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

Revitalising Riboflavin: Unveiling Its Timeless Significance in Human Physiology and Health

Since the early twentieth century, research on vitamins has revealed their therapeutic potential beyond their role as essential micronutrients. Riboflavin, known as vitamin B2, stands out for its unique characteristics. Despite numerous studies, riboflavin remains vital, with implications for human health. Abundantly present in various foods, riboflavin acts as a coenzyme in numerous enzymatic reactions crucial for human metabolism. Its role in energy production, erythrocyte synthesis, and vitamin metabolism underscores its importance in maintaining homeostasis. The impact of riboflavin extends to neurological function, skin health, and cardiovascular well-being, with adequate levels linked to reduced risks of various ailments. However, inadequate intake or physiological stress can lead to deficiency, a condition that poses serious health risks, including severe complications. This underscores the importance of maintaining sufficient levels of riboflavin for general wellness. The essential role of riboflavin in immune function further emphasises its significance for human health and vitality. This paper examines the diverse effects of riboflavin on health and stresses the importance of maintaining sufficient levels for overall well-being.

Genetic aetiologies of acute liver failure

Acute liver failure (ALF) is a rare, rapidly evolving, clinical syndrome with devastating consequences where definitive treatment is by emergency liver transplantation. Establishing a diagnosis can be challenging and, historically, the cause of ALF was unidentified in up to half of children. However, recent technological and clinical advances in genomic medicine have led to an increasing proportion being diagnosed with monogenic aetiologies of ALF. The conditions encountered include a diverse group of inherited metabolic disorders each with prognostic and treatment implications. Often these disorders are clinically indistinguishable and may even mimic disorders of immune regulation or red cell disorders. Rapid genomic sequencing for children with ALF is, therefore, a key component in the diagnostic work up today. This review focuses on the monogenic aetiologies of ALF.

The Ubiquinone-Ubiquinol Redox Cycle and Its Clinical Consequences: An Overview

Coenzyme Q10 (CoQ10) plays a key role in many aspects of cellular metabolism. For CoQ10 to function normally, continual interconversion between its oxidised (ubiquinone) and reduced (ubiquinol) forms is required. Given the central importance of this ubiquinone-ubiquinol redox cycle, this article reviews what is currently known about this process and the implications for clinical practice. In mitochondria, ubiquinone is reduced to ubiquinol by Complex I or II, Complex III (the Q cycle) re-oxidises ubiquinol to ubiquinone, and extra-mitochondrial oxidoreductase enzymes participate in the ubiquinone-ubiquinol redox cycle. In clinical terms, the outcome of deficiencies in various components associated with the ubiquinone-ubiquinol redox cycle is reviewed, with a particular focus on the potential clinical benefits of CoQ10 and selenium co-supplementation.

Complex mitochondrial disease caused by the mutation of COX10 in a toddler: a case-report study

Introduction and importance

Cytochrome C oxidase (COX) deficiency is an uncommon inherited metabolic disorder. It is identified by a lack of the COX, also known as Complex IV. This enzyme plays a crucial role in the rate-limiting and oxygen-accepting step of the respiratory chain within the subcellular structures called mitochondria. The deficiency of COX can either be restricted to skeletal muscle tissues or can impact multiple tissues throughout the body.

Case presentation

A 3-year-old girl was admitted due to muscle weakness and a decline in developmental milestones 7 days after a significant stressor. Leukodystrophy was observed in the brain magnetic resonance imaging, and genome sequencing identified a homozygous mutation in exon 1 and 7 of chromosome 17. This mutation led to a deficiency in COX10, which is a component of mitochondrial complex IV.

Clinical discussion

In the medical field, inherited metabolic disorders can be complex to diagnose due to overlapping symptoms with other conditions. Mitochondria's oxidative phosphorylation system, including the COX enzyme complex, plays a crucial role in energy production. Mitochondrial disorders, including COX deficiency, can present at various stages of life with diverse symptoms. Treatment options focus on supportive care and potential benefits from supplements like coenzyme-Q10 and small-molecule therapies targeting mitochondrial function. Identifying genetic mutations is key for advancing treatments in this area.

Conclusion

This report presents a unique case of developmental regression and muscle weakness in a paediatric patient, which can be attributed to a rare occurrence of type 3 nuclear mitochondrial complex IV 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.

Regulation of respiratory complex I assembly by FMN cofactor targeting

Respiratory complex I plays a crucial role in the mitochondrial electron transport chain and shows promise as a therapeutic target for various human diseases. While most studies focus on inhibiting complex I at the Q-site, little is known about inhibitors targeting other sites within the complex. In this study, we demonstrate that diphenyleneiodonium (DPI), a N-site inhibitor, uniquely affects the stability of complex I by reacting with its flavin cofactor FMN. Treatment with DPI blocks the final stage of complex I assembly, leading to the complete and reversible degradation of complex I in different cellular models. Growing cells in medium lacking the FMN precursor riboflavin or knocking out the mitochondrial flavin carrier gene SLC25A32 results in a similar complex I degradation. Overall, our findings establish a direct connection between mitochondrial flavin homeostasis and complex I stability and assembly, paving the way for novel pharmacological strategies to regulate respiratory complex I.

Biomolecules of Muscle Fatigue in Metabolic Myopathies

Metabolic myopathies are a group of genetic disorders that affect the normal functioning of muscles due to abnormalities in metabolic pathways. These conditions result in impaired energy production and utilization within muscle cells, leading to limitations in muscle function with concomitant occurrence of related signs and symptoms, among which fatigue is one of the most frequently reported. Understanding the underlying molecular mechanisms of muscle fatigue in these conditions is challenging for the development of an effective diagnostic and prognostic approach to test targeted therapeutic interventions. This paper outlines the key biomolecules involved in muscle fatigue in metabolic myopathies, including energy substrates, enzymes, ion channels, and signaling molecules. Potential future research directions in this field are also discussed.

