Ubidecarenone in the treatment of mitochondrial myopathies: a multi-center double-blind trial

Mitochondrial DNA replication and repair defects: Clinical phenotypes and therapeutic interventions

Mitochondria is a unique cellular organelle involved in multiple cellular processes and is critical for maintaining cellular homeostasis. This semi-autonomous organelle contains its circular genome - mtDNA (mitochondrial DNA), that undergoes continuous cycles of replication and repair to maintain the mitochondrial genome integrity. The majority of the mitochondrial genes, including mitochondrial replisome and repair genes, are nuclear-encoded. Although the repair machinery of mitochondria is quite efficient, the mitochondrial genome is highly susceptible to oxidative damage and other types of exogenous and endogenous agent-induced DNA damage, due to the absence of protective histones and their proximity to the main ROS production sites. Mutations in replication and repair genes of mitochondria can result in mtDNA depletion and deletions subsequently leading to mitochondrial genome instability. The combined action of mutations and deletions can result in compromised mitochondrial genome maintenance and lead to various mitochondrial disorders. Here, we review the mechanism of mitochondrial DNA replication and repair process, key proteins involved, and their altered function in mitochondrial disorders. The focus of this review will be on the key genes of mitochondrial DNA replication and repair machinery and the clinical phenotypes associated with mutations in these genes.

Secondary coenzyme Q deficiency in neurological disorders

Coenzyme Q (CoQ) is a ubiquitous lipid serving essential cellular functions. It is the only component of the mitochondrial respiratory chain that can be exogenously absorbed. Here, we provide an overview of current knowledge, controversies, and open questions about CoQ intracellular and tissue distribution, in particular in brain and skeletal muscle. We discuss human neurological diseases and mouse models associated with secondary CoQ deficiency in these tissues and highlight pharmacokinetic and anatomical challenges in exogenous CoQ biodistribution, recent improvements in CoQ formulations and imaging, as well as alternative therapeutical strategies to CoQ supplementation. The last section proposes possible mechanisms underlying secondary CoQ deficiency in human diseases with emphasis on neurological and neuromuscular disorders.

Current and Emerging Clinical Treatment in Mitochondrial Disease

Primary mitochondrial disease (PMD) is a group of complex genetic disorders that arise due to pathogenic variants in nuclear or mitochondrial genomes. Although PMD is one of the most prevalent inborn errors of metabolism, it often exhibits marked phenotypic variation and can therefore be difficult to recognise. Current treatment for PMD revolves around supportive and preventive approaches, with few disease-specific therapies available. However, over the last decade there has been considerable progress in our understanding of both the genetics and pathophysiology of PMD. This has resulted in the development of a plethora of new pharmacological and non-pharmacological therapies at varying stages of development. Many of these therapies are currently undergoing clinical trials. This review summarises the latest emerging therapies that may become mainstream treatment in the coming years. It is distinct from other recent reviews in the field by comprehensively addressing both pharmacological non-pharmacological therapy from both a bench and a bedside perspective. We highlight the current and developing therapeutic landscape in novel pharmacological treatment, dietary supplementation, exercise training, device use, mitochondrial donation, tissue replacement gene therapy, hypoxic therapy and mitochondrial base editing.

Cellular Consequences of Coenzyme Q10 Deficiency in Neurodegeneration of the Retina and Brain

Coenzyme Q10 (CoQ10) is a ubiquitous cofactor in the body, operating in the inner mitochondrial membrane, where it plays a vital role in the generation of adenosine triphosphate (ATP) through the electron transport chain (ETC). In addition to this, CoQ10 serves as an antioxidant, protecting the cell from oxidative stress by reactive oxygen species (ROS) as well as maintaining a proton (H+) gradient across lysosome membranes to facilitate the breakdown of cellular waste products. Through the process of ageing, the body becomes deficient in CoQ10, resulting in several systemic manifestations. On a cellular level, one of the consequences of CoQ10 deficiency is apoptosis, which can be visualised in tissues of the central nervous system (CNS). Diseases affecting the retina and brain such as age-related macular degeneration (AMD), glaucoma, Alzheimer's disease (AD) and Parkinson's disease (PD) have shown defects in cellular biochemical reactions attributed to reduced levels of CoQ10. Through further research into the pathogenesis of such conditions, the effects of CoQ10 deficiency can be counteracted through supplementation, early detection and intervention.

Parkin-mediated mitophagy and autophagy flux disruption in cellular models of MERRF syndrome

Mitochondrial diseases are considered rare genetic disorders characterized by defects in oxidative phosphorylation (OXPHOS). They can be provoked by mutations in nuclear DNA (nDNA) or mitochondrial DNA (mtDNA). MERRF (Myoclonic Epilepsy with Ragged-Red Fibers) syndrome is one of the most frequent mitochondrial diseases, principally caused by the m.8344A>G mutation in mtDNA, which affects the translation of all mtDNA-encoded proteins and therefore impairs mitochondrial function. In the present work, we evaluated autophagy and mitophagy flux in transmitochondrial cybrids and fibroblasts derived from a MERRF patient, reporting that Parkin-mediated mitophagy is increased in MERRF cell cultures. Our results suggest that supplementation with coenzyme Q₁₀ (CoQ), a component of the electron transport chain (ETC) and lipid antioxidant, prevents Parkin translocation to the mitochondria. In addition, CoQ acts as an enhancer of autophagy and mitophagy flux, which partially improves cell pathophysiology. The significance of Parkin-mediated mitophagy in cell survival was evaluated by silencing the expression of Parkin in MERRF cybrids. Our results show that mitophagy acts as a cell survival mechanism in mutant cells. To confirm these results in one of the main affected cell types in MERRF syndrome, mutant induced neurons (iNs) were generated by direct reprogramming of patients-derived skin fibroblasts. The treatment of MERRF iNs with Guttaquinon CoQ₁₀ (GuttaQ), a water-soluble derivative of CoQ, revealed a significant improvement in cell bioenergetics. These results indicate that iNs, along with fibroblasts and cybrids, can be utilized as reliable cellular models to shed light on disease pathomechanisms as well as for drug screening.

Metabolic Myopathies and the Respiratory System

Metabolic myopathies are a heterogeneous group of disorders characterized by inherited defects of enzymatic pathways involved in muscle cellular energetics and adenosine triphosphate synthesis. Skeletal and respiratory muscles are most affected. There are multiple mechanisms of disease. The age of onset and prognosis vary. Metabolic myopathies cause exercise intolerance, myalgia, and increase in muscle breakdown products during exercise. Some affect smooth muscle like the diaphragm and cause respiratory failure. The pathophysiology is complex and the evidence in literature to guide diagnosis and management is sparse. Treatment is limited. This review discusses the pathophysiology and diagnostic evaluation of these disorders.

