A novel mutation m.8561C>G in MT-ATP6/8 causing a mitochondrial syndrome with ataxia, peripheral neuropathy, diabetes mellitus, and hypergonadotropic hypogonadism

Variability of Clinical Phenotypes Caused by Isolated Defects of Mitochondrial ATP Synthase

Disorders of ATP synthase, the key enzyme in mitochondrial energy supply, belong to the most severe metabolic diseases, manifesting as early-onset mitochondrial encephalo-cardiomyopathies. Since ATP synthase subunits are encoded by both mitochondrial and nuclear DNA, pathogenic variants can be found in either genome. In addition, the biogenesis of ATP synthase requires several assembly factors, some of which are also hotspots for pathogenic variants. While variants of MT-ATP6 and TMEM70 represent the most common cases of mitochondrial and nuclear DNA mutations respectively, the advent of next-generation sequencing has revealed new pathogenic variants in a number of structural genes and TMEM70, sometimes with truly peculiar genetics. Here we present a systematic review of the reported cases and discuss biochemical mechanisms, through which they are affecting ATP synthase. We explore how the knowledge of pathophysiology can improve our understanding of enzyme biogenesis and function. Keywords: Mitochondrial diseases o ATP synthase o Nuclear DNA o Mitochondrial DNA o TMEM70.

Variants in Human ATP Synthase Mitochondrial Genes: Biochemical Dysfunctions, Associated Diseases, and Therapies

Mitochondrial ATP synthase (Complex V) catalyzes the last step of oxidative phosphorylation and provides most of the energy (ATP) required by human cells. The mitochondrial genes MT-ATP6 and MT-ATP8 encode two subunits of the multi-subunit Complex V. Since the discovery of the first MT-ATP6 variant in the year 1990 as the cause of Neuropathy, Ataxia, and Retinitis Pigmentosa (NARP) syndrome, a large and continuously increasing number of inborn variants in the MT-ATP6 and MT-ATP8 genes have been identified as pathogenic. Variants in these genes correlate with various clinical phenotypes, which include several neurodegenerative and multisystemic disorders. In the present review, we report the pathogenic variants in mitochondrial ATP synthase genes and highlight the molecular mechanisms underlying ATP synthase deficiency that promote biochemical dysfunctions. We discuss the possible structural changes induced by the most common variants found in patients by considering the recent cryo-electron microscopy structure of human ATP synthase. Finally, we provide the state-of-the-art of all therapeutic proposals reported in the literature, including drug interventions targeting mitochondrial dysfunctions, allotopic gene expression- and nuclease-based strategies, and discuss their potential translation into clinical trials.

Endocrine features of primary mitochondrial diseases

PURPOSE OF REVIEW

Primary mitochondrial diseases are one of the most prevalent groups of multisystem genetic disorders. Endocrinopathies associated with mitochondrial diseases may have clinical features that are distinct from the more common forms. We provide an overview of mitochondrial disorder genetics and phenotypes, focusing on recent studies regarding identification and treatment of associated endocrinopathies.

RECENT FINDINGS

Known endocrine phenotypes of mitochondrial disorders continue to expand, and now include growth hormone deficiency, hypogonadism, precocious puberty, hypoparathyroidism, hypo- and hyperthyroidism, diabetes, and adrenal insufficiency. Recent studies suggest several genotype-phenotype correlations, including those related to nuclear variants. Diagnosis is important, as special considerations should be made in the management of endocrinopathies in mitochondrial patients. Finally, new mitochondrial replacement strategies may soon be available for women interested in preventing mitochondrial disease transmission to offspring.

SUMMARY

Patients with multiple endocrinopathies or atypical endocrinopathies should be evaluated for primary mitochondrial disease, as a diagnosis may impact management of these individuals.

Quantification of Upper Limb Movements in Patients with Hereditary or Idiopathic Ataxia

Assessment of ataxic movements is usually based on clinical judgment. Technical devices can be employed in the quantification of ataxic movements in addition to clinical evaluation. The effect of maximal speed in upper limb movements in ataxia patients has not been quantified. The aim was to quantify upper limb movements in patients with hereditary or idiopathic ataxia and to find features of movement that are characteristic for ataxia. We examined 19 patients with degenerative ataxia and 21 healthy controls. An ad hoc system comprising a touch screen, an accelerometer, and a gyroscope was used to measure speed, angular acceleration, consistency, and accuracy of upper limb movements. The movements were quantified during finger-to-nose test that the patients were asked to perform at their own pace and as fast as possible. Disease severity was estimated by using the Scale for the Assessment and Rating of Ataxia (SARA). The mean SARA score of the patients was 13.5. Compared to the controls the performance of the patients was slow (p < 0.001) and arrhythmic (p < 0.001), but end-point accuracy on the touch screen was intact. The SARA score correlated with the standard deviation of amplitude of angular acceleration in Z-axis (F(1,17) = 15.00, p < 0.001 with R2 = 0.47). Upper limb movements of the patients with degenerative ataxia were slower and more arrhythmic than those in the controls. The patients retained spatial end-point accuracy.