New insights into the nutritional genomics of adult-onset riboflavin-responsive diseases

Riboflavin, or vitamin B2, is an essential nutrient that serves as a precursor to flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN). The binding of the FAD and/or FMN cofactors to flavoproteins is critical for regulating their assembly and activity. There are over 90 proteins in the human flavoproteome that regulate a diverse array of biochemical pathways including mitochondrial metabolism, riboflavin transport, ubiquinone and FAD synthesis, antioxidant signalling, one-carbon metabolism, nitric oxide signalling and peroxisome oxidative metabolism. The identification of patients with genetic variants in flavoprotein genes that lead to adult-onset pathologies remains a major diagnostic challenge. However, once identified, many patients with adult-onset inborn errors of metabolism demonstrate remarkable responses to riboflavin therapy. We review the structure:function relationships of mutant flavoproteins and propose new mechanistic insights into adult-onset riboflavin-responsive pathologies and metabolic dysregulations that apply to multiple biochemical pathways. We further address the vexing issue of how the inheritance of genetic variants in flavoprotein genes leads to an adult-onset disease with complex symptomologies and varying severities. We also propose a broad clinical framework that may not only improve the current diagnostic rates, but also facilitate a personalized approach to riboflavin therapy that is low cost, safe and lead to transformative outcomes in many patients.

Treatment of Mitochondrial Phenylalanyl-tRNa-Synthetase Deficiency (FARS2) with Oral Phenylalanine

OBJECTIVE

By loading transfer RNAs with their cognate amino acids, aminoacyl-tRNA synthetases (ARS) are essential for protein translation. Both cytosolic ARS1-deficiencies and mitochondrial ARS2 deficiencies can cause severe diseases. Amino acid supplementation has shown to positively influence the clinical course of four individuals with cytosolic ARS1 deficiencies. We hypothesize that this intervention could also benefit individuals with mitochondrial ARS2 deficiencies.

METHODS

This study was designed as a N-of-1 trial. Daily oral L-phenylalanine supplementation was used in a 3-year-old girl with FARS2 deficiency. A period without supplementation was implemented to discriminate the effects of treatment from age-related developments and continuing physiotherapy. Treatment effects were measured through a physiotherapeutic testing battery, including movement assessment battery for children, dynamic gait index, gross motor function measure 66, and quality of life questionnaires.

RESULTS

The individual showed clear improvement in all areas tested, especially in gross motor skills, movement abilities, and postural stability. In the period without supplementation, she lost newly acquired motor skills but regained these upon restarting supplementation. No adverse effects and good tolerance of treatment were observed.

INTERPRETATION AND CONCLUSION

Our positive results encourage further studies both on L-phenylalanine for other individuals with FARS2 deficiency and the exploration of this treatment rationale for other ARS2 deficiencies. Additionally, treatment costs were relatively low at 1.10 €/day.

The Efficacy of Whole Genome Sequencing and RNA-Seq in the Diagnosis of Whole Exome Sequencing Negative Patients with Complex Neurological Phenotypes

Whole-genome sequencing (WGS) is being increasingly utilized for the diagnosis of neurological disease by sequencing both the exome and the remaining 98 to 99% of the genetic code. In addition to more complete coverage, WGS can detect structural variants (SVs) and intronic variants (SNVs) that cannot be identified by whole exome sequencing (WES) or chromosome microarray (CMA). Other multi-omics tools, such as RNA sequencing (RNA-Seq), can be used in conjunction with WGS to functionally validate certain variants by detecting changes in gene expression and splicing. The objective of this retrospective study was to measure the diagnostic yield of duo/trio-based WGS and RNA-Seq in a cohort of 22 patients (20 families) with pediatric onset neurological phenotypes and negative or inconclusive WES results in lieu of reanalysis. WGS with RNA-Seq resulted in a definite diagnosis of an additional 25% of cases. Sixty percent of these solved cases arose from the identification of variants that were missed by WES. Variants that could not be unequivocally proven to be causative of the patients' condition were identified in an additional 5% of cases.

Clinical Reasoning: A 14-Year-Old Girl With Reversible Peripheral Neuropathy and Encephalopathy

A 14-year-old girl presented with acute ascending, symmetric numbness, and flaccid paralysis 3 weeks after a suspected gastrointestinal infection. She had experienced anorexia since this gastrointestinal episode. EMG showed a sensorimotor axonal polyneuropathy. Routine CSF analysis and serum-specific antibodies (antiganglioside and node of Ranvier-associated antibodies) were all negative. Laboratory investigations for possible etiologies revealed only mild metabolic perturbations. During her hospitalization, she developed mild cognitive deficits. Brain MRI showed bilateral symmetric basal ganglia lesions with hyperintensity on T2 fluid-attenuated inversion recovery, diffusion-weighted imaging hyperintensity, and corresponding apparent diffusion coefficient hypointensity, but without contrast enhancement. A more thorough and detailed history indicated exercise intolerance, and specific examinations subsequently revealed an underlying etiology. This case presentation discusses specific etiology of an acute-onset diffuse and symmetric neuropathy after an acquired injury in a teenager, emphasizing the need of a broad differential diagnosis in this condition.

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.

