Identical mitochondrial DNA deletion in mother with progressive external ophthalmoplegia and son with Pearson marrow-pancreas syndrome

Pearson syndrome: a multisystem mitochondrial disease with bone marrow failure

Pearson syndrome (PS) is a rare fatal mitochondrial disorder caused by single large-scale mitochondrial DNA deletions (SLSMDs). Most patients present with anemia in infancy. Bone marrow cytology with vacuolization in erythroid and myeloid precursors and ring-sideroblasts guides to the correct diagnosis, which is established by detection of SLSMDs. Non hematological symptoms suggesting a mitochondrial disease are often lacking at initial presentation, thus PS is an important differential diagnosis in isolated hypogenerative anemia in infancy. Spontaneous resolution of anemia occurs in two-third of patients at the age of 1-3 years, while multisystem non-hematological complications such as failure to thrive, muscle hypotonia, exocrine pancreas insufficiency, renal tubulopathy and cardiac dysfunction develop during the clinical course. Some patients with PS experience a phenotypical change to Kearns-Sayre syndrome. In the absence of curative therapy, the prognosis of patients with PS is dismal. Most patients die of acute lactic acidosis and multi-organ failure in early childhood. There is a great need for the development of novel therapies to alter the natural history of patients with PS.

Tackling Dysfunction of Mitochondrial Bioenergetics in the Brain

Oxidative phosphorylation (OxPhos) is the basic function of mitochondria, although the landscape of mitochondrial functions is continuously growing to include more aspects of cellular homeostasis. Thanks to the application of -omics technologies to the study of the OxPhos system, novel features emerge from the cataloging of novel proteins as mitochondrial thus adding details to the mitochondrial proteome and defining novel metabolic cellular interrelations, especially in the human brain. We focussed on the diversity of bioenergetics demand and different aspects of mitochondrial structure, functions, and dysfunction in the brain. Definition such as 'mitoexome', 'mitoproteome' and 'mitointeractome' have entered the field of 'mitochondrial medicine'. In this context, we reviewed several genetic defects that hamper the last step of aerobic metabolism, mostly involving the nervous tissue as one of the most prominent energy-dependent tissues and, as consequence, as a primary target of mitochondrial dysfunction. The dual genetic origin of the OxPhos complexes is one of the reasons for the complexity of the genotype-phenotype correlation when facing human diseases associated with mitochondrial defects. Such complexity clinically manifests with extremely heterogeneous symptoms, ranging from organ-specific to multisystemic dysfunction with different clinical courses. Finally, we briefly discuss the future directions of the multi-omics study of human brain disorders.

Mitochondrial diseases in adults

Mitochondrial medicine is a field that expanded exponentially in the last 30 years. Individually rare, mitochondrial diseases as a whole are probably the most frequent genetic disorder in adults. The complexity of their genotype-phenotype correlation, in terms of penetrance and clinical expressivity, natural history and diagnostic algorithm derives from the dual genetic determination. In fact, in addition to the about 1.500 genes encoding mitochondrial proteins that reside in the nuclear genome (nDNA), we have the 13 proteins encoded by the mitochondrial genome (mtDNA), for which 22 specific tRNAs and 2 rRNAs are also needed. Thus, besides Mendelian genetics, we need to consider all peculiarities of how mtDNA is inherited, maintained and expressed to fully understand the pathogenic mechanisms of these disorders. Yet, from the initial restriction to the narrow field of oxidative phosphorylation dysfunction, the landscape of mitochondrial functions impinging on cellular homeostasis, driving life and death, is impressively enlarged. Finally, from the clinical standpoint, starting from the neuromuscular field, where brain and skeletal muscle were the primary targets of mitochondrial dysfunction as energy-dependent tissues, after three decades virtually any subspecialty of medicine is now involved. We will summarize the key clinical pictures and pathogenic mechanisms of mitochondrial diseases in adults.

Detection of mitochondrial DNA (mtDNA) mutations

The maternally inherited mitochondrial DNA (mtDNA) is a circular 16,569bp double stranded DNA that encodes 37 genes, 24 of which (2 rRNAs and 22 tRNAs) are necessary for transcription and translation of 13 polypeptides that are all subunits of respiratory chain. Pathogenic mutations in mtDNA cause respiratory chain dysfunction, and are the underlying defect in an ever-increasing number of mtDNA-related encephalomyopathies with distinct phenotypes. In this chapter, we present an overview of mtDNA mutations and describe the molecular techniques currently employed in our laboratory to detect two types of mtDNA mutations: single-large-scale rearrangements and point mutations.

Cardiovascular Manifestations of Mitochondrial Disease

Genetic mitochondrial cardiomyopathies are uncommon causes of heart failure that may not be seen by most physicians. However, the prevalence of mitochondrial DNA mutations and somatic mutations affecting mitochondrial function are more common than previously thought. In this review, the pathogenesis of genetic mitochondrial disorders causing cardiovascular disease is reviewed. Treatment options are presently limited to mostly symptomatic support, but preclinical research is starting to reveal novel approaches that may lead to better and more targeted therapies in the future. With better understanding and clinician education, we hope to improve clinician recognition and diagnosis of these rare disorders in order to improve ongoing care of patients with these diseases and advance research towards discovering new therapeutic strategies to help treat these diseases.

