Mitochondrial DNA-related Disorders

Mitochondrial diseases are a group of disorders due to a mitochondrial respiratory chain deficiency. They may depend on mitochondrial genome (mtDNA-related disorders) as well as on a nuclear genome defect (nDNA-related disorders). mtDNA-related disorders encompass an increasing number of clinical pictures associated with more than 250 different provisional or confirmed pathogenic changes in mtDNA. Although some clinical syndromes are nosologically defined, most of the cases present with polymorphous phenotypes ranging from pure myopathy to multi-system involvement. Complexity of mitochondrial genetics is in part responsible for the extreme clinical intra- and inter-familial heterogeneity of this group of diseases. In this review, we briefly report an updated classification and overview the main clinical pictures of this class of diseases.     

Mitochondrial diseases in children including Leigh syndrome--biochemical and molecular background

Mitochondrial diseases in children are more frequently caused by mutations in nuclear DNA then in mtDNA. Special clinical phenotypes are associated with the mutations in SURF1 gene, in SCO2 gene and with mtDNA depletion syndromes. Leigh syndrome is the most common clinical presentation of various mitochondrial disorders during childhood. Elevation of lactate in blood, cerebrospinal fluid and urine is a simple biochemical marker of mitochondrial disorders but its specificity and sensitivity are low. Biochemical investigation of muscle biopsy and search for mitochondrial mutations remain a gold standard in the diagnosis. The standarized diagnostic criteria to establish level of diagnostic certainty (possible, probable, definite) are proposed to be used in practice; these include clinical features, neuroimaging and muscle biopsy investigations. Further research directions to improve our understanding of mitochondrial pathologies in children are suggested.     

The pathophysiology of stroke in mitochondrial disorders

Stroke occurs with an increased frequency in patients with mitochondrial disorders and is a characteristic feature of the MELAS phenotype. This article explores the proposed mechanisms by which mitochondrial dysfunction may contribute to both vascular and non-vascular strokes and stroke-like episodes. The clinical features, neuroimaging, and pathologic findings of MELAS are reviewed as evidence for a cytopathologic basis for stroke in mitochondrial disorders.     

Yeast models of human mitochondrial diseases: from molecular mechanisms to drug screening

Mitochondrial diseases are rare diseases most often linked to energy in the form of ATP-depletion. The high number of nuclear- and mitochondrial-DNA-encoded proteins (>500), required for ATP production and other crucial mitochondrial functions such as NADH re-oxidation, explains the increasing number of reported disorders. In recent years, yeast has revealed to be a powerful model to identify responsible genes, to study primary effects of pathogenic mutations and to determine the molecular mechanisms leading to mitochondrial disorders. However, the clinical management of patients with mitochondrial disorders is still essentially supportive. Here we review some of the most fruitful yeast mitochondrial disorder models and propose to subject these models to highthroughput chemical library screening to prospect new therapeutic drugs against mitochondrial diseases.     

Diagnostic challenges of mitochondrial DNA disorders

Although mitochondrial disorders are increasingly being recognized, confirming a specific diagnosis remains a great challenge due to the genetic and clinical heterogeneity of the disease. The heteroplasmic nature of most pathogenic mitochondrial DNA mutations and the uncertainties of the clinical significance of novel mutations pose additional complications in making a diagnosis. Suspicion of mitochondrial disease among patients with multiple, seemingly unrelated neuromuscular and multi-system disorders should ideally be confirmed by the finding of deleterious mutations in genes involving mitochondrial biogenesis and functions. The genetics are complex, as the primary mutation can be either in the nuclear or the mitochondrial DNA (mtDNA). MtDNA mutations are often maternally inherited, but can also be sporadic or secondary to mutations in nuclear-encoded mitochondrial-targeted genes. Several well-defined clinical syndromes associated with specific mutations have been described, yet the genotype-phenotype correlation is often unclear and most patients do not fit within any defined syndrome and even within a family the expressivity of the disease can be extremely variable. This article describes examples representing diagnostic challenges of mitochondrial diseases that include the limitations of the mutation detection method, the occurrence of mitochondrial disease in families with another known neuromuscular disorder, atypical clinical presentation, the lack of correlation between the degree of mutant heteroplasmy and the severity of the disease, variable penetrance, and nuclear gene defects causing mtDNA depletion.     

