Complete loss-of-function of the heart/muscle-specific adenine nucleotide translocator is associated with mitochondrial myopathy and cardiomyopathy

MtDNA-maintenance defects: syndromes and genes

A large group of mitochondrial disorders, ranging from early-onset pediatric encephalopathic syndromes to late-onset myopathy with chronic progressive external ophthalmoplegia (CPEOs), are inherited as Mendelian disorders characterized by disturbed mitochondrial DNA (mtDNA) maintenance. These errors of nuclear-mitochondrial intergenomic signaling may lead to mtDNA depletion, accumulation of mtDNA multiple deletions, or both, in critical tissues. The genes involved encode proteins belonging to at least three pathways: mtDNA replication and maintenance, nucleotide supply and balance, and mitochondrial dynamics and quality control. In most cases, allelic mutations in these genes may lead to profoundly different phenotypes associated with either mtDNA depletion or multiple deletions.

Human adenine nucleotide translocases physically and functionally interact with respirasomes

A network of interactions for human adenine nucleotide translocases, required for oxidative phosphorylation, is reported. Of particular interest is an evolutionarily conserved and functionally important association with respiratory supercomplexes, which is surprising because the respirasomes of yeast and mammals are different.

Members of the adenine nucleotide translocase (ANT) family exchange ADP for ATP across the mitochondrial inner membrane, an activity that is essential for oxidative phosphorylation (OXPHOS). Mutations in or dysregulation of ANTs is associated with progressive external ophthalmoplegia, cardiomyopathy, nonsyndromic intellectual disability, apoptosis, and the Warburg effect. Binding partners of human ANTs have not been systematically identified. The absence of such information has prevented a detailed molecular understanding of the assorted ANT-associated diseases, including insight into their disparate phenotypic manifestations. To fill this void, in this study, we define the interactomes of two human ANT isoforms. Analogous to its yeast counterpart, human ANTs associate with heterologous partner proteins, including the respiratory supercomplex (RSC) and other solute carriers. The evolutionarily conserved ANT–RSC association is particularly noteworthy because the composition, and thereby organization, of RSCs in yeast and human is different. Surprisingly, absence of the major ANT isoform only modestly impairs OXPHOS in HEK293 cells, indicating that the low levels of other isoforms provide functional redundancy. In contrast, pharmacological inhibition of OXPHOS expression and function inhibits ANT-dependent ADP/ATP exchange. Thus ANTs and the OXPHOS machinery physically interact and functionally cooperate to enhance ANT transport capacity and mitochondrial respiration.

Mitochondrial purine and pyrimidine metabolism and beyond

Carefully balanced deoxynucleoside triphosphate (dNTP) pools are essential for both nuclear and mitochondrial genome replication and repair. Two synthetic pathways operate in cells to produce dNTPs, e.g., the de novo and the salvage pathways. The key regulatory enzymes for de novo synthesis are ribonucleotide reductase (RNR) and thymidylate synthase (TS), and this process is considered to be cytosolic. The salvage pathway operates both in the cytosol (TK1 and dCK) and the mitochondria (TK2 and dGK). Mitochondrial dNTP pools are separated from the cytosolic ones owing to the double membrane structure of the mitochondria, and are formed by the salvage enzymes TK2 and dGK together with NMPKs and NDPK in postmitotic tissues, while in proliferating cells the mitochondrial dNTPs are mainly imported from the cytosol produced by the cytosolic pathways. Imbalanced mitochondrial dNTP pools lead to mtDNA depletion and/or deletions resulting in serious mitochondrial diseases. The mtDNA depletion syndrome is caused by deficiencies not only in enzymes in dNTP synthesis (TK2, dGK, p53R2, and TP) and mtDNA replication (mtDNA polymerase and twinkle helicase), but also in enzymes in other metabolic pathways such as SUCLA2 and SUCLG1, ABAT and MPV17. Basic questions are why defects in these enzymes affect dNTP synthesis and how important is mitochondrial nucleotide synthesis in the whole cell/organism perspective? This review will focus on recent studies on purine and pyrimidine metabolism, which have revealed several important links that connect mitochondrial nucleotide metabolism with amino acids, glucose, and fatty acid metabolism.

Mitochondrial DNA maintenance defects

The maintenance of mitochondrial DNA (mtDNA) depends on a number of nuclear gene-encoded proteins including a battery of enzymes forming the replisome needed to synthesize mtDNA. These enzymes need to be in balanced quantities to function properly that is in part achieved by exchanging intramitochondrial contents through mitochondrial fusion. In addition, mtDNA synthesis requires a balanced supply of nucleotides that is achieved by nucleotide recycling inside the mitochondria and import from the cytosol. Mitochondrial DNA maintenance defects (MDMDs) are a group of diseases caused by pathogenic variants in the nuclear genes involved in mtDNA maintenance resulting in impaired mtDNA synthesis leading to quantitative (mtDNA depletion) and qualitative (multiple mtDNA deletions) defects in mtDNA. Defective mtDNA leads to organ dysfunction due to insufficient mtDNA-encoded protein synthesis, resulting in an inadequate energy production to meet the needs of affected organs. MDMDs are inherited as autosomal recessive or dominant traits, and are associated with a broad phenotypic spectrum ranging from mild adult-onset ophthalmoplegia to severe infantile fatal hepatic failure. To date, pathogenic variants in 20 nuclear genes known to be crucial for mtDNA maintenance have been linked to MDMDs, including genes encoding enzymes of mtDNA replication machinery (POLG, POLG2, TWNK, TFAM, RNASEH1, MGME1, and DNA2), genes encoding proteins that function in maintaining a balanced mitochondrial nucleotide pool (TK2, DGUOK, SUCLG1, SUCLA2, ABAT, RRM2B, TYMP, SLC25A4, AGK, and MPV17), and genes encoding proteins involved in mitochondrial fusion (OPA1, MFN2, and FBXL4).

