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.     

Magnesium treatment for patients with refractory status epilepticus due to POLG1-mutations

Mutations in the gene encoding of the catalytic subunit of mtDNA polymerase gamma (POLG1) can cause typical Alpers' syndrome. Recently, a new POLG1 mutation phenotype was described, the so-called juvenile-onset Alpers' syndrome. This POLG1 mutation phenotype is characterized by refractory epilepsy with recurrent status epilepticus and episodes of epilepsia partialis continua, which often necessitate admission to the intensive care unit (ICU) and pose an important mortality risk. We describe two previously healthy unrelated teenage girls, who both were admitted with generalized tonic-clonic seizures and visual symptoms leading to a DNA-supported diagnosis of juvenile-onset Alpers' syndrome. Despite combined treatment with anti-epileptic drugs, both patients developed status epilepticus requiring admission to the ICU. Intravenous magnesium as anti-convulsant therapy was initiated, resulting in clinical and neurophysiological improvement and rapid extubation of both patients. Treating status epilepticus in juvenile-onset Alpers' syndrome with magnesium has not been described previously. Given the difficulties encountered while treating epilepsy in patients with this syndrome, magnesium therapy might be considered.     

POLG1 mutations associated with progressive encephalopathy in childhood

We have identified compound heterozygous missense mutations in POLG1, encoding the mitochondrial DNA polymerase gamma (Pol gamma), in 7 children with progressive encephalopathy from 5 unrelated families. The clinical features in 6 of the children included psychomotor regression, refractory seizures, stroke-like episodes, hepatopathy, and ataxia compatible with Alpers-Huttenlocher syndrome. Three families harbored a previously reported A467T substitution, which was found in compound with the earlier described G848S or the W748S substitution or a novel R574W substitution. Two families harbored the W748S change in compound with either of 2 novel mutations predicted to give an R232H or M1163R substitution. Muscle morphology showed mitochondrial myopathy with cytochrome c oxidase (COX)-deficient fibers in 4 patients. mtDNA analyses in muscle tissue revealed mtDNA depletion in 3 of the children and mtDNA deletions in the 2 sibling pairs. Neuropathologic investigation in 3 children revealed widespread cortical degeneration with gliosis and subcortical neuronal loss, especially in the thalamus, whereas there were only subcortical neurodegenerative findings in another child. The results support the concept that deletions as well as depletion of mtDNA are involved in the pathogenesis of Alpers-Huttenlocher syndrome and add 3 new POLG1 mutations associated with an early-onset neurodegenerative disease.     

Common mitochondrial DNA and POLG1 mutations are rare in the Chinese patients with adult-onset ataxia on Taiwan

BACKGROUND AND PURPOSE: Spinocerebellar ataxia (SCA) is a heterogeneous group of neurodegenerative disorders with common features of adult-onset cerebellar ataxia. Many patients with clinically suspected SCA are subsequently diagnosed with common SCA gene mutations. Previous reports suggest some common mitochondrial DNA (mtDNA) point mutations and mitochondrial DNA polymerase gene (POLG1) mutations might be additional underlying genetic causes of cerebellar ataxia. We tested whether mtDNA point mutations A3243G, A8344G, T8993G, and T8993C, or POLG1 mutations W748S and A467T are found in patients with adult-onset ataxia who did not have common SCA mutations. METHODS: Four hundred seventy-six unrelated patients with suspected SCA underwent genetic testing for SCA 1, 2, 3, 6, 7, 8, 10, 12, 17, and DRPLA gene mutations. After excluding these SCA mutations and patients with paternal transmission history, 265 patients were tested for mtDNA mutations A3243G, A8344G, T8993G, T8993C, and POLG1 W748S and A467T mutations. RESULTS: No mtDNA A3243G, A8344G, T8993G, T8993C, or POLG1 W748S and A467T mutation was detected in any of the 265 ataxia patients, suggesting that the upper limit of the 95% confidence interval for the prevalence of these mitochondrial mutations in Chinese patients with adult-onset non-SCA ataxia is no higher than 1.1%. CONCLUSIONS: The mtDNA mutations A3243G, A8344G, T8993G, T8993C, or POLG1 W748S and A467T are very rare causes of adult-onset ataxia in Taiwan. Routine screening for these mutations in ataxia patients with Chinese origin is of limited clinical value.     

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.     

Parieto-occipital lobe epilepsy caused by a POLG1 compound heterozygous A467T/W748S genotype

We describe a 16-year-old woman with a rare POLG1 A467T/W748S genotype, with a wide range of neurological manifestations, including focal parieto-occipital lobe seizures, migraine headaches, cerebellar ataxia, sensory-motor axonal neuropathy, and impairment of visual perception and cognitive function. Treatment of epilepsy in patients with a POLG1 compound heterozygous A467T/W748S genotype is very challenging; the epilepsy may preferentially respond to sodium channel blockers. The POLG1-related syndrome has a variable clinical course, and disease morbidity and mortality may be correlated with the genotype.     

