Complex I assembly function and fatty acid oxidation enzyme activity of ACAD9 both contribute to disease severity in ACAD9 deficiency

ACAD9 treatment with bezafibrate and nicotinamide riboside temporarily stabilizes cardiomyopathy and lactic acidosis

Pathogenic ACAD9 variants cause complex I deficiency. Patients presenting in infancy unresponsive to riboflavin have high mortality. A six-month-old infant presented with riboflavin unresponsive lactic acidosis and life-threatening cardiomyopathy. Treatment with high dose bezafibrate and nicotinamide riboside resulted in marked clinical improvement including reduced lactate and NT-pro-brain type natriuretic peptide levels, with stabilized echocardiographic measures. After a long stable period, the child succumbed from cardiac failure with infection at 10.5 months. Therapy was well tolerated. Peak bezafibrate levels exceeded its EC50. The clinical improvement with this treatment illustrates its potential, but weak PPAR agonist activity of bezafibrate limited its efficacy.

Immunodeficiency with susceptibility to lymphoma with complex genotype affecting energy metabolism (FBP1, ACAD9) and vesicle trafficking (RAB27A)

Introduction

Inborn errors of immunity (IEI) are characterized by a dysfunction of the immune system leading to increased susceptibility to infections, impaired immune regulation and cancer. We present a unique consanguineous family with a history of Hodgkin lymphoma, impaired EBV control and a late onset hemophagocytic lymphohistiocytosis (HLH).

Methods and results

Overall, family members presented with variable impairment of NK cell and cytotoxic T cell degranulation and cytotoxicity. Exome sequencing identified homozygous variants in RAB27A, FBP1 (Fructose-1,6-bisphosphatase 1) and ACAD9 (Acyl-CoA dehydrogenase family member 9). Variants in RAB27A lead to Griscelli syndrome type 2, hypopigmentation and HLH predisposition.

Discussion

Lymphoma is frequently seen in patients with hypomorphic mutations of genes predisposing to HLH. We hypothesize that the variants in FBP1 and ACAD9 might aggravate the clinical and immune phenotype, influence serial killing and lytic granule polarization by CD8 T cells. Understanding of the interplay between the multiple variants identified by whole exome sequencing (WES) is essential for correct interpretation of the immune phenotype and important for critical treatment decisions.

Severe Antenatal Hypertrophic Cardiomyopathy Secondary to ACAD9-Related Mitochondrial Complex I Deficiency

Introduction

Antenatal presentation of hypertrophic cardiomyopathy (HCM) is rare. We describe familial recurrence of antenatal HCM associated with intrauterine growth restriction and the diagnostic process undertaken.

Methods

Two pregnancies with antenatal HCM were followed up. Biological assessment including metabolic analyses, genetic analyses, and respiratory chain study was performed. We describe the clinical course of these two pregnancies, antenatal manifestations as well as specific histopathological findings, and review the literature.

Results

The assessment revealed a deficiency in complex I of the respiratory chain and two likely pathogenic variations in the ACAD9 gene.

Discussion and Conclusion

Antenatal HCM is rare and a diagnosis is not always made. In pregnancies presenting with cardiomyopathy and intrauterine growth restriction, ACAD9 deficiency should be considered as one of the potential underlying diagnoses, and ACAD9 molecular testing should be included among other prenatal investigations.

RNAseq Analysis of FABP4 Knockout Mouse Hippocampal Transcriptome Suggests a Role for WNT/β-Catenin in Preventing Obesity-Induced Cognitive Impairment

Microglial fatty-acid binding protein 4 (FABP4) is a regulator of neuroinflammation. We hypothesized that the link between lipid metabolism and inflammation indicates a role for FABP4 in regulating high fat diet (HFD)-induced cognitive decline. We have previously shown that obese FABP4 knockout mice exhibit decreased neuroinflammation and cognitive decline. FABP4 knockout and wild type mice were fed 60% HFD for 12 weeks starting at 15 weeks old. Hippocampal tissue was dissected and RNA-seq was performed to measure differentially expressed transcripts. Reactome molecular pathway analysis was utilized to examine differentially expressed pathways. Results showed that HFD-fed FABP4 knockout mice have a hippocampal transcriptome consistent with neuroprotection, including associations with decreased proinflammatory signaling, ER stress, apoptosis, and cognitive decline. This is accompanied by an increase in transcripts upregulating neurogenesis, synaptic plasticity, long-term potentiation, and spatial working memory. Pathway analysis revealed that mice lacking FABP4 had changes in metabolic function that support reduction in oxidative stress and inflammation, and improved energy homeostasis and cognitive function. Analysis suggested a role for WNT/β-Catenin signaling in the protection against insulin resistance, alleviating neuroinflammation and cognitive decline. Collectively, our work shows that FABP4 represents a potential target in alleviating HFD-induced neuroinflammation and cognitive decline and suggests a role for WNT/β-Catenin in this protection.

Novel biallelic mutations in TMEM126B cause splicing defects and lead to Leigh-like syndrome with severe complex I deficiency

Leigh syndrome (LS)/Leigh-like syndrome (LLS) is one of the most common mitochondrial disease subtypes, caused by mutations in either the nuclear or mitochondrial genomes. Here, we identified a novel intronic mutation (c.82-2 A > G) and a novel exonic insertion mutation (c.290dupT) in TMEM126B from a Chinese patient with clinical manifestations of LLS. In silico predictions, minigene splicing assays and patients’ RNA analyses determined that the c.82-2 A > G mutation resulted in complete exon 2 skipping, and the c.290dupT mutation provoked partial and complete exon 3 skipping, leading to translational frameshifts and premature termination. Functional analysis revealed the impaired mitochondrial function in patient-derived lymphocytes due to severe complex I content and assembly defect. Altogether, this is the first report of LLS in a patient carrying mutations in TMEM126B. Our data uncovers the functional effect and the molecular mechanism of the pathogenic variants c.82-2 A > G and c.290dupT, which expands the gene mutation spectrum of LLS and clinical spectrum caused by TMEM126B mutations, and thus help to clinical diagnosis of TMEM126B mutation‐related mitochondrial diseases.

