Mutation of the mitochondrial carrier SLC25A42 causes a novel form of mitochondrial myopathy in humans

SLC25A42 promotes gastric cancer growth by conferring ferroptosis resistance through enhancing CPT2-mediated fatty acid oxidation

Accumulating evidence has shown that the dysfunction of mitochondria, the multifunctional organelles in various cellular processes, is a pivotal event in the development of various diseases, including human cancers. Solute Carrier Family 25 Member 42 (SLC25A42) is a mitochondrial protein governing the transport of coenzyme A (CoA). However, the biological roles of SLC25A42 in human cancers are still unexplored. Here we uncovered that SLC25A42 is upregulated and correlated with a worse prognosis in GC patients. SLC25A42 promotes the proliferation of gastric cancer (GC) cells while suppresses apoptosis in vitro and in vivo. Mechanistically, SLC25A42 promotes the growth and inhibits apoptosis of GC cells by reprograming lipid metabolism. On the one hand, SLC25A42 enhances fatty acid oxidation-mediated mitochondrial respiration to provide energy for cell survival. On the other hand, SLC25A42 decreases the levels of free fatty acids and ROS to inhibit ferroptosis. Moreover, we found that SLC25A42 reprograms lipid metabolism in GC cells by upregulating the acetylation and thus the expression of CPT2. Collectively, our data reveal a critical oncogenic role of SLC25A42 in GCs and suggest that SLC25A42 represent a promising therapeutic target for GC.

Expanding the genetic and clinical spectrum of SLC25A42-associated disorders and testing of pantothenic acid to improve CoA level in vitro

SLC25A42 encodes the mitochondrial coenzyme A (CoA) transporter localized at the inner mitochondrial membrane. SLC25A42 deficiency leads to a congenital disease with a heterogeneous clinical presentation, including myopathy, developmental delay, lactic acidosis, and encephalopathy. Twenty-one patients have been described so far. In the current study, we report on the identification of new biallelic variants in SLC25A42 in three siblings. Patients presented with symmetrical T2 hyperintensity of the putamen with minor volume depression at the brain MRI, elevated lactate, reduced oxygen consumption rates in muscle and fibroblasts, and reduced CoA levels in fibroblasts. Administration of pantothenic acid led to clinical stabilization and increased CoA levels in fibroblasts, thus confirming a role for SLC25A42 in energy metabolism and CoA homeostasis.

Lipidomic analysis of human TANGO2-deficient cells suggests a lipid imbalance as a cause of TANGO2 deficiency disease

TANGO2 deficiency disease (TDD) is a multisystem disease caused by variants in the TANGO2 gene. Symptoms include neurodevelopmental delays, seizures and potentially lethal metabolic crises and cardiac arrhythmias. While the function of TANGO2 remains elusive, vitamin B5/pantothenic acid supplementation has been shown to alleviate symptoms in a fruit fly model and has also been used with success to treat individuals suffering from TDD. Since vitamin B5 is the precursor to the lipid activator coenzyme A (CoA), we hypothesized that TANGO2-deficient cells would display changes in the lipid profile compared to control and that these changes would be rescued by vitamin B5 supplementation. In addition, the specific changes seen might point to a pathway in which TANGO2 functions. Indeed, we found profound changes in the lipid profile of human TANGO2-deficient cells as well as an increased pool of free fatty acids in both human cells devoid of TANGO2 and Drosophila harboring a previously described TANGO2 loss of function allele. All these changes were reversed upon vitamin B5 supplementation. Pathway analysis showed significant increases in triglyceride as well as in lysophospholipid levels as the top enriched pathways in the absence of TANGO2. Consistent with a defect in triglyceride metabolism, we found changes in lipid droplet numbers and sizes in the absence of TANGO2 compared to control. Our data will allow for comparison between other model systems of TDD and the homing in on critical lipid imbalances that lead to the disease state.

