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

Mitochondrial DNA replication and repair defects: Clinical phenotypes and therapeutic interventions

Mitochondria is a unique cellular organelle involved in multiple cellular processes and is critical for maintaining cellular homeostasis. This semi-autonomous organelle contains its circular genome - mtDNA (mitochondrial DNA), that undergoes continuous cycles of replication and repair to maintain the mitochondrial genome integrity. The majority of the mitochondrial genes, including mitochondrial replisome and repair genes, are nuclear-encoded. Although the repair machinery of mitochondria is quite efficient, the mitochondrial genome is highly susceptible to oxidative damage and other types of exogenous and endogenous agent-induced DNA damage, due to the absence of protective histones and their proximity to the main ROS production sites. Mutations in replication and repair genes of mitochondria can result in mtDNA depletion and deletions subsequently leading to mitochondrial genome instability. The combined action of mutations and deletions can result in compromised mitochondrial genome maintenance and lead to various mitochondrial disorders. Here, we review the mechanism of mitochondrial DNA replication and repair process, key proteins involved, and their altered function in mitochondrial disorders. The focus of this review will be on the key genes of mitochondrial DNA replication and repair machinery and the clinical phenotypes associated with mutations in these genes.

Compound Heterozygosity for a Novel Frameshift Variant Causing Fatal Infantile Liver Failure and Genotype-Phenotype Correlation of POLG c.3286C>T Variant

A variant in the POLG gene is the leading cause of a heterogeneous group of mitochondrial disorders. No definitive treatment is currently available. Prenatal and newborn screening have the potential to improve clinical outcome of patients affected with POLG-related disorders. We reported a 4-month-old infant who presented with developmental delay, fever, and diarrhea. Within two weeks after hospital admission, the patient developed hepatic failure and died. Liver necropsy demonstrated an extensive loss of hepatocytes and bile duct proliferations. Trio-whole exome sequencing identified that the patient was compound heterozygous for a novel frameshift variant c.3102delG (p.Lys1035Serfs*59) and a common variant c.3286C>T (p.Arg1096Cys) in POLG (NM_002693.3) inherited from the mother and father, respectively. The c.3102delG (p.Lys1035Serfs*59) was a null variant and classified as pathogenic according to the American College of Medical Genetics and Genomics Standards and Guidelines. Prenatal genetic screenings using rapid whole exome sequencing successfully detected the heterozygous c.3286C>T variant in the following pregnancy and the normal alleles in the other one. Both children had been healthy. We reviewed all 34 cases identified with the POLG c.3286C>T variant and found that all 15 compound heterozygous cases had two missense variants except our patient who had the truncating variant and showed the earliest disease onset, rapid deterioration, and the youngest death. All homozygous cases had disease onset before age 2 and developed seizure. Here, we report a novel POLG variant expanding the genotypic spectrum, demonstrate the successful use of exome sequencing for prenatal and neonatal screenings of POLG-related disorders, and show the genotype-phenotype correlation of the common c.3286C>T variant.

MRI findings in SANDO variety of the ataxia-neuropathy spectrum with a novel mutation in POLG (c.3287G>T): A case report

Sensory ataxic neuropathy with dysarthria and ophthalmoparesis (SANDO) is an adult onset sensory ataxic neuropathy, dysarthria and chronic progressive external ophthalmoplegia associated with mutations in POLG1. We report a 38-year-old woman with a history of progressive gait instability and bilateral ptosis. Neurological examination found ataxia, ophthalmoplegia, and dysarthria. MRI showed bilateral thalamic and cerebellar lesions. A POLG related disorder was suspected and after DNA sequencing a SANDO with a novel mutation in POLG was confirmed.

Roles of the mitochondrial replisome in mitochondrial DNA deletion formation

Mitochondrial DNA (mtDNA) deletions are a common cause of human mitochondrial diseases. Mutations in the genes encoding components of the mitochondrial replisome, such as DNA polymerase gamma (Pol γ) and the mtDNA helicase Twinkle, have been associated with the accumulation of such deletions and the development of pathological conditions in humans. Recently, we demonstrated that changes in the level of wild-type Twinkle promote mtDNA deletions, which implies that not only mutations in, but also dysregulation of the stoichiometry between the replisome components is potentially pathogenic. The mechanism(s) by which alterations to the replisome function generate mtDNA deletions is(are) currently under debate. It is commonly accepted that stalling of the replication fork at sites likely to form secondary structures precedes the deletion formation. The secondary structural elements can be bypassed by the replication-slippage mechanism. Otherwise, stalling of the replication fork can generate single- and double-strand breaks, which can be repaired through recombination leading to the elimination of segments between the recombination sites. Here, we discuss aberrances of the replisome in the context of the two debated outcomes, and suggest new mechanistic explanations based on replication restart and template switching that could account for all the deletion types reported for patients.

Deep sequencing of mitochondrial DNA and characterization of a novel POLG mutation in a patient with arPEO

Objective

To determine the pathogenicity of a novel POLG mutation in a man with late-onset autosomal recessive progressive external ophthalmoplegia using clinical, molecular, and biochemical analyses.

Methods

A multipronged approach with detailed neurologic examinations, muscle biopsy analyses, molecular genetic studies, and in vitro biochemical characterization.

Results

The patient had slowly progressive bilateral ptosis and severely reduced horizontal and vertical gaze. Muscle biopsy showed slight variability in muscle fiber size, scattered ragged red fibers, and partial cytochrome c oxidase deficiency. Biallelic mutations were identified in the POLG gene encoding the catalytic A subunit of POLγ. One allele carried a novel mutation in the exonuclease domain (c.590T>C; p.F197S), and the other had a previously characterized null mutation in the polymerase domain (c.2740A>C; p.T914P). Biochemical characterization revealed that the novel F197S mutant protein had reduced exonuclease and DNA polymerase activities and confirmed that T914P was inactive. By deep sequencing of mitochondrial DNA (mtDNA) extracted from muscle, multiple large-scale rearrangements were mapped and quantified.

Conclusions

The patient's phenotype was caused by biallelic POLG mutations, resulting in one inactive POLγA protein (T914P) and one with decreased polymerase and exonuclease activity (F197S). The reduction in polymerase activity explains the presence of multiple pathogenic large-scale deletions in the patient's mtDNA.