An MCIA-like complex is required for mitochondrial complex I assembly and seed development in maize

During adaptive radiation, mitochondria have co-evolved with their hosts, leading to gain or loss of subunits and assembly factors of respiratory complexes. Plant mitochondrial complex I harbors ∼40 nuclear- and 9 mitochondrial-encoded subunits, and is formed by stepwise assembly during which different intermediates are integrated via various assembly factors. In mammals, the mitochondrial complex I intermediate assembly (MCIA) complex is required for building the membrane arm module. However, plants have lost almost all of the MCIA complex components, giving rise to the hypothesis that plants follow an ancestral pathway to assemble the membrane arm subunits. Here, we characterize a maize crumpled seed mutant, crk1, and reveal by map-based cloning that CRK1 encodes an ortholog of human complex I assembly factor 1, zNDUFAF1, the only evolutionarily conserved MCIA subunit in plants. zNDUFAF1 is localized in the mitochondria and accumulates in two intermediate complexes that contain complex I membrane arm subunits. Disruption of zNDUFAF1 results in severe defects in complex I assembly and activity, a cellular bioenergetic shift to aerobic glycolysis, and mitochondrial vacuolation. Moreover, we found that zNDUFAF1, the putative mitochondrial import inner membrane translocase ZmTIM17-1, and the isovaleryl-coenzyme A dehydrogenase ZmIVD1 interact each other, and could be co-precipitated from the mitochondria and co-migrate in the same assembly intermediates. Knockout of either ZmTIM17-1 or ZmIVD1 could lead to the significantly reduced complex I stability and activity as well as defective seeds. These results suggest that zNDUFAF1, ZmTIM17-1 and ZmIVD1 probably form an MCIA-like complex that is essential for the biogenesis of mitochondrial complex I and seed development in maize. Our findings also imply that plants and mammals recruit MCIA subunits independently for mitochondrial complex I assembly, highlighting the importance of parallel evolution in mitochondria adaptation to their hosts.

Molecular characteristics of the multi-functional FAO enzyme ACAD9 illustrate the importance of FADH2 /NADH ratios for mitochondrial ROS formation

A decade ago I postulated that ROS formation in mitochondria was influenced by different FADH₂ /NADH (F/N) ratios of catabolic substrates. Thus, fatty acid oxidation (FAO) would give higher ROS formation than glucose oxidation. Both the emergence of peroxisomes and neurons not using FAO, could be explained thus. ROS formation in NADH:ubiquinone oxidoreductase (Complex I) comes about by reverse electron transport (RET) due to high QH₂ levels, and scarcity of its electron-acceptor (Q) during FAO. The then new, unexpected, finding of an FAO enzyme, ACAD9, being involved in complex I biogenesis, hinted at connections in line with the hypothesis. Recent findings about ACAD9's role in regulation of respiration fit with predictions the model makes: cementing connections between ROS production and F/N ratios. I describe how ACAD9 might be central to reversing the oxidative damage in complex I resulting from FAO. This seems to involve two distinct, but intimately connected, ACAD9 characteristics: (i) its upregulation of complex I biogenesis, and (ii) releasing FADH₂ , with possible conversion into FMN, the crucial prosthetic group of complex I.

Characterization of exonic variants of uncertain significance in very long-chain acyl-CoA dehydrogenase identified through newborn screening

Very long-chain acyl-CoA dehydrogenase deficiency (VLCADD) is an autosomal recessive disease resulting from mutations in the ACADVL gene and is among the disorders tested for in newborn screening (NBS). Confirmatory sequencing following suspected VLCADD NBS results often identifies variants of uncertain significance (VUS) in the ACADVL gene, leading to uncertainty of diagnosis and providing effective treatment regimen. Currently, ACADVL has >300 VUSs in the ClinVar database that requiring characterization to determine potential pathogenicity. In this study, CRISPR/Cas9 genome editing was used to knock out ACADVL in HEK293T cells, and targeted deletion was confirmed by droplet digital polymerase chain reaction (PCR). No VLCAD protein was detected and an 84% decrease in enzyme activity using the electron transfer flavoprotein fluorescence reduction assay and C21-CoA as substrate was observed compared to control. Plasmids containing control or variant ACADVL coding sequence were transfected into the ACADVL null HEK293T. While transfection of control ACADVL restored VLCAD protein and enzyme activity, cells expressing the VLCAD Val283Ala mutant had 18% VLCAD enzyme activity and reduced protein compared to control. VLCAD Ile420Leu, Gly179Arg, and Gln406Pro produced protein comparable to control but 25%, 4%, and 5% VLCAD enzyme activity, respectively. Leu540Pro and Asp570_Ala572dup had reduced VLCAD protein and 10% and 3% VLCAD enzyme activity, respectively. VLCADD fibroblasts containing the same variations had decreased VLCAD protein and activity comparable to the transfection experiments. Generating ACADVL null HEK293T cell line allowed functional studies to determine pathogenicity of ACADVL exonic variants. This approach can be applied to multiple genes for other disorders identified through NBS.

Biochemical Markers for the Diagnosis of Mitochondrial Fatty Acid Oxidation Diseases

Mitochondrial fatty acid β-oxidation (FAO) contributes a large proportion to the body’s energy needs in fasting and in situations of metabolic stress. Most tissues use energy from fatty acids, particularly the heart, skeletal muscle and the liver. In the brain, ketone bodies formed from FAO in the liver are used as the main source of energy. The mitochondrial fatty acid oxidation disorders (FAODs), which include the carnitine system defects, constitute a group of diseases with several types and subtypes and with variable clinical spectrum and prognosis, from paucisymptomatic cases to more severe affectations, with a 5% rate of sudden death in childhood, and with fasting hypoketotic hypoglycemia frequently occurring. The implementation of newborn screening programs has resulted in new challenges in diagnosis, with the detection of new phenotypes as well as carriers and false positive cases. In this article, a review of the biochemical markers used for the diagnosis of FAODs is presented. The analysis of acylcarnitines by MS/MS contributes to improving the biochemical diagnosis, both in affected patients and in newborn screening, but acylglycines, organic acids, and other metabolites are also reported. Moreover, this review recommends caution, and outlines the differences in the interpretation of the biomarkers depending on age, clinical situation and types of samples or techniques.