Mitochondrial disease patient motivations and barriers to participate in clinical trials

Background

Clinical treatment trials are increasingly being designed in primary mitochondrial disease (PMD), a phenotypically and genetically heterogeneous collection of inherited multi- system energy deficiency disorders that lack effective therapy. We sought to identify motivating factors and barriers to clinical trial participation in PMD.

Methods

A survey study was conducted in two independent mitochondrial disease subject cohorts. A discovery cohort invited subjects with well-defined biochemical or molecularly- confirmed PMD followed at a single medical center (CHOP, n = 30/67 (45%) respondents). A replication cohort included self-identified PMD subjects in the Rare Disease Clinical Research Network (RDCRN) national contact registry (n = 290/1119 (26%) respondents). Five-point Likert scale responses were analyzed using descriptive and quantitative statistics. Experienced and prioritized symptoms for trial participation, and patient attitudes toward detailed aspects of clinical trial drug features and study design.

Results

PMD subjects experienced an average of 16 symptoms. Muscle weakness, chronic fatigue, and exercise intolerance were the lead symptoms encouraging trial participation.

Motivating trial design factors included a self-administered study drug; vitamin, antioxidant, natural or plant-derivative; pills; daily treatment; guaranteed treatment access during and after study; short travel distances; and late-stage (phase 3) participation. Relative trial participation barriers included a new study drug; discontinuation of current medications; disease progression; daily phlebotomy; and requiring participant payment. Treatment trial type or design preferences were not influenced by population age (pediatric versus adult), prior research trial experience, or disease severity.

Conclusions

These data are the first to convey clear PMD subject preferences and priorities to enable improved clinical treatment trial design that cuts across the complex diversity of disease. Partnering with rare disease patient communities is essential to effectively design robust clinical trials that engage patients and enable meaningful evaluation of emerging treatment interventions.

Mitochondrial disease patients' perception of dietary supplements' use

Surveys of mitochondrial disease physicians conducted through the Mitochondrial Medicine Society have shown that virtually all providers recommend a variety of dietary supplements as treatments to their patients in an effort to enhance energy production and reduce oxidative stress. In this survey, we asked patients and their parents about their experiences taking these dietary supplements for mitochondrial disease. The survey was disseminated through the North American Mitochondrial Disease Consortium (NAMDC) and the Rare Disease Clinical Research Network (RDCRN) registries and gathered 162 responses. The study ascertained each patient's mitochondrial disease diagnosis, dietary supplements used, adjunct therapy, and effects of the supplements on symptoms and health. Regardless of the specific underlying mitochondrial disease, the majority of the survey respondents stated they are or have been on dietary supplements. Most patients take more than four supplements primarily coenzyme Q10, l-carnitine, and riboflavin. The majority of patients taking supplements reported health benefits from the supplements. The onset of perceived benefits was between 2weeks to 3months of initiating intake. Supplements seem to be safe, with only 28% of patients experiencing mild side-effects and only 5.6% discontinuing their intake due to intolerance. Only 9% of patients had insurance coverage for their supplements and when paying out of pocket, 95% of them spend up to $500/month. Despite the use of concomitant therapies (prescribed medications, physical therapy, diet changes and other), 45.5% of patients think that dietary supplements are the only intervention improving their symptoms. Some limitations of this study include the retrospective collection of data probably associated with substantial recall bias, lack of longitudinal follow up to document pre- and post-supplement clinical status and second hand reports by parents for children which may reflect parents' subjective interpretation of symptoms severity and supplements effect rather than real patients' experience. More extensive prospective studies will help further elucidate this topic.

Coenzyme q10 therapy

For a number of years, coenzyme Q10 (CoQ10) was known for its key role in mitochondrial bioenergetics; later studies demonstrated its presence in other subcellular fractions and in blood plasma, and extensively investigated its antioxidant role. These 2 functions constitute the basis for supporting the clinical use of CoQ10. Also, at the inner mitochondrial membrane level, CoQ10 is recognized as an obligatory cofactor for the function of uncoupling proteins and a modulator of the mitochondrial transition pore. Furthermore, recent data indicate that CoQ10 affects the expression of genes involved in human cell signaling, metabolism and transport, and some of the effects of CoQ10 supplementation may be due to this property. CoQ10 deficiencies are due to autosomal recessive mutations, mitochondrial diseases, aging-related oxidative stress and carcinogenesis processes, and also statin treatment. Many neurodegenerative disorders, diabetes, cancer, and muscular and cardiovascular diseases have been associated with low CoQ10 levels as well as different ataxias and encephalomyopathies. CoQ10 treatment does not cause serious adverse effects in humans and new formulations have been developed that increase CoQ10 absorption and tissue distribution. Oral administration of CoQ10 is a frequent antioxidant strategy in many diseases that may provide a significant symptomatic benefit.

Coenzyme Q10 as a therapy for mitochondrial disease

Treatment of mitochondrial respiratory chain (MRC) disorders is extremely difficult, however, coenzyme Q10 (CoQ10) and its synthetic analogues are the only agents which have shown some therapeutic benefit to patients. CoQ10 serves as an electron carrier in the MRC as well as functioning as a potent lipid soluble antioxidant. CoQ10 supplementation is fundamental to the treatment of patients with primary defects in the CoQ10 biosynthetic pathway. The efficacy of CoQ10 and its analogues in the treatment of patients with MRC disorders not associated with a CoQ10 deficiency indicates their ability to restore electron flow in the MRC and/or increase mitochondrial antioxidant capacity may also be important contributory factors to their therapeutic potential.

