Clinical and molecular features of adPEO due to mutations in the Twinkle gene

Clinicogenetical Variants of Progressive External Ophthalmoplegia - An Especial Review of Non-ophthalmic Manifestations

Progressive external ophthalmoplegia (PEO) is a slowly progressive myopathy characterized by extraocular muscles involvement, leading to frozen eyes without diplopia. The pattern of inheritance may be mitochondrial, autosomal dominant or, rarely, autosomal recessive. Sporadic forms were also reported. Muscular involvement other than extraocular muscles may occur with varying degrees of weakness, but this mostly happens many years after the disease begins. There are also scattered data about systemic signs besides ophthalmoplegia. This article aims to review non-ophthalmic findings of PEO from a clinicogenetical point of view.

Twinkle mutations in two Chinese families with autosomal dominant progressive external ophthalmoplegia

Autosomal dominant progressive external ophthalmoplegia (adPEO) is a common adult onset mitochondrial disease caused by mutations in nuclear DNA (nDNA). Twinkle is one of the nuclear genes associated with adPEO. Clinical, histochemical, and molecular genetics findings of 6 patients from two Chinese families with adPEO were reported. Two point mutations (c.1423G>C, p.A475P and c.1061G>C, p.R354P) of Twinkle gene have been found. Multiple mtDNA deletions were also detected in patient's muscle and fibroblasts. This study confirms two mutations in Chinese adPEO families, which were first reported in the Chinese population.

Two novel mutations in PEO1 (twinkle) gene associated with chronic external ophthalmoplegia

Maintenance and replication of mitochondrial DNA require the concerted action of several factors encoded by nuclear genome. The mitochondrial helicase Twinkle is a key player of replisome machinery. Heterozygous mutations in its coding gene, PEO1, are associated with progressive external ophthalmoplegia (PEO) characterised by ptosis and ophthalmoparesis, with cytochrome c oxidase (COX)-deficient fibres, ragged-red fibres (RRF) and multiple mtDNA deletions in muscle. Here we describe clinical, histological and molecular features of two patients presenting with mitochondrial myopathy associated with PEO. PEO1 sequencing disclosed two novel mutations in exons 1 and 4 of the gene, respectively. Although mutations in PEO1 exon 1 have already been described, this is the first report of mutation occurring in exon 4.

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.

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.

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.

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.

Novel Twinkle gene mutation in autosomal dominant progressive external ophthalmoplegia and multisystem failure

A Saudi Arabian family presented with adult onset autosomal dominant progressive external ophthalmoplegia (adPEO) complicated by late onset reversible failure of the CNS, respiratory, hepatic, and endocrine systems. Clinical findings were suggestive of mitochondrial dysfunction and multiple mitochondrial DNA deletions were demonstrated on long range and real time polymerase chain reaction assays but not on Southern blotting. The disorder is caused by a novel heterozygous PEO1 mutation predicting a Leu360Gly substitution in the twinkle protein. The peculiar clinical presentation expands the variable phenotype observed in adPEO and Twinkle gene mutations.

Finding twinkle in the eyes of a 71-year-old lady: a case report and review of the genotypic and phenotypic spectrum of TWINKLE-related dominant disease

Progressive external ophthalmoplegia (PEO) can be caused by a disorder characterized by multiple mitochondrial DNA (mtDNA) deletions due to mutations in the TWINKLE gene, encoding a mtDNA helicase. We describe a 71-year-old woman who had developed PEO at age 55 years. She had cataracts, diabetes, paresthesias, cognitive defects, memory problems, hearing loss, and sensory ataxia. She had muscle weakness with ragged red fibers on biopsy. MRI showed static white matter changes. A c.908G>A substitution (p.R303Q) in the TWINKLE gene was identified. Multiple mtDNA deletions were detected in muscle but not blood by a PCR-based method, but not by Southern blot analysis. MtDNA copy number was maintained in blood and muscle. A systematic literature search was used to identify the genotypic and phenotypic spectrum of dominant TWINKLE-related disease. Patients were adults with PEO and symptoms including myopathy, neuropathy, dysarthria or dysphagia, sensory ataxia, and parkinsonism. Diabetes, cataract, memory loss, hearing loss, and cardiac problems were infrequent. All reported mutations clustered between amino acids 303 and 508 with no mutations at the N-terminal half of the gene. The TWINKLE gene should be analyzed in adults with PEO even in the absence of mtDNA deletions in muscle on Southern blot analysis, and of a family history for PEO. The pathogenic mutations identified 5' beyond the linker region suggest a functional role for this part of the protein despite the absence of a primase function in humans. In our patient, the pathogenesis involved multiple mtDNA deletions without reduction in mtDNA copy number.