Mitochondrial Dynamics and Insulin Secretion

Mitochondria are involved in the regulation of cellular energy metabolism, calcium homeostasis, and apoptosis. For mitochondrial quality control, dynamic processes, such as mitochondrial fission and fusion, are necessary to maintain shape and function. Disturbances of mitochondrial dynamics lead to dysfunctional mitochondria, which contribute to the development and progression of numerous diseases, including Type 2 Diabetes (T2D). Compelling evidence has been put forward that mitochondrial dynamics play a significant role in the metabolism-secretion coupling of pancreatic β cells. The disruption of mitochondrial dynamics is linked to defects in energy production and increased apoptosis, ultimately impairing insulin secretion and β cell death. This review provides an overview of molecular mechanisms controlling mitochondrial dynamics, their dysfunction in pancreatic β cells, and pharmaceutical agents targeting mitochondrial dynamic proteins, such as mitochondrial division inhibitor-1 (mdivi-1), dynasore, P110, and 15-oxospiramilactone (S3).

Neuropathy, Ataxia, and Retinitis Pigmentosa Syndrome

OBJECTIVES

To provide an overview about the phenotype, genotype, treatment, and outcome of neuropathy, ataxia, and retinitis pigmentosa (NARP) syndrome.

METHODS

Systematic review by application of appropriate search terms.

RESULTS

NARP syndrome is a syndromic mitochondrial disorder due to pathogenic variants in MT-ATP6. The canonical phenotypic features of NARP syndrome include proximal muscle weakness, axonal neuropathy, cerebellar ataxia, and retinitis pigmentosa. Noncanonical phenotypic features in NARP include epilepsy, cerebral or cerebellar atrophy, optic atrophy, cognitive impairment, dementia, sleep apnea syndrome, hearing impairment, renal insufficiency, and diabetes. So far, 10 pathogenic variants in MT-ATP6 have been associated with NARP, NARP-like syndrome, or NARP/maternally inherited Leigh overlap syndrome. Most pathogenic MT-ATP6 variants are missense, but a few truncating pathogenic variants have been reported. The most common variant responsible for NARP is the transversion m.8993T>G. Only symptomatic treatment for NARP syndrome is available. In most of the cases, patients die prematurely. Patients with late-onset NARP survive longer.

CONCLUSIONS

NARP is a rare, syndromic, monogenic mitochondrial disorder due to pathogenic variants in MT-ATP6. The nervous system and the eyes are most commonly affected. Although only symptomatic treatment is available, the outcome is usually fair.

Inflammatory Mechanisms of Diabetes and Its Vascular Complications

The main cause of death in patients with type 2 DM is cardiovascular complications resulting from the progression of atherosclerosis. The pathophysiology of the association between diabetes and its vascular complications is complex and multifactorial and closely related to the toxic effects of hyperglycemia that causes increased generation of reactive oxygen species and promotes the secretion of pro-inflammatory cytokines. Subsequent oxidative stress and inflammation are major factors of the progression of type 2 DM and its vascular complications. Data on the pathogenesis of the development of type 2 DM and associated cardiovascular diseases, in particular atherosclerosis, open up broad prospects for the further development of new diagnostic and therapeutic approaches.

Mitochondrial and metabolic dysfunction in ageing and age-related diseases

Organismal ageing is accompanied by progressive loss of cellular function and systemic deterioration of multiple tissues, leading to impaired function and increased vulnerability to death. Mitochondria have become recognized not merely as being energy suppliers but also as having an essential role in the development of diseases associated with ageing, such as neurodegenerative and cardiovascular diseases. A growing body of evidence suggests that ageing and age-related diseases are tightly related to an energy supply and demand imbalance, which might be alleviated by a variety of interventions, including physical activity and calorie restriction, as well as naturally occurring molecules targeting conserved longevity pathways. Here, we review key historical advances and progress from the past few years in our understanding of the role of mitochondria in ageing and age-related metabolic diseases. We also highlight emerging scientific innovations using mitochondria-targeted therapeutic approaches.

Mitophagy and mitochondrial dynamics in type 2 diabetes mellitus treatment

The prevalence of type 2 diabetes is associated with inflammatory bowels diseases, nonalcoholic steatohepatitis and even a spectrum of cancer such as colon cancer and liver cancer, resulting in a substantial healthcare burden on our society. Autophagy is a key regulator in metabolic homeostasis such as lipid metabolism, energy management and the balance of cellular mineral substances. Mitophagy is selective autophagy for clearing the damaged mitochondria and dysfunctional mitochondria. A myriad of evidence has demonstrated a major role of mitophagy in the regulation of type 2 diabetes and metabolic homeostasis. It is well established that defective mitophagy has been linked to the development of insulin resistance. Moreover, insulin resistance is further progressed to various diseases such as nephropathy, retinopathy and cardiovascular diseases. Concordantly, restoration of mitophagy will be a reliable and therapeutic target for type 2 diabetes. Recently, various phytochemicals have been proved to prevent dysfunctions of β-cells by mitophagy inductions during diabetes developments. In agreement with the above phenomenon, mitophagy inducers should be warranted as potential and novel therapeutic agents for treating diabetes. This review focuses on the role of mitophagy in type 2 diabetes relevant diseases and the pharmacological basis and therapeutic potential of autophagy regulators in type 2 diabetes.