Recurrent De Novo and Biallelic Variation of ATAD3A, Encoding a Mitochondrial Membrane Protein, Results in Distinct Neurological Syndromes

ATPase family AAA-domain containing protein 3A (ATAD3A) is a nuclear-encoded mitochondrial membrane protein implicated in mitochondrial dynamics, nucleoid organization, protein translation, cell growth, and cholesterol metabolism. We identified a recurrent de novo ATAD3A c.1582C>T (p.Arg528Trp) variant by whole-exome sequencing (WES) in five unrelated individuals with a core phenotype of global developmental delay, hypotonia, optic atrophy, axonal neuropathy, and hypertrophic cardiomyopathy. We also describe two families with biallelic variants in ATAD3A, including a homozygous variant in two siblings, and biallelic ATAD3A deletions mediated by nonallelic homologous recombination (NAHR) between ATAD3A and gene family members ATAD3B and ATAD3C. Tissue-specific overexpression of bor^R534W, the Drosophila mutation homologous to the human c.1582C>T (p.Arg528Trp) variant, resulted in a dramatic decrease in mitochondrial content, aberrant mitochondrial morphology, and increased autophagy. Homozygous null bor larvae showed a significant decrease of mitochondria, while overexpression of bor^WT resulted in larger, elongated mitochondria. Finally, fibroblasts of an affected individual exhibited increased mitophagy. We conclude that the p.Arg528Trp variant functions through a dominant-negative mechanism that results in small mitochondria that trigger mitophagy, resulting in a reduction in mitochondrial content. ATAD3A variation represents an additional link between mitochondrial dynamics and recognizable neurological syndromes, as seen with MFN2, OPA1, DNM1L, and STAT2 mutations.

Recurrent De Novo Dominant Mutations in SLC25A4 Cause Severe Early-Onset Mitochondrial Disease and Loss of Mitochondrial DNA Copy Number

Mutations in SLC25A4 encoding the mitochondrial ADP/ATP carrier AAC1 are well-recognized causes of mitochondrial disease. Several heterozygous SLC25A4 mutations cause adult-onset autosomal-dominant progressive external ophthalmoplegia associated with multiple mitochondrial DNA deletions, whereas recessive SLC25A4 mutations cause childhood-onset mitochondrial myopathy and cardiomyopathy. Here, we describe the identification by whole-exome sequencing of seven probands harboring dominant, de novo SLC25A4 mutations. All affected individuals presented at birth, were ventilator dependent and, where tested, revealed severe combined mitochondrial respiratory chain deficiencies associated with a marked loss of mitochondrial DNA copy number in skeletal muscle. Strikingly, an identical c.239G>A (p.Arg80His) mutation was present in four of the seven subjects, and the other three case subjects harbored the same c.703C>G (p.Arg235Gly) mutation. Analysis of skeletal muscle revealed a marked decrease of AAC1 protein levels and loss of respiratory chain complexes containing mitochondrial DNA-encoded subunits. We show that both recombinant AAC1 mutant proteins are severely impaired in ADP/ATP transport, affecting most likely the substrate binding and mechanics of the carrier, respectively. This highly reduced capacity for transport probably affects mitochondrial DNA maintenance and in turn respiration, causing a severe energy crisis. The confirmation of the pathogenicity of these de novo SLC25A4 mutations highlights a third distinct clinical phenotype associated with mutation of this gene and demonstrates that early-onset mitochondrial disease can be caused by recurrent de novo mutations, which has significant implications for the application and analysis of whole-exome sequencing data in mitochondrial disease.

Deficiency in the mouse mitochondrial adenine nucleotide translocator isoform 2 gene is associated with cardiac noncompaction

The mouse fetal and adult hearts express two adenine nucleotide translocator (ANT) isoform genes. The predominant isoform is the heart-muscle-brain ANT-isoform gene 1 (Ant1) while the other is the systemic Ant2 gene. Genetic inactivation of the Ant1 gene does not impair fetal development but results in hypertrophic cardiomyopathy in postnatal mice. Using a knockin X-linked Ant2 allele in which exons 3 and 4 are flanked by loxP sites combined in males with a protamine 1 promoter driven Cre recombinase we created females heterozygous for a null Ant2 allele. Crossing the heterozygous females with the Ant2(fl), PrmCre(+) males resulted in male and female ANT2-null embryos. These fetuses proved to be embryonic lethal by day E14.5 in association with cardiac developmental failure, immature cardiomyocytes having swollen mitochondria, cardiomyocyte hyperproliferation, and cardiac failure due to hypertrabeculation/noncompaction. ANTs have two main functions, mitochondrial-cytosol ATP/ADP exchange and modulation of the mitochondrial permeability transition pore (mtPTP). Previous studies imply that ANT2 biases the mtPTP toward closed while ANT1 biases the mtPTP toward open. It has been reported that immature cardiomyocytes have a constitutively opened mtPTP, the closure of which signals the maturation of cardiomyocytes. Therefore, we hypothesize that the developmental toxicity of the Ant2 null mutation may be the result of biasing the cardiomyocyte mtPTP to remain open thus impairing cardiomyocyte maturation and resulting in cardiomyocyte hyperproliferation and failure of trabecular maturation. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

Mitochondrial Function, Biology, and Role in Disease: A Scientific Statement From the American Heart Association

Cardiovascular disease is a major leading cause of morbidity and mortality in the United States and elsewhere. Alterations in mitochondrial function are increasingly being recognized as a contributing factor in myocardial infarction and in patients presenting with cardiomyopathy. Recent understanding of the complex interaction of the mitochondria in regulating metabolism and cell death can provide novel insight and therapeutic targets. The purpose of this statement is to better define the potential role of mitochondria in the genesis of cardiovascular disease such as ischemia and heart failure. To accomplish this, we will define the key mitochondrial processes that play a role in cardiovascular disease that are potential targets for novel therapeutic interventions. This is an exciting time in mitochondrial research. The past decade has provided novel insight into the role of mitochondria function and their importance in complex diseases. This statement will define the key roles that mitochondria play in cardiovascular physiology and disease and provide insight into how mitochondrial defects can contribute to cardiovascular disease; it will also discuss potential biomarkers of mitochondrial disease and suggest potential novel therapeutic approaches.

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.

