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.     

POLG mutations causing ophthalmoplegia, sensorimotor polyneuropathy, ataxia, and deafness

The authors identified two novel heterozygous missense transitions in the gene for the mitochondrial polymerase gammaA subunit (POLG) in a family with an autosomal recessive syndrome comprising progressive external ophthalmoplegia (PEO), polyneuropathy, ataxia, sensorineural hearing loss, and affective disorders. These mutations were not detected in 120 healthy control subjects.     

Association of novel POLG mutations and multiple mitochondrial DNA deletions with variable clinical phenotypes in a Spanish population

BACKGROUND: Both dominant and recessive mutations were reported in the gene encoding the mitochondrial (mt) DNA polymerase gamma (POLG) in patients with progressive external ophthalmoplegia (PEO). Phenotypes other than PEO were recently documented in patients with mutations in the POLG gene. OBJECTIVE: To screen patients with mitochondrial disease and multiple mtDNA deletions in muscle for mutations in the coding regions of the POLG, PEO1, and SLC25A4 genes. DESIGN: To identify the underlying molecular defect in a group of patients with multiple mtDNA deletions comparing their molecular genetic findings with those of healthy controls. PATIENTS: Twenty-four patients (16 men and 8 women) diagnosed with mitochondrial disease and having multiple mtDNA deletions in muscle by Southern blot analysis. Thirteen patients had PEO; 2 had PEO alone, 4 had PEO and myopathy, and 5 had PEO and multisystem involvement. Four patients had multisystem disease without PEO. The remaining 9 patients had isolated myopathy. DNA from 100 healthy individuals was also studied. RESULTS: No mutation was identified in the PEO1 or SLC25A4 genes. Nine POLG mutations were observed in 6 of 24 patients. Four novel mutations were detected and mapped in the linker region (M603L) and in the pol domain of the enzyme (R853W; D1184N; R1146C). Five patients with PEO had mutations: 2 were compound heterozygotes, 1 was homozygous, and another showed a mutation in a single allele. The remaining patient also showed a sole mutation and had an unusual phenotype lacking ocular involvement. CONCLUSIONS: POLG molecular defects were found in 25% of our patients with multiple mtDNA deletions and mitochondrial disease. The uncommon phenotype found in 1 of these patients stresses the clinical variability of patients harboring POLG mutations. Molecular studies in the POLG gene should be addressed in patients with mitochondrial disease, particularly in those with PEO, and multiple mtDNA deletions.     

Apraxia of lid opening mimicking ptosis in compound heterozygosity for A467T and W748S POLG1 mutations

Patients harboring A467T and W748S POLG1 mutations present with a broad variety of neurological phenotypes, including cerebellar ataxia, progressive external ophthalmoplegia (PEO), myoclonus, epilepsy, and peripheral neuropathy. With exception of ataxia and myoclonus, movement disorders are not typical features of POLG1 associated disorders. We report on two affected siblings compound heterozygous for A467T and W748S mutations, one suffering from choreoathetosis and apraxia of lid opening due to focal eyelid dystonia that mimicked progression of ptosis, resulting in functional blindness. So far, focal dystonia has not been reported in POLG1 mutation carriers, and should be considered when investigating patients with PEO and ptosis. Further studies on POLG1 mutations in focal dystonia are warranted. © 2008 Movement Disorder Society     

Recessive POLG mutations presenting with sensory and ataxic neuropathy in compound heterozygote patients with progressive external ophthalmoplegia

Autosomal recessive progressive external ophthalmoplegia is a mitochondrial disease characterized by accumulation of multiple large-scale deletions of mitochondrial DNA. We previously reported missense mutations in POLG, the gene encoding the mitochondrial DNA polymerase gamma in two nuclear families compatible with autosomal recessive progressive external ophthalmoplegia. Here, we report a novel POLG missense mutation (R627W) in a sporadic patient and we provide genetic support that all these POLG mutations are actually causal and recessive. The novel patient presented with sensory ataxic neuropathy and has the clinical triad of sensory ataxic neuropathy, dysarthria and ophthalmoparesis (SANDO). This is the first finding of a genetic cause of Sensory Ataxic Neuropathy, Dysarthria and Ophthalmoparesis and it implies that this disorder may actually be a variant of autosomal recessive progressive external ophthalmoplegia. Sensory neuropathy is the initial feature in Belgian compound heterozygote autosomal recessive progressive external ophthalmoplegia patients, all carrying the POLG A467T mutation, which occurs at a frequency of 0.6% in the Belgian population.     