Clonal expansion of mtDNA deletions: different disease models assessed by digital droplet PCR in single muscle cells

Deletions in mitochondrial DNA (mtDNA) are an important cause of human disease and their accumulation has been implicated in the ageing process. As mtDNA is a high copy number genome, the coexistence of deleted and wild-type mtDNA molecules within a single cell defines heteroplasmy. When deleted mtDNA molecules, driven by intracellular clonal expansion, reach a sufficiently high level, a biochemical defect emerges, contributing to the appearance and progression of clinical pathology. Consequently, it is relevant to determine the heteroplasmy levels within individual cells to understand the mechanism of clonal expansion. Heteroplasmy is reflected in a mosaic distribution of cytochrome c oxidase (COX)-deficient muscle fibers. We applied droplet digital PCR (ddPCR) to single muscle fibers collected by laser-capture microdissection (LCM) from muscle biopsies of patients with different paradigms of mitochondrial disease, characterized by the accumulation of single or multiple mtDNA deletions. By combining these two sensitive approaches, ddPCR and LCM, we document different models of clonal expansion in patients with single and multiple mtDNA deletions, implicating different mechanisms and time points for the development of COX deficiency in these molecularly distinct mitochondrial cytopathies.

Pearson syndrome

INTRODUCTION

Pearson syndrome (PS) is a sporadic and very rare syndrome classically associated with single large-scale deletions of mitochondrial DNA and characterized by refractory sideroblastic anemia during infancy. Areas covered: This review presents an analysis and interpretation of the published data that forms the basis for our understanding of PS. PubMed, Google Scholarand Thompson ISI Web of Knowledge were searched for relevant data. Expert commentary: PS is a very rare mitochodrial disease that involves different organs and systems. Clinical phenotype is extremely variable and may change over the course of disease itself with the possibility both of worsenings and improvements. Outcome is invariably lethal and at the moment no cure is available. Accurate supportive treatment and follow up program in centres with experience in mitochondrial diseases and marrow failure may positively influence quality and duration of life.

Mitochondrial-Nuclear Interactions Mediate Sex-Specific Transcriptional Profiles in Drosophila

The assembly and function of mitochondria require coordinated expression from two distinct genomes, the mitochondrial DNA (mtDNA) and nuclear DNA (nDNA). Mutations in either genome can be a source of phenotypic variation, yet their coexpression has been largely overlooked as a source of variation, particularly in the emerging paradigm of mitochondrial replacement therapy. Here we tested how the transcriptome responds to mtDNA and nDNA variation, along with mitonuclear interactions (mtDNA × nDNA) in Drosophila melanogaster. We used two mtDNA haplotypes that differ in a substantial number of single nucleotide polymorphisms, with >100 amino acid differences. We placed each haplotype on each of two D. melanogaster nuclear backgrounds and tested for transcription differences in both sexes. We found that large numbers of transcripts were differentially expressed between nuclear backgrounds, and that mtDNA type altered the expression of nDNA genes, suggesting a retrograde, trans effect of mitochondrial genotype. Females were generally more sensitive to genetic perturbation than males, and males demonstrated an asymmetrical effect of mtDNA in each nuclear background; mtDNA effects were nuclear-background specific. mtDNA-sensitive genes were not enriched in male- or female-limited expression space in either sex. Using a variety of differential expression analyses, we show the responses to mitonuclear covariation to be substantially different between the sexes, yet the mtDNA genes were consistently differentially expressed across nuclear backgrounds and sexes. Our results provide evidence that the main mtDNA effects can be consistent across nuclear backgrounds, but the interactions between mtDNA and nDNA can lead to sex-specific global transcript responses.

A complex genome-microRNA interplay in human mitochondria

Small noncoding regulatory RNA exist in wide spectrum of organisms ranging from prokaryote bacteria to humans. In human, a systematic search for noncoding RNA is mainly limited to the nuclear and cytosolic compartments. To investigate whether endogenous small regulatory RNA are present in cell organelles, human mitochondrial genome was also explored for prediction of precursor microRNA (pre-miRNA) and mature miRNA (miRNA) sequences. Six novel miRNA were predicted from the organelle genome by bioinformatics analysis. The structures are conserved in other five mammals including chimp, orangutan, mouse, rat, and rhesus genome. Experimentally, six human miRNA are well accumulated or deposited in human mitochondria. Three of them are expressed less prominently in Northern analysis. To ascertain their presence in human skeletal muscles, total RNA was extracted from enriched mitochondria by an immunomagnetic method. The expression of six novel pre-miRNA and miRNA was confirmed by Northern blot analysis; however, low level of remaining miRNA was found by sensitive Northern analysis. Their presence is further confirmed by real time RT-PCR. The six miRNA find their multiple targets throughout the human genome in three different types of software. The luciferase assay was used to confirm that MT-RNR2 gene was the potential target of hsa-miR-mit3 and hsa-miR-mit4.