The Mitochondrial Genome in Aging and Disease and the Future of Mitochondrial Therapeutics

Mitochondria are intracellular organelles that utilize nutrients to generate energy in the form of ATP by oxidative phosphorylation. Mitochondrial DNA (mtDNA) in humans is a 16,569 base pair double-stranded circular DNA that encodes for 13 vital proteins of the electron transport chain. Our understanding of the mitochondrial genome’s transcription, translation, and maintenance is still emerging, and human pathologies caused by mtDNA dysfunction are widely observed. Additionally, a correlation between declining mitochondrial DNA quality and copy number with organelle dysfunction in aging is well-documented in the literature. Despite tremendous advancements in nuclear gene-editing technologies and their value in translational avenues, our ability to edit mitochondrial DNA is still limited. In this review, we discuss the current therapeutic landscape in addressing the various pathologies that result from mtDNA mutations. We further evaluate existing gene therapy efforts, particularly allotopic expression and its potential to become an indispensable tool for restoring mitochondrial health in disease and aging.

A lack of a definite correlation between male sub-fertility and single nucleotide polymorphisms in sperm mitochondrial genes MT-CO3, MT-ATP6 and MT-ATP8

BACKGROUND

An inability of a man to conceive a potentially fertile woman after a year of unprotected intercourse is defined as male infertility. It is reported that 30-40% of males in their reproductive years have abnormalities in sperm production, either qualitatively or quantitatively, or both. However, genetic factors result in up to 15% of male infertility cases. The present study aimed to analyze the possible correlations between sub-fertility and polymorphisms in sperm mitochondrial CO3, ATP6 and ATP8 genes in sub-fertile men.

METHODS AND RESULTS

For 67 sub-fertile and 44 fertile male samples, Sanger sequencing of selected mitochondrial DNA genes was done. A total of twelve SNPs in the MT-CO3 gene: rs2248727, rs7520428, rs3134801, rs9743, rs28358272, rs2853824, rs2856985, rs2854139, rs41347846, rs28380140, rs3902407, and 28,411,821, fourteen SNPs in the MT-ATP6: rs2001031, rs2000975, rs2298011, rs7520428, rs9645429, rs112660509, rs6650105, rs6594033, rs6594034, rs6594035, rs3020563, rs28358887, rs2096044, and rs9283154, and ten SNPs in the MT-ATP8: rs9285835, rs9285836, rs9283154, rs8179289, rs121434446, rs1116906, rs2153588, rs1116905, rs1116907, and rs3020563 were detected in the case and control groups at different nucleotide positions. Only the rs7520428 in the MT-CO3 and MT-ATP6 showed a statistically significant difference between sub-fertile and fertile groups in the genotype's and allele's frequency test (P < 0.0001 for both).

CONCLUSION

The results of our study suggest that male sub-fertility is linked with rs7520428 SNP in MT-CO3 and MT-ATP6. The studied polymorphic variations in the MT-ATP8 gene, on the contrary, did not reveal any significant association with male sub-fertility.

The Mutations and Clinical Variability in Maternally Inherited Diabetes and Deafness: An Analysis of 161 Patients

Aims

To investigate the clinical features and mitochondrial mutations for maternally inherited diabetes and deafness.

Methods

PubMed, Embase, Medline, Web of Science, the China National Knowledge Infrastructure, and Wanfang were searched with the following search terms: "Maternally inherited diabetes and deafness" OR "MIDD" OR "Mitochondrial diabetes". The mutations and clinical features were analyzed. Correlation between the heteroplasmy levels of the m.3243A>G mutation in the peripheral blood and age at the onset of diabetes was conducted by Spearman test. The significance level was set as p < 0.05. Statistical analysis was performed using the Statistical Package for the Social Sciences version 26 for Windows.

Results

Totally 161 patients with 21 different mitochondrial mutations were enrolled. The most common mutation was the m.3243A>G mutation in 136 cases. Of 142 patients, 120 (84.51%) had family histories of diabetes or hearing loss. Hearing loss presented in 85.71% of the patients with mitochondrial mutations. Central nervous system diseases were found in 29.19%, myopathy in 22.98%, oculopathy in 23.60%, cardiac disease in 23.60%, and nephropathy in 13.66% of the patients. Forty-two of 101 (41.58%) patients were underweight. A significant negative correlation was found between the heteroplasmy levels of the m.3243A>G mutation in the peripheral blood and age at the onset of diabetes.

Conclusions

The young onset of diabetes with low or normal BMI, maternal inheritance, and presence of impairments of multiple systems should prompt a genetic testing in order to differentiate MIDD from other types of diabetes earlier.

An Overview of Mitochondrial Protein Defects in Neuromuscular Diseases

Neuromuscular diseases (NMDs) are dysfunctions that involve skeletal muscle and cause incorrect communication between the nerves and muscles. The specific causes of NMDs are not well known, but most of them are caused by genetic mutations. NMDs are generally progressive and entail muscle weakness and fatigue. Muscular impairments can differ in onset, severity, prognosis, and phenotype. A multitude of possible injury sites can make diagnosis of NMDs difficult. Mitochondria are crucial for cellular homeostasis and are involved in various metabolic pathways; for this reason, their dysfunction can lead to the development of different pathologies, including NMDs. Most NMDs due to mitochondrial dysfunction have been associated with mutations of genes involved in mitochondrial biogenesis and metabolism. This review is focused on some mitochondrial routes such as the TCA cycle, OXPHOS, and β-oxidation, recently found to be altered in NMDs. Particular attention is given to the alterations found in some genes encoding mitochondrial carriers, proteins of the inner mitochondrial membrane able to exchange metabolites between mitochondria and the cytosol. Briefly, we discuss possible strategies used to diagnose NMDs and therapies able to promote patient outcome.

