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Defects in the mitochondrial DNA replication enzyme, polymerase γ, are an important cause of mitochondrial disease with ∼25% of all adult diagnoses attributed to mutations in the POLG gene. Peripheral neuronopathy is often part of the clinical syndrome and can represent the most disabling feature. In spite of this, the molecular mechanisms underlying the neuronopathy remain to be elucidated and treatment strategies are limited. In the present study, we use a combined approach comprising clinical, electrophysiological, neuropathological and molecular genetic investigations to unravel the mechanisms underpinning peripheral neuronopathy in autosomal recessive polymerase γ-related disease. Electrophysiological assessments documented a dorsal root ganglionopathy in all 11 cases. Of the 11 cases, eight also showed changes consistent with motor fibre loss. Detailed neuropathological investigation of two patients confirmed the electrophysiological findings, revealing atrophy of posterior columns and striking neuronal cell loss from the dorsal root ganglia, which was accompanied by severe mitochondrial biochemical abnormalities involving respiratory chain complexes I and IV due to clonally-expanded mitochondrial DNA deletions and a significant reduction in mitochondrial DNA copy number in affected neurons. We propose that the respiratory chain defects, secondary to mitochondrial DNA deletion and depletion, are likely to be responsible for pathology observed in the dorsal root ganglion and the sensory ganglionopathy documented electrophysiologically.

Distinguishing pathogenic from polymorphic changes poses significant problems for geneticists and despite 30 years of postgenomic experience this remains the case in mitochondrial genetics. Base substitutions in mitochondrial tRNA (mt-tRNA) genes are particularly difficult, but important, because they are common causes of pathology and associated with high rates of transmission. Providing accurate genetic advice to patients and their families is of paramount importance in disease prevention, and brings into sharp focus the factors used to distinguish pathogenic from polymorphic variants. We have reevaluated our pathogenicity scoring system for mt-tRNA mutations following a considerable increase in the number reported since the system was devised in 2004. This allowed us to address notable issues including the underestimation of "definitely pathogenic" mutations resulting from insufficient data collection. We illustrate the robustness of our revised scoring system using novel pathogenic and previously reported polymorphic changes and conclude that while clear evidence from single-fiber and/or trans-mitochondrial cybrid studies remains the gold standard for assigning pathogenicity, our scoring system is valuable for deciding which mt-tRNA mutations to investigate further using these labor-intensive techniques., (© 2011 Wiley Periodicals, Inc.)

Background: Mitochondrial diseases are characterized by wide phenotypic and genetic variability, but presentations in adults with akinetic rigidity and hyperkinetic movement disorders are rare., Objectives: To describe clinically a subject with progressive neurodegeneration characterized by psychosis, dementia, and akinesia-rigidity, and to associate this phenotype with a novel mitochondrial transfer RNA(Phe) (tRNA(Phe)) (MTTF) mutation., Design, Setting, and Patient: Case description and detailed laboratory investigations of a 57-year-old woman at a university teaching hospital and a specialist mitochondrial diagnostic laboratory., Results: Histopathological findings indicated that an underlying mitochondrial abnormality was responsible for the subject's progressive neurological disorder, with mitochondrial genome sequencing revealing a novel m.586G>A MTTF mutation., Conclusions: The clinical phenotypes associated with mitochondrial disorders may include akinesia-rigidity and psychosis. Our findings further broaden the spectrum of neurological disease associated with mitochondrial tRNA(Phe) mutations.

Isolated complex I deficiency is the most frequently observed oxidative phosphorylation defect in children with mitochondrial disease, leading to a diverse range of clinical presentations, including Leigh syndrome. For most patients the genetic cause of the biochemical defect remains unknown due to incomplete understanding of the complex I assembly process. Nonetheless, a plethora of pathogenic mutations have been described to date in the seven mitochondrial-encoded subunits of complex I as well as in 12 of the nuclear-encoded subunits and in six assembly factors. Whilst several mitochondrial DNA mutations are recurrent, the majority of these mutations are reported in single families. We have sequenced core structural and functional nuclear-encoded subunits of complex I in a cohort of 34 paediatric patients with isolated complex I deficiency, identifying pathogenic mutations in 6 patients. These included a novel homozygous NDUFS1 mutation in an Asian child with Leigh syndrome, a previously identified NDUFS8 mutation (c.236C>T, p.P79L) in a second Asian child with Leigh-like syndrome and six novel, compound heterozygous NDUFS2 mutations in four white Caucasian patients with Leigh or Leigh-like syndrome. Three of these children harboured an identical NDUFS2 mutation (c.875T>C, p.M292T), which was also identified in conjunction with a novel NDUFS2 splice site mutation (c.866+4A>G) in a fourth Caucasian child who presented to a different diagnostic centre, with microsatellite and single nucleotide polymorphism analyses indicating that this was due to an ancient common founder event. Our results confirm that NDUFS2 is a mutational hotspot in Caucasian children with isolated complex I deficiency and recommend the routine diagnostic investigation of this gene in patients with Leigh or Leigh-like phenotypes.