Clinical and genetic heterogeneity in progressive external ophthalmoplegia due to mutations in polymerase gamma

BACKGROUND: The mendelian forms of progressive external ophthalmoplegia (PEO) associated with multiple mitochondrial DNA deletions are clinically heterogeneous disorders transmitted as dominant or recessive traits. Autosomal dominant PEO is caused by mutations in at least 3 genes: adenine nucleotide translocator-1 (ANT1), encoding the muscle-specific adenine nucleotide translocator; chromosome 10 open reading frame 2 (C10orf2), encoding Twinkle helicase; and polymerase gamma (POLG), encoding the alpha subunit of polymerase gamma. Mutations in POLG can also cause autosomal recessive PEO, which is often associated with multisystemic disorders. OBJECTIVE AND METHODS: To further investigate the frequency and genotype-phenotype correlations of mutations in the POLG gene, we used single-stranded conformational polymorphism analysis and direct sequencing to screen 30 patients with familial or sporadic PEO and multiple mitochondrial DNA deletions in muscle but without mutations in ANT1 and C10orf2. RESULTS: Four unrelated patients had novel POLG mutations. A woman with PEO and mental retardation had a heterozygous Gly1076Val mutation. Two patients, one with PEO, exercise intolerance, and gastrointestinal dysmotility and the other with PEO, neuropathy, deafness, and hypogonadism, both had a Pro587Leu change. The fourth patient, who was compound heterozygous for Ala889Thr and Arg579Trp mutations, had PEO, gastrointestinal dysmotility, and neuropathy. These mutations were not detected in 120 healthy control alleles. CONCLUSIONS: Our results demonstrate that POLG mutations account for a substantial proportion of patients (13%) with PEO and multiple mitochondrial DNA deletions and cause both clinically and genetically heterogeneous disorders.     

Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE)-like phenotype: an expanded clinical spectrum of POLG1 mutations

The aim of the study was to determine the prevalence of MNGIE-like phenotype in patients with recessive POLG1 mutations. Mutations in the POLG1 gene, which encodes for the catalytic subunit of the mitochondrial DNA polymerase gamma essential for mitochondrial DNA replication, cause a wide spectrum of mitochondrial disorders. Common phenotypes associated with POLG1 mutations include Alpers syndrome, ataxia-neuropathy syndrome, and progressive external ophthalmoplegia (PEO). Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) is an autosomal recessive disorder characterized by severe gastrointestinal dysmotility, cachexia, PEO and/or ptosis, peripheral neuropathy, and leukoencephalopathy. MNGIE is caused by TYMP mutations. Rare cases of MNGIE-like phenotype have been linked to RRM2B mutations. Recently, POLG1 mutations were identified in a family with clinical features of MNGIE but no leukoencephalopathy. The coding regions and exon-intron boundaries of POLG1 were sequence analyzed in patients suspected of POLG1 related disorders. Clinical features of 92 unrelated patients with two pathogenic POLG1 alleles were carefully reviewed. Three patients, accounting for 3.3% of all patients with two pathogenic POLG1 mutations, were found to have clinical features consistent with MNGIE but no leukoencephalopathy. Patient 1 carries p.W748S and p.R953C; patient 2 is homozygous for p.W748S, and patient 3 is homozygous for p.A467T. In addition, patient 2 has a similarly affected sibling with the same POLG1 genotype. POLG1 mutations may cause MNGIE-like syndrome, but the lack of leukoencephalopathy and the normal plasma thymidine favor POLG1 mutations as responsible molecular defect.     

