Maternally inherited Leigh syndrome

iPSC models of mitochondrial diseases

Mitochondrial diseases are historically difficult to study. They cause multi-systemic defects with prevalent impairment of hard-to-access tissues such as the brain and the heart. Furthermore, they suffer from a paucity of conventional model systems, especially because of the challenges associated with mitochondrial DNA (mtDNA) engineering. Consequently, most mitochondrial diseases are currently untreatable. Human induced pluripotent stem cells (iPSCs) represent a promising approach for developing human model systems and assessing therapeutic avenues in a patient- and tissue-specific context. iPSCs are being increasingly used to investigate mitochondrial diseases, either for dissecting mutation-specific defects within two-dimensional (2D) or three-dimensional (3D) progenies or for unveiling the impact of potential treatment options. Here, we review how iPSC-derived 2D cells and 3D organoid models have been applied to the study of mitochondrial diseases caused by either nuclear or mtDNA defects. We anticipate that the field of iPSC-driven modeling of mitochondrial diseases will continue to grow, likely leading to the development of innovative platforms for treatment discovery and toxicity that could benefit the patient community suffering from these debilitating disorders with highly unmet medical needs.

Syndromic Retinitis Pigmentosa: A Narrative Review

Retinitis pigmentosa (RP) encompasses inherited retinal dystrophies, appearing either as an isolated eye condition or as part of a broader systemic syndrome, known as syndromic RP. In these cases, RP includes systemic symptoms impacting other organs, complicating diagnosis and management. This review highlights key systemic syndromes linked with RP, such as Usher, Bardet-Biedl, and Alström syndromes, focusing on genetic mutations, inheritance, and clinical symptoms. These insights support clinicians in recognizing syndromic RP early. Ocular signs like nystagmus and congenital cataracts may indicate systemic disease, prompting genetic testing. Conversely, systemic symptoms may necessitate eye exams, even if vision symptoms are absent. Understanding the systemic aspects of these syndromes emphasizes the need for multidisciplinary collaboration among ophthalmologists, pediatricians, and other specialists to optimize patient care. The review also addresses emerging genetic therapies aimed at both visual and systemic symptoms, though more extensive studies are required to confirm their effectiveness. Overall, by detailing the genetic and clinical profiles of syndromic RP, this review seeks to aid healthcare professionals in diagnosing and managing these complex conditions more effectively, enhancing patient outcomes through timely, specialized intervention.

Intracellular calcium dysregulation in heart and brain diseases: Insights from induced pluripotent stem cell studies

The central nervous system (CNS) plays a role in regulating heart rate and myocardial contractility through sympathetic and parasympathetic nerves, and the heart can impact the functional equilibrium of the CNS through feedback signals. Although heart and brain diseases often coexist and mutually influence each other, the potential links between heart and brain diseases remain unclear due to a lack of reliable models of these relationships. Induced pluripotent stem cells (iPSCs), which can differentiate into multiple functional cell types, stem cell biology and regenerative medicine may offer tools to clarify the mechanisms of these relationships and facilitate screening of effective therapeutic agents. Because calcium ions play essential roles in regulating both the cardiovascular and nervous systems, this review addresses how recent iPSC disease models reveal how dysregulation of intracellular calcium might be a common pathological factor underlying the relationships between heart and brain diseases.

Modeling mitochondrial DNA diseases: from base editing to pluripotent stem-cell-derived organoids

Mitochondrial DNA (mtDNA) diseases are multi-systemic disorders caused by mutations affecting a fraction or the entirety of mtDNA copies. Currently, there are no approved therapies for the majority of mtDNA diseases. Challenges associated with engineering mtDNA have in fact hindered the study of mtDNA defects. Despite these difficulties, it has been possible to develop valuable cellular and animal models of mtDNA diseases. Here, we describe recent advances in base editing of mtDNA and the generation of three-dimensional organoids from patient-derived human-induced pluripotent stem cells (iPSCs). Together with already available modeling tools, the combination of these novel technologies could allow determining the impact of specific mtDNA mutations in distinct human cell types and might help uncover how mtDNA mutation load segregates during tissue organization. iPSC-derived organoids could also represent a platform for the identification of treatment strategies and for probing the in vitro effectiveness of mtDNA gene therapies. These studies have the potential to increase our mechanistic understanding of mtDNA diseases and may open the way to highly needed and personalized therapeutic interventions.

Mitochondrial encephalomyopathy

Mitochondrial dysfunction, especially perturbation of oxidative phosphorylation and adenosine triphosphate (ATP) generation, disrupts cellular homeostasis and is a surprisingly frequent cause of central and peripheral nervous system pathology. Mitochondrial disease is an umbrella term that encompasses a host of clinical syndromes and features caused by in excess of 300 different genetic defects affecting the mitochondrial and nuclear genomes. Patients with mitochondrial disease can present at any age, ranging from neonatal onset to late adult life, with variable organ involvement and neurological manifestations including neurodevelopmental delay, seizures, stroke-like episodes, movement disorders, optic neuropathy, myopathy, and neuropathy. Until relatively recently, analysis of skeletal muscle biopsy was the focus of diagnostic algorithms, but step-changes in the scope and availability of next-generation sequencing technology and multiomics analysis have revolutionized mitochondrial disease diagnosis. Currently, there is no specific therapy for most types of mitochondrial disease, although clinical trials research in the field is gathering momentum. In that context, active management of epilepsy, stroke-like episodes, dystonia, brainstem dysfunction, and Parkinsonism are all the more important in improving patient quality of life and reducing mortality.

Attempts to understand the mechanisms of mitochondrial diseases: The reverse genetics of mouse models for mitochondrial disease

BACKGROUND

Mitochondrial disease is a general term for a disease caused by a decline in mitochondrial function. The pathology of this disease is extremely diverse and complex, and the mechanism of its pathogenesis is still unknown. Using mouse models that develop the disease via the same processes as in humans is the easiest path to understanding the underlying mechanism. However, creating a mouse model is extremely difficult due to the lack of technologies that enable editing of mitochondrial DNA (mtDNA).