Genotypes of chronic progressive external ophthalmoplegia in a large adult-onset cohort

Chronic progressive external ophthalmoplegia (CPEO) is a common presentation of mitochondrial disease. We performed a retrospective evaluation of the molecular genetic testing and genotype-phenotype correlations in a large cohort of adult-onset CPEO patients (N = 111). One hundred percent of patients tested had at least one mitochondrial DNA (mtDNA) deletion. Genetic testing of nuclear genes encoding mitochondrial proteins identified pathogenic/likely pathogenic variants likely to be associated with CPEO in 7.6% of patients. As expected, the nuclear gene most associated with DNA variation was POLG. A single likely pathogenic mitochondrial DNA variant (m.12278T>C) was identified in two unrelated patients. No significant differences were noted in the clinical phenotypes of patients with pathogenic or likely pathogenic nuclear variants in comparison to those with negative nuclear gene testing. Analysis of deletion size and heteroplasmy in muscle-derived mtDNA showed significant correlations with age of symptom onset but not disease severity (number of canonical CPEO features). Results suggest that smaller mtDNA deletions (p = 0.0127, r² = 0.1201) and higher heteroplasmy of single mtDNA deletions (p = 0.0112, r² = 0.2483) are associated with an earlier age of onset in CPEO patients.

Phenotypic spectrum of SLC25A4 mutations

There is no comprehensive overview concerning the phenotypic variability in patients carrying SLC25A4 mutations available. Therefore, the aim of the present review was to summarise and discuss recent findings concerning the clinical presentation and phenotypic heterogeneity of SLC25A4 mutations. The study was conducted by systematically reviewing the literature using the search terms 'mitochondrial', "myopathy', 'nuclear DNA', 'mitochondrial DNA', in combination with 'SLC25A4' or 'AAC1'. The results indicated that the phenotypic heterogeneity in patients carrying a SLC25A4 mutation is broader than so far anticipated. Patients carrying a SLC25A4 mutation not only manifest as encephalo-myo-cardiomyopathy but also with scoliosis, cataract, depression, headache, hydrocephalus or arterial hypertension. SLC25A4 mutations may result in mtDNA depletion or multiple mitochondrial (mt)DNA deletions. SLC25A4-associated mtDNA depletion presents with the more severe phenotype and the worse outcome than patients with multiple mtDNA deletions. Depletion syndrome due to SLC25A4 mutations is associated with congenital respiratory insufficiency requiring mechanical ventilation with poor prognosis in the majority of the cases. Mutations in the SLC25A4 gene manifest phenotypically with multiorgan abnormalities in addition to encephalo-myo-cardiomyopathy. SLC25A4 mutations, causing mtDNA depletion, present with a more severe phenotype, including respiratory insufficiency and more widespread cerebral disease than mutations causing multiple mtDNA deletions.

DNA Polymerase Beta Participates in Mitochondrial DNA Repair

We have detected DNA polymerase beta (Polβ), known as a key nuclear base excision repair (BER) protein, in mitochondrial protein extracts derived from mammalian tissue and cells. Manipulation of the N-terminal sequence affected the amount of Polβ in the mitochondria. Using Polβ fragments, mitochondrion-specific protein partners were identified, with the interactors functioning mainly in DNA maintenance and mitochondrial import. Of particular interest was the identification of the proteins TWINKLE, SSBP1, and TFAM, all of which are mitochondrion-specific DNA effectors and are known to function in the nucleoid. Polβ directly interacted functionally with the mitochondrial helicase TWINKLE. Human kidney cells with Polβ knockout (KO) had higher endogenous mitochondrial DNA (mtDNA) damage. Mitochondrial extracts derived from heterozygous Polβ mouse tissue and KO cells had lower nucleotide incorporation activity. Mouse-derived Polβ null fibroblasts had severely affected metabolic parameters. Indeed, gene knockout of Polβ caused mitochondrial dysfunction, including reduced membrane potential and mitochondrial content. We show that Polβ is a mitochondrial polymerase involved in mtDNA maintenance and is required for mitochondrial homeostasis.

MtDNA-maintenance defects: syndromes and genes

A large group of mitochondrial disorders, ranging from early-onset pediatric encephalopathic syndromes to late-onset myopathy with chronic progressive external ophthalmoplegia (CPEOs), are inherited as Mendelian disorders characterized by disturbed mitochondrial DNA (mtDNA) maintenance. These errors of nuclear-mitochondrial intergenomic signaling may lead to mtDNA depletion, accumulation of mtDNA multiple deletions, or both, in critical tissues. The genes involved encode proteins belonging to at least three pathways: mtDNA replication and maintenance, nucleotide supply and balance, and mitochondrial dynamics and quality control. In most cases, allelic mutations in these genes may lead to profoundly different phenotypes associated with either mtDNA depletion or multiple deletions.

Synergistic Effects of the in cis T251I and P587L Mitochondrial DNA Polymerase γ Disease Mutations

Human mitochondrial DNA (mtDNA) polymerase γ (Pol γ) is the only polymerase known to replicate the mitochondrial genome. The Pol γ holoenzyme consists of the p140 catalytic subunit (POLG) and the p55 homodimeric accessory subunit (POLG2), which enhances binding of Pol γ to DNA and promotes processivity of the holoenzyme. Mutations within POLG impede maintenance of mtDNA and cause mitochondrial diseases. Two common POLG mutations usually found in cis in patients primarily with progressive external ophthalmoplegia generate T251I and P587L amino acid substitutions. To determine whether T251I or P587L is the primary pathogenic allele or whether both substitutions are required to cause disease, we overproduced and purified WT, T251I, P587L, and T251I + P587L double variant forms of recombinant Pol γ. Biochemical characterization of these variants revealed impaired DNA binding affinity, reduced thermostability, diminished exonuclease activity, defective catalytic activity, and compromised DNA processivity, even in the presence of the p55 accessory subunit. However, physical association with p55 was unperturbed, suggesting intersubunit affinities similar to WT. Notably, although the single mutants were similarly impaired, a dramatic synergistic effect was found for the double mutant across all parameters. In conclusion, our analyses suggest that individually both T251I and P587L substitutions functionally impair Pol γ, with greater pathogenicity predicted for the single P587L variant. Combining T251I and P587L induces extreme thermal lability and leads to synergistic nucleotide and DNA binding defects, which severely impair catalytic activity and correlate with presentation of disease in patients.

