Concise Review: Heteroplasmic Mitochondrial DNA Mutations and Mitochondrial Diseases: Toward iPSC-Based Disease Modeling, Drug Discovery, and Regenerative Therapeutics

Mitochondrial impairment and synaptic dysfunction are associated with neurological defects in iPSCs-derived cortical neurons of MERRF patients

BACKGROUND

Myoclonic epilepsy with ragged-red fibers (MERRF) syndrome is a rare inherited mitochondrial disease mainly caused by the m.8344A > G mutation in mitochondrial tRNALys gene, and usually manifested as complex neurological disorders and muscle weakness. Currently, the pathogenic mechanism of this disease has not yet been resolved, and there is no effective therapy for MERRF syndrome. In this study, MERRF patients-derived iPSCs were used to model patient-specific neurons for investigation of the pathogenic mechanism of neurological disorders in mitochondrial disease.

METHODS

MERRF patient-derived iPSCs were differentiated into excitatory glutamatergic neurons to unravel the effects of the m.8344A > G mutation on mitochondrial bioenergetic function, neural-lineage differentiation and neuronal function. By the well-established differentiation protocol and electrophysiological activity assay platform, we examined the pathophysiological behaviors in cortical neurons of MERRF patients.

RESULTS

We have successfully established the iPSCs-derived neural progenitor cells and cortical-like neurons of patients with MERRF syndrome that retained the heteroplasmy of the m.8344A > G mutation from the patients' skin fibroblasts and exhibited the phenotype of the mitochondrial disease. MERRF neural cells harboring the m.8344A > G mutation exhibited impaired mitochondrial bioenergetic function, elevated ROS levels and imbalanced expression of antioxidant enzymes. Our findings indicate that neural immaturity and synaptic protein loss led to the impairment of neuronal activity and plasticity in MERRF neurons harboring the m.8344A > G mutation. By electrophysiological recordings, we monitored the in vivo neuronal behaviors of MERRF neurons and found that neurons harboring a high level of the m.8344A > G mutation exhibited impairment of the spontaneous and evoked potential-stimulated neuronal activities.

CONCLUSIONS

We demonstrated for the first time the link of mitochondrial impairment and synaptic dysfunction to neurological defects through impeding synaptic plasticity in excitatory neurons derived from iPSCs of MERRF patients harboring the m.8344A > G mutation. This study has provided new insight into the pathogenic mechanism of the tRNALys gene mutation of mtDNA, which is useful for the development of a patient-specific iPSCs platform for disease modeling and screening of new drugs to treat patients with MERRF syndrome.

Role of Bioactive Compounds in the Regulation of Mitochondrial Dysfunctions in Brain and Age-Related Neurodegenerative Diseases

Mitochondria are multifunctional organelles that participate in a wide range of metabolic processes, including energy production and biomolecule synthesis. The morphology and distribution of intracellular mitochondria change dynamically, reflecting a cell’s metabolic activity. Oxidative stress is defined as a mismatch between the body’s ability to neutralise and eliminate reactive oxygen and nitrogen species (ROS and RNS). A determination of mitochondria failure in increasing oxidative stress, as well as its implications in neurodegenerative illnesses and apoptosis, is a significant developmental process of focus in this review. The neuroprotective effects of bioactive compounds linked to neuronal regulation, as well as related neuronal development abnormalities, will be investigated. In conclusion, the study of secondary components and the use of mitochondrial features in the analysis of various neurodevelopmental diseases has enabled the development of a new class of mitochondrial-targeted pharmaceuticals capable of alleviating neurodegenerative disease states and enabling longevity and healthy ageing for the vast majority of people.

