Commentary: Human mitochondrial cytopathies

Mitochondria provide energy (ATP) for all eukaryotic cells except mature erythrocytes and keratinocytes. They are abundant in cells that expend much energy, such as muscle, exocrine pancreas, nervous system, and heart cells, and motile sperm. Many mitochondrial enzymes are encoded by nuclear DNA and imported into the mitochondria. Like bacteria, mitochondria possess their own DNA and ribosomes. They are fueled by fatty acids and pyruvate, and through acetyl-coA enzyme can use fats, carbohydrates, and proteins as energy sources, producing ATP for cells. A high index of suspicion for mitochondrial mutations enables clinicians to recognize these unusual and rare disorders and provide proper genetic counseling. Mitochondrial cytopathies include a diverse group of diseases, affecting many organs, especially skeletal muscle and central nervous system, and are associated with abnormal mitochondria in skeletal muscle known as ragged red fibers. Mitochondrial DNA mutations are detectable in peripheral blood.     

Mitochondrial diseases--an expanding spectrum of disorders and affected genes

Mitochondrial diseases are a heterogeneous group of disorders caused by the impairment of the mitochondrial oxidative phosphorylation system which have been associated with various mutations of the mitochondrial DNA (mtDNA) and nuclear gene mutations. The clinical phenotypes are very diverse and the spectrum is still expanding. This review gives an overview of the principal clinical phenotypes and the molecular genetic basis of mitochondrial disorders identified so far.     

Dilation of the aortic root in mitochondrial disease patients

Mitochondrial cytopathies are genetically, clinically, and biochemically heterogeneous disorders due to defects of oxidative phosphorylation. The heart is highly energy dependent and therefore particularly vulnerable to defects of energy production. Hypertrophic and dilated cardiomyopathy and left ventricular noncompaction are the main cardiac manifestations occurring in mitochondrial cytopathies. Here we report ten patients with mitochondrial cytopathy presenting with dilation of the aorta. This clinical feature has not been previously reported to be associated with mitochondrial disease. The long term consequences of this observation are unknown and follow-up studies are needed to clarify the impact of this finding in the population of subjects with mitochondrial cytopathies. The mechanism(s) involved in the pathogenesis of this complication are unknown and may be potentially implicated also in the pathogenesis of other more common etiologies of aortic aneurysmal disease.     

线粒体tRNA^ser（UCN））基因突变致线粒体脑肌病1例并文献复习

目的分析1例线粒体细胞病患儿的临床表现及其基因突变特点。方法对1例临床诊断为线粒体脑肌病患儿归纳总结其临床表现及实验室检查结果,并运用PCR法扩增患儿外周血线粒体基因3243、8344和8993热点突变及已报道的62个常见突变位点所在片段,对扩增片段采用直接测序方法检测突变。在某医院年度体检中选择70名无血缘关系的健康成人作为正常对照,采用PCR-RFLP方法进行多态性分析。结果男性患儿,1岁9个月时出现持续高乳酸血症、反复严重代谢性酸中毒和高氨血症,头颅CT扫描显示双侧额顶叶对称性空泡样低密度灶,考虑线粒体性脑肌病;脑萎缩。2岁时死亡。患儿外周血线粒体基因3243、8344和8993热点突变及已报道的62个常见突变位点均未见突变,患儿线粒体tRNAser（UCN）基因存在7496 T→C突变。为证实在正常人群中线粒体tRNAser（UCN）基因是否存在7496 T→C突变,70名正常对照组皆未发现这一位点突变。结论线粒体脑肌病可以表现为代谢紊乱和神经损伤,应提高警惕。线粒体tRNAser（UCN）基因7496 T→C突变可能导致线粒体细胞病。该突变尚未见报道。     

Cardiomyopathies due to defective energy metabolism: morphological and functional features