Mitochondrial diseases

By convention, the term "mitochondrial diseases" refers to disorders of the mitochondrial respiratory chain, which is the only metabolic pathway in the cell that is under the dual control of the mitochondrial genome (mtDNA) and the nuclear genome (nDNA). Therefore, a genetic classification of the mitochondrial diseases distinguishes disorders due to mutations in mtDNA, which are governed by the relatively lax rules of mitochondrial genetics, and disorders due to mutations in nDNA, which are governed by the stricter rules of mendelian genetics. Mutations in mtDNA can be divided into those that impair mitochondrial protein synthesis in toto and those that affect any one of the 13 respiratory chain subunits encoded by mtDNA. Essential clinical features for each group of diseases are reviewed. Disorders due to mutations in nDNA are more abundant not only because most respiratory chain subunits are nucleus-encoded but also because correct assembly and functioning of the respiratory chain require numerous steps, all of which are under the control of nDNA. These steps (and related diseases) include: (i) synthesis of assembly proteins; (ii) intergenomic signaling; (iii) mitochondrial importation of nDNA-encoded proteins; (iv) synthesis of inner mitochondrial membrane phospholipids; (v) mitochondrial motility and fission.     

Mitochondrial Optic Neuropathies: How Two Genomes may Kill the Same Cell Type?

Ocular involvement is a prevalent feature in mitochondrial diseases. Leber’s hereditary optic neuropathy (LHON) and dominant optic atrophy (DOA) are both non-syndromic optic neuropathies with a mitochondrial etiology. LHON is associated with point mutations in the mitochondrial DNA (mtDNA), which affect subunit genes of complex I. The majority of DOA patients harbor mutations in the nuclear-encoded protein OPA1, which is targeted to mitochondria and participates to cristae organization and mitochondrial network dynamics. In both disorders the retinal ganglion cells (RGCs) are specific cellular targets of the degenerative process. We here review the clinical features and the genetic bases, and delineate the possible common pathomechanism for both these disorders.     

Pathogenic expression of homoplasmic mtDNA mutations needs a complex nuclear-mitochondrial interaction

Here we define a category of human, maternally inherited disorders that are characterized by a homoplasmic mtDNA pathogenic mutation with variable penetrance and a stereotypical clinical expression, usually restricted to a single tissue. Examples of such disorders include Leber's hereditary optic neuropathy, mitochondrial non-syndromic sensorineural hearing loss, and a form of mitochondrial hypertrophic cardiomyopathy. The mtDNA mutation is necessary, but not sufficient to induce the pathology, and multiple lines of evidence suggest a two-locus genetic model involving a primary mitochondrial mutation and a nuclear modifier. The nuclear modifier does not induce any pathology per se, but it contributes to the pathogenic effect of the mitochondrial mutation. The nuclear modifier could be a common functional polymorphism in a tissue-specific protein, possibly with mitochondrial location.     

Human diseases with impaired mitochondrial protein synthesis

Mitochondrial respiratory chain deficiencies represent one of the major causes of metabolic disorders that are related to genetic defects in mitochondrial or nuclear DNA. The mitochondrial protein synthesis allows the synthesis of the 13 respiratory chain subunits encoded by mtDNA. Altogether, about 100 different proteins are involved in the translation of the 13 proteins encoded by the mitochondrial genome emphasizing the considerable investment required to maintain mitochondrial genetic system. Mitochondrial protein synthesis deficiency can be caused by mutations in any component of the translation apparatus including tRNA, rRNA and proteins. Mutations in mitochondrial rRNA and tRNAs have been first identified in various forms of mitochondrial disorders. Moreover abnormal translation due to mutation in nuclear genes encoding tRNA-modifying enzymes, ribosomal proteins, aminoacyl-tRNA synthetases, elongation and termination factors and translational activators have been successively described. These deficiencies are characterized by a huge clinical and genetic heterogeneity hampering to establish genotype-phenotype correlations and an easy diagnosis. One can hypothesize that a new technique for gene identification, such as exome sequencing will rapidly allow to expand the list of genes involved in abnormal mitochondrial protein synthesis.     

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.     