Mitochondrial (mt-) tRNA (MTT) gene mutations are an important cause of human morbidity and are associated with a wide range of pathology, from isolated organ-specific diseases such as myopathy or hearing loss, through to multisystem disorders with encephalopathy, gastrointestinal dysmotility, and life-threatening cardiomyopathy. Our understanding of how MTT mutations cause disease remains poor and progress has been hampered by the complex interaction of genotype with phenotype that can result in patients who harbor the same mutation exhibiting starkly contrasting phenotypes, whereas other (genetically heterogeneous) patients manifest clinically identical syndromes. A further complexity is the highly polymorphic nature of mitochondrial DNA (mtDNA), which must temper any reflex assumptions of pathogenicity for novel MTT substitutions. Nevertheless significant progress is being made and we shall review the methods employed to identify and characterize MTT mutations as pathogenic. Also important is our understanding of the molecular processes involved and we shall discuss the data available on two of the most studied MTT mutations (m.8344A > G and m.3243A > G) as well as other potential pathogenic mechanisms. Knowledge of factors influencing the inheritance of MTT mutations, and therefore the likelihood of disease transmission, is of particular importance to female patients. At present, the factors determining transmission remain elusive, but we shall examine several possible mechanisms and discuss the evidence for each. Finally, a number of different yeast and mouse models are currently used to investigate mitochondrial disease and we will assess the importance of and difficulties associated with each model as well as the future of possible therapies for patients with mitochondrial disease., (Copyright © 2010 John Wiley & Sons, Inc.)

Objective: To describe an unusual clinical phenotype in an adult harboring 2 compound heterozygous polymerase γ (POLG) mutations., Design: Case report., Setting: University-based outpatient neurology clinic and pathology and genetics laboratory., Patient: A 27-year-old man presenting with isolated distal myopathy of the upper extremities in the absence of sensory disturbances., Results: Histochemical analysis of a muscle biopsy specimen showed numerous cytochrome c oxidase-deficient fibers. Molecular analysis revealed marked depletion of muscle mitochondrial DNA in the absence of multiple mitochondrial DNA deletions. Sequence analysis of the POLG gene revealed heterozygous sequence variants in compound c.1156C>T (p.R386C) and c.2794C>T (p.H932Y) segregating with clinical disease in the family. The p.R386C change appears to be a novel mutation., Conclusion: Our case broadens the phenotypic spectrum of disorders associated with POLG mutations and highlights the complex relationship between genotype and phenotype in POLG-related disease.

Mitochondrial transfer RNA (mt-tRNA) mutations are the commonest mitochondrial (mtDNA) mutations to cause human disease. The majority of mt-tRNA mutations are heteroplasmic and while some exhibit maternal transmission within families, many others are only seen as sporadic mutations. Using the available clinical, biochemical and genetic data from published pathogenic mt-tRNA mutations, we have explored several different factors thought to influence the transmission of mt-tRNA mutations. Our data show that the most important factor in predicting whether a mutation is transmitted to offspring is whether the mt-tRNA mutation is selected against in a rapidly replicating tissue such as blood. This suggests that those mt-tRNA mutations which exert a major phenotype in dividing cells are unlikely to be inherited. This is entirely compatible with recent observations on the mitochondrial genetic bottleneck in early development and has important implications for families with mt-tRNA disease.

