Novel POLG1 mutations associated with neuromuscular and liver phenotypes in adults and children.

BACKGROUND: The POLG1 gene encodes the catalytic subunit of DNA polymerase gamma, essential for mitochondrial DNA replication and repair. Mutations in POLG1 have been linked to a spectrum of clinical phenotypes, and may account for up to 25% of all adult presentations of mitochondrial disease. METHODS AND RESULTS: We present 14 patients, with characteristic features of mitochondrial disease including progressive external ophthalmoplegia (PEO) and Alpers-Huttenlocher syndrome and laboratory findings indicative of mitochondrial dysfunction, including cytochrome c oxidase (COX) deficiency and multiple deletions or depletion of the mitochondrial DNA. Four novel POLG1 missense substitutions (p.R597W, p.L605R, p.G746S, p.A862T), are described, together with the first adult patient with a recently described polymerase domain mutation (p.R1047W). All novel changes were rare in a control population and affected highly conserved amino acids. CONCLUSION: The addition of these substitutions-including the first report of a dinucleotide mutation (c.1814_1815TT>GC)-to the growing list of defects further confirms the importance of POLG1 mutations as the underlying abnormality in a range of neurological presentations.

Cytochrome c oxidase deficient muscle fibres: substantial variation in their proportions within skeletal muscles from patients with mitochondrial myopathy.

Mitochondrial DNA (mtDNA) disease is a common cause of myopathy and the presence of histochemically demonstrated cytochrome c oxidase (COX) deficiency is an extremely useful diagnostic feature. However, there is currently no quantitative information regarding the variability of COX deficiency within or between muscles. This study addresses this issue by studying a number of skeletal muscle samples obtained at post-mortem from three patients with mitochondrial disease due to established mitochondrial DNA defects. COX deficient muscle fibres were enumerated in sections of these muscles and analysed according to patient, individual muscle, position within a particular muscle and sample size. Descriptive statistics were generated followed by an analysis of variance (ANOVA) to assess the effect of these parameters on the mean percentage of COX deficient fibres. We observed statistically significant variation in the percentage of COX deficient fibres within individual muscles from each patient for samples sizes of between 100 and 400 fibres. Our results have implications for the way in which biopsies of skeletal muscle are used for the assessment of disease severity, progression and response to treatment.

Somatic mitochondrial DNA mutations in adult-onset leukaemia.

Mitochondrial genome instability has recently been demonstrated in a wide variety of human tumours and is implicated in the development of the myelodysplastic syndromes, a heterogeneous group of haematological disorders with an increased risk of malignant transformation. We therefore investigated the incidence of somatic mitochondrial DNA (mtDNA) mutations in patients with adult-onset leukaemia. We sequenced the entire mitochondrial genome from both normal tissue (buccal epithelial cells) and the leukaemia from 24 patients with adult-onset leukaemia. Somatic mtDNA mutation was present in nine individuals ( approximately 40%) and in each case the tumour genome differed from the normal genome sequence by a single sequence change. Using PCR-RFLP analysis and real-time PCR, we have studied in detail the mutation present in one patient with acute lymphatic leukaemia, demonstrating that the mutation is associated specifically with the leukaemia.

Neuropathological and histochemical changes in a multiple mitochondrial DNA deletion disorder.

The identification of cytochrome c oxidase (COX)-deficient/succinate dehydrogenase (SDH)- positive cells using sequential histochemistry has proved important in the identification of cells with high mitochondrial DNA (mtDNA) mutant load. We demonstrate large numbers of COX-deficient/SDH-positive neurons in a mosaic pattern throughout the CNS of a patient with a multiple mtDNA deletion disorder. This patient had prominent central and peripheral nervous system involvement with marked cerebellar ataxia, a parkinsonian extra-pyramidal movement disorder, external ophthalmoplegia, dysphagia, and a severe peripheral neuropathy. There was degeneration of myelin tracts in the cerebellum and dorsal spinal columns, diffuse astrocytosis, and selective neuronal degeneration particularly in the midbrain and cerebral microvacuolation. The proportional distribution of the COX-deficient neurons did not always correlate directly with the degree of neuropathological damage with regions of high neuronal loss having relatively low proportions of these cells. Other clinically affected CNS regions have high levels of COX-deficient neurons without significant cell loss. The role of these COX-deficient neurons in causing neuronal degeneration and clinical symptoms is discussed.

Cytochrome c oxidase deficient cells accumulate in the hippocampus and choroid plexus with age.