Coenzyme Q(10) supplementation reverses age-related impairments in spatial learning and lowers protein oxidation

Coenzyme Q10 (CoQ) is widely available as a dietary supplement and remains under consideration as a treatment for age-associated neurodegenerative conditions. However, no studies have determined if supplementation, initiated relatively late in life, could have beneficial effects on mild functional impairments associated with normal brain aging. Accordingly, the current study assessed the effect of CoQ intake in older mice for which cognitive and psychomotor impairments were already evident. Separate groups of young (3.5 months) and relatively old mice (17.5 months) were fed a control diet or a diet supplemented with low (0.72 mg/g) or high (2.81 mg/g) concentrations of CoQ for 15 weeks. After 6 weeks, the mice were given tests for spatial learning (Morris water maze), spontaneous locomotor activity, motor coordination, and startle reflex. Age-related impairments in cognitive and psychomotor functions were evident in the 17.5-month-old mice fed the control diet, and the low-CoQ diet failed to affect any aspect of the impaired performance. However, in the Morris water maze test, old mice on the high-CoQ diet swam to the safe platform with greater efficiency than the mice on the control diet. The old mice supplemented with the high-CoQ diet did not show improvement when spatial performance was measured using probe trials and failed to show improvement in other tests of behavioral performance. Protein oxidative damage was decreased in the mitochondria from the heart, liver, and skeletal muscle of the high-CoQ-supplemented mice and, to some extent, in the brain mitochondria. Contrasting with the deleterious effect of long-term CoQ supplementation initiated during young adulthood previously published, this study suggests that CoQ improves spatial learning and attenuates oxidative damage when administered in relatively high doses and delayed until early senescence, after age-related declines have occurred. Thus, in individuals with age-associated symptoms of cognitive decline, high-CoQ intake may be beneficial.

New treatments for mitochondrial disease-no time to drop our standards

Mitochondrial dysfunction is a common cause of inherited multisystem disease that often involves the nervous system. Despite major advances in our understanding of the pathophysiology of mitochondrial diseases, clinical management of these conditions remains largely supportive. Using a systematic approach, we identified 1,039 publications on treatments for mitochondrial diseases, only 35 of which included observations on more than five patients. Reports of a positive outcome on the basis of a biomarker of unproven clinical significance were more common in nonrandomized and nonblinded studies, suggesting a publication bias toward positive but poorly executed studies. Although trial design is improving, there is a critical need to develop new biomarkers of mitochondrial disease. In this Perspectives article, we make recommendations for the design of future treatment trials in mitochondrial diseases. Patients and physicians should no longer rely on potentially biased data, with the associated costs and risks.

Coenzyme Q10 depletion in medical and neuropsychiatric disorders: potential repercussions and therapeutic implications

Coenzyme Q10 (CoQ10) is an antioxidant, a membrane stabilizer, and a vital cofactor in the mitochondrial electron transport chain, enabling the generation of adenosine triphosphate. It additionally regulates gene expression and apoptosis; is an essential cofactor of uncoupling proteins; and has anti-inflammatory, redox modulatory, and neuroprotective effects. This paper reviews the known physiological role of CoQ10 in cellular metabolism, cell death, differentiation and gene regulation, and examines the potential repercussions of CoQ10 depletion including its role in illnesses such as Parkinson's disease, depression, myalgic encephalomyelitis/chronic fatigue syndrome, and fibromyalgia. CoQ10 depletion may play a role in the pathophysiology of these disorders by modulating cellular processes including hydrogen peroxide formation, gene regulation, cytoprotection, bioenegetic performance, and regulation of cellular metabolism. CoQ10 treatment improves quality of life in patients with Parkinson's disease and may play a role in delaying the progression of that disorder. Administration of CoQ10 has antidepressive effects. CoQ10 treatment significantly reduces fatigue and improves ergonomic performance during exercise and thus may have potential in alleviating the exercise intolerance and exhaustion displayed by people with myalgic encepholamyletis/chronic fatigue syndrome. Administration of CoQ10 improves hyperalgesia and quality of life in patients with fibromyalgia. The evidence base for the effectiveness of treatment with CoQ10 may be explained via its ability to ameliorate oxidative stress and protect mitochondria.

Diagnosis and treatment of mitochondrial myopathies

Mitochondrial disorders are a heterogeneous group of disorders resulting from primary dysfunction of the respiratory chain. Muscle tissue is highly metabolically active, and therefore myopathy is a common element of the clinical presentation of these disorders, although this may be overshadowed by central neurological features. This review is aimed at a general medical and neurologist readership and provides a clinical approach to the recognition, investigation, and treatment of mitochondrial myopathies. Emphasis is placed on practical management considerations while including some recent updates in the field.

Treatment of hereditary optic neuropathies

The hereditary optic neuropathies are inherited disorders in which optic nerve dysfunction is a prominent feature in the phenotypic expression of disease. Optic neuropathy may be primarily an isolated finding, such as in Leber hereditary optic neuropathy and dominant optic atrophy, or part of a multisystem disorder. The pathophysiological mechanisms underlying the hereditary optic neuropathies involve mitochondrial dysfunction owing to mutations in mitochondrial or nuclear DNA that encodes proteins essential to mitochondrial function. Effective treatments are limited, and current management includes therapies directed at enhancing mitochondrial function and preventing oxidative damage, as well as genetic counselling, and supportive and symptomatic measures. New therapies, including gene therapy, are emerging via animal models and human clinical trials. Leber hereditary optic neuropathy, in particular, provides a unique model for testing promising treatments owing to its characteristic sequential bilateral involvement and the accessibility of target tissue within the eye. Lessons learned from treatment of the hereditary optic neuropathies may have therapeutic implications for other disorders of presumed mitochondrial dysfunction. In this Review, the natural history of the common inherited optic neuropathies, the presumed pathogenesis of several of these disorders, and the literature to date regarding potential therapies are summarized.

Treatment for mitochondrial disorders

BACKGROUND

Mitochondrial respiratory chain disorders are the most prevalent group of inherited neurometabolic diseases. They present with central and peripheral neurological features usually in association with other organ involvement including the eye, the heart, the liver, and kidneys, diabetes mellitus and sensorineural deafness. Current treatment is largely supportive and the disorders progress relentlessly causing significant morbidity and premature death. Vitamin supplements, pharmacological agents and exercise therapy have been used in isolated cases and small clinical trials, but the efficacy of these interventions is unclear. The first review was carried out in 2003, and identified six clinical trials. This major update was carried out to identify new studies and grade the original studies for potential bias in accordance with revised Cochrane Collaboration guidelines.

OBJECTIVES

To determine whether there is objective evidence to support the use of current treatments for mitochondrial disease.

SEARCH METHODS

We searched the Cochrane Neuromuscular Disease Group Specialized Register (4 July 2011), CENTRAL (2011, Issue 2, MEDLINE (1966 to July 2011), and EMBASE (January 1980 to July 2011), and contacted experts in the field.

