In vitro supplementation with deoxynucleoside monophosphates rescues mitochondrial DNA depletion

Guanylate Kinase 1 Deficiency: A Novel and Potentially Treatable Mitochondrial DNA Depletion/Deletions Disease

OBJECTIVE

Mitochondrial DNA (mtDNA) depletion/deletions syndrome (MDDS) comprises a group of diseases caused by primary autosomal defects of mtDNA maintenance. Our objective was to study the etiology of MDDS in 4 patients who lack pathogenic variants in known genetic causes.

METHODS

Whole exome sequencing of the probands was performed to identify pathogenic variants. We validated the mitochondrial defect by analyzing mtDNA, mitochondrial dNTP pools, respiratory chain activities, and GUK1 activity. To confirm pathogenicity of GUK1 deficiency, we expressed 2 GUK1 isoforms in patient cells.

RESULTS

We identified biallelic GUK1 pathogenic variants in all 4 probands who presented with ptosis, ophthalmoparesis, and myopathic proximal limb weakness, as well as variable hepatopathy and altered T-lymphocyte profiles. Muscle biopsies from all probands showed mtDNA depletion, deletions, or both, as well as reduced activities of mitochondrial respiratory chain enzymes. GUK1 encodes guanylate kinase, originally identified as a cytosolic enzyme. Long and short isoforms of GUK1 exist. We observed that the long isoform is intramitochondrial and the short is cytosolic. In probands' fibroblasts, we noted decreased GUK1 activity causing unbalanced mitochondrial dNTP pools and mtDNA depletion in both replicating and quiescent fibroblasts indicating that GUK1 deficiency impairs de novo and salvage nucleotide pathways. Proband fibroblasts treated with deoxyguanosine and/or forodesine, a purine phosphatase inhibitor, ameliorated mtDNA depletion, indicating potential pharmacological therapies.

INTERPRETATION

Primary GUK1 deficiency is a new and potentially treatable cause of MDDS. The cytosolic isoform of GUK1 may contribute to the T-lymphocyte abnormality, which has not been observed in other MDDS disorders. ANN NEUROL 2024;96:1209-1224.

Nucleoside supplements as treatments for mitochondrial DNA depletion syndrome

Introduction: In mitochondrial DNA (mtDNA) depletion syndrome (MDS), patients cannot maintain sufficient mtDNA for their energy needs. MDS presentations range from infantile encephalopathy with hepatopathy (Alpers syndrome) to adult chronic progressive external ophthalmoplegia. Most are caused by nucleotide imbalance or by defects in the mtDNA replisome. There is currently no curative treatment available. Nucleoside therapy is a promising experimental treatment for TK2 deficiency, where patients are supplemented with exogenous deoxypyrimidines. We aimed to explore the benefits of nucleoside supplementation in POLG and TWNK deficient fibroblasts. Methods: We used high-content fluorescence microscopy with software-based image analysis to assay mtDNA content and membrane potential quantitatively, using vital dyes PicoGreen and MitoTracker Red CMXRos respectively. We tested the effect of 15 combinations (A, T, G, C, AT, AC, AG, CT, CG, GT, ATC, ATG, AGC, TGC, ATGC) of deoxynucleoside supplements on mtDNA content of fibroblasts derived from four patients with MDS (POLG1, POLG2, DGUOK, TWNK) in both a replicating (10% dialysed FCS) and quiescent (0.1% dialysed FCS) state. We used qPCR to measure mtDNA content of supplemented and non-supplemented fibroblasts following mtDNA depletion using 20 µM ddC and after 14- and 21-day recovery in a quiescent state. Results: Nucleoside treatments at 200 µM that significantly increased mtDNA content also significantly reduced the number of cells remaining in culture after 7 days of treatment, as well as mitochondrial membrane potential. These toxic effects were abolished by reducing the concentration of nucleosides to 50 µM. In POLG1 and TWNK cells the combination of ATGC treatment increased mtDNA content the most after 7 days in non-replicating cells. ATGC nucleoside combination significantly increased the rate of mtDNA recovery in quiescent POLG1 cells following mtDNA depletion by ddC. Conclusion: High-content imaging enabled us to link mtDNA copy number with key read-outs linked to patient wellbeing. Elevated G increased mtDNA copy number but severely impaired fibroblast growth, potentially by inhibiting purine synthesis and/or causing replication stress. Combinations of nucleosides ATGC, T, or TC, benefited growth of cells harbouring POLG mutations. These combinations, one of which reflects a commercially available preparation, could be explored further for treatment of POLG patients.

