Mitochondrial Dysfunction and Calcium Dysregulation in Leigh Syndrome Induced Pluripotent Stem Cell Derived Neurons.

Leigh syndrome (LS) is the most frequent infantile mitochondrial disorder (MD) and is characterized by neurodegeneration and astrogliosis in the basal ganglia or the brain stem. At present, there is no cure or treatment for this disease, partly due to scarcity of LS models. Current models generally fail to recapitulate important traits of the disease. Therefore, there is an urgent need to develop new human in vitro models. Establishment of induced pluripotent stem cells (iPSCs) followed by differentiation into neurons is a powerful tool to obtain an in vitro model for LS. Here, we describe the generation and characterization of iPSCs, neural stem cells (NSCs) and iPSC-derived neurons harboring the mtDNA mutation m.13513G>A in heteroplasmy. We have performed mitochondrial characterization, analysis of electrophysiological properties and calcium imaging of LS neurons. Here, we show a clearly compromised oxidative phosphorylation (OXPHOS) function in LS patient neurons. This is also the first report of electrophysiological studies performed on iPSC-derived neurons harboring an mtDNA mutation, which revealed that, in spite of having identical electrical properties, diseased neurons manifested mitochondrial dysfunction together with a diminished calcium buffering capacity. This could lead to an overload of cytoplasmic calcium concentration and the consequent cell death observed in patients. Importantly, our results highlight the importance of calcium homeostasis in LS pathology.

The Challenge of Bringing iPSCs to the Patient.

The implementation of induced pluripotent stem cells (iPSCs) in biomedical research more than a decade ago, resulted in a huge leap forward in the highly promising area of personalized medicine. Nowadays, we are even closer to the patient than ever. To date, there are multiple examples of iPSCs applications in clinical trials and drug screening. However, there are still many obstacles to overcome. In this review, we will focus our attention on the advantages of implementing induced pluripotent stem cells technology into the clinics but also commenting on all the current drawbacks that could hinder this promising path towards the patient.

iPSCs, a Future Tool for Therapeutic Intervention in Mitochondrial Disorders: Pros and Cons.

Mitochondrial disorders, although individually are rare, taken together constitute a big group of diseases that share a defect in the oxidative phosphorylation system. Up to now, the development of therapies for these diseases is very slow and ineffective due in part to the lack of appropriate disease models. Therefore, there is an urgent need for the discovery of new therapeutic interventions. Regarding this, the generation of induced pluripotent stem cells (iPSCs) has opened new expectations in the regenerative medicine field. However, special cares and considerations must be taken into account previous to a replacement therapy. J. Cell. Physiol. 231: 2317-2318, 2016. © 2016 Wiley Periodicals, Inc.

Differences in reactive oxygen species production explain the phenotypes associated with common mouse mitochondrial DNA variants.

Common mitochondrial DNA (mtDNA) haplotypes in humans and mice have been associated with various phenotypes, including learning performance and disease penetrance. Notably, no influence of mtDNA haplotype in cell respiration has been demonstrated. Here, using cell lines carrying four different common mouse mtDNA haplotypes in an identical nuclear background, we show that the similar level of respiration among the cell lines is only apparent and is a consequence of compensatory mechanisms triggered by different production of reactive oxygen species. We observe that the respiration capacity per molecule of mtDNA in cells with the NIH3T3 or NZB mtDNA is lower than in those with the C57BL/6J, CBA/J or BALB/cJ mtDNA. In addition, we have determined the genetic element underlying these differences. Our data provide insight into the molecular basis of the complex phenotypes associated with common mtDNA variants and anticipate a relevant contribution of mtDNA single nucleotide polymorphisms to phenotypic variability in humans.

Evolution meets disease: penetrance and functional epistasis of mitochondrial tRNA mutations.