Energy Metabolism and Intracellular pH Alteration in Neural Spheroids Carrying Down Syndrome

Brain diseases including Down syndrome (DS/TS21) are known to be characterized by changes in cellular metabolism. To adequately assess such metabolic changes during pathological processes and to test drugs, methods are needed that allow monitoring of these changes in real time with minimally invasive effects. Thus, the aim of our work was to study the metabolic status and intracellular pH of spheroids carrying DS using fluorescence microscopy and FLIM. For metabolic analysis we measured the fluorescence intensities, fluorescence lifetimes and the contributions of the free and bound forms of NAD(P)H. For intracellular pH assay we measured the fluorescence intensities of SypHer-2 and BCECF. Data were processed with SPCImage and Fiji-ImageJ. We demonstrated the predominance of glycolysis in TS21 spheroids compared with normal karyotype (NK) spheroids. Assessment of the intracellular pH indicated a more alkaline intracellular pH in the TS21 spheroids compared to NK spheroids. Using fluorescence imaging, we performed a comprehensive comparative analysis of the metabolism and intracellular pH of TS21 spheroids and showed that fluorescence microscopy and FLIM make it possible to study living cells in 3D models in real time with minimally invasive effects.

Cellular Models for Primary CoQ Deficiency Pathogenesis Study

Primary coenzyme Q₁₀ (CoQ) deficiency includes a heterogeneous group of mitochondrial diseases characterized by low mitochondrial levels of CoQ due to decreased endogenous biosynthesis rate. These diseases respond to CoQ treatment mainly at the early stages of the disease. The advances in the next generation sequencing (NGS) as whole-exome sequencing (WES) and whole-genome sequencing (WGS) have increased the discoveries of mutations in either gene already described to participate in CoQ biosynthesis or new genes also involved in this pathway. However, these technologies usually provide many mutations in genes whose pathogenic effect must be validated. To functionally validate the impact of gene variations in the disease’s onset and progression, different cell models are commonly used. We review here the use of yeast strains for functional complementation of human genes, dermal skin fibroblasts from patients as an excellent tool to demonstrate the biochemical and genetic mechanisms of these diseases and the development of human-induced pluripotent stem cells (hiPSCs) and iPSC-derived organoids for the study of the pathogenesis and treatment approaches.

Potential of Mitochondrial Genome Editing for Human Fertility Health

Mitochondrial DNA (mtDNA) encodes vital proteins and RNAs for the normal functioning of the mitochondria. Mutations in mtDNA leading to mitochondrial dysfunction are relevant to a large spectrum of diseases, including fertility disorders. Since mtDNA undergoes rather complex processes during gametogenesis and fertilization, clarification of the changes and functions of mtDNA and its essential impact on gamete quality and fertility during this process is of great significance. Thanks to the emergence and rapid development of gene editing technology, breakthroughs have been made in mitochondrial genome editing (MGE), offering great potential for the treatment of mtDNA-related diseases. In this review, we summarize the features of mitochondria and their unique genome, emphasizing their inheritance patterns; illustrate the role of mtDNA in gametogenesis and fertilization; and discuss potential therapies based on MGE as well as the outlook in this field.

Down syndrome is an oxidative phosphorylation disorder

Down syndrome is the most common genomic disorder of intellectual disability and is caused by trisomy of chromosome 21. Several genes in this chromosome repress mitochondrial biogenesis. The goal of this study was to evaluate whether early overexpression of these genes may cause a prenatal impairment of oxidative phosphorylation negatively affecting neurogenesis. Reduction in the mitochondrial energy production and a lower mitochondrial function have been reported in diverse tissues or cell types, and also at any age, including early fetuses, suggesting that a defect in oxidative phosphorylation is an early and general event in Down syndrome individuals. Moreover, many of the medical conditions associated with Down syndrome are also frequently found in patients with oxidative phosphorylation disease. Several drugs that enhance mitochondrial biogenesis are nowadays available and some of them have been already tested in mouse models of Down syndrome restoring neurogenesis and cognitive defects. Because neurogenesis relies on a correct mitochondrial function and critical periods of brain development occur mainly in the prenatal and early neonatal stages, therapeutic approaches intended to improve oxidative phosphorylation should be provided in these periods.