Juvenile cobalamin deficiency in a 17-year-old child with autonomic dysfunction and skin changes

We report a rare case of juvenile cobalamin deficiency who presented at the age of 17 years. He was underweight and had skin changes, normocytic anemia, and autonomic dysfunction, which led to adynamic ileus and acute postrenal failure. The expected macrocytosis was masked by an underlying alpha-thalassemia trait. The patient had an excellent response to parenteral cobalamin treatment.

Celiac disease in siblings with Pearson syndrome

Pearson syndrome (PS) is a rare mitochondrial disorder characterized by sideroblastic anemia and exocrine pancreas deficiency as a result of mitochondrial DNA deletion or deletion-duplication. A 21-year-old woman and 11-year-old brother who had been diagnosed as PS at the age of 18 and 8, respectively, were admitted to our hospital with the complaints of chronic diarrhea while using exogenous pancreas enzymes. Diarrhea was present in both for a long time and attributed to existing PS. Endoscopic and laboratory examinations revealed celiac disease (CD). After gluten-free diet, their symptoms were resolved. Detailed family investigation revealed that parents and the sister of the siblings were free of both PS and CD. To our knowledge, there has been no previous report of the presence of these 2 disorders, PS and CD, in the same patient. CD should be screened and treated appropriately in cases with PS.

Pearson syndrome in the neonatal period: two case reports and review of the literature

Pearson syndrome is a multiorgan mitochondrial cytopathy that results from defective oxidative phosphorylation owing to mitochondrial DNA deletions. Prognosis is severe and death occurs in infancy or early childhood. This article describes 2 cases with a severe neonatal onset of the disease. A review of the literature reveals the atypical presentation of the disease in the neonatal period, which is often overlooked and underdiagnosed.

New evidence confirms that the mitochondrial bottleneck is generated without reduction of mitochondrial DNA content in early primordial germ cells of mice

In mammals, observations of rapid shifts in mitochondrial DNA (mtDNA) variants between generations have led to the creation of the bottleneck theory for the transmission of mtDNA. The bottleneck could be attributed to a marked decline of mtDNA content in germ cells giving rise to the next generation, to a small effective number of mtDNA segregation units resulting from homoplasmic nucleoids rather than the single mtDNA molecule serving as the units of segregation, or to the selective transmission of a subgroup of the mtDNA population to the progeny. We have previously determined mtDNA copy number in single germ cells and shown that the bottleneck occurs without the reduction in germline mtDNA content. Recently one study suggested that the bottleneck is driven by a remarkable decline of mtDNA copies in early primordial germ cells (PGCs), while another study reported that the mtDNA genetic bottleneck results from replication of a subpopulation of the mtDNA genome during postnatal oocyte maturation and not during embryonic oogenesis, despite a detected a reduction in mtDNA content in early PGCs. To clarify these contradictory results, we examined the mtDNA copy number in PGCs isolated from transgenic mice expressing fluorescent proteins specifically in PGCs as in the aforementioned two other studies. We provide clear evidence to confirm that no remarkable reduction in mtDNA content occurs in PGCs and reinforce that the bottleneck is generated without reduction of mtDNA content in germ cells.

Deletion-mutant mtDNA increases in somatic tissues but decreases in female germ cells with age

The proportions of mutant and wild-type mtDNA are crucial in determining the severity of mitochondrial diseases. It has been generally considered that deletion-mutant mtDNA has replication advantages and accumulates with time. Here, we examine the tissue-by-tissue proportions of mutant mtDNA with a 4696-bp deletion (DeltamtDNA) and wild-type mtDNA in mitochondrial disease model mice (mito-mice). Comparison of the proportions of DeltamtDNA in each tissue at various ages showed that the rate of accumulation of DeltamtDNA differed among tissues. The heart, skeletal muscles, kidney, liver, testis, and ovary showed increases in the proportion of DeltamtDNA with age, but the pancreas, spleen, brain, and blood showed only a slight or no increase in proportion. In contrast to the somatic tissues, however, the germ cells of female mito-mice and resultant offspring showed a strong decrease in DeltamtDNA with maternal age. The decrease was so acute that some offspring showed complete disappearance of DeltamtDNA, even though their elder brothers and sisters had high proportions of DeltamtDNA. Female germ cells have a machinery that prevents the inheritance of defective mtDNA to the following generation since germ cells are kept for a long time until they are ovulated.