SELECTION CRITERIA

We included randomised controlled trials (including cross-over studies). Two of the authors independently selected abstracts for further detailed review. Further review was performed independently by all five authors to decide which trials fit the inclusion criteria and graded risk of bias. Participants included males and females of any age with a confirmed diagnosis of mitochondrial disease based upon muscle histochemistry, respiratory chain complex analysis of tissues or cell lines or DNA studies. Interventions included any pharmacological agent, dietary modification, nutritional supplement, exercise therapy or other treatment. The review authors excluded studies at high risk of bias in any category. The primary outcome measures included an change in muscle strength and/or endurance, or neurological clinical features. Secondary outcome measures included quality of life assessments, biochemical markers of disease and negative outcomes.

DATA COLLECTION AND ANALYSIS

Two of the authors (GP and PFC) independently identified studies for further evaluation from all abstracts within the search period. For those studies identified for further review, all five authors then independently assessed which studies met the entry criteria. For the included studies, we extracted details of the number of randomised participants, treatment, study design, study category, allocation concealment and other risk of bias criteria, and participant characteristics. Analysis was based on intention-to-treat data. We planned to use meta-analysis, but this did not prove necessary.

MAIN RESULTS

The authors reviewed 1335 abstracts, and from these identified 21 potentially eligible abstracts. Upon detailed review, 12 studies fulfilled the entry criteria. Of these, eight were new studies that had been published since the previous version of this review. Two studies which were included in the previous version of this review were excluded because of potential for bias. The comparability of the included studies is extremely low because of differences in the specific diseases studied, differences in the therapeutic agents used, dosage, study design, and outcomes. The methodological quality of included studies was generally high, although risk of bias was unclear in random sequence generation and allocation concealment for most studies. Otherwise, the risk of bias was low for most studies in the other categories. Serious adverse events were uncommon, except for peripheral nerve toxicity in a long-term trial of dichloroacetate (DCA) in adults.One trial studied high-dose coenzyme Q10 without clinically meaningful improvement (although there were multiple biochemical, physiologic, and neuroimaging outcomes, in 30 participants). Three trials used creatine monohydrate alone, with one reporting evidence of improved measures of muscle strength and post-exercise lactate, but the other two reported no benefit (total of 38 participants). One trial studied the effects of a combination of coenzyme Q10, creatine monohydrate, and lipoic acid and reported a statistically significant improvement in biochemical markers and peak ankle dorsiflexion strength, but overall no clinical improvement in 16 participants. Five trials studied the effects of DCA: three trials in children showed a statistically significant improvement in secondary outcome measures of mitochondrial metabolism (venous lactate in three trials, and magnetic resonance spectroscopy (MRS) in one trial; total of 63 participants). One trial of short-term DCA in adults demonstrated no clinically relevant improvement (improved venous lactate but no change in physiologic, imaging, or questionnaire findings, in eight participants). One longer-term DCA trial in adults was terminated prematurely due to peripheral nerve toxicity without clinical benefit (assessments included the GATE score, venous lactate and MRS, in 30 participants). One trial using dimethylglycine showed no significant effect (measurements of venous lactate and oxygen consumption (VO(2)) in five participants). One trial using a whey-based supplement showed statistically significant improvement in markers of free radical reducing capacity but no clinical benefit (assessments included the Short Form 36 Health Survey (SF-36) questionnaire and UK Medical Research Council (MRC) muscle strength, in 13 participants).

AUTHORS' CONCLUSIONS

Despite identifying eight new trials there is currently no clear evidence supporting the use of any intervention in mitochondrial disorders. Further research is needed to establish the role of a wide range of therapeutic approaches. We suggest further research should identify novel agents to be tested in homogeneous study populations with clinically relevant primary endpoints.

A randomized trial of coenzyme Q10 in mitochondrial disorders

Case reports and open-label studies suggest that coenzyme Q(10) (CoQ(10)) treatment may have beneficial effects in mitochondrial disease patients; however, controlled trials are warranted to clinically prove its effectiveness. Thirty patients with mitochondrial cytopathy received 1200 mg/day CoQ(10) for 60 days in a randomized, double-blind, cross-over trial. Blood lactate, urinary markers of oxidative stress, body composition, activities of daily living, quality of life, forearm handgrip strength and oxygen desaturation, cycle exercise cardiorespiratory variables, and brain metabolites were measured. CoQ(10) treatment attenuated the rise in lactate after cycle ergometry, increased (∽1.93 ml) VO(2)/kg lean mass after 5 minutes of cycling (P < 0.005), and decreased gray matter choline-containing compounds (P < 0.05). Sixty days of moderate- to high-dose CoQ(10) treatment had minor effects on cycle exercise aerobic capacity and post-exercise lactate but did not affect other clinically relevant variables such as strength or resting lactate.

Mitochondrial disease: when the powerhouse goes awry

Knowledge of and interest in mitochondrial disease is relatively recent. The first diagnosis of a mitochondrial cytopathy occurred in 1962. Since then, researchers have elucidated hundreds of types of mutant mitochondrial DNA and mapped their repercussions. This article walks the reader through the basics of mitochondrial function--normal and mutant. Mitochondrial cytopathies appear first in organs with high-energy needs; once a threshold of mutant mitochondrial DNA is exceeded, manifestations can be single or multisystem conditions. Although these conditions are currently incurable, some therapies aimed at enhancing mitochondrial function or scavenging free radicals may help individual patients. Certain nonpharmacological therapies should be employed in all patients. Research findings suggest that mitochondrial functioning may have a greater role in other pathologies than previously assumed.

Coenzyme Q10 and Neurological Diseases

Coenzyme Q10 (CoQ10, or ubiquinone) is a small electron carrier of the mitochondrial respiratory chain with antioxidant properties. CoQ10 supplementation has been widely used for mitochondrial disorders. The rationale for using CoQ10 is very powerful when this compound is primary decreased because of defective synthesis. Primary CoQ10 deficiency is a treatable condition, so heightened "clinical awareness" about this diagnosis is essential. CoQ10 and its analogue, idebenone, have also been widely used in the treatment of other neurodegenerative disorders. These compounds could potentially play a therapeutic role in Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, Friedreich's ataxia, and other conditions which have been linked to mitochondrial dysfunction. This article reviews the physiological roles of CoQ10, as well as the rationale and the role in clinical practice of CoQ10 supplementation in different neurological diseases, from primary CoQ10 deficiency to neurodegenerative disorders.

