Adult-onset Progressive External Ophthalmoplegia Research Articles

Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations.

Autosomal dominant mutations in the TWNK gene, which encodes a mitochondrial DNA helicase, cause adult-onset progressive external ophthalmoplegia (PEO) and PEO-plus presentations. In this retrospective observational study, we describe clinical and complementary data from 25 PEO patients with mutations in TWNK recruited from the Hospital 12 de Octubre Mitochondrial Disorders Laboratory Database. The mean ages of onset and diagnosis were 43 and 63 years, respectively. Family history was positive in 22 patients. Ptosis and PEO (92% and 80%) were the most common findings. Weakness was present in 48%, affecting proximal limbs, neck, and bulbar muscles. Exercise intolerance was present in 28%. Less frequent manifestations were cardiac (24%) and respiratory (4%) involvement, neuropathy (8%), ataxia (4%), and parkinsonism (4%). Only 28% had mild hyperCKemia. All 19 available muscle biopsies showed signs of mitochondrial dysfunction. Ten different TWNK mutations were identified, with c.1361T>G (p.Val454Gly) and c.1070G>C (p.Arg357Pro) being the most common. Before definitive genetic confirmation, 56% of patients were misdiagnosed (36% with myasthenia, 20% with oculopharyngeal muscle dystrophy). Accurate differential diagnosis and early confirmation with appropriately chosen complementary studies allow genetic counseling and the avoidance of unnecessary treatments. Thus, mitochondrial myopathies must be considered in PEO/PEO-plus presentations, and particularly, TWNK is an important cause when positive family history is present.

Identification and Characterization of New RNASEH1 Mutations Associated With PEO Syndrome and Multiple Mitochondrial DNA Deletions.

Mitochondrial DNA (mtDNA) depletion and deletion syndrome encompasses a group of disorders caused by mutations in genes involved in mtDNA replication and maintenance. The clinical phenotype ranges from fatal infantile hepatocerebral forms to mild adult onset progressive external ophthalmoplegia (PEO). We report the case of a patient with PEO and multiple mtDNA deletions, with two new homozygous mutations in RNASEH1. The first mutation (c.487T>C) is located in the same catalytic domain as the four previously reported mutations, and the second (c.258_260del) is located in the connection domain, where no mutations have been reported. In silico study of the mutations predicted only the first mutation as pathogenic, but functional studies showed that both mutations cause loss of ribonuclease H1 activity. mtDNA replication dysfunction was demonstrated in patient fibroblasts, which were unable to recover normal mtDNA copy number after ethidium bromide-induced mtDNA depletion. Our results demonstrate the pathogenicity of two new RNASEH1 variants found in a patient with PEO syndrome, multiple deletions, and mild mitochondrial myopathy.

A complex genomic locus drives mtDNA replicase POLG expression to its disease-related nervous system regions.

DNA polymerase gamma (POLG), the mtDNA replicase, is a common cause of mitochondrial neurodegeneration. Why POLG defects especially cause central nervous system (CNS) diseases is unknown. We discovered a complex genomic regulatory locus for POLG, containing three functional CNS‐specific enhancers that drive expression specifically in oculomotor complex and sensory interneurons of the spinal cord, completely overlapping with the regions showing neuronal death in POLG patients. The regulatory locus also expresses two functional RNAs, LINC00925‐ RNA and MIR9‐3, which are coexpressed with POLG. The MIR9‐3 targets include NR2E1, a transcription factor maintaining neural stem cells in undifferentiated state, and MTHFD2, the regulatory enzyme of mitochondrial folate cycle, linking POLG expression to stem cell differentiation and folate metabolism. Our evidence suggests that distant genomic non‐coding regions contribute to regulation of genes encoding mitochondrial proteins. Such genomic arrangement of POLG locus, driving expression to CNS regions affected in POLG patients, presents a potential mechanism for CNS‐specific manifestations in POLG disease.