The chronological accumulation of mitochondrial dysfunction has been proposed as a potential mechanism in the physiological processes of aging. Cytochrome c oxidase deficient, succinate dehydrogenase positive muscle fibers containing high copy numbers of a mitochondrial DNA mutation are a pathological hallmark of mitochondrial DNA disorders. We show that there is an age-related increase in cytochrome c oxidase-deficient cells in both hippocampal pyramidal neurons and choroid plexus epithelial cells. We suggest that these cells contribute to the cell death and dysfunction in CNS aging.

Mitochondrial enzyme-deficient hippocampal neurons and choroidal cells in AD.

OBJECTIVE: To determine whether hippocampal neurons and choroidal epithelial cells demonstrate a mitochondrial enzyme deficiency in AD more frequently than in normal aging. BACKGROUND: High levels of mutant mitochondrial DNA (mtDNA) cause a deficiency in cytochrome c oxidase (COX) (complex IV activity) because three of its 13 subunits are encoded for by mtDNA. In contrast, succinate dehydrogenase (SDH) (complex II activity) remains intact because all of its subunits are nuclear encoded. The histologic hallmark of cells containing high levels of mtDNA mutation in both primary mtDNA disorders and normal aging muscle is the presence of COX-deficient SDH-positive cells. METHODS: The authors applied a sequential histochemical method for COX and SDH to hippocampal sections in 17 AD and 17 age-matched control brains. This confers the advantages of both looking at individual cells in situ and measuring the actual mitochondrial complex activity rather than simply the complex quantity. RESULTS: COX-deficient SDH-positive hippocampal neurons and choroidal epithelial cells are more prevalent in patients with AD than in controls. In addition the COX-deficient SDH-positive choroidal cells are associated with an enlargement in size. CONCLUSION: This increase in number of COX-deficient SDH-positive hippocampal pyramidal neurons and choroid epithelial cells provides strong evidence that a substantial mitochondrial enzyme activity defect occurs in individual cells more frequently in AD than in normal aging and that mitochondria may play a significant role in the pathogenesis of AD.

Mitochondrial DNA mutations in disease and ageing.

The chronological accumulation of mitochondrial DNA mutations has been proposed as a potential mechanism in the physiological processes of ageing and age-related disease. We discuss the evidence behind this theory and relate some of the ageing mitochondrial changes to mitochondrial DNA disorders. In particular, we describe the aggregation of cytochrome c oxidase-deficient cells in both skeletal muscle and the CNS in normal ageing as seen in the mitochondrial DNA disorders. These mitochondrial enzyme-deficient cells have been shown to occur in significant quantities in both muscle and CNS in patients with mitochondrial DNA disorders. In both ageing and mtDNA disorder muscle these cytochrome c-deficient fibres contain high levels of a single mutant strain of mitochondrial DNA. Whether these mutations are a primary or secondary event in the physiology of ageing remains to be determined.

Role of mitochondrial DNA mutations in disease and aging.

Since Harman in 1972 first proposed a role in the process of aging for the mitochondrial genome, a wealth of evidence has been accumulated to support this theory. We discuss the hereditary mitochondrial DNA disorders, which we believe may give insight into both normal aging and neurodegenerative conditions. We then review the evidence for the role of mitochondrial DNA mutations in both aging and age-related disorders and also discuss new approaches for investigating the mitochondrial genome at a single cell level, by observing the activity of the mitochondrial enzyme cytochrome c oxidase.

Analysis of mitochondrial DNA mutations : deletions.

Although the precise mechanisms of the aging process remain poorly understood, a plausible theory for cellular dysfunction and deterioration during aging involves mitochondria (1, 2). The major function of mitochondria is to generate energy for cellular processes in the form of ATP by oxidative phosphorylation. Mitochondria contain their own DNA (mtDNA), a small 16.5 kb circular molecule that encodes 13 essential polypeptides of the mitochondrial respiratory chain, as well as 2 rRNAs and 22 tRNAs required for intramitochondrial protein synthesis (3). The mitochondrial respiratory chain is a series of five, multisubunit protein complexes located within the inner mitochondrial membrane. The first four of these (complexes I-IV) reoxidize reduced cofactors (NADH and FADH(2)) generated by the oxidation of foodstuffs, thereby generating an electrochemical gradient across the inner mitochondrial membrane which is harnessed by the fifth complex, the ATP synthetase, to drive the formation of ATP.

Hypertrophic olivary degeneration on magnetic resonance imaging in mitochondrial syndromes associated with POLG and SURF1 mutations.