About half of the mitochondrial DNA (mtDNA) mutations causing diseases in humans occur in tRNA genes. Particularly intriguing are those pathogenic tRNA mutations than can reach homoplasmy and yet show very different penetrance among patients. These mutations are scarce and, in addition to their obvious interest for understanding human pathology, they can be excellent experimental examples to model evolution and fixation of mitochondrial tRNA mutations. To date, the only source of this type of mutations is human patients. We report here the generation and characterization of the first mitochondrial tRNA pathological mutation in mouse cells, an m.3739G>A transition in the mitochondrial mt-Ti gene. This mutation recapitulates the molecular hallmarks of a disease-causing mutation described in humans, an m.4290T>C transition affecting also the human mt-Ti gene. We could determine that the pathogenic molecular mechanism, induced by both the mouse and the human mutations, is a high frequency of abnormal folding of the tRNA(Ile) that cannot be charged with isoleucine. We demonstrate that the cells harboring the mouse or human mutant tRNA have exacerbated mitochondrial biogenesis triggered by an increase in mitochondrial ROS production as a compensatory response. We propose that both the nature of the pathogenic mechanism combined with the existence of a compensatory mechanism can explain the penetrance pattern of this mutation. This particular behavior can allow a scenario for the evolution of mitochondrial tRNAs in which the fixation of two alleles that are individually deleterious can proceed in two steps and not require the simultaneous mutation of both.

Hereditary Optic Neuropathies: A Systematic Review on the Interplay between Biomaterials and Induced Pluripotent Stem Cells.

Hereditary optic neuropathies (HONs) such as dominant optic atrophy (DOA) and Leber Hereditary Optic Neuropathy (LHON) are mitochondrial diseases characterized by a degenerative loss of retinal ganglion cells (RGCs) and are a cause of blindness worldwide. To date, there are only limited disease-modifying treatments for these disorders. The discovery of induced pluripotent stem cell (iPSC) technology has opened several promising opportunities in the field of HON research and the search for therapeutic approaches. This systematic review is focused on the two most frequent HONs (LHON and DOA) and on the recent studies related to the application of human iPSC technology in combination with biomaterials technology for their potential use in the development of RGC replacement therapies with the final aim of the improvement or even the restoration of the vision of HON patients. To this purpose, the combination of natural and synthetic biomaterials modified with peptides, neurotrophic factors, and other low- to medium-molecular weight compounds, mimicking the ocular extracellular matrices, with human iPSC or iPSC-derived cell retinal progenitors holds enormous potential to be exploited in the near future for the generation of transplantable RGC populations.

Cardiac dysfunction in mitochondrial disease. Clinical and molecular features.

BACKGROUND: Mitochondrial disorders (MD) are multisystem diseases that arise as a result of dysfunction of the oxidative phosphorylation system. The predominance of neuromuscular manifestations in MD could mask the presence of other clinical phenotypes such as cardiac dysfunction. Reported here is a retrospective study, the main objective of which was to characterize the clinical and molecular features of a cohort of patients with cardiomyopathy and MD. METHODS AND RESULTS: Hospital charts of 2,520 patients, evaluated for presumed MD were reviewed. The clinical criterion for inclusion in this study was the presence of a cardiac disturbance accompanied by a mitochondrial dysfunction. Only 71 patients met this criterion. The mitochondrial genome (mtDNA) could be sequenced only in 45 and the pathogenicity of 2 of the found changes was investigated using transmitochondrial cybrids. Three nucleotide changes in mtDNA that may be relevant and 3 with confirmed pathogenicity were identified but no mutations were found in the 13 nuclear genes analyzed. CONCLUSIONS: The mtDNA should be sequenced in patients with cardiac dysfunction accompanied by symptoms suggestive of MD; databases should be carefully and periodically screened to discard mitochondrial variants that could be associated with MD; functional assays are necessary to classify mitochondrial variants as pathogenic or polymorphic; and additional efforts must be made in order to identify nuclear genes that can explain some as yet uncharacterized molecular features of mitochondrial cardiomyopathy.

Mitochondrial haplogroups associated with end-stage heart failure and coronary allograft vasculopathy in heart transplant patients.

AIMS: Mitochondrial haplogroups are known to influence individual predisposition to a wide spectrum of metabolic and degenerative diseases, including ischaemic cardiovascular diseases. We have examined the influence of the mitochondrial DNA (mtDNA) background on the development of human end-stage heart failure (HF) in patients undergoing heart transplantation. The influence of mtDNA haplogroups on the incidence of transplant-related complications, mainly cardiac allograft vasculopathy (CAV), and on post-transplant survival was also studied. METHODS AND RESULTS: The most common mitochondrial haplogroups in European populations were genotyped in 450 heart transplant recipients, 248 heart transplant donors, and 206 healthy controls. Mitochondrial haplogroups were determined by PCR amplification of short mtDNA fragments, followed by restriction fragment length polymorphism analysis. After adjustment for age and sex the frequency of haplogroup H was significantly higher in heart transplant recipients than in controls [OR: 1.86 (95% confidence intervals, CI: 1.27-2.74), P= 0.014], and in heart donors [OR: 1.47 (95% CI: 0.99-2.19), P= 0.032]. Likewise, haplogroup Uk was found significantly more frequently among CAV patients than in non-CAV heart allograft recipients [OR: 4.1 (95% CI: 1.51-11.42), P= 0.042]. Finally, heart donor haplogroups had no influence on the morbidity or mortality after heart transplantation. CONCLUSIONS: Mitochondrial haplogroups behave like risk factors for the progress to end-stage HF in a Spanish cardiac transplant population. Mitochondrial DNA variants may have some influence on the appearance of cardiac transplant complications.