A Mother's Story, Mitogenome Relationships in the Genus Rupicapra

Simple Summary

Two species of chamois (Rupicapra rupicapra and R. pyrenaica) are currently recognized by taxonomy and further subdivided into seven and three subspecies, respectively. However, recent research based on molecular markers finds this classification questionable. We aim to increase the resolution of published research on chamois phylogeny by including mitogenomes of all available subspecies, including the previously unpublished mitogenomes of R. r. balcanica and R. r. tatrica subspecies. The inferred phylogeny based on the full mitogenomes confirms the previously reported genus subdivision in three clades and its monophyletic positioning within the Caprinae. Phylogeny and taxonomy of Rupicapra species thus remain controversial prompting for the inclusion of archeological remains to solve the controversy.

Abstract

Although the two species of chamois (Rupicapra rupicapra and R. pyrenaica) are currently classified as least-concern by the IUCN (International Union for Conservation of Nature), inconsistencies on the subspecies classification reported in literature make it challenging to assess the conservation status of the single subspecies. Previous studies relying on mitochondrial genes, sometimes in combination with nuclear or Y-chromosome markers, reported the presence of clusters corresponding to the geographic distribution but highlighting ambiguities in the genus phylogeny. Here we report novel de novo assembled sequences of the mitochondrial genome from nine individuals, including previously unpublished R. r. balcanica and R. r. tatrica subspecies, and use them to untangle the genus phylogeny. Our results based on the full mitogenome inferred phylogeny confirm the previously reported genus subdivision in three clades and its monophyletic positioning within the Caprinae. Phylogeny and taxonomy of Rupicapra species thus remain controversial prompting for the inclusion of archeological remains to solve the controversy.

Current progress with mammalian models of mitochondrial DNA disease

Mitochondrial disorders make up a large class of heritable diseases that cause a broad array of different human pathologies. They can affect many different organ systems, or display very specific tissue presentation, and can lead to illness either in childhood or later in life. While the over 1200 genes encoded in the nuclear DNA play an important role in human mitochondrial disease, it has been known for over 30 years that mutations of the mitochondria's own small, multicopy DNA chromosome (mtDNA) can lead to heritable human diseases. Unfortunately, animal mtDNA has resisted transgenic and directed genome editing technologies until quite recently. As such, animal models to aid in our understanding of these diseases, and to explore preclinical therapeutic research have been quite rare. This review will discuss the unusual properties of animal mitochondria that have hindered the generation of animal models. It will also discuss the existing mammalian models of human mtDNA disease, describe the methods employed in their generation, and will discuss recent advances in the targeting of DNA-manipulating enzymes to the mitochondria and how these may be employed to generate new models.

HmtVar: a new resource for human mitochondrial variations and pathogenicity data

Interest in human mitochondrial genetic data is constantly increasing among both clinicians and researchers, due to the involvement of mitochondrial DNA (mtDNA) in a number of physiological and pathological processes. Thanks to new sequencing technologies and modern databases, the large amount of information on mtDNA variability may be exploited to gain insights into the relationship between mtDNA variants, phenotypes and diseases. To facilitate this process, we have developed the HmtVar resource, a variant-focused database that allows the exploration of a dataset of over 40 000 human mitochondrial variants. Mitochondrial variation data, initially gathered from the HmtDB platform, are integrated with in-house pathogenicity assessments based on various evaluation criteria and with a set of additional annotations from third-party resources. The result is a comprehensive collection of information of crucial importance for human mitochondrial variation studies and investigation of common and rare diseases in which the mitochondrion may be involved. HmtVar is accessible at https://www.hmtvar.uniba.it and data may be retrieved using either a web interface through the Query page or a state-of-the-art API for programmatic access.