SCOPE OF REVIEW

This paper outlines the complex pathogenesis of mitochondrial disease, and the difficulties in producing relevant mouse models. Then, the paper provides a detailed discussion on several mice created with mutations in mtDNA. The paper also introduces the pathology of mouse models with mutations including knockouts of nuclear genes that directly affect mitochondrial function.

MAJOR CONCLUSIONS

Several mice with mtDNA mutations and those with nuclear DNA mutations have been established. Although these models help elucidate the pathological mechanism of mitochondrial disease, they lack sufficient diversity to enable a complete understanding. Considering the variety of factors that affect the cause and mechanism of mitochondrial disease, it is necessary to account for this background diversity in mouse models as well.

GENERAL SIGNIFICANCE

Mouse models are indispensable for understanding the pathological mechanism of mitochondrial disease, as well as for searching new treatments. There is a need for the creation and examination of mouse models with more diverse mutations and altered nuclear backgrounds and breeding environments.

The m.9143T>C Variant: Recurrent Infections and Immunodeficiency as an Extension of the Phenotypic Spectrum in MT-ATP6 Mutations?

Pathogenic variants in the MT-ATP6 are a well-known cause for maternally inherited mitochondrial disorders associated with a wide range of clinical phenotypes. Here, we present a 31- year old female with insulin-dependent diabetes mellitus, recurrent lactic acidosis and ketoacidosis recurrent infections with suspected immunodeficiency with T cell lymphopenia and hypogammaglobulinemia as well as proximal tetraparesis with severe muscle and limb pain and rapid physical exhaustion. Muscle biopsy and respiratory chain activities were normal. Single-exome sequencing revealed a variant in the MT-ATP6 gene: m.9143T>C. Analysis of further specimen of the index and mother (segregation studies) revealed the highest mutation load in muscle (99% level of mtDNA heteroplasmy) of the index patient. Interestingly, acute metabolic and physical decompensation during recurrent illness was documented to be a common clinical feature in patients with MT-ATP6 variants. However, it was not mentioned as a key symptom. Thus, we suggest that the clinical spectrum might be expanded in ATP6-associated diseases.

Novel insights into the functional metabolic impact of an apparent de novo m.8993T>G variant in the MT-ATP6 gene associated with maternally inherited form of Leigh Syndrome

In this study, we report a novel perpective of metabolic consequences for the m.8993T>G variant using fibroblasts from a proband with clinical symptoms compatible with Maternally Inherited Leigh Syndrome (MILS). Definitive diagnosis was corroborated by mitochondrial DNA testing for the pathogenic variant m.8993T>G in MT-ATP6 subunit by Sanger sequencing. The long-range PCR followed by massively parallel sequencing method detected the near homoplasmic m.8993T>G variant at 83% in the proband's fibroblasts and at 0.4% in the mother's fibroblasts. Our results are compatible with very low levels of germline heteroplasmy or an apparent de novo mutation. Our mitochondrial morphometric analysis reveals severe defects in mitochondrial cristae structure in the proband's fibroblasts. Our live-cell mitochondrial respiratory analyses show impaired oxidative phosphorylation with decreased spare respiratory capacity in response to energy stress in the proband's fibroblasts. We detected a diminished glycolysis with a lessened glycolytic capacity and reserve, revealing a stunted ability to switch to glycolysis upon full inhibition of OXPHOS activities. This dysregulated energy reprogramming results in a defective interplay between OXPHOS and glycolysis during an energy crisis. Our study sheds light on the potential pathophysiologic mechanism leading to chronic energy crisis in this MILS patient harboring the m.8993T>G variant.

ATP Synthase Diseases of Mitochondrial Genetic Origin

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

Novel reproductive technologies to prevent mitochondrial disease

BACKGROUND

The use of nuclear transfer (NT) has been proposed as a novel reproductive treatment to overcome the transmission of maternally-inherited mitochondrial DNA (mtDNA) mutations. Pathogenic mutations in mtDNA can cause a wide-spectrum of life-limiting disorders, collectively known as mtDNA disease, for which there are currently few effective treatments and no known cures. The many unique features of mtDNA make genetic counselling challenging for women harbouring pathogenic mtDNA mutations but reproductive options that involve medical intervention are available that will minimize the risk of mtDNA disease in their offspring. This includes PGD, which is currently offered as a clinical treatment but will not be suitable for all. The potential for NT to reduce transmission of mtDNA mutations has been demonstrated in both animal and human models, and has recently been clinically applied not only to prevent mtDNA disease but also for some infertility cases. In this review, we will interrogate the different NT techniques, including a discussion on the available safety and efficacy data of these technologies for mtDNA disease prevention. In addition, we appraise the evidence for the translational use of NT technologies in infertility.

OBJECTIVE AND RATIONALE

We propose to review the current scientific evidence regarding the clinical use of NT to prevent mitochondrial disease.

SEARCH METHODS

The scientific literature was investigated by searching PubMed database until Jan 2017. Relevant documents from Human Fertilisation and Embryology Authority as well as reports from both the scientific and popular media were also implemented. The above searches were based on the following key words: 'mitochondria', 'mitochondrial DNA'; 'mitochondrial DNA disease', 'fertility'; 'preimplantation genetic diagnosis', 'nuclear transfer', 'mitochondrial replacement' and 'mitochondrial donation'.

OUTCOMES

While NT techniques have been shown to effectively reduce the transmission of heteroplasmic mtDNA variants in animal models, and increasing evidence supports their use to prevent the transmission of human mtDNA disease, the need for robust, long-term evaluation is still warranted. Moreover, prenatal screening would still be strongly advocated in combination with the use of these IVF-based technologies. Scientific evidence to support the use of NT and other novel reproductive techniques for infertility is currently lacking.

WIDER IMPLICATIONS

It is mandatory that any new ART treatments are first adequately assessed in both animal and human models before the cautious implementation of these new therapeutic approaches is clinically undertaken. There is growing evidence to suggest that the translation of these innovative technologies into clinical practice should be cautiously adopted only in highly selected patients. Indeed, given the limited safety and efficacy data, close monitoring of any offspring remains paramount.