Two novel POLG1 mutations in a patient with progressive external ophthalmoplegia, levodopa-responsive pseudo-orthostatic tremor and parkinsonism

Different mutations, or combinations of mutations, in POLG1, the gene encoding pol gammaA, the catalytic subunit of mitochondrial DNA polymerase, are associated with a spectrum of clinical presentations including autosomal dominant or recessive progressive external ophthalmoplegia (PEO), juvenile-onset ataxia and epilepsy, and Alpers-Huttenlocher syndrome. Parkinsonian features have been reported as a late complication of POLG1-associated dominant PEO. Good response to levodopa or dopamine agonists, reduced dopamine uptake in the corpus striatum and neuronal loss of the Substantia Nigra pars compacta have been documented in a few cases. Here we report two novel mutations in POLG1 in a compound heterozygous patient with autosomal recessive PEO, followed by pseudo-orthostatic tremor evolving into levodopa-responsive parkinsonism. These observations support the hypothesis that mtDNA dysfunction is engaged in the pathogenesis of idiopathic Parkinson's disease.     

Autosomal disorders of mitochondrial DNA maintenance

Mitochondrial DNA (mtDNA) is maternally inherited. After birth, secondary mtDNA defects can arise. MtDNA depletion is a reduction in the amount of mtDNA in particular tissues. Multiple deletions of mtDNA accumulate as somatic mutations in mainly postmitotic tissues. These disorders of mtDNA maintenance frequently show Mendelian inheritance. Positional cloning has identified several genes involved in the control of mtDNA stability. Recessive mutations in the genes ECGF1, dGK, TK2, SUCLA2 and POLG cause mtDNA depletion syndromes (MDS). Generally, MDS has infantile onset tissue specific features. Mutations in the genes ECGF1, ANT1, C10orf2 and POLG are associated with multiple mtDNA deletions. The nature of these mutations is dominant in ANT1, C10orf2 and POLG and recessive in ECGF1, C10orf2 and POLG. Mutations in these genes frequently cause progressive external ophthalmoplegia (PEO). However clinical heterogeneity results in different neurological syndromes with considerable overlap. The most common features are PEO, neuropathy, myopathy, ataxia, epilepsy and hepatopathy.     

Mutations of mitochondrial DNA polymerase gammaA are a frequent cause of autosomal dominant or recessive progressive external ophthalmoplegia

One form of familial progressive external ophthalmoplegia with multiple mitochondrial DNA deletions recently has been associated with mutations in POLG1, the gene encoding pol gammaA, the catalytic subunit of mitochondrial DNA polymerase. We screened the POLG1 gene in several PEO families and identified five different heterozygous missense mutations of POLG1 in 10 autosomal dominant families. Recessive mutations were found in three families. Our data show that mutations of POLG1 are the most frequent cause of familial progressive external ophthalmoplegia associated with accumulation of multiple mitochondrial DNA deletions, accounting for approximately 45% of our family cohort.     

Sensory ataxic neuropathy with ophthalmoparesis caused by POLG mutations

Mutations in POLG gene are responsible for a wide spectrum of clinical disorders with altered mitochondrial DNA (mtDNA) integrity, including mtDNA multiple deletions and depletion. Sensory ataxic neuropathy with ophthalmoparesis (SANDO) caused by mutations in POLG gene, fulfilling the clinical triad of sensory ataxic neuropathy, dysarthria and/or dysphagia and ophthalmoparesis, has described in a few reports. Here we described five cases of adult onset autosomal recessive sensory ataxic neuropathy with ophthalmoplegia. All patients had ataxia, neuropathy, myopathy, and progressive external ophthalmoplegia (PEO). The muscle pathology revealed ragged-red and cytochrome c oxidase (COX) negative fibers in three patients. However, deficiencies in the activities of mitochondrial respiratory chain enzyme complexes were not detected in any of the patients' muscle samples. Multiple deletions of mtDNA were detected in blood and muscle specimens but mtDNA depletion was not found. Due to these diagnostic difficulties, POLG-related syndromes are definitively diagnosed based on the presence of deleterious mutations in the POLG gene.     

Sudden unexpected death with rare compound heterozygous variants in PRICKLE1

neurogenetics - Progressive myoclonus epilepsy-ataxia syndrome (EPM5) is an autosomal recessive form of progressive myoclonus epilepsy that has been associated with a homozygous missense mutation...     