Patient-specific neural progenitor cells derived from induced pluripotent stem cells offer a promise of good models for mitochondrial disease

Mitochondria are the primary generators of ATP in eukaryotic cells through the process of oxidative phosphorylation. Mitochondria are also involved in several other important cellular functions including regulation of intracellular Ca²⁺, cell signaling and apoptosis. Mitochondrial dysfunction causes disease and since it is not possible to perform repeated studies in humans, models are essential to enable us to investigate the mechanisms involved. Recently, the discovery of induced pluripotent stem cells (iPSCs), made by reprogramming adult somatic cells (Takahashi and Yamanaka 2006; Yamanaka and Blau 2010), has provided a unique opportunity for studying aspects of disease mechanisms in patient-specific cells and tissues. Reprogramming cells to neuronal lineage such as neural progenitor cells (NPCs) generated from the neural induction of reprogrammed iPSCs can thus provide a useful model for investigating neurological disease mechanisms including those caused by mitochondrial dysfunction. In addition, NPCs display a huge clinical potential in drug screening and therapeutics.

Osteoblastic differentiation improved by bezafibrate-induced mitochondrial biogenesis in deciduous tooth-derived pulp stem cells from a child with Leigh syndrome

Leigh syndrome is a highly heterogeneous condition caused by pathological mutations in either nuclear or mitochondrial DNA regions encoding molecules involved in mitochondrial oxidative phosphorylation, in which many organs including the brain can be affected. Among these organs, a high incidence of poor bone health has been recognized in primary mitochondrial diseases including Leigh syndrome. However, the direct association between mitochondrial dysfunction and poor bone health has not been fully elucidated. Mitochondrial biosynthesis is a potential therapeutic target for this syndrome, as it can ameliorate the impairment of oxidative phosphorylation without altering these gene mutations. A recent study has shown the impaired osteogenesis in the dental pulp stem cells derived from the deciduous teeth of a child with Leigh syndrome, harboring the heteroplasmic mutation G13513A in the mitochondrial DNA region encoding the ND5 subunit of the respiratory chain complex I. The present study aimed to investigate whether mitochondrial biogenesis could be a therapeutic target for improving osteogenesis, using the same stem cells in a patient-specific cellular model. For this purpose, bezafibrate was used because it has been reported to induce mitochondrial biogenesis as well as to improve bone metabolism and osteoporosis. Bezafibrate clearly improved the differentiation of patient-derived stem cells into osteoblasts and the mineralization of differentiated osteoblasts. The mRNA expression of peroxisome proliferator-activated receptor-gamma coactivator-1α, ATP production, and mitochondrial Ca²⁺ levels were all significantly increased by bezafibrate in the patient-derived cells. In addition, the increased amount and morphological shift from the fragmentary to network shape associated with DRP1 downregulation were also observed in the bezafibrate-treated patient-derived cells. These results suggest that mitochondrial biogenesis may be a potential therapeutic target for improving osteogenesis in patients with Leigh syndrome, and bezafibrate may be one of the candidate treatment agents.

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Dental pulp stem cells from a child with Leigh syndrome have impaired osteogenesis.

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Bezafibrate-PGC-1α pathway improves osteogenesis via mitochondrial biogenesis.

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Bezafibrate also induces DRP1 downregulation and mitochondrial network formation.

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Dental pulp stem cells may help to establish treatment strategies for Leigh syndrome.

Ectopic hTERT expression facilitates reprograming of fibroblasts derived from patients with Werner syndrome as a WS cellular model