Clinical, Genetic, and Histological Characterization of Patients with Rare Neuromuscular and Mitochondrial Diseases Presenting with Different Cardiomyopathy Phenotypes

Cardiomyopathies are mostly determined by genetic mutations affecting either cardiac muscle cell structure or function. Nevertheless, cardiomyopathies may also be part of complex clinical phenotypes in the spectrum of neuromuscular (NMD) or mitochondrial diseases (MD). The aim of this study is to describe the clinical, molecular, and histological characteristics of a consecutive cohort of patients with cardiomyopathy associated with NMDs or MDs referred to a tertiary cardiomyopathy clinic. Consecutive patients with a definitive diagnosis of NMDs and MDs presenting with a cardiomyopathy phenotype were described. Seven patients were identified: two patients with ACAD9 deficiency (Patient 1 carried the c.1240C>T (p.Arg414Cys) homozygous variant in ACAD9; Patient 2 carried the c.1240C>T (p.Arg414Cys) and the c.1646G>A (p.Ar549Gln) variants in ACAD9); two patients with MYH7-related myopathy (Patient 3 carried the c.1325G>A (p.Arg442His) variant in MYH7; Patient 4 carried the c.1357C>T (p.Arg453Cys) variant in MYH7); one patient with desminopathy (Patient 5 carried the c.46C>T (p.Arg16Cys) variant in DES); two patients with mitochondrial myopathy (Patient 6 carried the m.3243A>G variant in MT-TL1; Patient 7 carried the c.253G>A (p.Gly85Arg) and the c.1055C>T (p.Thr352Met) variants in MTO1). All patients underwent a comprehensive cardiovascular and neuromuscular evaluation, including muscle biopsy and genetic testing. This study described the clinical phenotype of rare NMDs and MDs presenting as cardiomyopathies. A multidisciplinary evaluation, combined with genetic testing, plays a main role in the diagnosis of these rare diseases, and provides information about clinical expectations, and guides management.

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.

Currently available therapies in mitochondrial disease

Mitochondrial diseases are a heterogeneous group of multisystem disorders caused by impaired mitochondrial function. These disorders occur at any age and involve any tissue, typically affecting organs highly dependent on aerobic metabolism. Diagnosis and management are extremely difficult due to various underlying genetic defects and a wide range of clinical symptoms. Preventive care and active surveillance are strategies to try to reduce morbidity and mortality by timely treatment of organ-specific complications. More specific interventional therapies are in early phases of development and no effective treatment or cure currently exists. A variety of dietary supplements have been utilized based on biological logic. For several reasons, few randomized controlled trials have been completed to assess the efficacy of these supplements. The majority of the literature on supplement efficacy represents case reports, retrospective analyses and open-label studies. We briefly review selected supplements that have some degree of clinical research support. In mitochondrial diseases, potential triggers of metabolic decompensation or medications that are potentially toxic to mitochondrial function should be avoided. We shortly summarize current recommendations on safe medication in mitochondrial diseases. Finally, we focus on the frequent and debilitating symptoms of exercise intolerance and fatigue and their management including physical training strategies.

Loss of mitochondrial fatty acid β-oxidation protein short-chain Enoyl-CoA hydratase disrupts oxidative phosphorylation protein complex stability and function

Short-chain enoyl-CoA hydratase 1 (ECHS1) is involved in the second step of mitochondrial fatty acid β-oxidation (FAO), catalysing the hydration of short-chain enoyl-CoA esters to short-chain 3-hyroxyl-CoA esters. Genetic deficiency in ECHS1 (ECHS1D) is associated with a specific subset of Leigh Syndrome, a disease typically caused by defects in oxidative phosphorylation (OXPHOS). Here, we examined the molecular pathogenesis of ECHS1D using a CRISPR/Cas9 edited human cell 'knockout' model and fibroblasts from ECHS1D patients. Transcriptome analysis of ECHS1 'knockout' cells showed reductions in key mitochondrial pathways, including the tricarboxylic acid cycle, receptor-mediated mitophagy and nucleotide biosynthesis. Subsequent proteomic analyses confirmed these reductions and revealed additional defects in mitochondrial oxidoreductase activity and fatty acid β-oxidation. Functional analysis of ECHS1 'knockout' cells showed reduced mitochondrial oxygen consumption rates when metabolising glucose or OXPHOS complex I-linked substrates, as well as decreased complex I and complex IV enzyme activities. ECHS1 'knockout' cells also exhibited decreased OXPHOS protein complex steady-state levels (complex I, complex III₂ , complex IV, complex V and supercomplexes CIII₂ /CIV and CI/CIII₂ /CIV), which were associated with a defect in complex I assembly. Patient fibroblasts exhibit varied reduction of mature OXPHOS complex steady-state levels, with defects detected in CIII₂ , CIV, CV and the CI/CIII₂ /CIV supercomplex. Overall, these findings highlight the contribution of defective OXPHOS function, in particular complex I deficiency, to the molecular pathogenesis of ECHS1D.

Mitochondrial Unfolded Protein Response and Integrated Stress Response as Promising Therapeutic Targets for Mitochondrial Diseases

The development and application of high-throughput omics technologies have enabled a more in-depth understanding of mitochondrial biosynthesis metabolism and the pathogenesis of mitochondrial diseases. In accordance with this, a host of new treatments for mitochondrial disease are emerging. As an essential pathway in maintaining mitochondrial proteostasis, the mitochondrial unfolded protein response (UPR^mt) is not only of considerable significance for mitochondrial substance metabolism but also plays a fundamental role in the development of mitochondrial diseases. Furthermore, in mammals, the integrated stress response (ISR) and UPR^mt are strongly coupled, functioning together to maintain mitochondrial function. Therefore, ISR and UPR^mt show great application prospects in the treatment of mitochondrial diseases. In this review, we provide an overview of the molecular mechanisms of ISR and UPR^mt and focus on them as potential targets for mitochondrial disease therapy.