Cardiomyopathies are defined as diseases of the myocardium associated with cardiac dysfunction and are classified by morphological characteristics as hypertrophic (HCM), dilated (DCM) arrhithmogenic right ventricular (ARVC) and restrictive cardiomyopathy. These were once considered as specific diagnoses but there is now considerable evidence that many different gene mutations can cause these pathologies. In recent years, big emphasis has been given to the possibility that deregulation of cardiac metabolism may play a role in the mechanisms that lead to cardiac maladaptive remodelling. Cardiac energy metabolism is tightly controlled in mammalian organisms during development and in response to diverse dietary, physiologic, and pathologic conditions. The cardiac phenotype of many genetic diseases caused by mutations in proteins involved in mitochondrial energy production and/or homeostasis, underscores the importance of energetic pathway on cardiac function. For example, inborn errors in nuclear-encoded mitochondrial fatty acid oxidation (FAO) pathway enzymes and defects in fatty acid uptake are an important cause of childhood HCM and sudden death. Abnormalities in mitochondrial respiratory chain function, particularly those caused by mitochondrial DNA (mtDNA) mutations, are responsible for a heterogeneous group of clinical disorders, including isolated HCM. Mitochondrial cardiomyopathies (MCM) are characterized by an adverse clinical course with biventricular dilation and failure, even at a young age. Mutations in genes encoding the gamma2 subunit of AMP-activated protein kinase (PRKAG2), alpha-galactosidase A (GLA) and lysosome-associated membrane proteine-2 (LAMP2) can cause profound myocardial hypertrophy in association with electrophysiological defects. Unlike HCM due to sarcomere gene mutations, which is characterized by myofiber disarray and fibrosis, large cytosolic vacuoles characterize cardiomyopathy due to defect in energy metabolism. Ultrastructural analysis revealed massive mitochondrial proliferation in MCM and glycogen in complexes with protein and/or lipids in cardiomyopathy due to PRKAG2, GLA and LAMP2 mutations.     

Mitochondrial diseases: molecular mechanisms, clinical presentations and diagnosis investigations

Mitochondrial diseases are relatively common inherited metabolic diseases due to mitochondrial respiratory chain dysfunction. Their clinical presentation is extremely diverse, multisystemic or confined to a single tissue, sporadic or transmitted, by maternal or mendelian inheritance. The diagnosis of mitochondrial disorders is difficult. It is based upon several types of clues both clinical (family history, type of symptoms but also their association in syndromic presentation,...) and biological (alteration of the lactate metabolism, brain imaging, morphological alterations especially of muscle tissue). The diagnosis relies upon the demonstration of a defect of the respiratory chain activities and/or upon the identification of the underlying genetic alteration. Molecular diagnosis remains quite difficult and up to-date concerns essentially mitochondrial DNA mutations. On one hand, clinical and biological presentations as well as enzymatic defects lack specificity. On the other hand, candidate genes are very numerous and part of them are probably still unknown.     

Mitochondrial disorders: genetics, counseling, prenatal diagnosis and reproductive options

Most patients with mitochondrial disorders are diagnosed by finding a respiratory chain enzyme defect or a mutation in the mitochondrial DNA (mtDNA). The provision of accurate genetic counseling and reproductive options to these families is complicated by the unique genetic features of mtDNA that distinguish it from Mendelian genetics. These include maternal inheritance, heteroplasmy, the threshold effect, the mitochondrial bottleneck, tissue variation, and selection. Although we still have much to learn about mtDNA genetics, it is now possible to provide useful guidance to families with an mtDNA mutation or a respiratory chain enzyme defect. We describe a range of current reproductive options that may be considered for prevention of transmission of mtDNA mutations, including the use of donor oocytes, prenatal diagnosis (by chorionic villus sampling or amniocentesis), and preimplantation genetic diagnosis, plus possible future options such as nuclear transfer and cytoplasmic transfer. For common mtDNA mutations associated with mitochondrial cytopathies (such as NARP, Leigh Disease, MELAS, MERRF, Leber's Hereditary Optic Neuropathy, CPEO, Kearns-Sayre syndrome, and Pearson syndrome), we summarize the available data on recurrence risk and discuss the relative advantages and disadvantages of reproductive options.     

