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Autosomal recessive mitochondrial ataxic syndrome due to mitochondrial polymerase γ mutations

Objective: To investigate three families and one sporadic case with a recessively inherited ataxic syndrome. Methods: Clinical and genetic studies were performed in six individuals. Southern blotting and real time PCR were used to detect deletions of mtDNA and mutations in the POLG gene were identified using a combination of DHPLC and direct DNA sequencing. Results: The patients have a distinctive, progressive disorder that starts with episodic symptoms such as migraine-like headache or epilepsy. Ataxia, which is a combination of central and peripheral disease, develops later as does ophthalmoplegia. The commonest form of epilepsy was focal and involved the occipital lobes. Myoclonus was common and patients have a high risk of status epilepticus. MRI typically shows signal changes in the central cerebellum, olivary nuclei, occipital cortex, and thalami. COX negative muscle fibers were found in four of six; in one patient these were rare and in another absent. Multiple mtDNA deletions were identified in all patients, although in two these were not apparent on Southern blotting and real time PCR was required to demonstrate the defect. Two families were homozygous for a previously described POLG mutation, G1399A (A467T). One family and the sporadic case had the same two new mutations, a G to C at position 1491 (Q497H) and a G to C at 2243 (W748S). Conclusions: Mutations in POLG cause a recessively inherited syndrome with episodic features and progressive ataxia. Characteristic changes on MRI are seen and although skeletal muscle may appear morphologically normal, multiple mtDNA deletions can be detected using real-time PCR.

Resistance training in patients with single, large-scale deletions of mitochondrial DNA

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.

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 (≈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.

Melas associated with mutations in the polg1 gene.

MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes) syndrome is a characteristic mitochondrial disorder caused by point mutations in the mitochondrial genome (mtDNA).1 In addition to primary mtDNA defects, mutations in nuclear genes essential for mtDNA maintenance are emerging as important causes of mitochondrial disease leading to secondary mtDNA changes including mtDNA depletion and multiple mtDNA deletions. Of particular interest are mutations in the mtDNA polymerase γ ( POLG1 ) gene, which are associated with a phenotypic spectrum ranging from Alpers syndrome to recessive ataxia and late-onset progressive external ophthalmoplegia (PEO).2–5 We describe a patient with strokelike episodes typical of MELAS due to mutation of POLG1 .nn### Case history.nnA 28-year-old man was admitted to hospital at age 23 years because of acute onset of head jerking to the left side and vomiting, preceded by several days of visual scintillations and right-sided headache. He had been well until 2 years previously when it was noted that he had problems with coordination and concentration. Neurologic examination revealed left homonymous hemianopia, areflexia, and sensory ataxia. A few hours after admission, he developed left-sided focal seizures with secondary generalization leading to status epilepticus, which was successfully treated with clonazepam and …

Mitochondrial DNA deletion in “identical” twin brothers

Defects of mitochondrial DNA (mtDNA) are an important cause of genetic disease in humans.1 The defect may take the form of a deletion or point mutation, and clinical features range from severe neonatal illness to mild muscle weakness later in life.2 In a recent epidemiology study, single large scale mtDNA deletion disorders represented approximately 25% of adult patients with mtDNA disease.3 The three main syndromes observed in these patients are Pearson’s syndrome (PS), a severe systemic illness of childhood associated with sideroblastic anaemia; Kearns-Sayre syndrome (KSS) which is also associated with multiple system involvement; and chronic progressive external ophthalmoplegia (CPEO) in which the predominant clinical problem relates to an eye movement disturbance and ptosis. In both KSS and PS the mtDNA deletion is present in many tissues,4–6 whereas in CPEO, the mtDNA deletion is found in muscle but not other tissues.5,7nnIn patients with single large scale mtDNA deletions, it is not established at which stage in development the mtDNA deletion occurs. For the vast majority of cases, there is no family history and as described above, the mtDNA deletion may be limited to a single tissue. Recently, we had the opportunity to study 31 year old twin brothers who were genetically identical in terms of nuclear markers, but one had developed CPEO whereas the other was completely asymptomatic. We hoped that by studying these brothers we might gain further insight into the origin of the mtDNA deletion.nn### Case reportsnnThe twin brothers were born 2 months premature, and the first twin (the affected brother) required a forceps delivery. Although no specific problems were documented immediately after birth, the affected twin (patient 1) was noted to have a slightly asymmetric skull (probably related to the delivery) and a slight squint. He was, however, very athletic …