Deoxyribonucleoside treatment rescues EtBr-induced mtDNA depletion in iPSC-derived neural stem cells with POLG mutations

Mutations in POLG, the gene encoding the catalytic subunit of the mitochondrial DNA (mtDNA) polymerase gamma (Pol-γ), lead to diseases driven by defective mtDNA maintenance. Despite being the most prevalent cause of mitochondrial disease, treatments for POLG-related disorders remain elusive. In this study, we used POLG patient-induced pluripotent stem cell (iPSC)-derived neural stem cells (iNSCs), one homozygous for the POLG mutation c.2243G>C and one compound heterozygous with c.2243G>C and c.1399G>A, and treated these iNSCs with ethidium bromide (EtBr) to study the rate of depletion and repopulation of mtDNA. In addition, we investigated the effect of deoxyribonucleoside (dNs) supplementation on mtDNA maintenance during EtBr treatment and post-treatment repopulation in the same cells. EtBr-induced mtDNA depletion occurred at a similar rate in both patient and control iNSCs, however, restoration of mtDNA levels was significantly delayed in iNSCs carrying the compound heterozygous POLG mutations. In contrast, iNSC with the homozygous POLG mutation recovered their mtDNA at a rate similar to controls. When we treated cells with dNs, we found that this reduced EtBr-induced mtDNA depletion and significantly increased repopulation rates in both patient iNSCs. These observations are consistent with the hypothesis that mutations in POLG impair mtDNA repopulation also within intact neural lineage cells and suggest that those with compound heterozygous mutation have a more severe defect of mtDNA synthesis. Our findings further highlight the potential for dNs to improve mtDNA replication in the presence of POLG mutations, suggesting that this may offer a new therapeutic modality for mitochondrial diseases caused by disturbed mtDNA homeostasis.

Drug Drop Test: How to Quickly Identify Potential Therapeutic Compounds for Mitochondrial Diseases Using Yeast Saccharomyces cerevisiae

Mitochondrial diseases (MDs) refer to a group of clinically and genetically heterogeneous pathologies characterized by defective mitochondrial function and energy production. Unfortunately, there is no effective treatment for most MDs, and current therapeutic management is limited to relieving symptoms. The yeast Saccharomyces cerevisiae has been efficiently used as a model organism to study mitochondria-related disorders thanks to its easy manipulation and well-known mitochondrial biogenesis and metabolism. It has been successfully exploited both to validate alleged pathogenic variants identified in patients and to discover potential beneficial molecules for their treatment. The so-called "drug drop test", a phenotype-based high-throughput screening, especially if coupled with a drug repurposing approach, allows the identification of molecules with high translational potential in a cost-effective and time-saving manner. In addition to drug identification, S. cerevisiae can be used to point out the drug's target or pathway. To date, drug drop tests have been successfully carried out for a variety of disease models, leading to very promising results. The most relevant aspect is that studies on more complex model organisms confirmed the effectiveness of the drugs, strengthening the results obtained in yeast and demonstrating the usefulness of this screening as a novel approach to revealing new therapeutic molecules for MDs.