Ketogenic Diet Treatment of Defects in the Mitochondrial Malate Aspartate Shuttle and Pyruvate Carrier

The mitochondrial malate aspartate shuttle system (MAS) maintains the cytosolic NAD+/NADH redox balance, thereby sustaining cytosolic redox-dependent pathways, such as glycolysis and serine biosynthesis. Human disease has been associated with defects in four MAS-proteins (encoded by MDH1, MDH2, GOT2, SLC25A12) sharing a neurological/epileptic phenotype, as well as citrin deficiency (SLC25A13) with a complex hepatopathic-neuropsychiatric phenotype. Ketogenic diets (KD) are high-fat/low-carbohydrate diets, which decrease glycolysis thus bypassing the mentioned defects. The same holds for mitochondrial pyruvate carrier (MPC) 1 deficiency, which also presents neurological deficits. We here describe 40 (18 previously unreported) subjects with MAS-/MPC1-defects (32 neurological phenotypes, eight citrin deficiency), describe and discuss their phenotypes and genotypes (presenting 12 novel variants), and the efficacy of KD. Of 13 MAS/MPC1-individuals with a neurological phenotype treated with KD, 11 experienced benefits—mainly a striking effect against seizures. Two individuals with citrin deficiency deceased before the correct diagnosis was established, presumably due to high-carbohydrate treatment. Six citrin-deficient individuals received a carbohydrate-restricted/fat-enriched diet and showed normalisation of laboratory values/hepatopathy as well as age-adequate thriving. We conclude that patients with MAS-/MPC1-defects are amenable to dietary intervention and that early (genetic) diagnosis is key for initiation of proper treatment and can even be lifesaving.

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.

The ins and outs of the flavin mononucleotide cofactor of respiratory complex I

The flavin mononucleotide (FMN) cofactor of respiratory complex I occupies a key position in the electron transport chain. Here, the electrons coming from NADH start the sequence of oxidoreduction reactions, which drives the generation of the proton-motive force necessary for ATP synthesis. The overall architecture and the general catalytic proprieties of the FMN site are mostly well established. However, several aspects regarding the complex I flavin cofactor are still unknown. For example, the flavin binding to the N-module, the NADH-oxidizing portion of complex I, lacks a molecular description. The dissociation of FMN from the enzyme is beginning to emerge as an important regulatory mechanism of complex I activity and ROS production. Finally, how mitochondria import and metabolize FMN is still uncertain. This review summarizes the current knowledge on complex I flavin cofactor and discusses the open questions for future research.

Molecular Genetics Overview of Primary Mitochondrial Myopathies

Mitochondrial disorders are the most common inherited conditions, characterized by defects in oxidative phosphorylation and caused by mutations in nuclear or mitochondrial genes. Due to its high energy request, skeletal muscle is typically involved. According to the International Workshop of Experts in Mitochondrial Diseases held in Rome in 2016, the term Primary Mitochondrial Myopathy (PMM) should refer to those mitochondrial disorders affecting principally, but not exclusively, the skeletal muscle. The clinical presentation may include general isolated myopathy with muscle weakness, exercise intolerance, chronic ophthalmoplegia/ophthalmoparesis (cPEO) and eyelids ptosis, or multisystem conditions where there is a coexistence with extramuscular signs and symptoms. In recent years, new therapeutic targets have been identified leading to the launch of some promising clinical trials that have mainly focused on treating muscle symptoms and that require populations with defined genotype. Advantages in next-generation sequencing techniques have substantially improved diagnosis. So far, an increasing number of mutations have been identified as responsible for mitochondrial disorders. In this review, we focused on the principal molecular genetic alterations in PMM. Accordingly, we carried out a comprehensive review of the literature and briefly discussed the possible approaches which could guide the clinician to a genetic diagnosis.

An Overview of Mitochondrial Protein Defects in Neuromuscular Diseases

Neuromuscular diseases (NMDs) are dysfunctions that involve skeletal muscle and cause incorrect communication between the nerves and muscles. The specific causes of NMDs are not well known, but most of them are caused by genetic mutations. NMDs are generally progressive and entail muscle weakness and fatigue. Muscular impairments can differ in onset, severity, prognosis, and phenotype. A multitude of possible injury sites can make diagnosis of NMDs difficult. Mitochondria are crucial for cellular homeostasis and are involved in various metabolic pathways; for this reason, their dysfunction can lead to the development of different pathologies, including NMDs. Most NMDs due to mitochondrial dysfunction have been associated with mutations of genes involved in mitochondrial biogenesis and metabolism. This review is focused on some mitochondrial routes such as the TCA cycle, OXPHOS, and β-oxidation, recently found to be altered in NMDs. Particular attention is given to the alterations found in some genes encoding mitochondrial carriers, proteins of the inner mitochondrial membrane able to exchange metabolites between mitochondria and the cytosol. Briefly, we discuss possible strategies used to diagnose NMDs and therapies able to promote patient outcome.

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.

Patient Care Standards for Primary Mitochondrial Disease in Australia. An Australian adaptation of the Mitochondrial Medicine Society recommendations

This document provides consensus‐based recommendations for general physicians and primary care physicians who diagnose and manage patients with mitochondrial diseases (MD). It builds on previous international guidelines, with particular emphasis on clinical management in the Australian setting. This statement was prepared by a working group of medical practitioners, nurses and allied health professionals with clinical expertise and experience in managing Australian patients with MD. As new treatments and management plans emerge, these consensus‐based recommendations will continue to evolve, but current standards of care are summarised in this document.

