Mitochondrial DNA polymerase-gamma and human disease

Sequence variants in four candidate genes (NIPSNAP1, GBAS, CHCHD1 and METT11D1) in patients with combined oxidative phosphorylation system deficiencies

The oxidative phosphorylation (OXPHOS) system, comprising five enzyme complexes, is located in the inner membrane of mitochondria and is the final biochemical pathway in oxidative ATP production. Defects in this energy-generating system can cause a wide range of clinical symptoms; these diseases are often progressive and multisystemic. Numerous genes have been implicated in OXPHOS deficiencies and many mutations have been described. However, in a substantial number of patients with decreased enzyme activities of two or more OXPHOS complexes, no mutations in the mitochondrial DNA or in nuclear genes known to be involved in these disorders have been found. In this study, four nuclear candidate genes--NIPSNAP1, GBAS, CHCHD1 and METT11D1--were screened for mutations in 22 patients with a combined enzymatic deficiency of primarily the OXPHOS complexes I, III and IV to determine whether a mutation in one of these genes could explain the mitochondrial disorder. For each variant not yet reported as a polymorphism, 100 control samples were screened for the presence of the variant. This way we identified 14 new polymorphisms and 2 presumably non-pathogenic mutations. No mutations were found that could explain the mitochondrial disorder in the patients investigated in this study. Therefore, the genetic defect in these patients must be located in other nuclear genes involved in mtDNA maintenance, transcription or translation, in import, processing or degradation of nuclear encoded mitochondrial proteins, or in assembly of the OXPHOS system.

CAG-repeat variant in the polymerase γ gene and male infertility in the Chinese population: a meta-analysis

Several studies have reported a relationship between the length of the CAG-repeat in the polymerase γ (POLG) gene and male infertility. However, other studies have not reproduced this result. In our study, the POLG-CAG-repeat length was analyzed in 535 healthy individuals from six Chinese Han populations living in different provinces. The frequencies of 10-CAG alleles and genotypes were high (97.38 and 94.13%, respectively), with no significant difference among the six Chinese Han populations. Furthermore, we determined the distribution of the POLG-CAG-repeat in 150 infertile men and 126 fertile men. Our study suggested that the distributions of POLG-CAG-repeat alleles and genotypes were not significantly different between infertile (95.67 and 92.67%, respectively) and fertile men (97.22 and 94.44%, respectively). In a subsequent meta-analysis, combining our data with data from previous studies, a comparison of the CAG-repeat alleles in fertile versus infertile men showed no obvious risk for male infertility associated with any particular allele (pooled odds ratio (OR)=0.94; 95% confidence interval (CI): 0.60-1.48). The significance level was not attained with any of the following genetic models: homozygote comparison (not 10/not 10 versus 10/10: OR=1.34; 95% CI: 0.66-2.72), heterozygote comparison (10/not 10 versus 10/10: OR=1.04; 95% CI: 0.78-1.38), dominant model comparison (not 10/not 10+10/not 10 versus 10/10: OR=1.08; 95% CI: 0.79-1.47) and recessive genetic comparison (not 10/not 10 versus 10/not 10+10/10: OR=1.31; 95% CI: 0.68-2.55). In conclusion, there is no significant difference of the frequencies of POLG-CAG-repeat variants among six Chinese Han populations, and this polymorphism may not be associated with Chinese male infertility. On the basis of a meta-analysis, there is no obvious association between CAG-repeat variants of the POLG gene and male infertility.

[Autosomal recessive cerebellar ataxias]

INTRODUCTION

Autosomal recessive cerebellar ataxias (ARCA) are heterogeneous and complex inherited neurodegenerative diseases that may affect the cerebellum and/or the spinocerebellar tract, the posterior column of the spinal cord and the peripheral nerves. Cerebellar ataxia is frequently proeminent and mostly associated with several neurological or extra-neurological signs, leading to a major disability before the age of 30.

STATE OF ART

Friedreich's ataxia (FRDA) is clearly the most frequent ARCA and several rarer entities have been described during the past fifteen years such as ataxia with oculomotor apraxia type 1 (AOA1) and type 2 (AOA2), ataxia with vitamin E deficiency (AVED) and autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS). The ACAR are characterized by both allelic and non-allelic genetic heterogeneity. They may be divided into three groups: spino-cerebellar ataxia with pure sensory neuropathy; cerebellar ataxia with sensori-motor axonal neuropathy; pure cerebellar ataxia (i.e. ataxia of purely cerebellar origin that may be associated with other symptoms). Common physiological pathways are involved in several ARCA, such as DNA repair deficiency (AOA1, ataxia telangiectasia [AT]…), RNA termination disorder (AOA2), mitochondrial defect (FRDA, sensory ataxic neuropathy with dysarthria and ophthalmoplegia [Sando]…), lipoprotein assembly defects (AVED, abetalipoproteinemia [ABL]), chaperon protein disorders (ARSACS, Marinesco-Sjögren syndrome [MSS]) or peroxysomal diseases (Refsum disease [RD]).

PERSPECTIVES

New nanotechnology methods and high throughput gene analysis as well as bioinformatics should lead to the identification of several new ARCAs in the next few years despite the rarity of these entities. However, the challenge of the next decades will be the discovery of efficient treatments for these disabling neurodegenerative disorders.

CONCLUSION

Clinicians should be aware of the more frequent ARCAs, especially FRDA, in addition to ARCAs for which treatment is available (FRDA, AVED, ABL and RD for instance).