Experimental therapy for mitochondrial diseases

Mitochondrial diseases are extremely heterogeneous genetic disorders due to faulty oxidative phosphorylation (OxPhos). No cure is currently available for these conditions, beside supportive interventions aimed at relieving complications. Mitochondria are under a double genetic control carried out by the mitochondrial DNA (mtDNA) and by nuclear DNA. Thus, not surprisingly, mutations in either genome can cause mitochondrial disease. Although mitochondria are usually associated with respiration and ATP synthesis, they play fundamental roles in a large number of other biochemical, signaling, and execution pathways, each being a potential target for therapeutic interventions. These can be classified as general therapies, i.e., potentially applicable to a number of different mitochondrial conditions, or therapies tailored to a single disease, i.e., personalized approaches, such as gene therapy, cell therapy, and organ replacement. Mitochondrial medicine is a particularly lively research field, and the last few years witnessed a steady increase in the number of clinical applications. This chapter will present the most recent therapeutic attempts emerged from preclinical work and an update of the currently ongoing clinical applications. We think that we are starting a new era in which the etiologic treatment of these conditions is becoming a realistic option.

Mitochondrial DNA maintenance defects: potential therapeutic strategies

Mitochondrial DNA (mtDNA) replication depends on the mitochondrial import of hundreds of nuclear encoded proteins that control the mitochondrial genome maintenance and integrity. Defects in these processes result in an expanding group of disorders called mtDNA maintenance defects that are characterized by mtDNA depletion and/or multiple mtDNA deletions with variable phenotypic manifestations. As it applies for mitochondrial disorders in general, current treatment options for mtDNA maintenance defects are limited. Lately, with the development of model organisms, improved understanding of the pathophysiology of these disorders, and a better knowledge of their natural history, the number of preclinical studies and existing and planned clinical trials has been increasing. In this review, we discuss recent preclinical studies and current and future clinical trials concerning potential therapeutic options for the different mtDNA maintenance defects.

Entecavir competitively inhibits deoxyguanosine and deoxyadenosine phosphorylation in isolated mitochondria and the perfused rat heart

Deoxyguanosine kinase (dGK) is reported responsible for the phosphorylation of deoxyadenosine (dA) and deoxyguanosine (dG) in the mitochondrial purine salvage pathway. Antiviral nucleoside analogs known as nucleoside reverse transcriptase inhibitors (NRTIs) must be phosphorylated by host enzymes for the analog to become active. We address the possibility that NRTI purine analogs may be competitive inhibitors of dGK. From a group of such analogs, we demonstrate that entecavir (ETV) competitively inhibited the phosphorylation of dG and dA in rat mitochondria. Mitochondria from the brain, heart, kidney, and liver showed a marked preference for phosphorylation of dG over dA (10-30-fold) and ETV over dA (2.5-4-fold). We found that ETV inhibited the phosphorylation of dG with an IC50 of 15.3 ± 2.2 μM and that ETV and dG were both potent inhibitors of dA phosphorylation with IC50s of 0.034 ± 0.007 and 0.028 ± 0.006 μM, respectively. In addition, the phosphorylation of dG and ETV followed Michaelis-Menten kinetics and each competitively inhibited the phosphorylation of the other. We observed that the kinetics of dA phosphorylation were strikingly different from those of dG phosphorylation, with an exponentially lower affinity for dGK and no effect of dA on dG or ETV phosphorylation. Finally, in an isolated heart perfusion model, we demonstrated that dG, dA, and ETV were phosphorylated and dG phosphorylation was inhibited by ETV. Taken together, these data demonstrate that dGK is inhibited by ETV and that the primary role of dGK is in the phosphorylation of dG rather than dA.

Saccharomyces cerevisiae as a Tool for Studying Mutations in Nuclear Genes Involved in Diseases Caused by Mitochondrial DNA Instability