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.

[Molecular medicine: pathobiochemistry as the key to personalized treatment of inherited diseases]

Genetische Defekte werden vielfach noch als Schicksal empfunden, mit dem man sich Zeit seines Lebens abfinden muss. Es stimmt, dass vererbte Anlagen in vielen Fällen zu schweren Krankheiten führen, allerdings stimmt es auch, dass der Anteil von genetischen Defekten, bei denen eine Therapieoption besteht, stetig wächst und sich der Ausbruch von Krankheitssymptomen bei einigen davon bestenfalls gänzlich verhindern lässt. Die Kenntnis des genauen molekularen Krankheitsmechanismus liefert oft die Grundlage für einen Therapieansatz. Zum Auffinden des genetischen Defekts haben die Möglichkeiten der genomweiten Sequenzierung und ihr mittlerweile breiter Einsatz in der Diagnostik entscheidend beigetragen. Nach dem Nachweis einer genetischen Veränderung braucht es aber noch die Untersuchung der pathobiochemischen Konsequenzen auf zellulärer und systemischer Ebene. Dabei handelt es sich oft um einen längeren Prozess, da der volle Umfang von Funktionsausfällen nicht immer auf Anhieb erkennbar ist. Bei metabolischen Defekten kann die Therapie ein Auffüllen von fehlenden Produkten oder eine Reduktion von giftigen Substraten sein. Oft lässt sich auch die Restfunktion von betroffenen „pathways“ verbessern. Neuerdings haben Therapien mit direkter Korrektur des betroffenen Gendefekts Einzug in die therapeutische Anwendung gefunden. Da die ersten Krankheitssymptome in vielen Fällen früh im Leben auftreten, trifft die Kinderheilkunde eine Vorreiterrolle in der Entwicklung von Therapieansätzen.

Ketogenic diet for mitochondrial disease: a systematic review on efficacy and safety

Background

No curative therapy for mitochondrial disease (MD) exists, prioritizing supportive treatment for symptom relief. In animal and cell models ketones decrease oxidative stress, increase antioxidants and scavenge free radicals, putting ketogenic diets (KDs) on the list of management options for MD. Furthermore, KDs are well-known, safe and effective treatments for epilepsy, a frequent symptom of MD. This systematic review evaluates efficacy and safety of KD for MD.

Methods

We searched Pubmed, Cochrane, Embase and Cinahl (November 2020) with search terms linked to MD and KD. From the identified records, we excluded studies on Pyruvate Dehydrogenase Complex deficiency. From these eligible reports, cases without a genetically confirmed diagnosis and cases without sufficient data on KD and clinical course were excluded. The remaining studies were included in the qualitative analysis.

Results

Only 20 cases (14 pediatric) from the 694 papers identified met the inclusion criteria (one controlled trial (n = 5), 15 case reports). KD led to seizure control in 7 out of 8 cases and improved muscular symptoms in 3 of 10 individuals. In 4 of 20 cases KD reversed the clinical phenotype (e.g. cardiomyopathy, movement disorder). In 5 adults with mitochondrial DNA deletion(s) related myopathy rhabdomyolysis led to cessation of KD. Three individuals with POLG mutations died while being on KD, however, their survival was not different compared to individuals with POLG mutations without KD.

Conclusion

Data on efficacy and safety of KD for MD is too scarce for general recommendations. KD should be considered in individuals with MD and therapy refractory epilepsy, while KD is contraindicated in mitochondrial DNA deletion(s) related myopathy. When considering KD for MD the high rate of adverse effects should be taken into account, but also spectacular improvements in individual cases. KD is a highly individual management option in this fragile patient group and requires an experienced team. To increase knowledge on this—individually—promising management option more (prospective) studies using adequate outcome measures are crucial.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-021-01927-w.

Epilepsy in Mitochondrial Diseases-Current State of Knowledge on Aetiology and Treatment

Mitochondrial diseases are a heterogeneous group of diseases resulting from energy deficit and reduced adenosine triphosphate (ATP) production due to impaired oxidative phosphorylation. The manifestation of mitochondrial disease is usually multi-organ. Epilepsy is one of the most common manifestations of diseases resulting from mitochondrial dysfunction, especially in children. The onset of epilepsy is associated with poor prognosis, while its treatment is very challenging, which further adversely affects the course of these disorders. Fortunately, our knowledge of mitochondrial diseases is still growing, which gives hope for patients to improve their condition in the future. The paper presents the pathophysiology, clinical picture and treatment options for epilepsy in patients with mitochondrial disease.

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.

Treatable inherited metabolic disorders causing intellectual disability: 2021 review and digital app

BACKGROUND

The Treatable ID App was created in 2012 as digital tool to improve early recognition and intervention for treatable inherited metabolic disorders (IMDs) presenting with global developmental delay and intellectual disability (collectively 'treatable IDs'). Our aim is to update the 2012 review on treatable IDs and App to capture the advances made in the identification of new IMDs along with increased pathophysiological insights catalyzing therapeutic development and implementation.

METHODS

Two independent reviewers queried PubMed, OMIM and Orphanet databases to reassess all previously included disorders and therapies and to identify all reports on Treatable IDs published between 2012 and 2021. These were included if listed in the International Classification of IMDs (ICIMD) and presenting with ID as a major feature, and if published evidence for a therapeutic intervention improving ID primary and/or secondary outcomes is available. Data on clinical symptoms, diagnostic testing, treatment strategies, effects on outcomes, and evidence levels were extracted and evaluated by the reviewers and external experts. The generated knowledge was translated into a diagnostic algorithm and updated version of the App with novel features.