Hypertrophic olivary degeneration (HOD) is associated with lesions within the dento-rubro-olivary pathway or Guillain-Mollaret triangle and may be associated clinically with palatal tremor. Here we report HOD on brain magnetic resonance (MR) imaging in three patients with progressive mitochondrial syndromes in the absence of palatal tremor. Two of the patients were found to have identical compound heterozygous mutations in the POLG gene, encoding the catalytic subunit of the mitochondrial DNA polymerase-γ, but presented with different clinical phenotypes. The first patient displayed the clinical syndrome of sensory ataxia, neuropathy, dysarthria, and ophthalmoparesis (SANDO), while the second patient was affected by a neurological disorder consisting of an ophthalmoplegia, myopathy, and neuropathy. The third case was a child with Leigh syndrome due to SURF1 gene mutations, who presented with a generalized tremor. We discuss the brain MR imaging findings in these three cases along with a literature review on the MR features of previously reported cases of patients with POLG gene mutations and Leigh disease due to SURF1 gene mutations. Our findings suggest that the presence of HOD, in the appropriate clinical setting, should alert the clinician to the possibility of a mitochondrial disorder and the need to screen for mutations in POLG and SURF1 genes.

The clinical, histochemical, and molecular spectrum of PEO1 (Twinkle)-linked adPEO.

BACKGROUND: Mutations in the Twinkle (PEO1) gene are a recognized cause of autosomal dominant progressive external ophthalmoplegia (adPEO), resulting in the accumulation of multiple mitochondrial DNA (mtDNA) deletions and cytochrome c oxidase (COX)-deficient fibers in skeletal muscle secondary to a disorder of mtDNA maintenance. Patients typically present with isolated extraocular muscle involvement, with little apparent evidence of the clinical heterogeneity documented in other mtDNA maintenance disorders, in particular POLG-related disease. METHODS: We reviewed the clinical, histochemical, and molecular genetics analysis of 33 unreported patients from 26 families together with all previous cases described in the literature to define the clinical phenotype associated with PEO1 mutations. RESULTS: Ptosis and ophthalmoparesis were almost universal clinical features among this cohort, with 52% (17/33) reporting fatigue and 33% (11/33) having mild proximal myopathy. Features consistent with CNS involvement were rarely described; however, in 24% (8/33) of the patients, cardiac abnormalities were reported. Mitochondrial histochemical changes observed in muscle showed remarkable variability, as did the secondary mtDNA deletions, which in some patients were only detected by PCR-based assays and not Southern blotting. Moreover, we report 7 novel PEO1 variants. CONCLUSIONS: Our data suggest a shared clinical phenotype with variable mild multiorgan involvement, and that the contribution of PEO1 mutations as a cause of adPEO may well be underestimated. Direct sequencing of the PEO1 gene should be considered in adPEO patients prior to muscle biopsy.

Homoplasmy, heteroplasmy, and mitochondrial dystonia.

BACKGROUND: In clinical practice, mitochondrial disease is seldom considered until a variable combination of seizures, alteration in tone, muscle weakness, and developmental problems is evident. However, it is not uncommon for one symptom to occur in isolation and dominate the clinical phenotype. We report six patients from two families where dystonia was the principal clinical manifestation. A mitochondrial etiology was considered in each case because of the association of dystonia with other less prominent clinical features such as epilepsy. METHODS: Histochemical and biochemical analyses were undertaken in skeletal muscle biopsies from individuals in both families. Sequencing of skeletal muscle mtDNA was also performed and suspected mutations were quantified by hot last cycle PCR-RFLP or primer extension assay. Functional consequences of one of the mutations were investigated by measurement of steady state levels of mitochondrial tRNA. RESULTS: Two distinct mitochondrial pathologies were identified: a novel, homoplasmic mitochondrial tRNA(Cys) (MTTC) mutation and the primary, m.11778G>A Leber hereditary optic neuropathy (LHON) mutation. The mild nature of both mutations has permitted very high levels of mutated mtDNA to accumulate. Patients with the mutation in the MTTC gene have no wild type mtDNA detectable and although the LHON mutation is heteroplasmic in the patients we report, it is commonly observed to be homoplasmic. CONCLUSIONS: The mitochondrial etiology identified in these patients emphasizes the pathologic potential of homoplasmic mutations and has important implications for the investigation and genetic counseling of families where dystonia is the principal clinical feature. We advocate that mitochondrial disease should be given serious consideration in patients with familial, progressive dystonia, particularly when additional neurologic features such as epilepsy are present.

Catastrophic presentation of mitochondrial disease due to a mutation in the tRNA(His) gene.

The authors describe a patient who presented with headache, seizures, and severe cerebral edema in whom they identified a novel mutation in the mitochondrial (mt-) tRNA(His) gene. This G12147A transition is heteroplasmic, predicted to disrupt a highly conserved base pair, and segregates with the cytochrome c oxidase deficiency in single muscle fibers.