Molecular mechanism of interactions between ACAD9 and binding partners in mitochondrial respiratory complex I assembly

The dual function protein ACAD9 catalyzes α,β-dehydrogenation of fatty acyl-CoA thioesters in fatty acid β-oxidation and is an essential chaperone for mitochondrial respiratory complex I (CI) assembly. ACAD9, ECSIT, and NDUFAF1 interact to form the core mitochondrial CI assembly complex. Current studies examine the molecular mechanism of ACAD9/ECSIT/NDUFAF1interactions. ACAD9 binds to the carboxy-terminal half and NDUFAF1 to the amino-terminal half of ECSIT. Binary complexes are unstable and aggregate easily, while the ACAD9/ECSIT/NDUFAF1 ternary complex is soluble and highly stable. Molecular modeling and small-angle X-ray scattering studies identified intra-complex interaction sites and binding sites for other assembly factors. Binding of ECSIT at the ETF binding site in the amino-terminal domain of ACAD9 is consistent with observed loss of FAD and enzymatic activity and demonstrates that the two functions of ACAD9 are mutually exclusive. Mapping of 42 known pathogenic mutations onto the homology-modeled ACAD9 structure provides structural insights into pathomechanisms of CI deficiency.

Targeting the mitochondrial trifunctional protein restrains tumor growth in oxidative lung carcinomas

Metabolic reprogramming is a common hallmark of cancer, but a large variability in tumor bioenergetics exists between patients. Using high-resolution respirometry on fresh biopsies of human lung adenocarcinoma, we identified 2 subgroups reflected in the histologically normal, paired, cancer-adjacent tissue: high (OX+) mitochondrial respiration and low (OX-) mitochondrial respiration. The OX+ tumors poorly incorporated [18F]fluorodeoxy-glucose and showed increased expression of the mitochondrial trifunctional fatty acid oxidation enzyme (MTP; HADHA) compared with the paired adjacent tissue. Genetic inhibition of MTP altered OX+ tumor growth in vivo. Trimetazidine, an approved drug inhibitor of MTP used in cardiology, also reduced tumor growth and induced disruption of the physical interaction between the MTP and respiratory chain complex I, leading to a cellular redox and energy crisis. MTP expression in tumors was assessed using histology scoring methods and varied in negative correlation with [18F]fluorodeoxy-glucose incorporation. These findings provide proof-of-concept data for preclinical, precision, bioenergetic medicine in oxidative lung carcinomas.

Development and characterization of a mouse model for Acad9 deficiency

Acyl CoA Dehydrogenase 9 (ACAD9) is a member of the family of flavoenzymes that catalyze the dehydrogenation of acyl-CoAs to 2,3 enoyl-CoAs in mitochondrial fatty acid oxidation (FAO). Inborn errors of metabolism of all family members, including ACAD9, have been described in humans, and represent significant causes of morbidity and mortality particularly in children. ACAD9 deficiency leads to a combined defect in fatty acid oxidation and oxidative phosphorylation (OXPHOS) due to a dual role in the pathways. In addition to its function in mitochondrial FAO, ACAD9 has a second function as one of 14 factors responsible for assembly of complex I of the electron transport chain (ETC). Considerable controversy remains over the relative role of these two functions in normal physiology and the disparate clinical findings described in patients with ACAD9 deficiency. To better understand the normal function of ACAD9 and the pathophysiology of its deficiency, several knock out mouse models were developed. Homozygous total body knock out appeared to be lethal as no ACAD9 animals were obtained. Cre-lox technology was then used to generate tissue-specific deletion of the gene. Cardiac-specific ACAD9 deficient animals had severe neonatal cardiomyopathy and died by 17 days of age. They had severe mitochondrial dysfunction in vitro. Muscle-specific mutants were viable but exhibited muscle weakness. Additional studies of heart muscle from the cardiac specific deficient animals were used to examine the evolutionarily conserved signaling Intermediate in toll pathway (ECSIT) protein, a known binding partner of ACAD9 in the electron chain complex I assembly pathway. As expected, ECSIT levels were significantly reduced in the absence of ACAD9 protein, consistent with the demonstrated impairment of the complex I assembly. The various ACAD9 deficient animals should serve as useful models for development of novel therapeutics for this disorder.

Characterization of variants of uncertain significance in isovaleryl-CoA dehydrogenase identified through newborn screening: An approach for faster analysis

INTRODUCTION

Clinical standard of care for newborn screening (NBS) is acylcarnitine metabolites quantitation by tandem mass spectrometry (MS/MS) from dried blood spots. Follow up sequencing often results in identification of one or more variants of uncertain significance (VUS). Isovaleric acidemia (IVA) is an autosomal recessive inborn error of metabolism caused by deficiency of isovaleryl-CoA dehydrogenase (IVDH) in the Leu catabolism pathway. Many IVD mutations are characterized as VUS complicating IVA clinical diagnoses and treatment. We present a testing platform approach to confirm the functional implication of VUS identified in newborns with IVA applicable to multiple inborn errors of metabolism identified by NBS.

METHODS

An IVD null HEK293T cell culture model was generated by using a dual sgRNA CRISPR/Cas9 genome-editing strategy targeting IVD exons 2-3. Clonal cell lines were confirmed by a combination of genomic breakpoint sequencing and droplet digital PCR. The IVD null model had no IVDH antigen signal and 96% reduction in IVDH enzyme activity. The IVD null model was transfected with vectors containing control or variant IVD and functional assays were performed to determine variant pathogenicity.

RESULTS

c.149G > C (p.Arg50Pro; precursor numbering), c.986T > C (p.Met329Thr), and c.1010G > A (p.Arg337Gln), c.1179del394 f. mutant proteins had reduced IVDH protein and activity. c.932C > T (p.Ala311Val), c.707C > T (p.Thr236Ile), and c.1232G > A (p.Arg411Gln) had stable IVDH protein, but no enzyme activity. c.521T > G (p.Val174Gly) had normal IVDH protein and activity. IVD variant transfection results confirmed results from IVA fibroblasts containing the same variants.

CONCLUSIONS

We have developed an IVD null HEK293T cell line to rapidly allow determination of VUS pathogenicity following identification of novel alleles by clinical sequencing following positive NBS results for suspected IVA. We suggest similar models can be generated via genome-editing for high throughput assessment of VUS function for a multitude of inborn errors of metabolism and can ideally supplement NBS programs.

Optic neuropathy linked to ACAD9 pathogenic variants: A potentially riboflavin-responsive disorder?

Mitochondrial complex I (CI) deficiencies (OMIM 252010) are the commonest inherited mitochondrial disorders in children. Acyl-CoA dehydrogenase 9 (ACAD9) is a flavoenzyme involved chiefly in CI assembly and possibly in fatty acid oxidation. Biallelic pathogenic variants result in CI dysfunction, with a phenotype ranging from early onset and sometimes fatal mitochondrial encephalopathy with lactic acidosis to late-onset exercise intolerance. Cardiomyopathy is often associated. We report a patient with childhood-onset optic and peripheral neuropathy without cardiac involvement, related to CI deficiency. Genetic analysis revealed compound heterozygous pathogenic variants in ACAD9, expanding the clinical spectrum associated to ACAD9 mutations. Importantly, riboflavin treatment (15 mg/kg/day) improved long-distance visual acuity and demonstrated significant rescue of CI activity in vitro.