P31 Genotypic and phenotypic heterogeneity in adult-onset progressive external ophthalmoplegia (PEO) with mitochondrial DNA instability: a systematic review

P31 Genotypic and phenotypic heterogeneity in adult-onset progressive external ophthalmoplegia (PEO) with mitochondrial DNA instability: a systematic review

P32 Mutations in SPG7 cause chronic progressive external ophthalmoplegia through disordered mtDNA maintenance

P32 Mutations in SPG7 cause chronic progressive external ophthalmoplegia through disordered mtDNA maintenance

Adult-onset Mendelian PEO Associated with Mitochondrial Disease

Progressive external ophthalmoplegia (PEO) is an eye movement disorder characterised by paresis of the extra ocular muscles and muscle restricted multiple mitochondrial DNA (mtDNA) deletions. Classification of patients is particularly difficult due to overlapping phenotypes and a poor genotype-phenotype relationship. Despite the identification of several nuclear encoded genes causing PEO, over half of patients with clinically confirmed PEO do not have a genetic diagnosis. To systematically review genotypic and phenotypic correlates of published cases of adult-onset PEO. Patients were identified from interrogation of articles from Scopus, Medline via PubMed, and Genetic Abstracts databases using electronic searches (1st January 1970 to 8th November 2013). Reference lists and UniProt entries were also manually checked for additional articles. Twelve nuclear encoded genes were identified (TYMP, SLC25A4, POLG, C10ORF2, OPA1, POLG2, RRM2B, TK2, DGUOK, MPV17, MGME1, and DNA2) systematically from 583 patients. At the time of writing, mutations in SPG7 and AFG3L2 genes were reported to be associated with ophthalmoparesis and multiple mtDNA deletions in fourteen additional adult-onset PEO patients, bringing the total number of known genes to fourteen. Diagnostic yield is still critically dependent on the meticulous clinical and biochemical characterisation of patients. Understanding the intimate relationship between genotype and phenotype remains a fundamental challenge. The results of this systematic review provide guidance to both patients and clinician about future prognosis, and will serve, in future, to assess methods of disease prevention and evaluation of targeted therapeutic strategies.

Subnormal levels of POLγA cause inefficient initiation of light-strand DNA synthesis and lead to mitochondrial DNA deletions and autosomal dominant progressive external ophthalmoplegia

The POLG1 gene encodes the catalytic subunit of mitochondrial DNA (mtDNA) polymerase γ (POLγ). We here describe a sibling pair with adult-onset progressive external ophthalmoplegia, cognitive impairment and mitochondrial myopathy characterized by DNA depletion and multiple mtDNA deletions. The phenotype is due to compound heterozygous POLG1 mutations, T914P and the intron mutation c.3104 + 3A > T. The mutant genes produce POLγ isoforms with heterozygous phenotypes that fail to synthesize longer DNA products in vitro. However, exon skipping in the c.3104 + 3A > T mutant is not complete, and the presence of low levels of wild-type POLγ explains patient survival. To better understand the underlying pathogenic mechanisms, we characterized the effects of POLγ depletion in vitro and found that leading-strand DNA synthesis is relatively undisturbed. In contrast, initiation of lagging-strand DNA synthesis is ineffective at lower POLγ concentrations that uncouples leading strand from lagging-strand DNA synthesis. In vivo, this effect leads to prolonged exposure of the heavy strand in its single-stranded conformation that in turn can cause the mtDNA deletions observed in our patients. Our findings, thus, suggest a molecular mechanism explaining how POLγ mutations can cause mtDNA deletions in vivo.

Cardiac screening investigations in adult‐onset progressive external ophthalmoplegia patients

Patients with mitochondrial myopathies may develop cardiac complications such as cardiomyopathy and/or cardiac conduction defects. To identify these potentially life-threatening and treatable conditions, it is common practice to screen patients intermittently with electrocardiography and echocardiography. The optimal time interval for such screening investigations is unknown. We developed this study to review our screening results in adult-onset patients with progressive external ophthalmoplegia (PEO). This study was a retrospective review of PEO patients with 5 years or more of cardiac screening investigations who did not have any cardiac symptoms. Fifteen patients were included, and cardiomyopathy was identified on screening echocardiogram in 1 patient. Four patients had other abnormalities identified, which were unrelated to their mitochondrial myopathy. Only 1 patient in 15 developed cardiac complications related to mitochondrial disease during 5 years of follow-up. We suggest that a screening interval of 3-5 years is probably appropriate for adult-onset PEO patients who do not have cardiac symptoms.