Mitochondrial DNA (mtDNA) maintenance is critical for oxidative phosphorylation (OXPHOS) since some subunits of the respiratory chain complexes are mitochondrially encoded. Pathological mutations in nuclear genes involved in the mtDNA metabolism may result in a quantitative decrease in mtDNA levels, referred to as mtDNA depletion, or in qualitative defects in mtDNA, especially in multiple deletions. Since, in the last decade, most of the novel mutations have been identified through whole-exome sequencing, it is crucial to confirm the pathogenicity by functional analysis in the appropriate model systems. Among these, the yeast Saccharomyces cerevisiae has proved to be a good model for studying mutations associated with mtDNA instability. This review focuses on the use of yeast for evaluating the pathogenicity of mutations in six genes, MPV17/SYM1, MRM2/MRM2, OPA1/MGM1, POLG/MIP1, RRM2B/RNR2, and SLC25A4/AAC2, all associated with mtDNA depletion or multiple deletions. We highlight the techniques used to construct a specific model and to measure the mtDNA instability as well as the main results obtained. We then report the contribution that yeast has given in understanding the pathogenic mechanisms of the mutant variants, in finding the genetic suppressors of the mitochondrial defects and in the discovery of molecules able to improve the mtDNA stability.

A Yeast-Based Repurposing Approach for the Treatment of Mitochondrial DNA Depletion Syndromes Led to the Identification of Molecules Able to Modulate the dNTP Pool

Mitochondrial DNA depletion syndromes (MDS) are clinically heterogenous and often severe diseases, characterized by a reduction of the number of copies of mitochondrial DNA (mtDNA) in affected tissues. In the context of MDS, yeast has proved to be both an excellent model for the study of the mechanisms underlying mitochondrial pathologies and for the discovery of new therapies via high-throughput assays. Among the several genes involved in MDS, it has been shown that recessive mutations in MPV17 cause a hepatocerebral form of MDS and Navajo neurohepatopathy. MPV17 encodes a non selective channel in the inner mitochondrial membrane, but its physiological role and the nature of its cargo remains elusive. In this study we identify ten drugs active against MPV17 disorder, modelled in yeast using the homologous gene SYM1. All ten of the identified molecules cause a concomitant increase of both the mitochondrial deoxyribonucleoside triphosphate (mtdNTP) pool and mtDNA stability, which suggests that the reduced availability of DNA synthesis precursors is the cause for the mtDNA deletion and depletion associated with Sym1 deficiency. We finally evaluated the effect of these molecules on mtDNA stability in two other MDS yeast models, extending the potential use of these drugs to a wider range of MDS patients.

Therapy Prospects for Mitochondrial DNA Maintenance Disorders

Mitochondrial DNA depletion and multiple deletions syndromes (MDDS) constitute a group of mitochondrial diseases defined by dysfunctional mitochondrial DNA (mtDNA) replication and maintenance. As is the case for many other mitochondrial diseases, the options for the treatment of these disorders are rather limited today. Some aggressive treatments such as liver transplantation or allogeneic stem cell transplantation are among the few available options for patients with some forms of MDDS. However, in recent years, significant advances in our knowledge of the biochemical pathomechanisms accounting for dysfunctional mtDNA replication have been achieved, which has opened new prospects for the treatment of these often fatal diseases. Current strategies under investigation to treat MDDS range from small molecule substrate enhancement approaches to more complex treatments, such as lentiviral or adenoassociated vector-mediated gene therapy. Some of these experimental therapies have already reached the clinical phase with very promising results, however, they are hampered by the fact that these are all rare disorders and so the patient recruitment potential for clinical trials is very limited.

The nucleotide prodrug CERC-913 improves mtDNA content in primary hepatocytes from DGUOK-deficient rats

Loss-of-function mutations in the deoxyguanosine kinase (DGUOK) gene result in a mitochondrial DNA (mtDNA) depletion syndrome. DGUOK plays an important role in converting deoxyribonucleosides to deoxyribonucleoside monophosphates via the salvage pathway for mtDNA synthesis. DGUOK deficiency manifests predominantly in the liver; the most common cause of death is liver failure within the first year of life and no therapeutic options are currently available. in vitro supplementation with deoxyguanosine or deoxyguanosine monophosphate (dGMP) were reported to rescue mtDNA depletion in DGUOK-deficient, patient-derived fibroblasts and myoblasts. CERC-913, a novel ProTide prodrug of dGMP, was designed to bypass defective DGUOK while improving permeability and stability relative to nucleoside monophosphates. To evaluate CERC-913 for its ability to rescue mtDNA depletion, we developed a primary hepatocyte culture model using liver tissue from DGUOK-deficient rats. DGUOK knockout rat hepatocyte cultures exhibit severely reduced mtDNA copy number (~10%) relative to wild type by qPCR and mtDNA content remains stable for up to 8 days in culture. CERC-913 increased mtDNA content in DGUOK-deficient hepatocytes up to 2.4-fold after 4 days of treatment in a dose-dependent fashion, which was significantly more effective than dGMP at similar concentrations. These early results suggest primary hepatocyte culture is a useful model for the study of mtDNA depletion syndromes and that CERC-913 treatment can improve mtDNA content in this model.