Levodopa response in Parkinsonism with multiple mitochondrial DNA deletions

We report a patient with an autosomal dominant chronic progressive external ophthalmoplegia phenotype associated with multiple mtDNA deletions in muscle from a family in which linkage analysis excluded mutations in DNA polymerase gamma (POLG), adenine nucleotide translocase (ANT-1) or C10orf2 (Twinkle). She presented with prominent Parkinsonism characterized by prolonged benefit from levodopa (L-dopa) and the later development of L-dopa induced dyskinesias and motor fluctuations. Thus L-dopa responsiveness, L-dopa induced dyskinesias and motor fluctuations may also occur in atypical Parkinsonism of mitochondrial disease, just as they may in multiple system atrophy.

The role of mitochondrial DNA mutations in mammalian aging

Mitochondrial DNA (mtDNA) accumulates both base-substitution mutations and deletions with aging in several tissues in mammals. Here, we examine the evidence supporting a causative role for mtDNA mutations in mammalian aging. We describe and compare human diseases and mouse models associated with mitochondrial genome instability. We also discuss potential mechanisms for the generation of these mutations and the means by which they may mediate their pathological consequences. Strategies for slowing the accumulation and attenuating the effects of mtDNA mutations are discussed.

Mitochondrial diseases: a nosological update

Mitochondrial diseases are disorders caused by impairment of the mitochondrial respiratory chain, characterized by clinical-genetic heterogeneity and frequent multisystemic involvement. It is difficult to establish a precise genotype/phenotype correlation and obtain a definitive nosology. Today's genetic classification distinguishes disorders caused by defects in the mitochondrial genome (sporadic or maternally-inherited) from disorders caused by defects in the nuclear genome (autosomally-inherited). We report an updated classification, briefly review the main clinical syndromes and describe the most recent genetic knowledge.

Differential phenotypes of active site and human autosomal dominant progressive external ophthalmoplegia mutations in Drosophila mitochondrial DNA helicase expressed in Schneider cells

We report the cloning and molecular analysis of Drosophila mitochondrial DNA helicase (d-mtDNA helicase) homologous to human TWINKLE, which encodes one of the genes responsible for autosomal dominant progressive external ophthalmoplegia. An RNA interference construct was designed that reduces expression of d-mtDNA helicase to an undetectable level in Schneider cells. RNA interference knockdown of d-mtDNA helicase decreases the copy number of mitochondrial DNA (mtDNA) approximately 5-fold. In a corollary manner, overexpression of d-mtDNA helicase increases mtDNA levels 1.4-fold. Overexpression of helicase active site mutants K388A and D483A results in a severe depletion of mtDNA and a dominant negative lethal phenotype. Overexpression of mutants analogous to human autosomal dominant progressive external ophthalmoplegia mutations shows differential effects. Overexpression of I334T and A442P mutants yields a dominant negative effect as for the active site mutants. In contrast, overexpression of A326T, R341Q, and W441C mutants results in increased mtDNA copy number, as observed with wild-type overexpression. Our dominant negative analysis of d-mtDNA helicase in cultured cells provides a tractable model for understanding human autosomal dominant progressive external ophthalmoplegia mutations.

Twinkle, the Mitochondrial Replicative DNA Helicase, Is Widespread in the Eukaryotic Radiation and May Also Be the Mitochondrial DNA Primase in Most Eukaryotes

Recently, the human protein responsible for replicative mtDNA helicase activity was identified and designated Twinkle. Twinkle has been implicated in autosomal dominant progressive external ophthalmoplegia (adPEO), a mitochondrial disorder characterized by mtDNA deletions. The Twinkle protein appears to have evolved from an ancestor shared with the bifunctional primase-helicase found in the T-odd bacteriophages. However, the question has been raised as to whether human Twinkle possesses primase activity, due to amino acid sequence divergence and absence of a zinc-finger motif thought to play an integral role in DNA binding. To date, a primase protein participating in mtDNA replication has not been identified in any eukaryote. Here we investigate the wider phylogenetic distribution of Twinkle by surveying and analyzing data from ongoing EST and genome sequencing projects. We identify Twinkle homologues in representatives from five of six major eukaryotic assemblages ("supergroups") and present the sequence of the complete Twinkle gene from two members of Amoebozoa, a supergroup of amoeboid protists at the base of the opisthokont (fungal/metazoan) radiation. Notably, we identify conserved primase motifs including the zinc finger in all Twinkle sequences outside of Metazoa. Accordingly, we propose that Twinkle likely serves as the primase as well as the helicase for mtDNA replication in most eukaryotes whose genome encodes it, with the exception of Metazoa.