Disease Variants of the Human Mitochondrial DNA Helicase Encoded by C10orf2 Differentially Alter Protein Stability, Nucleotide Hydrolysis, and Helicase Activity

Missense mutations in the human C10orf2 gene, encoding the mitochondrial DNA (mtDNA) helicase, co-segregate with mitochondrial diseases such as adult-onset progressive external ophthalmoplegia, hepatocerebral syndrome with mtDNA depletion syndrome, and infantile-onset spinocerebellar ataxia. To understand the biochemical consequences of C10orf2 mutations, we overproduced wild type and 20 mutant forms of human mtDNA helicase in Escherichia coli and developed novel schemes to purify the recombinant enzymes to near homogeneity. A combination of molecular crowding, non-ionic detergents, Mg(2+) ions, and elevated ionic strength was required to combat insolubility and intrinsic instability of certain mutant variants. A systematic biochemical assessment of the enzymes included analysis of DNA binding affinity, DNA helicase activity, the kinetics of nucleotide hydrolysis, and estimates of thermal stability. In contrast to other studies, we found that all 20 mutant variants retain helicase function under optimized in vitro conditions despite partial reductions in DNA binding affinity, nucleotide hydrolysis, or thermal stability for some mutants. Such partial defects are consistent with the delayed presentation of mitochondrial diseases associated with mutation of C10orf2.

Novel Twinkle (PEO1) gene mutations in mendelian progressive external ophthalmoplegia

Multiple deletions of mitochondrial DNA (mtDNA) are associated with different mitochondrial disorders inherited as autosomal dominant and recessive traits. Causative mutations have been found in five genes, mainly involved in mtDNA replication and stability. They include POLG1, the gene encoding the catalytic subunit of mtDNA polymerase (pol gamma), POLG2 encoding its accessory subunit, ANT1 coding the adenine nucleotide translocator and PEO1 which codes for Twinkle, the mitochondrial helicase. Finally OPA1 missense mutations are involved in phenotypes presenting optic atrophy as a major feature.To define the relative contribution of POLG1, POLG2, ANT1 and PEO1 genes to the mtDNA multiple deletion syndromes, we analysed them in a cohort of 67 probands showing accumulation of multiple mtDNA deletions in muscle. The patients were predominantly affected with a mitochondrial myopathy with or without progressive external ophthalmoplegia (PEO). Genetic analysis revealed that 1) PEO1 has a major role in determining familial PEO, since it accounts for 26.8% of familial cases, followed by ANT1 (14.6%) and POLG1 (9.8%); 2) no mutations in any of the known genes were found in 53.7% of probands of this series. Six novel missense mutations contributing to the mutational load of PEO1 gene (p.R334P, p.W315S, p. S426N, p.W474S, p.F478I, p.E479K) were associated with an adult onset PEO phenotype.

Mutant mitochondrial helicase Twinkle causes multiple mtDNA deletions and a late-onset mitochondrial disease in mice

Defects of mitochondrial DNA (mtDNA) maintenance have recently been associated with inherited neurodegenerative and muscle diseases and the aging process. Twinkle is a nuclear-encoded mtDNA helicase, dominant mutations of which cause adult-onset progressive external ophthalmoplegia (PEO) with multiple mtDNA deletions. We have generated transgenic mice expressing mouse Twinkle with PEO patient mutations. Multiple mtDNA deletions accumulate in the tissues of these mice, resulting in progressive respiratory dysfunction and chronic late-onset mitochondrial disease starting at 1 year of age. The muscles of the mice faithfully replicate all of the key histological, genetic, and biochemical features of PEO patients. Furthermore, the mice have progressive deficiency of cytochrome c oxidase in distinct neuronal populations. These "deletor" mice do not, however, show premature aging, indicating that subtle accumulation of mtDNA deletions and progressive respiratory chain dysfunction are not sufficient to accelerate aging. This model is a valuable tool for therapy development and testing for adult-onset mitochondrial disorders.

Complex genetic counselling and prenatal analysis in a woman with external ophthalmoplegia and deleted mtDNA

Single large mitochondrial DNA deletions (DeltamtDNA) are usually spontaneously occurring and cause a wide variety of symptoms, ranging from severe infantile multisystem disorders to adult onset progressive external ophthalmoplegia (PEO). There is always heteroplasmy with a mixture of normal and mutant mtDNA and the levels usually vary widely between tissues. There is at present insufficient scientific basis for accurate genetic counselling of women with DeltamtDNA, but it is reasonable to assume that DeltamtDNA can be transmitted if it is present in the female germ cells. Here, we present the results of prenatal analysis in a woman with DeltamtDNA and PEO. No DeltamtDNA was detected by Southern blot and PCR analyses of chorionic villi from the first trimester of pregnancy, in cord blood obtained at birth or in peripheral blood from the child at six months of age. This makes it unlikely that the child will develop a severe infantile mitochondrial disorder due to transmission of high levels of DeltamtDNA. However, the complex mitochondrial genetics and the limited access to human tissues makes it impossible to exclude transmission of low levels of DeltamtDNA that possibly could cause disease later in life.