Status epilepticus in children with Alpers' disease caused by POLG1 mutations: EEG and MRI features

Purpose:   Refractory convulsive status epilepticus in infancy and childhood is a rare emergency situation. Metabolic disorders frequently underlie this condition, in particular Alpers' disease caused by POLG1 mutations. Status epilepticus may be the first symptom. A pathognomonic electroencephalography (EEG) signature may facilitate diagnosis of Alpers' disease and allow timely avoidance of valproic acid, which is contraindicated in this disorder because it may trigger fatal liver failure.
Patients:   We present five patients with Alpers' disease caused by mutations in POLG1 . Age of onset ranged from 7 months to 10 years. Three of the five children died after 3 to 12 months after onset of status epilepticus. Two of these had liver failure associated with use of valproic acid; liver transplantation in one child did not prevent a fatal neurologic outcome.
Results:   Convulsive status epilepticus was the first obvious sign of Alpers' disease in all children. All had focal clonic and complex-focal seizures; four of them developed epilepsia partialis continua. In four children, initial EEG showed unilateral occipital rhythmic high-amplitude delta with superimposed (poly)spikes (RHADS). Magnetic resonance imaging (MRI) revealed cortical and thalamic involvement in all, although there were only discrete abnormalities in one child. Metabolic investigations remained normal in three children.
Conclusion:   Alpers' disease is an important differential diagnosis in childhood refractory convulsive status epilepticus. Its EEG hallmark of RHADS is important for timely diagnosis, management, and counseling.     

Parkinsonism associated with the homozygous W748S mutation in the POLG1 gene

Parkinsonism has been described in patients with mutations in POLG1 gene. The W748S mutation is one of the most common mutations in this gene and it has been found to be a frequent cause of autosomal recessive ataxia in adults and the Alpers syndrome in children. We found the W748S mutation in a 65-year-old man with a late-onset syndrome consisting of ataxia, parkinsonism, ophthalmoplegia, peripheral neuropathy, and sensorineural hearing loss. Parkinsonism is one of the phenotypic features associated also with the W748S mutation.     

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.     

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     

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.     

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.     

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.     

Rapidly progressive neurological deterioration in a child with Alpers syndrome exhibiting a previously unremarkable brain MRI

Alpers syndrome is a fatal disorder due to mutations in the POLG gene encoding the catalytic subunit of mitochondrial DNA polymerase gamma (Pol gamma) involved in mitochondrial DNA (mtDNA) replication. We describe a case of Alpers syndrome due to POLG mutations, with rapidly progressive course, a fatal outcome, and an essentially normal brain MRI in the early oligo-symptomatic phase. Our observation suggests that Alpers syndrome should be considered even in patients with an initially unremarkable brain MRI. The patient was found to harbor the p.Q497H, p.W748S and p.E1143G mutations in cis on one allele, and a fourth mutation, the p.G848S on the other allele. Although the individual mutations detected in the presented case have been previously reported, the specific genotype formed by the particular combination of these is novel.     

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.     

Mitochondrial DNA depletion syndrome due to mutations in the RRM2B gene

Mitochondrial DNA depletion syndrome (MDS) is characterized by a reduction in mtDNA copy number and has been associated with mutations in eight nuclear genes, including enzymes involved in mitochondrial nucleotide metabolism (POLG, TK2, DGUOK, SUCLA2, SUCLG1, PEO1) and MPV17. Recently, mutations in the RRM2B gene, encoding the p53-controlled ribonucleotide reductase subunit, have been described in seven infants from four families, who presented with various combinations of hypotonia, tubulopathy, seizures, respiratory distress, diarrhea, and lactic acidosis. All children died before 4 months of age. We sequenced the RRM2B gene in three unrelated cases with unexplained severe mtDNA depletion. The first patient developed intractable diarrhea, profound weakness, respiratory distress, and died at 3 months. The other two unrelated patients had a much milder phenotype and are still alive at ages 27 and 36 months. All three patients had lactic acidosis and severe depletion of mtDNA in muscle. Muscle histochemistry showed RRF and COX deficiency. Sequencing the RRM2B gene revealed three missense mutations and two single nucleotide deletions in exons 6, 8, and 9, confirming that RRM2B mutations are important causes of MDS and that the clinical phenotype is heterogeneous and not invariably fatal in infancy.     

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.     

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 approximately 10% 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.     

Mutations of ANT1, Twinkle,and POLG1 in sporadic progressive external ophthalmoplegia (PEO)

To verify the impact of mutations in ANT1, Twinkle, and POLG1 genes in sporadic progressive external ophthalmoplegia associated with multiple mitochondrial DNA (mtDNA) deletions, DNA samples from 15 Italian and 12 British patients were screened. Mutations in ANT1 were found in one patient, in Twinkle in two patients, and in POLG1 in seven patients. Irrespective of the inheritance mode, screening of these genes should be performed in all patients with progressive external ophthalmoplegia with multiple mtDNA deletions.     

Clonally expanded mitochondrial DNA mutations in epileptic individuals with mutated DNA polymerase gamma

The instability of the mitochondrial genome in individuals harboring pathogenic mutations in the catalytic subunit of mitochondrial DNA (mtDNA) polymerase gamma (POLG) is well recognized, but the underlying molecular mechanisms remain to be elucidated. In 5 pediatric patients with severe myoclonic epilepsy and valproic acid-induced liver failure, we identified 1 novel and 4 previously described pathogenic mutations in the linker region of this enzyme. Although muscle biopsies in these patients showed unremarkable histologic features, postmortem liver tissue available from 1 individual exhibited large cytochrome c oxidase-negative areas. These cytochrome c oxidase-negative areas contained 4-fold less mtDNA than cytochrome c oxidase-positive areas. Decreased copy numbers of mtDNA were observed not only in the liver, skeletal muscle, and brain but also in blood samples from all patients. There were also patient-specific patterns of multiple mtDNA deletions in different tissues, and in 2 patients, there were clonally expanded mtDNA point mutations. The low amount of deleted mtDNA molecules makes it unlikely that the deletions contribute significantly to the general biochemical defect. The clonal expansion of a few individual-specific deletions and point mutations indicates an accelerated segregation of early mtDNA mutations that likely are a consequence of low mtDNA copy numbers. Moreover, these results suggest a potential diagnostic approach for identifying mtDNA depletion in patients.