Cardiac and skeletal muscles show molecularly distinct responses to cancer cachexia

Cancer cachexia is a systemic, paraneoplastic syndrome seen in patients with advanced cancer. There is growing interest in the altered muscle pathophysiology experienced by cachectic patients. This study reports the microarray analysis of gene expression in cardiac and skeletal muscle in the colon 26 (C26) carcinoma mouse model of cancer cachexia. A total of 268 genes were found to be differentially expressed in cardiac muscle tissue, compared with nontumor-bearing controls. This was fewer than the 1,533 genes that changed in cachectic skeletal muscle. In addition to different numbers of genes changing, different cellular functions were seen to change in each tissue. The cachectic heart showed signs of inflammation, similar to cachectic skeletal muscle, but did not show the upregulation of ubiquitin-dependent protein catabolic processes or downregulation of genes involved in cellular energetics and muscle regeneration that characterizes skeletal muscle cachexia. Quantitative PCR was used to investigate a subset of inflammatory genes in the cardiac and skeletal muscle of independent cachectic samples; this revealed that B4galt1, C1s, Serpina3n, and Vsig4 were significantly upregulated in cardiac tissue, whereas C1s and Serpina3n were significantly upregulated in skeletal tissue. Our skeletal muscle microarray results were also compared with those from three published microarray studies and found to be consistent in terms of the genes differentially expressed and the functional processes affected. Our study highlights that skeletal and cardiac muscles are affected differently in the C26 mouse model of cachexia and that therapeutic strategies cannot assume that both muscle types will show a similar response.

The EuroBioBank Network: 10 years of hands-on experience of collaborative, transnational biobanking for rare diseases

The EuroBioBank (EBB) network (www.eurobiobank.org) is the first operating network of biobanks in Europe to provide human DNA, cell and tissue samples as a service to the scientific community conducting research on rare diseases (RDs). The EBB was established in 2001 to facilitate access to RD biospecimens and associated data; it obtained funding from the European Commission in 2002 (5th framework programme) and started operation in 2003. The set-up phase, during the EC funding period 2003-2006, established the basis for running the network; the following consolidation phase has seen the growth of the network through the joining of new partners, better network cohesion, improved coordination of activities, and the development of a quality-control system. During this phase the network participated in the EC-funded TREAT-NMD programme and was involved in planning of the European Biobanking and Biomolecular Resources Research Infrastructure. Recently, EBB became a partner of RD-Connect, an FP7 EU programme aimed at linking RD biobanks, registries, and bioinformatics data. Within RD-Connect, EBB contributes expertise, promotes high professional standards, and best practices in RD biobanking, is implementing integration with RD patient registries and 'omics' data, thus challenging the fragmentation of international cooperation on the field.

A cytosolic network suppressing mitochondria-mediated proteostatic stress and cell death

Mitochondria are multifunctional organelles whose dysfunction leads to neuromuscular degeneration and ageing. The multi-functionality poses a great challenge for understanding the mechanisms by which mitochondrial dysfunction causes specific pathologies. Among the leading mitochondrial mediators of cell death are energy depletion, free radical production, defects in iron-sulfur cluster biosynthesis, the release of pro-apoptotic and non-cell-autonomous signalling molecules, and altered stress signalling. Here we identify a new pathway of mitochondria-mediated cell death in yeast. This pathway was named mitochondrial precursor over-accumulation stress (mPOS), and is characterized by aberrant accumulation of mitochondrial precursors in the cytosol. mPOS can be triggered by clinically relevant mitochondrial damage that is not limited to the core machineries of protein import. We also discover a large network of genes that suppress mPOS, by modulating ribosomal biogenesis, messenger RNA decapping, transcript-specific translation, protein chaperoning and turnover. In response to mPOS, several ribosome-associated proteins were upregulated, including Gis2 and Nog2, which promote cap-independent translation and inhibit the nuclear export of the 60S ribosomal subunit, respectively. Gis2 and Nog2 upregulation promotes cell survival, which may be part of a feedback loop that attenuates mPOS. Our data indicate that mitochondrial dysfunction contributes directly to cytosolic proteostatic stress, and provide an explanation for the association between these two hallmarks of degenerative diseases and ageing. The results are relevant to understanding diseases (for example, spinocerebellar ataxia, amyotrophic lateral sclerosis and myotonic dystrophy) that involve mutations within the anti-degenerative network.

Misfolding of mutant adenine nucleotide translocase in yeast supports a novel mechanism of Ant1-induced muscle diseases

Missense mutations in membrane proteins cause dominant diseases by unknown mechanisms. Pathogenic mutations in adenine nucleotide translocase 1 affect protein folding and the assembly of multiple protein complexes. The misfolding of one single protein is therefore sufficient to induce proteostatic stress on the membrane.

Approximately one-third of proteins in the cell reside in the membrane. Mutations in membrane proteins can induce conformational changes and expose nonnative polar domains/residues to the lipid environment. The molecular effect of the resulting membrane stress is poorly defined. Adenine nucleotide translocase 1 (Ant1) is a mitochondrial inner membrane protein involved in ATP/ADP exchange. Missense mutations in the Ant1 isoform cause autosomal dominant progressive external ophthalmoplegia (adPEO), cardiomyopathy, and myopathy. The mechanism of the Ant1-induced pathologies is highly debated. Here we show that equivalent mutations in the yeast Aac2 protein cause protein misfolding. Misfolded Aac2 drastically affects the assembly and stability of multiple protein complexes in the membrane, which ultimately inhibits cell growth. Despite causing similar proteostatic damages, the adPEO- but not the cardiomyopathy/myopathy-type Aac2 proteins form large aggregates. The data suggest that the Ant1-induced diseases belong to protein misfolding disorders. Protein homeostasis is subtly maintained on the mitochondrial inner membrane and can be derailed by the misfolding of one single protein with or without aggregate formation. This finding could have broad implications for understanding other dominant diseases (e.g., retinitis pigmentosa) caused by missense mutations in membrane proteins.

Two Novel Mutations in the SLC25A4 Gene in a Patient with Mitochondrial Myopathy

In a 28-year-old male with a mild mitochondrial myopathy manifesting as exercise intolerance and early signs of cardiomyopathy without muscle weakness or ophthalmoplegia, we identified two novel mutations in the SLC25A4 gene: c.707G>C in exon 3 (p.(R236P)) and c.116_137del in exon 2 (p.(Q39Lfs*14)). Serum lactate levels at rest were elevated (12.7 mM). Both the patient's father and brother were heterozygous carriers of the c.707G>C mutation and were asymptomatic. The second mutation causes a 22 bp deletion leading to a frame shift likely giving rise to a premature stop codon and nonsense-mediated decay (NMD). The segregation of the mutations could not be tested directly as the mother had died before. However, indirect evidence from NMD experiments showed that the two mutations were situated on two different alleles in the patient. This case is unique compared to other previously reported patients with either progressive external ophthalmoplegia (PEO) or clear hypertrophic cardiomyopathy with exercise intolerance and/or muscle weakness carrying recessive mutations leading to a complete absence of the SLC25A4 protein. Most likely in our patient, although severely reduced, SLC25A4 is still partially present and functional.