The genetics and pathology of mitochondrial disease

Mitochondria are double-membrane-bound organelles that are present in all nucleated eukaryotic cells and are responsible for the production of cellular energy in the form of ATP. Mitochondrial function is under dual genetic control - the 16.6-kb mitochondrial genome, with only 37 genes, and the nuclear genome, which encodes the remaining ∼1300 proteins of the mitoproteome. Mitochondrial dysfunction can arise because of defects in either mitochondrial DNA or nuclear mitochondrial genes, and can present in childhood or adulthood in association with vast clinical heterogeneity, with symptoms affecting a single organ or tissue, or multisystem involvement. There is no cure for mitochondrial disease for the vast majority of mitochondrial disease patients, and a genetic diagnosis is therefore crucial for genetic counselling and recurrence risk calculation, and can impact on the clinical management of affected patients. Next-generation sequencing strategies are proving pivotal in the discovery of new disease genes and the diagnosis of clinically affected patients; mutations in >250 genes have now been shown to cause mitochondrial disease, and the biochemical, histochemical, immunocytochemical and neuropathological characterization of these patients has led to improved diagnostic testing strategies and novel diagnostic techniques. This review focuses on the current genetic landscape associated with mitochondrial disease, before focusing on advances in studying associated mitochondrial pathology in two, clinically relevant organs - skeletal muscle and brain. © 2016 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Novel mutation in C10orf2 associated with multiple mtDNA deletions, chronic progressive external ophthalmoplegia and premature aging

Chronic progressive external ophthalmoplegia (CPEO) is caused by defects in both mitochondrial and nuclear genes, however, the causal genetic factors in large number of patients remains undetermined. Therefore, our aim was to screen 12 unrelated patients with CPEO for mutation/multiple deletions in mtDNA and mutations in the coding regions of C10orf2, which is essential for mtDNA replication. Histopathological study of muscle biopsy revealed cytochrome c oxidase-deficient fibers and ragged blue fibers in all the patients. Long-range PCR of DNA from skeletal muscle revealed multiple mtDNA deletions in all the 12 patients. Further, sequencing coding regions of C10orf2 revealed three variants in three different patients, of which two were novel (c.1964G>A/p.G655D; c.204G>A/p.G68G) variants and one was reported (c.1052A>G/p. N351S). Sequencing of other nuclear genes that are associated with CPEO and multiple mtDNA deletions, such as; POLG1, POLG2, TK2, ANT1, DGUOK, MPV17 and RRM2B did not reveal any pathogenic mutation in patients with C10orf2 mutation. Since in silico analyses revealed p.G655D could be a potentially pathogenic and it was absent in 200 healthy controls, p.G655D could be the causative factor for CPEO. Therefore, we suggest that C10orf2 gene should be screened in CPEO individuals with multiple mtDNA deletions, which might help in prognosis of this disease and appropriate genetic counseling.

Large-scale mitochondrial DNA deletion underlying familial multiple system atrophy of the cerebellar subtype

A family with mitochondrial inheritance of multiple system atrophy of the cerebellar subtype. MRI brain shows significant cerebellar atrophy with mild pontine atrophy and the classical hot cross bun sign in Pons. The muscle biopsy was indicative of mitochondrial myopathy. Mitochondrial DNA analysis revealed a low‐level large mtDNA deletion, m.3264_1607del12806 bp.

Sporadic PEO caused by a novel POLG variation and a Twinkle mutation: digenic inheritance?

Progressive external ophthalmoplegia (PEO) with multiple deletions of mitochondrial DNA (mtDNA) is associated with several mutations in nuclear genes. They include POLG, POLG2, ANT1, C10orf2/Twinkle, and OPA1. However, digenic inheritance in mitochondrial disorders has been documented in a few cases over the years. Here we describe an 80-year-old man with sporadic PEO associated with mtDNA deletions. Sequencing of the POLG revealed a novel heterozygous mutation (c.2831A>G; p.Glu944Gly), predicted in silico as damaging, in the patient who also carried a heterozygous mutation in C10orf2/Twinkle (c.1142T>C; p.Leu381Pro). This case provides a second report of a PEO with different mutations in the POLG and C10orf2/Twinkle genes, supporting the hypothesis that the PEO phenotype can be determined by the co-existence of two abnormalities in separate genes, both involved in the maintenance and stability of mtDNA. Finally, this study expands the spectrum of POLG mutations and highlights the need to sequence the whole set of nuclear genes associated with PEO and multiple mtDNA deletions.

Protecting the mitochondrial powerhouse

Mitochondria are the oxygen-consuming power plants of cells. They provide a critical milieu for the synthesis of many essential molecules and allow for highly efficient energy production through oxidative phosphorylation. The use of oxygen is, however, a double-edged sword that on the one hand supplies ATP for cellular survival, and on the other leads to the formation of damaging reactive oxygen species (ROS). Different quality control pathways maintain mitochondria function including mitochondrial DNA (mtDNA) replication and repair, fusion-fission dynamics, free radical scavenging, and mitophagy. Further, failure of these pathways may lead to human disease. We review these pathways and propose a strategy towards a treatment for these often untreatable disorders.

Mitochondrial Diseases Part III: Therapeutic interventions in mouse models of OXPHOS deficiencies

Mitochondrial defects are the cause of numerous disorders affecting the oxidative phosphorylation system (OXPHOS) in humans leading predominantly to neurological and muscular degeneration. The molecular origin, manifestations, and progression of mitochondrial diseases have a broad spectrum, which makes very challenging to find a globally effective therapy. The study of the molecular mechanisms underlying the mitochondrial dysfunction indicates that there is a wide range of pathways, enzymes and molecules that can be potentially targeted for therapeutic purposes. Therefore, focusing on the pathology of the disease is essential to design new treatments. In this review, we will summarize and discuss the different therapeutic interventions tested in some mouse models of mitochondrial diseases emphasizing the molecular mechanisms of action and their potential applications.

Peripheral neuropathy predicts nuclear gene defect in patients with mitochondrial ophthalmoplegia

Mitochondrial ophthalmoplegia is a genetically heterogeneous disorder. Horga et al. investigate whether peripheral neuropathy can predict the underlying genetic defect in patients with progressive external ophthalmoplegia. Results indicate that neuropathy is highly predictive of a nuclear DNA defect and that it is rarely associated with single mitochondrial DNA deletions.