Atypical case of Wolfram syndrome revealed through targeted exome sequencing in a patient with suspected mitochondrial disease

Background  

Mitochondrial diseases comprise a diverse set of clinical disorders that affect multiple organ systems with varying severity and age of onset. Due to their clinical and genetic heterogeneity, these diseases are difficult to diagnose. We have developed a targeted exome sequencing approach to improve our ability to properly diagnose mitochondrial diseases and apply it here to an individual patient. Our method targets mitochondrial DNA (mtDNA) and the exons of 1,600 nuclear genes involved in mitochondrial biology or Mendelian disorders with multi-system phenotypes, thereby allowing for simultaneous evaluation of multiple disease loci.     

Mitochondrial aminoacyl-tRNA synthetases in human disease

Mitochondrial aminoacyl-tRNA synthetases (mtARSs) are essential in the process of transferring genetic information from mitochondrial DNA to the complexes of the oxidative phosphorylation system. These synthetases perform an integral step in the initiation of mitochondrial protein synthesis by charging tRNAs with their cognate amino acids. All mtARSs are encoded by nuclear genes, nine of which have recently been described as disease genes for mitochondrial disorders. Unexpectedly, the clinical presentations of these diseases are highly specific to the affected synthetase. Encephalopathy is the most common manifestation but again with gene-specific outcomes. Other clinical presentations include myopathy with anemia, cardiomyopathy, tubulopathy and hearing loss with female ovarian dysgenesis. Here we review the described mutation types and the associated patient phenotypes. The identified mutation spectrum suggests that only mutation types that allow some residual tRNA-charging activity can result in the described mtARS diseases but the molecular mechanisms behind the selective tissue involvement are not currently understood.     

Morphological Findings of Extraocular Myopathy with Chronic Progressive External Ophthalmoplegia

Mitochondrial diseases are a large group of disorders resulting from mutations of nuclear DNA (nDNA) and mitochondrial DNA (mtDNA). Patients present clinically with multiple manifestations, including myopathies and multiple system disorders. Establishing a specific diagnosis often requires extensive clinical and laboratory evaluation. In this study of 2 adult patients with presumptive mitochondrial disease, the authors have identified distinctive morphological changes in medial rectus muscle biopsies that confirm the diagnosis of chronic progressive external ophthalmoplegia (CPEO). These findings demonstrate the usefulness of electron microscopy using medial rectus muscle in the diagnosis of adult patients with a slowly progressive course of mild skeletal weakness and CPEO.     

Approaches and limitations to gene therapy for mitochondrial diseases

Mitochondrial dysfunction may be caused by mutations in either the nuclear and/or the mitochondrial genome. Since 1988, mitochondrial DNA mutations have been linked to retinopathies, myopathies, neurodegenerative diseases, and possibly normal aging. Adequate drug therapies for these disorders have yet to be discovered. Therefore, gene therapy must be considered as a possible alternative. In this review, we will discuss the possibilities and the problems associated with gene therapy for mitochondrial disorders.     

The contribution of mitochondria to common disorders

Mitochondrial dysfunction secondary to mitochondrial and nuclear DNA mutations has been associated with energy deficiency in multiple organ systems and a variety of severe, often fatal, clinical syndromes. Although the production of energy is indeed the primary function of mitochondria, attention has also been directed toward their role producing reactive oxygen and nitrogen species and the subsequent widespread deleterious effects of these intermediates. The generation of toxic reactive intermediates has been implicated in a number of relatively common disorders, including neurodegenerative diseases, diabetes, and cancer. Understanding the role mitochondrial dysfunction plays in the pathogenesis of common disorders has provided unique insights into a number of diseases and offers hope for potential new therapies.     

Defects of Intergenomic Communication: Where Do We Stand?