Novel POLG1 mutations in a patient with adult-onset progressive external ophthalmoplegia and encephalopathy

Mutations in POLG1 are an important cause of human mitochondrial disease. We describe a woman who presented with bilateral ptosis and external ophthalmoplegia at 64 years of age. Neurological examination revealed symptoms of diffuse encephalopathy. The symptoms were progressive and at 67 years she was severely cognitively impaired, had severe bilateral ptosis and complete external ophthalmoplegia. Frequent cytochrome c oxidase-negative fibres were detected in muscle. Electrophysiological examination revealed myopathic changes and axonal neuropathy. Standard laboratory tests were normal. Brain CT showed general, moderate cortical atrophy. Molecular analysis of muscle DNA revealed multiple mitochondrial DNA deletions. Sequencing of the entire POLG1 gene revealed two changes c.2993C>T (p.998S>L) and c.3550G>C (p.1184D>H). Both mutations are previously unreported and confirmed to be compound heterozygous. Late-onset progressive external ophthalmoplegia with severe encephalopathy is an unusual combination in patients with POLG1 mutations. POLG-associated disease should be considered in any patient with unexplained or unusual neurological features.

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.

Variations of the CAG trinucleotide repeat in DNA polymerase γ (POLG1) is associated with Parkinson's disease in Sweden

DNA polymerase gamma (POLG1) is coding for the catalytic subunit of the heterotrimeric mitochondrial DNA polymerase and involved in replication and repair of mitochondrial DNA. In addition to its 5' to 3' polymerase activity, POLG1 has a 3' to 5' exonuclease activity important in the repair process. Mitochondrial dysfunction has been implicated in neurodegenerative disorders like Parkinson's disease (PD). Dopamine neurons, which degenerate in PD, are believed to be particularly susceptible to mitochondrial dysfunction, which makes POLG1 a possible candidate gene for the disease. POLG1 has a polyglutamine tract (poly-Q) in the N-terminal, encoded by a CAG sequence in exon 2. Most commonly the poly-Q tract comprises 10 repeats (10Q, frequency >80%) or moderately common 11Q (frequency 6-12%); however the composition of poly-Q alleles has been reported to vary from 6Q to 14Q. We analyzed this POLG1 trinucleotide repeat in a Swedish PD case-control material and detected variations from 5Q to 15Q. We report a significant association between the non-10/11Q repeats with PD (p=0.002). In silico analysis of poly-Q length effect on mRNA folding energy show a decrease in energy for <10/11Q mRNA (4.6%) and an increase for >10/11Q mRNA (4.8%) compared to 10/11Q mRNA. Our results strengthen the evidence for involvement of POLG1 and mitochondrial dysfunction in PD.

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

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

Five mutations of mitochondrial DNA polymerase-gamma (POLG) are not a prevalent etiology for spontaneous 46,XX primary ovarian insufficiency

The study objective was to determine if mutations in mitochondrial DNA polymerase gamma (POLG) are associated with spontaneous 46,XX primary ovarian insufficiency (sPOI) using restriction fragment length polymorphism analysis of genomic DNA. Of 201 women with 46,XX sPOI analyzed, we found only one case (0.5%, 95% confidence interval 0-3%) of heterozygosity for a POLG mutation, suggesting that this is not a common genetic etiology for this form of infertility.

Mitochondrial DNA depletion and its correlation with TFAM, TFB1M, TFB2M and POLG in human diffusely infiltrating astrocytomas

Mitochondrial DNA (mtDNA) alterations and their clinical and pathological implications have been analyzed in several human malignancies. A marked decrease in mtDNA copy number along with the increase in malignancy was observed in diffusely infiltrating astrocytomas (24 WHO grade II, 18 grade III, and 78 grade IV or GBM) compared to non-neoplastic brain tissues, being mostly depleted in GBM. Although high relative gene expression levels of mtDNA replication regulators (mitochondrial polymerase catalytic subunit (POLG), transcription factors A (TFAM), B1 (TFB1M) and B2 (TFB2M)) were detected, it cannot successfully revert the mtDNA depletion observed in our samples. On the other hand, a strong correlation among the expression levels of mitochondrial transcription factors corroborates with the TFAM role in the direct control of TFB1M and TFB2M during initiation of mtDNA replication. POLG expression was related to decreased mtDNA copy number, and its overexpression associated with TFAM expression levels also have an impact on long-term survival among GBM patients, interpreted as a potential predictive factor for better prognosis.

The role of mitochondrial DNA copy number in mammalian fertility

Mammalian mitochondrial DNA (mtDNA) is a small, maternally inherited genome that codes for 13 essential proteins in the respiratory chain. Mature oocytes contain more than 150 000 copies of mtDNA, at least an order of magnitude greater than the number in most somatic cells, but sperm contain only approximately 100 copies. Mitochondrial oxidative phosphorylation has been suggested to be an important determinant of oocyte quality and sperm motility; however, the functional significance of the high mtDNA copy number in oocytes, and of the low copy number in sperm, remains unclear. To investigate the effects of mtDNA copy number on fertility, we genetically manipulated mtDNA copy number in the mouse by deleting one copy of Tfam, an essential component of the mitochondrial nucleoid, at different stages of germline development. We show that males can tolerate at least a threefold reduction in mtDNA copy number in their sperm without impaired fertility, and in fact, they preferentially transmit a deleted Tfam allele. Surprisingly, oocytes with as few as 4000 copies of mtDNA can be fertilized and progress normally through preimplantation development to the blastocyst stage. The mature oocyte, however, has a critical postimplantation developmental threshold of 40 000-50 000 copies of mtDNA in the mature oocyte. These observations suggest that the high mtDNA copy number in the mature oocyte is a genetic device designed to distribute mitochondria and mtDNAs to the cells of the early postimplantation embryo before mitochondrial biogenesis and mtDNA replication resumes, whereas down-regulation of mtDNA copy number is important for normal sperm function.