The induced pluripotent stem cell (iPSC) technology has provided a unique opportunity to develop disease-specific models and personalized treatment for genetic disorders, and is well suitable for the study of Werner syndrome (WS), an autosomal recessive disease with adult onset of premature aging caused by mutations in the RecQ like helicase (WRN) gene. WS-derived fibroblasts were previously shown to be able to generate iPSCs; however, it remains elusive how WS-derived iPSCs behave and whether they are able to mimic the disease-specific phenotype. The present study was designed to address these issues. Unexpectedly, we found that a specific WS fibroblast line of homozygous truncation mutation was difficult to be reprogrammed by using the Yamanaka factors even under hypoxic conditions due to their defect in induction of hTERT, the catalytic unit of telomerase. Ectopic expression of hTERT restores the ability of this WS fibroblast line to form iPSCs, although with a low efficiency. To examine the phenotype of WRN-deficient pluripotent stem cells, we also generated WRN knockout human embryonic stem (ES) cells by using the CRISPR/Cas9 method. The iPSCs derived from WS-hTERT cells and WRN-/- ESCs are fully pluripotent, express pluripotent markers and can differentiate into three germ layer cells; however, WS-iPSCs and WRN-/- ESCs show S phase defect in cell cycle progression. Moreover, WS-iPSCs and WRN-/- ESCs, like WS patient-derived fibroblasts, remain hypersensitive to topoisomerase inhibitors. Collectively, WS-derived iPSCs and WRN-/- ESCs mimic the intrinsic disease phenotype, which may serve as a suitable disease model, whereas not be good for a therapeutic purpose without gene correction.

Preventing Mitochondrial Diseases: Embryo-Sparing Donor-Independent Options

Mutant mitochondrial DNA gives rise to a broad range of incurable inborn maladies. Prevention may now be possible by replacing the mutation-carrying mitochondria of zygotes or oocytes at risk with donated unaffected counterparts. However, mitochondrial replacement therapy is being held back by theological, ethical, and safety concerns over the loss of human zygotes and the involvement of a donor. These concerns make it plain that the identification, validation, and regulatory adjudication of novel embryo-sparing donor-independent technologies remains a pressing imperative. This Opinion highlights three emerging embryo-sparing donor-independent options that stand to markedly allay theological, ethical, and safety concerns raised by mitochondrial replacement therapy.

Genetic and epigenetic factors which modulate differentiation propensity in human pluripotent stem cells

BACKGROUND

Human pluripotent stem cell (hPSC) lines are known to have a bias in their differentiation. This gives individual cell lines a propensity to preferentially differentiate towards one germ layer or cell type over others. Chromosomal aberrations, mitochondrial mutations, genetic diversity and epigenetic variance are the main drivers of this phenomenon, and can lead to a wide range of phenotypes.

OBJECTIVE AND RATIONALE

Our aim is to provide a comprehensive overview of the different factors which influence differentiation propensity. Specifically, we sought to highlight known genetic variances and their mechanisms, in addition to more general observations from larger abnormalities. Furthermore, we wanted to provide an up-to-date list of a growing number of predictive indicators which are able to identify differentiation propensity before the initiation of differentiation. As differentiation propensity can lead to difficulties in both research as well as clinical translation, our thorough overview could be a useful tool.

SEARCH METHODS

Combinations of the following key words were applied as search criteria in the PubMed database: embryonic stem cells, induced pluripotent stem cells, differentiation propensity (also: potential, efficiency, capacity, bias, variability), epigenetics, chromosomal abnormalities, genetic aberrations, X chromosome inactivation, mitochondrial function, mitochondrial metabolism, genetic diversity, reprogramming, predictive marker, residual stem cell, clinic. Only studies in English were included, ranging from 2000 to 2017, with a majority ranging from 2010 to 1017. Further manuscripts were added from cross-references.

OUTCOMES

Differentiation propensity is affected by a wide variety of (epi)genetic factors. These factors clearly lead to a loss of differentiation capacity, preference towards certain cell types and oftentimes, phenotypes which begin to resemble cancer. Broad changes in (epi)genetics, such as aneuploidies or wide-ranging modifications to the epigenetic landscape tend to lead to extensive, less definite changes in differentiation capacity, whereas more specific abnormalities often have precise ramifications in which certain cell types become more preferential. Furthermore, there appears to be a greater, though often less considered, contribution to differentiation propensity by factors such as mitochondria and inherent genetic diversity. Varied differentiation capacity can also lead to potential consequences in the clinical translation of hPSC, including the occurrence of residual undifferentiated stem cells, and the transplantation of potentially transformed cells.