Mitochondrial DNA sequence characteristics modulate the size of the genetic bottleneck

With a combined carrier frequency of 1:200, heteroplasmic mitochondrial DNA (mtDNA) mutations cause human disease in ∼1:5000 of the population. Rapid shifts in the level of heteroplasmy seen within a single generation contribute to the wide range in the severity of clinical phenotypes seen in families transmitting mtDNA disease, consistent with a genetic bottleneck during transmission. Although preliminary evidence from human pedigrees points towards a random drift process underlying the shifting heteroplasmy, some reports describe differences in segregation pattern between different mtDNA mutations. However, based on limited observations and with no direct comparisons, it is not clear whether these observations simply reflect pedigree ascertainment and publication bias. To address this issue, we studied 577 mother–child pairs transmitting the m.11778G>A, m.3460G>A, m.8344A>G, m.8993T>G/C and m.3243A>G mtDNA mutations. Our analysis controlled for inter-assay differences, inter-laboratory variation and ascertainment bias. We found no evidence of selection during transmission but show that different mtDNA mutations segregate at different rates in human pedigrees. m.8993T>G/C segregated significantly faster than m.11778G>A, m.8344A>G and m.3243A>G, consistent with a tighter mtDNA genetic bottleneck in m.8993T>G/C pedigrees. Our observations support the existence of different genetic bottlenecks primarily determined by the underlying mtDNA mutation, explaining the different inheritance patterns observed in human pedigrees transmitting pathogenic mtDNA mutations.

Understanding structure, function, and mutations in the mitochondrial ATP synthase

The mitochondrial ATP synthase is a multimeric enzyme complex with an overall molecular weight of about 600,000 Da. The ATP synthase is a molecular motor composed of two separable parts: F₁ and F_o. The F₁ portion contains the catalytic sites for ATP synthesis and protrudes into the mitochondrial matrix. F_o forms a proton turbine that is embedded in the inner membrane and connected to the rotor of F₁. The flux of protons flowing down a potential gradient powers the rotation of the rotor driving the synthesis of ATP. Thus, the flow of protons though F_o is coupled to the synthesis of ATP. This review will discuss the structure/function relationship in the ATP synthase as determined by biochemical, crystallographic, and genetic studies. An emphasis will be placed on linking the structure/function relationship with understanding how disease causing mutations or putative single nucleotide polymorphisms (SNPs) in genes encoding the subunits of the ATP synthase, will affect the function of the enzyme and the health of the individual. The review will start by summarizing the current understanding of the subunit composition of the enzyme and the role of the subunits followed by a discussion on known mutations and their effect on the activity of the ATP synthase. The review will conclude with a summary of mutations in genes encoding subunits of the ATP synthase that are known to be responsible for human disease, and a brief discussion on SNPs.

Identification and biochemical characterization of the novel mutation m.8839G>C in the mitochondrial ATP6 gene associated with NARP syndrome

Mutations in the ATP6 gene are reported to be associated with Leber hereditary optic neuropathy, bilateral striatal necrosis, coronary atherosclerosis risk and neuropathy, ataxia and retinitis pigmentosa (NARP)/maternally inherited Leigh syndromes. Here, we present a patient with NARP syndrome, in whom a previously undescribed mutation was detected in the ATP6 gene: m.8839G>C. Several observations support the concept that m.8839G>C is pathogenically involved in the clinical phenotype of this patient: (1) the mutation was heteroplasmic in muscle; (2) mutation load was higher in the symptomatic patient than in the asymptomatic carriers; (3) cybrids carrying this mutation presented lower cell proliferation, increased mitochondrial DNA (mtDNA) copy number, increased steady-state OxPhos protein levels and decreased mitochondrial membrane potential with respect to isogenic wild-type cybrids; (4) this change was not observed in 2959 human mtDNAs from different mitochondrial haplogroups; (5) the affected amino acid was conserved in all the ATP6 sequences analyzed; and (6) using in silico prediction, the mutation was classified as 'probably damaging'. However, measurement of ATP synthesis showed no differences between wild-type and mutated cybrids. Thus, we suggest that m.8839G>C may lower the efficiency between proton translocation within F0 and F1 rotation, required for ATP synthesis. Further experiments are needed to fully characterize the molecular mechanisms involved in m.8839G>C pathogenicity.

Frequencies of mtDNA mutations in primary tissues of colorectal adenopolyps

To investigate the potential role of mtDNA alterations during the onset of colorectal cancer, the occurrence of mtDNA variants in colorectal adenomatous (Tubular, Tubulovillous, and Villous) polyps, were studied. High resolution endonucleases and PCR-based sequence were applied to examine mtDNA variants in the ND and ATPase genes of 64 primary tissues of colorectal adeno-polyps and their matched normal controls. Forty-two variants were observed and 57% (24/42) were not previously reported in the MITODAT reference. Fifty-eight percent of these variants were germline and homoplasmic transitions. The distribution of observed mtDNA variants includes: 31% (13/42) tubular, 52% (22/42) tubulovillous, 45% (19/42) villous, and 45% (19/42) cancer (including FAP and JVP). Notably, an unreported germline variant in the ATPase 8 gene at nucleotide position (np) G8573A was observed in tubulovillous adenomas tissues. The results suggest that some specific mtDNA variants may serve as a potential biomarker for colorectal adenomatous polyps.

Mitochondrial membrane potential and ATP production in primary disorders of ATP synthase

Studies of fibroblasts with primary defects in mitochondrial ATP synthase (ATPase) due to heteroplasmic mtDNA mutations in the ATP6 gene, affecting protonophoric function or synthesis of subunit a, show that at high mutation loads, mitochondrial membrane potential DeltaPsi(m) at state 4 is normal, but ADP-induced discharge of DeltaPsi(m) is impaired and ATP synthesis at state 3-ADP is decreased. Increased DeltaPsi(m) and low ATP synthesis is also found when the ATPase content is diminished by altered biogenesis of the enzyme complex. Irrespective of the different pathogenic mechanisms, elevated DeltaPsi(m) in primary ATPase disorders could increase mitochondrial production of reactive oxygen species and decrease energy provision.

