion, our report on a patient with TXN2 deficiency suggests an important role of reactive oxygen species homeostasis for human neuronal maintenance and energy metabolism.

Genetic cause and prevalence of hydroxyprolinemia.

BACKGROUND: Hydroxyprolinemia is an inborn error of amino acid degradation that is considered a non-disease. Known for more than 50 years, its genetic cause and prevalence have remained unclear. In MS/MS newborn screening, the mass spectrum of hydroxyproline cannot be differentiated from isoleucine and leucine causing false positive newborn screening test results for maple syrup urine disease (MSUD). METHODS: We studied two siblings with hydroxyprolinemia via exome sequencing and confirmed the candidate gene in five further individuals with hydroxyprolinemia, who were all characterized biochemically and clinically. The prevalence was calculated based on the number of individuals with hydroxyprolinemia detected via MS/MS newborn screening at our centre from 2003 to 2014. RESULTS: In six cases, we identified homozygous or compound heterozygous mutations in PRODH2 as the underlying genetic cause of hydroxyprolinemia. One individual was heterozygous for a deletion in PRODH2 and had an intermittent biochemical phenotype with partial normalization of hydroxyproline concentrations. In one further individual with persistent hydroxyprolinemia no mutation in PRODH2 was found, raising the possibility of another defect of hydroxyproline degradation yet to be identified as the underlying cause of hydroxyprolinemia. Plasma hydroxyproline concentrations were clearly elevated in all individuals with biallelic mutations in PRODH2. All studied individuals remained asymptomatic, giving further evidence that hydroxyprolinemia is a benign condition. The estimated prevalence of hydroxyprolinemia in Germany is about one in 47,300 newborns. CONCLUSION: Our results establish mutations in PRODH2 as a cause of human hydroxyprolinemia via impaired dehydrogenation of hydroxyproline to delta1-pyroline-3-hydroxy-5-carboxylic acid, and we suggest PRODH2 be renamed HYPDH. Hydroxyprolinemia is an autosomal-recessively inherited benign condition. It is a frequent cause of false positive screening results for MSUD, the prevalence being about 2.5 times higher than that of MSUD.

Recurrent acute liver failure due to NBAS deficiency: phenotypic spectrum, disease mechanisms, and therapeutic concepts.

BACKGROUND: Acute liver failure (ALF) in infancy and childhood is a life-threatening emergency and in about 50% the etiology remains unknown. Recently biallelic mutations in NBAS were identified as a new molecular cause of ALF with onset in infancy, leading to recurrent acute liver failure (RALF). METHODS: The phenotype and medical history of 14 individuals with NBAS deficiency was studied in detail and functional studies were performed on patients' fibroblasts. RESULTS: The phenotypic spectrum of NBAS deficiency ranges from isolated RALF to a multisystemic disease with short stature, skeletal dysplasia, immunological abnormalities, optic atrophy, and normal motor and cognitive development resembling SOPH syndrome. Liver crises are triggered by febrile infections; they become less frequent with age but are not restricted to childhood. Complete recovery is typical, but ALF crises can be fatal. Antipyretic therapy and induction of anabolism including glucose and parenteral lipids effectively ameliorates the course of liver crises. Patients' fibroblasts showed an increased sensitivity to high temperature at protein and functional level and a disturbed tethering of vesicles, pointing at a defect of intracellular transport between the endoplasmic reticulum and Golgi. CONCLUSIONS: Mutations in NBAS cause a complex disease with a wide clinical spectrum ranging from isolated RALF to a multisystemic phenotype. Thermal susceptibility of the syntaxin 18 complex is the basis of fever dependency of ALF episodes. NBAS deficiency is the first disease related to a primary defect of retrograde transport. Identification of NBAS deficiency allows optimized therapy of liver crises and even prevention of further episodes.

Clinical implementation of RNA sequencing for Mendelian disease diagnostics.

BACKGROUND: Lack of functional evidence hampers variant interpretation, leaving a large proportion of individuals with a suspected Mendelian disorder without genetic diagnosis after whole genome or whole exome sequencing (WES). Research studies advocate to further sequence transcriptomes to directly and systematically probe gene expression defects. However, collection of additional biopsies and establishment of lab workflows, analytical pipelines, and defined concepts in clinical interpretation of aberrant gene expression are still needed for adopting RNA sequencing (RNA-seq) in routine diagnostics. METHODS: We implemented an automated RNA-seq protocol and a computational workflow with which we analyzed skin fibroblasts of 303 individuals with a suspected mitochondrial disease that previously underwent WES. We also assessed through simulations how aberrant expression and mono-allelic expression tests depend on RNA-seq coverage. RESULTS: We detected on average 12,500 genes per sample including around 60% of all disease genes-a coverage substantially higher than with whole blood, supporting the use of skin biopsies. We prioritized genes demonstrating aberrant expression, aberrant splicing, or mono-allelic expression. The pipeline required less than 1 week from sample preparation to result reporting and provided a median of eight disease-associated genes per patient for inspection. A genetic diagnosis was established for 16% of the 205 WES-inconclusive cases. Detection of aberrant expression was a major contributor to diagnosis including instances of 50% reduction, which, together with mono-allelic expression, allowed for the diagnosis of dominant disorders caused by haploinsufficiency. Moreover, calling aberrant splicing and variants from RNA-seq data enabled detecting and validating splice-disrupting variants, of which the majority fell outside WES-covered regions. CONCLUSION: Together, these results show that streamlined experimental and computational processes can accelerate the implementation of RNA-seq in routine diagnostics.

Arabidopsis thaliana alternative dehydrogenases: a potential therapy for mitochondrial complex I deficiency? Perspectives and pitfalls.