Strategies for fighting mitochondrial diseases

Mitochondrial diseases are extremely heterogeneous genetic conditions characterized by faulty oxidative phosphorylation (OXPHOS). OXPHOS deficiency can be the result of mutation in mtDNA genes, encoding either proteins (13 subunits of the mitochondrial complexes I, III, IV and V) or the tRNA and rRNA components of the in situ mtDNA translation. The remaining mitochondrial disease genes are in the nucleus, encoding proteins with a huge variety of functions, from structural subunits of the mitochondrial complexes, to factors involved in their formation and regulation, components of the mtDNA replication and expression machinery, biosynthetic enzymes for the biosynthesis or incorporation of prosthetic groups, components of the mitochondrial quality control and proteostasis, enzymes involved in the clearance of toxic compounds, factors involved in the formation of the lipid milieu, etc. These different functions represent potential targets for 'general' therapeutic interventions, as they may be adapted to a number of different mitochondrial conditions. This is in contrast with 'tailored', personalized therapeutic approaches, such as gene therapy, cell therapy and organ replacement, that can be useful only for individual conditions. This review will present the most recent concepts emerged from preclinical work and the attempts to translate them into the clinics. The common notion that mitochondrial disorders have no cure is currently challenged by a massive effort of scientists and clinicians, and we do expect that thanks to this intensive investigation work and tangible results for the development of strategies amenable to the treatment of patients with these tremendously difficult conditions are not so far away.

Mitochondrial DNA Depletion Syndromes

Mitochondrial disorders present in a myriad of ways, which causes them to be included in the differential diagnosis for many patients with undiagnosed disease. A subset of mitochondrial disorders are caused by intrinsic defects in the mitochondrial replication machinery, leading to loss of cellular mitochondrial content over time. The diagnosis of mitochondrial disease is complex. Several best-practice guides are available that enable a higher likelihood of detecting a mitochondrial disorder. The application of genomic sequencing and advanced physiologic analysis of the electron transport chain can offer more definitive evidence of mitochondrial dysfunction.

Increased dNTP pools rescue mtDNA depletion in human POLG-deficient fibroblasts

Polymerase γ catalytic subunit (POLG) gene encodes the enzyme responsible for mitochondrial DNA (mtDNA) synthesis. Mutations affecting POLG are the most prevalent cause of mitochondrial disease because of defective mtDNA replication and lead to a wide spectrum of clinical phenotypes characterized by mtDNA deletions or depletion. Enhancing mitochondrial deoxyribonucleoside triphosphate (dNTP) synthesis effectively rescues mtDNA depletion in different models of defective mtDNA maintenance due to dNTP insufficiency. In this study, we studied mtDNA copy number recovery rates following ethidium bromide-forced depletion in quiescent fibroblasts from patients harboring mutations in different domains of POLG. Whereas control cells spontaneously recovered initial mtDNA levels, POLG-deficient cells experienced a more severe depletion and could not repopulate mtDNA. However, activation of deoxyribonucleoside (dN) salvage by supplementation with dNs plus erythro-9-(2-hydroxy-3-nonyl) adenine (inhibitor of deoxyadenosine degradation) led to increased mitochondrial dNTP pools and promoted mtDNA repopulation in all tested POLG-mutant cells independently of their specific genetic defect. The treatment did not compromise POLG fidelity because no increase in multiple deletions or point mutations was detected. Our study suggests that physiologic dNTP concentration limits the mtDNA replication rate. We thus propose that increasing mitochondrial dNTP availability could be of therapeutic interest for POLG deficiency and other conditions in which mtDNA maintenance is challenged.-Blázquez-Bermejo, C., Carreño-Gago, L., Molina-Granada, D., Aguirre, J., Ramón, J., Torres-Torronteras, J., Cabrera-Pérez, R., Martín, M. Á., Domínguez-González, C., de la Cruz, X., Lombès, A., García-Arumí, E., Martí, R., Cámara, Y. Increased dNTP pools rescue mtDNA depletion in human POLG-deficient fibroblasts.