Riboflavin in Neurological Diseases: A Narrative Review

Riboflavin is classified as one of the water-soluble B vitamins. It is part of the functional group of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) cofactors and is required for numerous flavoprotein-catalysed reactions. Riboflavin has important antioxidant properties, essential for correct cell functioning. It is required for the conversion of oxidised glutathione to the reduced form and for the mitochondrial respiratory chain as complexes I and II contain flavoprotein reductases and electron transferring flavoproteins. Riboflavin deficiency has been demonstrated to impair the oxidative state of the body, especially in relation to lipid peroxidation status, in both animal and human studies. In the nervous system, riboflavin is essential for the synthesis of myelin and its deficiency can determine the disruption of myelin lamellae. The inherited condition of restricted riboflavin absorption and utilisation, reported in about 10-15% of world population, warrants further investigation in relation to its association with the main neurodegenerative diseases. Several successful trials testing riboflavin for migraine prevention were performed, and this drug is currently classified as a Level B medication for migraine according to the American Academy of Neurology evidence-based rating, with evidence supporting its efficacy. Brown-Vialetto-Van Laere syndrome and Fazio-Londe diseases are now renamed as "riboflavin transporter deficiency" because these are autosomal recessive diseases caused by mutations of SLC52A2 and SLC52A3 genes that encode riboflavin transporters. High doses of riboflavin represent the mainstay of the therapy of these diseases and high doses of riboflavin should be rapidly started as soon as the diagnosis is suspected and continued lifelong. Remarkably, some mitochondrial diseases respond to supplementation with riboflavin. These include multiple acyl-CoA-dehydrogenase deficiency (which is caused by ETFDH gene mutations in the majority of the cases, or mutations in the ETFA and ETFB genes in a minority), mutations of ACAD9 gene, mutations of AIFM1 gene, mutations of the NDUFV1 and NDUFV2 genes. Therapeutic riboflavin administration has been tried in other neurological diseases, including stroke, multiple sclerosis, Friedreich's ataxia and Parkinson's disease. Unfortunately, the design of these clinical trials was not uniform, not allowing to accurately assess the real effects of this molecule on the disease course. In this review we analyse the properties of riboflavin and its possible effects on the pathogenesis of different neurological diseases, and we will review the current indications of this vitamin as a therapeutic intervention in neurology.

Bezafibrate activation of PPAR drives disturbances in mitochondrial redox bioenergetics and decreases the viability of cells from patients with VLCAD deficiency

Very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency is the most common inborn long-chain fatty acid oxidation (FAO) disorder. VLCAD deficiency is characterized by distinct phenotypes. The severe phenotypes are potentially life-threatening and affect the heart or liver, with a comparatively milder phenotype characterized by myopathic symptoms. There is an unmet clinical need for effective treatment options for the myopathic phenotype. The molecular mechanisms driving the gradual decrease in mitochondrial function and associated alterations of muscle fibers are unclear. The peroxisome proliferator-activated receptor (PPAR) pan-agonist bezafibrate is a potent modulator of FAO and multiple other mitochondrial functions and has been proposed as a potential medication for myopathic cases of long-chain FAO disorders. In vitro experiments have demonstrated the ability of bezafibrate to increase VLCAD expression and activity. However, the outcome of small-scale clinical trials has been controversial. We found VLCAD deficient patient fibroblasts to have an increased oxidative stress burden and deranged mitochondrial bioenergetic capacity, compared to controls. Applying heat stress under fasting conditions to bezafibrate pretreated patient cells, caused a marked further increase of mitochondrial superoxide levels. Patient cells failed to maintain levels of the essential thiol peptide antioxidant glutathione and experienced a decrease in cellular viability. Our findings indicate that chronic PPAR activation is a plausible initiator of long-term pathogenesis in VLCAD deficiency. Our findings further implicate disruption of redox homeostasis as a key pathogenic mechanism in VLCAD deficiency and support the notion that a deranged thiol metabolism might be an important pathogenic factor in VLCAD deficiency.

Assembly of The Mitochondrial Complex I Assembly Complex Suggests a Regulatory Role for Deflavination

Fatty acid β‐oxidation (FAO) and oxidative phosphorylation (OXPHOS) are mitochondrial redox processes that generate ATP. The biogenesis of the respiratory Complex I, a 1 MDa multiprotein complex that is responsible for initiating OXPHOS, is mediated by assembly factors including the mitochondrial complex I assembly (MCIA) complex. However, the organisation and the role of the MCIA complex are still unclear. Here we show that ECSIT functions as the bridging node of the MCIA core complex. Furthermore, cryo‐electron microscopy together with biochemical and biophysical experiments reveal that the C‐terminal domain of ECSIT directly binds to the vestigial dehydrogenase domain of the FAO enzyme ACAD9 and induces its deflavination, switching ACAD9 from its role in FAO to an MCIA factor. These findings provide the structural basis for the MCIA complex architecture and suggest a unique molecular mechanism for coordinating the regulation of the FAO and OXPHOS pathways to ensure an efficient energy production.

Successful pregnancy in a patient with mitochondrial cardiomyopathy due to ACAD9 deficiency

Acyl-CoA dehydrogenase family member 9 (ACAD9) is an enzyme essential for the assembly of mitochondrial respiratory chain complex I. ACAD9 deficiency can cause lactic acidosis, myopathy, cardiomyopathy, intellectual disability, and early demise. We present a patient with mitochondrial myopathy, hypertrophic cardiomyopathy, and epilepsy due to recessive ACAD9 mutations. A muscle biopsy depicted ragged red fibers, and decreased activity of complex I of the respiratory chain. Treatment with riboflavin was initiated at the age of 4 years due to complex I deficiency (before the genetic diagnosis), resulting in symptomatic improvement of the cardiomyopathy, exercise intolerance, and lactate levels. A novel homozygous ACAD9 mutation was found: c.398G>A; p.Ser133Asn at the age of 23 years. Three years later she sustained a normal pregnancy, and gave birth to a healthy baby girl delivered by an elective Cesarean section. To the best of our knowledge, this is the first description of a successful pregnancy and delivery in a patient with this rare mitochondrial disease.