Identification Drug Targets for Oxaliplatin-Induced Cardiotoxicity without Affecting Cancer Treatment through Inter Variability Cross-Correlation Analysis (IVCCA)

The successful treatment of side effects of chemotherapy faces two major limitations: the need to avoid interfering with pathways essential for the cancer-destroying effects of the chemotherapy drug, and the need to avoid helping tumor progression through cancer promoting cellular pathways. To address these questions and identify new pathways and targets that satisfy these limitations, we have developed the bioinformatics tool Inter Variability Cross-Correlation Analysis (IVCCA). This tool calculates the cross-correlation of differentially expressed genes, analyzes their clusters, and compares them across a vast number of known pathways to identify the most relevant target(s). To demonstrate the utility of IVCCA, we applied this platform to RNA-seq data obtained from the hearts of the animal models with oxaliplatin-induced CTX. RNA-seq of the heart tissue from oxaliplatin treated mice identified 1744 differentially expressed genes with False Discovery Rate (FDR) less than 0.05 and fold change above 1.5 across nine samples. We compared the results against traditional gene enrichment analysis methods, revealing that IVCCA identified additional pathways potentially involved in CTX beyond those detected by conventional approaches. The newly identified pathways such as energy metabolism and several others represent promising target for therapeutic intervention against CTX, while preserving the efficacy of the chemotherapy treatment and avoiding tumor proliferation. Targeting these pathways is expected to mitigate the damaging effects of chemotherapy on cardiac tissues and improve patient outcomes by reducing the incidence of heart failure and other cardiovascular complications, ultimately enabling patients to complete their full course of chemotherapy with improved quality of life and survival rates.

Membrane transporters in cell physiology, cancer metabolism and drug response

By controlling the passage of small molecules across lipid bilayers, membrane transporters influence not only the uptake and efflux of nutrients, but also the metabolic state of the cell. With more than 450 members, the Solute Carriers (SLCs) are the largest transporter super-family, clustering into families with different substrate specificities and regulatory properties. Cells of different types are, therefore, able to tailor their transporter expression signatures depending on their metabolic requirements, and the physiological importance of these proteins is illustrated by their mis-regulation in a number of disease states. In cancer, transporter expression is heterogeneous, and the SLC family has been shown to facilitate the accumulation of biomass, influence redox homeostasis, and also mediate metabolic crosstalk with other cell types within the tumour microenvironment. This Review explores the roles of membrane transporters in physiological and malignant settings, and how these roles can affect drug response, through either indirect modulation of sensitivity or the direct transport of small-molecule therapeutic compounds into cells.

Mitochondrial heterogeneity in diseases

As key organelles involved in cellular metabolism, mitochondria frequently undergo adaptive changes in morphology, components and functions in response to various environmental stresses and cellular demands. Previous studies of mitochondria research have gradually evolved, from focusing on morphological change analysis to systematic multiomics, thereby revealing the mitochondrial variation between cells or within the mitochondrial population within a single cell. The phenomenon of mitochondrial variation features is defined as mitochondrial heterogeneity. Moreover, mitochondrial heterogeneity has been reported to influence a variety of physiological processes, including tissue homeostasis, tissue repair, immunoregulation, and tumor progression. Here, we comprehensively review the mitochondrial heterogeneity in different tissues under pathological states, involving variant features of mitochondrial DNA, RNA, protein and lipid components. Then, the mechanisms that contribute to mitochondrial heterogeneity are also summarized, such as the mutation of the mitochondrial genome and the import of mitochondrial proteins that result in the heterogeneity of mitochondrial DNA and protein components. Additionally, multiple perspectives are investigated to better comprehend the mysteries of mitochondrial heterogeneity between cells. Finally, we summarize the prospective mitochondrial heterogeneity-targeting therapies in terms of alleviating mitochondrial oxidative damage, reducing mitochondrial carbon stress and enhancing mitochondrial biogenesis to relieve various pathological conditions. The possibility of recent technological advances in targeted mitochondrial gene editing is also discussed.

Targeting mitochondrial impairment for the treatment of cardiovascular diseases: From hypertension to ischemia-reperfusion injury, searching for new pharmacological targets