Mitochondrial encephalopathy, lactic acidosis, and strokelike episodes: basic concepts, clinical phenotype, and therapeutic management of MELAS syndrome

Since the initial description almost 25 years ago, the syndrome of mitochondrial encephalopathy, lactic acidosis, and strokelike episodes (MELAS) has been a useful model to study the complex interplay of factors that define mitochondrial disease. This syndrome, most commonly caused by an A-to-G transition mutation at position 3243 of the mitochondrial genome, is typified by characteristic neurological manifestations including seizures, encephalopathy, and strokelike episodes, as well as other frequent secondary manifestations including short stature, cognitive impairment, migraines, depression, cardiomyopathy, cardiac conduction defects, and diabetes mellitus. In this review, we discuss the history, pathogenesis, clinical features, and diagnostic and management strategies of mitochondrial disease in general and of MELAS in particular. We explore features of mitochondrial genetics, including the concepts of heteroplasmy, mitotic segregation, and threshold effect, as a basis for understanding the variability and complicated inheritance patterns seen with this group of diseases. We also describe systemic manifestations of MELAS-associated mutations, including cardiac, renal, endocrine, gastrointestinal, and endothelial abnormalities and pathology, as well as the hypothetical role of derangements to COX enzymatic function in driving the unique pathology and clinical manifestations of MELAS. Although therapeutic options for MELAS and other mitochondrial diseases remain limited, and recent trials have been disappointing, we also consider current and potential therapeutic modalities.

The uptake and distribution of coenzyme Q(10)

This review describes recent advances in our understanding of the uptake and distribution of coenzyme Q10 (CoQ10) in cells, animals, and humans. These advances have provided evidence of important pharmacokinetic factors, such as non-linear absorption and enterohepatic recirculation, and may facilitate the development of new CoQ10 formulations. Studies providing data which support the claim of tissue uptake of exogenous CoQ10 are also discussed. Improved CoQ10 dosing and drug level monitoring guidelines are suggested for adult and pediatric patient populations. Future CoQ10 research should consider uptake and distribution factors to determine cost-benefit relationships.

The evidence basis for coenzyme Q therapy in oxidative phosphorylation disease

The evidence supporting a treatment benefit for coenzyme Q10 (CoQ10) in primary mitochondrial disease (mitochondrial disease) whilst positive is limited. Mitochondrial disease in this context is defined as genetic disease causing an impairment in mitochondrial oxidative phosphorylation (OXPHOS). There are no treatment trials achieving the highest Level I evidence designation. Reasons for this include the relative rarity of mitochondrial disease, the heterogeneity of mitochondrial disease, the natural cofactor status and easy 'over the counter availability' of CoQ10 all of which make funding for the necessary large blinded clinical trials unlikely. At this time the best evidence for efficacy comes from controlled trials in common cardiovascular and neurodegenerative diseases with mitochondrial and OXPHOS dysfunction the etiology of which is most likely multifactorial with environmental factors playing on a background of genetic predisposition. There remain questions about dosing, bioavailability, tissue penetration and intracellular distribution of orally administered CoQ10, a compound which is endogenously produced within the mitochondria of all cells. In some mitochondrial diseases and other commoner disorders such as cardiac disease and Parkinson's disease low mitochondrial or tissue levels of CoQ10 have been demonstrated providing an obvious rationale for supplementation. This paper discusses the current state of the evidence supporting the use of CoQ10 in mitochondrial disease.

Enrichment of coenzyme Q10 in plasma and blood cells: defense against oxidative damage

Coenzyme Q10 (CoQ10) concentration in blood cells was analyzed by HPLC and compared to plasma concentration before, during, and after CoQ10 (3 mg/kg/day) supplementation to human probands. Lymphocyte DNA 8-hydroxydeoxy-guanosine (8-OHdG), a marker of oxidative stress, was analyzed by Comet assay.

Subjects supplemented with CoQ10 showed a distinct response in plasma concentrations after 14 and 28 days. Plasma levels returned to baseline values 12 weeks after treatment stopped. The plasma concentration increase did not affect erythrocyte levels. However, after CoQ10 supplementation, the platelet level increased; after supplementation stopped, the platelet level showed a delayed decrease. A positive correlation was shown between the plasma CoQ10 level and platelet and white blood cell CoQ10 levels. During CoQ10 supplementation, delayed formation of 8-OHdG in lymphocyte DNA was observed; this effect was long-lasting and could be observed even 12 weeks after supplementation stopped. Intracellular enrichment may support anti-oxidative defense mechanisms.

Beneficial effects of creatine, CoQ10, and lipoic acid in mitochondrial disorders

Mitochondrial disorders share common cellular consequences: (1) decreased ATP production; (2) increased reliance on alternative anaerobic energy sources; and (3) increased production of reactive oxygen species. The purpose of the present study was to determine the effect of a combination therapy (creatine monohydrate, coenzyme Q(10), and lipoic acid to target the above-mentioned cellular consequences) on several outcome variables using a randomized, double-blind, placebo-controlled, crossover study design in patients with mitochondrial cytopathies. Three patients had mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), four had mitochondrial DNA deletions (three patients with chronic progressive external ophthalmoplegia and one with Kearns-Sayre syndrome), and nine had a variety of other mitochondrial diseases not falling into the two former groups. The combination therapy resulted in lower resting plasma lactate and urinary 8-isoprostanes, as well as attenuation of the decline in peak ankle dorsiflexion strength in all patient groups, whereas higher fat-free mass was observed only in the MELAS group. Together, these results suggest that combination therapies targeting multiple final common pathways of mitochondrial dysfunction favorably influence surrogate markers of cellular energy dysfunction. Future studies with larger sample sizes in relatively homogeneous groups will be required to determine whether such combination therapies influence function and quality of life.

Maternally inherited diabetes and deafness in a North American kindred: tips for making the diagnosis and review of unique management issues

CONTEXT

Mutations in mitochondrial DNA are rare etiologies of adult-onset diabetes mellitus (DM) that merit identification to 1) prevent iatrogenic lactic acidosis, 2) prompt appropriate screening of affected patients and their families, 3) provide genetic counseling, and 4) provide an opportunity to investigate strategies for preventing diabetes.

OBJECTIVE

The objective of this study is to raise awareness of this rare form of adult-onset nonobese DM so that these patients are identified and provided with appropriate care.

PATIENTS

We describe a kindred in which four of seven siblings have adult-onset DM and sensorineural hearing loss with a confirmed genetic mutation at position 3243 in the tRNA. Two other siblings in this kindred demonstrate different phenotypes of mitochondrial disease.

INTERVENTION

The proband was treated with coenzyme Q10 for 1 yr.

OUTCOME MEASURES

Outcome measures included stress thallium exercise testing and audiometry testing.