Progressive external ophthalmoplegia is a common clinical feature in mitochondrial disease caused by nuclear DNA defects and single, large-scale mitochondrial DNA deletions and is less frequently associated with point mutations of mitochondrial DNA. Peripheral neuropathy is also a frequent manifestation of mitochondrial disease, although its prevalence and characteristics varies considerably among the different syndromes and genetic aetiologies. Based on clinical observations, we systematically investigated whether the presence of peripheral neuropathy could predict the underlying genetic defect in patients with progressive external ophthalmoplegia. We analysed detailed demographic, clinical and neurophysiological data from 116 patients with genetically-defined mitochondrial disease and progressive external ophthalmoplegia. Seventy-eight patients (67%) had a single mitochondrial DNA deletion, 12 (10%) had a point mutation of mitochondrial DNA and 26 (22%) had mutations in either POLG, C10orf2 or RRM2B, or had multiple mitochondrial DNA deletions in muscle without an identified nuclear gene defect. Seventy-seven patients had neurophysiological studies; of these, 16 patients (21%) had a large-fibre peripheral neuropathy. The prevalence of peripheral neuropathy was significantly lower in patients with a single mitochondrial DNA deletion (2%) as compared to those with a point mutation of mitochondrial DNA or with a nuclear DNA defect (44% and 52%, respectively; P < 0.001). Univariate analyses revealed significant differences in the distribution of other clinical features between genotypes, including age at disease onset, gender, family history, progressive external ophthalmoplegia at clinical presentation, hearing loss, pigmentary retinopathy and extrapyramidal features. However, binomial logistic regression analysis identified peripheral neuropathy as the only independent predictor associated with a nuclear DNA defect (P = 0.002; odds ratio 8.43, 95% confidence interval 2.24–31.76). Multinomial logistic regression analysis identified peripheral neuropathy, family history and hearing loss as significant predictors of the genotype, and the same three variables showed the highest performance in genotype classification in a decision tree analysis. Of these variables, peripheral neuropathy had the highest specificity (91%), negative predictive value (83%) and positive likelihood ratio (5.87) for the diagnosis of a nuclear DNA defect. These results indicate that peripheral neuropathy is a rare finding in patients with single mitochondrial DNA deletions but that it is highly predictive of an underlying nuclear DNA defect. This observation may facilitate the development of diagnostic algorithms. We suggest that nuclear gene testing may enable a more rapid diagnosis and avoid muscle biopsy in patients with progressive external ophthalmoplegia and peripheral neuropathy.

Mitochondrial DNA disease-molecular insights and potential routes to a cure

Mitochondrial DNA diseases are common neurological conditions caused by mutations in the mitochondrial genome or nuclear genes responsible for its maintenance. Current treatments for these disorders are focussed on the management of the symptoms, rather than the correction of biochemical defects caused by the mutation. This review focuses on the molecular effects of mutations, the symptoms they cause and current work focusing on the development of targeted treatments for mitochondrial DNA disease.

[A case of sensory ataxic neuropathy, dysarthria, and ophthalmoparesis with multiple mitochondrial DNA deletions]

We report the case of a 62-year-old man with sensory ataxic neuropathy, dysarthria, and ophthalmoparesis (SANDO). He developed gait disturbance at 54 years of age, muscle weakness at 56 years, and difficulty hearing at 58 years. His brother had muscle weakness in both legs from age 20 years, and was diagnosed with Charcot-Marie-Tooth disease because he had muscle weakness of the four extremities, decreased CMAP and SNAP amplitudes on peripheral nerve conduction tests, and loss of large myelinated fibers and onion-bulb formations on sural nerve biopsy. His brother died aged 46 years, but no accurate cause of death was identified. Neurological examination of the present patient revealed bilateral ptosis, external ophthalmoparesis, dysarthria, dysphagia, sensorineural hearing loss, mild weakness and atrophy of proximal muscles in all four limbs, severe sensory ataxia, and disturbance of deep sensation in his legs. He showed elevation of lactate and pyruvate levels in cerebrospinal fluid and serum. An aerobic exercise test disclosed a marked increase in lactate and pyruvate levels in serum. On nerve conduction study, amplitudes of CMAP and SNAP, and F wave-evoked frequency were decreased. Needle electromyography showed chronic neurogenic patterns with fibrillation potentials in the extremity muscles. Head MRI demonstrated T2 prolonged lesions in the bilateral basal ganglia, while brain MRS revealed a small lactate peak. Biopsy of his left lateral vastus muscle showed ragged-red fibers and group atrophy, and some muscle fibers had decreased cytochrome c activity. Left sural nerve biopsy revealed a marked loss of large myelinated fibers, and some onion-bulb formations. Genetic testing disclosed a large mtDNA deletion in the biopsied muscle. Among nuclear genes, we found point mutations in ANT-1 (exon 1 c.105G>A, 5' untranslated region) and POLG-1 (exon 4, c.1218G>A, p. and exon 23 c.3920C>T, p.A1217V). We diagnosed SANDO. This is the first case of SANDO with large mitochondrial DNA deletions in Japanese.

Mitochondrial genetics

INTRODUCTION

In the last 10 years the field of mitochondrial genetics has widened, shifting the focus from rare sporadic, metabolic disease to the effects of mitochondrial DNA (mtDNA) variation in a growing spectrum of human disease. The aim of this review is to guide the reader through some key concepts regarding mitochondria before introducing both classic and emerging mitochondrial disorders.

SOURCES OF DATA

In this article, a review of the current mitochondrial genetics literature was conducted using PubMed (http://www.ncbi.nlm.nih.gov/pubmed/). In addition, this review makes use of a growing number of publically available databases including MITOMAP, a human mitochondrial genome database (www.mitomap.org), the Human DNA polymerase Gamma Mutation Database (http://tools.niehs.nih.gov/polg/) and PhyloTree.org (www.phylotree.org), a repository of global mtDNA variation.

AREAS OF AGREEMENT

The disruption in cellular energy, resulting from defects in mtDNA or defects in the nuclear-encoded genes responsible for mitochondrial maintenance, manifests in a growing number of human diseases.

AREAS OF CONTROVERSY

The exact mechanisms which govern the inheritance of mtDNA are hotly debated.

GROWING POINTS

Although still in the early stages, the development of in vitro genetic manipulation could see an end to the inheritance of the most severe mtDNA disease.

Mitochondrial disease in childhood: nuclear encoded

Primary mitochondrial disorders are clinically and genetically heterogeneous, caused by an alteration(s) in either mitochondrial DNA or nuclear DNA, and affect the respiratory chain's ability to undergo oxidative phosphorylation, leading to decreased production of adenosine triphosphophate and subsequent energy failure. These disorders may present at any age, but children tend to have an acute onset of disease compared with subacute or slowly progressive presentation in adults. Varying organ involvement also contributes to the phenotypic spectrum seen in these disorders. The childhood presentation of primary mitochondrial disease is mainly due to nuclear DNA mutations, with mitochondrial DNA mutations being less frequent in childhood and more prominent in adulthood disease. The clinician should be aware of the pediatric presentation of mitochondrial disease and have an understanding of the myriad of nuclear genes responsible for these disorders. The nuclear genes can be best understood by utilizing a classification system of location and function within the mitochondria.