Mitochondria and mitochondrial proteins represent a group of promising pharmacological target candidates in the search of new molecular targets and drugs to counteract the onset of hypertension and more in general cardiovascular diseases (CVDs). Indeed, several mitochondrial pathways result impaired in CVDs, showing ATP depletion and ROS production as common traits of cardiac tissue degeneration. Thus, targeting mitochondrial dysfunction in cardiomyocytes can represent a successful strategy to prevent heart failure. In this context, the identification of new pharmacological targets among mitochondrial proteins paves the way for the design of new selective drugs. Thanks to the advances in omics approaches, to a greater availability of mitochondrial crystallized protein structures and to the development of new computational approaches for protein 3D-modelling and drug design, it is now possible to investigate in detail impaired mitochondrial pathways in CVDs. Furthermore, it is possible to design new powerful drugs able to hit the selected pharmacological targets in a highly selective way to rescue mitochondrial dysfunction and prevent cardiac tissue degeneration. The role of mitochondrial dysfunction in the onset of CVDs appears increasingly evident, as reflected by the impairment of proteins involved in lipid peroxidation, mitochondrial dynamics, respiratory chain complexes, and membrane polarization maintenance in CVD patients. Conversely, little is known about proteins responsible for the cross-talk between mitochondria and cytoplasm in cardiomyocytes. Mitochondrial transporters of the SLC25A family, in particular, are responsible for the translocation of nucleotides (e.g., ATP), amino acids (e.g., aspartate, glutamate, ornithine), organic acids (e.g. malate and 2-oxoglutarate), and other cofactors (e.g., inorganic phosphate, NAD⁺, FAD, carnitine, CoA derivatives) between the mitochondrial and cytosolic compartments. Thus, mitochondrial transporters play a key role in the mitochondria-cytosol cross-talk by leading metabolic pathways such as the malate/aspartate shuttle, the carnitine shuttle, the ATP export from mitochondria, and the regulation of permeability transition pore opening. Since all these pathways are crucial for maintaining healthy cardiomyocytes, mitochondrial carriers emerge as an interesting class of new possible pharmacological targets for CVD treatments.

Severe inbreeding, increased mutation load and gene loss-of-function in the critically endangered Devils Hole pupfish

Small populations with limited range are often threatened by inbreeding and reduced genetic diversity, which can reduce fitness and exacerbate population decline. One of the most extreme natural examples is the Devils Hole pupfish (Cyprinodon diabolis), an iconic and critically endangered species with the smallest known range of any vertebrate. This species has experienced severe declines in population size over the last 30 years and suffered major bottlenecks in 2007 and 2013, when the population shrunk to 38 and 35 individuals, respectively. Here, we analysed 30 resequenced genomes of desert pupfishes from Death Valley, Ash Meadows and surrounding areas to examine the genomic consequences of small population size. We found extremely high levels of inbreeding (FROH = 0.34-0.81) and an increased amount of potentially deleterious genetic variation in the Devils Hole pupfish as compared to other species, including unique, fixed loss-of-function alleles and deletions in genes associated with sperm motility and hypoxia. Additionally, we successfully resequenced a formalin-fixed museum specimen from 1980 and found that the population was already highly inbred prior to recent known bottlenecks. We thus document severe inbreeding and increased mutation load in the Devils Hole pupfish and identify candidate deleterious variants to inform management of this conservation icon.

Mitochondrial transport and metabolism of the vitamin B-derived cofactors thiamine pyrophosphate, coenzyme A, FAD and NAD+ , and related diseases: A review

Multiple mitochondrial matrix enzymes playing key roles in metabolism require cofactors for their action. Due to the high impermeability of the mitochondrial inner membrane, these cofactors need to be synthesized within the mitochondria or be imported, themselves or one of their precursors, into the organelles. Transporters belonging to the protein family of mitochondrial carriers have been identified to transport the coenzymes: thiamine pyrophosphate, coenzyme A, FAD and NAD+ , which are all structurally similar to nucleotides and derived from different B-vitamins. These mitochondrial cofactors bind more or less tightly to their enzymes and, after having been involved in a specific reaction step, are regenerated, spontaneously or by other enzymes, to return to their active form, ready for the next catalysis round. Disease-causing mutations in the mitochondrial cofactor carrier genes compromise not only the transport reaction but also the activity of all mitochondrial enzymes using that particular cofactor and the metabolic pathways in which the cofactor-dependent enzymes are involved. The mitochondrial transport, metabolism and diseases of the cofactors thiamine pyrophosphate, coenzyme A, FAD and NAD+ are the focus of this review.