Molecular epidemiology of hereditary ataxia in Finland

BACKGROUND

The genetics of cerebellar ataxia is complex. Hundreds of causative genes have been identified, but only a few cause more than single cases. The spectrum of ataxia-causing genes differs considerably between populations. The aim of the study was to investigate the molecular epidemiology of ataxia in the Finnish population.

PATIENTS AND METHODS

All patients in hospital database were reviewed for the diagnosis of unspecified ataxia. Acquired ataxias and nongenetic ataxias such as those related to infection, trauma or stroke were excluded. Sixty patients with sporadic ataxia with unknown etiology and 36 patients with familial ataxia of unknown etiology were recruited in the study. Repeat expansions in the SCA genes (ATXN1, 2, 3, 7, 8/OS, CACNA1A, TBP), FXN, and RFC1 were determined. Point mutations in POLG, SPG7 and in mitochondrial DNA (mtDNA) were investigated. In addition, DNA from 8 patients was exome sequenced.

RESULTS

A genetic cause of ataxia was found in 33 patients (34.4%). Seven patients had a dominantly inherited repeat expansion in ATXN8/OS. Ten patients had mitochondrial ataxia resulting from mutations in nuclear mitochondrial genes POLG or RARS2, or from a point mutation m.8561C > G or a single deletion in mtDNA. Interestingly, five patients were biallelic for the recently identified pathogenic repeat expansion in RFC1. All the five patients presented with the phenotype of cerebellar ataxia, neuropathy, and vestibular areflexia (CANVAS). Moreover, screening of 54 patients with Charcot-Marie-Tooth neuropathy revealed four additional patients with biallelic repeat expansion in RFC1, but none of them had cerebellar symptoms.

CONCLUSIONS

Expansion in ATXN8/OS results in the majority of dominant ataxias in Finland, while mutations in RFC1 and POLG are the most common cause of recessive ataxias. Our results suggest that analysis of RFC1 should be included in the routine diagnostics of idiopathic ataxia and Charcot-Marie-Tooth polyneuropathy.

Mitochondria and Antibiotics: For Good or for Evil?

The discovery and application of antibiotics in the common clinical practice has undeniably been one of the major medical advances in our times. Their use meant a drastic drop in infectious diseases-related mortality and contributed to prolonging human life expectancy worldwide. Nevertheless, antibiotics are considered by many a double-edged sword. Their extensive use in the past few years has given rise to a global problem: antibiotic resistance. This factor and the increasing evidence that a wide range of antibiotics can damage mammalian mitochondria, have driven a significant sector of the medical and scientific communities to advise against the use of antibiotics for purposes other to treating severe infections. Notwithstanding, a notorious number of recent studies support the use of these drugs to treat very diverse conditions, ranging from cancer to neurodegenerative or mitochondrial diseases. In this context, there is great controversy on whether the risks associated to antibiotics outweigh their promising beneficial features. The aim of this review is to provide insight in the topic, purpose for which the most relevant findings regarding antibiotic therapies have been discussed.

Secondary manifestations of mitochondrial disorders

Mitochondrial disorders (MIDs) are a heterogeneous group of genetic metabolic diseases due to mutations in the mitochondrial DNA (mtDNA) or in the nuclear DNA (nDNA) (Rahman and Rahman, 2018). Some affected genes encode proteins with various functions, or structural RNAs such as transfer RNAs (tRNAs) and ribosomal RNAs (rRNAs). MIDs may also be caused by mutations in non-coding regions (e.g., D-loop of mtDNA) (Rahman and Rahman, 2018). Proteins involved in MIDs include enzymes, assembling factors, transport proteins, signaling proteins, pore proteins, and fusion/fission proteins (Gorman et al., 2016). The pathways most frequently affected by mutations in "mitochondrial genes" are the respiratory chain and the oxidative phosphorylation. Dysfunction of many other pathways (e.g., β-oxidation, pyruvate-dehydrogenase complex, and heme synthesis) may also manifest as MIDs (Hu et al., 2019). The estimated prevalence of MIDs is at least 1:5000 (Ng and Turnbull, 2016).

Strategies for mitochondrial gene editing

Mitochondria, as the energy factory of cells, participate in metabolism processes and play a critical role in the maintenance of human life activities. Mitochondria belong to semi-automatic organelles, which have their own genome different from nuclear genome. Mitochondrial DNA (mtDNA) mutations can cause a series of diseases and threaten human health. However, an effective approach to edit mitochondrial DNA, though long-desired, is lacking. In recent years, gene editing technologies, represented by restriction endonucleases (RE) technology, zinc finger nuclease (ZFN) technology, transcription activator-like effector nuclease (TALEN) technology, CRISPR system and pAgo-based system have been comprehensively explored, but the application of these technologies in mitochondrial gene editing is still to be explored and optimized. The present study highlights the progress and limitations of current mitochondrial gene editing technologies and approaches, and provides insights for development of novel strategies for future attempts.