Mitochondrial diseases: therapeutic approaches

Therapy of mitochondrial encephalomyopathies (defined restrictively as defects of the mitochondrial respiratory chain) is woefully inadequate, despite great progress in our understanding of the molecular bases of these disorders. In this review, we consider sequentially several different therapeutic approaches. Palliative therapy is dictated by good medical practice and includes anticonvulsant medication, control of endocrine dysfunction, and surgical procedures. Removal of noxious metabolites is centered on combating lactic acidosis, but extends to other metabolites. Attempts to bypass blocks in the respiratory chain by administration of electron acceptors have not been successful, but this may be amenable to genetic engineering. Administration of metabolites and cofactors is the mainstay of real-life therapy and is especially important in disorders due to primary deficiencies of specific compounds, such as carnitine or coenzyme Q10. There is increasing interest in the administration of reactive oxygen species scavengers both in primary mitochondrial diseases and in neurodegenerative diseases directly or indirectly related to mitochondrial dysfunction. Aerobic exercise and physical therapy prevent or correct deconditioning and improve exercise tolerance in patients with mitochondrial myopathies due to mitochondrial DNA (mtDNA) mutations. Gene therapy is a challenge because of polyplasmy and heteroplasmy, but interesting experimental approaches are being pursued and include, for example, decreasing the ratio of mutant to wild-type mitochondrial genomes (gene shifting), converting mutated mtDNA genes into normal nuclear DNA genes (allotopic expression), importing cognate genes from other species, or correcting mtDNA mutations with specific restriction endonucleases. Germline therapy raises ethical problems but is being considered for prevention of maternal transmission of mtDNA mutations. Preventive therapy through genetic counseling and prenatal diagnosis is becoming increasingly important for nuclear DNA-related disorders. Progress in each of these approaches provides some glimmer of hope for the future, although much work remains to be done.

Hematological manifestations of primary mitochondrial disorders

At onset mitochondrial disorders (MID) frequently manifest as a mono-organic problem but turn into multisystem disease during the disease course in most of the cases. Organs/tissues most frequently affected in MID are the cerebrum, peripheral nerves, and the skeletal muscle. Additionally, most of the inner organs may be affected alone or in combination. Hematological manifestations of MID include aplastic, megaloblastic, or sideroblastic anemia, leukopenia, neutropenia, thrombocytopenia, or pancytopenia. In single cases either permanent or recurrent eosinophilia has been observed. Hematological abnormalities may occur together with syndromic or nonsyndromic MIDs. Syndromic MIDs, in which hematological manifestations predominate, are the Pearson syndrome (pancytopenia), Kearns-Sayre syndrome (anemia), Barth syndrome (neutropenia), and the autosomal recessive mitochondrial myopathy, lactic acidosis and sideroblastic anemia syndrome. In single cases with Leigh's syndrome, MERRF (myoclonic epilepsy and ragged-red fiber) syndrome, Leber's hereditary optic neuropathy, and Friedreich's ataxia anemia has been described. Anemia, leukopenia, thrombocytopenia, eosinophilia, or pancytopenia can frequently also be found in nonsyndromic MIDs with or without involvement of other tissues. Therapy of blood cell involvement in MID comprises application of antioxidants, vitamins, iron, bone marrow-stimulating factors, or substitution of cells.

The neurological evolution of Pearson syndrome: case report and literature review

BACKGROUND

Pearson syndrome (PS) is an uncommon specific syndrome among mitochondrial diseases. It has unique clinical presentations.

AIMS

The purpose of this article is to clarify the neurological evolution, neuroimage findings, molecular genetic analysis and outcomes in PS cases with neurologic manifestations.

METHODS

We described the clinical progress of a female patient who was diagnosed as PS with a novel 6.0 kbp mitochondrial DNA deletion. She had typical clinical features of PS in early infancy followed by multiple organs involvement after the age of 1 year. At age 3, Kearns-Sayre syndrome (KSS) and Leigh syndrome (LS) developed. We also reviewed PS cases reported in the literature and analyzed the neurological evolution.

RESULTS

Total 55 PS cases, including our index case, had been reported. Among them, 11 cases had detailed clinical descriptions in terms of hypotonia, developmental delay, ataxia or tremor. In whom, PS might evolve into KSS and/or LS: three cases evolving into KSS; one case on the transition of KSS; three cases evolve into LS; our index case has both presentations. The neuroimage findings of PS were quite different which might be from normal to specific abnormal findings over the cerebral white matter, cerebellum, basal ganglion and brainstem. Among those cases, the molecular analysis revealed large-scale mitochondrial deletion around 3.1-6.0kbp. The outcome of PS was opposite: either early death before age 4 or survived beyond age 7.

CONCLUSIONS

The neurological features of PS have potential evolution changes that are from normal, mild neurological deficits to special mitochondrial syndromes: KSS and LS. Closely monitoring neurological symptoms, arranging eye fundus examinations and neuroimaging studies in cases with changes of neurological signs are crucial.

Chronic progressive ophthalmoplegia with large-scale mtDNA rearrangement: can we predict progression?

The prognosis of chronic progressive ophthalmoplegia with large-scale mitochondrial DNA (mtDNA) may strikingly vary from mild slowly progressive myopathy to severe multi-organ involvement. Evaluation of the disease course at the beginning of the disease is reputed impossible. To address the existence of predictive prognostic clues of these diseases, we classified 69 patients with chronic progressive ophthalmoplegia and large size mtDNA deletion into two groups according to the presence of manifestations from brain, inner ear or retina. These manifestations were present in 29 patients (CPEO/+N group) and absent in 40 patients (CPEO/-N group). We retrospectively established the clinical history of the patients and characterized their genetic alteration (amount of residual normal mtDNA molecules, site, size and percentage of the mtDNA deletion in 116 DNA samples from muscle, blood, urinary and buccal cells). In both clinical groups, the disease was progressive and heart conduction defects were frequent. We show that the CPEO/+N phenotype segregated with severe prognosis in term of rate of progression, multi-organs involvement and rate of survival. Age at onset appeared a predictive factor. The risk to develop a CPEO/+N phenotype was high when onset was before 9 years of age and low when onset was after 20 years of age. The presence and proportion of the mtDNA deletion in blood was also significantly associated with the CPEO/+N phenotype. This study is the first to establish the natural history of chronic ophthalmoplegia with mtDNA deletion in a large series of patients and to look for parameters potentially predictive of the patients' clinical course.