Mitochondrial DNA and genetic disease

From their very beginning to the present day, mitochondria have evolved to become a crucial organelle within the cell. The mitochondrial genome encodes only 37 genes, but its compact structure and minimal redundancy results in mutations on the mitochondrial genome being an important cause of genetic disease. In the present chapter we describe the up-to-date knowledge about mitochondrial DNA structure and function, and describe some of the consequences of defective function including disease and aging.

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.

POLG1 polyglutamine tract variants associated with Parkinson's disease

A possible role of allelic variation of the mitochondrial DNA polymerase gamma (POLG1) gene in Parkinson's disease (PD) has been suggested. First, POLG1 missense mutations have been found in patients with familial parkinsonism and mitochondrial myopathy. Second, increased frequency of rare alleles of the POLG1 CAG-repeat (poly-Q) has been found in Finnish idiopathic apparently sporadic PD patients, but conflicting reports exist. The POLG1 poly-Q exhibits one major allele with 10 repeats (10Q, frequency >/=80%) and several less common alleles such as 11Q (frequency 6-9%), 6Q-9Q and 12Q-14Q (frequencies <4%). It is not known, whether the poly-Q variation modulates POLG1 function. Here we sequenced the poly-Q in 641 North American Caucasian PD patients and 292 controls. Caucasian literature controls were also used. Normal allele was defined either as 10/11Q or as 10Q according to the previous literature. The frequency of the non-10/11Q alleles in cases was not significantly different from the controls. Variant alleles defined as non-10Q were significantly increased in the PD patients compared to the North American controls (17.6% vs. 12.3%, p=0.004) as well as compared to the larger set of 897 controls (17.6% vs. 13.2%, p=0.0007). These results suggest that POLG1 poly-Q alleles other than the conserved 10Q allele may increase susceptibility to PD. This finding may be attributable to a beneficial function of the 10Q repeat protein or linkage disequilibrium between the 10Q allele and another variation within or close to POLG1. Other large case-control studies and analyses on functional differences of POLG1 poly-Q variants are warranted.

Mouse models of mtDNA replication diseases

Defective mitochondrial DNA (mtDNA) replication is a common cause of human disease in children and adults. mtDNA replication relies on a large set of nuclear-encoded proteins that either belong to the replication machinery itself, or participate in the nucleotide pool regulation. Identification of patient mutations in the corresponding genes has revealed that dysfunctional mtDNA replication can cause highly variable disease phenotypes. We describe here the strategies that have been undertaken to generate mouse models for mtDNA replication diseases. Such models are essential tools for understanding the consequences of mtDNA replication defects on different tissues and on the metabolism of the whole organism.

Marked mitochondrial DNA depletion associated with a novel SUCLG1 gene mutation resulting in lethal neonatal acidosis, multi-organ failure, and interrupted aortic arch

The aim of this study was to identify the causative genetic lesion in two apparently unrelated newborns having lethal lactic acidosis, multi-organ failure and congenital malformations including interrupted aortic arch, who exhibited mild methylmalonic aciduria, combined mitochondrial respiratory chain deficiency, and marked muscle mitochondrial DNA depletion. A novel mutation in the SUCLG1 gene was identified. Phenotype severity in Succinate-CoA ligase dysfunction appears to be more correlated to the muscle mtDNA content than to the tissue distribution of the heterodimer subunits. Prominent impairment of mitochondrial respiratory chain may result in deep ravages in developmental tissues leading to multiple organ failure and malformations.

Measurement of kinetic parameters of human platelet DNA polymerase gamma

Synthesis of mitochondrial DNA is performed by DNA polymerase gamma. Mutations in POLG, the gene encoding the catalytic subunit of DNA polymerase gamma, are a major cause of neurological disease. A large proportion of patients carry rare nucleotide substitutions leading to single amino acid changes. Confirming that these replacements are pathogenic can be problematic without biochemical evidence. Here, we provide a hands-on protocol for an in vitro kinetic assay of DNA polymerase gamma which allows assessment of the K(m) and V(max) for the incoming nucleotide of the polymerization reaction. To avoid measurement of contaminating nuclear DNA polymerases, platelet extracts are used since platelets do not contain a nucleus. Moreover, platelets have the advantage of being obtainable relatively non-invasively. Polymerization activity is determined by measurement of the incorporation of radioactive thymidine 5'-triphosphate (dTTP) on the homopolymeric RNA substrate poly(rA).oligo(dT)(12-18). To further minimize nuclear DNA polymerase activity, aphidicolin, an inhibitor of most nuclear DNA polymerases, is included in the reaction. In addition, reactions are carried out in the absence and presence of the competitive inhibitor of DNA polymerase gamma, 2',3'-dideoxythymidine 5'-triphosphate (ddTTP), to allow calculation of the ddTTP-sensitive incorporation. With this method, platelets from healthy control subjects extracted with 3% Triton X-100 showed a K(m) for dTTP of 1.42 microM and a V(max) of 0.83 pmol min(-1)mg(-1).