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

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

Twinkle helicase is essential for mtDNA maintenance and regulates mtDNA copy number

Mechanisms of mitochondrial DNA (mtDNA) maintenance have recently gained wide interest owing to their role in inherited diseases as well as in aging. Twinkle is a new mitochondrial 5'-3' DNA helicase, defects of which we have previously shown to underlie a mitochondrial disease, progressive external ophthalmoplegia with multiple mtDNA deletions. Mouse Twinkle is highly similar to the human counterpart, suggesting conserved function. Here, we have characterized the mouse Twinkle gene and expression profile and report that the expression patterns are not conserved between human and mouse, but are synchronized with the adjacent gene MrpL43, suggesting a shared promoter. To elucidate the in vivo role of Twinkle in mtDNA maintenance, we generated two transgenic mouse lines overexpressing wild-type Twinkle. We could demonstrate for the first time that increased expression of Twinkle in muscle and heart increases mtDNA copy number up to 3-fold higher than controls, more than any other factor reported to date. Additionally, we utilized cultured human cells and observed that reduced expression of Twinkle by RNA interference mediated a rapid drop in mtDNA copy number, further supporting the in vivo results. These data demonstrate that Twinkle helicase is essential for mtDNA maintenance, and that it may be a key regulator of mtDNA copy number in mammals.

Parkinsonism, premature menopause, and mitochondrial DNA polymerase gamma mutations: clinical and molecular genetic study

BACKGROUND

Mutations in the gene encoding mitochondrial DNA polymerase gamma (POLG), the enzyme that synthesises mitochondrial DNA (mtDNA), have been associated with a mitochondrial disease-autosomal dominant or recessive progressive external ophthalmoplegia-and multiple deletions of mtDNA. Mitochondrial dysfunction is also suspected to participate in the pathogenesis of Parkinson's disease. However, no primary gene defects affecting mitochondrial proteins causing mendelian transmission of parkinsonism have been characterised. We aimed to analyse the gene sequence of POLG in patients with progressive external ophthalmoplegia and their healthy relatives.

METHODS

In seven families of various ethnic origins we assessed patients with progressive external ophthalmoplegia and unaffected individuals by clinical, biochemical, morphological, and molecular genetic characterisation and positron emission tomography (PET).

FINDINGS

We recorded mutations in POLG in members of all seven families. Clinical assessment showed significant cosegregation of parkinsonism with POLG mutations (p<0.0001), and PET findings were consistent with dopaminergic neuron loss. Post-mortem examination in two individuals showed loss of pigmented neurons and pigment phagocytosis in substantia nigra without Lewy bodies. Furthermore, most women with progressive external ophthalmoplegia had early menopause-before age 35 years. The POLG gene defect resulted in secondary accumulation of mtDNA deletions in patients' tissues.

INTERPRETATION

Dysfunction of mitochondrial POLG causes a severe progressive multisystem disorder including parkinsonism and premature menopause, which are not typical of mitochondrial disease. Cosegregation of parkinsonism and POLG mutations in our families suggests that when defective, this gene can underlie mendelian transmission of parkinsonism.

RELEVANCE TO PRACTICE

Awareness that mitochondrial POLG mutations can underlie parkinsonism is important for clinicians working in diagnosis of movement disorders, as well as for studies of the genetics of Parkinson's disease. Further, progressive external ophthalmoplegia with muscle weakness and neuropathy can mask symptoms of parkinsonism, and clinicians should pay special attention to detect and treat parkinsonism in those individuals.

Two families with autosomal dominant progressive external ophthalmoplegia

OBJECTIVES

We report here the clinical and genetic features of two new families with autosomal dominant progressive external ophthalmoplegia (adPEO).

PATIENTS AND METHODS

The examination of index patients included a detailed clinical characterisation, histological analysis of muscle biopsy specimens, and genetic testing of mitochondrial and nuclear DNA extracted from muscle and leucocytes.