Prevention of mitochondrial disease inheritance by assisted reproductive technologies: prospects and challenges

BACKGROUND

Mitochondrial diseases are caused by the mutations in both nuclear and mitochondrial DNA (mtDNA) and the treatment options for patients who have mitochondrial disease are rather limited. Mitochondrial DNA is transmitted maternally and does not follow a Mendelian pattern of inheritance. Since reliable and predictable detection of mitochondrial disorders in embryos and oocytes is unattainable at present, an alternative approach to this problem has emerged as partial or complete replacement of mutated mtDNA with the wild-type mtDNA through embryo manipulations. Currently available methods to achieve this goal are germinal vesicle transfer (GVT), metaphase chromosome transfer (CT), pronuclear transfer (PNT) and ooplasmic transfer (OT).

SCOPE OF REVIEW

We summarize the state of the art regarding these technologies and discuss the implications of recent advances in the field for clinical practice.

MAJOR CONCLUSIONS

CT, PNT and GVT techniques hold promise to prevent transmission of mutant mtDNA through ARTs. However, it is clear that mtDNA heteroplasmy in oocytes, embryos and offspring produced by these methods remains as a legitimate concern.

GENERAL SIGNIFICANCE

New approaches to eliminate transmission of mutant mtDNA certainly need to be explored in order to bring the promise of clinical application for the treatment of mitochondrial disorders. This article is part of a Special Issue entitled Biochemistry of Mitochondria, Life and Intervention 2010.

Mouse models for nuclear DNA-encoded mitochondrial complex I deficiency

Mitochondrial diseases are a group of heterogeneous pathologies with decreased cellular energy production as a common denominator. Defects in the oxidative phosphorylation (OXPHOS) system, the most frequent one in humans being isolated complex I deficiency (OMIM 252010), underlie this disturbed-energy generation. As biogenesis of OXPHOS complexes is under dual genetic control, with complex II being the sole exception, mutations in both nuclear DNA (nDNA) and mitochondrial DNA (mtDNA) are found. Increasing knowledge is becoming available with respect to the pathophysiology and cellular consequences of OXPHOS dysfunction. This aids the rational design of new treatment strategies. Recently, the first successful treatment trials were carried out in patient-derived cell lines. In these studies chemical compounds were used that target cellular aberrations induced by complex I dysfunction. Before the field of human clinical trials is entered, it is necessary to study the effects of these compounds with respect to toxicity, pharmacokinetics and therapeutic potential in suitable animal models. Here, we discuss two recent mouse models for nDNA-encoded complex I deficiency and their tissue-specific knock-outs.

Mitochondrial DNA 11777C>A mutation associated Leigh syndrome: case report with a review of the previously described pedigrees

Here, we present a male infant with clinical signs of typical Leigh syndrome. The first symptom, myoclonus was presented already on the 5th day of life; at 7 months of age limb convulsions and cerebral paresis with hypotonia were developed. At the age of 11 months, MRI verified increased signal intensity in the entire mesencephalon and medulla oblongata; while gray matter proton spectroscopy revealed presence of lactate increase in the brain. At age of 17 months, the child died in cardiorespiratory arrest. After autopsy, the diagnosis of Leigh syndrome was established; using DNA isolated from skeletal muscle and liver, heteroplasmic (>50%) mitochondrial 11777C>A was detected in the fourth subunit of NADH dehydrogenase enzyme (MTND4) encoding gene, which causes Arg --> Ser replacement. The mutation was also detected in low copy number in blood of mother. Albeit this mutation type is well recognized as a typical mtDNA mutation, according the reports available on the PubMed, this mutation was described only in four patients with wide phenotypic variations; here, we reviewed the characteristic clinical features of them. Taken together, the earliest onset of symptoms, the nature of the first presentation signs, the most rapid progression, the character of minor additional symptoms, and the early fatal outcome differentiate the phenotypic variant of the proband presented here from cases reported so far by others.

Identification of ataxia-associated mtDNA mutations (m.4052T>C and m.9035T>C) and evaluation of their pathogenicity in transmitochondrial cybrids

The potential pathogenicity of two homoplasmic mtDNA point mutations, 9035T>C and 4452T>C, found in a family afflicted with maternally transmitted cognitive developmental delay, learning disability, and progressive ataxia was evaluated using transmitochondrial cybrids. We confirmed that the 4452T>C transition in tRNA(Met) represented a polymorphism; however, 9035T>C conversion in the ATP6 gene was responsible for a defective F(0)-ATPase. Accordingly, mutant cybrids had a reduced oligomycin-sensitive ATP hydrolyzing activity. They had less than half of the steady-state content of ATP and nearly an 8-fold higher basal level of reactive oxygen species (ROS). Mutant cybrids were unable to cope with additional insults, i.e., glucose deprivation or tertiary-butyl hydroperoxide, and they succumbed to either apoptotic or necrotic cell death. Both of these outcomes were prevented by the antioxidants CoQ(10) and vitamin E, suggesting that the abnormally high levels of ROS were the triggers of cell death. In conclusion, the principal metabolic defects, i.e., energy deficiency and ROS burden, resulted from the 9035T>C mutation and could be responsible for the development of clinical symptoms in this family. Furthermore, antioxidant therapy might prove helpful in the management of this disease.

Maternally inherited Leigh syndrome: T8993G mutation in a Tunisian family

Leigh syndrome is a genetically heterogeneous, neurodegenerative disorder that predominantly affects children and leads to death within months or years. Mutations causing this disease have been found in both mitochondrial and nuclear DNA. The present report describes a Tunisian family with a maternally inherited Leigh syndrome harboring the mitochondrial T8993G mutation in the ATPase 6 gene. Polymerase chain reaction-restriction fragment length polymorphism analysis with the MspI restriction endonuclease, quantified with a digital image analyzer and gel documentation system, showed that the T8993G mutation was present with a high percentage in the blood of the three patients tested. This mutation was also detected in the asymptomatic mothers of these three patients (90, 96, and 60%). Two novel mitochondrial mutations were identified in the mitochondrial ATP6 gene -- T8741G (L72R) and A8795G (H90R) -- and three novel polymorphisms. Altogether, Leigh syndrome presenting the T8993G mutation in the ATPase 6 gene with variable heteroplasmic loads (44-98%) in a single Tunisian family is a novel finding.