Multi-system neurological disease is common in patients with OPA1 mutations

Additional neurological features have recently been described in seven families transmitting pathogenic mutations in OPA1, the most common cause of autosomal dominant optic atrophy. However, the frequency of these syndromal 'dominant optic atrophy plus' variants and the extent of neurological involvement have not been established. In this large multi-centre study of 104 patients from 45 independent families, including 60 new cases, we show that extra-ocular neurological complications are common in OPA1 disease, and affect up to 20% of all mutational carriers. Bilateral sensorineural deafness beginning in late childhood and early adulthood was a prominent manifestation, followed by a combination of ataxia, myopathy, peripheral neuropathy and progressive external ophthalmoplegia from the third decade of life onwards. We also identified novel clinical presentations with spastic paraparesis mimicking hereditary spastic paraplegia, and a multiple sclerosis-like illness. In contrast to initial reports, multi-system neurological disease was associated with all mutational subtypes, although there was an increased risk with missense mutations [odds ratio = 3.06, 95% confidence interval = 1.44-6.49; P = 0.0027], and mutations located within the guanosine triphosphate-ase region (odds ratio = 2.29, 95% confidence interval = 1.08-4.82; P = 0.0271). Histochemical and molecular characterization of skeletal muscle biopsies revealed the presence of cytochrome c oxidase-deficient fibres and multiple mitochondrial DNA deletions in the majority of patients harbouring OPA1 mutations, even in those with isolated optic nerve involvement. However, the cytochrome c oxidase-deficient load was over four times higher in the dominant optic atrophy + group compared to the pure optic neuropathy group, implicating a causal role for these secondary mitochondrial DNA defects in disease pathophysiology. Individuals with dominant optic atrophy plus phenotypes also had significantly worse visual outcomes, and careful surveillance is therefore mandatory to optimize the detection and management of neurological disability in a group of patients who already have significant visual impairment.

Mitochondrial DNA mutations and human disease

Mitochondrial disorders are a group of clinically heterogeneous diseases, commonly defined by a lack of cellular energy due to oxidative phosphorylation (OXPHOS) defects. Since the identification of the first human pathological mitochondrial DNA (mtDNA) mutations in 1988, significant efforts have been spent in cataloguing the vast array of causative genetic defects of these disorders. Currently, more than 250 pathogenic mtDNA mutations have been identified. An ever-increasing number of nuclear DNA mutations are also being reported as the majority of proteins involved in mitochondrial metabolism and maintenance are nuclear-encoded. Understanding the phenotypic diversity and elucidating the molecular mechanisms at the basis of these diseases has however proved challenging. Progress has been hampered by the peculiar features of mitochondrial genetics, an inability to manipulate the mitochondrial genome, and difficulties in obtaining suitable models of disease. In this review, we will first outline the unique features of mitochondrial genetics before detailing the diseases and their genetic causes, focusing specifically on primary mtDNA genetic defects. The functional consequences of mtDNA mutations that have been characterised to date will also be discussed, along with current and potential future diagnostic and therapeutic advances.

Mutations in mitochondrial DNA polymerase-gamma promote breast tumorigenesis

Decreased mitochondrial oxidative phosphorylation (OXPHOS) is one of the hallmarks of cancer. To date, the identity of nuclear gene(s) responsible for decreased OXPHOS in tumors remains unknown. It is also unclear whether mutations in nuclear gene(s) responsible for decreased OXPHOS affect tumorigenesis. Polymerase-gamma (POLG) is the only DNA polymerase known to function in human mitochondria. Mutations in POLG are known to cause mitochondrial DNA (mtDNA) depletion and decreased OXPHOS, resulting in mtDNA depletion syndrome in humans. We therefore sequenced all coding exons (2-23) and flanking intron/splice junctions of POLG in breast tumors. We found that the POLG gene was mutated in 63% of breast tumors. We identified a total of 17 mutations across the POLG gene. Mutations were found in all three domains of the POLG protein, including T251I (the exonuclease domain), P587L (the linker region) and E1143G (the polymerase domain). We identified two novel mutations that include one silent (A703A) and one missense (R628Q) mutation in the evolutionarily conserved POLG linker region. In addition, we identified three novel mutations in the intronic region. Our study also revealed that mtDNA was depleted in breast tumors. Consistently, mutant POLG, when expressed in breast cancer cells, induced a depletion of mtDNA, decreased mitochondrial activity, decreased mitochondrial membrane potential, increased levels of reactive oxygen species and increased Matrigel invasion. Together, our study provides the first comprehensive analysis of the POLG gene mutation in human cancer and suggests a function for POLG (1) in decreased OXPHOS in cancers and (2) in promoting tumorigenicity.

Two novel POLG mutations causing hepatic mitochondrial DNA depletion with recurrent hypoketotic hypoglycaemia and fatal liver dysfunction

BACKGROUND

Inherited mtDNA depletion syndromes (MDS) are a group of severe mitochondrial disorders resulting from defects in nucleus-encoded factors and often associated with severe or fatal liver failure.

PATIENT

In this article, we describe the case of an 18-month-old patient with recurrent hypoketotic hypoglycaemia and fatal hepatic dysfunction with liver mtDNA depletion.

METHODS

The assessment of mtDNA copy number was performed on leucocytes, liver and muscle biopsy by Quantitative Real Time PCR and total RNA from liver biopsy was used as a template to amplify the cDNA of the POLG1 gene.

RESULTS

Sequence analysis identified two previously undescribed mutations (1868T>G and 2263A>G) located in the gene coding the catalytic subunit of mitochondrial DNA polymerase gamma (POLG), predicting an L623W and K755E amino acid change, respectively. Both mutations were located in the highly conserved linker region of the protein and were absent in more than 200 healthy unrelated control subjects. The identification of these two mutations allowed us to perform genetic counselling and prenatal diagnosis.

CONCLUSION

Our data further expand the spectrum of POLG1 gene mutations and the unique phenotype reported (late onset isolated liver disease without lactic acidosis) increase the variability of clinical presentations associated with mutations in this gene.

[Colorectal cancer 2D-proteomics: identification of altered protein expression]

Modern proteomic techniques make it possible to identify numerous changes in protein expression in tumor in comparison to normal tissues. Despite the wide application of proteomics in current studies, identification of proteins with stable concentration differences in normal and cancer cells remains rather difficult. The current study was directed to the search of new potential protein colorectal cancer markers using comparative proteomics of protein extracts obtained from primary tumors and adjacent normal tissues. This widespread neoplasm is characterized by lack of evident symptoms at early stages of cancerogenesis. It is highly important to develop fast and sensitive methods of molecular diagnostics. We studied paired cancerous and normal clinical tissue samples from 11 patients with colorectal adenocarcinomas by comparative 2-D PAGE and MALDI-TOF mass-spectrometry identification. Sixteen proteins with stable differential expression were selected and identified, including 13 overexpressed and 3 downregulated proteins. In summary, we describe the discovery overexpression of GPD1 and RRBP1 proteins and lack of expression for HNRNPH1 and SERPINB6 proteins which are new candidate biomarkers of colon cancer.