Understanding the role of OXPHOS dysfunction in the pathogenesis of ECHS1 deficiency

Mitochondria provide the main source of energy for eukaryotic cells, oxidizing fatty acids and sugars to generate ATP. Mitochondrial fatty acid β-oxidation (FAO) and oxidative phosphorylation (OXPHOS) are two key pathways involved in this process. Disruption of FAO can cause human disease, with patients commonly presenting with liver failure, hypoketotic glycaemia and rhabdomyolysis. However, patients with deficiencies in the FAO enzyme short-chain enoyl-CoA hydratase 1 (ECHS1) are typically diagnosed with Leigh syndrome, a lethal form of subacute necrotizing encephalomyelopathy that is normally associated with OXPHOS dysfunction. Furthermore, some ECHS1-deficient patients also exhibit secondary OXPHOS defects. This sequela of FAO disorders has long been thought to be caused by the accumulation of inhibitory fatty acid intermediates. However, new evidence suggests that the mechanisms involved are more complex, and that disruption of OXPHOS protein complex biogenesis and/or stability is also involved. In this review, we examine the clinical, biochemical and genetic features of all ECHS1-deficient patients described to date. In particular, we consider the secondary OXPHOS defects associated with ECHS1 deficiency and discuss their possible contribution to disease pathogenesis.

Multi-omics studies in cellular models of methylmalonic acidemia and propionic acidemia reveal dysregulation of serine metabolism

BACKGROUND

Methylmalonic acidemia (MMA) and propionic acidemia (PA) are related disorders of mitochondrial propionate metabolism, caused by defects in methylmalonyl-CoA mutase (MUT) and propionyl-CoA carboxylase (PCC), respectively. These biochemical defects lead to a complex cascade of downstream metabolic abnormalities, and identification of these abnormal pathways has important implications for understanding disease pathophysiology. Using a multi-omics approach in cellular models of MMA and PA, we identified serine and thiol metabolism as important areas of metabolic dysregulation.

METHODS

We performed global proteomic analysis of fibroblasts and untargeted metabolomics analysis of plasma from individuals with MMA to identify novel pathways of dysfunction. We probed these novel pathways in CRISPR-edited, MUT and PCCA null HEK293 cell lines via targeted metabolomics, gene expression analysis, and flux metabolomics tracing utilization of ¹³C-glucose.

RESULTS

Proteomic analysis of fibroblasts identified upregulation of multiple proteins involved in serine synthesis and thiol metabolism including: phosphoserine amino transferase (PSAT1), cystathionine beta synthase (CBS), and mercaptopyruvate sulfurtransferase (MPST). Metabolomics analysis of plasma revealed significantly increased levels of cystathionine and glutathione, central metabolites in thiol metabolism. CRISPR-edited MUT and PCCA HEK293 cells recapitulate primary defects of MMA and PA and have upregulation of transcripts associated with serine and thiol metabolism including PSAT1. ¹³C-glucose flux metabolomics in MUT and PCCA null HEK293 cells identified increases in serine de novo biosynthesis, serine transport, and abnormal downstream TCA cycle utilization.

CONCLUSION

We identified abnormal serine metabolism as a novel area of cellular dysfunction in MMA and PA, thus introducing a potential new target for therapeutic investigation.

Maternal diet deficient in riboflavin induces embryonic death associated with alterations in the hepatic proteome of duck embryos

Background

Maternal riboflavin deficiency (RD) induces embryonic death in poultry. The underlying mechanisms, however, remain to be established and an overview of molecular alterations at the protein level is still lacking. We investigated embryonic hepatic proteome changes induced by maternal RD to explain embryonic death.

Methods

A total of 80 45-week-old breeding female ducks were divided into two groups of 40 birds each, and all birds were raised individually for 8 weeks. All the female ducks received either a RD or a riboflavin adequate (control, CON) diet, which supplemented the basal diet with 0 or 10 mg riboflavin /kg of diet respectively.

Results

The riboflavin concentrations of maternal plasma and egg yolk, as well as egg hatchability declined markedly in the RD group compared to those in the CON group after 2 weeks, and declined further over time. The hepatic proteome of E13 viable embryos from 8-week fertile eggs showed that 223 proteins were upregulated and 366 proteins were downregulated (> 1.5-fold change) in the RD group compared to those in the CON group. Pathway analysis showed that differentially expressed proteins were mainly enriched in the fatty acid beta-oxidation, electron transport chain (ETC), and tricarboxylic acid (TCA) cycle. Specifically, all the proteins involved in the fatty acid beta-oxidation and ETC, as well as six out of seven proteins involved in the TCA cycle, were diminished in the RD group, indicating that these processes could be impaired by RD.

Conclusion

Maternal RD leads to embryonic death of offspring and is associated with impaired energy generation processes, indicated by a number of downregulated proteins involved in the fatty acid beta-oxidation, ETC, and TCA cycle in the hepatic of duck embryos. These findings contribute to our understanding of the mechanisms of liver metabolic disorders due to maternal RD.

Electronic supplementary material

The online version of this article (10.1186/s12986-019-0345-8) contains supplementary material, which is available to authorized users.

Human diseases associated with defects in assembly of OXPHOS complexes

The structural biogenesis and functional proficiency of the multiheteromeric complexes forming the mitochondrial oxidative phosphorylation system (OXPHOS) require the concerted action of a number of chaperones and other assembly factors, most of which are specific for each complex. Mutations in a large number of these assembly factors are responsible for mitochondrial disorders, in most cases of infantile onset, typically characterized by biochemical defects of single specific complexes. In fact, pathogenic mutations in complex-specific assembly factors outnumber, in many cases, the repertoire of mutations found in structural subunits of specific complexes. The identification of patients with specific defects in assembly factors has provided an important contribution to the nosological characterization of mitochondrial disorders, and has also been a crucial means to identify a huge number of these proteins in humans, which play an essential role in mitochondrial bioenergetics. The wide use of next generation sequencing (NGS) has led to and will allow the identifcation of additional components of the assembly machinery of individual complexes, mutations of which are responsible for human disorders. The functional studies on patients' specimens, together with the creation and characterization of in vivo models, are fundamental to better understand the mechanisms of each of them. A new chapter in this field will be, in the near future, the discovery of mechanisms and actions underlying the formation of supercomplexes, molecular structures formed by the physical, and possibly functional, interaction of some of the individual respiratory complexes, particularly complex I (CI), III (CIII), and IV (CIV).