The GABA transaminase, ABAT, is essential for mitochondrial nucleoside metabolism

ABAT is a key enzyme responsible for catabolism of principal inhibitory neurotransmitter γ-aminobutyric acid (GABA). We report an essential role for ABAT in a seemingly unrelated pathway, mitochondrial nucleoside salvage, and demonstrate that mutations in this enzyme cause an autosomal recessive neurometabolic disorder and mtDNA depletion syndrome (MDS). We describe a family with encephalomyopathic MDS caused by a homozygous missense mutation in ABAT that results in elevated GABA in subjects' brains as well as decreased mtDNA levels in subjects' fibroblasts. Nucleoside rescue and co-IP experiments pinpoint that ABAT functions in the mitochondrial nucleoside salvage pathway to facilitate conversion of dNDPs to dNTPs. Pharmacological inhibition of ABAT through the irreversible inhibitor Vigabatrin caused depletion of mtDNA in photoreceptor cells that was prevented through addition of dNTPs in cell culture media. This work reveals ABAT as a connection between GABA metabolism and nucleoside metabolism and defines a neurometabolic disorder that includes MDS.

Physiological and pathological roles of the mitochondrial permeability transition pore in the heart

Prolonged mitochondrial permeability transition pore (MPTP) opening results in mitochondrial energetic dysfunction, organelle swelling, rupture, and typically a type of necrotic cell death. However, acute opening of the MPTP has a critical physiologic role in regulating mitochondrial Ca(2+) handling and metabolism. Despite the physiological and pathological roles that the MPTP orchestrates, the proteins that comprise the pore itself remain an area of ongoing investigation. Here, we will discuss the molecular composition of the MPTP and its role in regulating cardiac physiology and disease. A better understanding of MPTP structure and function will likely suggest novel cardioprotective therapeutic approaches.

Power(2): the power of yeast genetics applied to the powerhouse of the cell

The budding yeast Saccharomyces cerevisiae has served as a remarkable model organism for numerous seminal discoveries in biology. This paradigm extends to the mitochondria, a central hub for cellular metabolism, where studies in yeast have helped to reinvigorate the field and launch an exciting new era in mitochondrial biology. Here we discuss a few recent examples in which yeast research has laid a foundation for our understanding of evolutionarily conserved mitochondrial processes and functions, from key factors and pathways involved in the assembly of oxidative phosphorylation (OXPHOS) complexes to metabolite transport, lipid metabolism, and interorganelle communication. We also highlight new areas of yeast mitochondrial biology that are likely to aid in our understanding of the mitochondrial etiology of disease in the future.

Mitochondrial Diseases Part II: Mouse models of OXPHOS deficiencies caused by defects in regulatory factors and other components required for mitochondrial function

Mitochondrial disorders are defined as defects that affect the oxidative phosphorylation system (OXPHOS). They are characterized by a heterogeneous array of clinical presentations due in part to a wide variety of factors required for proper function of the components of the OXPHOS system. There is no cure for these disorders owing to our poor knowledge of the pathogenic mechanisms of disease. To understand the mechanisms of human disease numerous mouse models have been developed in recent years. Here we summarize the features of several mouse models of mitochondrial diseases directly related to those factors affecting mtDNA maintenance, replication, transcription, translation as well as other proteins that are involved in mitochondrial dynamics and quality control which affect mitochondrial OXPHOS function without being intrinsic components of the system. We discuss how these models have contributed to our understanding of mitochondrial diseases and their pathogenic mechanisms.

The ADP/ATP carrier and its relationship to oxidative phosphorylation in ancestral protist trypanosoma brucei

The highly conserved ADP/ATP carrier (AAC) is a key energetic link between the mitochondrial (mt) and cytosolic compartments of all aerobic eukaryotic cells, as it exchanges the ATP generated inside the organelle for the cytosolic ADP. Trypanosoma brucei, a parasitic protist of medical and veterinary importance, possesses a single functional AAC protein (TbAAC) that is related to the human and yeast ADP/ATP carriers. However, unlike previous studies performed with these model organisms, this study showed that TbAAC is most likely not a stable component of either the respiratory supercomplex III+IV or the ATP synthasome but rather functions as a physically separate entity in this highly diverged eukaryote. Therefore, TbAAC RNA interference (RNAi) ablation in the insect stage of T. brucei does not impair the activity or arrangement of the respiratory chain complexes. Nevertheless, RNAi silencing of TbAAC caused a severe growth defect that coincides with a significant reduction of mt ATP synthesis by both substrate and oxidative phosphorylation. Furthermore, TbAAC downregulation resulted in a decreased level of cytosolic ATP, a higher mt membrane potential, an elevated amount of reactive oxygen species, and a reduced consumption of oxygen in the mitochondria. Interestingly, while TbAAC has previously been demonstrated to serve as the sole ADP/ATP carrier for ADP influx into the mitochondria, our data suggest that a second carrier for ATP influx may be present and active in the T. brucei mitochondrion. Overall, this study provides more insight into the delicate balance of the functional relationship between TbAAC and the oxidative phosphorylation (OXPHOS) pathway in an early diverged eukaryote.

Downregulation of adenine nucleotide translocator 1 exacerbates tumor necrosis factor-α-mediated cardiac inflammatory responses

Inflammation contributes significantly to cardiac dysfunction. Although the initial phase of inflammation is essential for repair and healing, excessive proinflammatory cytokines are detrimental to the heart. We found that adenine nucleotide translocator isoform-1 (ANT1) protein levels were significantly decreased in the inflamed heart of C57BL/6 mice following cecal ligation and puncture. To understand the molecular mechanisms involved, we performed small-interfering RNA-mediated knockdown of ANT1 and studied tumor necrosis factor-α (TNFα)-induced inflammatory responses in myocardium-derived H9c2 cells and cardiomyocytes. ANT1 knockdown significantly increased swollen mitochondria and mitochondrial reactive oxygen species, concomitant with increased TNFα-induced NF-κB reporter gene activity and interleukin-6 and TNFα expression. A mitochondrial-targeted antioxidant mito-TEMPO attenuated TNFα-induced mitochondrial reactive oxygen species, NF-κB reporter gene activity, and cytokine expression in ANT1 knockdown cells. Interestingly, TNFα or lipopolysaccharide (LPS) treatment significantly decreased ANT1 protein levels, suggesting a feed-forward regulation of proinflammatory cytokine expression activated by ANT1 downregulation. These data suggest that ANT1 downregulation contributes to cardiac inflammation post-cecal ligation and puncture. Preventing ANT1 downregulation could provide a novel molecular target to temper cardiac inflammation.