Preimplantation genetic diagnosis for mitochondrial DNA disorders: ethical guidance for clinical practice

Although morally acceptable in theory, preimplantation genetic diagnosis (PGD) for mitochondrial DNA (mtDNA) disorders raises several ethical questions in clinical practice. This paper discusses the major conditions for good clinical practice. Our starting point is that PGD for mtDNA mutations should as far as possible be embedded in a scientific research protocol. For every clinical application of PGD for mtDNA disorders, it is not only important to avoid a 'high risk of serious harm' to the future child, but also to consider to what extent it would be possible, desirable and proportional to try to reduce the health risks and minimize harm. The first issue we discuss is oocyte sampling, which may point out whether PGD is feasible for a specific couple. The second issue is whether one blastomere represents the genetic composition of the embryo as a whole -- and how this could (or should) be investigated. The third issue regards the cutoff points below which embryos are considered to be eligible for transfer. We scrutinize how to determine these cutoff points and how to use these cutoff points in clinical practice -- for example, when parents ask to take more or less risks. The fourth issue regards the number of cycles that can (or should) justifiably be carried out to find the best possible embryo. Fifth, we discuss whether follow-up studies should be conducted, particularly the genetic testing of children born after IVF/PGD. Finally, we offer the main information that is required to obtain a truly informed consent.

PGD to reduce reproductive risk: the case of mitochondrial DNA disorders

This paper discusses the pros and cons of introducing PGD for mitochondrial DNA (mtDNA) disorders such as NARP (Neurogenic muscle weakness, Ataxia, Retinis Pigmentosa)/Leigh, MELAS (Mitochondrial myopathy, Encephalopathy, Lactic acidosis, and Stroke-like episodes), private mtDNA mutations and LHON (Leber Hereditary Optic Neuropathy). Although there is little experience with PGD for mtDNA disorders, it is reasonable to assume that in many cases, the best one can achieve is the selection of the 'least' affected embryos for transfer. So instead of 'promising' parents a healthy child, PGD in these cases can only aim at reducing reproductive risk. From an ethical point of view, this raises challenging questions about parental and medical responsibilities. The main argument in favour of PGD is that it offers couples at risk the opportunity of reducing their chances of having a severely affected child. Potential objections are manifold, but we conclude that none of them supplies convincing moral arguments to regard risk-reducing PGD as unacceptable. Nevertheless, introducing this new application of PGD in clinical practice will raise further complex issues of determining conditions for its responsible use.

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.

Molecular-clinical correlations in a family with variable tissue mitochondrial DNA T8993G mutant load

Unlike many pathogenic mitochondrial DNA mutations, the T8993G mutation associated with Leigh syndrome (LS) and neurogenic muscle weakness, ataxia, retinitis pigmentosa (NARP) typically shows little variation in mutant load between different tissue types. We describe the molecular and clinical findings in a family with variable disease severity and tissue T8993G mutant loads. Real-time ARMS qPCR testing showed that two brothers with features of NARP and LS had high mutant loads (>90%) in all tissues tested, similar to previously reported cases. Their sister, who has mild speech delay but attends normal school, was found to have a relatively high mutant load (mean 93%) in tissues derived from endoderm (buccal mucosa) and mesoderm (blood and skin fibroblasts). However, in tissue derived from ectoderm (hair bulbs), she carried a considerably lower proportion of mutant mtDNA. Because both surface ectoderm, which gives rise to outer epithelia and hair, and neuroectoderm, which gives rise to the central nervous system, are derived from ectoderm, it is tempting to speculate that the mutant load detected in the oligosymptomatic sister's hair bulbs is a reflection of the brain mutant load. We conclude that significant variation in tissue mutant load may occur in at least some individuals that harbor the T8993G mutation. This adds additional complexity to genetic counseling and prenatal diagnosis in such instances. Given the shared embryonic origin of hair bulbs and brain, we recommend performing hair bulb mtDNA analysis in asymptomatic or oligosymptomatic individuals that have high blood mutant loads in order to understand better the genotype-phenotype correlations related to the T8993G mutation.

Mitochondrial encephalomyopathies: diagnostic approach

Mitochondrial diseases have extremely heterogeneous clinical presentations due to the ubiquitous nature of mitochondria and the dual genetic control of the respiratory chain. Thus, mitochondrial disorders can be multisystemic (mitochondrial encephalomyopathies) or confined to a single tissue, and they can be sporadic or transmitted by mendelian or maternal inheritance. Mendelian disorders are usually inherited as autosomal recessive traits, tend to present earlier in life, and usually "breed true" in each family. By contrast, mitochondrial DNA-related diseases usually start later and vary in their presentation within members of the same family. Precise diagnosis is often a challenge; we go through the traditional steps of the diagnostic process, trying to highlight clues to mitochondrial dysfunction in the family history, physical and neurological examinations, routine and special laboratory tests, and histo-chemical and biochemical results of the muscle biopsy. The ultimate goal is to reach, whenever possible, a definitive molecular diagnosis, which permits rational genetic counseling and a prenatal diagnosis.

Reduced respiratory control with ADP and changed pattern of respiratory chain enzymes as a result of selective deficiency of the mitochondrial ATP synthase

The F(o)F(1)-ATPase, a multisubunit protein complex of the inner mitochondrial membrane, produces most of the ATP in mammalian cells. Mitochondrial diseases as a result of a dysfunction of ATPase can be caused by mutations in mitochondrial DNA-encoded ATPase subunit a or rarely by an ATPase defect of nuclear origin. Here we present a detailed functional and immunochemical analysis of a new case of selective and generalized ATPase deficiency found in an Austrian patient. The defect manifested with developmental delay, muscle hypotonia, failure to thrive, ptosis, and varying lactic acidemia (up to 12 mmol/L) beginning from the neonatal period. A low-degree dilated cardiomyopathy of the left ventricle developed between the age of 1 and 2 y. A >90% decrease in oligomycin-sensitive ATPase activity and an 86% decrease in the content of the ATPase complex was found in muscle mitochondria. It was associated with a significant decrease of ADP-stimulated respiration of succinate (1.5-fold) and respiratory control with ADP (1.7-fold) in permeabilized muscle fibers, and with a slight decrease of the respiratory chain complex I and compensatory increase in the content of complexes III and IV. The same ATPase deficiency without an increase in respiratory chain complexes was found in fibroblasts, suggesting a generalized defect with tissue-specific manifestation. Absence of any mutations in mitochondrial ATP6 and ATP8 genes indicates a nuclear origin of the defect.