Clinical, biochemical and genetic spectrum of 70 patients with ACAD9 deficiency: is riboflavin supplementation effective?

BACKGROUND

Mitochondrial acyl-CoA dehydrogenase family member 9 (ACAD9) is essential for the assembly of mitochondrial respiratory chain complex I. Disease causing biallelic variants in ACAD9 have been reported in individuals presenting with lactic acidosis and cardiomyopathy.

RESULTS

We describe the genetic, clinical and biochemical findings in a cohort of 70 patients, of whom 29 previously unpublished. We found 34 known and 18 previously unreported variants in ACAD9. No patients harbored biallelic loss of function mutations, indicating that this combination is unlikely to be compatible with life. Causal pathogenic variants were distributed throughout the entire gene, and there was no obvious genotype-phenotype correlation. Most of the patients presented in the first year of life. For this subgroup the survival was poor (50% not surviving the first 2 years) comparing to patients with a later presentation (more than 90% surviving 10 years). The most common clinical findings were cardiomyopathy (85%), muscular weakness (75%) and exercise intolerance (72%). Interestingly, severe intellectual deficits were only reported in one patient and severe developmental delays in four patients. More than 70% of the patients were able to perform the same activities of daily living when compared to peers.

CONCLUSIONS

Our data show that riboflavin treatment improves complex I activity in the majority of patient-derived fibroblasts tested. This effect was also reported for most of the treated patients and is mirrored in the survival data. In the patient group with disease-onset below 1 year of age, we observed a statistically-significant better survival for patients treated with riboflavin.

Evaluation of mitochondrial bioenergetics, dynamics, endoplasmic reticulum-mitochondria crosstalk, and reactive oxygen species in fibroblasts from patients with complex I deficiency

Mitochondrial complex I (CI) deficiency is the most frequent cause of oxidative phosphorylation (OXPHOS) disorders in humans. In order to benchmark the effects of CI deficiency on mitochondrial bioenergetics and dynamics, respiratory chain (RC) and endoplasmic reticulum (ER)-mitochondria communication, and superoxide production, fibroblasts from patients with mutations in the ND6, NDUFV1 or ACAD9 genes were analyzed. Fatty acid metabolism, basal and maximal respiration, mitochondrial membrane potential, and ATP levels were decreased. Changes in proteins involved in mitochondrial dynamics were detected in various combinations in each cell line, while variable changes in RC components were observed. ACAD9 deficient cells exhibited an increase in RC complex subunits and DDIT3, an ER stress marker. The level of proteins involved in ER-mitochondria communication was decreased in ND6 and ACAD9 deficient cells. |ΔΨ| and cell viability were further decreased in all cell lines. These findings suggest that disruption of mitochondrial bioenergetics and dynamics, ER-mitochondria crosstalk, and increased superoxide contribute to the pathophysiology in patients with ACAD9 deficiency. Furthermore, treatment of ACAD9 deficient cells with JP4-039, a novel mitochondria-targeted reactive oxygen species, electron and radical scavenger, decreased superoxide level and increased basal and maximal respiratory rate, identifying a potential therapeutic intervention opportunity in CI deficiency.

Severe riboflavin deficiency induces alterations in the hepatic proteome of starter Pekin ducks

Suboptimal vitamin B2 status is encountered globally. Riboflavin deficiency depresses growth and results in a fatty liver. The underlying mechanisms remain to be established and an overview of molecular alterations is lacking. We investigated hepatic proteome changes induced by riboflavin deficiency to explain its effects on growth and hepatic lipid metabolism. In all, 360 1-d-old Pekin ducks were divided into three groups of 120 birds each, with twelve replicates and ten birds per replicate. For 21 d, the ducks were fed ad libitum a control diet (CAL), a riboflavin-deficient diet (RD) or were pair-fed with the control diet to the mean daily intake of the RD group (CPF). When comparing RD with CAL and CPF, growth depression, liver enlargement, liver lipid accumulation and enhanced liver SFA (C6 : 0, C12 : 0, C16 : 0, C18 : 0) were observed. In RD, thirty-two proteins were enhanced and thirty-one diminished (>1·5-fold) compared with CAL and CPF. Selected proteins were confirmed by Western blotting. The diminished proteins are mainly involved in fatty acid β-oxidation and the mitochondrial electron transport chain (ETC), whereas the enhanced proteins are mainly involved in TAG and cholesterol biosynthesis. RD causes liver lipid accumulation and growth depression probably by impairing fatty acid β-oxidation and ETC. These findings contribute to our understanding of the mechanisms of liver lipid metabolic disorders due to RD.

Inflexibility of AMPK-mediated metabolic reprogramming in mitochondrial disease

Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome is most commonly caused by the A3243G mutation of mitochondrial DNA. The capacity to utilize fatty acid or glucose as a fuel source and how such dynamic switches of metabolic fuel preferences and transcriptional modulation of adaptive mechanism in response to energy deficiency in MELAS syndrome have not been fully elucidated. The fibroblasts from patients with MELAS syndrome demonstrated a remarkable deficiency of electron transport chain complexes I and IV, an impaired cellular biogenesis under glucose deprivation, and a decreased ATP synthesis. In situ analysis of the bioenergetic properties of MELAS cells demonstrated an attenuated fatty acid oxidation that concomitantly occurred with impaired mitochondrial respiration, while energy production was mostly dependent on glycolysis. Furthermore, the transcriptional modulation was mediated by the AMP-activated protein kinase (AMPK) signaling pathway, which activated its downstream modulators leading to a subsequent increase in glycolytic flux through activation of pyruvate dehydrogenase. In contrast, the activities of carnitine palmitoyltransferase for fatty acid oxidation and acetyl-CoA carboxylase-1 for fatty acid synthesis were reduced and transcriptional regulation factors for biogenesis were not altered. These results provide novel information that MELAS cells lack the adaptive mechanism to switch fuel source from glucose to fatty acid, as glycolysis rates increase in response to energy deficiency. The aberrant secondary cellular responses to disrupted metabolic homeostasis mediated by AMPK signaling pathway may contribute to the development of the clinical phenotype.

Advances in the Understanding and Treatment of Mitochondrial Fatty Acid Oxidation Disorders

Purpose of review

This review focuses on advances made in the past three years with regards to understanding the mitochondrial fatty acid oxidation (FAO) pathway, the pathophysiological ramifications of genetic lesions in FAO enzymes, and emerging therapies for FAO disorders.