Mutations in MT-ATP6 are a frequent cause of adult-onset spinocerebellar ataxia

Adult-onset ataxias are a genetically and clinically heterogeneous group of movement disorders. In addition to nuclear gene mutations, sequence changes have also been described in the mitochondrial genome. Here, we present findings of mutation analysis of the mitochondrial gene MT-ATP6. We analyzed 94 patients with adult-onset spinocerebellar ataxia (SCA), including 34 sporadic cases. In all patients, common sequence changes found in SCAs such as repeat expansions and point mutations had been excluded previously. We found pathogenic MT-ATP variants in five of these patients (5.32%), two of whom were sporadic. Four of the five mutations have not previously been described in ataxias. All but one of these mutations affect transmembrane helices of subunit-α of ATP synthase. Two mutations (p.G16S, and p.P18S) disrupt transmembrane helix 1 (TMH1), one mutation (p.G167D) affects TMH5, and another one (p.L217P) TMH6. The fifth mutation (p.T96A) describes an amino acid change in close proximity to transmembrane helix 3 (TMH3). The level of heteroplasmy was either complete or very high ranging from 87 to 99%. The high prevalence of pathogenic MT-ATP6 variants suggests that analysis of this gene should be included in the routine workup of both hereditary and sporadic ataxias.

The ATP Synthase Deficiency in Human Diseases

Human diseases range from gene-associated to gene-non-associated disorders, including age-related diseases, neurodegenerative, neuromuscular, cardiovascular, diabetic diseases, neurocognitive disorders and cancer. Mitochondria participate to the cascades of pathogenic events leading to the onset and progression of these diseases independently of their association to mutations of genes encoding mitochondrial protein. Under physiological conditions, the mitochondrial ATP synthase provides the most energy of the cell via the oxidative phosphorylation. Alterations of oxidative phosphorylation mainly affect the tissues characterized by a high-energy metabolism, such as nervous, cardiac and skeletal muscle tissues. In this review, we focus on human diseases caused by altered expressions of ATP synthase genes of both mitochondrial and nuclear origin. Moreover, we describe the contribution of ATP synthase to the pathophysiological mechanisms of other human diseases such as cardiovascular, neurodegenerative diseases or neurocognitive disorders.

ATP synthase deficiency due to m.8528T>C mutation - a novel cause of severe neonatal hyperammonemia requiring hemodialysis

OBJECTIVES

Hyperammonemia in a newborn is a serious condition, which requires prompt intervention as it can lead to severe neurological impairment and death if left untreated. The most common causes of hyperammonemia in a newborn are acute liver failure and inherited metabolic disorders. Several mitochondrial disorders have been described as a cause of severe neonatal hyperammonemia.

CASE PRESENTATION

Here we describe a new case of adenosine-triphosphate (ATP) synthase deficiency due to m.8528T>C mutation as a novel cause of severe neonatal hyperammonemia. So far six patients with this mutation have been described but none of them was reported to need hemodialysis in the first days of life.

CONCLUSION

This broadens the so far known differential diagnosis of severe neonatal hyperammonemia requiring hemodialysis.

Blackout in the powerhouse: clinical phenotypes associated with defects in the assembly of OXPHOS complexes and the mitoribosome

Mitochondria produce the bulk of the energy used by almost all eukaryotic cells through oxidative phosphorylation (OXPHOS) which occurs on the four complexes of the respiratory chain and the F₁–F₀ ATPase. Mitochondrial diseases are a heterogenous group of conditions affecting OXPHOS, either directly through mutation of genes encoding subunits of OXPHOS complexes, or indirectly through mutations in genes encoding proteins supporting this process. These include proteins that promote assembly of the OXPHOS complexes, the post-translational modification of subunits, insertion of cofactors or indeed subunit synthesis. The latter is important for all 13 of the proteins encoded by human mitochondrial DNA, which are synthesised on mitochondrial ribosomes. Together the five OXPHOS complexes and the mitochondrial ribosome are comprised of more than 160 subunits and many more proteins support their biogenesis. Mutations in both nuclear and mitochondrial genes encoding these proteins have been reported to cause mitochondrial disease, many leading to defective complex assembly with the severity of the assembly defect reflecting the severity of the disease. This review aims to act as an interface between the clinical and basic research underpinning our knowledge of OXPHOS complex and ribosome assembly, and the dysfunction of this process in mitochondrial disease.

Mitochondrial disorders of the OXPHOS system

Mitochondrial disorders are among the most frequent inborn errors of metabolism, their primary cause being the dysfunction of the oxidative phosphorylation system (OXPHOS). OXPHOS is composed of the electron transport chain (ETC), formed by four multimeric enzymes and two mobile electron carriers, plus an ATP synthase [also called complex V (cV)]. The ETC performs the redox reactions involved in cellular respiration while generating the proton motive force used by cV to synthesize ATP. OXPHOS biogenesis involves multiple steps, starting from the expression of genes encoded in physically separated genomes, namely the mitochondrial and nuclear DNA, to the coordinated assembly of components and cofactors building each individual complex and eventually the supercomplexes. The genetic cause underlying around half of the diagnosed mitochondrial disease cases is currently known. Many of these cases result from pathogenic variants in genes encoding structural subunits or additional factors directly involved in the assembly of the ETC complexes. Here, we review the historical and most recent findings concerning the clinical phenotypes and the molecular pathological mechanisms underlying this particular group of disorders.