WIDER IMPLICATIONS

As hPSC continue to advance towards the clinic, our understanding of them progresses as well. As a result, the challenges faced become more numerous, but also more clear. If the transition to the clinic is to be achieved with a minimum number of potential setbacks, thorough evaluation of the cells will be an absolute necessity. Altered differentiation propensity represents at least one such hurdle, for which researchers and eventually clinicians will need to find solutions. Already, steps are being taken to tackle the issue, though further research will be required to evaluate any long-term risks it poses.

Mitochondrial DNA Heteroplasmy and Purifying Selection in the Mammalian Female Germ Line

Inherited mutations in the mitochondrial (mt)DNA are a major cause of human disease, with approximately 1 in 5000 people affected by one of the hundreds of identified pathogenic mtDNA point mutations or deletions. Due to the severe, and often untreatable, symptoms of many mitochondrial diseases, identifying how these mutations are inherited from one generation to the next has been an area of intense research in recent years. Despite large advances in our understanding of this complex process, many questions remain unanswered, with one of the most hotly debated being whether or not purifying selection acts against pathogenic mutations during germline development.

Targeted elimination of mutant mitochondrial DNA in MELAS-iPSCs by mitoTALENs

Mitochondrial diseases are maternally inherited heterogeneous disorders that are primarily caused by mitochondrial DNA (mtDNA) mutations. Depending on the ratio of mutant to wild-type mtDNA, known as heteroplasmy, mitochondrial defects can result in a wide spectrum of clinical manifestations. Mitochondria-targeted endonucleases provide an alternative avenue for treating mitochondrial disorders via targeted destruction of the mutant mtDNA and induction of heteroplasmic shifting. Here, we generated mitochondrial disease patient-specific induced pluripotent stem cells (MiPSCs) that harbored a high proportion of m.3243A>G mtDNA mutations and caused mitochondrial encephalomyopathy and stroke-like episodes (MELAS). We engineered mitochondrial-targeted transcription activator-like effector nucleases (mitoTALENs) and successfully eliminated the m.3243A>G mutation in MiPSCs. Off-target mutagenesis was not detected in the targeted MiPSC clones. Utilizing a dual fluorescence iPSC reporter cell line expressing a 3243G mutant mtDNA sequence in the nuclear genome, mitoTALENs displayed a significantly limited ability to target the nuclear genome compared with nuclear-localized TALENs. Moreover, genetically rescued MiPSCs displayed normal mitochondrial respiration and energy production. Moreover, neuronal progenitor cells differentiated from the rescued MiPSCs also demonstrated normal metabolic profiles. Furthermore, we successfully achieved reduction in the human m.3243A>G mtDNA mutation in porcine oocytes via injection of mitoTALEN mRNA. Our study shows the great potential for using mitoTALENs for specific targeting of mutant mtDNA both in iPSCs and mammalian oocytes, which not only provides a new avenue for studying mitochondrial biology and disease but also suggests a potential therapeutic approach for the treatment of mitochondrial disease, as well as the prevention of germline transmission of mutant mtDNA.

Comparative performance analysis of human iPSC-derived and primary neural progenitor cells (NPC) grown as neurospheres in vitro

