NDUFA4 (Renamed COXFA4) Is a Cytochrome-c Oxidase Subunit

Groundbreaking work by Kadenbach and colleagues in the 1980s revealed the presence of 13 subunits in the mammalian mitochondrial cytochrome-c oxidase (COX; Complex IV). This observation stood the test of time until 2012 when it was demonstrated that NDUFA4, a polypeptide previously attributed to mitochondrial Complex I, was a 14th subunit of COX. In his recent opinion article, Kadenbach argued that NDUFA4 is not a subunit of COX. However, based on the findings that NDUFA4 deficiency results in a severe loss of COX activity and that NDUFA4 represents a stoichiometric component of the individual COX complex, we reason that NDUFA4 is a bona fide COX subunit and propose renaming it as COX subunit FA4 (COXFA4)

Designing clinical trials for rare diseases: unique challenges and opportunities

Orphan drug development is a rapidly expanding field. Nevertheless, clinical trials for rare diseases can present inherent challenges. Optimal study design and partnerships between academia and industry are therefore required for the successful development, delivery and clinical approval of effective therapies in this group of disorders

Clinical and Molecular Genetic Studies in Mitochondrial Disease

Up to a third of adults attending the Queen Square UK NHS Specialised Service for Rare Mitochondrial Disease out-patient clinic remain genetically undetermined. This thesis describes research which aimed to establish the molecular basis of mitochondrial disease in these patients and evaluate genotype/phenotype correlations. Both novel clinical syndromes and molecular causes of disease were identified. Furthermore, new insights into the respiratory chain (RC) protein structure were elucidated. Novel clinical phenotypes: Three previously unrecognised mitochondrial disease phenotypes were characterised. First, m.9185T>C in MT-ATP6, encoding subunit 6 of ATP synthase (complex V), was detected in a pedigree exhibiting matrilineal inheritance. Presentation was with axonal Charcot-Marie-Tooth (CMT2) disease. Further screening of 270 patients with genetically unclassified CMT2 demonstrated three additional families harbouring the same mutation, thus proving a causal link between reduced complex V activity and impaired axonal function. Second, a severe distal myopathy was observed in two unrelated patients with de novo dominant POLG mutations. Finally, COX10 mutations were linked to adult cytochrome c oxidase (COX) deficiency. Despite a complex multisystem phenotype comprising short stature, proximal myopathy, fatigue, sensorineural hearing loss, pigmentary maculopathy, renal Fanconi syndrome and premature ovarian failure, the patient’s clinical severity was considerably milder than fatal COX10-related infantile disease. Nuclear gene mutations: Three major experimental strategies were employed to locate mutations in the nuclear genes of adults with clinically and/or biochemically suspected mitochondrial disease. First, a candidate gene approach identified RRM2B mutations in 2/33 patients with multiple mitochondrial DNA deletions. Second, whole-exome sequencing confirmed COX10 mutations can cause adult mitochondrial disease. Finally, combined homozygosity mapping/whole-exome sequencing in a consanguineous family led to the discovery that NDUFA4 mutations cause COX-deficient Leigh syndrome. This example of ‘back-translation’ led to the discovery that NDUFA4, previously considered to be a complex I subunit, is actually an important component of the COX enzyme complex

Moving Towards Clinical Trials for Mitochondrial Diseases

Primary mitochondrial diseases represent some of the most common and severe inherited metabolic disorders, affecting approximately 1 in 4300 live births. The clinical and molecular diversity typified by mitochondrial diseases has contributed to the lack of licensed disease‐modifying therapies available. Management for the majority of patients is primarily supportive. The failure of clinical trials in mitochondrial disease partly relates to the inefficacy of compounds studied. However, it is also likely to be a consequence of the significant challenges faced by clinicians and researchers when designing trials for mitochondrial diseases, which have historically been hampered by a lack of natural history data, biomarkers and outcome measures to detect a treatment effect. Encouragingly, over the past decade there have been significant advances in therapy development for mitochondrial diseases, with many small molecules now transitioning from preclinical to early phase human interventional studies. In this review, we present the treatments and management strategies currently available to people with mitochondrial disease. We evaluate the challenges and potential solutions to trial design and highlight the emerging pharmacological and genetic strategies that are moving from the laboratory to clinical trials for this group of disorders

Applying genomic and transcriptomic advances to mitochondrial medicine

Next-generation sequencing (NGS) has increased our understanding of the molecular basis of many primary mitochondrial diseases (PMDs). Despite this progress, many patients with suspected PMD remain without a genetic diagnosis, which restricts their access to in-depth genetic counselling, reproductive options and clinical trials, in addition to hampering efforts to understand the underlying disease mechanisms. Although they represent a considerable improvement over their predecessors, current methods for sequencing the mitochondrial and nuclear genomes have important limitations, and molecular diagnostic techniques are often manual and time consuming. However, recent advances in genomics and transcriptomics offer realistic solutions to these challenges. In this Review, we discuss the current genetic testing approach for PMDs and the opportunities that exist for increased use of whole-genome NGS of nuclear and mitochondrial DNA (mtDNA) in the clinical environment. We consider the possible role for long-read approaches in sequencing of mtDNA and in the identification of novel nuclear genomic causes of PMDs. We examine the expanding applications of RNA sequencing, including the detection of cryptic variants that affect splicing and gene expression and the interpretation of rare and novel mitochondrial transfer RNA variants