High mitochondrial DNA T8993G mutation (<90%) without typical features of Leigh's and NARP syndromes

Neuropathy, ataxia, and retinitis pigmentosa (NARP) syndrome and maternally inherited Leigh's syndrome have been associated with T8993G point mutations in the mitochondrial adenosine triphosphatase 6 gene. Typically, NARP syndrome is characterized by developmental delay, seizures, dementia, retinitis pigmentosa, ataxia, sensory neuropathy, and proximal weakness. Usually, there is a correlation between the percentage of mutated mitochondrial DNA and clinical severity, and when mutated mitochondrial DNA is > 90%, it is often seen with Leigh's syndrome. We now report a family with mitochondrial DNA T8993G mutation in eight living members, five with mutant mitochondrial DNA >90% and one with 20% mutant mitochondrial DNA. However, their clinical features include variable combinations of seizures, behavior problems, learning disability, mental retardation, sensorineural deafness, cerebellar ataxia, and proximal muscle weakness. No retinitis pigmentosa was found in all eight living members, including a 56-year-old grandmother. Only one dead female relative was diagnosed with Leigh's syndrome on the neuropathologic examination at age 22 years, when she died of an accident. High mitochondrial DNA T8993G mutation is not always associated with typical features of Leigh's and NARP syndromes.

Barth's syndrome-like disorder: a new phenotype with a maternally inherited A3243G substitution of mitochondrial DNA (MELAS mutation)

An Argentine male child died at 4.5 years of age of a lethal mitochondrial disease associated with a MELAS mutation and a Barth syndrome-like presentation. The child had severe failure to thrive from the early months and for approximately two years thereafter. In addition, the patient had severely delayed gross motor milestones, marked muscle weakness, and dilated cardiomyopathy that progressed to congestive heart failure. He also had persistently elevated urinary levels of 3-methylglutaconic and 2-ethylhydracrylic acids and low blood levels of cholesterol. Detailed histopathologic evaluation of the skeletal muscle biopsy showed high activity of succinate dehydrogenase, a generalized decrease of COX activity, and abundant ragged-red fibers. Electron microscopic studies revealed multiple mitochondrial abnormalities in lymphocytes and monocytes, in the striated muscle, and in the postmortem samples (muscle, heart, liver, and brain). Biochemical analysis showed a pronounced and constant lactic acidosis, and abnormal urinary organic acid excretion (unchanged in the fasting and postprandial states). In addition, in CSF there was a marked increase of lactate and beta-hydroxybutyrate (beta-HOB) and also a high systemic ratio beta-HOB/acetoacetate. Enzymatic assay of the respiratory chain in biopsied muscle showed 10% of complex I activity and 24% of complex IV activity compared with controls. Molecular studies of the mitochondrial genome revealed an A to G mutation at nucleotide pair 3243 in mitochondrial DNA, a well-known pathogenetic mutation (MELAS mutation) in all the patient's tissues and also in the blood specimens of the probands mother and sibs (4 of 5). The diagnosis of MELAS mutation was reinforced by the absence of an identifiable mutation in the X-linked G4.5 gene of the propositus. The present observation gives additional evidence of the variable clinical expression of mtDNA mutations in humans and demonstrates that all clinical variants deserve adequate investigation to establish a primary defect. It also suggests adding Barth-like syndrome to the list of phenotypes with the MELAS mutation.

Mitochondrial DNA mutations in the pathogenesis of human disease

The coding sequence for the human mitochondrial genome (mtDNA) was published in 1981. Within a decade, the first pathogenic mtDNA mutations were described in humans with sporadic and maternally inherited disease. The last ten years has seen a profusion of reports describing new pathogenic mutations associated with a diverse range of clinical phenotypes. Although we have seen great advances in our understanding of the molecular mechanisms involved in the pathogenesis of mtDNA disease, we are only just beginning to tackle some of the more difficult questions. In this review we describe recent advances in our understanding of mtDNA disease and highlight ways that this knowledge might lead to novel therapies in the future.

G8363A mutation in the mitochondrial DNA transfer ribonucleic acidLys gene: another cause of Leigh syndrome

We identified a G-->A transition at nt-8363 in the mitochondrial DNA transfer ribonucleic acidLys gene in blood and muscle from a 13-month-old girl who had clinical and neuroradiologic evidence of Leigh syndrome and died at age 27 months. The mutation was less abundant in the same tissues from the patient's mother, who developed myoclonus epilepsy with ragged red fibers (MERRF) in her late 20s. In both mother and daughter, muscle histochemistry showed ragged red and cytochrome c oxidase-negative fibers and biochemical analysis showed partial defects of multiple respiratory-chain enzymes. A maternal half-sister of the proband had died at 2.5 years of age from neuropathologically proven Leigh syndrome. The G8363A mutation, which previously had been associated with cardiomyopathy and hearing loss, MERRF, and multiple lipomas, also should be included in the differential diagnosis of maternally inherited Leigh syndrome.