Developmental neurotoxicity (DNT) testing performed in rats is resource-intensive (costs, time, animals) and bears the issue of species extrapolation. Thus, reliable alternative human-based approaches are needed for predicting neurodevelopmental toxicity. Human induced pluripotent stem cells (hiPSCs) represent a basis for an alternative method possibly being part of an alternative DNT testing strategy. Here, we compared two hiPSC neural induction protocols resulting in 3D neurospheres: one using noggin and one cultivating cells in neural induction medium (NIM protocol). Performance of Nestin⁺/SOX2⁺ hiPSC-derived neural progenitor cells (NPCs) was compared to primary human NPCs. Generally, primary hNPCs first differentiate into Nestin⁺ and/or GFAP⁺ radial glia-like cells, while the hiPSC-derived NPCs (hiPSC-NPC) first differentiate into βIII-Tubulin⁺ neurons suggesting an earlier developmental stage of hiPSC-NPC. In the 'Neurosphere Assay', NIM generated hiPSC-NPC produced neurons with higher performance than with the noggin protocol. After long-term differentiation, hiPSC-NPC form neuronal networks, which become electrically active on microelectrode arrays after 85days. Finally, methylmercury chloride inhibits hiPSC-NPC and hNPC migration with similar potencies. hiPSC-NPCs-derived neurospheres seem to be useful for DNT evaluation representing early neural development in vitro. More system characterization by compound testing is needed to gain higher confidence in this method.

May I Cut in? Gene Editing Approaches in Human Induced Pluripotent Stem Cells

In the decade since Yamanaka and colleagues described methods to reprogram somatic cells into a pluripotent state, human induced pluripotent stem cells (hiPSCs) have demonstrated tremendous promise in numerous disease modeling, drug discovery, and regenerative medicine applications. More recently, the development and refinement of advanced gene transduction and editing technologies have further accelerated the potential of hiPSCs. In this review, we discuss the various gene editing technologies that are being implemented with hiPSCs. Specifically, we describe the emergence of technologies including zinc-finger nuclease (ZFN), transcription activator-like effector nuclease (TALEN), and clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 that can be used to edit the genome at precise locations, and discuss the strengths and weaknesses of each of these technologies. In addition, we present the current applications of these technologies in elucidating the mechanisms of human development and disease, developing novel and effective therapeutic molecules, and engineering cell-based therapies. Finally, we discuss the emerging technological advances in targeted gene editing methods.

Low dose resveratrol ameliorates mitochondrial respiratory dysfunction and enhances cellular reprogramming

Mitochondrial disease is associated with a wide variety of clinical presentations, even among patients carrying heteroplasmic mitochondrial DNA (mtDNA) mutations, probably because of variations in mutant mtDNA proportions at the tissue and organ levels. Although several case reports and clinical trials have assessed the effectiveness of various types of drugs and supplements for the treatment of mitochondrial diseases, there are currently no cures for these conditions. In this study, we demonstrated for the first time that low dose resveratrol (RSV) ameliorated mitochondrial respiratory dysfunction in patient-derived fibroblasts carrying homoplasmic mtDNA mutations. Furthermore, low dose RSV also facilitated efficient cellular reprogramming of the patient-derived fibroblasts into induced pluripotent stem cells, partly due to improved cellular viability. Our results highlight the potential of RSV as a new therapeutic drug candidate for the treatment of mitochondrial diseases.

Mitochondrial respiratory dysfunction disturbs neuronal and cardiac lineage commitment of human iPSCs

Mitochondrial diseases are genetically heterogeneous and present a broad clinical spectrum among patients; in most cases, genetic determinants of mitochondrial diseases are heteroplasmic mitochondrial DNA (mtDNA) mutations. However, it is uncertain whether and how heteroplasmic mtDNA mutations affect particular cellular fate-determination processes, which are closely associated with the cell-type-specific pathophysiology of mitochondrial diseases. In this study, we established two isogenic induced pluripotent stem cell (iPSC) lines each carrying different proportions of a heteroplasmic m.3243A>G mutation from the same patient; one exhibited apparently normal and the other showed most likely impaired mitochondrial respiratory function. Low proportions of m.3243A>G exhibited no apparent molecular pathogenic influence on directed differentiation into neurons and cardiomyocytes, whereas high proportions of m.3243A>G showed both induced neuronal cell death and inhibited cardiac lineage commitment. Such neuronal and cardiac maturation defects were also confirmed using another patient-derived iPSC line carrying quite high proportion of m.3243A>G. In conclusion, mitochondrial respiratory dysfunction strongly inhibits maturation and survival of iPSC-derived neurons and cardiomyocytes; our presenting data also suggest that appropriate mitochondrial maturation actually contributes to cellular fate-determination processes during development.