Nucleoside supplementation modulates mitochondrial DNA copy number in the dguok -/- zebrafish

Deoxyguanosine kinase (dGK) is an essential rate-limiting component of the mitochondrial purine nucleotide salvage pathway, encoded by the nuclear gene encoding deoxyguanosine kinase (DGUOK). Mutations in DGUOK lead to mitochondrial DNA (mtDNA) depletion typically in the liver and brain, causing a hepatocerebral phenotype. Previous work has shown that in cultured DGUOK patient cells it is possible to rescue mtDNA depletion by increasing substrate amounts for dGK. In this study we developed a mutant dguok zebrafish (Danio rerio) line using CRISPR/Cas9 mediated mutagenesis; dguok-/- fish have significantly reduced mtDNA levels compared with wild-type (wt) fish. When supplemented with only one purine nucleoside (dGuo), mtDNA copy number in both mutant and wt juvenile animals was significantly reduced, contrasting with previous cell culture studies, possibly because of nucleotide pool imbalance. However, in adult dguok-/- fish we detected a significant increase in liver mtDNA copy number when supplemented with both purine nucleosides. This study further supports the idea that nucleoside supplementation has a potential therapeutic benefit in mtDNA depletion syndromes by substrate enhancement of the purine nucleoside salvage pathway and might improve the liver pathology in patients.

Towards a therapy for mitochondrial disease: an update

Preclinical work aimed at developing new therapies for mitochondrial diseases has recently given new hopes and opened unexpected perspectives for the patients affected by these pathologies. In contrast, only minor progresses have been achieved so far in the translation into the clinics. Many challenges are still ahead, including the need for a better characterization of the pharmacological effects of the different approaches and the design of appropriate clinical trials with robust outcome measures for this extremely heterogeneous, rare, and complex group of disorders. In this review, we will discuss the most important achievements and the major challenges in this very dynamic research field.

Combined use of Saccharomyces cerevisiae, Caenorhabditis elegans and patient fibroblasts leads to the identification of clofilium tosylate as a potential therapeutic chemical against POLG-related diseases

Mitochondria are organelles that have their own DNA (mitochondrial DNA, mtDNA) whose maintenance is necessary for the majority of ATP production in eukaryotic cells. Defects in mtDNA maintenance or integrity are responsible for numerous diseases. The DNA polymerase γ (POLG) ensures proper mtDNA replication and repair. Mutations in POLG are a major cause of mitochondrial disorders including hepatic insufficiency, Alpers syndrome, progressive external ophthalmoplegia, sensory neuropathy and ataxia. Mutations in POLG are also associated with parkinsonism. To date, no effective therapy is available. Based on the conservation of mitochondrial function from yeast to human, we used Saccharomyces cerevisiae and Caenorhabditis elegans as first pass filters to identify a chemical that suppresses mtDNA instability in cultured fibroblasts of a POLG-deficient patient. We showed that this unsuspected compound, clofilium tosylate (CLO), belonging to a class of anti-arrhythmic agents, prevents mtDNA loss of all yeast mitochondrial polymerase mutants tested, improves behavior and mtDNA content of polg-1-deficient worms and increases mtDNA content of quiescent POLG-deficient fibroblasts. Furthermore, the mode of action of the drug seems conserved as CLO increases POLG steady-state level in yeast and human cells. Two other anti-arrhythmic agents (FDA-approved) sharing common pharmacological properties and chemical structure also show potential benefit for POLG deficiency in C. elegans. Our findings provide evidence of the first mtDNA-stabilizing compound that may be an effective pharmacological alternative for the treatment of POLG-related diseases.