Update on nuclear mitochondrial genes and neurologic disorders

The majority of primary mitochondrial disorders are due to nuclear gene mutations, not aberrations within the mitochondrial genome. The nervous system is frequently involved due to its high-energy demands. Many nonspecific neurologic symptoms may be present in mitochondrial disease; however, there are well-recognized red flags that should alert the clinician to the possibility of mitochondrial disease. There is an ever increasing number of nuclear gene mutations discovered that play a role in primary mitochondrial disease and its neurologic symptomatology. Neurologists need to be aware of the wide neurologic presentation, the red-flag symptoms, and the nuclear gene mutations involved in the pathophysiology of mitochondrial disease to diagnose and manage this patient population.

Neurologic disorders due to mitochondrial DNA mutations

The mitochondrial DNA (mtDNA) is a compact genome inherited through the maternal lineage. Mutations in mtDNA lead to many of the earliest identified syndromic mitochondrial diseases and display a diverse range of age of onset, symptoms, and outcomes-from isolated childhood onset vision or hearing loss to a multisystemic neurodegenerative disorder with strokes, neuropathy, ophthalmoparesis, and epilepsy beginning at any age. As a heterogeneous group, mitochondrial diseases represent one of the most common metabolic disorders in children and adults, frequently seen by both pediatric and adult specialists. Although the myriad of diseases can make diagnosis seems daunting, the need for extensive supportive care and treatment (the latter for at least a select few mitochondrial disorders) and a rapid and accurate recognition of these disorders is necessary. Here, we provide a review of the most common mitochondrial disease syndromes due to mtDNA mutations.

Molecular diagnosis of infantile mitochondrial disease with targeted next-generation sequencing

Advances in next-generation sequencing (NGS) promise to facilitate diagnosis of inherited disorders. Although in research settings NGS has pinpointed causal alleles using segregation in large families, the key challenge for clinical diagnosis is application to single individuals. To explore its diagnostic use, we performed targeted NGS in 42 unrelated infants with clinical and biochemical evidence of mitochondrial oxidative phosphorylation disease. These devastating mitochondrial disorders are characterized by phenotypic and genetic heterogeneity, with more than 100 causal genes identified to date. We performed "MitoExome" sequencing of the mitochondrial DNA (mtDNA) and exons of ~1000 nuclear genes encoding mitochondrial proteins and prioritized rare mutations predicted to disrupt function. Because patients and healthy control individuals harbored a comparable number of such heterozygous alleles, we could not prioritize dominant-acting genes. However, patients showed a fivefold enrichment of genes with two such mutations that could underlie recessive disease. In total, 23 of 42 (55%) patients harbored such recessive genes or pathogenic mtDNA variants. Firm diagnoses were enabled in 10 patients (24%) who had mutations in genes previously linked to disease. Thirteen patients (31%) had mutations in nuclear genes not previously linked to disease. The pathogenicity of two such genes, NDUFB3 and AGK, was supported by complementation studies and evidence from multiple patients, respectively. The results underscore the potential and challenges of deploying NGS in clinical settings.

Phenotypic and genotypic variability in Alpers syndrome

BACKGROUND

Alpers syndrome is one of the most common phenotypes of mitochondrial disorders in early childhood and has been associated with pathogenic mutations in POLG1.

AIMS

To investigate the phenotypic-genotypic correlations in Alpers syndrome and to identify potential differences among patients with Alpers syndrome with or without pathogenic POLG1 mutations.

METHODS

Patients with the phenotype of Alpers syndrome who were referred to our pediatric hospital during 1984-2007 and were diagnosed with mitochondrial encephalomyopathy underwent further biochemical, morphological and genetic investigations.

RESULTS

A total of 19 patients were included in the study, of whom six had pathogenic POLG1 mutations including a novel mutation (c.907 G>A, p.Gly303Arg). Complete mtDNA sequencing in the subgroup without POLG1 mutations showed 5 novel and 5 very rare mtDNA variants considered as rare polymorphisms. Compared to POLG1(-) patients, the POLG1(+) patients more frequently had seizures at onset, which often became refractory. Ataxia and stroke-like episodes were much more common, while microcephaly and spasticity were encountered almost solely in the POLG1(-) group. Hepatic and ophthalmological involvement developed in 79% and 88% of patients, respectively. Most of the patients in both groups had predominant deficiency of complex I. In addition to the major degenerative changes in the cerebral cortex, the basal ganglia, thalamus and white matter were also involved to variable extent.

CONCLUSION

Alpers syndrome is a heterogeneous syndrome that should be considered in patients with early-onset progressive cortical encephalopathy regardless of liver involvement. The phenotype is different depending on the presence or absence of POLG1 mutations.

Identification of a novel Twinkle mutation in a family with infantile onset spinocerebellar ataxia by whole exome sequencing

Whole exome sequencing combined with homozygosity mapping comprises a genetic diagnostic tool to identify genetic defects in families with multiple affected members, compatible with presumed autosomal recessively inherited neurometabolic/neurogenetic disease. These tools were applied to a family with two individuals manifesting ataxia, associated with peripheral sensory neuropathy, athetosis, seizures, deafness, and ophthalmoplegia. A novel homozygous missense mutation c.1366C>G (L456V) in C10orf2 (the Twinkle gene) was identified, confirming infantile onset spinocerebellar ataxia in the probands. Signs in infantile onset spinocerebellar ataxia follow a fairly distinct pattern, affecting early development, followed by ataxia and loss of skills. However, this very rare disease was previously reported only in Finland. We suggest that infantile onset spinocerebellar ataxia should be more frequently considered in the differential diagnosis of neurometabolic diseases in childhood. Next-generation sequencing and its use along with homozygosity mapping offer highly promising techniques for molecular diagnosis, especially in small families affected with very rare neurometabolic disorders such as infantile onset spinocerebellar ataxia.

Progressive external ophthalmoplegia in southwestern Finland: a clinical and genetic study

BACKGROUND

Progressive external ophthalmoplegia (PEO) is a common phenotype of mitochondrial disease. Molecular etiologies include sporadic, large-scale deletions in mitochondrial DNA (mtDNA), multiple mtDNA deletions secondary to autosomal dominant or recessive mutations and mtDNA point mutations.