Disease mutations in the human mitochondrial DNA polymerase thumb subdomain impart severe defects in mitochondrial DNA replication

Forty-five different point mutations in POLG, the gene encoding the catalytic subunit of the human mitochondrial DNA polymerase (pol gamma), cause the early onset mitochondrial DNA depletion disorder, Alpers syndrome. Sequence analysis of the C-terminal polymerase region of pol gamma revealed a cluster of four Alpers mutations at highly conserved residues in the thumb subdomain (G848S, c.2542g-->a; T851A, c.2551a-->g; R852C, c.2554c-->t; R853Q, c.2558g-->a) and two Alpers mutations at less conserved positions in the adjacent palm subdomain (Q879H, c.2637g-->t and T885S, c.2653a-->t). Biochemical characterization of purified, recombinant forms of pol gamma revealed that Alpers mutations in the thumb subdomain reduced polymerase activity more than 99% relative to the wild-type enzyme, whereas the palm subdomain mutations retained 50-70% wild-type polymerase activity. All six mutant enzymes retained physical and functional interaction with the pol gamma accessory subunit (p55), and none of the six mutants exhibited defects in misinsertion fidelity in vitro. However, differential DNA binding by these mutants suggests a possible orientation of the DNA with respect to the polymerase during catalysis. To our knowledge this study represents the first structure-function analysis of the thumb subdomain in pol gamma and examines the consequences of mitochondrial disease mutations in this region.

Genetic calcium signaling abnormalities in the central nervous system: seizures, migraine, and autism

The calcium ion is one of the most versatile, ancient, and universal of biological signaling molecules, known to regulate physiological systems at every level from membrane potential and ion transporters to kinases and transcription factors. Disruptions of intracellular calcium homeostasis underlie a host of emerging diseases, the calciumopathies. Cytosolic calcium signals originate either as extracellular calcium enters through plasma membrane ion channels or from the release of an intracellular store in the endoplasmic reticulum (ER) via inositol triphosphate receptor and ryanodine receptor channels. Therefore, to a large extent, calciumopathies represent a subset of the channelopathies, but include regulatory pathways and the mitochondria, the major intracellular calcium repository that dynamically participates with the ER stores in calcium signaling, thereby integrating cellular energy metabolism into these pathways, a process of emerging importance in the analysis of the neurodegenerative and neuropsychiatric diseases. Many of the calciumopathies are common complex polygenic diseases, but leads to their understanding come most prominently from rare monogenic channelopathy paradigms. Monogenic forms of common neuronal disease phenotypes-such as seizures, ataxia, and migraine-produce a constitutionally hyperexcitable tissue that is susceptible to periodic decompensations. The gene families and genetic lesions underlying familial hemiplegic migraine, FHM1/CACNA1A, FHM2/ATP1A2, and FHM3/SCN1A, and monogenic mitochondrial migraine syndromes, provide a robust platform from which genes, such as CACNA1C, which encodes the calcium channel mutated in Timothy syndrome, can be evaluated for their role in autism and bipolar disease.

Mitochondrial DNA level, but not active replicase, is essential for Caenorhabditis elegans development

A number of studies showed that the development and the lifespan of Caenorhabditis elegans is dependent on mitochondrial function. In this study, we addressed the role of mitochondrial DNA levels and mtDNA maintenance in development of C. elegans by analyzing deletion mutants for mitochondrial polymerase gamma (polg-1(ok1548)). Surprisingly, even though previous studies in other model organisms showed necessity of polymerase gamma for embryonic development, homozygous polg-1(ok1548) mutants had normal development and reached adulthood without any morphological defects. However, polg-1 deficient animals have a seriously compromised gonadal function as a result of severe mitochondrial depletion, leading to sterility and shortened lifespan. Our results indicate that the gonad is the primary site of mtDNA replication, whilst the mtDNA of adult somatic tissues mainly stems from the developing embryo. Furthermore, we show that the mtDNA copy number shows great plasticity as it can be almost tripled as a response to the environmental stimuli. Finally, we show that the mtDNA copy number is an essential limiting factor for the worm development and therefore, a number of mechanisms set to maintain mtDNA levels exist, ensuring a normal development of C. elegans even in the absence of the mitochondrial replicase.

Mouse models of mitochondrial DNA defects and their relevance for human disease

Qualitative and quantitative changes in mitochondrial DNA (mtDNA) have been shown to be common causes of inherited neurodegenerative and muscular diseases, and have also been implicated in ageing. These diseases can be caused by primary mtDNA mutations, or by defects in nuclear-encoded mtDNA maintenance proteins that cause secondary mtDNA mutagenesis or instability. Furthermore, it has been proposed that mtDNA copy number affects cellular tolerance to environmental stress. However, the mechanisms that regulate mtDNA copy number and the tissue-specific consequences of mtDNA mutations are largely unknown. As post-mitotic tissues differ greatly from proliferating cultured cells in their need for mtDNA maintenance, and as most mitochondrial diseases affect post-mitotic cell types, the mouse is an important model in which to study mtDNA defects. Here, we review recently developed mouse models, and their contribution to our knowledge of mtDNA maintenance and its role in disease.