DNA polymerase γ and disease: what we have learned from yeast

Mip1 is the Saccharomyces cerevisiae DNA polymerase γ (Pol γ), which is responsible for the replication of mitochondrial DNA (mtDNA). It belongs to the family A of the DNA polymerases and it is orthologs to human POLGA. In humans, mutations in POLG(1) cause many mitochondrial pathologies, such as progressive external ophthalmoplegia (PEO), Alpers' syndrome, and ataxia-neuropathy syndrome, all of which present instability of mtDNA, which results in impaired mitochondrial function in several tissues with variable degrees of severity. In this review, we summarize the genetic and biochemical knowledge published on yeast mitochondrial DNA polymerase from 1989, when the MIP1 gene was first cloned, up until now. The role of yeast is particularly emphasized in (i) validating the pathological mutations found in human POLG and modeled in MIP1, (ii) determining the molecular defects caused by these mutations and (iii) finding the correlation between mutations/polymorphisms in POLGA and mtDNA toxicity induced by specific drugs. We also describe recent findings regarding the discovery of molecules able to rescue the phenotypic defects caused by pathological mutations in Mip1, and the construction of a model system in which the human Pol γ holoenzyme is expressed in yeast and complements the loss of Mip1.

Emerging concepts in the therapy of mitochondrial disease

Mitochondrial disorders are an important group of genetic conditions characterized by impaired oxidative phosphorylation. Mitochondrial disorders come with an impressive variability of symptoms, organ involvement, and clinical course, which considerably impact the quality of life and quite often shorten the lifespan expectancy. Although the last 20 years have witnessed an exponential increase in understanding the genetic and biochemical mechanisms leading to disease, this has not resulted in the development of effective therapeutic approaches, amenable of improving clinical course and outcome of these conditions to any significant extent. Therapeutic options for mitochondrial diseases still remain focused on supportive interventions aimed at relieving complications. However, new therapeutic strategies have recently been emerging, some of which have shown potential efficacy at the pre-clinical level. This review will present the state of the art on experimental therapy for mitochondrial disorders.

Development of pharmacological strategies for mitochondrial disorders

Mitochondrial diseases are an unusually genetically and phenotypically heterogeneous group of disorders, which are extremely challenging to treat. Currently, apart from supportive therapy, there are no effective treatments for the vast majority of mitochondrial diseases. Huge scientific effort, however, is being put into understanding the mechanisms underlying mitochondrial disease pathology and developing potential treatments. To date, a variety of treatments have been evaluated by randomized clinical trials, but unfortunately, none of these has delivered breakthrough results. Increased understanding of mitochondrial pathways and the development of many animal models, some of which are accurate phenocopies of human diseases, are facilitating the discovery and evaluation of novel prospective treatments. Targeting reactive oxygen species has been a treatment of interest for many years; however, only in recent years has it been possible to direct antioxidant delivery specifically into the mitochondria. Increasing mitochondrial biogenesis, whether by pharmacological approaches, dietary manipulation or exercise therapy, is also currently an active area of research. Modulating mitochondrial dynamics and mitophagy and the mitochondrial membrane lipid milieu have also emerged as possible treatment strategies. Recent technological advances in gene therapy, including allotopic and transkingdom gene expression and mitochondrially targeted transcription activator-like nucleases, have led to promising results in cell and animal models of mitochondrial diseases, but most of these techniques are still far from clinical application.

