The spectrum of clinical disease caused by the A467T and W748S POLG mutations: A study of 26 cases

Department of Neurology, Institute of Clinical Medicine, University of Bergen and Haukeland University Hospital Bergen, Norway.
Brain (Impact Factor: 9.2). 08/2006; 129(Pt 7):1685-92. DOI: 10.1093/brain/awl097
Source: PubMed


We studied 26 patients belonging to 20 families with a disorder caused by mutations in the POLG gene. The patients were homozygous for 1399 G/A or 2243 G/C (giving the amino acid changes A467T and W748S, respectively) or compound heterozygotes for these two mutations. Irrespective of genotype, the patients exhibited a progressive neurological disorder usually starting in their teens and characterized by epilepsy, headache, ataxia, neuropathy, myoclonus and late onset ophthalmoplegia. However, major differences in survival were seen depending on genotype, with compound heterozygotes having a significantly shorter survival time than patients homozygous either for the A467T or W748S (P = 0.006). Epilepsy occurred in 22 of the 26 patients and in the majority of these there was an occipital EEG focus. Episodes of both generalized and focal motor status epilepticus were common and highly resistant to treatment, even with generalized anaesthesia. Status epilepticus was the recorded cause of death in 9 of 11 patients. Liver failure was the sole cause of death in two patients and evolved terminally in six others, all but one of whom were being treated with sodium valproate. Two patients underwent liver transplantation, but only one survived. Delayed psychomotor development and subsequent cognitive decline also occurs. This study demonstrates the clinical spectrum of a disorder that combines features of Alpers' syndrome and a later onset mitochondrial spinocerebellar ataxia with epilepsy and headache. Patients with this disorder are at high risk of death from status epilepticus and from liver failure, if exposed to sodium valproate. Each mutation appears capable of producing a disorder that is recessively inherited, although we also find evidence in one patient suggesting that heterozygotes may manifest. Compound heterozygotes have a significantly more severe phenotype raising the possibility of a dominant negative effect.