Mitochondrial DNA deletion in a girl with Fanconi's syndrome

We report a sporadic large-scale mitochondrial deletion in a paediatric patient with Fanconi's syndrome. Renal biopsy disclosed chronic interstitial nephritis. Ultrastructural examination of the renal tissue showed many giant atypical mitochondria. Histochemical stains revealed markedly reduced cytochrome c oxidase (COX). Genetic analysis disclosed a novel mitochondrial deletion of 7.3 kb in both peripheral blood and renal tissue. Mitochondrial diseases have heterogeneous clinical phenotypes; mutation analysis has proved to be an effective tool in confirming the diagnosis.

Parafoveolar intraretinal crystals in pearson syndrome

PURPOSE

To describe the occurrence of bilateral parafoveolar intraretinal crystals in a patient with Pearson syndrome (bone marrow-pancreas syndrome).

METHODS

A patient with an established diagnosis of Pearson syndrome underwent complete ophthalmic examination and optical coherence tomography.

RESULTS

Fine, refractile, crystalline, parafoveolar intraretinal deposits were seen bilaterally. There was no evidence of pigmentary retinopathy or ophthalmoplegia.

CONCLUSIONS

Intraretinal crystals in the macula may be seen in Pearson syndrome and may not cause a substantial decrease in visual acuity. Pearson syndrome should be considered in the differential diagnosis of parafoveolar crystalline retinopathy.

Quantification of total mitochondrial DNA and the 4977-bp common deletion in Pearson's syndrome lymphoblasts using a fluorogenic 5'-nuclease (TaqMan) real-time polymerase chain reaction assay and plasmid external calibration standards

This study describes a multiplex real-time polymerase chain reaction (PCR) assay that quantifies total mitochondrial DNA (mtDNA(total)) and mtDNA bearing the 4977-base pair 'common deletion' (deltamtDNA4977) in lymphoblasts derived from an individual diagnosed with Pearson's syndrome. The method is unique in its use of plasmids as external quantification standards and its use of multiplex conditions. Standards are validated by comparison with purified mtDNA amplification curves and by the fact that curves are largely unaffected by nuclear DNA (nucDNA). Finally, slopes of standard curves and unknowns are shown to be similar to each other and to theoretical predictions. From these data, mtDNA(total) in these cells is calculated to be 3258 (+723/-592) copies per cell while deltamtDNA4977 averages 232 (+136/-86) copies per cell or 7% (+4.65/-2.81).

Mitochondrial DNA analysis in clinical laboratory diagnostics

Mitochondrial disorders are increasingly being diagnosed, especially among patients with multiple, seemingly unrelated, neuromuscular and multi-sytem disorders. The genetics are complex, in particular as the primary mutation can be either on the nuclear or the mitochondrial DNA (mtDNA). mtDNA mutations are often maternally inherited, but can be sporadic or secondary to autosomally inherited mutations in nuclear genes that regulate mtDNA biosynthesis. mtDNA mutations demonstrate extreme variable expressivity in terms of clinical manifestations and severity, even within a family. Disease is often episodic. Several well-defined clinical syndromes associated with specific mutations are described, yet the genotype-phenotype correlation is fair at best and most patients do not fit within any defined syndrome and have rare or novel mutations. In most patients, mutant and wild-type mtDNA coexist ("heteroplasmy"), although homoplasmic mtDNA mutations also are known. "Standard" mtDNA clinical diagnostics usually consists of a PCR-based assay to detect a small number of relatively common point mutations and Southern blotting (or PCR) for large (>500 bp) rearrangements. In selected cases testing negative, additional analyses can include real-time PCR for mtDNA depletion, and full mtDNA genome screening for the detection of rare and novel point mutations by a variety of methods. Prenatal diagnosis is problematic in most cases.

Mitochondrial DNA and disease

The small circle of mitochondrial DNA (mtDNA) present in all human cells has proven to be a veritable Pandora's box of pathogenic mutations and rearrangements. In this review, we summarize the distinctive rules of mitochondrial genetics (maternal inheritance, mitotic segregation, heteroplasmy and threshold effect), stress the relatively high prevalence of mtDNA-related diseases, and consider recent additions to the already long list of pathogenic mutations (especially mutations affecting protein-coding genes). We then discuss more controversial issues, including the functional or pathological role of mtDNA haplotypes, the pathogenicity of homoplasmic mutations and the still largely obscure pathophysiology of mtDNA mutations.

Risk of developing a mitochondrial DNA deletion disorder

BACKGROUND

Pathogenic mitochondrial DNA (mtDNA) mutations are found in at least one in 8000 individuals. No effective treatment for mtDNA disorders is available, making disease prevention important. Many patients with mtDNA disease harbour a single pathogenic mtDNA deletion, but the risk factors for new cases and disease recurrence are not known.