Mitochondrial transporters of the SLC25 family and associated diseases: a review

To date, 14 inherited diseases (including phenotypes) associated to mitochondrial transporters of the SLC25 family have been well characterized biochemically and genetically. They are rare metabolic disorders caused by mutations in the SLC25 nuclear genes that encode mitochondrial carriers, a superfamily of 53 proteins in humans that shuttle a variety of solutes across the mitochondrial membrane. Mitochondrial carriers vary considerably in the nature and size of the substrates they transport, the modes of transport and driving forces. However, their substrate translocation mechanism at the molecular level is thought to be basically the same. Herein, the main structural and functional properties of the SLC25 mitochondrial carriers and the known carrier-related diseases are presented. Two of these disorders, ADP/ATP carrier deficiency and phosphate carrier deficiency, are caused by defects of the two mitochondrial carriers that provide mitochondria with ADP and phosphate, the substrates of oxidative phosphorylation; these disorders therefore are characterized by defective energy production by mitochondria. The mutations of SLC25 carrier genes involved in other cellular functions cause carnitine/acylcarnitine carrier deficiency, HHH syndrome, aspartate/glutamate isoform 1 and 2 deficiencies, congenital Amish microcephaly, neuropathy with bilateral striatal necrosis, congenital sideroblastic anemia, neonatal epileptic encephalopathy, and citrate carrier deficiency; these disorders are characterized by specific metabolic dysfunctions depending on the role of the defective carrier in intermediary metabolism.

Phenotypic rescue of a Drosophila model of mitochondrial ANT1 disease

A point mutation in the Drosophila gene that codes for the major adult isoform of adenine nuclear translocase (ANT) represents a model for human diseases that are associated with ANT insufficiency [stress-sensitive B¹ (sesB¹)]. We characterized the organismal, bioenergetic and molecular phenotype of sesB¹ flies then tested strategies to compensate the mutant phenotype. In addition to developmental delay and mechanical-stress-induced seizures, sesB¹ flies have an impaired response to sound, defective male courtship, female sterility and curtailed lifespan. These phenotypes, excluding the latter two, are shared with the mitoribosomal protein S12 mutant, tko^25t. Mitochondria from sesB¹ adults showed a decreased respiratory control ratio and downregulation of cytochrome oxidase. sesB¹ adults exhibited ATP depletion, lactate accumulation and changes in gene expression that were consistent with a metabolic shift towards glycolysis, characterized by activation of lactate dehydrogenase and anaplerotic pathways. Females also showed downregulation of many genes that are required for oogenesis, and their eggs, although fertilized, failed to develop to the larval stages. The sesB¹ phenotypes of developmental delay and mechanical-stress-induced seizures were alleviated by an altered mitochondrial DNA background. Female sterility was substantially rescued by somatic expression of alternative oxidase (AOX) from the sea squirt Ciona intestinalis, whereas AOX did not alleviate developmental delay. Our findings illustrate the potential of different therapeutic strategies for ANT-linked diseases, based on alleviating metabolic stress.

Genetic deletion of the mitochondrial phosphate carrier desensitizes the mitochondrial permeability transition pore and causes cardiomyopathy

The mitochondrial phosphate carrier (PiC) is critical for ATP synthesis by serving as the primary means for mitochondrial phosphate import across the inner membrane. In addition to its role in energy production, PiC is hypothesized to have a role in cell death as either a component or a regulator of the mitochondrial permeability transition pore (MPTP) complex. Here, we have generated a mouse model with inducible and cardiac-specific deletion of the Slc25a3 gene (PiC protein). Loss of PiC protein did not prevent MPTP opening, suggesting it is not a direct pore-forming component of this complex. However, Slc25a3 deletion in the heart blunted MPTP opening in response to Ca(2+) challenge and led to a greater Ca(2+) uptake capacity. This desensitization of MPTP opening due to loss or reduction in PiC protein attenuated cardiac ischemic-reperfusion injury, as well as partially protected cells in culture from Ca(2+) overload induced death. Intriguingly, deletion of the Slc25a3 gene from the heart long-term resulted in profound hypertrophy with ventricular dilation and depressed cardiac function, all features that reflect the cardiomyopathy observed in humans with mutations in SLC25A3. Together, these results demonstrate that although the PiC is not a direct component of the MPTP, it can regulate its activity, suggesting a novel therapeutic target for reducing necrotic cell death. In addition, mice lacking Slc25a3 in the heart serve as a novel model of metabolic, mitochondrial-driven cardiomyopathy.

Genomics and genetics in the biology of adaptation to exercise

This article is devoted to the role of genetic variation and gene-exercise interactions in the biology of adaptation to exercise. There is evidence from genetic epidemiology research that DNA sequence differences contribute to human variation in physical activity level, cardiorespiratory fitness in the untrained state, cardiovascular and metabolic response to acute exercise, and responsiveness to regular exercise. Methodological and technological advances have made it possible to undertake the molecular dissection of the genetic component of complex, multifactorial traits, such as those of interest to exercise biology, in terms of tissue expression profile, genes, and allelic variants. The evidence from animal models and human studies is considered. Data on candidate genes, genome-wide linkage results, genome-wide association findings, expression arrays, and combinations of these approaches are reviewed. Combining transcriptomic and genomic technologies has been shown to be more powerful as evidenced by the development of a recent molecular predictor of the ability to increase VO2max with exercise training. For exercise as a behavior and physiological fitness as a state to be major players in public health policies will require that the role of human individuality and the influence of DNA sequence differences be understood. Likewise, progress in the use of exercise in therapeutic medicine will depend to a large extent on our ability to identify the favorable responders for given physiological properties to a given exercise regimen.