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Available from: Charalampos Tzoulis
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    • "Three patients had infantile onset encephalopathy with severe epilepsy and strokelike episodes. The clinical and genetic features of all patients are summarized in Table 1 and have been described elsewhere in detail.8,9 "
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    ABSTRACT: Objective Polymerase gamma (POLG) mutations are a common cause of mitochondrial disease and have also been linked to neurodegeneration and aging. We studied the molecular mechanisms underlying POLG-related neurodegeneration using post-mortem tissue from a large number of patients.Methods Clinical information was available from all subjects. Formalin-fixed and frozen brain tissue from 15 patients and 23 controls were studied employing a combination of histopathology, immunohistochemistry and molecular studies of microdissected neurons.ResultsThe primary consequence of POLG mutation in neurons is mitochondrial DNA depletion. This was already present in infants with little evidence of neuronal loss or mitochondrial dysfunction. With longer disease duration, we found an additional, progressive accumulation of mitochondrial DNA deletions and point mutations accompanied by increasing numbers of complex I deficient neurons. Progressive neurodegeneration primarily affected the cerebellar systems and dopaminergic cells of the substantia nigra. Superimposed on this chronic process were acute, focal cortical lesions that correlated with epileptogenic foci and which showed massive neuronal loss.InterpretationPOLG mutations appear to compromise neuronal respiration via a combination of early and stable depletion and a progressive somatic mutagenesis of the mitochondrial genome. This leads to two distinct, but overlapping biological processes: a chronic neurodegeneration reflected clinically by progressive ataxia and cognitive impairment, and an acute focal neuronal necrosis that appears related to the presence of epileptic seizures. Our findings offer an explanation of the acute-on-chronic clinical course of this common mitochondrial encephalopathy. ANN NEUROL 2014. © 2014 American Neurological Association
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    • "Mutations in POLG, the gene encoding the catalytic POLG subunit, are known to cause mitochondrial DNA deletions in neurons (Reeve et al., 2008) and respiratory complex I deficiency (Lax et al., 2012a). Clinically, POLG mutations cause a wide spectrum of disease including myopathy, ataxia, epileptic encephalopathy and liver disease (Van Goethem et al., 2001; Ferrari et al., 2005; Hakonen et al., 2005; Tzoulis et al., 2006). Parkinsonism, including levodoparesponsive Parkinson's disease, has been described (Cottrell et al., 2000; Luoma et al., 2004; Davidzon et al., 2006; Reeve et al., 2012) but is an uncommon manifestation. "
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    ABSTRACT: The role of mitochondria in the pathogenesis of neurodegeneration is an area of intense study. It is known that defects in proteins involved in mitochondrial quality control can cause Parkinson's disease, and there is increasing evidence linking mitochondrial dysfunction, and particularly mitochondrial DNA abnormalities, to neuronal loss in the substantia nigra. Mutations in the catalytic subunit of polymerase gamma are among the most common causes of mitochondrial disease and owing to its role in mitochondrial DNA homeostasis, polymerase gamma defects are often considered a paradigm for mitochondrial diseases generally. Yet, despite this, parkinsonism is uncommon with polymerase gamma defects. In this study, we investigated structural and functional changes in the substantia nigra of 11 patients with polymerase gamma encephalopathy. We characterized the mitochondrial DNA abnormalities and examined the respiratory chain in neurons of the substantia nigra. We also investigated nigrostriatal integrity and function using a combination of post-mortem and in vivo functional studies with dopamine transporter imaging and positron emission tomography. At the cellular level, dopaminergic nigral neurons of patients with polymerase gamma encephalopathy contained a significantly lower copy number of mitochondrial DNA (depletion) and higher levels of deletions than normal control subjects. A selective and progressive complex I deficiency was seen and this was associated with a severe and progressive loss of the dopaminergic neurons of the pars compacta. Dopamine transporter imaging and positron emission tomography showed that the degree of nigral neuronal loss and nigrostriatal depletion were severe and appeared greater even than that seen in idiopathic Parkinson's disease. Despite this, however, none of our patients showed any signs of parkinsonism. The additional presence of both thalamic and cerebellar dysfunction in our patients suggested that these may play a role in counteracting the effects of basal ganglia dysfunction and prevent the development of clinical parkinsonism.
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    • "The predominance of liver involvement in the younger child has led to liver transplantation in some patients, but the outcome has been poor and is not recommended for POLG-related disease because of neurological involvement (Tzoulis et al. 2006; Wong et al. 2008). Even though seizures in POLG patients can be resistant to anti-seizure medications, the use of valproic acid should be avoided, because of induction of liver failure (Horvath et al. 2006; Tzoulis et al. 2006; Chinnery and Zeviani 2008). With a paucity of options known to treat POLG-related mitochondrial diseases, it is important to use basic research to understand genetic and environmental changes that can suppress the molecular phenotype related to POLG mutations. "
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    ABSTRACT: The inability to replicate mitochondrial genomes (mtDNA) by the mitochondrial DNA polymerase (pol γ) leads to a subset of mitochondrial diseases. Many mutations in POLG, the gene that encodes pol γ, have been associated with mitochondrial diseases such as myocerebrohepatopathy spectrum (MCHS) disorders, Alpers-Huttenlocher syndrome, myoclonic epilepsy myopathy sensory ataxia (MEMSA), ataxia neuropathy spectrum (ANS), and progressive external ophthalmoplegia (PEO). This chapter explores five important topics in POLG-related disease: (1) clinical symptoms that identify and distinguish POLG-related diseases, (2) molecular characterization of defects in polymerase activity by POLG disease variants, (3) the importance of holoenzyme formation in disease presentation, (4) the role of pol γ exonuclease activity and mutagenesis in disease and aging, and (5) novel approaches to therapy and avoidance of toxicity based on primary research in pol γ replication.
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