METHODS

We did a multicentre study of 226 families in which a single mtDNA deletion had been identified in the proband, including patients with chronic progressive external ophthalmoplegia, Kearns Sayre syndrome, or Pearson's syndrome. We studied the relation between maternal age and the risk of unaffected mothers having an affected child, and determined the recurrence risks among the siblings and offspring of affected individuals.

FINDINGS

We noted no relation between maternal age and the risk of unaffected mothers having children with an mtDNA deletion disorder. None of the 251 siblings of the index cases developed clinical features of mtDNA disease. Risk of recurrence among the offspring of affected women was 4.11% (95% CI 0.86-11.54, or one in 117 to one in nine births). Only one of the mothers who had an affected child had a duplication of mtDNA in skeletal muscle.

INTERPRETATION

Unlike nuclear chromosomal rearrangements, incidence of mtDNA deletion disorders does not increase with maternal age, and unaffected mothers are unlikely to have more than one affected child. Affected women were previously thought to have a negligible chance of having clinically affected offspring, but the actual risk is, on average, about one in 24 births.

The diagnosis of mitochondrial muscle disease

Mitochondrial respiratory chain abnormalities are an important cause of neuromuscular disease and may be due to defects of either the mitochondrial or nuclear genome. On account of the clinical and genetic heterogeneity exhibited by the mitochondrial myopathies, their investigation and diagnosis remains a challenge, requiring a combination of techniques including muscle histochemistry, biochemical assessment of respiratory chain function and molecular genetic studies. Here, we describe a step-by-step approach to the clinical and laboratory diagnosis of mitochondrial muscle disease, highlighting the many potential problems that can hinder reaching the correct diagnosis.

Mitochondrial Encephalomyopathies: Therapeutic Approach

Therapy for mitochondrial diseases is woefully inadequate. How-ever, lack of cure does not equate with lack of treatment. In this review, we consider sequentially several different therapeutic approaches. Palliative therapy is dictated by good medical practice and includes anticonvulsant medication, control of endocrine dysfunction, and surgical procedures. Removal of noxious metabolites is centered on combating lactic acidosis, but it extends to other metabolites, such as thymidine in patients with the mitochondrial neurogastrointestinal encephalomyopathy syndrome. Attempts to bypass blocks in the respiratory chain by administration of artificial electron acceptors have not been successful, but this concept may be amenable to genetic engineering. Administration of metabolites and cofactors is the mainstay of real-life therapy and includes both components of the respiratory chain and other natural compounds. There is increasing interest in the administration of reactive oxygen species scavengers both in primary mitochondrial diseases and in neurodegenerative diseases directly or indirectly related to mitochondrial dysfunction. Aerobic exercise and physical therapy prevent or correct deconditioning and improve exercise tolerance in patients with mitochondrial myopathies due to mtDNA mutations. Gene therapy is a challenge because of polyplasmy and heteroplasmy, but interesting experimental approaches are being pursued and include, for example, decreasing the ratio of mutant to wild-type mitochondrial genomes (gene shifting), converting mutated mtDNA genes into normal nDNA genes (allotropic expression), importing cognate genes from other species, or correcting mtDNA mutations with specific restriction endonucleases. Germline therapy raises ethical problems but is being seriously considered to prevent maternal transmission of mtDNA mutations. Preventive therapy through genetic counseling and prenatal diagnosis is still limited for mtDNA-related disorders but is becoming increasingly important for nDNA-related disorders.

Gastrointestinal manifestations of mitochondrial disease

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

The genetics of inherited sideroblastic anemias

The sideroblastic anemias are a heterogeneous group of acquired and inherited bone marrow disorders defined by the presence of pathologic iron deposits in erythroblast mitochondria. While the pathogenesis of almost all cases of acquired sideroblastic anemia is unknown, the molecular genetic basis for several of the inherited forms have now been described. Initially, mutations in ALAS2 in X-linked sideroblastic anemia (XLSA) focused attention on the heme biosynthetic pathway as a primary cause of sideroblastic anemia. However, the subsequent description of the genes involved in XLSA with ataxia, thiamine-responsive megaloblastic anemia, and Pearson marrow-pancreas syndrome have implicated other pathways, including mitochondrial oxidative phosphorylation, thiamine metabolism, and iron-sulfur cluster biosynthesis, as primary defects in sideroblastic anemias that may only secondarily impact heme metabolism.

Mitochondrial diseases

Mitochondrial disorders are caused by deficient respiratory chain function, resulting in a complex series of pathophysiological events. Genetic counselling is complicated because the respiratory chain subunits are encoded by both nuclear and mitochondrial DNA genes. Only a minority of the nuclear genes involved in mitochondrial function have been identified, and even fewer are associated with human mitochondrial disease. Mutations in mitochondrial DNA are particularly challenging because of the complexities of mitochondrial genetics: the mitochondrial DNA is strictly maternally inherited; there are 10(3)-10(4) copies of mitochondrial DNA in somatic cells; affected individuals often have a mixture of normal and mutated mitochondrial DNA (mitochondrial DNA heteroplasmy), the level of mutated mitochondrial DNA (the mitochondrial DNA mutation load) may vary widely between different maternally related individuals, between tissues and with time; a particular minimal threshold of mutated mitochondrial DNA is required to impair respiratory chain function; and there is not always a good correlation between mutant load and phenotype.