RESULTS

After 1 yr of treatment of with coenzyme Q10, repeat stress thallium testing demonstrated improvement in the exercise tolerance of the proband from 7-12 min. Audiometry testing did not demonstrate a change in the rate of hearing decline.

CONCLUSION

Maternally inherited diabetes and deafness is a rare cause of DM that is important to diagnose because of the unique management issues and associated comorbidities. This work highlights clues to the identification of this rare monogenic form of adult- onset diabetes.

The mitochondrial myopathy encephalopathy, lactic acidosis with stroke-like episodes (MELAS) syndrome: a review of treatment options

Mitochondrial encephalomyopathies are a multisystemic group of disorders that are characterised by a wide range of biochemical and genetic mitochondrial defects and variable modes of inheritance. Among this group of disorders, the mitochondrial myopathy, encephalopathy, lactic acidosis with stroke-like episodes (MELAS) syndrome is one of the most frequently occurring, maternally inherited mitochondrial disorders. As the name implies, stroke-like episodes are the defining feature of the MELAS syndrome, often occurring before the age of 15 years. The clinical course of this disorder is highly variable, ranging from asymptomatic, with normal early development, to progressive muscle weakness, lactic acidosis, cognitive dysfunction, seizures, stroke-like episodes, encephalopathy and premature death. This syndrome is associated with a number of point mutations in the mitochondrial DNA, with over 80% of the mutations occurring in the dihydrouridine loop of the mitochondrial transfer RNA(Leu(UUR)) [tRNA(Leu)((UUR))] gene. The pathophysiology of the disease is not completely understood; however, several different mechanisms are proposed to contribute to this disease. These include decreased aminoacylation of mitochondrial tRNA, resulting in decreased mitochondrial protein synthesis; changes in calcium homeostasis; and alterations in nitric oxide metabolism. Currently, no consensus criteria exist for treating the MELAS syndrome or mitochondrial dysfunction in other diseases. Many of the therapeutic strategies used have been adopted as the result of isolated case reports or limited clinical studies that have included a heterogeneous population of patients with the MELAS syndrome, other defects in oxidative phosphorylation or lactic acidosis due to disorders of pyruvate metabolism. Current approaches to the treatment of the MELAS syndrome are based on the use of antioxidants, respiratory chain substrates and cofactors in the form of vitamins; however, no consistent benefits have been observed with these treatments.

Point mutation tRNA(Ser(UCN)) in a child with hearing loss and myoclonus epilepsy

We report on a family with a 12-year-old boy who suffered from a maternally inherited syndrome characterized by a combination of sensorineural hearing loss, myoclonus epilepsy, ataxia, severe psychomotor retardation, short stature, and diabetes mellitus. First, he showed a muscular hypotonia with hearing loss; later, he developed a myoclonus epilepsy, growth failure, and severe psychomotor retardation. At the age of 10 years, he developed diabetes mellitus. After initiation of combined ubiquinone and vitamin C treatment, we observed a progression in psychomotor development. Lactate and pyruvate levels in blood and cerebrospinal fluid were normal. No ragged red fibers or ultrastructural abnormalities were seen in a skeletal muscle biopsy. Biochemical assays of respiratory chain complex activities revealed decreased activity of complexes I and IV. By sequence analysis of mitochondrial DNA encoding transfer ribonucleic acids (RNAs), a homoplasmic T to C substitution at nucleotide position 7512 was found affecting a highly conserved base pair in the tRNA(ser(UCN)) acceptor stem. Asymptomatic family members of the maternal line were heteroplasmic for the mutation in blood samples. Analysis of mitochondrial DNA in patients with hearing loss and myoclonus epilepsy is recommended, even in the absence of laboratory findings. Therapeutically, ubiquinone and antioxidants can be beneficial.

Clinical aspects of coenzyme Q10: an update

PURPOSE OF REVIEW

Coenzyme Q10 is administered for an ever-widening range of disorders, therefore it is timely to illustrate the latest findings with special emphasis on areas in which this therapeutic approach is completely new. These findings also give further insight into the biochemical mechanisms underlying clinical involvement of coenzyme Q10.

RECENT FINDINGS

Cardiovascular properties of coenzyme Q10 have been further addressed, namely regarding myocardial protection during cardiac surgery, end-stage heart failure, pediatric cardiomyopathy and in cardiopulmonary resuscitation. The vascular aspects of coenzyme Q10 addressing the important field of endothelial function are briefly examined. The controversial issue of the statin/coenzyme Q10 relationship has been investigated in preliminary studies in which the two substances were administered simultaneously. Work on different neurological diseases, involving mitochondrial dysfunction and oxidative stress, highlights some of the neuroprotective mechanisms of coenzyme Q10. A 4-year follow-up on 10 Friedreich's Ataxia patients treated with coenzyme Q10 and vitamin E showed a substantial improvement in cardiac and skeletal muscle bioenergetics and heart function. Mitochondrial dysfunction likely plays a role in the pathophysiology of migraine as well as age-related macular degeneration and a therapy including coenzyme Q10 produced significant improvement. Finally, the effect of coenzyme Q10 was evaluated in the treatment of asthenozoospermia.

SUMMARY

The latest findings highlight the beneficial role of coenzyme Q10 as coadjuvant in the treatment of syndromes, characterized by impaired mitochondrial bioenergetics and increased oxidative stress, which have a high social impact. Besides their clinical significance, these data give further insight into the biochemical mechanisms of coenzyme Q10 activity.

Coenzyme Q-10 treatment of patients with a 7445A--->G mitochondrial DNA mutation stops the progression of hearing loss

CONCLUSION

CoQ 10 may be helpful in delaying the progression of hearing loss in patients with the 7445A-->G mitochondrial mutation. Objective. To assess the effect of an antioxidant drug (Coenzyme Q-10) on the hearing level of patients with the mitochondrial DNA 7445A-->G mutation and associated sensorineural hearing loss (SNHL). Material and methods. We identified three patients with bilateral non-syndromic SNHL harboring the mitochondrial 7445A-->G mutation. Two patients had a family history of hearing loss with a strong matrilineal inheritance. The other patient did not have a family history of hearing loss. Two patients (1 with familial and 1 with sporadic SNHL) received treatment with 75 mg of Coenzyme Q-10 (CoQ10) twice a day for 1 year. The remaining patient with a familial form of hearing loss did not agree to take the treatment. Average bone conduction pure-tone thresholds for 0.5, 1, 2 and 4 kHz were obtained before and after diagnosis of mitochondrial hearing loss, and before and after treatment with CoQ10. Results. CoQ10-treated patients did not show any additional deterioration of their SNHL after 12 (familial case) and 13 months (sporadic case). The progression rate of SNHL was 6 dB/year in the 2 years prior to initiation of treatment in the familial case who received CoQ10 treatment. One year after being diagnosed with mitochondrial hearing loss, the patient who refused CoQ10 treatment exhibited an 11-dB deterioration of his hearing thresholds. There were no side-effects related to treatment with CoQ10.