The mitochondrial ADP/ATP carrier (SLC25 family): pathological implications of its dysfunction

In aerobic eukaryotic cells, the high energy metabolite ATP is generated mainly within the mitochondria following the process of oxidative phosphorylation. The mitochondrial ATP is exported to the cytoplasm using a specialized transport protein, the ADP/ATP carrier, to provide energy to the cell. Any deficiency or dysfunction of this membrane protein leads to serious consequences on cell metabolism and can cause various diseases such as muscular dystrophy. Described as a decisive player in the programmed cell death, it was recently shown to play a role in cancer. The objective of this review is to summarize the current knowledge of the involvement of the ADP/ATP carrier, encoded by the SLC25A4, SLC25A5, SLC25A6 and SLC25A31 genes, in human diseases and of the efforts made at designing different model systems to study this carrier and the associated pathologies through biochemical, genetic, and structural approaches.

Adenine nucleotide translocase, mitochondrial stress, and degenerative cell death

Mitochondria are intracellular organelles involved in ATP synthesis, apoptosis, calcium signaling, metabolism, and the synthesis of critical metabolic cofactors. Mitochondrial dysfunction is associated with age-related degenerative diseases. How mitochondrial dysfunction causes cell degeneration is not well understood. Recent studies have shown that mutations in the adenine nucleotide translocase (Ant) cause aging-dependent degenerative cell death (DCD) in yeast, which is sequentially manifested by inner membrane stress, mitochondrial DNA (mtDNA) loss, and progressive loss of cell viability. Ant is an abundant protein primarily involved in ADP/ATP exchange across the mitochondrial inner membrane. It also mediates basal proton leak and regulates the mitochondrial permeability transition pore. Missense mutations in the human Ant1 cause several degenerative diseases which are commonly manifested by fractional mtDNA deletions. Multiple models have been proposed to explain the Ant1-induced pathogenesis. Studies from yeast have suggested that in addition to altered nucleotide transport properties, the mutant proteins cause a global stress on the inner membrane. The mutant proteins likely interfere with general mitochondrial biogenesis in a dominant-negative manner, which secondarily destabilizes mtDNA. More recent work revealed that the Ant-induced DCD is suppressed by reduced cytosolic protein synthesis. This finding suggests a proteostatic crosstalk between mitochondria and the cytosol, which may play an important role for cell survival during aging.

Thymidine kinase and mtDNA depletion in human cardiomyopathy: epigenetic and translational evidence for energy starvation

This study addresses how depletion of human cardiac left ventricle (LV) mitochondrial DNA (mtDNA) and epigenetic nuclear DNA methylation promote cardiac dysfunction in human dilated cardiomyopathy (DCM) through regulation of pyrimidine nucleotide kinases. Samples of DCM LV and right ventricle (n = 18) were obtained fresh at heart transplant surgery. Parallel samples from nonfailing (NF) controls (n = 12) were from donor hearts found unsuitable for clinical use. We analyzed abundance of mtDNA and nuclear DNA (nDNA) using qPCR. LV mtDNA was depleted in DCM (50%, P < 0.05 each) compared with NF. No detectable change in RV mtDNA abundance occurred. DNA methylation and gene expression were determined using microarray analysis (GEO accession number: GSE43435). Fifty-seven gene promoters exhibited DNA hypermethylation or hypomethylation in DCM LVs. Among those, cytosolic thymidine kinase 1 (TK1) was hypermethylated. Expression arrays revealed decreased abundance of the TK1 mRNA transcript with no change in transcripts for other relevant thymidine metabolism enzymes. Quantitative immunoblots confirmed decreased TK1 polypeptide steady state abundance. TK1 activity remained unchanged in DCM samples while mitochondrial thymidine kinase (TK2) activity was significantly reduced. Compensatory TK activity was found in cardiac myocytes in the DCM LV. Diminished TK2 activity is mechanistically important to reduced mtDNA abundance and identified in DCM LV samples here. Epigenetic and genetic changes result in changes in mtDNA and in nucleotide substrates for mtDNA replication and underpin energy starvation in DCM.

A mitochondrial bioenergetic etiology of disease

The classical Mendelian genetic perspective has failed to adequately explain the biology and genetics of common metabolic and degenerative diseases. This is because these diseases are primarily systemic bioenergetic diseases, and the most important energy genes are located in the cytoplasmic mitochondrial DNA (mtDNA). Therefore, to understand these "complex" diseases, we must investigate their bioenergetic pathophysiology and consider the genetics of the thousands of copies of maternally inherited mtDNA, the more than 1,000 nuclear DNA (nDNA) bioenergetic genes, and the epigenomic and signal transduction systems that coordinate these dispersed elements of the mitochondrial genome.

Severity of cardiomyopathy associated with adenine nucleotide translocator-1 deficiency correlates with mtDNA haplogroup

Mutations of both nuclear and mitochondrial DNA (mtDNA)-encoded mitochondrial proteins can cause cardiomyopathy associated with mitochondrial dysfunction. Hence, the cardiac phenotype of nuclear DNA mitochondrial mutations might be modulated by mtDNA variation. We studied a 13-generation Mennonite pedigree with autosomal recessive myopathy and cardiomyopathy due to an SLC25A4 frameshift null mutation (c.523delC, p.Q175RfsX38), which codes for the heart-muscle isoform of the adenine nucleotide translocator-1. Ten homozygous null (adenine nucleotide translocator-1(-/-)) patients monitored over a median of 6 years had a phenotype of progressive myocardial thickening, hyperalaninemia, lactic acidosis, exercise intolerance, and persistent adrenergic activation. Electrocardiography and echocardiography with velocity vector imaging revealed abnormal contractile mechanics, myocardial repolarization abnormalities, and impaired left ventricular relaxation. End-stage heart disease was characterized by massive, symmetric, concentric cardiac hypertrophy; widespread cardiomyocyte degeneration; overabundant and structurally abnormal mitochondria; extensive subendocardial interstitial fibrosis; and marked hypertrophy of arteriolar smooth muscle. Substantial variability in the progression and severity of heart disease segregated with maternal lineage, and sequencing of mtDNA from five maternal lineages revealed two major European haplogroups, U and H. Patients with the haplogroup U mtDNAs had more rapid and severe cardiomyopathy than those with haplogroup H.