Approaches to finding the molecular basis of mitochondrial oxidative phosphorylation disorders

Inherited disorders of mitochondrial oxidative phosphorylation are the most common group of inborn errors of metabolism and cause a wide range of clinical presentations. Mitochondrial DNA encodes 13 protein subunits required for oxidative phosphorylation plus 22 transfer RNAs and two ribosomal RNAs, and mutations in most of these genes cause human disease. Nuclear genes encode most of the protein subunits and all other proteins required for mitochondrial biogenesis and mitochondrial DNA replication and expression. Mutations in 64 nuclear genes and 34 mitochondrial genes are now known to cause mitochondrial disease and many novel mitochondrial disease genes await discovery. The genetic complexity of oxidative phosphorylation means that maternal, autosomal recessive, autosomal dominant and X-linked modes of inheritance can occur, along with de novo mutations. This complexity presents a challenge in planning efficient molecular genetic diagnosis of patients with suspected mitochondrial disease. In some situations, clinical phenotype can be strongly predictive of the underlying genotype. However, more often this is not the case and it is usually helpful, particularly with pediatric patients, to determine whether the activity of one or more of the individual oxidative phosphorylation enzymes is deficient before proceeding with mutation analysis. In this review we will summarize the genetic bases of mitochondrial disease and discuss some approaches to integrate information from clinical presentation, laboratory findings, family history, and imaging to guide molecular investigation.

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

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

An analysis of enzyme kinetics data for mitochondrial DNA strand termination by nucleoside reverse transcription inhibitors

Nucleoside analogs used in antiretroviral treatment have been associated with mitochondrial toxicity. The polymerase-γ hypothesis states that this toxicity stems from the analogs' inhibition of the mitochondrial DNA polymerase (polymerase-γ) leading to mitochondrial DNA (mtDNA) depletion. We have constructed a computational model of the interaction of polymerase-γ with activated nucleoside and nucleotide analog drugs, based on experimentally measured reaction rates and base excision rates, together with the mtDNA genome size, the human mtDNA sequence, and mitochondrial dNTP concentrations. The model predicts an approximately 1000-fold difference in the activated drug concentration required for a 50% probability of mtDNA strand termination between the activated di-deoxy analogs d4T, ddC, and ddI (activated to ddA) and the activated forms of the analogs 3TC, TDF, AZT, FTC, and ABC. These predictions are supported by experimental and clinical data showing significantly greater mtDNA depletion in cell culture and patient samples caused by the di-deoxy analog drugs. For zidovudine (AZT) we calculated a very low mtDNA replication termination probability, in contrast to its reported mitochondrial toxicity in vitro and clinically. Therefore AZT mitochondrial toxicity is likely due to a mechanism that does not involve strand termination of mtDNA replication.

While HIV/AIDS therapy is very successful at controlling HIV infection, the therapy must continue for the remainder of the patient's life. Approximately one-fourth of these patients suffer from serious drug toxicity problems. It is generally believed that the toxicity of these drugs is caused by damage to mitochondria, the “power plants” of every cell. But we do not know exactly how this damage occurs. The most common explanation is that these drugs damage mitochondria in the same way that they control the virus, by interfering with DNA replication. We tested that idea by analyzing data for the interaction of several AIDS drugs with the mitochondrial DNA polymerase, the protein responsible for copying mitochondrial DNA. By using a detailed simulation of the mitochondrial DNA polymerase, we show that some of these drugs do interact well enough with the mitochondrial DNA polymerase to lead to toxic effects. However, many of these drugs, including the commonly used drug AZT, had very little toxic effect in this simulation although AZT often causes toxicity in patients. This indicates that the toxicity of AZT occurs through some other process and not through the direct interruption of mitochondrial DNA replication.

Mouse models of oxidative phosphorylation defects: powerful tools to study the pathobiology of mitochondrial diseases

Defects in the oxidative phosphorylation system (OXPHOS) are responsible for a group of extremely heterogeneous and pleiotropic pathologies commonly known as mitochondrial diseases. Although many mutations have been found to be responsible for OXPHOS defects, their pathogenetic mechanisms are still poorly understood. An important contribution to investigate the in vivo function of several mitochondrial proteins and their role in mitochondrial dysfunction, has been provided by mouse models. Thanks to their genetic and physiologic similarity to humans, mouse models represent a powerful tool to investigate the impact of pathological mutations on metabolic pathways. In this review we discuss the main mouse models of mitochondrial disease developed, focusing on the ones that directly affect the OXPHOS system.

Enhancement of apoptotic damage of squamous cell carcinoma cells by inhibition of the mitochondrial DNA repairing system

Mitochondrial DNA (mtDNA) repair systems are thought to be associated with the susceptibility of cancer cells to anticancer agents. The present study investigated the relationship between the susceptibility to gamma-rays and the mtDNA repair ability of oral squamous cell carcinoma (OSC) cell lines. The levels of 8-hydroxy-2'-deoxyguanosine (8-OHdG) and mtDNA common deletion in both nuclear and mitochondrial DNA of OSC-2, OSC-3 and OSC-6 cells (radio-sensitive cell lines) after gamma-ray-irradiation were higher than those of OSC-1, OSC-4 and OSC-5 cells (radio-resistant cell lines). Compared with OSC-2, OSC-3 and OSC-6 cells, OSC-1, OSC-4 and OSC-5 cells had higher levels of activity of phosphoinositide-3 kinase (PI-3K)/Akt and more strongly expressed 8-hydroxyguanine DNA glycosylase (OGG1), DNA polymerase gamma (POLG) and mitochondrial transcription factor A (Tfam). Down-regulation of these mtDNA-repair-associated molecules by the RNA interference technique enhanced the susceptibility of OSC-2 and OSC-5 cells to gamma-rays, and the expression of Tfam and POLG was down-regulated by inhibitors of PI-3K/Akt signaling. These results indicate that the inhibition of mtDNA repair capacity by PI-3K/Akt signal inhibitors and OGG1 down-regulator in cancer cells may be a useful strategy for cancer treatment when combined with ionizing irradiation and chemotherapeutic drugs.