An expanding number of autosomal diseases has been associated with mitochondrial DNA (mtDNA) depletion and multiple deletions. These disorders have been classified as defects of intergenomic communication because mutations of the nuclear DNA are thought to disrupt the normal cross-talk that regulates the integrity and quantity of mtDNA. In 1989, autosomal dominant progressive external ophthalmoplegia with multiple deletions of mitochondrial DNA was the first of these disorders to be identified.Two years later, mtDNA depletion syndrome was initially reported in infants with severe hepatopathy or myopathy. The causes of these diseases are still unclear, but genetic linkage studies have identified three chromosomal loci for AD-PEO. Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE), an autosomal recessive disorder associated with both mtDNA depletion and multiple deletions, is now known to be due to loss-of-function mutations in the gene encoding thymidine phosphorylase. Increased plasma thymidine levels in MNGIE patients suggest that imbalanced nucleoside and nucleotide pools in mitochondria may lead to impaired replication of mtDNA. Future research will certainly lead to the identification of additional genetic causes of intergenomic communication defects and will likely provide insight into the normal "dialogue" between the two genomes.     

Mitochondrial gene mutations in familial non-insulin-dependent diabetes mellitus in Taiwan

Mitochondrial gene mutations are found to cause certain forms of diabetes mellitus and related syndromes. To study the prevalence of mitochondrial gene mutations in subjects with non-insulin-dependent diabetes mellitus (NIDDM) in Taiwan, 23 pedigrees with multiple siblings affected with NIDDM were consecutively collected from patients living in northern Taiwan. The A-to-G mutation at position 3243 np in the tRNALeu gene and the mutation at position 8344 were screened by PCR-RFLP methods and confirmed by direct DNA sequence analysis. Among 23 NIDDM pedigrees, one pedigree was found to carry the 3243 np mutation. There was no 8344 np mutation in this series. Clinical features of this pedigree were consistent with mitochondrial disease in terms of maternal transmission, relatively early onset, non-obesity, insulin-requirement and association with hearing impairment. There was no correlation between the degree of heteroplasmy of mitochondrial gene mutation in leukocyte DNA and clinical severity. We conclude that a mitochondrial gene defect is an important genetic factor in familial cases with NIDDM in Taiwan.     

New insights into structure and function of mitochondria and their role in aging and disease

This review covers some novel findings on mitochondrial biochemistry and discusses diseases due to mitochondrial DNA mutations as a model of the changes occurring during physiological aging. The random collision model of organization of the mitochondrial respiratory chain has been recently challenged on the basis of findings of supramolecular organization of respiratory chain complexes. The source of superoxide in Complex I is discussed on the basis of laboratory experiments using a series of specific inhibitors and is presumably iron sulfur center N2. Maternally inherited diseases due to mutations of structural genes in mitochondrial DNA are surveyed as a model of alterations mimicking those occurring during normal aging. The molecular defects in senescence are surveyed on the basis of the "Mitochondrial Theory of Aging", establishing mitochondrial DNA somatic mutations, caused by accumulation of oxygen radical damage, to be at the basis of cellular senescence. Mitochondrial production of reactive oxygen species increases with aging and mitochondrial DNA mutations and deletions accumulate and may be responsible for oxidative phosphorylation defects. Evidence is presented favoring the mitochondrial theory, with primary mitochondrial alterations, although the problem is made more complex by changes in the cross-talk between nuclear and mitochondrial DNA.     

Novel POLG mutations in progressive external ophthalmoplegia mimicking mitochondrial neurogastrointestinal encephalomyopathy

Autosomal recessive progressive external ophthalmoplegia (PEO) is one clinical disorder associated with multiple mitochondrial DNA deletions and can be caused by missense mutations in POLG, the gene encoding the mitochondrial DNA polymerase gamma. Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) is another autosomal recessive disorder associated with PEO and multiple deletions of mitochondrial DNA in skeletal muscle. In several patients this disorder is caused by loss of function mutations in the gene encoding thymidine phosphorylase (TP). We report a recessive family with features of MNGIE but no leukoencephalopathy in which two patients carry three missense mutations in POLG, of which two are novel mutations (N846S and P587L). The third mutation was previously reported as a recessive POLG mutation (T251I). This finding indicates the need for POLG sequencing in patients with features of MNGIE without TP mutations.     