Creation of an iPSC-Based Skeletal Muscle Model of McArdle Disease Harbouring the Mutation c.2392T>C (p.Trp798Arg) in the PYGM Gene.

McArdle disease is a rare autosomal recessive condition caused by mutations in the PYGM gene. This gene encodes the skeletal muscle isoform of glycogen phosphorylase or myophosphorylase. Patients with McArdle disease have an inability to obtain energy from their muscle glycogen stores, which manifests as a marked exercise intolerance. Nowadays, there is no cure for this disorder and recommendations are intended to prevent and mitigate symptoms. There is great heterogeneity among the pathogenic variants found in the PYGM gene, and there is no obvious correlation between genotypes and phenotypes. Here, we present the generation of the first human iPSC-based skeletal muscle model harbouring the second most frequent mutation in PYGM in the Spanish population: NM_005609.4: c.2392T>C (p.Trp798Arg). To this end, iPSCs derived from a McArdle patient and a healthy control were both successfully differentiated into skeletal muscle cells using a small molecule-based protocol. The created McArdle skeletal muscle model was validated by confirming distinctive biochemical aspects of the disease such as the absence of myophosphorylase, the most typical biochemical feature of these patients. This model will be very valuable for use in future high-throughput pharmacological screenings.

MidA is a putative methyltransferase that is required for mitochondrial complex I function.

Dictyostelium and human MidA are homologous proteins that belong to a family of proteins of unknown function called DUF185. Using yeast two-hybrid screening and pull-down experiments, we showed that both proteins interact with the mitochondrial complex I subunit NDUFS2. Consistent with this, Dictyostelium cells lacking MidA showed a specific defect in complex I activity, and knockdown of human MidA in HEK293T cells resulted in reduced levels of assembled complex I. These results indicate a role for MidA in complex I assembly or stability. A structural bioinformatics analysis suggested the presence of a methyltransferase domain; this was further supported by site-directed mutagenesis of specific residues from the putative catalytic site. Interestingly, this complex I deficiency in a Dictyostelium midA(-) mutant causes a complex phenotypic outcome, which includes phototaxis and thermotaxis defects. We found that these aspects of the phenotype are mediated by a chronic activation of AMPK, revealing a possible role of AMPK signaling in complex I cytopathology.

Hereditary Optic Neuropathies: Induced Pluripotent Stem Cell-Based 2D/3D Approaches.

Inherited optic neuropathies share visual impairment due to the degeneration of retinal ganglion cells (RGCs) as the hallmark of the disease. This group of genetic disorders are caused by mutations in nuclear genes or in the mitochondrial DNA (mtDNA). An impaired mitochondrial function is the underlying mechanism of these diseases. Currently, optic neuropathies lack an effective treatment, and the implementation of induced pluripotent stem cell (iPSC) technology would entail a huge step forward. The generation of iPSC-derived RGCs would allow faithfully modeling these disorders, and these RGCs would represent an appealing platform for drug screening as well, paving the way for a proper therapy. Here, we review the ongoing two-dimensional (2D) and three-dimensional (3D) approaches based on iPSCs and their applications, taking into account the more innovative technologies, which include tissue engineering or microfluidics.

Generation of the iPSC line IISHDOi007-A from peripheral blood mononuclear cells from a patient with McArdle disease harbouring the mutation c.2392 T > C; p.Trp798Arg.

Peripheral blood mononuclear cells (PBMCs) from a McArdle patient carrying a homozygous mutation in the PYGM gene: c.2392 T > C; p.Trp798Arg were used for the generation of the human iPSC line, IISHDOi007-A. For the delivery of the reprogramming factors Oct3/4, Sox2, Klf4, and c-Myc, a non-integrative methodology that implies the use of Sendai virus has been applied.

Derivation of a human DOA iPSC line, IISHDOi006-A, with a mutation in the ACO2 gene: c.1999G>A; p.Glu667Lys.