DNA polymerase gamma and mitochondrial disease: understanding the consequence of POLG mutations

DNA polymerase gamma is the only known DNA polymerase in human mitochondria and is essential for mitochondrial DNA replication and repair. It is well established that defects in mtDNA replication lead to mitochondrial dysfunction and disease. Over 160 coding variations in the gene encoding the catalytic subunit of DNA polymerase gamma (POLG) have been identified. Our group and others have characterized a number of the more common and interesting mutations, as well as those disease mutations in the DNA polymerase gamma accessory subunit. We review the results of these studies, which provide clues to the mechanisms leading to the disease state.

Populationsgenetische mitochondriale DNA-Daten

Der populationsgenetische Aspekt der Nutzung mitochondrialer DNA in der Forensik und medizinischen Genetik bezieht sich implizit auf die gesamte Datengrundlage und die mtDNA-Phylogenie, von der in Hinblick auf die zu untersuchenden Fragestellungen gezielt Teile ausgesondert werden. Wir heben besonders jene Aspekte hervor, die in der Vergangenheit bei vielen Untersuchungen nicht adäquat berücksichtigt wurden.

Sensory ataxic neuropathy with ophthalmoparesis caused by POLG mutations

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

Depletion of mitochondrial DNA in fibroblast cultures from patients with POLG1 mutations is a consequence of catalytic mutations

We investigated clinical and cellular phenotypes of 24 children with mutations in the catalytic (alpha) subunit of the mitochondrial DNA (mtDNA) gamma polymerase (POLG1). Twenty-one had Alpers syndrome, the commonest severe POLG1 autosomal recessive phenotype, comprising hepatoencephalopathy and often mtDNA depletion. The cellular mtDNA content reflected the genotype more closely than did clinical features. Patients with tissue depletion of mtDNA all had at least one allele with either a missense mutation in a catalytic domain or a nonsense mutation. Four out of 12 patients exhibited a progressive, mosaic pattern of mtDNA depletion in cultured fibroblasts. All these patients had mutations in a catalytic domain in both POLG1 alleles, in either the polymerase or exonuclease domain or both. The tissue mtDNA content of patients who had two linker mutations was normal, and their phenotypes the mildest. Epilepsy and/or movement disorder were major features in all 21. Previous studies have implicated replication stalling as a mechanism for mtDNA depletion. The mosaic cellular depletion that we have demonstrated in cell cultures may be a manifestation of severe replication stalling. One patient with a severe cellular and clinical phenotype was a compound heterozygote with POLG1 mutations in the polymerase and exonuclease domain intrans. This suggests that POLG1 requires both polymerase and 3'-5' exonuclease activity in the same molecule. This is consistent with current functional models for eukaryotic DNA polymerases, which alternate between polymerizing and editing modes, as determined by competition between these two active sites for the 3' end of the DNA.

Inherited mitochondrial diseases of DNA replication

Mitochondrial genetic diseases can result from defects in mitochondrial DNA (mtDNA) in the form of deletions, point mutations, or depletion, which ultimately cause loss of oxidative phosphorylation. These mutations may be spontaneous, maternally inherited, or a result of inherited nuclear defects in genes that maintain mtDNA. This review focuses on our current understanding of nuclear gene mutations that produce mtDNA alterations and cause mitochondrial depletion syndrome (MDS), progressive external ophthalmoplegia (PEO), ataxia-neuropathy, or mitochondrial neurogastrointestinal encephalomyopathy (MNGIE). To date, all of these etiologic nuclear genes fall into one of two categories: genes whose products function directly at the mtDNA replication fork, such as POLG, POLG2, and TWINKLE, or genes whose products supply the mitochondria with deoxynucleotide triphosphate pools needed for DNA replication, such as TK2, DGUOK, TP, SUCLA2, ANT1, and possibly the newly identified MPV17.

What causes mitochondrial DNA deletions in human cells?

Mitochondrial DNA (mtDNA) deletions are a primary cause of mitochondrial disease and are likely to have a central role in the aging of postmitotic tissues. Understanding the mechanism of the formation and subsequent clonal expansion of these mtDNA deletions is an essential first step in trying to prevent their occurrence. We review the previous literature and recent results from our own laboratories, and conclude that mtDNA deletions are most likely to occur during repair of damaged mtDNA rather than during replication. This conclusion has important implications for prevention of mtDNA disease and, potentially, for our understanding of the aging process.

Generation of rho0 cells utilizing a mitochondrially targeted restriction endonuclease and comparative analyses

Eukaryotic cells devoid of mitochondrial DNA (ρ⁰ cells) were originally generated under artificial growth conditions utilizing ethidium bromide. The chemical is known to intercalate preferentially with the mitochondrial double-stranded DNA thereby interfering with enzymes of the replication machinery. ρ⁰ cell lines are highly valuable tools to study human mitochondrial disorders because they can be utilized in cytoplasmic transfer experiments. However, mutagenic effects of ethidium bromide onto the nuclear DNA cannot be excluded. To foreclose this mutagenic character during the development of ρ⁰ cell lines, we developed an extremely mild, reliable and timesaving method to generate ρ⁰ cell lines within 3–5 days based on an enzymatic approach. Utilizing the genes for the restriction endonuclease EcoRI and the fluorescent protein EGFP that were fused to a mitochondrial targeting sequence, we developed a CMV-driven expression vector that allowed the temporal expression of the resulting fusion enzyme in eukaryotic cells. Applied on the human cell line 143B.TK⁻ the active protein localized to mitochondria and induced the complete destruction of endogenous mtDNA. Mouse and rat ρ⁰ cell lines were also successfully created with this approach. Furthermore, the newly established 143B.TK⁻ ρ⁰ cell line was characterized in great detail thereby releasing interesting insights into the morphology and ultra structure of human ρ⁰ mitochondria.