RESULTS

Index patients in both families presented with PEO and developed other clinical disease manifestations, such as myopathy and cardiomyopathy (patient 1) and axonal neuropathy, diabetes mellitus, hearing loss, and myopathy (patient 2), later in the course of illness. Both patients had ragged red fibres on muscle histology. Southern blot of mtDNA from muscle of patient 2 showed multiple deletions. In this case, a novel heterozygous missense mutation F485L was identified in the nuclear encoded putative mitochondrial helicase Twinkle. The mutation co-segregated with the clinical phenotype in the family and was not detected in 150 control chromosomes. In the other index patient, sequencing of ANT1, C10orf2 (encoding for Twinkle), and POLG1 did not reveal pathogenic mutations.

CONCLUSIONS

Our cases illustrate the clinical variability of adPEO, add a novel pathogenic mutation in Twinkle (F485L) to the growing list of genetic abnormalities in adPEO, and reinforce the relevance of other yet unidentified genes in mtDNA maintenance and pathogenesis of adPEO.

Mitochondriopathies

Mitochondriopathies (MCPs) are either due to sporadic or inherited mutations in nuclear or mitochondrial DNA located genes (primary MCPs), or due to exogenous factors (secondary MCPs). MCPs usually show a chronic, slowly progressive course and present with multiorgan involvement with varying onset between birth and late adulthood. Although several proteins with signalling, assembling, transport, enzymatic function can be impaired in MCP, most frequently the activity of the respiratory chain (RC) protein complexes is primarily or secondarily affected, leading to impaired oxygen utilization and reduced energy production. MCPs represent a diagnostic challenge because of their wide variation in presentation and course. Systems frequently affected in MCP are the peripheral nervous system (myopathy, polyneuropathy, lactacidosis), brain (leucencephalopathy, calcifications, stroke-like episodes, atrophy with dementia, epilepsy, upper motor neuron signs, ataxia, extrapyramidal manifestations, fatigue), endocrinium (short stature, hyperhidrosis, diabetes, hyperlipidaemia, hypogonadism, amenorrhoea, delayed puberty), heart (impulse generation or conduction defects, cardiomyopathy, left ventricular non-compaction heart failure), eyes (cataract, glaucoma, pigmentary retinopathy, optic atrophy), ears (deafness, tinnitus, peripheral vertigo), guts (dysphagia, vomiting, diarrhoea, hepatopathy, pseudo-obstruction, pancreatitis, pancreas insufficiency), kidney (renal failure, cysts) and bone marrow (sideroblastic anaemia). Apart from well-recognized syndromes, MCP should be considered in any patient with unexplained progressive multisystem disorder. Although there is actually no specific therapy and cure for MCP, many secondary problems require specific treatment. The rapidly increasing understanding of the pathophysiological background of MCPs may further facilitate the diagnostic approach and open perspectives to future, possibly causative therapies.

Progressive external ophthalmoplegia characterized by multiple deletions of mitochondrial DNA: unraveling the pathogenesis of human mitochondrial DNA instability and the initiation of a genetic classification

Over the last decade, many sporadic and familial cases have been reported with multiple deletions of mitochondrial DNA (mtDNA) in postmitotic tissues. Most patients suffer from progressive external ophthalmoplegia (PEO) and may have a nuclear gene defect that predisposes to the accumulation of mtDNA deletions. Recently, positional cloning has led to the discovery of mutations in four such nuclear genes. Some mutations are dominant and others recessive. In all autosomal mutations, defective mtDNA replication and/or repair are probably responsible for the generation of secondary mtDNA deletions. There are also data suggestive of a prominent pathogenic role for disturbed nucleotide metabolism. We here present a tentative genotype-phenotype correlation. Since clinical presentations are heterogeneous and overlap with different previously described clinical syndromes, we advocate the use of a genetic, instead of a clinical, classification of disorders with multiple mtDNA deletions.

A novel Twinkle gene mutation in autosomal dominant progressive external ophthalmoplegia

Autosomal dominant progressive external ophthalmoplegia is a common neurological presentation of mitochondrial disease and is characterised by multiple deletions of mitochondrial DNA in muscle. We describe a family with autosomal dominant progressive external ophthalmoplegia caused by a novel heterozygous A to C transversion at nucleotide 956 of the Twinkle gene. The deltoid muscle biopsy of the index case revealed sparse respiratory deficient cells. Multiple mitochondrial DNA deletions were clearly evident in the index case by both long-range and real-time polymerase chain reaction assays but not by Southern blotting, highlighting the diagnostic difficulties associated with characterising patients with multiple mitochondrial DNA deletions.