Mitochondrial encephalomyopathies: Clinical and molecular analysis

The classification of mitochondrial encephalomyopathies relied upon clinical, biochemical, and histological features until the discovery of mitochondrial DNA defects in 1988. Since then, an outburst of molecular genetic information has aided our understanding of the pathogenesis and the classification of these heterogeneous disorders. Novel concepts of maternal inheritance, mitochondrial DNA (mtDNA) heteroplasmy, tissue distribution, and threshold have explained many of the clinical characteristics. The discovery of point mutations, large-scale mtDNA deletions, duplications, and autosomally inherited disorders with multiple mtDNA deletions have revealed new genetic phenomena. Despite our rapidly expanding understanding of the molecular genetic defects, many questions remain to be explored to fill the gap in our knowledge of the relationship between genotype and clinical phenotype.     

Molecular genetic and clinical aspects of mitochondrial disorders in childhood

Mitochondrial OXPHOS disorders are caused by mutations in mitochondrial or nuclear genes, which directly or indirectly affect mitochondrial oxidative phosphorylation (OXPHOS). Primary mtDNA abnormalities in children are due to rearrangements (deletions or duplications) and point mutations or insertions. Mutations in the nuclear-encoded polypeptide subunits of OXPHOS result in complex I and II deficiency, whereas mutations in the nuclear proteins involved in the assembly of OXPHOS subunits cause defects in complexes I, III, IV, and V. Here, we review recent progress in the identification of mitochondrial and nuclear gene defects and the associated clinical manifestations of these disorders in childhood.     

The neurological presentations of childhood and adult mitochondrial disease: established syndromes and phenotypic variations

Mitochondrial disorders are caused by mutations of nuclear or mitochondrial DNA encoded genes involved in oxidative phosphorylation (OXPHOS). Mutations in these critical genes are associated with specific clinical syndromes with diverse presentations (DiMauro and Schon, 2003. NEJM 348, 2656; Hart et al., 2002. Mitochondrial Disorders in Neurology). Since mitochondria are present in many of our organs and play a key role in energy metabolism, mitochondrial encephalomyopathies often present as multisystem disorders which may manifest with neurologic, cardiac, endocrine, gastrointestinal, hepatic, renal and/or hematologic involvement (DiMauro and Bonilla, 1997. The Molecular and Genetic Basis of Neurological Disease; Vu et al., 2002. Neurol. Clin. N. Am. 20, 809). This chapter will address adult and childhood onset mitochondrial DNA encephalomyopathies, characteristic clinical presentations, as well as their molecular characterization.     

Next generation molecular diagnosis of mitochondrial disorders

Mitochondrial disorders are by far the most genetically heterogeneous group of diseases, involving two genomes, the 16.6 kb mitochondrial genome and ~ 1500 genes encoded in the nuclear genome. For maternally inherited mitochondrial DNA disorders, a complete molecular diagnosis requires several different methods for the detection and quantification of mtDNA point mutations and large deletions. For mitochondrial disorders caused by autosomal recessive, dominant, and X-linked nuclear genes, the diagnosis has relied on clinical, biochemical, and molecular studies to point to a group of candidate genes followed by stepwise Sanger sequencing of the candidate genes one-by-one. The development of Next Generation Sequencing (NGS) has revolutionized the diagnostic approach. Using massively parallel sequencing (MPS) analysis of the entire mitochondrial genome, mtDNA point mutations and deletions can be detected and quantified in one single step. The NGS approach also allows simultaneous analyses of a group of genes or the whole exome, thus, the mutations in causative gene(s) can be identified in one-step. New approaches make genetic analyses much faster and more efficient. Huge amounts of sequencing data produced by the new technologies brought new challenges to bioinformatics, analytical pipelines, and interpretation of numerous novel variants. This article reviews the clinical utility of next generation sequencing for the molecular diagnoses of complex dual genome mitochondrial disorders.     

A Comprehensive Genomic Analysis Reveals the Genetic Landscape of Mitochondrial Respiratory Chain Complex Deficiencies

Mitochondrial disorders have the highest incidence among congenital metabolic disorders characterized by biochemical respiratory chain complex deficiencies. It occurs at a rate of 1 in 5,000 births, and has phenotypic and genetic heterogeneity. Mutations in about 1,500 nuclear encoded mitochondrial proteins may cause mitochondrial dysfunction of energy production and mitochondrial disorders. More than 250 genes that cause mitochondrial disorders have been reported to date. However exact genetic diagnosis for patients still remained largely unknown. To reveal this heterogeneity, we performed comprehensive genomic analyses for 142 patients with childhood-onset mitochondrial respiratory chain complex deficiencies. The approach includes whole mtDNA and exome analyses using high-throughput sequencing, and chromosomal aberration analyses using high-density oligonucleotide arrays. We identified 37 novel mutations in known mitochondrial disease genes and 3 mitochondria-related genes (MRPS23, QRSL1, and PNPLA4) as novel causative genes. We also identified 2 genes known to cause monogenic diseases (MECP2 and TNNI3) and 3 chromosomal aberrations (6q24.3-q25.1, 17p12, and 22q11.21) as causes in this cohort. Our approaches enhance the ability to identify pathogenic gene mutations in patients with biochemically defined mitochondrial respiratory chain complex deficiencies in clinical settings. They also underscore clinical and genetic heterogeneity and will improve patient care of this complex disorder.     