Mitochondrial DNA variants in genomic data: diagnostic uplifts and predictive implications

A broad spectrum of rare disease presentations can now be investigated by analysing mitochondrial DNA (mtDNA) variants from whole-genome sequencing (WGS) data. However, mtDNA mutations may cause unanticipated, extended phenotypes and have reproductive implications. We recommend that these be considered by patients and clinicians before embarking on WGS

Cardiolipin, Mitochondria, and Neurological Disease

Over the past decade, it has become clear that lipid homeostasis is central to cellular metabolism. Lipids are particularly abundant in the central nervous system (CNS) where they modulate membrane fluidity, electric signal transduction, and synaptic stabilization. Abnormal lipid profiles reported in Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and traumatic brain injury (TBI), are further support for the importance of lipid metablism in the nervous system. Cardiolipin (CL), a mitochondria-exclusive phospholipid, has recently emerged as a focus of neurodegenerative disease research. Aberrant CL content, structure, and localization are linked to impaired neurogenesis and neuronal dysfunction, contributing to aging and the pathogenesis of several neurodegenerative diseases, such as AD and PD. Furthermore, the highly tissue-specific acyl chain composition of CL confers it significant potential as a biomarker to diagnose and monitor the progression in several neurological diseases. CL also represents a potential target for pharmacological strategies aimed at treating neurodegeneration. Given the equipoise that currently exists between CL metabolism, mitochondrial function, and neurological disease, we review the role of CL in nervous system physiology and monogenic and neurodegenerative disease pathophysiology, in addition to its potential application as a biomarker and pharmacological target

Discussion of off-target and tentative genomic findings may sometimes be necessary to allow evaluation of their clinical significance

We discuss a case where clinical genomic investigation of muscle weakness unexpectedly found a genetic variant that might (or might not) predispose to kidney cancer. We argue that despite its off-target and uncertain nature, this variant should be discussed with the man who had the test, not because it is medical information, but because this discussion would allow the further clinical evaluation that might lead it to becoming so. We argue that while prominent ethical debates around genomics often take 'results' as a starting point and ask questions as to whether to look for and how to react to them, the construction of genomic results is fraught with ethical complexity, although often couched as a primarily technical problem. We highlight the need for greater focus on, and appreciation of, the ethical work undertaken daily by scientists and clinicians working in genomic medicine and discuss how public conversations around genomics need to adapt to prepare future patients for potentially uncertain and unexpected outcomes from clinical genomic tests

Mitochondrial Strokes: Diagnostic Challenges and Chameleons

Mitochondrial stroke-like episodes (SLEs) are a hallmark of mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS). They should be suspected in anyone with an acute/subacute onset of focal neurological symptoms at any age and are usually driven by seizures. Suggestive features of an underlying mitochondrial pathology include evolving MRI lesions, often originating within the posterior brain regions, the presence of multisystemic involvement, including diabetes, deafness, or cardiomyopathy, and a positive family history. The diagnosis of MELAS has important implications for those affected and their relatives, given it enables early initiation of appropriate treatment and genetic counselling. However, the diagnosis is frequently challenging, particularly during the acute phase of an event. We describe four cases of mitochondrial strokes to highlight the considerable overlap that exists with other neurological disorders, including viral and autoimmune encephalitis, ischemic stroke, and central nervous system (CNS) vasculitis, and discuss the clinical, laboratory, and imaging features that can help distinguish MELAS from these differential diagnoses

Single deletions in mitochondrial DNA--molecular mechanisms and disease phenotypes in clinical practice.

Over 20 years ago single clonal deletions were the first mitochondrial DNA (mtDNA) genetic defects described in association with human disease. Since then very large numbers of children and adults harbouring such deletions have been described and it is clear they are an important cause of human mitochondrial disease. However, there still remain many important challenges in relation to our understanding of mechanisms leading to deletion formation and propagation and in relation to the factors determining the complex and varying relationship between genotype and clinical phenotype. Although multidisciplinary team care is essential and can improve quality of life and outcomes for patients, a definitive molecular treatment for single mtDNA deletions remains an important translational research goal. Patients with mtDNA deletions exhibit a very wide range of different clinical phenotypes with marked variation in age at onset and disease severity. Single mtDNA deletions may enter into the differential diagnosis of many different paediatric and adult presentations across a wide range of medical specialties, although neurological presentations are amongst the most common. In this review, we examine the molecular mechanisms underpinning mtDNA replication and we consider the hypotheses proposed to explain the formation and propagation of single large-scale mtDNA deletions. We also describe the range of clinical features associated with single mtDNA deletions, outline a molecular diagnostic approach and discuss current management including the role of aerobic and resistance exercise training programmes