Mitochondrial "dysmorphology" in variant classification

Mitochondrial disorders are challenging to diagnose. Exome sequencing has greatly enhanced the diagnostic precision of these disorders although interpreting variants of uncertain significance (VUS) remains a formidable obstacle. Whether specific mitochondrial morphological changes can aid in the classification of these variants is unknown. Here, we describe two families (four patients), each with a VUS in a gene known to affect the morphology of mitochondria through a specific role in the fission-fusion balance. In the first, the missense variant in MFF, encoding a fission factor, was associated with impaired fission giving rise to a characteristically over-tubular appearance of mitochondria. In the second, the missense variant in DNAJA3, which has no listed OMIM phenotype, was associated with fragmented appearance of mitochondria consistent with its published deficiency states. In both instances, the highly specific phenotypes allowed us to upgrade the classification of the variants. Our results suggest that, in select cases, mitochondrial "dysmorphology" can be helpful in interpreting variants to reach a molecular 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.

Coenzyme a Biochemistry: From Neurodevelopment to Neurodegeneration

Coenzyme A (CoA) is an essential cofactor in all living organisms. It is involved in a large number of biochemical processes functioning either as an activator of molecules with carbonyl groups or as a carrier of acyl moieties. Together with its thioester derivatives, it plays a central role in cell metabolism, post-translational modification, and gene expression. Furthermore, recent studies revealed a role for CoA in the redox regulation by the S-thiolation of cysteine residues in cellular proteins. The intracellular concentration and distribution in different cellular compartments of CoA and its derivatives are controlled by several extracellular stimuli such as nutrients, hormones, metabolites, and cellular stresses. Perturbations of the biosynthesis and homeostasis of CoA and/or acyl-CoA are connected with several pathological conditions, including cancer, myopathies, and cardiomyopathies. In the most recent years, defects in genes involved in CoA production and distribution have been found in patients affected by rare forms of neurodegenerative and neurodevelopmental disorders. In this review, we will summarize the most relevant aspects of CoA cellular metabolism, their role in the pathogenesis of selected neurodevelopmental and neurodegenerative disorders, and recent advancements in the search for therapeutic approaches for such diseases.

The SLC25 Carrier Family: Important Transport Proteins in Mitochondrial Physiology and Pathology

Members of the mitochondrial carrier family (SLC25) transport a variety of compounds across the inner membrane of mitochondria. These transport steps provide building blocks for the cell and link the pathways of the mitochondrial matrix and cytosol. An increasing number of diseases and pathologies has been associated with their dysfunction. In this review, the molecular basis of these diseases is explained based on our current understanding of their transport mechanism.

SLC25A42-associated mitochondrial encephalomyopathy: Report of additional founder cases and functional characterization of a novel deletion

SLC25A42 is the main transporter of coenzyme A (CoA) into mitochondria. To date, 15 individuals have been reported to have one of two bi-allelic homozygous missense variants in the SLC25A42 as the cause of mitochondrial encephalomyopathy, of which 14 of them were of Saudi origin and share the same founder variant, c.871A > G:p.Asn291Asp. The other subject was of German origin with a variant at canonical splice site, c.380 + 2 T > A. Here, we describe the clinical manifestations and the disease course in additional six Saudi patients from four unrelated consanguineous families. While five patients have the Saudi founder p.Asn291Asp variant, one subject has a novel deletion. Functional analyses on fibroblasts obtained from this patient revealed that the deletion causes significant decrease in mitochondrial oxygen consumption and ATP production compared to healthy individuals. Moreover, extracellular acidification rate revealed significantly reduced glycolysis, glycolytic capacity, and glycolytic reserve as compared to control individuals. There were no changes in the mitochondrial DNA (mtDNA) content of patient fibroblasts. Immunoblotting experiments revealed significantly diminished protein expression due to the deletion. In conclusion, we report additional patients with SLC25A42-associated mitochondrial encephalomyopathy. Our study expands the molecular spectrum of this condition and provides further evidence of mitochondrial dysfunction as a central cause of pathology. We therefore propose that this disorder should be included in the differential diagnosis of any patient with an unexplained motor and speech delay, recurrent encephalopathy with metabolic acidosis, intermittent or persistent dystonia, lactic acidosis, basal ganglia lesions and, especially, of Arab ethnicity. Finally, deep brain stimulation should be considered in the management of patients with life altering dystonia.