Mitochondrial DNA mutations at nucleotide 8993 show a lack of tissue- or age-related variation

Two pathogenic mitochondrial DNA mutations, a T-to-G substitution (8993T > G) and a T-to-C substitution (8993T > C), at nucleotide 8993 have been reported. We describe 13 pedigrees with mitochondrial DNA mutations at nucleotide 8993; 10 pedigrees with the 8993T > G mutation and three with the 8993T > C mutation. Prenatal diagnosis of the nucleotide 8993 mutations is technically possible. However, there are three major concerns: (i) that there is variation in mutant loads among tissues; (ii) that the mutant load in a tissue may change over time; and (iii) that the genotype-phenotype correlation is not clearly understood. We have used the 13 pedigrees to determine specifically the extent of tissue- and age-related variation of the two mutations at nucleotide 8993 in the mitochondrial DNA. The tissue variation was investigated by analysing two or more different tissues from a total of 18 individuals. The age-related variation of the mutation was investigated by comparing the amount of both mutations in blood taken at birth and at a later age. No substantial tissue variation was found, nor was there any substantial change in the proportion of either mutation over periods of 8-23 years in the four individuals studied. In addition, we noted that two features were remarkably common in families with nucleotide 8993 mutations, namely (i) unexplained infant death (8 cases in 13 pedigrees); and (ii) de novo mutations (5 of the 10 8993T > G pedigrees).

Genetic counseling and prenatal diagnosis for the mitochondrial DNA mutations at nucleotide 8993

Mitochondrial genetics is complicated by heteroplasmy, or mutant load, which may be from 1%-99%, and thus may produce a gene dosage-type effect. Limited data are available for genotype/phenotype correlations in disorders caused by mtDNA mutations; therefore, prenatal diagnosis for mtDNA mutations has been hindered by an inability to predict accurately the clinical severity expected from a mutant load measured in fetal tissue. After reviewing 44 published and 12 unpublished pedigrees, we considered the possibility of prenatal diagnosis for two common mtDNA mutations at nucleotide 8993. We related the severity of symptoms to the mutant load and predicted the clinical outcome of a given mutant load. We also used the available data to generate empirical recurrence risks for genetic counseling, which may be used in conjunction with prenatal diagnosis.

Modeling the Leigh syndrome nt8993 T-->C mutation in Escherichia coli F1F0 ATP synthase

The mutations in human mitochondrial DNA at nt8993 are associated with a range of neuromuscular disorders. One mutation encodes a proline in place of a leucine conserved in all animal mitochondrial ATPase-6 subunits and bacterial a subunits of F1F0 ATP synthases. This conserved site is leu-156 and leu-207 in humans and Escherichia coli, respectively. An aleu-207-->pro substitution mutation has been constructed in the E. coli F1F0 ATP synthase in order to model the biochemical basis of the human disease mutation. The phenotype of the aleu-207-->pro substitution has been compared to that of the previously studied aleu-207-->arg substitution (Hartzog and Cain, 1993, Journal of Biological Chemistry 268, 12250-12252). The leu-207-->pro mutation resulted in approximately a 35% decrease in the number of intact enzyme complexes as determined by N, N'-dicyclohexylcarbodiimide-sensitive membrane associated ATP hydrolysis activity and western analysis using an anti-a subunit antibody. A 75% reduction in the efficiency of proton translocation through F1F0 ATP synthase was observed in ATP-driven proton pumping assays. Interestingly, the loss in F1F0 ATP synthase activity resulting from the leu-207-->pro substitution was markedly less dramatic than had been observed for the leu-207-->arg mutation studied earlier. By analogy, the human enzyme may also be affected by the leu-156-->pro substitution to a lesser extent than the leu-156-->arg substitution, and this would account for the milder clinical manifestations of the human leu-156-->pro disease mutations.

Neonatal presentations of mitochondrial metabolic disorders

Because of the high energy requirements of the growing neonate, disorders of mitochondrial metabolism caused by defects in fatty acid oxidation, pyruvate metabolism, and the respiratory chain may often present in the neonatal period. Common neonatal presentations are hypotonia, lethargy, feeding and respiratory difficulties, failure to thrive, psychomotor delay, seizures, and vomiting. Laboratory clues include alterations in the levels of lactate, pyruvate (and the lactate/pyruvate ratio), glucose, and ketone bodies. Diagnosis usually depends on specific enzyme assays or on molecular genetic analysis. Without treatment, most infants die in the first few days or months of life. In the last decade, there have been significant advances in the understanding of the molecular basis of these disorders. This review discusses the major subgroups of mitochondrial disorders, focusing on defects of pyruvate oxidation, the Krebs cycle, and the respiratory chain. Disorders caused by respiratory chain defects may involve nuclear DNA, mitochondrial DNA, or intergenomic signaling. Recognition and early diagnosis of these conditions are important in the genetic counseling of these families.

Clinical and laboratory findings in referrals for mitochondrial DNA analysis

BACKGROUND

Increasingly, mutations of mitochondrial DNA (mtDNA) are being considered when investigating the aetiology of neurological diseases in childhood. However, they are often difficult to predict clinically.

METHOD

Mitochondrial DNA analysis was carried out on 190 children from 1992 to 1996. Most patients were screened for large scale rearrangements and point mutations at nucleotide positions 3243, 3271, 8344, and 8993.

RESULTS

Mutations were found in only 15 patients (7.9%) and were either large scale rearrangements (seven patients) or point mutations at nucleotide position 3243 (eight patients). Other point mutations were screened for depending on the clinical picture. The age of symptom onset was significantly older in children with an mtDNA mutation (mean 7.0 years) compared with children without a mutation (mean 2.8 years). Neither Leigh's syndrome (28 cases) nor severe infantile lactic acidosis (12 cases) was associated with mtDNA mutation. Only three clinical features were significantly associated with an mtDNA mutation: progressive external ophthalmoplegia, myopathy, and pigmentary retinopathy. Family history was valuable: the point mutation at nucleotide 3243 (but not the large scale rearrangements) was associated with maternal inheritance; and consanguinity was not associated with mtDNA mutations. The only investigation that provided specific evidence of an underlying mtDNA mutation was histochemical staining of muscle biopsy specimens. The large scale mutations associated with Kearns-Sayre syndrome and progressive external ophthalmoplegia were found in DNA from muscle only, not leucocyte DNA; whereas point mutations were found in leucocyte DNA.

CONCLUSIONS

Even among children seen at a neurogenetic referral centre, mtDNA mutations were very uncommon. Muscle biopsy was the only investigation to provide evidence of mtDNA abnormality.