Late-onset ptosis and myopathy in a patient with a heterozygous insertion in POLG2

Polymerase gamma 1 (POLG) mutations are a frequent cause of both autosomal dominant and recessive complex neurological phenotypes. In contrast, only a single pathogenic mutation in one patient was reported in POLG2 so far. Here we describe a 62-year-old woman, carrying a novel heterozygous sequence variant in the POLG2 gene. She developed bilateral ptosis at 30 years of age, followed by exercise intolerance, muscle weakness and mild CK increase in her late forties. Muscle histology and respiratory chain activities were normal. Southern blot and long range PCR detected multiple mtDNA deletions, but no depletion in muscle DNA. Sequencing of POLG, PEO1, ANT1, OPA1 and RRM2B showed normal results. A novel heteroallelic 24 bp insertion (c.1207_1208ins24) was detected in POLG2. This 24 bp insertion into exon 7 causes missplicing and loss of exon 7 in myoblast cDNA. We did not detect POLG2 mutations in 62 patients with multiple mtDNA deletions in muscle DNA, suggesting that POLG2 mutations may represent a rare cause of autosomal dominant PEO.     

Progressive external ophthalmoplegia and vision and hearing loss in a patient with mutations in POLG2 and OPA1

OBJECTIVE: To describe the clinical features, muscle pathological characteristics, and molecular studies of a patient with a mutation in the gene encoding the accessory subunit (p55) of polymerase gamma (POLG2) and a mutation in the OPA1 gene. DESIGN: Clinical examination and morphological, biochemical, and molecular analyses. SETTING: Tertiary care university hospitals and molecular genetics and scientific computing laboratory. PATIENT: A 42-year-old man experienced hearing loss, progressive external ophthalmoplegia (PEO), loss of central vision, macrocytic anemia, and hypogonadism. His family history was negative for neurological disease, and his serum lactate level was normal. RESULTS: A muscle biopsy specimen showed scattered intensely succinate dehydrogenase-positive and cytochrome-c oxidase-negative fibers. Southern blot of muscle mitochondrial DNA showed multiple deletions. The results of screening for mutations in the nuclear genes associated with PEO and multiple mitochondrial DNA deletions, including those in POLG (polymerase gamma gene), ANT1 (gene encoding adenine nucleotide translocator 1), and PEO1, were negative, but sequencing of POLG2 revealed a G1247C mutation in exon 7, resulting in the substitution of a highly conserved glycine with an alanine at codon 416 (G416A). Because biochemical analysis of the mutant protein showed no alteration in chromatographic properties and normal ability to protect the catalytic subunit from N-ethylmaleimide, we also sequenced the OPA1 gene and identified a novel heterozygous mutation (Y582C). CONCLUSION: Although we initially focused on the mutation in POLG2, the mutation in OPA1 is more likely to explain the late-onset PEO and multisystem disorder in this patient.     

Progressive external ophthalmoplegia and multiple mitochondrial DNA deletions

Progressive external ophthalmoplegia (PEO) with secondary accumulation of multiple deletions of mitochondrial DNA (mtDNA) clinically resembles disorders due to primary mutations of mtDNA but follows a Mendelian inheritance pattern. The disorder belongs to an interesting group of diseases in which both the nuclear and the mitochondrial genome are involved in the pathology. Both autosomal dominant (adPEO) and recessive (arPEO) variants of this disorder occur. Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) patients may have multiple mtDNA deletions and/or depletion of mtDNA. Recent reports of mutations in Thymidine Phosphorylase in MNGIE, and of mutations in adenine nucleotide translocator (ANT1), Twinkle and mitochondrial DNA polymerase gamma (POLG) in adPEO, have lead to new insights in the pathogenesis of these disorders of mtDNA maintenance. We also identified POLG mutations in two families with arPEO, which underlines the crucial role of the mtDNA replication machinery for mtDNA maintenance.     

A new POLG1 mutation with peo and severe axonal and demyelinating sensory–motor neuropathy