Computational studies of the mitochondrial carrier family SLC25. Present status and future perspectives

The members of the mitochondrial carrier family, also known as solute carrier family 25 (SLC25), are transmembrane proteins involved in the translocation of a plethora of small molecules between the mitochondrial intermembrane space and the matrix. These transporters are characterized by three homologous domains structure and a transport mechanism that involves the transition between different conformations. Mutations in regions critical for these transporters’ function often cause several diseases, given the crucial role of these proteins in the mitochondrial homeostasis. Experimental studies can be problematic in the case of membrane proteins, in particular concerning the characterization of the structure–function relationships. For this reason, computational methods are often applied in order to develop new hypotheses or to support/explain experimental evidence. Here the computational analyses carried out on the SLC25 members are reviewed, describing the main techniques used and the outcome in terms of improved knowledge of the transport mechanism. Potential future applications on this protein family of more recent and advanced in silico methods are also suggested.

20,000 picometers under the OMM: diving into the vastness of mitochondrial metabolite transport

The metabolic compartmentalization enabled by mitochondria is key feature of many cellular processes such as energy conversion to ATP production, redox balance, and the biosynthesis of heme, urea, nucleotides, lipids, and others. For a majority of these functions, metabolites need to be transported across the impermeable inner mitochondrial membrane by dedicated carrier proteins. Here, we examine the substrates, structural features, and human health implications of four mitochondrial metabolite carrier families: the SLC25A family, the mitochondrial ABCB transporters, the mitochondrial pyruvate carrier (MPC), and the sideroflexin proteins.

Diseases Caused by Mutations in Mitochondrial Carrier Genes SLC25: A Review

In the 1980s, after the mitochondrial DNA (mtDNA) had been sequenced, several diseases resulting from mtDNA mutations emerged. Later, numerous disorders caused by mutations in the nuclear genes encoding mitochondrial proteins were found. A group of these diseases are due to defects of mitochondrial carriers, a family of proteins named solute carrier family 25 (SLC25), that transport a variety of solutes such as the reagents of ATP synthase (ATP, ADP, and phosphate), tricarboxylic acid cycle intermediates, cofactors, amino acids, and carnitine esters of fatty acids. The disease-causing mutations disclosed in mitochondrial carriers range from point mutations, which are often localized in the substrate translocation pore of the carrier, to large deletions and insertions. The biochemical consequences of deficient transport are the compartmentalized accumulation of the substrates and dysfunctional mitochondrial and cellular metabolism, which frequently develop into various forms of myopathy, encephalopathy, or neuropathy. Examples of diseases, due to mitochondrial carrier mutations are: combined D-2- and L-2-hydroxyglutaric aciduria, carnitine-acylcarnitine carrier deficiency, hyperornithinemia-hyperammonemia-homocitrillinuria (HHH) syndrome, early infantile epileptic encephalopathy type 3, Amish microcephaly, aspartate/glutamate isoform 1 deficiency, congenital sideroblastic anemia, Fontaine progeroid syndrome, and citrullinemia type II. Here, we review all the mitochondrial carrier-related diseases known until now, focusing on the connections between the molecular basis, altered metabolism, and phenotypes of these inherited disorders.

Regulation of coenzyme A levels by degradation: the 'Ins and Outs'

Coenzyme A (CoA) is the predominant acyl carrier in mammalian cells and a cofactor that plays a key role in energy and lipid metabolism. CoA and its thioesters (acyl-CoAs) regulate a multitude of metabolic processes at different levels: as substrates, allosteric modulators, and via post-translational modification of histones and other non-histone proteins. Evidence is emerging that synthesis and degradation of CoA are regulated in a manner that enables metabolic flexibility in different subcellular compartments. Degradation of CoA occurs through distinct intra- and extracellular pathways that rely on the activity of specific hydrolases. The pantetheinase enzymes specifically hydrolyze pantetheine to cysteamine and pantothenate, the last step in the extracellular degradation pathway for CoA. This reaction releases pantothenate in the bloodstream, making this CoA precursor available for cellular uptake and de novo CoA synthesis. Intracellular degradation of CoA depends on specific mitochondrial and peroxisomal Nudix hydrolases. These enzymes are also active against a subset of acyl-CoAs and play a key role in the regulation of subcellular (acyl-)CoA pools and CoA-dependent metabolic reactions. The evidence currently available indicates that the extracellular and intracellular (acyl-)CoA degradation pathways are regulated in a coordinated and opposite manner by the nutritional state and maximize the changes in the total intracellular CoA levels that support the metabolic switch between fed and fasted states in organs like the liver. The objective of this review is to update the contribution of these pathways to the regulation of metabolism, physiology and pathology and to highlight the many questions that remain open.