The mitochondrial transporter family SLC25: identification, properties and physiopathology

SLC25 is a large family of nuclear-encoded transporters embedded in the inner mitochondrial membrane and in a few cases other organelle membranes. The members of this superfamily are widespread in eukaryotes and involved in numerous metabolic pathways and cell functions. They can be easily recognized by their striking sequence features, i.e., a tripartite structure, six transmembrane α-helices and a 3-fold repeated signature motifs. SLC25 members vary greatly in the nature and size of their transported substrates, modes of transport (i.e., uniport, symport or antiport) and driving forces, although the molecular mechanism of substrate translocation may be basically the same. Based on substrate specificity, 24 subfamilies, well conserved throughout evolution, have been functionally characterized mainly by transport assays upon heterologous gene expression, purification and reconstitution into liposomes. Several other SLC25 family members remain to be characterized. In recent years mutations in the SLC25 genes have been shown to be responsible for 11 diseases, highlighting the important role of SLC25 in metabolism.

Impaired transport of nucleotides in a mitochondrial carrier explains severe human genetic diseases

The mitochondrial ADP/ATP carrier (AAC) is a prominent actor in the energetic regulation of the cell, importing ADP into the mitochondria and exporting ATP toward the cytoplasm. Severe genetic diseases have been ascribed to specific mutations in this membrane protein. How minute, well-localized modifications of the transporter impact the function of the mitochondria remains, however, largely unclear. Here, for the first time, the relationship between all documented pathological mutations of the AAC and its transport properties is established. Activity measurements combined synergistically with molecular-dynamics simulations demonstrate how all documented pathological mutations alter the binding affinity and the translocation kinetics of the nucleotides. Throwing a bridge between the pathologies and their molecular origins, these results reveal two distinct mechanisms responsible for AAC-related genetic disorders, wherein the mutations either modulate the association of the nucleotides to the carrier by modifying its electrostatic signature or reduce its conformational plasticity.

Deficiency of the mitochondrial phosphate carrier presenting as myopathy and cardiomyopathy in a family with three affected children

In a family three children presented with severe neonatal lactic acidosis, hypertrophic cardiomyopathy and generalised muscular hypotonia. One child died in infancy, two survived a clinically severe neonatal period. At an age of 9 and 17years, respectively, they present with exercise intolerance, proximal muscle weakness, non-progressive hypertrophic cardiomyopathy and normal mental development. In a muscle biopsy normal activity of respiratory chain enzymes was found; however the amount of the mitochondrial phosphate carrier was decreased. This protein is expressed in two tissue-specific isoforms generated by mutually exclusive alternative splicing of the SLC25A3 gene transcript. We identified a homozygous mutation c.158-9A>G located in the 5'-intron next to exon 3A specific for heart and skeletal muscle. This creates a novel splice site resulting in a more than 95% decrease of the wild type allele.

Adenine nucleotide translocase 1 deficiency results in dilated cardiomyopathy with defects in myocardial mechanics, histopathological alterations, and activation of apoptosis

OBJECTIVES

the aim of this study was to test the hypothesis that chronic mitochondrial energy deficiency causes dilated cardiomyopathy, we characterized the hearts of age-matched young and old adenine nucleotide translocator (ANT)1 mutant and control mice.

BACKGROUND

ANTs export mitochondrial adenosine triphosphate into the cytosol and have a role in the regulation of the intrinsic apoptosis pathway. Mitochondrial energy deficiency has been hypothesized, on the basis of indirect evidence, to be a factor in the pathophysiology of dilated cardiomyopathies. Ant1 inactivation should limit adenosine triphosphate for contraction and calcium transport, thereby resulting in early cardiac dysfunction with later dilation and heart failure.

METHODS

we conducted a multiyear study of 73 mutant (Ant1-/-) and 57 control (Ant1+/+) mice, between the ages of 2 and 21 months. Hearts were characterized by cardiac anatomy, echocardiographic imaging with velocity vector analysis, histopathology, and apoptosis assays.

RESULTS

the Ant1-/- mice developed a distinctive concentric dilated cardiomyopathy, characterized by substantial myocardial hypertrophy and ventricular dilation, with cardiac function declining earlier in age as compared to control mice. Left ventricular circumferential, radial, and rotational mechanics were reduced even in the younger mutants with preserved systolic function. Histopathologic analysis demonstrated increased myocyte hypertrophy, fibrosis, and calcification in the mutant mice as compared with control mice. Furthermore, increased cytoplasmic cytochrome c levels and caspase 3 activation were observed in the mutant mice.

CONCLUSIONS

our results demonstrate that mitochondrial energy deficiency is sufficient to cause dilated cardiomyopathy, confirming that energy defects are a factor in this disease. Energy deficiency initially leads to early mechanical dysfunction before a decline in left ventricular systolic function. Chronic energy deficiency with age then leads to heart failure. Our results now allow us to use the Ant1-/- mouse model for testing new therapies for ANT1 mutant patients.

Mitochondrial medicine: to a new era of gene therapy for mitochondrial DNA mutations

Mitochondrial disorders can no longer be ignored in most medical disciplines. Such disorders include specific and widespread organ involvement, with tissue degeneration or tumor formation. Primary or secondary actors, mitochondrial dysfunctions also play a role in the aging process. Despite progresses made in identification of their molecular bases, nearly everything remains to be done as regards therapy. Research dealing with mitochondrial physiology and pathology has >20 years of history around the world. We are involved, as are many other laboratories, in the challenge of finding ways to fight these diseases. However, our main limitation is the scarcety of animal models required for both understanding the molecular mechanisms underlying the diseases and evaluating therapeutic strategies. This is especially true for diseases due to mutations in mitochondrial DNA (mtDNA), since an authentic genetic model of mtDNA mutations is technically a very difficult task due to both the inability of manipulating the mitochondrial genome of living mammalian cells and to its multicopy nature. This has led researchers in the field to consider the prospect of gene therapy approaches that can roughly be divided into three groups: (1) import of wild-type copies or relevant sections of DNA or RNA into mitochondria, (2) manipulation of mitochondrial genetic content, and (3) rescue of a defect by expression of an engineered gene product from the nucleus (allotopic or xenotropic expression). We briefly introduce these concepts and indicate where promising progress has been made in the last decade.