Strategies for treating mitochondrial disorders: an update

Mitochondrial diseases are a heterogeneous group of disorders resulting from primary dysfunction of the respiratory chain due to both nuclear and mitochondrial DNA mutations. The wide heterogeneity of biochemical dysfunctions and pathogenic mechanisms typical of this group of diseases has hindered therapy trials; therefore, available treatment options remain limited. Therapeutic strategies aimed at increasing mitochondrial functions (by enhancing biogenesis and electron transport chain function), improving the removal of reactive oxygen species and noxious metabolites, modulating aberrant calcium homeostasis and repopulating mitochondrial DNA could potentially restore the respiratory chain dysfunction. The challenge that lies ahead is the translation of some promising laboratory results into safe and effective therapies for patients. In this review we briefly update and discuss the most feasible therapeutic approaches for mitochondrial diseases.

Administration of deoxyribonucleosides or inhibition of their catabolism as a pharmacological approach for mitochondrial DNA depletion syndrome

Mitochondrial DNA (mtDNA) depletion syndrome (MDS) is characterized by a reduction in mtDNA copy number and consequent mitochondrial dysfunction in affected tissues. A subgroup of MDS is caused by mutations in genes that disrupt deoxyribonucleotide metabolism, which ultimately leads to limited availability of one or several deoxyribonucleoside triphosphates (dNTPs), and subsequent mtDNA depletion. Here, using in vitro experimental approaches (primary cell culture of deoxyguanosine kinase-deficient cells and thymidine-induced mtDNA depletion in culture as a model of mitochondrial neurogastrointestinal encephalomyopathy, MNGIE), we show that supplements of those deoxyribonucleosides (dNs) involved in each biochemical defect (deoxyguanosine or deoxycytidine, dCtd) prevents mtDNA copy number reduction. Similar effects can be obtained by specific inhibition of dN catabolism using tetrahydrouridine (THU; inhibitor of cytidine deaminase) or immucillin H (inhibitor of purine nucleoside phosphorylase). In addition, using an MNGIE animal model, we provide evidence that mitochondrial dNTP content can be modulated in vivo by systemic administration of dCtd or THU. In spite of the severity associated with diseases due to defects in mtDNA replication, there are currently no effective therapeutic options available. Only in the case of MNGIE, allogeneic hematopoietic stem cell transplantation has proven efficient as a long-term therapeutic strategy. We propose increasing cellular availability of the deficient dNTP precursor by direct administration of the dN or inhibition of its catabolism, as a potential treatment for mtDNA depletion syndrome caused by defects in dNTP metabolism.

Mitochondrial DNA depletion syndromes: review and updates of genetic basis, manifestations, and therapeutic options

Mitochondrial DNA (mtDNA) depletion syndromes (MDS) are a genetically and clinically heterogeneous group of autosomal recessive disorders that are characterized by a severe reduction in mtDNA content leading to impaired energy production in affected tissues and organs. MDS are due to defects in mtDNA maintenance caused by mutations in nuclear genes that function in either mitochondrial nucleotide synthesis (TK2, SUCLA2, SUCLG1, RRM2B, DGUOK, and TYMP) or mtDNA replication (POLG and C10orf2). MDS are phenotypically heterogeneous and usually classified as myopathic, encephalomyopathic, hepatocerebral or neurogastrointestinal. Myopathic MDS, caused by mutations in TK2, usually present before the age of 2 years with hypotonia and muscle weakness. Encephalomyopathic MDS, caused by mutations in SUCLA2, SUCLG1, or RRM2B, typically present during infancy with hypotonia and pronounced neurological features. Hepatocerebral MDS, caused by mutations in DGUOK, MPV17, POLG, or C10orf2, commonly have an early-onset liver dysfunction and neurological involvement. Finally, TYMP mutations have been associated with mitochondrial neurogastrointestinal encephalopathy (MNGIE) disease that typically presents before the age of 20 years with progressive gastrointestinal dysmotility and peripheral neuropathy. Overall, MDS are severe disorders with poor prognosis in the majority of affected individuals. No efficacious therapy is available for any of these disorders. Affected individuals should have a comprehensive evaluation to assess the degree of involvement of different systems. Treatment is directed mainly toward providing symptomatic management. Nutritional modulation and cofactor supplementation may be beneficial. Liver transplantation remains controversial. Finally, stem cell transplantation in MNGIE disease shows promising results.