Respiratory chain supercomplexes set the threshold for respiration defects in human mtDNA mutant cybrids

Mitochondrial DNA (mtDNA) mutations cause heterogeneous disorders in humans. MtDNA exists in multiple copies per cell, and mutations need to accumulate beyond a critical threshold to cause disease, because coexisting wild-type mtDNA can complement the genetic defect. A better understanding of the molecular determinants of functional complementation among mtDNA molecules could help us shedding some light on the mechanisms modulating the phenotypic expression of mtDNA mutations in mitochondrial diseases. We studied mtDNA complementation in human cells by fusing two cell lines, one containing a homoplasmic mutation in a subunit of respiratory chain complex IV, COX I, and the other a distinct homoplasmic mutation in a subunit of complex III, cytochrome b. Upon cell fusion, respiration is recovered in hybrids cells, indicating that mitochondria fuse and exchange genetic and protein materials. Mitochondrial functional complementation occurs frequently, but with variable efficiency. We have investigated by native gel electrophoresis the molecular organization of the mitochondrial respiratory chain in complementing hybrid cells. We show that the recovery of mitochondrial respiration correlates with the presence of supramolecular structures (supercomplexes) containing complexes I, III and IV. We suggest that critical amounts of complexes III or IV are required in order for supercomplexes to form and provide mitochondrial functional complementation. From these findings, supercomplex assembly emerges as a necessary step for respiration, and its defect sets the threshold for respiratory impairment in mtDNA mutant cells.     

Diseases caused by nuclear genes affecting mtDNA stability

Diseases caused by nuclear genes that affect mitochondrial DNA (mtDNA) stability are an interesting group of mitochondrial disorders, involving both cellular genomes. In these disorders, a primary nuclear gene defect causes secondary mtDNA loss or deletion formation, which leads to tissue dysfunction. Therefore, the diseases clinically resemble those caused by mtDNA mutations, but follow a Mendelian inheritance pattern. Several clinical entities associated with multiple mtDNA deletions have been characterized, the most frequently described being autosomal dominant progressive external ophthalmoplegia (adPEO). MtDNA depletion syndrome (MDS) is a severe disease of childhood, in which tissue-specific loss of mtDNA is seen. Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) patients may have multiple mtDNA deletions and/or mtDNA depletion. Recent reports of thymidine phosphorylase mutations in MNGIE and adenine nucleotide translocator mutations in adPEO have given new insights into the mechanisms of mtDNA maintenance in mammals. The common mechanism underlying both of these gene defects could be disturbed mitochondrial nucleoside pools, the building blocks of mtDNA. Future studies on MNGIE and adPEO pathogenesis, and identification of additional gene defects in adPEO and MDS will provide further understanding about the mammalian mtDNA maintenance and the crosstalk between the nuclear and mitochondrial genomes.     

Complex I deficiency: clinical features, biochemistry and molecular genetics

Complex I deficiency is the most frequent mitochondrial disorder presenting in childhood, accounting for up to 30% of cases. As with many mitochondrial disorders, complex I deficiency is characterised by marked clinical and genetic heterogeneity, leading to considerable diagnostic challenges for the clinician, not least because of the involvement of two genomes. The most prevalent clinical presentations include Leigh syndrome, leukoencephalopathy and other early-onset neurodegenerative disorders; fatal infantile lactic acidosis; hypertrophic cardiomyopathy; and exercise intolerance. Causative genetic defects may involve the seven mitochondrial-encoded or 38 nuclear-encoded subunits of the enzyme, or any of an increasing number of assembly factors implicated in the correct biosynthesis of complex I within the inner mitochondrial membrane. In this review, we discuss recent advances in knowledge of the structure, function and assembly of complex I and how these advances, together with new high-throughput genetic screening techniques, have translated into improved genetic diagnosis for affected patients and their families. Approximately 25% of cases have mitochondrial DNA mutations, while a further ～25% have mutations in a nuclear subunit or in one of nine known assembly factors. We also present a systematic review of all published cases of nuclear-encoded complex I deficiency, including 117 cases with nuclear subunit mutations and 55 with assembly factor mutations, and highlight clinical, radiological and biochemical clues that may expedite genetic diagnosis.