Respiratory Chain Complex Disorganization Impairs Mitochondrial and Cellular Integrity: Phenotypic Variation in Cytochrome c Oxidase Deficiency

The relationships between the molecular abnormalities in mitochondrial respiratory chain complexes and their negative contributions to mitochondrial and cellular functions have been proved to be essential for better understandings in mitochondrial medicine. Herein, we established the method to identify disease phenotypic differences among patients with muscle histopathological cytochrome c oxidase (COX) deficiency, as one of the representative clinical features in mitochondrial diseases, by using patients' myoblasts that are derived from biopsied skeletal muscle tissues. We identified two obviously different severities in molecular diagnostic criteria of COX deficiency among patients: structurally stable, but functionally mild/moderate defect and severe functional defect with the disrupted COX holoenzyme structure. COX holoenzyme disorganization actually triggered several mitochondrial dysfunctions, including the decreased ATP level, the increased oxidative stress level, and the damaged membrane potential level, all of which lead to the deteriorated cellular growth, the accelerated cellular senescence, and the induced apoptotic cell death. Our cell-based in vitro diagnostic approaches would be widely applicable to understanding patient-specific pathomechanism in various types of mitochondrial diseases, including other respiratory chain complex deficiencies and other mitochondrial metabolic enzyme deficiencies.

The genetics and pathology of mitochondrial disease

Mitochondria are double-membrane-bound organelles that are present in all nucleated eukaryotic cells and are responsible for the production of cellular energy in the form of ATP. Mitochondrial function is under dual genetic control - the 16.6-kb mitochondrial genome, with only 37 genes, and the nuclear genome, which encodes the remaining ∼1300 proteins of the mitoproteome. Mitochondrial dysfunction can arise because of defects in either mitochondrial DNA or nuclear mitochondrial genes, and can present in childhood or adulthood in association with vast clinical heterogeneity, with symptoms affecting a single organ or tissue, or multisystem involvement. There is no cure for mitochondrial disease for the vast majority of mitochondrial disease patients, and a genetic diagnosis is therefore crucial for genetic counselling and recurrence risk calculation, and can impact on the clinical management of affected patients. Next-generation sequencing strategies are proving pivotal in the discovery of new disease genes and the diagnosis of clinically affected patients; mutations in >250 genes have now been shown to cause mitochondrial disease, and the biochemical, histochemical, immunocytochemical and neuropathological characterization of these patients has led to improved diagnostic testing strategies and novel diagnostic techniques. This review focuses on the current genetic landscape associated with mitochondrial disease, before focusing on advances in studying associated mitochondrial pathology in two, clinically relevant organs - skeletal muscle and brain. © 2016 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Review: Central nervous system involvement in mitochondrial disease

Mitochondrial respiratory chain defects are an important cause of inherited disorders affecting approximately 1 in 5000 people in the UK population. Collectively these disorders are termed ‘mitochondrial diseases’ and they result from either mitochondrial DNA mutations or defects in nuclear DNA. Although they are frequently multisystem disorders, neurological deficits are particularly common, wide‐ranging and disabling for patients. This review details the manifold neurological impairments associated with mitochondrial disease, and describes the efforts to understand how they arise and progressively worsen in patients with mitochondrial disease. We describe advances in our understanding of disease pathogenesis through detailed neuropathological studies and how this has spurred the development of cellular and animal models of disease. We underscore the importance of continued clinical, molecular genetic, neuropathological and animal model studies to fully characterize mitochondrial diseases and understand mechanisms of neurodegeneration. These studies are instrumental for the next phase of mitochondrial research that has a particular emphasis on finding novel ways to treat mitochondrial disease to improve patient care and quality of life.