Inherited Metabolic Disorders in Adults: A view from Saudi Arabia

The incidence of inherited metabolic disorders (IMD) in Saudi Arabia is one of the highest in the world. Early diagnosis and advances in the treatment of these diseases have led to improved survival of these patients resulting in a rapidly growing number of adults with IMD. This is the first report from a single tertiary care center, on the experience of managing a large cohort of adult patients with a wide range of IMD. We describe the common IMD seen in adult patients in Saudi Arabia, highlighting the variations from the Caucasian populations, and unique challenges in providing care to these adults. We mention the pitfalls causing the delay in the diagnosis particularly in cases of late-onset IMD in adults. We also discuss some unusual complications seen in adult patients during the course of their disease. We describe the role of genetic prevention services in Saudi Arabia and the importance of research in this field.

Emerging Roles in the Biogenesis of Cytochrome c Oxidase for Members of the Mitochondrial Carrier Family

The mitochondrial carrier family (MCF) is a group of transport proteins that are mostly localized to the inner mitochondrial membrane where they facilitate the movement of various solutes across the membrane. Although these carriers represent potential targets for therapeutic application and are repeatedly associated with human disease, research on the MCF has not progressed commensurate to their physiologic and pathophysiologic importance. Many of the 53 MCF members in humans are orphans and lack known transport substrates. Even for the relatively well-studied members of this family, such as the ADP/ATP carrier and the uncoupling protein, there exist fundamental gaps in our understanding of their biological roles including a clear rationale for the existence of multiple isoforms. Here, we briefly review this important family of mitochondrial carriers, provide a few salient examples of their diverse metabolic roles and disease associations, and then focus on an emerging link between several distinct MCF members, including the ADP/ATP carrier, and cytochrome c oxidase biogenesis. As the ADP/ATP carrier is regarded as the paradigm of the entire MCF, its newly established role in regulating translation of the mitochondrial genome highlights that we still have a lot to learn about these metabolite transporters.

A Mutation in the Mitochondrial Aspartate/Glutamate Carrier Leads to a More Oxidizing Intramitochondrial Environment and an Inflammatory Myopathy in Dutch Shepherd Dogs

BACKGROUND

Inflammatory myopathies are characterized by infiltration of inflammatory cells into muscle. Typically, immune-mediated disorders such as polymyositis, dermatomyositis and inclusion body myositis are diagnosed.

OBJECTIVE

A small family of dogs with early onset muscle weakness and inflammatory muscle biopsies were investigated for an underlying genetic cause.

METHODS

Following the histopathological diagnosis of inflammatory myopathy, mutational analysis including whole genome sequencing, functional transport studies of the mutated and wild-type proteins, and metabolomic analysis were performed.

RESULTS

Whole genome resequencing identified a pathological variant in the SLC25A12 gene, resulting in a leucine to proline substitution at amino acid 349 in the mitochondrial aspartate-glutamate transporter known as the neuron and muscle specific aspartate glutamate carrier 1 (AGC1). Functionally reconstituting recombinant wild-type and mutant AGC1 into liposomes demonstrated a dramatic decrease in AGC1 transport activity and inability to transfer reducing equivalents from the cytosol into mitochondria. Targeted, broad-spectrum metabolomic analysis from affected and control muscles demonstrated a proinflammatory milieu and strong support for oxidative stress.

CONCLUSIONS

This study provides the first description of a metabolic mechanism in which ablated mitochondrial glutamate transport markedly reduced the import of reducing equivalents into mitochondria and produced a highly oxidizing and proinflammatory muscle environment and an inflammatory myopathy.