Human iPSC line, IISHDOi006-A, was obtained from fibroblasts of a patient with Dominant Optic Atrophy (DOA) carrying a heterozygous mutation in the gene ACO2: c.1999G>A; p.Glu667Lys. Reprogramming factors Oct3/4, Sox2, Klf4, and c-Myc were delivered using a non-integrative methodology that involves the use of Sendai virus.

Derivation of an aged mouse induced pluripotent stem cell line, IISHDOi005-A.

A mouse iPSC line, IISHDOi005-A, generated from fibroblasts obtained from a mouse C57BL/6J with an age of 1 year and a half, has been obtained. For this purpose, reprogramming factors Oct3/4, Sox2, Klf4, and c-Myc were delivered using Sendai virus.

Generation of a human iPSC line, IISHDOi002-A, with a 46, XY/47, XYY mosaicism and belonging to an African mitochondrial haplogroup.

We have generated a human iPSC line, IISHDOi002-A, from commercial primary normal human dermal fibroblasts belonging to an African mitochondrial haplogroup (L3), and with a 46, XY/47, XYY mosaicism. For this purpose, reprogramming factors Oct3/4, Sox2, Klf4 and cMyc were delivered using a non-integrative methodology that involves the use of Sendai virus.

Establishment of a human iPSC line, IISHDOi004-A, from a patient with Usher syndrome associated with the mutation c.2276G>T; p.Cys759Phe in the USH2A gene.

A human iPSC line, IISHDOi004-A, from fibroblasts obtained from a patient with Usher syndrome, harboring a homozygous mutation in the USH2A gene (c.2276G>T; p.Cys759Phe) has been generated. Reprogramming factors Oct3/4, Sox2, Klf4, and c-Myc were delivered using Sendai virus.

Establishment of a human DOA 'plus' iPSC line, IISHDOi003-A, with the mutation in the OPA1 gene: c.1635C>A; p.Ser545Arg.

We have generated a human iPSC line IISHDOi003-A from fibroblasts of a patient with a dominant optic atrophy 'plus' phenotype, harbouring a heterozygous mutation, c.1635C>A; p.Ser545Arg, in the OPA1 gene. Reprogramming factors Oct3/4, Sox2, Klf4, and c-Myc were delivered using Sendai virus.

Establishment of a human iPSC line (IISHDOi001-A) from a patient with McArdle disease.

Human iPSC line IISHDOi001-A was generated from fibroblasts of a patient with McArdle disease harbouring the mutation, c.148C>T; p.Arg50Ter, in the PYGM gene. Reprogramming factors Oct3/4, Sox2, Klf4, and c-Myc were delivered using Sendai virus.

m.6267G>A: a recurrent mutation in the human mitochondrial DNA that reduces cytochrome c oxidase activity and is associated with tumors.

Complete sequencing of the mitochondrial genome of 13 cell lines derived from a variety of human cancers revealed nine novel mitochondrial DNA (mtDNA) variations. One of them, m.6267G>A, is a recurrent mutation that introduces the Ala122Thr substitution in the mitochondrially encoded cytochrome c oxidase I (MT-CO1): p.MT-CO1: Ala122Thr (GenBank: NP_536845.1). Biochemical analysis of the original cell lines and the transmitochondrial cybrids generated by transferring mitochondrial DNAs to a common nuclear background, indicate that cytochrome c oxidase (COX) activity, respiration, and growth in galactose are impaired by the m.6267G>A mutation. This mutation, found twice in the cancer cell lines included in this study, has been also encountered in one out of 63 breast cancer samples, one out of 64 colon cancer samples, one out of 260 prostate cancer samples, and in one out of 15 pancreatic cancer cell lines. In all instances the m.6267G>A mutation was associated to different mtDNA haplogroups. These findings, contrast with the extremely low frequency of the m.6267G>A mutation in the normal population (1:2264) and its apparent absence in other pathologies, strongly suggesting that the m.6267G>A missense mutation is a recurrent mutation specifically associated with cancer.

Generation of a human iPSC line from a patient with Leigh syndrome.

Human iPSC line LND554SV.3 was generated from heteroplasmic fibroblasts of a patient with Leigh syndrome carrying a mutation in the MT-ND5 gene (m.13513GNA; p.D393N). Reprogramming factors Oct3/4, Sox2, Klf4,and cMyc were delivered using a non-integrative methodology that involves the use of Sendai virus.