RESULTS

Our review identified 116 treatable IDs (139 genes), of which 44 newly identified, belonging to 17 ICIMD categories. The most frequent therapeutic interventions were nutritional, pharmacological and vitamin and trace element supplementation. Evidence level varied from 1 to 3 (trials, cohort studies, case-control studies) for 19% and 4-5 (case-report, expert opinion) for 81% of treatments. Reported effects included improvement of clinical deterioration in 62%, neurological manifestations in 47% and development in 37%.

CONCLUSION

The number of treatable IDs identified by our literature review increased by more than one-third in eight years. Although there has been much attention to gene-based and enzyme replacement therapy, the majority of effective treatments are nutritional, which are relatively affordable, widely available and (often) surprisingly effective. We present a diagnostic algorithm (adjustable to local resources and expertise) and the updated App to facilitate a swift and accurate workup, prioritizing treatable IDs. Our digital tool is freely available as Native and Web App (www.treatable-id.org) with several novel features. Our Treatable ID endeavor contributes to the Treatabolome and International Rare Diseases Research Consortium goals, enabling clinicians to deliver rapid evidence-based interventions to our rare disease patients.

Adolescent-Onset and Adult-Onset Vitamin-Responsive Neurogenetic Diseases: A Review

Importance

Vitamin-responsive inherited diseases are among the rare genetic disorders with a specific pharmacological treatment. Many of these conditions have a prominent neurological phenotype that is mainly reported in children. Being rare and often strikingly different in adult-onset forms, they are still poorly known in the medical fields specific to adults.

Observation

This article reviews all articles reporting cases of patients with a genetically confirmed inherited vitamin-responsive neurological disease and neurological onset after the age of 10 years. On this basis, 24 different diseases are described, involving vitamins A, B1, B2, B3, B6, B8, B9, B12, E, and tetrahydrobiopterin (BH4). Information such as clinical symptoms, disease course, imaging studies, biochemical alterations, and response to treatment present an overall picture of these patients.

Conclusions and Relevance

Vitamin-responsive neurogenetic diseases represent a group of rare conditions that are probably underdiagnosed in adults and may have a dramatic response to treatment when started early in the course of the disease. In this review, main features of the adult-onset forms are defined and simple key messages are provided to help identify clinical situations when specific diagnostic tests should be performed and/or vitamins should be promptly administered.

Blackout in the powerhouse: clinical phenotypes associated with defects in the assembly of OXPHOS complexes and the mitoribosome

Mitochondria produce the bulk of the energy used by almost all eukaryotic cells through oxidative phosphorylation (OXPHOS) which occurs on the four complexes of the respiratory chain and the F₁–F₀ ATPase. Mitochondrial diseases are a heterogenous group of conditions affecting OXPHOS, either directly through mutation of genes encoding subunits of OXPHOS complexes, or indirectly through mutations in genes encoding proteins supporting this process. These include proteins that promote assembly of the OXPHOS complexes, the post-translational modification of subunits, insertion of cofactors or indeed subunit synthesis. The latter is important for all 13 of the proteins encoded by human mitochondrial DNA, which are synthesised on mitochondrial ribosomes. Together the five OXPHOS complexes and the mitochondrial ribosome are comprised of more than 160 subunits and many more proteins support their biogenesis. Mutations in both nuclear and mitochondrial genes encoding these proteins have been reported to cause mitochondrial disease, many leading to defective complex assembly with the severity of the assembly defect reflecting the severity of the disease. This review aims to act as an interface between the clinical and basic research underpinning our knowledge of OXPHOS complex and ribosome assembly, and the dysfunction of this process in mitochondrial disease.

Mitochondrial Complexome Profiling

Complexome profiling combines blue native gel electrophoresis (BNE) and quantitative mass spectrometry to define an entire protein interactome of a cell, an organelle, or a biological membrane preparation. The method allows the identification of protein assemblies with low abundance and detects dynamic processes of protein complex assembly. Applications of complexome profiling range from the determination of complex subunit compositions, assembly of single protein complexes, and supercomplexes to comprehensive differential studies between patients or disease models. This chapter describes the workflow of complexome profiling from sample preparation, mass spectrometry to data analysis with a bioinformatics tool.

Current progress in the therapeutic options for mitochondrial disorders

Mitochondrial disorders manifest enormous genetic and clinical heterogeneity - they can appear at any age, present with various phenotypes affecting any organ, and display any mode of inheritance. What mitochondrial diseases do have in common, is impairment of respiratory chain activity, which is responsible for more than 90% of energy production within cells. While diagnostics of mitochondrial disorders has been accelerated by introducing Next-Generation Sequencing techniques in recent years, the treatment options are still very limited. For many patients only a supportive or symptomatic therapy is available at the moment. However, decades of basic and preclinical research have uncovered potential target points and numerous compounds or interventions are now subjects of clinical trials. In this review, we focus on current and emerging therapeutic approaches towards the treatment of mitochondrial disorders. We focus on small compounds, metabolic interference, such as endurance training or ketogenic diet and also on genomic approaches.

Mitochondrial medicine therapies: rationale, evidence, and dosing guidelines

PURPOSE OF REVIEW

Primary mitochondrial disease is a highly heterogeneous but collectively common inherited metabolic disorder, affecting at least one in 4300 individuals. Therapeutic management of mitochondrial disease typically involves empiric prescription of enzymatic cofactors, antioxidants, and amino acid and other nutrient supplements, based on biochemical reasoning, historical experience, and consensus expert opinion. As the field continues to rapidly advance, we review here the preclinical and clinical evidence, and specific dosing guidelines, for common mitochondrial medicine therapies to guide practitioners in their prescribing practices.