Phenotypic spectrum of SLC25A4 mutations

There is no comprehensive overview concerning the phenotypic variability in patients carrying SLC25A4 mutations available. Therefore, the aim of the present review was to summarise and discuss recent findings concerning the clinical presentation and phenotypic heterogeneity of SLC25A4 mutations. The study was conducted by systematically reviewing the literature using the search terms 'mitochondrial', "myopathy', 'nuclear DNA', 'mitochondrial DNA', in combination with 'SLC25A4' or 'AAC1'. The results indicated that the phenotypic heterogeneity in patients carrying a SLC25A4 mutation is broader than so far anticipated. Patients carrying a SLC25A4 mutation not only manifest as encephalo-myo-cardiomyopathy but also with scoliosis, cataract, depression, headache, hydrocephalus or arterial hypertension. SLC25A4 mutations may result in mtDNA depletion or multiple mitochondrial (mt)DNA deletions. SLC25A4-associated mtDNA depletion presents with the more severe phenotype and the worse outcome than patients with multiple mtDNA deletions. Depletion syndrome due to SLC25A4 mutations is associated with congenital respiratory insufficiency requiring mechanical ventilation with poor prognosis in the majority of the cases. Mutations in the SLC25A4 gene manifest phenotypically with multiorgan abnormalities in addition to encephalo-myo-cardiomyopathy. SLC25A4 mutations, causing mtDNA depletion, present with a more severe phenotype, including respiratory insufficiency and more widespread cerebral disease than mutations causing multiple mtDNA deletions.

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

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

Expanding the phenotype of SLC25A42-associated mitochondrial encephalomyopathy

SLC25A42 gene encodes an inner mitochondrial membrane protein that imports Coenzyme A into the mitochondrial matrix. A mutation in this gene was recently reported in a subject born to consanguineous parents who presented with mitochondrial myopathy with muscle weakness and lactic acidosis. In this report, we present 12 additional individuals with the same founder mutation who presented with variable manifestations ranging from asymptomatic lactic acidosis to a severe phenotype characterized by developmental regression and epilepsy. Our report confirms the link between SLC25A42 and mitochondrial disease in humans, and suggests that pathogenic variants in SLC25A42 should be interpreted with the understanding that the associated phenotype may be highly variable.

Mitochondrial oxodicarboxylate carrier deficiency is associated with mitochondrial DNA depletion and spinal muscular atrophy-like disease

Purpose

Members of the mitochondrial carrier family (SLC25) transport metabolites, nucleotides, co-factors and inorganic ions across the mitochondrial inner membrane.

Methods

We identified a pathogenic variant in a novel mitochondrial carrier gene in a patient by whole exome sequencing. The pathogenicity of the mutation was studied by transport assays, computer modelling followed by targeted metabolic testing and in vitro studies in human fibroblasts and neurons.

Results

The patient carries a homozygous pathogenic variant c.695A>G; p.(Lys232Arg) in the SLC25A21 gene, encoding the mitochondrial oxodicarboxylate carrier, and developed spinal muscular atrophy and mitochondrial myopathy. Transport assays show that the mutation renders SLC25A21 dysfunctional and 2-oxoadipate cannot be imported into the mitochondrial matrix. Computer models of central metabolism predicted that impaired transport of oxodicarboxylate disrupts the pathways of lysine and tryptophan degradation, and causes accumulation of 2-oxoadipate, pipecolic acid and quinolinic acid, which was confirmed in the patient’s urine by targeted metabolomics. Exposure to 2-oxoadipate and quinolinic acid decreased the level of mitochondrial complexes in neuronal cells (SH-SY5Y) and induced apoptosis.

Conclusion

Mitochondrial oxodicarboxylate carrier deficiency leads to mitochondrial dysfunction and the accumulation of oxoadipate and quinolinic acid, which in turn cause toxicity in spinal motor neurons leading to spinal muscular atrophy-like disease.

An overview of combined D-2- and L-2-hydroxyglutaric aciduria: functional analysis of CIC variants

Combined D-2- and L-2-hydroxyglutaric aciduria (D/L-2-HGA) is a devastating neurometabolic disorder, usually lethal in the first years of life. Autosomal recessive mutations in the SLC25A1 gene, which encodes the mitochondrial citrate carrier (CIC), were previously detected in patients affected with combined D/L-2-HGA. We showed that transfection of deficient fibroblasts with wild-type SLC25A1 restored citrate efflux and decreased intracellular 2-hydroxyglutarate levels, confirming that deficient CIC is the cause of D/L-2-HGA. We developed and implemented a functional assay and applied it to all 17 missense variants detected in a total of 26 CIC-deficient patients, including eight novel cases, showing reduced activities of varying degrees. In addition, we analyzed the importance of residues affected by these missense variants using our existing scoring system. This allowed not only a clinical and biochemical overview of the D/L-2-HGA patients but also phenotype-genotype correlation studies.