Background: Pathogenic mutations of the human mitochondrial genome are associated with well-characterized, progressive neurological syndromes, with mutations in the transfer RNA genes being particularly prominent., Objective: To describe a novel mitochondrial transfer RNA(Pro) gene mutation in a woman with a myoclonic epilepsy with ragged-red fibers-like disease. Design, Setting, and Patient Case report of a 49-year-old woman presenting with a myoclonic epilepsy with ragged-red fibers-like disease comprising myoclonic jerks, cerebellar ataxia, and proximal muscle weakness., Results: Histochemical analysis of a muscle biopsy revealed numerous cytochrome-c oxidase-deficient, ragged-red fibers, while biochemical studies indicated decreased activity of respiratory chain complex I. Molecular investigation of mitochondrial DNA revealed a new heteroplasmic mutation in the TpsiC stem of the mitochondrial transfer RNA(Pro) gene that segregated with cytochrome-c oxidase deficiency in single muscle fibers., Conclusions: Our case serves to illustrate the ever-evolving phenotypic spectrum of mitochondrial DNA disease and the importance of performing comprehensive mitochondrial genetic studies in the absence of common mitochondrial DNA mutations.

Dramatic tissue variation in mitochondrial heteroplasmy has been found to exist in patients with sporadic mitochondrial DNA (mtDNA) mutations. Despite high abundance in mature skeletal muscle, levels of the causative mutation are low or undetectable in satellite cells. The activation of these typically quiescent mitotic cells and subsequent shifting of wild-type mtDNA templates to mature muscle have been proposed as a means of restoring a more normal mitochondrial genotype and function in these patients. Because resistance exercise is known to serve as a stimulus for satellite cell induction within active skeletal muscle, this study sought to assess the therapeutic potential of resistance training in eight patients with single, large-scale mtDNA deletions by assessing: physiological determinants of peak muscle strength and oxidative capacity and muscle biopsy-derived measures of damage, mtDNA mutation load, level of oxidative impairment and satellite cell numbers. Our results show that 12 weeks of progressive overload leg resistance training led to: (i) increased muscle strength; (ii) myofibre damage and regeneration; (iii) increased proportion of neural cell adhesion molecule (NCAM)-positive satellite cells; (iv) improved muscle oxidative capacity. Taken together, we believe these findings support the hypothesis of resistance exercise-induced mitochondrial gene-shifting in muscle containing satellite cells which have low or absent levels of deleted mtDNA. Further investigation is warranted to refine parameters of the exercise training protocol in order to maximize the training effect on mitochondrial genotype and treatment potential for patients with selected, sporadic mutations of mtDNA in skeletal muscle.

Mutations in nuclear genes involved in mitochondrial DNA (mtDNA) maintenance cause a wide range of clinical phenotypes associated with the secondary accumulation of multiple mtDNA deletions in affected tissues. The majority of families with autosomal dominant progressive external ophthalmoplegia (PEO) harbour mutations in genes encoding one of three well-characterized proteins--pol gamma, Twinkle or Ant 1. Here we show that a heterozygous mis-sense mutation in OPA1 leads to multiple mtDNA deletions in skeletal muscle and a mosaic defect of cytochrome c oxidase (COX). The disorder presented with visual failure and optic atrophy in childhood, followed by PEO, ataxia, deafness and a sensory-motor neuropathy in adult life. COX-deficient skeletal muscle fibres contained supra-threshold levels of multiple mtDNA deletions, and genetic linkage, sequencing and expression analysis excluded POLG1, PEO1 and SLC25A4, the gene encoding Ant 1, as the cause. This demonstrates the importance of OPA1 in mtDNA maintenance, and implicates OPA1 in diseases associated with secondary defects of mtDNA.

The 3243A > G mutation is one of the most frequently observed mutations of mitochondrial DNA (mtDNA), and is associated with numerous clinical presentations including mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS), progressive external ophthalmoplegia (PEO) and diabetes and deafness. The routine diagnosis of the 3243A > G mutation in blood is difficult as mutation levels are known to decrease in this tissue over time, while in some patients it may be absent. We have directly compared the levels of the 3243A > G mutation in skeletal muscle, blood and urinary epithelial cells in 18 patients and observed a striking correlation between the mutation load in postmitotic muscle and urinary epithelium, a mitotic tissue. These data strongly support the use of urinary epithelial cells as the tissue of choice in the noninvasive diagnosis of the 3243A > G mutation.