Diagnosis and management of MELAS

Mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) is the most common maternally inherited mitochondrial disease. An A-->G mutation in the transfer RNA(Leu(UUR)) gene at position 3243 of the mitochondrial DNA accounts for most MELAS cases. The transient nature of the stroke-like episodes is reflected in abnormalities on neuroimaging. The cardinal laboratory abnormalities include elevated serum lactate during the acute episodes and respiratory enzyme defects in skeletal muscle. Muscle biopsy also helps confirm the diagnosis by identifying abnormal proliferation of mitochondria. Although current treatment options for MELAS are largely supportive, several therapeutic approaches have been attempted with limited success. Genetic counseling is an important component of patient management in MELAS. Newer reproductive technologies hold promise for reducing the recurrence of MELAS in subsequent generations. Advances in research into gene therapy offer hope of treatment for the future.

Coenzyme Q 10 Improves Lactic Acidosis, Strokelike Episodes, and Epilepsy in a Patient With MELAS (Mitochondrial Myopathy, Encephalopathy, Lactic Acidosis, and Strokelike episodes)

Mitochondrial encephalomyopathies encompass a group of disorders that have impaired oxidative metabolism in skeletal muscles and central nervous system. Many compounds have been used in clinical trials on mitochondrial diseases, but the outcomes have been variable. It remains controversial whether treatment of mitochondrial diseases with coenzyme Q 10 is effective. This paper describes a case of mitochondrial myopathy, encephalopathy, lactic acidosis, strokelike episodes, and exercise intolerance successfully treated with coenzyme Q 10. Efficacy of this therapy in this patient is correlated to control of lactic acidosis and serum creatine kinase levels. Disappointingly, larger studies with coenzyme Q 10 failed to demonstrate a clear beneficial effect on the entire study population with regard to clinical improvement or several parameters of the oxidative metabolism. They suggest that the use of coenzyme Q in treatment of mitochondrial diseases should be confined to protocols. There is a confounding variation in phenotype and genotype, and the natural history of the disorders in individual patients is not accurately predictable. The unpredictable a priori efficacy of therapy suggests that a long-term trial of oral coenzyme Q may be warranted.

Effect of high-dose vitamins, coenzyme Q and high-fat diet in paediatric patients with mitochondrial diseases

We reviewed the medical records of all patients with confirmed mitochondrial diseases treated with any or all of thiamin, riboflavin, coenzyme Q, vitamin C (approximately 10 mg/kg per day) and a high-fat diet (50-60% of caloric intake) between 1997 and 2003. There were 15 patients (9 male): 10 had enzymatic deficiency and 10 had a molecular diagnosis. Age at diagnosis was 11 months to 17 years 10 months. Treatment was commenced at time of clinical diagnosis in 12 patients. Follow-up period was 3 days to 7 years (median 22 months). Improvement was reported in 9 patients, of whom 4 attained further developmental skills, but this was only temporary in 6 patients. Five patients died during the follow-up period (3 days to 7 years). Patients with the 3243A > G mutation showed no significant change in the course of their disease, except for fewer migraine attacks. Of the six patients who had seizures, one has had a significant reduction in the severity of the seizures and one has had no further seizures. Plasma lactate levels were noncontributory. We conclude that high-dose vitamin and cofactor treatment and, where applicable, high-fat diet, are well tolerated and possibly effective in the short term, but ineffective in the longer term.

Mitochondrial encephalopathy

Mitochondrial disorders cause a wide spectrum of diseases in children. Their presentation is nonspecific with encephalomyopathy, failure to thrive, seizures, ophthalmoplegia, and sensorineural hearing loss. These disorders are progressive and are aggravated by fever and infections. They can be caused by mutations in nDNA or mtDNA. Diagnosis requires a complex battery of clinical studies coupled with diagnostic findings on muscle biopsy (abnormal structure, histochemistry, or enzyme studies) or DNA testing. Therapy for mitochondrial disorders remains largely ineffective.

Gastrointestinal manifestations of mitochondrial disease

Although non-specific gastrointestinal and hepatic symptoms are commonly found in most mitochondrial disorders, they are among the cardinal manifestations of several primary mitochondrial diseases, such as: mitochondrial neurogastrointestinal encephalomyopathy; mitochondrial DNA depletion syndrome; Alpers syndrome; and Pearson syndrome. Management of these heterogeneous disorders includes the empiric supplementation with various "mitochondrial cocktails," supportive therapies, and avoidance of drugs and conditions known to have a detrimental effect on the respiratory chain. There is a great need for improved methods of treatment and controlled clinical trials of existing therapies. Liver transplantation is successful in acquired cases; however neuromuscular involvement in primary mitochondrial disorders should be a contraindication for liver transplantation.

Nutritional cofactor treatment in mitochondrial disorders

Mitochondrial disorders are degenerative diseases characterized by a decrease in the ability of mitochondria to supply cellular energy requirements. Substantial progress has been made in defining the specific biochemical defects and underlying molecular mechanisms, but limited information is available about the development and evaluation of effective treatment approaches. The goal of nutritional cofactor therapy is to increase mitochondrial adenosine 5'-triphosphate production and slow or arrest the progression of clinical symptoms. Accumulation of toxic metabolites and reduction of electron transfer activity have prompted the use of antioxidants, electron transfer mediators (which bypass the defective site), and enzyme cofactors. Metabolic therapies that have been reported to produce a positive effect include Coenzyme Q(10) (ubiquinone); other antioxidants such as ascorbic acid, vitamin E, and lipoic acid; riboflavin; thiamin; niacin; vitamin K (phylloquinone and menadione); creatine; and carnitine. A literature review of the use of these supplements in mitochondrial disorders is presented.