Mitochondrial ND1 T4216C and ND2 C5178A mutations are associated with maternally transmitted diabetes mellitus

Mutations in mitochondrial DNA (mtDNA) are important causes for type 2 diabetes mellitus (T2DM). To investigate the association between mtDNA mutations/variants and diabetes, we reported here clinical, genetic and biochemical characterization of a Chinese pedigree with maternally transmitted T2DM. Using PCR and direct sequencing analysis of mitochondrial genomes from the matrilineal relatives, we identified two potential pathogenic mutations, m.T4216C (p.Y304H) and m.C5178A (p.L237M) in the ND1 and ND2 genes, respectively, together with a set of genetic polymorphisms belonging to the human mitochondrial haplogroup D4b. Moreover, by isolating and analyzing polymononuclear leukocytes generated from the T2DM patients and controls, we identified lower levels of mitochondrial membrane potential and ATP production in T2DM patients than in the controls, in contrast, a significantly higher level of reactive oxygen species was observed in the T2DM patients carrying both of the m.T4216C and m.C5178A mutations (p < 0.05 for all). In addition, the plasma levels of malondialdehyde and 8-hydroxydeoxyguanosine in the T2DM patients markedly increased, while the level of superoxide dismutase decreased (p < 0.05 for all). Taken together, our data indicated that the ND1 T4216C and ND2 C5178A mutations may lead to oxidative stress and impair the mitochondrial function, and this, in turn, might have been involved in the pathogenesis and progression of T2DM in this pedigree. Thus, our study provides novel insight into the pathophysiology of T2DM that is manifested by mitochondrial dysfunction.

Human Mitochondrial Pathologies of the Respiratory Chain and ATP Synthase: Contributions from Studies of Saccharomyces cerevisiae

The ease with which the unicellular yeast Saccharomyces cerevisiae can be manipulated genetically and biochemically has established this organism as a good model for the study of human mitochondrial diseases. The combined use of biochemical and molecular genetic tools has been instrumental in elucidating the functions of numerous yeast nuclear gene products with human homologs that affect a large number of metabolic and biological processes, including those housed in mitochondria. These include structural and catalytic subunits of enzymes and protein factors that impinge on the biogenesis of the respiratory chain. This article will review what is currently known about the genetics and clinical phenotypes of mitochondrial diseases of the respiratory chain and ATP synthase, with special emphasis on the contribution of information gained from pet mutants with mutations in nuclear genes that impair mitochondrial respiration. Our intent is to provide the yeast mitochondrial specialist with basic knowledge of human mitochondrial pathologies and the human specialist with information on how genes that directly and indirectly affect respiration were identified and characterized in yeast.

Novel Point Mutations in Mitochondrial MT-CO2 Gene May Be Risk Factors for Coronary Artery Disease

A wide range of genetic and environmental interactions are involved in the development of coronary artery disease (CAD). Considerable evidence suggests that mitochondrial DNA mutations are associated with heart failure. In this work, we examined the possible mutations in hotspot mitochondrial genes and their association with Iranian patients with coronary artery disease. In this case-control study, nucleotide variations were investigated in 109 patients with coronary atherosclerosis and 105 control subjects with no family history of cardiovascular disease. The molecular analysis of related mitochondrial genes was performed by polymerase chain reaction sequencing. Our results showed 25 nucleotide variations (10 missense mutations, 9 synonymous polymorphisms, and 6 variants in tRNA genes) that for the first time were presented in coronary artery disease. Our results suggest that novel heteroplasmic m.8231 C>A mutation is involved in CAD (p = 0.007). These nucleotide variations suggest the role of mitochondrial mutations as a predisposing factor which in combination with environmental risk factors may affect the pathogenesis of coronary atherosclerosis. So, further investigation is needed for a better understanding of the pathogenesis and predisposing effects of these variations on the disease.

Delineating MT-ATP6-associated disease: From isolated neuropathy to early onset neurodegeneration

Objective

To delineate the phenotypic and genotypic spectrum in carriers of mitochondrial MT-ATP6 mutations in a large international cohort.

Methods

We analyzed in detail the clinical, genetical, and neuroimaging data from 132 mutation carriers from national registries and local databases from Europe, USA, Japan, and China.

Results

We identified 113 clinically affected and 19 asymptomatic individuals with a known pathogenic MT-ATP6 mutation. The most frequent mutations were m.8993 T > G (53/132, 40%), m.8993 T > C (30/132, 23%), m.9176 T > C (30/132, 23%), and m.9185 T > C (12/132, 9%). The degree of heteroplasmy was high both in affected (mean 95%, range 20%–100%) and unaffected individuals (mean 73%, range 20%–100%). Age at onset ranged from prenatal to the age of 75 years, but almost half of the patients (49/103, 48%) became symptomatic before their first birthday. In 28 deceased patients, the median age of death was 14 months. The most frequent symptoms were ataxia (81%), cognitive dysfunction (49%), neuropathy (48%), seizures (37%), and retinopathy (14%). A diagnosis of Leigh syndrome was made in 55% of patients, whereas the classic syndrome of neuropathy, ataxia, and retinitis pigmentosa (NARP) was rare (8%).

Conclusions

In this currently largest series of patients with mitochondrial MT-ATP6 mutations, the phenotypic spectrum ranged from asymptomatic to early onset multisystemic neurodegeneration. The degree of mutation heteroplasmy did not reliably predict disease severity. Leigh syndrome was found in more than half of the patients, whereas classic NARP syndrome was rare. Oligosymptomatic presentations were rather frequent in adult-onset patients, indicating the need to include MT-ATP6 mutations in the differential diagnosis of both ataxias and neuropathies.