Bacteria, yeast, worms, and flies: exploiting simple model organisms to investigate human mitochondrial diseases

The extensive conservation of mitochondrial structure, composition, and function across evolution offers a unique opportunity to expand our understanding of human mitochondrial biology and disease. By investigating the biology of much simpler model organisms, it is often possible to answer questions that are unreachable at the clinical level. Here, we review the relative utility of four different model organisms, namely the bacterium Escherichia coli, the yeast Saccharomyces cerevisiae, the nematode Caenorhabditis elegans, and the fruit fly Drosophila melanogaster, in studying the role of mitochondrial proteins relevant to human disease. E. coli are single cell, prokaryotic bacteria that have proven to be a useful model system in which to investigate mitochondrial respiratory chain protein structure and function. S. cerevisiae is a single-celled eukaryote that can grow equally well by mitochondrial-dependent respiration or by ethanol fermentation, a property that has proven to be a veritable boon for investigating mitochondrial functionality. C. elegans is a multicellular, microscopic worm that is organized into five major tissues and has proven to be a robust model animal for in vitro and in vivo studies of primary respiratory chain dysfunction and its potential therapies in humans. Studied for over a century, D. melanogaster is a classic metazoan model system offering an abundance of genetic tools and reagents that facilitates investigations of mitochondrial biology using both forward and reverse genetics. The respective strengths and limitations of each species relative to mitochondrial studies are explored. In addition, an overview is provided of major discoveries made in mitochondrial biology in each of these four model systems.

Mitochondrial diseases and the role of the yeast models

Nowadays, mitochondrial diseases are recognized and studied with much attention and they cannot be considered anymore as 'rare diseases'. Yeast has been an instrumental organism to understand the genetic and molecular aspects of the many roles of mitochondria within the cells. Thanks to the general conservation of mitochondrial genes and pathways between human and yeast, it can also be used to model some diseases. In this review, we focus on the most recent topics, exemplifying those for which yeast models have been especially valuable.

PGC-1 coactivators in cardiac development and disease

The beating heart requires a constant flux of ATP to maintain contractile function, and there is increasing evidence that energetic defects contribute to the development of heart failure. The last 10 years have seen a resurgent interest in cardiac intermediary metabolism and a dramatic increase in our understanding of transcriptional networks that regulate cardiac energetics. The PPAR-γ coactivator (PGC)-1 family of proteins plays a central role in these pathways. The mechanisms by which PGC-1 proteins regulate transcriptional networks and are regulated by physiological cues, as well as the roles they play in cardiac development and disease, are reviewed here.

Mitochondrial DNA mutations in disease and aging

The human mitochondrial genome involves over 1,000 genes, dispersed across the maternally inherited mitochondrial DNA (mtDNA) and the biparentally inherited nuclear DNA (nDNA). The mtDNA encodes 13 core proteins that determine the efficiency of the mitochondrial energy-generating system, oxidative phosphorylation (OXPHOS), plus the RNA genes for their translation within the mitochondrion. The mtDNA has a very high mutation rate, which results in three classes of clinically relevant mtDNA mutations: recently deleterious germline line mutations resulting in mitochondrial disease; ancient regional variants, a subset of which permitted humans to adapt to differences in their energetic environments; and somatic mutations that accumulate with age eroding mitochondrial energy production and providing the aging clock. Mutations in nDNA-encoded OXPHOS structural genes can also cause mitochondrial disease, and alterations in nDNA mitochondrial biogenesis genes can destabilize the mtDNA and lead to clinical phenotypes. Finally, when combined, nonpathogenic nDNA and mtDNA protein variants can be functionally incompatible and cause disease. The essential functions of the conserved mtDNA proteins and their high mutation rate raise the question as to why the cumulative mtDNA genetic load does not result in species extinction. Studies of mice harboring deleterious mtDNA mutations have shown that the mammalian ovary selectively eliminates the most deleterious mtDNA mutations. However, milder mtDNA mutations are transmitted through the ovary and the female germline and introduced into the general population. This unique genetic system provides a flexible method for generating genetic variation in cellular and organismal energetics that permits species to adapt to alterations in their regional energetic environment.

Bedside-to-Bench conference: research agenda for idiopathic fatigue and aging

The American Geriatrics Society, with support from the National Institute on Aging and the John A. Hartford Foundation, held its fifth Bedside-to-Bench research conference, "Idiopathic Fatigue and Aging," to provide participants with opportunities to learn about cutting-edge research developments, draft recommendations for future research, and network with colleagues and leaders in the field. Fatigue is a symptom that older persons, especially by those with chronic diseases, frequently experience. Definitions and prevalence of fatigue may vary across studies, across diseases, and even between investigators and patients. The focus of this review is on physical fatigue, recognizing that there are other related domains of fatigue (such as cognitive fatigue). Many definitions of fatigue involve a sensation of "low" energy, suggesting that fatigue could be a disorder of energy balance. Poor energy utilization efficiency has not been considered in previous studies but is likely to be one of the most important determinants of fatigue in older individuals. Relationships between activity level, capacity for activity, a tolerable rate of activity, and a tolerable fatigue threshold or ceiling underlie a notion of fatiguability. Mechanisms probably contributing to fatigue in older adults include decline in mitochondrial function, alterations in brain neurotransmitters, oxidative stress, and inflammation. The relationships between muscle function and fatigue are complex. A number of diseases (such as cancer) are known to cause fatigue and may serve as models for how underlying impaired physiological processes contribute to fatigue, particularly those in which energy utilization may be an important factor. A further understanding of fatigue will require two key strategies: to develop and refine fatigue definitions and measurement tools and to explore underlying mechanisms using animal and human models.

Mouse models of mtDNA replication diseases

Defective mitochondrial DNA (mtDNA) replication is a common cause of human disease in children and adults. mtDNA replication relies on a large set of nuclear-encoded proteins that either belong to the replication machinery itself, or participate in the nucleotide pool regulation. Identification of patient mutations in the corresponding genes has revealed that dysfunctional mtDNA replication can cause highly variable disease phenotypes. We describe here the strategies that have been undertaken to generate mouse models for mtDNA replication diseases. Such models are essential tools for understanding the consequences of mtDNA replication defects on different tissues and on the metabolism of the whole organism.