METHODS

We studied the prevalence and clinical and genetic characteristics of PEO in a defined population in southwestern Finland. A total of 620 patients were first identified from the patient registry at the Turku University Hospital over an 18-year period. The medical records of these patients were scrutinized, and those with clinical features compatible with PEO were ascertained.

RESULTS

We identified 10 patients with possible PEO. The patients were examined clinically, and DNA was analyzed for mtDNA deletions and for the m.3243A>G and m.8344A>G mtDNA point mutations. The ANT1, PEO1, POLG1 and POLG2 genes were sequenced. We confirmed the clinical diagnosis of PEO in 6 patients. Large-scale mtDNA deletions were detected in 3 out of 6 PEO patients and mutations in the POLG1 gene in 1 out of 6. We did not find any mutations in the ANT1, PEO1 or POLG2 genes.

CONCLUSIONS

Our results suggest that molecular investigation of patients with PEO, either sporadic or familial, should start with an analysis for mtDNA deletions, followed by an analysis of the POLG1 gene.

A novel POLG gene mutation in a patient with SANDO

The human mitochondrial genome is replicated by DNA polymerase γ, which is encoded by polymerase γ gene (POLG1) on chromosome 15q25. Patients with POLG1 mutations usually present as Alpers' syndrome or progressive external ophthalmoplegia. Our patient was a 48-year old woman with sensory ataxic neuropathy, dysarthria, ophthalmoplegia, and dysphagia. Sequence analysis revealed that she has two heterozygous missense mutations in the POLG1, a c.1774C>T substitution in exon 10, which results in a p.L591F amino acid change; and a c.3286C>T substitution in exon 21, which results in a p.R1096C amino acid change. The 1774C>T substitution is a novel mutation. Previously described adult patients with one mutation in exon 10 and the other in exon 21 of POLG1 had presented with progressive external ophthalmoplegia. We now describe a patient with mutations in the same exons but suffering from the more complex clinical syndrome of sensory ataxic neuropathy, dysarthria, ophthalmoplegia.

Mitochondrial disease in pregnancy: a systematic review

Mitochondrial diseases are heterogeneous in clinical presentation and genotype. The incidence of known pathogenic mitochondrial DNA mutations in the general population is 1 in 500. Little is known about the implications of pregnancy for women with mitochondrial disease. We undertook a systematic review of the literature on mitochondrial disease in pregnancy. Ten case reports were identified. The most common complications were threatened preterm labour (5 women) and preeclampsia (4 women). Two women experienced magnesium sulphate toxicity. Pregnancy had a varied effect on mitochondrial disease with some women being asymptomatic; others developing mild symptoms such as exercise intolerance or muscle weakness which resolved postnatally; and others developed more serious, persistent symptoms such as symptomatic Wolff-Parkinson-White syndrome, persistent paraesthesia and focal segmental glomerulosclerosis. Women with mitochondrial disease appear to be at increased risk of complications during pregnancy and labour but further prospective cohort studies are needed.

SLC25A4 and C10ORF2 Mutations in Autosomal Dominant Progressive External Ophthalmoplegia

Background and Purpose

Progressive external ophthalmoplegia (PEO) with Mendelian inheritance is a heterogeneous group of diseases associated with multiple deletions of mitochondrial DNA (mtDNA), which results from the disturbed replication and maintenance of mtDNA secondary to the mutations of nuclear genes including POLG, SLC25A4, C10ORF2, POLG2, OPA1, and RRM2B. The aim of this study was to identify the genetic defects underlying the pathology and clinical features in two Korean kindreds with autosomal dominant PEO.

Methods

Two pathologically proven PEO patients with a clear autosomal dominant pattern of inheritance were selected. To exclude a large-scale rearrangement, a long-range polymerase chain reaction (PCR) was performed using DNA extracted from biopsied muscle tissue taken from each patient. All coding regions and exon-intron boundaries of POLG, SLC25A4, C10ORF2, and POLG2 were amplified by PCR and directly sequenced.

Results

One patient showed multiple deletions of mtDNA on long-range PCR analysis, and two known heterozygous missense mutations in SLC25A4 (p.Asp104Gly) and C10ORF2 (p.Glu479Lys) were identified in each patient. The p.Asp104Gly mutation in SLC25A4 was identified in the patient with an early onset, slowly progressive, pure PEO phenotype, while the p.Glu479Lys mutation in C10ORF2 was identified in the other patient, with a late-onset disease and PEO plus phenotype.

Conclusions

Two mutations affecting nuclear genes were identified in Korean patients with autosomal dominant PEO. Further studies are necessary to identify the clear pathogenetic mechanisms and establish genotype-phenotype correlations in autosomal dominant PEO.

The importance of mitochondria in age-related and inherited eye disorders

Mitochondria are critical for ocular function as they represent the major source of a cell's supply of energy and play an important role in cell differentiation and survival. Mitochondrial dysfunction can occur as a result of inherited mitochondrial mutations (e.g. Leber's hereditary optic neuropathy and chronic progressive external ophthalmoplegia) or stochastic oxidative damage which leads to cumulative mitochondrial damage and is an important factor in age-related disorders (e.g. age-related macular degeneration, cataract and diabetic retinopathy). Mitochondrial DNA (mtDNA) instability is an important factor in mitochondrial impairment culminating in age-related changes and pathology, and in all regions of the eye mtDNA damage is increased as a consequence of aging and age-related disease. It is now apparent that the mitochondrial genome is a weak link in the defenses of ocular cells since it is susceptible to oxidative damage and it lacks some of the systems that protect the nuclear genome, such as nucleotide excision repair. Accumulation of mitochondrial mutations leads to cellular dysfunction and increased susceptibility to adverse events which contribute to the pathogenesis of numerous sporadic and chronic disorders in the eye.

Retinal function and structure in Ant1-deficient mice

PURPOSE

Mutations in ANT, a mitochondrial ATP transporter, are typically associated with myopathy. Because of the high metabolic demands of the retina, the authors examined whether elimination of the Ant1 isoform in a transgenic mouse affects retinal function or morphology.

METHODS

RT-PCR was used to confirm Ant1 expression in retinas of wild-type (WT) or Ant1(-/-) mice. Full-field ERGs were used to test retinal function under dark- and light-adapted conditions and the recovery of the photoresponse to a bright flash. Using histologic methods, the authors assessed the retinal location of ANT and ANT1-β-gal reporter protein, mitochondrial activity with cytochrome c oxidase (COX) and succinate dehydrogenase (SDH) staining, retinal layer thickness, and bipolar cell types using Chx10 and recoverin.