Recent findings

FAO has now been recognized to play a key energetic role in pulmonary surfactant synthesis, T-cell differentiation and memory, and the response of the proximal tubule to kidney injury. Patients with FAO disorders may face defects in these cellular systems as they age. Aspirin, statins, and nutritional supplements modulate the rate of FAO under normal conditions and could be risk factors for triggering symptoms in patients with FAO disorders. Patients have been identified with mutations in the ACAD9 and ECHS1 genes, which may represent new FAO disorders. New interventions for long-chain FAODs are in clinical trials. Finally, post-translational modifications that regulate fatty acid oxidation protein activities have been characterized that represent important new therapeutic targets.

Summary

Recent research has led to a deeper understanding of FAO. New therapeutic avenues are being pursued that may ultimately cause a paradigm shift for patient care.

Severe defect in mitochondrial complex I assembly with mitochondrial DNA deletions in ACAD9-deficient mild myopathy

INTRODUCTION

Acyl-coenzyme A dehydrogenase 9 (ACAD9) has a role in mitochondrial complex I (CI) assembly. Only a few patients who carry ACAD9 mutations have been reported. They mainly present with severe hypertrophic cardiomyopathy, although a minority have only mild isolated myopathy. Although the secondary factors influencing disease severity have not been elucidated, conservation of CI assembly and residual enzymatic activity have been suggested as explanations for the mild phenotypes associated with ACAD9 mutations.

METHODS

We report a novel homozygous ACAD9 mutation (c.1240C>T; p.Arg414Cys) in a 34-year-old woman who presented with non-progressive myopathy.

RESULTS

We show that this ACAD9 mutation led to a severe defect in CI assembly in the patient's muscle. Furthermore, the impact of CI deficiency is confirmed by accumulation of mitochondrial DNA deletions.

CONCLUSION

Our data suggest that a major defect of CI assembly is not responsible for a severe phenotype. Muscle Nerve 55: 919-922, 2017.

Lifetime exercise intolerance with lactic acidosis as key manifestation of novel compound heterozygous ACAD9 mutations causing complex I deficiency

We report a 36-year-old female having lifetime exercise intolerance and lactic acidosis with nausea associated with novel compound heterozygous Acyl-CoA dehydrogenase 9 gene (ACAD9) mutations (p.Ala390Thr and p.Arg518Cys). ACAD9 is an assembly factor for the mitochondrial respiratory chain complex I. ACAD9 mutations are recognized as frequent causes of complex I deficiency. Our patient presented with exercise intolerance, rapid fatigue, and nausea since early childhood. Mild physical workload provoked the occurrence of nausea and vomiting repeatedly. Her neurological examination, laboratory findings and muscle biopsy demonstrated no abnormalities. A bicycle spiroergometry provoked significant lactic acidosis during and following exercise pointing towards a mitochondrial disorder. Subsequently, the analysis of respiratory chain enzyme activities in muscle revealed severe isolated complex I deficiency. Candidate gene sequencing revealed two novel heterozygous ACAD9 mutations. This patient report expands the mutational and phenotypic spectrum of diseases associated with mutations in ACAD9.

An atypical presentation of ACAD9 deficiency: Diagnosis by whole exome sequencing broadens the phenotypic spectrum and alters treatment approach

Acyl-CoA dehydrogenase 9 (ACAD9), linked to chromosome 3q21.3, is one of a family of multimeric mitochondrial flavoenzymes that catalyze the degradation of fatty acyl-CoA from the carnitine shuttle via β-oxidation (He et al. 2007). ACAD9, specifically, is implicated in the processing of palmitoyl-CoA and long-chain unsaturated substrates, but unlike other acyl-CoA dehydrogenases (ACADs), it has a significant role in mitochondrial complex I assembly (Nouws et al. 2010 & 2014). Mutations in this enzyme typically cause mitochondrial complex I deficiency, as well as a mild defect in long chain fatty acid metabolism (Haack et al. 2010, Kirby et al. 2004, Mcfarland et al. 2003, Nouws et al. 2010 & 2014). The clinical phenotype of ACAD9 deficiency and the associated mitochondrial complex I deficiency reflect this unique duality, and symptoms are variable in severity and onset. Patients classically present with cardiac dysfunction due to hypertrophic cardiomyopathy. Other common features include Leigh syndrome, macrocephaly, and liver disease (Robinson et al. 1998).

We report the case of an 11-month old girl presenting with microcephaly, dystonia, and lactic acidosis, concerning for a mitochondrial disorder, but atypical for ACAD9 deficiency. Muscle biopsy showed mitochondrial proliferation, but normal mitochondrial complex I activity. The diagnosis of ACAD9 deficiency was not initially considered, due both to these findings and to her atypical presentation. Biochemical assay for ACAD9 deficiency is not clinically available. Family trio-based whole exome sequencing (WES) identified 2 compound heterozygous mutations in the ACAD9 gene. This discovery led to optimized treatment of her mitochondrial dysfunction, and supplementation with riboflavin, resulting in clinical improvement.

There have been fewer than 25 reported cases of ACAD9 deficiency in the literature to date. We review these and compare them to the unique features of our patient. ACAD9 deficiency should be considered in the differential diagnosis of patients with lactic acidosis, seizures, and other symptoms of mitochondrial disease, including those with normal mitochondrial enzyme activities. This case demonstrates the utility of WES, in conjunction with biochemical testing, for the appropriate diagnosis and treatment of disorders of energy metabolism.

Evidence of a wide spectrum of cardiac involvement due to ACAD9 mutations: Report on nine patients

Acyl-CoA dehydrogenase 9 (ACAD9) is a mitochondrial protein involved in oxidative phosphorylation complex I biogenesis. This protein also exhibits acyl-CoA dehydrogenase (ACAD) activity. ACAD9-mutated patients have been reported to suffer from primarily heart, muscle, liver, and nervous system disorders. ACAD9 mutation is suspected in cases of elevated lactic acid levels combined with complex I deficiency, and confirmed by ACAD9 gene analysis. At least 18 ACAD9-mutated patients have previously been reported, usually displaying severe cardiac involvement. We retrospectively studied nine additional patients from three unrelated families with a wide spectrum of cardiac involvement between the families as well as the patients from the same families. All patients exhibited elevated lactate levels. Deleterious ACAD9 mutations were identified in all patients except one for whom it was not possible to recover DNA. To our knowledge, this is one of the first reports on isolated mild ventricular hypertrophy due to ACAD9 mutation in a family with moderate symptoms during adolescence. This report also confirms that dilated cardiomyopathy may occur in conjunction with ACAD9 mutation and that some patients may respond clinically to riboflavin treatment. Of note, several patients suffered from patent ductus arteriosus (PDA), with one exhibiting a complex congenital heart defect. It is yet unknown whether these cardiac manifestations were related to ACAD9 mutation. In conclusion, this disorder should be suspected in the presence of lactic acidosis, complex I deficiency, and any cardiac involvement, even mild.