Recessive germline SDHA and SDHB mutations causing leukodystrophy and isolated mitochondrial complex II deficiency

Background Isolated complex II deficiency is a rare form of mitochondrial disease, accounting for approximately 2% of all respiratory chain deficiency diagnoses. The succinate dehydrogenase (SDH) genes (SDHA, SDHB, SDHC and SDHD) are autosomally-encoded and transcribe the conjugated heterotetramers of complex II via the action of two known assembly factors (SDHAF1 and SDHAF2). Only a handful of reports describe inherited SDH gene defects as a cause of paediatric mitochondrial disease, involving either SDHA (Leigh syndrome, cardiomyopathy) or SDHAF1 (infantile leukoencephalopathy). However, all four SDH genes, together with SDHAF2, have known tumour suppressor functions, with numerous germline and somatic mutations reported in association with hereditary cancer syndromes, including paraganglioma and pheochromocytoma. Methods and results Here, we report the clinical and molecular investigations of two patients with histochemical and biochemical evidence of a severe, isolated complex II deficiency due to novel SDH gene mutations; the first patient presented with cardiomyopathy and leukodystrophy due to compound heterozygous p.Thr508Ile and p.Ser509Leu SDHA mutations, while the second patient presented with hypotonia and leukodystrophy with elevated brain succinate demonstrated by MR spectroscopy due to a novel, homozygous p.Asp48Val SDHB mutation. Western blotting and BN-PAGE studies confirmed decreased steady-state levels of the relevant SDH subunits and impairment of complex II assembly. Evidence from yeast complementation studies provided additional support for pathogenicity of the SDHB mutation. Conclusions Our report represents the first example of SDHB mutation as a cause of inherited mitochondrial respiratory chain disease and extends the SDHA mutation spectrum in patients with isolated complex II deficiency.

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.

POLG1, C10ORF2, and ANT1 mutations are uncommon in sporadic progressive external ophthalmoplegia with multiple mitochondrial DNA deletions

The authors sequenced POLG1, C10ORF2, and ANT1 in 38 sporadic progressive external ophthalmoplegia patients with multiple mitochondrial DNA (mtDNA) deletions. Causative mutations were identified in approximately10% of cases, with two unrelated individuals harboring a novel premature stop codon mutation (1356T>G). None had a mutation in C10ORF2 or ANT1. In the majority of patients, the primary nuclear genetic defect is likely to affect other unknown genes important for mtDNA maintenance.

Mitochondrial DNA mutations in the haematopoietic system

examined in PBMC samples from five patients during the course of allo-SCT. Interestingly, HTLV-I tax expression was at a very low level in all samples (Figure 2). In the carrier state, HTLV-Iinfected T cells are considered to be polyclonal and to express tax gene, which is at least partially responsible for their immortalization. Such immortalized T cells, derived from recipient and/or donor cells, possibly exist in ATL patients receiving allo-SCT. Tax protein is an immunodominant target antigen recognized by virus-specific T cells, and tax-expressing cells were reported to be suppressed by immune system in HTLV-I carriers. However, in the severe immunodeficient state such as post-SCT conditions, tax-expressing cells may expand like lymphoproliferative disorder (LPD) caused by Epstein–Barr virus (EBV). In the present study, we found that the expression of tax mRNA was scarcely detectable in all samples tested during allo-SCT. To our knowledge, polyclonal expansion of HTLV-I-infected cells has not yet been reported in patients receiving allo-SCT, including the present patients. Taken together, the expansion of such tax-expressing cells appeared to seldom occur during the course of allo-SCT. In conclusion, both HTLV-I proviral load and sIL-2R level were able to predict relapse in patients with ATL receiving alloSCT. Close and simultaneous monitoring of these two markers would be useful to guide the timely introduction of immunetherapeutic interventions.