Identical mitochondrial DNA deletion in a woman with ocular myopathy and in her son with pearson syndrome

Single deletions of mitochondrial DNA (mtDNA) are associated with three major clinical conditions: Kearns-Sayre syndrome, a multisystem disorder; Pearson syndrome (PS), a disorder of the hematopoietic system; and progressive external ophthalmoplegia (PEO), primarily affecting the ocular muscles. Typically, single mtDNA deletions are sporadic events, since the mothers, siblings, and offspring of affected individuals are unaffected. We studied a woman who presented with PEO, ptosis, and weakness of pharyngeal, facial, neck, and limb muscles. She had two unaffected children, but another of her children, an infant son, had sideroblastic anemia, was diagnosed with PS, and died at age 1 year. Morphological analysis of a muscle biopsy sample from the mother showed cytochrome c oxidase-negative ragged-red fibers-a typical pattern in patients with mtDNA deletions. Southern blot analysis using multiple restriction endonucleases and probed with multiple mtDNA fragments showed that both the mother and her infant son harbored an identical 5,355-bp single deletion in mtDNA, without flanking direct repeats. The deletion was the only abnormal species of mtDNA identified in both patients, and there was no evidence for duplications. We conclude that, although the vast majority of single large-scale deletions in mtDNA are sporadic, in rare cases, single deletions can be transmitted through the germline.

Diagnosis and management of mitochondrial diseases

Mitochondrial dysfunction should be considered in the differential diagnosis of any progressive multisystem disorder. The diagnosis is most challenging when only one symptom is present. In contrast, the diagnosis is easier to consider when two or more seemingly unrelated symptoms are present, involving more than one organ system. It is important to consider the diagnosis of a mitochondrial disorder when dealing with an unexplained association of symptoms, with an early onset and progressive course involving seemingly unrelated organs. The investigation can be relatively straightforward if a person has a recognizable phenotype and if it is possible to identify a known pathogenic mtDNA mutation. The difficulty arises when no known mtDNA defect can be found or when the clinical abnormalities are complex and not easily matched to those of more common mitochondrial disorders. In summary: A full mitochondrial evaluation often is warranted in children with a complex neurologic picture or a single neurologic symptom and other system involvement. When the presentation is classic for a maternally inherited mitochondrial syndrome, such as MELAS, MERRF, or Leber's hereditary optic neuropathy, appropriate mtDNA studies should be obtained first. When the clinical picture is classic for a nuclear DNA inherited syndrome and the gene or linkage is known, such as MNGIE, the clinician should proceed with genetic studies. When the clinical picture is nonspecific but highly suggestive of a mitochondrial disorder, the clinician should start with plasma or CSF lactic acid, ketone bodies, plasma acylcarnitines, and urinary organic acids. If these studies are abnormal, the clinician should proceed with muscle biopsy and assessment of the respiratory chain enzymes. Normal plasma or CSF lactic acid does not rule out a mitochondrial disorder.

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.

Fluorescence in situ hybridization analysis of peripheral blood cells in Pearson marrow-pancreas syndrome

We used a dual-color fluorescence in situ hybridization technique to estimate deleted mitochondrial DNA at a single-cell level and determine any correlation with the disease progression in lymphocytes from patients with Pearson marrow-pancreas syndrome. The method demonstrated a shift in heteroplasmy, paralleling the hematologic improvement.

Mitochondrial respiratory chain disorders and the liver

Mitochondrial respiratory chain disorders are an established cause of liver failure in early childhood but they are probably under-diagnosed, partly due to under-recognition and partly due to the difficulty of investigation. It is particularly important to look for mitochondrial disorders if the liver disease presents with hypoglycaemia and lactic acidaemia or if it is accompanied by neurological, muscle or renal tubular abnormalities. Respiratory chain defects have been demonstrated in a number of patients who die of liver failure following severe epilepsy; this includes at least some cases of Alpers syndrome or 'progressive neuronal degeneration of childhood'. In mitochondrial liver disease, histology usually shows steatosis, often accompanied by fibrosis, cholestasis and loss of hepatocytes. Unless the clinical picture suggests a particular syndrome, such as Pearson syndrome, biochemical assays and histochemistry should be the initial investigations. Ideally, investigations should be carried out on liver as well as more standard tissues, such as muscle, since defects can be tissue-specific. Nuclear defects and mtDNA point mutations are probably responsible for many cases of mitochondrial liver disease but, as yet, the only identified molecular abnormalities are mtDNA rearrangements and mtDNA depletion. Treatment of mitochondrial liver disease is unsatisfactory. If the disease is confined to the liver, transplantation may be appropriate but in several patients transplantation has been followed by the appearance of disease in other organs, particularly the brain.