RECENT FINDINGS

Since publication of Mitochondrial Medicine Society guidelines for mitochondrial medicine therapies management in 2009, data has emerged to support consideration for using additional therapeutic agents and discontinuation of several previously used agents. Preclinical animal modeling data have indicated a lack of efficacy for vitamin C as an antioxidant for primary mitochondrial disease, but provided strong evidence for vitamin E and N-acetylcysteine. Clinical data have suggested L-carnitine may accelerate atherosclerotic disease. Long-term follow up on L-arginine use as prophylaxis against or acute treatment for metabolic strokes has provided more data supporting its clinical use in individuals with mitochondrial encephalopathy with lactic acidosis and stroke-like episodes (MELAS) syndrome and Leigh syndrome. Further, several precision therapies have been developed for specific molecular causes and/or shared clinical phenotypes of primary mitochondrial disease.

SUMMARY

We provide a comprehensive update on mitochondrial medicine therapies based on current evidence and our single-center clinical experience to support or refute their use, and provide detailed dosing guidelines, for the clinical management of mitochondrial disease. The overarching goal of empiric mitochondrial medicines is to utilize therapies with favorable benefit-to-risk profiles that may stabilize and enhance residual metabolic function to improve cellular resiliency and slow clinical disease progression and/or prevent acute decompensation.

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.

The genetic basis of isolated mitochondrial complex II deficiency

Mitochondrial complex II (succinate:ubiquinone oxidoreductase) is the smallest complex of the oxidative phosphorylation system, a tetramer of just 140 kDa. Despite its diminutive size, it is a key complex in two coupled metabolic pathways - it oxidises succinate to fumarate in the tricarboxylic acid cycle and the electrons are used to reduce FAD to FADH₂, ultimately reducing ubiquinone to ubiquinol in the respiratory chain. The biogenesis and assembly of complex II is facilitated by four ancillary proteins, all of which are autosomally-encoded. Numerous pathogenic defects have been reported which describe two broad clinical manifestations, either susceptibility to cancer in the case of single, heterozygous germline variants, or a mitochondrial disease presentation, almost exclusively due to bi-allelic recessive variants and associated with an isolated complex II deficiency. Here we present a compendium of pathogenic gene variants that have been documented in the literature in patients with an isolated mitochondrial complex II deficiency. To date, 61 patients are described, harbouring 32 different pathogenic variants in four distinct complex II genes: three structural subunit genes (SDHA, SDHB and SDHD) and one assembly factor gene (SDHAF1). Many pathogenic variants result in a null allele due to nonsense, frameshift or splicing defects however, the missense variants that do occur tend to induce substitutions at highly conserved residues in regions of the proteins that are critical for binding to other subunits or substrates. There is phenotypic heterogeneity associated with defects in each complex II gene, similar to other mitochondrial diseases.

Riboflavin Deficiency-Implications for General Human Health and Inborn Errors of Metabolism

As an essential vitamin, the role of riboflavin in human diet and health is increasingly being highlighted. Insufficient dietary intake of riboflavin is often reported in nutritional surveys and population studies, even in non-developing countries with abundant sources of riboflavin-rich dietary products. A latent subclinical riboflavin deficiency can result in a significant clinical phenotype when combined with inborn genetic disturbances or environmental and physiological factors like infections, exercise, diet, aging and pregnancy. Riboflavin, and more importantly its derivatives, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), play a crucial role in essential cellular processes including mitochondrial energy metabolism, stress responses, vitamin and cofactor biogenesis, where they function as cofactors to ensure the catalytic activity and folding/stability of flavoenzymes. Numerous inborn errors of flavin metabolism and flavoenzyme function have been described, and supplementation with riboflavin has in many cases been shown to be lifesaving or to mitigate symptoms. This review discusses the environmental, physiological and genetic factors that affect cellular riboflavin status. We describe the crucial role of riboflavin for general human health, and the clear benefits of riboflavin treatment in patients with inborn errors of metabolism.

Mitochondrial Diseases: Hope for the Future

Mitochondrial diseases are clinically heterogeneous disorders caused by a wide spectrum of mutations in genes encoded by either the nuclear or the mitochondrial genome. Treatments for mitochondrial diseases are currently focused on symptomatic management rather than improving the biochemical defect caused by a particular mutation. This review focuses on the latest advances in the development of treatments for mitochondrial disease, both small molecules and gene therapies, as well as methods to prevent transmission of mitochondrial disease through the germline.

Advances in the treatment of mitochondrial epilepsies

Epilepsy is frequently a severe and sinister symptom in primary mitochondrial diseases, a group of more than 350 different genetic disorders characterized by mitochondrial dysfunction and extreme clinical and biochemical heterogeneity. Mitochondrial epilepsy is notoriously difficult to manage, principally because the vast majority of primary mitochondrial diseases currently lack effective therapies. Treating the underlying mitochondrial disorder is likely to be a more effective strategy than using traditional antiepileptic drugs. This review, initially presented at the 7th London-Innsbruck Colloquium on Status Epilepticus and Acute Seizures at the Francis Crick Institute in London, summarizes the currently available and emerging therapies for mitochondrial epilepsy. Potentially treatable mitochondrial diseases include disorders of coenzyme Q₁₀ biosynthesis and a group of mitochondrial respiratory chain complex I subunit and assembly factor defects that respond to riboflavin (vitamin B2). Approaches that have been adopted in actively recruiting clinical trials include redox modulation, harnessing mitochondrial biogenesis, using rapamycin to target mitophagy, nucleoside supplementation, and gene and cell therapies. Most of the clinical trials are at an early stage (Phase 1 or 2) and none of the currently active trials is specifically targeting mitochondrial epilepsy. This article is part of the Special Issue "Proceedings of the 7th London-Innsbruck Colloquium on Status Epilepticus and Acute Seizures".