Leigh syndrome, cytochrome C oxidase deficiency and hypsarrhythmia with infantile spasms

A rare patient with infantile spasms, hypsarrhythmia, cytochrome c oxidase deficiency and Leigh syndrome is reported. Although rare, infantile spasms and Leigh syndrome may occur simultaneously. Leigh syndrome should be included in the differential diagnosis of infantile spasms.

Mitochondrial disease associated with the T8993G mutation of the mitochondrial ATPase 6 gene: a clinical, biochemical, and molecular study in six families

AIM

To contribute to the establishment of a rational clinical, neuroradiological, and molecular approach to neurogenic muscle weakness, ataxia, and retinitis pigmentosa (NARP) and maternally inherited Leigh's syndrome (MILS).

METHODS AND RESULTS

The T8993G mutation in the mitochondrial genome was found in several maternal members of six pedigrees, whose clinical status ranged from no symptoms to severe infantile subacute necrotising encephalomyelopathy (Leigh's disease). In one case a MELAS-like syndrome was documented both clinically and neuroradiologically. Relevant genetic features of the series were anticipation of symptoms through subsequent generations, and the presence of several cases in whom the mutation apparently occurred recently or was new. A uniform distribution of the mutation in many tissues was shown in one patient subjected to necropsy. In general, a good correlation was found between clinical severity and mutation heteroplasmy in readily accessible tissues, such as lymphocytes or fibroblasts. By contrast, a consistent reduction of the mitochondrial ATPase activity, to about half of the normal values, was found in most of the clinically affected cases, irrespective of the amount of mutant mitochondrial DNA.

CONCLUSIONS

Although the measurement of ATP hydrolysis in cultured fibroblasts was a reliable, and sometimes instrumental, means to identify T8993G positive patients, the relation between the mutation and the oxidative phosphorylation defect is probably very complex, and its understanding requires more complex biochemical analysis.

A near homoplasmic T8993G mtDNA mutation in a patient with atypic Leigh syndrome not present in the mother's tissues

We report an interesting case of a T8993G mutation in the muscle mtDNA of a 2-year-old girl who presented with myoclonia, developmental delay, increased lactate concentration in CSF and cerebral MRI abnormalities without retinopathy, suggesting an atypical form of Leigh syndrome. Search for mutant molecules in the blood and skin fibroblasts of the patient's healthy mother as well as in the blood of the also unaffected patient's sister was unsuccessful. This "sporadic' case of 8993 mtDNA mutation may be due either to a de novo mutation arising during oogenesis (or early embryogenesis) or to a mutation pre-existing in oocytes in a few mtDNA molecules and selected through a narrow bottleneck mechanism. Whatever the mechanism involved, this observation illustrates a complete shift of the mtDNA type in only one generation: from 0 to nearly 100% of mtDNA molecules with the T8993G mutation.

Impairment of central and peripheral myelin in mitochondrial diseases

Clinical or sub-clinical impairment of central and peripheral myelin is often part of the overlapping multisystem disorders associated with a variety of mitochondrial (mt)DNA abnormalities. Suboptimal energy metabolism of the oligodendrocytes and Schwann cells carrying mitochondrial defects may cause insufficient production of myelin. Further, edema, vascular and toxic factors may directly damage myelin. The recognition that certain mtDNA point mutations are associated with inflammatory demyelination of the central nervous system suggests that additional mechanisms besides degeneration need to be considered in the development of some forms of myelin damage.

Biochemical analysis of fibroblasts from patients with cytochrome c oxidase-associated Leigh syndrome

Cultured skin fibroblasts from four patients with Leigh syndrome and cytochrome c oxidase deficiency were studied. Mitochondrial DNA (mtDNA) analysis excluded large-scale deletions and known point mutations associated with Leigh syndrome. The COX activities were reduced to 18-44% of healthy probands, when measured in the presence of laurylmaltoside. COX activity from patients was shown to be more temperature sensitive than COX activity from control cells. In order to determine the subunit composition of COX immunoblotting studies were performed using mono- and polyclonal antibodies to distinct subunits. A monoclonal antibody to subunit IV crossreacted with two unknown proteins of higher apparent molecular weight in mitochondria from three patients, but not in mitochondria from control and the fourth patient. Quantification of immunoreactivity revealed a decrease of subunits II/III and IV parallel to the determined enzyme activity. In contrast, a variable amount of subunit VIIa (and/or VIIb) was found in mitochondria from different patients. The results indicate a defective COX holoenzyme complex in patients with Leigh syndrome and suggest different molecular origins of the defect.

Leigh syndrome associated with mitochondrial DNA 8993 T-->G mutation and ragged-red fibers

We report a 6-month-old girl with episodic hyperventilation, myoclonus, disturbed consciousness, and lactic acidosis. Brain sonogram revealed progressive ventriculomegaly, and MRI showed symmetric low densities over the putamen bilaterally with diffuse cortical cystic lesions. Ragged-red fibers were noted in the muscle biopsy. Molecular analysis revealed a heteroplasmic T-->G mutation at nucleotide position (np) 8993 of mitochondrial DNA (mtDNA). The proportion of the mutant mtDNA in the muscle of the proband was estimated to be 86%. Her mother and maternal uncle also harbored the same mutant mtDNA (54 and 48% in their leukocytes, respectively). One of her sisters carried 64% mutant mtDNA in her leukocytes, but another sister did not. These results suggest that this mutant mtDNA is transmitted through the maternal lineage in this family in a randomly segregated manner. To our knowledge, this is the first report of a Chinese patient with Leigh syndrome associated with the T-->G substitution at np 8993 of mtDNA.

Leigh syndrome and partial deficit of cytochrome c oxidase associated with epilepsia partialis continua

Epilepsia partialis continua (EPC) has previously been reported in only two patients with Leigh syndrome (LS). We report here a subject in whom LS and partial deficit of cytochrome c oxidase (COX) were associated with EPC. Epilepsy in this subject appeared when neurologic impairment was clearly evident and MRI showed cortical lesions typically associated with congenital lactic acidosis.