Mitochondrial myopathies and encephalomyopathies

Defects of mitochondrial metabolism result in a wide variety of human disorders, which can present at any time from infancy to late adulthood and involve virtually any tissue either alone or in combination. Abnormalities of the electron transport and oxidative phosphorylation (OXPHOS) system are probably the most common cause of mitochondrial diseases. Thirteen of the protein subunits of OXPHOS are encoded by mitochondrial DNA (mtDNA) and mutations of this genome are important causes of OXPHOS deficiency. The link between genotype and phenotype with respect to mtDNA mutations is not clear: the same mutation may result in a variety of phenotypes, and the same phenotype may be seen with a variety of different mtDNA mutations. The pathogenesis of mtDNA mutations is unclear although OXPHOS and ATP deficiency, and free radical generation, are thought to contribute to tissue dysfunction. There is now strong evidence for mitochondrial dysfunction in neurodegenerative disorders. In some cases, e.g. Friedreich's ataxia, hereditary spastic paraplegia, this is a result of a mutation of a nuclear gene encoding a mitochondrial protein, whilst in others, e.g. Huntington's disease, amyotrophic lateral sclerosis, the OXPHOS defect is secondary to events induced by a mutation in a nuclear gene encoding a non-mitochondrial protein. In yet a third group, e.g. Parkinson's disease, Alzheimer's disease, the relationship of the mitochondrial defect to aetiology and pathogenesis is unclear.

Common deletion of mitochondrial DNA in a 5-year-old girl with failure to thrive, ptosis, ophthalmoplegia and ragged-red fibers

A girl aged 4 years and 10 months presented with failure to thrive, ptosis, ragged-red fibers and the common 4.9 kb mitochondrial DNA deletion. She had elevated serum lactic and pyruvic acids. The onset was at around 18 months. There were no signs of retinitis, and abnormal renal, liver or pancreatic functions. She later developed mild ophthalmoplegia at 6 years of age. Additional features of chronic progressive external ophthalmoplegia (CPEO) or Kearns-Sayre syndrome (KSS) are the conditions that should be watched and investigated in the long-term follow-up of this girl.

Mitochondrial encephalomyopathies: the enigma of genotype versus phenotype

Over the past decade a large body of evidence has accumulated implicating defects of human mitochondrial DNA in the pathogenesis of a group of disorders known collectively as the mitochondrial encephalomyopathies. Although impaired oxidative phosphorylation is likely to represent the final common pathway leading to cellular dysfunction in these diseases, fundamental issues still remain elusive. Perhaps the most challenging of these is to understand the mechanisms which underlie the complex relationship between genotype and phenotype. Here we examine this relationship and discuss some of the factors which are likely to be involved.

Mitochondria and childhood liver diseases

The newly recognized mitochondrial hepatopathies should be considered in the differential diagnosis of acute and chronic liver disease in childhood. It may appear as neonatal liver failure, delayed onset liver failure in early childhood or as a multisystemic process. Comparison of features of several of the known primary mitochondrial hepatopathies is provided in Table 5. Secondary mitochondrial hepatopathies are examples of the critical importance of mitochondrial function in the pathogenesis of liver injury. Our improved understanding of the role of the mitochondria in cellular necrosis and apoptosis opens the way for development of new therapeutic approaches to several hepatic disorders. Primary mitochondrial hepatopathies (especially the respiratory chain defects) should be considered in any child with liver disease and neuromuscular involvement, multisystemic disease, lactic acidosis or rapidly progressive disease, and when hepatic steatosis is the dominant histologic finding on examination of a liver specimen. Current therapies are inadequate; improved therapeutic strategies are needed for these disorders. Some patients with respiratory chain defects limited to the liver have had successful liver transplantation. This field is in evolution and will undoubtedly provide new and important developments as the next millennium begins.

Evidence from human oocytes for a genetic bottleneck in an mtDNA disease

We have examined oocytes from a patient with Kearn-Sayre syndrome caused by mtDNA rearrangements. In mtDNA diseases, mutant and wild-type mtDNA frequently coexist in affected individuals (the condition of heteroplasmy). The proportion of mutant mtDNA transmitted from mother to offspring is variable because of a genetic bottleneck, and the "dose" of mutant mtDNA received influences the severity of the phenotype. The feasibility of prenatal diagnosis is critically dependent on the nature and timing of this bottleneck. Significant levels of rearranged mtDNA were detectable in the majority of the patient's oocytes, by use of multiplex PCR, with wide variation, in the levels of mutant and wild-type molecules, between individual oocytes. We also used length variation in a homopolymeric C tract, which is often heteroplasmic in normal controls, to identify founder subpopulations of mtDNAs in this patient's oocytes. We present direct evidence that the number of segregating units (n) is three to five orders of magnitude less than the number of mitochondria in the human female oocyte. In some cases, the best estimate of n may correspond to a single mitochondrion, if it is assumed that intergenerational transmission of mtDNA can be treated as a single sampling event. The bottleneck appears to contribute a major component of the variable transmission from mother to oocyte, in this patient and in a control. That this bottleneck had occurred by the time that oocytes were mature advances the prospects for prenatal diagnosis of mtDNA diseases.