Successful treatment of infantile-onset ACAD9-related cardiomyopathy with a combination of sodium pyruvate, beta-blocker, and coenzyme Q10

Mitochondrial acyl-CoA dehydrogenase 9 (ACAD9) deficiency is one of the common causes of respiratory chain complex I deficiency, which is characterized by cardiomyopathy, lactic acidemia, and muscle weakness. Infantile cardiomyopathy is the most common phenotype and is usually lethal by the age of 5 years. Riboflavin treatment is known to be effective in ~65% of the patients; however, the remaining are unresponsive to riboflavin and are in need of additional treatment measures. In this report, we describe a patient with ACAD9 deficiency who developed progressive cardiomyopathy at 8 months of age. As the patient's left ventricular ejection fraction (LVEF) kept decreasing to 45.4% at 1 year 8 months, sodium pyruvate treatment was introduced together with a beta-blocker and coenzyme Q10. This resulted in a steady improvement, with full and sustained normalization of cardiac function without riboflavin. The therapy, therefore, might be a useful addition for the treatment of ACAD9 deficiency.

Disorders of riboflavin metabolism

Riboflavin (vitamin B2), a water-soluble vitamin, is an essential nutrient in higher organisms as it is not endogenously synthesised, with requirements being met principally by dietary intake. Tissue-specific transporter proteins direct riboflavin to the intracellular machinery responsible for the biosynthesis of the flavocoenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). These flavocoenzymes play a vital role in ensuring the functionality of a multitude of flavoproteins involved in bioenergetics, redox homeostasis, DNA repair, chromatin remodelling, protein folding, apoptosis, and other physiologically relevant processes. Hence, it is not surprising that the impairment of flavin homeostasis in humans may lead to multisystem dysfunction including neuromuscular disorders, anaemia, abnormal fetal development, and cardiovascular disease. In this review, we provide an overview of riboflavin absorption, transport, and metabolism. We then focus on the clinical and biochemical features associated with biallelic FLAD1 mutations leading to FAD synthase deficiency, the only known primary defect in flavocoenzyme synthesis, in addition to providing an overview of clinical disorders associated with nutritional deficiency of riboflavin and primary defects of riboflavin transport. Finally, we give a brief overview of disorders of the cellular flavoproteome. Because riboflavin therapy may be beneficial in a number of primary or secondary disorders of the cellular flavoproteome, early recognition and prompt management of these disorders is imperative.

An acyl-CoA dehydrogenase microplate activity assay using recombinant porcine electron transfer flavoprotein

Acyl-CoA dehydrogenases (ACADs) play key roles in the mitochondrial catabolism of fatty acids and branched-chain amino acids. All nine characterized ACAD enzymes use electron transfer flavoprotein (ETF) as their redox partner. The gold standard for measuring ACAD activity is the anaerobic ETF fluorescence reduction assay, which follows the decrease of pig ETF fluorescence as it accepts electrons from an ACAD in vitro. Although first described 35 years ago, the assay has not been widely used due to the need to maintain an anaerobic assay environment and to purify ETF from pig liver mitochondria. Here, we present a method for expressing recombinant pig ETF in E coli and purifying it to homogeneity. The recombinant protein is virtually pure after one chromatography step, bears higher intrinsic fluorescence than the native enzyme, and provides enhanced activity in the ETF fluorescence reduction assay. Finally, we present a simplified protocol for removing molecular oxygen that allows adaption of the assay to a 96-well plate format. The availability of recombinant pig ETF and the microplate version of the ACAD activity assay will allow wide application of the assay for both basic research and clinical diagnostics.

[Mitochondrial diseases]

Mitochondrial diseases (MD) are caused by mutations in the mitochondrial DNA or nuclear DNA. The clinical manifestation is often most severe in tissues with high energy demands. The most common MDs are Leber's hereditary optic neuropathy (LHON), chronic progressive external ophthalmoplegia (CPEO) and mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS). Therapeutic approaches for MD include bridging of respiratory chain defects, pharmacological stimulation of mitochondrial metabolism, supplementation of deficient factors and symptomatic treatment. Initial gene therapeutic approaches for causal treatment have already reached the clinical development stage. This article provides an introduction to MD, a summary of the most important syndromes and an overview over established and innovative therapeutic approaches.

Lipid Myopathies

Disorders of lipid metabolism affect several tissues, including skeletal and cardiac muscle tissues. Lipid myopathies (LM) are rare multi-systemic diseases, which most often are due to genetic defects. Clinically, LM can have acute or chronic clinical presentation. Disease onset can occur in all ages, from early stages of life to late-adult onset, showing with a wide spectrum of clinical symptoms. Muscular involvement can be fluctuant or stable and can manifest as fatigue, exercise intolerance and muscular weakness. Muscular atrophy is rarely present. Acute muscular exacerbations, resulting in rhabdomyolysis crisis are triggered by several factors. Several classifications of lipid myopathies have been proposed, based on clinical involvement, biochemical defect or histopathological findings. Herein, we propose a full revision of all the main clinical entities of lipid metabolism disorders with a muscle involvement, also including some those disorders of fatty acid oxidation (FAO) with muscular symptoms not included among previous lipid myopathies classifications.