A novel variant m.8561C>T in the overlapping region of MT-ATP6 and MT-ATP8 in a child with early-onset severe neurological signs

Among mitochondrial diseases, isolated complex V (CV) deficiency represents a rare cause of respiratory chain (RC) dysfunction. In mammalian mitochondrial DNA (mtDNA), MT-ATP6 partly overlaps with MT-ATP8 making double mutations possible, yet extremely rarely reported principally in patients with cardiomyopathy. Here, we report a novel m.8561 C>T substitution in the overlapping region of MT-ATP6 and MT-ATP8 in a child with early-onset ataxia, psychomotor delay and microcephaly, enlarging the clinical manifestations spectrum associated with CV deficiency.

Mitochondrial DNA 7908-8816 region mutations in maternally inherited essential hypertensive subjects in China

Background

Nuclear genes or family-based mitochondrial screening have been the focus of genetic studies into essential hypertension. Studies into the role of mitochondria in sporadic Chinese hypertensives are lacking. The objective of the study was to explore the relationship between mitochondrial DNA (mtDNA) variations and the development of maternally inherited essential hypertension (MIEH) in China.

Methods

Yangzhou residents who were outpatients or in-patients at the Department of Cardiology in Northern Jiangsu People’s Hospital (Jiangsu, China) from June 2009 to June 2015 were recruited in a 1:1 case control study of 600 gender-matched Chinese MIEH subjects and controls. Genomic DNA was isolated from whole blood cells. The most likely sites for hypertension were screened using oligodeoxynucleotides at positions 7908–8816, purified and subsequently analyzed by direct sequencing according to the revised consensus Cambridge sequence. The frequency, density, type and conservative evolution of mtDNA variations were comprehensively analyzed.

Results

We found a statistical difference between the two groups for body mass index, waist circumference, abdominal circumference, triglyceride, low-density lipoprotein cholesterol, fasting blood glucose, uric acid, creatinine and blood urea nitrogen (P < 0.05). More amino-acid changes and RNA variants were found in MIEH subjects than the controls (P < 0.01). The detection system simultaneously identified 40 different heteroplasmic or homoplasmic mutations in 4 genes: COXII, tRNA^Lys, ATP8 and ATP 6. The mtDNA variations were mainly distributed in regions of ATP6 binding sites, and the site of highest mutation frequency was m. 8414C > T. Three changes in single bases (C8414T in ATP8, A8701G in ATP6 and G8584A in ATP6) were significantly different in the MIEH patients and the controls (P < 0.001). The m.8273_8281del mutation was identified from 59 MIEH patients.

Conclusions

Our results indicate that novel mtDNA mutations may be involved in the pathological process of MIEH, and mitochondrial genetic characteristics were identified in MIEH individuals.

Electronic supplementary material

The online version of this article (10.1186/s12920-018-0408-0) contains supplementary material, which is available to authorized users.

The biochemical characterization of a missense mutation m.8914C>T in ATP6 gene associated with mitochondrial encephalomyopathy

Mutations in ATP6 gene are frequent causes of mitochondrial encephalomyopathies. ATP6 gene encodes one subunit of complexⅤ. The present study described a missense mutation in ATP6 gene in a 8-year-old Chinese boy with mitochondrial encephalomyopathy. We identified one missense mutation in ATP6 gene (m.8914C>T) by mitochondrial DNA sequencing. This mutation altered the amino acid proline in serine, and alterative protein is predicted to be harmful. The mutation load in blood sample of patient is 59.49%. Activity of all mitochondrial complexes in blood are normal, however, the total function of mitochondrial oxidative phosphorylation were declined (including pathwayⅠ, pathwayⅡ and pathwayⅣ). The missense mutation (m.8914C>T) in ATP6 gene could result in abnormal function of complexV and is related with mitochondrial encephalomyopathy.

Investigation of frequent somatic mutations of MTND5 gene in gastric cancer cell lines and tissues

The present study investigated the single nucleotide variants (SNVs) in mitochondrial DNA (mtDNA) of 13 paired gastric cancer tissue samples and seven gastric cancer cell lines using direct sequencing analysis of the MTND5 region. Results showed that nuclear mitochondrial pseudogenes (NUMTs) and mitochondrial copy number affected the detection of the SNV frequency in gastric cancer tissue and cell line samples using high-throughput sequencing technique. The heteroplasmic point mutation C12474T and G12835A happened in AGS and BGC823 cell lines, respectively. A total of seven SNVs were found in three paired gastric cancer tissue samples, including five heteroplasmic point mutations (A12406G, C12705T, T12882C, G12501A, and A12584G) and two homoplasmic point mutations (G12561A and C13590T). Gastric cancer tissue sample 16 exhibited the highest SNVs frequency with four SNVs (np 12406, np 12705, np 12882, and np 12501), whereas no SNVs or SNPs were detected in the tissue sample 4. SNP 12705 turned out to be an SNV in gastric cancer tissue sample 16. SNV 12338 detected by exome sequencing approach appeared to be an SNP in this study.