RESULTS

Ant1(-/-) mice had supernormal ERG b-waves under both dark- and light-adapted conditions. X-Gal staining was detected in a subset of cells within the inner retina. The following characteristics were normal in Ant1(-/-) mice compared with age-matched WT mice: recovery of the photoresponse, COX and SDH activity, retinal morphology, and bipolar cell morphology.

CONCLUSIONS

The presence of ANT1 in a subset of inner retinal cells accompanied by supernormal ERG responses suggests that ANT1 may be localized to hyperpolarizing bipolar cells. However, the immunohistochemical techniques used here did not show any differences in bipolar cells. Moderate functional changes coupled with a lack of detectable morphologic changes suggest that ANT1 is not essential for ATP transport in the retina.

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.

Sensory ataxic neuropathy with dysarthria and ophthalmoparesis (SANDO) in late life due to compound heterozygous POLG mutations

Missense mutations in the gene for polymerase gamma 1 (POLG1) cause a number of phenotypically heterogeneous mitochondrial diseases, most commonly progressive external ophthalmoplegia, and are characterized by the accumulation of multiple, large-scale deletions of mitochondrial DNA. The triad of sensory ataxic neuropathy, dysarthria, and ophthalmoparesis (SANDO) has been demonstrated in a small subset of patients with POLG1 mutations. We report a sporadic case of an 80-year-old compound heterozygote man who presented with SANDO and was found to have three known pathogenic mutations in the POLG1 gene (p.T251I/p.P587L/p.G848S). To our knowledge, none of these mutations have been demonstrated previously in SANDO. This patient's late presentation illustrates that a mitochondrial disorder should be considered regardless of age if the clinical symptoms warrant.

A cluster of pathogenic mutations in the 3'-5' exonuclease domain of DNA polymerase gamma defines a novel module coupling DNA synthesis and degradation

Mutations in DNA polymerase gamma (pol g), the unique replicase inside mitochondria, cause a broad and complex spectrum of diseases in human. We have used Mip1, the yeast pol g, as a model enzyme to characterize six pathogenic pol g mutations. Four mutations clustered in a highly conserved 3'-5' exonuclease module are localized in the DNA-binding channel in close vicinity to the polymerase domain. They result in an increased frequency of point mutations and high instability of the mitochondrial DNA (mtDNA) in yeast cells, and unexpectedly for mutator mutations in the exonuclease domain, they favour exonucleolysis versus polymerization. This trait is associated with highly decreased DNA-binding affinity and poorly processive DNA synthesis. Our data show for the first time that a 3'-5' exonuclease module of pol g plays a crucial role in the coordination of the polymerase and exonuclease functions and they strongly suggest that in patients the disease is not caused by defective proofreading but results from poor mtDNA replication generated by a severe imbalance between DNA synthesis and degradation.

The clinical, histochemical, and molecular spectrum of PEO1 (Twinkle)-linked adPEO

BACKGROUND

Mutations in the Twinkle (PEO1) gene are a recognized cause of autosomal dominant progressive external ophthalmoplegia (adPEO), resulting in the accumulation of multiple mitochondrial DNA (mtDNA) deletions and cytochrome c oxidase (COX)-deficient fibers in skeletal muscle secondary to a disorder of mtDNA maintenance. Patients typically present with isolated extraocular muscle involvement, with little apparent evidence of the clinical heterogeneity documented in other mtDNA maintenance disorders, in particular POLG-related disease.

METHODS

We reviewed the clinical, histochemical, and molecular genetics analysis of 33 unreported patients from 26 families together with all previous cases described in the literature to define the clinical phenotype associated with PEO1 mutations.

RESULTS

Ptosis and ophthalmoparesis were almost universal clinical features among this cohort, with 52% (17/33) reporting fatigue and 33% (11/33) having mild proximal myopathy. Features consistent with CNS involvement were rarely described; however, in 24% (8/33) of the patients, cardiac abnormalities were reported. Mitochondrial histochemical changes observed in muscle showed remarkable variability, as did the secondary mtDNA deletions, which in some patients were only detected by PCR-based assays and not Southern blotting. Moreover, we report 7 novel PEO1 variants.

CONCLUSIONS

Our data suggest a shared clinical phenotype with variable mild multiorgan involvement, and that the contribution of PEO1 mutations as a cause of adPEO may well be underestimated. Direct sequencing of the PEO1 gene should be considered in adPEO patients prior to muscle biopsy.

Rare autosomal dominant POLG1 mutation in a family with metabolic strokes, posterior column spinal degeneration, and multi-endocrine disease

DNA POLG is the only mitochondrial DNA polymerase and is encoded by nuclear DNA. Depending on the location and inheritance, mutations in POLG1, the catalytic subunit, can cause symptoms including severe infantile epilepsy, metabolic strokes, chronic ataxia, neuropathy, and ophthalmoplegia. We reviewed medical records and conducted extensive interviews with the family of identical twin probands with a mutation in the linker region of DNA polymerase gamma 1 (POLG1) (G517V) and discuss postmortem findings from their grandmother. Both twins developed type I diabetes, adrenal insufficiency, hypothyroidism, and psychiatric problems in addition to neurological difficulties including bilateral basal ganglia infarcts, headaches, and seizures. The maternal grandmother, now deceased, had psychosis and balance problems, and postmortem findings include lacunar infarcts in the basal ganglia (caudate nucleus, putamen, and globus pallidus) and posterior spinal column degeneration. We discuss novel aspects of their presentation and implications for practice.

Caenorhabditis elegans, a pluricellular model organism to screen new genes involved in mitochondrial genome maintenance

The inheritance of functional mitochondria depends on faithful replication and transmission of mitochondrial DNA (mtDNA). A large and heterogeneous group of human disorders is associated with mitochondrial genome quantitative and qualitative anomalies. Several nuclear genes have been shown to account for these severe OXPHOS disorders. However, in several cases, the disease-causing mutations still remain unknown. Caenorhabditis elegans has been largely used for studying various biological functions because this multicellular organism has short life cycle and is easy to grow in the laboratory. Mitochondrial functions are relatively well conserved between human and C.elegans, and heteroplasmy exists in this organism as in human. C. elegans therefore represents a useful tool for studying mtDNA maintenance. Suppression by RNA interference of genes involved in mtDNA replication such as polg-1, encoding the mitochondrial DNA polymerase, results in reduced mtDNA copy number but in a normal phenotype of the F1 worms. By combining RNAi of genes involved in mtDNA maintenance and EtBr exposure, we were able to reveal a strong and specific phenotype (developmental larval arrest) associated to a severe decrease of mtDNA copy number. Moreover, we tested and validated the screen efficiency for human orthologous genes encoding mitochondrial nucleoid proteins. This allowed us to identify several genes that seem to be closely related to mtDNA maintenance in C. elegans. This work reports a first step in the further development of a large-scale screening in C. elegans that should allow to identify new genes of mtDNA maintenance whose human orthologs will obviously constitute new candidate genes for patients with quantitative or qualitative mtDNA anomalies.