NGS Technologies as a Turning Point in Rare Disease Research , Diagnosis and Treatment

Approximately 25-50 million Americans, 30 million Europeans, and 8% of the Australian population have a rare disease. Rare diseases are thus a common problem for clinicians and account for enormous healthcare costs worldwide due to the difficulty of establishing a specific diagnosis. In this article, we review the milestones achieved in our understanding of rare diseases since the emergence of next-generation sequencing (NGS) technologies and analyze how these advances have influenced research and diagnosis. The first half of this review describes how NGS has changed diagnostic workflows and provided an unprecedented, simple way of discovering novel disease-associated genes. We focus particularly on metabolic and neurodevelopmental disorders. NGS has enabled cheap and rapid genetic diagnosis, highlighted the relevance of mosaic and de novo mutations, brought to light the wide phenotypic spectrum of most genes, detected digenic inheritance or the presence of more than one rare disease in the same patient, and paved the way for promising new therapies. In the second part of the review, we look at the limitations and challenges of NGS, including determination of variant causality, the loss of variants in coding and non-coding regions, and the detection of somatic mosaicism variants and epigenetic mutations, and discuss how these can be overcome in the near future.

The promiscuous enzyme medium-chain 3-keto-acyl-CoA thiolase triggers a vicious cycle in fatty-acid beta-oxidation

Mitochondrial fatty-acid beta-oxidation (mFAO) plays a central role in mammalian energy metabolism. Multiple severe diseases are associated with defects in this pathway. Its kinetic structure is characterized by a complex wiring of which the functional implications have hardly been explored. Repetitive cycles of reversible reactions, each cycle shortening the fatty acid by two carbon atoms, evoke competition between intermediates of different chain lengths for a common set of 'promiscuous' enzymes (enzymes with activity towards multiple substrates). In our validated kinetic model of the pathway, substrate overload causes a steep and detrimental flux decline. Here, we unravel the underlying mechanism and the role of enzyme promiscuity in it. Comparison of alternative model versions elucidated the role of promiscuity of individual enzymes. Promiscuity of the last enzyme of the pathway, medium-chain ketoacyl-CoA thiolase (MCKAT), was both necessary and sufficient to elicit the flux decline. Subsequently, Metabolic Control Analysis revealed that MCKAT had insufficient capacity to cope with high substrate influx. Next, we quantified the internal metabolic regulation, revealing a vicious cycle around MCKAT. Upon substrate overload, MCKAT's ketoacyl-CoA substrates started to accumulate. The unfavourable equilibrium constant of the preceding enzyme, medium/short-chain hydroxyacyl-CoA dehydrogenase, worked as an amplifier, leading to accumulation of upstream CoA esters, including acyl-CoA esters. These acyl-CoA esters are at the same time products of MCKAT and inhibited its already low activity further. Finally, the accumulation of CoA esters led to a sequestration of free CoA. CoA being a cofactor for MCKAT, its sequestration limited the MCKAT activity even further, thus completing the vicious cycle. Since CoA is also a substrate for distant enzymes, it efficiently communicated the 'traffic jam' at MCKAT to the entire pathway. This novel mechanism provides a basis to explore the role of mFAO in disease and elucidate similar principles in other pathways of lipid metabolism.

Discoveries, metabolic roles and diseases of mitochondrial carriers: A review

Mitochondrial carriers (MCs) are a superfamily of nuclear-encoded proteins that are mostly localized in the inner mitochondrial membrane and transport numerous metabolites, nucleotides, cofactors and inorganic anions. Their unique sequence features, i.e., a tripartite structure, six transmembrane α-helices and a three-fold repeated signature motif, allow MCs to be easily recognized. This review describes how the functions of MCs from Saccharomyces cerevisiae, Homo sapiens and Arabidopsis thaliana (listed in the first table) were discovered after the genome sequence of S. cerevisiae was determined in 1996. In the genomic era, more than 50 previously unknown MCs from these organisms have been identified and characterized biochemically using a method consisting of gene expression, purification of the recombinant proteins, their reconstitution into liposomes and transport assays (EPRA). Information derived from studies with intact mitochondria, genetic and metabolic evidence, sequence similarity, phylogenetic analysis and complementation of knockout phenotypes have guided the choice of substrates that were tested in the transport assays. In addition, the diseases associated to defects of human MCs have been briefly reviewed. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.