BACKGROUND: Complex I (CI or NADH:ubiquinone oxidoreductase) deficiency is the most frequent cause of mitochondrial respiratory chain defect. Successful attempts to rescue CI function by introducing an exogenous NADH dehydrogenase, such as the NDI1 from Saccharomyces cerevisiae (ScNDI1), have been reported although with drawbacks related to competition with CI. In contrast to ScNDI1, which is permanently active in yeast naturally devoid of CI, plant alternative NADH dehydrogenases (NDH-2) support the oxidation of NADH only when the CI is metabolically inactive and conceivably when the concentration of matrix NADH exceeds a certain threshold. We therefore explored the feasibility of CI rescue by NDH-2 from Arabidopsis thaliana (At) in human CI defective fibroblasts. RESULTS: We showed that, other than ScNDI1, two different NDH-2 (AtNDA2 and AtNDB4) targeted to the mitochondria were able to rescue CI deficiency and decrease oxidative stress as indicated by a normalization of SOD activity in human CI-defective fibroblasts. We further demonstrated that when expressed in human control fibroblasts, AtNDA2 shows an affinity for NADH oxidation similar to that of CI, thus competing with CI for the oxidation of NADH as opposed to our initial hypothesis. This competition reduced the amount of ATP produced per oxygen atom reduced to water by half in control cells. CONCLUSIONS: In conclusion, despite their promising potential to rescue CI defects, due to a possible competition with remaining CI activity, plant NDH-2 should be regarded with caution as potential therapeutic tools for human mitochondrial diseases.

A Homozygous Splice Site Mutation in SLC25A42, Encoding the Mitochondrial Transporter of Coenzyme A, Causes Metabolic Crises and Epileptic Encephalopathy.

SLC25A42 is an inner mitochondrial membrane protein which has been shown to transport coenzyme A through a lipid bilayer in vitro. A homozygous missense variant in this gene has been recently reported in 13 subjects of Arab descent presenting with mitochondriopathy with variable clinical manifestations. By exome sequencing, we identified two additional individuals carrying rare variants in this gene. One subject was found to carry the previously reported missense variant in homozygous state, while the second subject carried a homozygous canonical splice site variant resulting in a splice defect. With the identification of two additional cases, we corroborate the association between rare variants in SLC25A42 and a clinical presentation characterized by myopathy, developmental delay, lactic acidosis, and encephalopathy. Furthermore, we highlight the biochemical consequences of the splice defect by measuring a mild decrease of coenzyme A content in SLC25A42-mutant fibroblasts.

Assessing Mitochondrial Bioenergetics in Isolated Mitochondria from Various Mouse Tissues Using Seahorse XF96 Analyzer.

Working with isolated mitochondria is the gold standard approach to investigate the function of the electron transport chain in tissues, free from the influence of other cellular factors. In this chapter, we outline a detailed protocol to measure the rate of oxygen consumption (OCR) with the high-throughput analyzer Seahorse XF96. More importantly, this protocol wants to provide practical tips for handling many different samples at once, and take a real advantage of using a high-throughput system. As a proof of concept, we have isolated mitochondria from brain, heart, liver, muscle, kidney, and lung of a wild-type mouse, and measured basal respiration (State II), ADP-stimulated respiration (State III), non-ADP-stimulated respiration (State IVo), and FCCP-stimulated respiration (State IIIu) using respiratory substrates specific to the respiratory chain complex I (RCCI) and complex II (RCCII). Mitochondrial purification and Seahorse runs were performed in less than eight working hours.

A Case of Beta-propeller Protein-associated Neurodegeneration due to a Heterozygous Deletion of WDR45.

BACKGROUND: Static encephalopathy of childhood with neurodegeneration in adulthood is a phenotypically distinctive, X-linked dominant subtype of neurodegeneration with brain iron accumulation (NBIA). WDR45 mutations were recently identified as causal. WDR45 encodes a beta-propeller scaffold protein with a putative role in autophagy, and the disease has been renamed beta-propeller protein-associated neurodegeneration (BPAN). CASE REPORT: Here we describe a female patient suffering from a classical BPAN phenotype due to a novel heterozygous deletion of WDR45. An initial gene panel and Sanger sequencing approach failed to uncover the molecular defect. Based on the typical clinical and neuroimaging phenotype, quantitative polymerase chain reaction of the WDR45 coding regions was undertaken, and this showed a reduction of the gene dosage by 50% compared with controls. DISCUSSION: An extended search for deletions should be performed in apparently WDR45-negative cases presenting with features of NBIA and should also be considered in young patients with predominant intellectual disabilities and hypertonia/parkinsonism/dystonia.

Genetic diagnosis of Mendelian disorders via RNA sequencing.

Across a variety of Mendelian disorders, ∼50-75% of patients do not receive a genetic diagnosis by exome sequencing indicating disease-causing variants in non-coding regions. Although genome sequencing in principle reveals all genetic variants, their sizeable number and poorer annotation make prioritization challenging. Here, we demonstrate the power of transcriptome sequencing to molecularly diagnose 10% (5 of 48) of mitochondriopathy patients and identify candidate genes for the remainder. We find a median of one aberrantly expressed gene, five aberrant splicing events and six mono-allelically expressed rare variants in patient-derived fibroblasts and establish disease-causing roles for each kind. Private exons often arise from cryptic splice sites providing an important clue for variant prioritization. One such event is found in the complex I assembly factor TIMMDC1 establishing a novel disease-associated gene. In conclusion, our study expands the diagnostic tools for detecting non-exonic variants and provides examples of intronic loss-of-function variants with pathological relevance.