POLG1 p.R722H mutation associated with multiple mtDNA deletions and a neurological phenotype

Background

The c.2447G>A (p.R722H) mutation in the gene POLG1 of the catalytic subunit of human mitochondrial polymerase gamma has been previously found in a few occasions but its pathogenicity has remained uncertain. We set out to ascertain its contribution to neuromuscular disease.

Methods

Probands from two families with probable mitochondrial disease were examined clinically, muscle and buccal epithelial DNA were analyzed for mtDNA deletions, and the POLG1, POLG2, ANT1 and Twinkle genes were sequenced.

Results

An adult proband presented with progressive external ophthalmoplegia, sensorineural hearing impairment, diabetes mellitus, dysphagia, a limb myopathy and dementia. Brain MRI showed central and cortical atrophy, and ¹⁸F-deoxyglucose PET revealed reduced glucose uptake. Histochemical analysis of muscle disclosed ragged red fibers and cytochrome c oxidase-negative fibers. Electron microscopy showed subsarcolemmal aggregates of morphologically normal mitochondria. Multiple mtDNA deletions were found in the muscle, and sequencing of the POLG1 gene revealed a homozygous c.2447G>A (p.R722H) mutation. His two siblings were also homozygous with respect to the p.R722H mutation and presented with dementia and sensorineural hearing impairment. In another family the p.R722H mutation was found as compound heterozygosity with the common p.W748S mutation in two siblings with mental retardation, ptosis, epilepsy and psychiatric symptoms. The estimated carrier frequency of the p.R722H mutation was 1:135 in the Finnish population. No mutations in POLG2, ANT1 and Twinkle genes were found. Analysis of the POLG1 sequence by homology modeling supported the notion that the p.R722H mutation is pathogenic.

Conclusions

The recessive c.2447G>A (p.R722H) mutation in the linker region of the POLG1 gene is pathogenic for multiple mtDNA deletions in muscle and is associated with a late-onset neurological phenotype as a homozygous state. The onset of the disease can be earlier in compound heterozygotes.

The human mitochondrial replication fork in health and disease

Mitochondria are organelles whose main function is to generate power by oxidative phosphorylation. Some of the essential genes required for this energy production are encoded by the mitochondrial genome, a small circular double stranded DNA molecule. Human mtDNA is replicated by a specialized machinery distinct from the nuclear replisome. Defects in the mitochondrial replication machinery can lead to loss of genetic information by deletion and/or depletion of the mtDNA, which subsequently may cause disturbed oxidative phosphorylation and neuromuscular symptoms in patients. We discuss here the different components of the mitochondrial replication machinery and their role in disease. We also review the mode of mammalian mtDNA replication.

A novel POLG gene mutation in 4 children with Alpers-like hepatocerebral syndromes

OBJECTIVE

To describe a novel POLG missense mutation (c.3218C>T; p.P1073L) that, in association with 2 previously described mutations, caused an Alpers-like hepatocerebral syndrome in 4 children.

DESIGN

Genotype-phenotype correlation.

SETTING

Tertiary care universities.

PATIENTS

Four children, 2 related and 2 unrelated, with the novel p.P1073L mutation (all patients) and either the p.A467T (2 patients), p.G848S (1 patient), or p.W748S (1 patient) mutation presented with psychomotor delay, encephalopathy, and liver failure.

INTERVENTIONS

Detailed clinical and laboratory examinations including brain magnetic resonance imaging, muscle biopsy, measurement of mitochondrial DNA, and sequencing of the POLG gene.

MAIN OUTCOME MEASURES

Definition of clinical variability.

RESULTS

All 4 patients had psychomotor delay, seizures, and liver disease. Three patients had severe gastrointestinal dysmotility, which may be associated with the new p.P1073L mutation.

CONCLUSIONS

The heterozygous presence of the novel p.P1073L mutation in trans with another recessive POLG mutation causes a hepatocerebral disorder identical or very similar to Alpers syndrome. This adds to the already striking clinical heterogeneity of POLG mutations. In the Belgian patients, the familial occurrence without consanguinity is related to the high frequency of the recessive p.A467T and p.W748S mutations in northwestern Europe and reveals a pitfall for diagnosis and genetic counseling.

mip1 containing mutations associated with mitochondrial disease causes mutagenesis and depletion of mtDNA in Saccharomyces cerevisiae

DNA polymerase gamma (pol gamma) is responsible for replication and repair of mitochondrial DNA (mtDNA). Over 150 mutations in POLG (which encodes pol gamma) have been discovered in patients with mitochondrial disorders including Alpers, progressive external ophthalmoplegia and ataxia-neuropathy syndrome. However, the severity and dominance of many POLG disease-associated mutations are unclear, because they have been reported in sporadic cases. To understand the consequences of pol gamma disease-associated mutations in vivo, we identified dominant and recessive changes in mtDNA mutagenesis, depletion and mitochondrial dysfunction caused by 31 mutations in the conserved regions of the gene, MIP1, which encodes the Saccharomyces cerevisiae ortholog of human pol gamma. Twenty mip1 mutant enzymes were shown to disrupt mtDNA replication and may be sufficient to cause disease. Previously uncharacterized sporadic mutations, Q308H, R807C, G1076V, R1096H and S1104C, caused decreased polymerase activity leading to mtDNA depletion and mitochondrial dysfunction. We present evidence showing a limited role of point mutagenesis by these POLG mutations in mitochondrial dysfunction and disease progression. Instead, most mitochondrial defective mip1 mutants displayed reduced or depleted mtDNA. We also determined that the severity of the phenotype of the mip1 mutant strain correlates with the age of onset of disease associated with the human ortholog. Finally, we demonstrated that increasing nucleotide pools by overexpression of ribonucleotide reductase (RNR1) suppressed mtDNA replication defects caused by several dominant mip1 mutations, and the orthologous human mutations revealed severe nucleotide binding defects.