Human diseases associated with defects in assembly of OXPHOS complexes

The structural biogenesis and functional proficiency of the multiheteromeric complexes forming the mitochondrial oxidative phosphorylation system (OXPHOS) require the concerted action of a number of chaperones and other assembly factors, most of which are specific for each complex. Mutations in a large number of these assembly factors are responsible for mitochondrial disorders, in most cases of infantile onset, typically characterized by biochemical defects of single specific complexes. In fact, pathogenic mutations in complex-specific assembly factors outnumber, in many cases, the repertoire of mutations found in structural subunits of specific complexes. The identification of patients with specific defects in assembly factors has provided an important contribution to the nosological characterization of mitochondrial disorders, and has also been a crucial means to identify a huge number of these proteins in humans, which play an essential role in mitochondrial bioenergetics. The wide use of next generation sequencing (NGS) has led to and will allow the identifcation of additional components of the assembly machinery of individual complexes, mutations of which are responsible for human disorders. The functional studies on patients' specimens, together with the creation and characterization of in vivo models, are fundamental to better understand the mechanisms of each of them. A new chapter in this field will be, in the near future, the discovery of mechanisms and actions underlying the formation of supercomplexes, molecular structures formed by the physical, and possibly functional, interaction of some of the individual respiratory complexes, particularly complex I (CI), III (CIII), and IV (CIV).

Gene therapy for the mitochondrial genome: Purging mutations, pacifying ailments

In the recent years, the reported cases of mitochondrial disorders have reached a colossal number. These disorders spawn a sundry of pathological conditions, which lead to pernicious symptoms and even fatality. Due to the unpredictable etiologies, mitochondrial diseases are putatively referred to as "mystondria" (mysterious diseases of mitochondria). Although present-day research has greatly improved our understanding of mitochondrial disorders, effective therapeutic interventions are still at the precursory stage. The conundrum becomes further complicated because these pathologies might occur due to either mitochondrial DNA (mtDNA) mutations or due to mutations in the nuclear DNA (nDNA), or both. While correcting nDNA mutations by using gene therapy (replacement of defective genes by delivering wild-type (WT) ones into the host cell, or silencing a dominant mutant allele that is pathogenic) has emerged as a promising strategy to address some mitochondrial diseases, the complications in correcting the defects of mtDNA in order to renovate mitochondrial functions have remained a steep challenge. In this review, we focus specifically on the selective gene therapy strategies that have demonstrated prospects in targeting the pathological mutations in the mitochondrial genome, thereby treating mitochondrial ailments.

Advances in the Molecular Pathophysiology, Genetics, and Treatment of Primary Ovarian Insufficiency

Primary ovarian insufficiency (POI) affects ∼1% of women before 40 years of age. The recent leap in genetic knowledge obtained by next generation sequencing (NGS) together with animal models has further elucidated its molecular pathogenesis, identifying novel genes/pathways. Mutations of >60 genes emphasize high genetic heterogeneity. Genome-wide association studies have revealed a shared genetic background between POI and reproductive aging. NGS will provide a genetic diagnosis leading to genetic/therapeutic counseling: first, defects in meiosis or DNA repair genes may predispose to tumors; and second, specific gene defects may predict the risk of rapid loss of a persistent ovarian reserve, an important determinant in fertility preservation. Indeed, a recent innovative treatment of POI by in vitro activation of dormant follicles proved to be successful.

ATP Synthase Diseases of Mitochondrial Genetic Origin

Devastating human neuromuscular disorders have been associated to defects in the ATP synthase. This enzyme is found in the inner mitochondrial membrane and catalyzes the last step in oxidative phosphorylation, which provides aerobic eukaryotes with ATP. With the advent of structures of complete ATP synthases, and the availability of genetically approachable systems such as the yeast Saccharomyces cerevisiae, we can begin to understand these molecular machines and their associated defects at the molecular level. In this review, we describe what is known about the clinical syndromes induced by 58 different mutations found in the mitochondrial genes encoding membrane subunits 8 and a of ATP synthase, and evaluate their functional consequences with respect to recently described cryo-EM structures.

Recent Advances in Mitochondrial Disease

Mitochondrial disease is a challenging area of genetics because two distinct genomes can contribute to disease pathogenesis. It is also challenging clinically because of the myriad of different symptoms and, until recently, a lack of a genetic diagnosis in many patients. The last five years has brought remarkable progress in this area. We provide a brief overview of mitochondrial origin, function, and biology, which are key to understanding the genetic basis of mitochondrial disease. However, the primary purpose of this review is to describe the recent advances related to the diagnosis, genetic basis, and prevention of mitochondrial disease, highlighting the newly described disease genes and the evolving methodologies aimed at preventing mitochondrial DNA disease transmission.

Mitochondrial dynamics in type 2 diabetes: Pathophysiological implications

Mitochondria play a key role in maintaining cellular metabolic homeostasis. These organelles have a high plasticity and are involved in dynamic processes such as mitochondrial fusion and fission, mitophagy and mitochondrial biogenesis. Type 2 diabetes is characterised by mitochondrial dysfunction, high production of reactive oxygen species (ROS) and low levels of ATP. Mitochondrial fusion is modulated by different proteins, including mitofusin-1 (MFN1), mitofusin-2 (MFN2) and optic atrophy (OPA-1), while fission is controlled by mitochondrial fission 1 (FIS1), dynamin-related protein 1 (DRP1) and mitochondrial fission factor (MFF). PARKIN and (PTEN)-induced putative kinase 1 (PINK1) participate in the process of mitophagy, for which mitochondrial fission is necessary. In this review, we discuss the molecular pathways of mitochondrial dynamics, their impairment under type 2 diabetes, and pharmaceutical approaches for targeting mitochondrial dynamics, such as mitochondrial division inhibitor-1 (mdivi-1), dynasore, P110 and 15-oxospiramilactone. Furthermore, we discuss the pathophysiological implications of impaired mitochondrial dynamics, especially in type 2 diabetes.