Background Progressive external ophthalmoplegia (PEO) is a mitochondrial disorder associated with defective enzymatic activities of oxidative phosphorylation (OXPHOS), depletion of mitochondrial DNA (mtDNA) and/or accumulation of mtDNA mutations and deletions. Recent positional cloning studies have linked the disease to four different chromosomal loci. Mutations in POLG1 are a frequent cause of this disorder. Methods We describe two first–cousins: the propositus presented with PEO,mitochondrial myopathy and neuropathy, whereas his cousin showed a Charcot– Marie–Tooth phenotype. Neurophysiological studies, peroneal muscle and sural nerve biopsies, and molecular studies of mtDNA maintenance genes (ANT1, Twinkle, POLG1, TP) and non dominant CMT–related genes (GDAP1, LMNA, GJB1) were performed. Results A severe axonal degeneration was found in both patients whereas hypomyelination was observed only in the patient with PEO whose muscle biopsy specimen also showed defective OXPHOS and multiple mtDNA deletions. While no pathogenetic mutations in GDAP1, LMNA, and GJB1 were found, we identified a novel homozygous POLG1 mutation (G763R) in the PEO patient. The mutation was heterozygous in his healthy relatives and in his affected cousin. Conclusions A homozygous POLG1 mutation might explain PEO with mitochondrial abnormalities in skeletal muscle in our propositus, and it might have aggravated his axonal and hypomyelinating sensory–motor neuropathy. Most likely, his cousin had an axonal polyneuropathy with CMT phenotype of still unknown etiology.     

Sensory ataxic neuropathy due to a novel C10Orf2 mutation with probable germline mosaicism

The authors describe siblings with progressive external ophthalmoplegia (PEO) due to a novel heterozygous A to G transition at nucleotide 955 of C10Orf2 (Twinkle). The mutation was not identified in parents' blood, hair follicles, buccal mucosa, or urinary epithelium, indicating germ line mosaicism. One sibling presented with sensory ataxic neuropathy, dysarthria, and ophthalmoparesis (SANDO), a phenotype previously associated with the POLG1 gene, highlighting the clinical overlap in autosomal PEO.     

Neuromelanin MRI in a family with mitochondrial parkinsonism harboring a Y955C mutation in POLG1

BackgroundProgressive external ophthalmoplegia (PEO) and parkinsonism can be caused by genetic mutations that affect mitochondrial DNA (mtDNA) maintenance. We characterized parkinsonism in a family with dominantly inherited PEO.MethodsWe conducted clinical, histological and genetic analyses on two affected members suffering from PEO and parkinsonism, and reviewed the cases in the literature. To clarify parkinsonism related to multiple mtDNA deletions, we used 3-T neuromelanin magnetic resonance imaging (MRI) to assess signal changes in the substantia nigra (SN) and locus ceruleus (LC) in our patients, and compared the results to those observed in idiopathic Parkinson's disease (iPD) (n = 35).ResultsWe report the first case of a Japanese family harboring a heterozygous p.Y955C mutation in POLG1. The clinical features of parkinsonism related to the Y955C mutation in a total of 16 patients, including our two cases, are indistinguishable from iPD. However, neuromelanin MRI showed a distinct pattern in our cases compared to iPD. The neuromelanin imaging results were consistent with the neuropathological findings reported in cases of POLG1 mutations, in which neurons of the SN were profoundly affected while those in the LC were preserved.ConclusionsOur results suggest that 3-T neuromelanin MRI may be useful for differentiating POLG1 mutation-associated parkinsonism from iPD, and that POLG1 mutations may cause selective neuronal loss in the SN via a mechanism different from that of iPD.     

POLG mutations and Alpers syndrome

Alpers-Huttenlocher syndrome (AHS) an autosomal recessive hepatocerebral syndrome of early onset, has been associated with mitochondrial DNA (mtDNA) depletion and mutations in polymerase gamma gene (POLG). We have identified POLG mutations in four patients with hepatocerebral syndrome and mtDNA depletion in liver, who fulfilled criteria for AHS. All were compound heterozygous for the G848S and W748S mutations, previously reported in patients with progressive external ophtalmoplegia or ataxia. We conclude that AHS should be included in the clinical spectrum of mtDNA depletion and is often associated with POLG mutations, which can cause either multiple mtDNA deletions or mtDNA depletion.     

Alpers syndrome with prominent white matter changes

Alpers syndrome is a fatal neurogenetic disorder caused by the mutations in POLG1 gene encoding the mitochondrial DNA polymerase gamma (polgamma). Two missense variants, c.248T > C (p.L83P), c.2662G > A (p.G888S) in POLG1 were detected in a 10-year-old Chinese girl with refractory seizures, acute liver failure after exposure to valproic acid, cortical blindness, and psychomotor regression. The pathology of left occipital lobe showed neuronal loss, spongiform degeneration, astrocytosis, and demyelination. In addition, there were prominent white matter changes in a series of brain magnetic resonance imaging (MRI) and increased immunological factors in CSF.