Coenzyme Q10 reduces the toxicity of rotenone in neuronal cultures by preserving the mitochondrial membrane potential

Defects in mitochondrial energy metabolism due to respiratory chain disorders lead to a decrease in mitochondrial membrane potential (DeltaPsim) and induce apoptosis. Since coenzyme Q10 (CoQ10) plays a dual role as an antioxidant and bioenergetic agent in the respiratory chain, it has attracted increasing attention concerning the prevention of apoptosis in mitochondrial diseases. In this study the potential of CoQ10 to antagonize the apoptosis-inducing effects of the respiratory chain inhibitor rotenone was explored by video-enhanced microscopy in SH-SY5Y neuroblastoma cells. The cationic fluorescent dye JC-1 which exhibits potential-dependent accumulation in mitochondria was used as an indicator to monitor changes in DeltaPsim. The relative changes in fluorescence intensity after incubation with rotenone for 15 minutes were calculated. Pre-treatment with CoQ10 (10 or 100 microM) for 48 h led to a significant reduction of rotenone-induced loss of DeltaPsim. These results suggest, that cytoprotection by CoQ10 may be mediated by raising cellular resistance against the initiating steps of apoptosis, namely the decrease of DeltaPsim. Whether these data may provide new directions for the development of neuroprotective strategies has to be investigated in future studies.

Nutritional and exercise-based therapies in the treatment of mitochondrial disease

PURPOSE OF REVIEW

This review will critically summarize the nutritional and exercise-based interventions that have been used to treat mitochondrial disease, with a focus on the biochemical or molecular rationale for their use as well as recent advances in the field.

RECENT FINDINGS

Many nutritional-based treatment strategies have been used in an attempt to target energy impairment and its sequelae. Recently, coenzyme Q10, idebenone and triacylglycerol have been shown to bypass defective respiratory enzymes or scavenge free radicals, whereas creatine monohydrate has provided an alternative energy source. Thiamine has been used to decrease lactate levels and increase flux through aerobic metabolism, and riboflavin has been used as a precursor to complexes I and II. Several therapies employing various antioxidants in combination with other supplements have been effective at targeting several of the final common pathways of mitochondrial disease. Miscellaneous supplements, such as L-arginine and uridine, have also had recent success. However, although positive responses have been reported with these agents, many reports have shown no benefit, and there is widespread disparity in the literature. An alternative approach to treatment is exercise training. Both resistance and endurance exercise training have had positive outcomes in patients with mitochondrial disease, although several questions remain to be answered.

SUMMARY

There is no currently recognized treatment for mitochondrial disease. Future clinical trials are needed, as well as research into the potential for in-vitro screening of various compounds within affected cells from patients. Until this time, an accurate diagnosis will facilitate treatment on a case-by-case basis.

A practical approach to the diagnosis and management of MELAS: case report and review

BACKGROUND

Mitochondrial encephalomyopathy, lactic acidosis with stroke-like episodes (MELAS) is a mitochondrial disorder and an important diagnostic consideration in the young patient with nonhemorrhagic stroke. Its presentation is varied and diagnosis is based on early recognition of the clinical features and correct interpretation of laboratory and radiologic studies.

SUMMARY

In this article, we report a patient with MELAS and review the clinical, laboratory, and neuroradiologic features of the condition. In the young patient with multiple stroke-like episodes in different vascular territories and neuroradiologic features of transient abnormalities in varying regions, laboratory testing for MELAS must be performed. The presence of ragged red fibers in skeletal muscle and biochemical demonstration of defects in mitochondrial respiratory enzymes strongly support the diagnosis. Molecular genetic testing for abnormalities in mitochondrial DNA will confirm the diagnosis. The importance of a thorough assessment of family history is also emphasized. The basic principles of mitochondrial genetics and the common point mutations and rearrangements of mitochondrial DNA associated with MELAS are reviewed. Although treatment options are limited, several therapeutic agents have been studied.

CONCLUSIONS

The diagnosis of MELAS should be considered in the young patient with stroke, especially when accompanied by other clinical features such as seizures, encephalopathy, and muscle weakness. Laboratory evaluation can provide an accurate diagnosis, especially when the appropriate mitochondrial DNA studies are performed. Genetic counseling should be provided to patients with MELAS associated with mitochondrial DNA point mutations. Better understanding of the molecular basis of the condition may result in the development of effective treatment strategies.

Mitochondrial neurogastrointestinal encephalomyopathy: diagnostic features of two patients

Mitochondrial neurogastrointestinal encephalomyopathy is a rare, multisystem disorder characterized by gastrointestinal dysmotility, ptosis, neurologic findings (e.g., peripheral neuropathy), leukoencephalopathy, and thin body habitus. Gastrointestinal motility studies and skeletal muscle biopsy are recommended diagnostic tools. We report two patients that highlight the diagnostic characteristics of this rare entity.

Maternal transmission of diabetes

Type 2 diabetes mellitus represents a heterogeneous group of conditions characterized by impaired glucose homeostasis. The disorder runs in families but the mechanism underlying this is unknown. Many, but not all, studies have suggested that mothers are excessively implicated in the transmission of the disorder. A number of possible genetic phenomena could explain this observation, including the exclusively maternal transmission of mitochondrial DNA (mtDNA). It is now apparent that mutations in mtDNA can indeed result in maternally inherited diabetes. Although several mutations have been implicated, the strongest evidence relates to a point substitution at nucleotide position 3243 (A to G) in the mitochondrial tRNA(leu(UUR)) gene. Mitochondrial diabetes is commonly associated with nerve deafness and often presents with progressive non-autoimmune beta-cell failure. Specific treatment with Coenzyme Q10 or L-carnitine may be beneficial. Several rodent models of mitochondrial diabetes have been developed, including one in which mtDNA is specifically depleted in the pancreatic islets. Apart from severe, pathogenic mtDNA mutations, common polymorphisms in mtDNA may contribute to variations of insulin secretory capacity in normal individuals. Mitochondrial diabetes accounts for less than 1% of all diabetes and other mechanisms must underlie the maternal transmission of Type 2 diabetes. Possibilities include the role of maternally controlled environments, imprinted genes and epigenetic phenomena.

Epidemiology and treatment of mitochondrial disorders

The last ten years have seen a huge increase in the number of different genetic defects found in patients with mitochondrial disorders, but the true impact of mitochondrial disease is only just becoming apparent. Mitochondrial diseases are far more common than was anticipated. Although there have also been major advances in our understanding of mitochondrial pathology, the clinical management of patients with mitochondrial disease is largely supportive. In this article, we focus on primary disorders of the mitochondrial respiratory chain and mtDNA defects. We review the available epidemiological data, outline current strategies for the management of mitochondrial disease, and highlight new therapeutic approaches that may prove useful in the future.