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.

The origin of the supernumerary subunits and assembly factors of complex I: A treasure trove of pathway evolution

We review and document the evolutionary origin of all complex I assembly factors and nine supernumerary subunits from protein families. Based on experimental data and the conservation of critical residues we identify a spectrum of protein function conservation between the complex I representatives and their non-complex I homologs. This spectrum ranges from proteins that have retained their molecular function but in which the substrate specificity may have changed or have become more specific, like NDUFAF5, to proteins that have lost their original molecular function and critical catalytic residues like NDUFAF6. In between are proteins that have retained their molecular function, which however appears unrelated to complex I, like ACAD9, or proteins in which amino acids of the active site are conserved but for which no enzymatic activity has been reported, like NDUFA10. We interpret complex I evolution against the background of molecular evolution theory. Complex I supernumerary subunits and assembly factors appear to have been recruited from proteins that are mitochondrial and/or that are expressed when complex I is active. Within the evolution of complex I and its assembly there are many cases of neofunctionalization after gene duplication, like ACAD9 and TMEM126B, one case of subfunctionalization: ACPM1 and ACPM2 in Yarrowia lipolytica, and one case in which a complex I protein itself appears to have been the source of a new protein from another complex: NDUFS6 gave rise to cytochrome c oxidase subunit COX4/COX5b. Complex I and its assembly can therewith be regarded as a treasure trove for pathway evolution. This article is part of a Special Issue entitled Respiratory complex I, edited by Volker Zickermann and Ulrich Brandt.

Combined defects in oxidative phosphorylation and fatty acid β-oxidation in mitochondrial disease

Mitochondria provide the main source of energy to eukaryotic cells, oxidizing fats and sugars to generate ATP. Mitochondrial fatty acid β-oxidation (FAO) and oxidative phosphorylation (OXPHOS) are two metabolic pathways which are central to this process. Defects in these pathways can result in diseases of the brain, skeletal muscle, heart and liver, affecting approximately 1 in 5000 live births. There are no effective therapies for these disorders, with quality of life severely reduced for most patients. The pathology underlying many aspects of these diseases is not well understood; for example, it is not clear why some patients with primary FAO deficiencies exhibit secondary OXPHOS defects. However, recent findings suggest that physical interactions exist between FAO and OXPHOS proteins, and that these interactions are critical for both FAO and OXPHOS function. Here, we review our current understanding of the interactions between FAO and OXPHOS proteins and how defects in these two metabolic pathways contribute to mitochondrial disease pathogenesis.

Lethal Neonatal Progression of Fetal Cardiomegaly Associated to ACAD9 Deficiency

ACAD9 (acyl-CoA dehydrogenase 9) is an essential factor for the mitochondrial respiratory chain complex I assembly. ACAD9, a member of acyl-CoA dehydrogenase family, has high homology with VLCAD (very long-chain acyl-CoA dehydrogenase) and harbors a homodimer structure. Recently, patients with ACAD9 deficiency have been described with a wide clinical spectrum ranging from severe lethal form to moderate form with exercise intolerance.We report here a prenatal presentation with intrauterine growth retardation and cardiomegaly, with a fatal outcome shortly after birth. Compound heterozygous mutations, a splice-site mutation - c.1030-1G>T and a missense mutation - c.1249C>T; p.Arg417Cys, were identified in the ACAD9 gene. Their effect on protein structure and expression level was investigated. Protein modeling suggested a functional effect of the c.1030-1G>T mutation generating a non-degraded truncated protein and the p.Arg417Cys, creating an aberrant dimer. Our results underscore the crucial role of ACAD9 protein for cardiac function.

The Biochemistry and Physiology of Mitochondrial Fatty Acid β-Oxidation and Its Genetic Disorders

Mitochondrial fatty acid β-oxidation (FAO) is the major pathway for the degradation of fatty acids and is essential for maintaining energy homeostasis in the human body. Fatty acids are a crucial energy source in the postabsorptive and fasted states when glucose supply is limiting. But even when glucose is abundantly available, FAO is a main energy source for the heart, skeletal muscle, and kidney. A series of enzymes, transporters, and other facilitating proteins are involved in FAO. Recessively inherited defects are known for most of the genes encoding these proteins. The clinical presentation of these disorders may include hypoketotic hypoglycemia, (cardio)myopathy, arrhythmia, and rhabdomyolysis and illustrates the importance of FAO during fasting and in hepatic and (cardio)muscular function. In this review, we present the current state of knowledge on the biochemistry and physiological functions of FAO and discuss the pathophysiological processes associated with FAO disorders.

Characterization of clinically identified mutations in NDUFV1, the flavin-binding subunit of respiratory complex I, using a yeast model system

Dysfunctions in mitochondrial complex I (NADH:ubiquinone oxidoreductase) are both genetically and clinically highly diverse and a major cause of human mitochondrial diseases. The genetic determinants of individual clinical cases are increasingly being described, but how these genetic defects affect complex I on the molecular and cellular level, and have different clinical consequences in different individuals, is little understood. Furthermore, without molecular-level information innocent genetic variants may be misassigned as pathogenic. Here, we have used a yeast model system (Yarrowia lipolytica) to study the molecular consequences of 16 single amino acid substitutions, classified as pathogenic, in the NDUFV1 subunit of complex I. NDUFV1 binds the flavin cofactor that oxidizes NADH and is the site of complex I-mediated reactive oxygen species production. Seven mutations caused loss of complex I expression, suggesting they are detrimental but precluding further study. In two variants complex I was fully assembled but did not contain any flavin, and four mutations led to functionally compromised enzymes. Our study provides a molecular rationale for assignment of all these variants as pathogenic. However, three variants provided complex I that was functionally equivalent to the wild-type enzyme, challenging their assignment as pathogenic. By combining structural, bioinformatic and functional data, a simple scoring system for the initial evaluation of future NDUFV1 variants is proposed. Overall, our results broaden understanding of how mutations in this centrally important core subunit of complex I affect its function and provide a basis for understanding the role of NDUFV1 mutations in mitochondrial dysfunction.