Developing a systematic approach to the diagnosis and classification of mitochondrial disease

The accurate diagnosis and classification of mitochondrial diseases are essential first steps in understanding the natural history and true health care burden imposed by these protean and devastating disorders. Epidemiologic studies place the incidence of genetic forms of mitochondrial disease between 1 in 2000 and 1 in 5000 live births. Symptoms may not appear for years after birth, even when inherited. Once they occur, however, the course is often relentlessly progressive. Diagnosis requires a combination of clinical and laboratory studies that are applied systematically. DNA analysis and respiratory chain studies remain the mainstays of diagnosis, but several other disciplines may contribute to achieving diagnostic confidence when a single study is suggestive but inconclusive. A comprehensive classification system for mitochondrial diseases has not yet been developed. The current International Classification of Diseases, 10th Revision (ICD-10) includes just 10 codes for mitochondrial disorders. Supplementary data of 347 proposed ICD-10 codes is included to assist with the development of a more comprehensive system for the diagnosis and classification of mitochondrial disease.     

Mitochondriale Erkrankungen

As outlined in other articles of this issue mitochondrial medicine is a complex area in clinical genetics. Due to the wide variability of clinical presentation in both pediatric and adult patients there are frequent constellations of symptoms that may suggest an underlying mitochondrial disorder. This is a challenge in genetic counseling because basically all patterns of inheritance have to be taken into account??including maternal transmission??but a straightforward genetic testing to confirm or exclude the suggested diagnosis is hampered by the immense genetic heterogeneity of the mitochondrial disease spectrum. This article focuses on a diagnostic strategy and specific aspects of genetic counseling in mitochondrial disorders.     

Epidemiology of A3243G, the mutation for mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes: prevalence of the mutation in an adult population.

Mitochondrial diseases are characterized by considerable clinical variability and are most often caused by mutations in mtDNA. Because of the phenotypic variability, epidemiological studies of the frequency of these disorders have been difficult to perform. We studied the prevalence of the mtDNA mutation at nucleotide 3243 in an adult population of 245,201 individuals. This mutation is the most common molecular etiology of MELAS syndrome (mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes), one of the clinical entities among the mitochondrial disorders. Patients with diabetes mellitus, sensorineural hearing impairment, epilepsy, occipital brain infarct, ophthalmoplegia, cerebral white-matter disease, basal-ganglia calcifications, hypertrophic cardiomyopathy, or ataxia were ascertained on the basis of defined clinical criteria and family-history data. A total of 615 patients were identified, and 480 samples were examined for the mutation. The mutation was found in 11 pedigrees, and its frequency was calculated to be >=16. 3/100,000 in the adult population (95% confidence interval 11.3-21. 4/100,000). The mutation had arisen in the population at least nine times, as determined by mtDNA haplotyping. Clinical evaluation of the probands revealed a syndrome that most frequently consisted of hearing impairment, cognitive decline, and short stature. The high prevalence of the common MELAS mutation in the adult population suggests that mitochondrial disorders constitute one of the largest diagnostic categories of neurogenetic diseases.     

线粒体功能缺陷和神经系统疾病

线粒体呼吸链缺陷一直被认为是诱发线粒体疾病的重要因素,这有助于研究人员阐释其遗传和临床多样性。然而,线粒体的其他功能也具有重要意义,包括蛋白质运输、细胞器动力学和细胞凋亡。调控这些功能的基因缺陷不仅导致神经和精神疾病,而且还导致年龄相关的神经变性疾病。因此,引起越来越多的关注。在讨论呼吸链缺陷引起相关神经系统疾病的一些致病难题后,就线粒体动力学改变引起的相关神经系统疾病病因和常见神经变性疾病的病理生理机制作一综述。