Mitochondrial DNA content, an inaccurate biomarker of mitochondrial alteration in human immunodeficiency virus-related lipodystrophy

Lipoatrophy is a prevalent side effect of antiretroviral treatment of human immunodeficiency virus (HIV) infection. Its mechanisms are still disputed but include mitochondrial toxicity and, in particular, mitochondrial DNA (mtDNA) depletion induced by nucleoside reverse transcriptase inhibitors. To obtain an integrated evaluation of the mitochondrial alteration in lipoatrophy, we investigated the DNA, RNA, and protein levels in 15 samples of abdominal subcutaneous adipose tissue from HIV-infected patients with peripheral lipoatrophy and compared the results with those for 15 samples from age- and body mass index-matched controls. The DNA and RNA analyses used PCR-based techniques, while proteins were quantified with enzyme-linked immunosorbent assay and measurement of activities with spectrophotometric assays. Depletion of mtDNA and mtDNA-encoded MT-CO2 mRNA was present, but normal levels of mtDNA-dependent activity (cytochrome c oxidase) and protein (MT-CO2p) showed that it was compensated for. An increase in nuclear-DNA-dependent mitochondrial activities (citrate synthase and malate dehydrogenase) and protein (COX4I1p), as well as transcriptional up-regulation of nuclear-DNA-encoded mitochondrial genes (COX4I1 and UCP2), demonstrated increased mitochondrial biogenesis. However, the expression of the known transcription factors of mitochondrial biogenesis (TFAM, NRF1, GABPA, PPARGC1A, PPARGC1B, and PPRC1) was normal or decreased. Increased amounts of activated caspase 3 and of DDIT3 mRNA showed the induction of apoptosis and oxidative stress, respectively. The mtDNA content did not correlate with any other mitochondrial parameter. In conclusion, mtDNA content does not appear to be an accurate biomarker of mitochondrial alteration in lipoatrophic adipose tissue. The preservation of mtDNA-dependent mitochondrial functions occurred despite severe mtDNA depletion. The presence of significant oxidative stress and apoptosis did not correlate with the mtDNA content.

Real-time detection and quantification of mitochondrial mutations with oligonucleotide primers containing locked nucleic acid

BACKGROUND

The phenotypic expression of disorders caused by point mutations, deletions or depletions within the mitochondrial genome (mtDNA) is heterogeneous. This relates to the phenomena of heteroplasmy, tissue threshold as well as the distribution of mutant DNA among tissues. Hence, the diagnostics of these disorders demands highly specific, sensitive and quantitative methods.

METHODS

We have developed an allele-specific quantitative real-time PCR method for the detection of two of the most prevalent disease causing mitochondrial mutations, m.3243A>G (MELAS) and m.8993T>G (NARP). Locked Nucleic Acid (LNA) modified primers were used to obtain high allele specificity. In order to monitor mtDNA depletion a real-time method for mtDNA/nuclear DNA copy number ratio determination was developed.

RESULTS

Rapid and sensitive detection and quantification of MELAS and NARP mtDNA alleles were achieved. Heteroplasmy levels as low as 0.01% could be detected, and the mtDNA/nuclear DNA ratio could be determined.

CONCLUSIONS

The present method that allows simultaneous determination of heteroplasmy levels and mtDNA/nuclear DNA copy number ratio, will provide a useful tool in molecular diagnostics and in future epidemiological studies of mitochondrial diseases.

Mitochondrial DNA damage and impaired base excision repair during epileptogenesis

Oxidative stress and mitochondrial dysfunction are acute consequences of status epilepticus (SE). However, the role of mitochondrial oxidative stress and genomic instability during epileptogenesis remains unknown. Using the kainate animal model of temporal lobe epilepsy, we investigated oxidative mitochondrial DNA (mtDNA) damage and changes in the mitochondrial base excision repair pathway (mtBER) in the rat hippocampus for a period of 3 months after SE. Acute seizure activity caused a time-dependent increase in mitochondrial, but not nuclear 8-hydroxy-2-deoxyguanosine (8-OHdG/2dG) levels and a greater frequency of mtDNA lesions. This was accompanied by increased mitochondrial H2O2 production and a transient decrease in mtDNA repair capacity. The mtBER proteins 8-oxoguanine glycosylase (Ogg1) and DNA polymerase gamma (Pol gamma) demonstrated elevated expression at mRNA and protein levels shortly after SE and this was followed by a gradual improvement in mtDNA repair capacity. Recurrent seizures associated with the chronic phase of epilepsy coincided with the accumulation of mtDNA damage, increased mitochondrial H2O2 levels, decreased expression of Ogg1 and Pol gamma and impaired mtDNA repair capacity. Together, increased oxidative mtDNA damage, mitochondrial H2O2 production and alterations in the mtBER pathway provide evidence for mitochondrial oxidative stress in epilepsy and suggest that mitochondrial injury may contribute to epileptogenesis.

Mitochondrial disorders

PURPOSE OF REVIEW

Mitochondrial disorders are increasingly acknowledged as a major category in clinical neurology. In this review we highlight the most recent advances in the field, including the characterization of new disease genes, new physiopathological insights, and the role of mitochondrial dysfunction in neurodegeneration.

RECENT FINDINGS

Substantial progress has been made on the genetic basis and pathogenic mechanisms in disorders associated with altered mitochondrial DNA stability and expression. These defects include a wide spectrum of neurological conditions caused by genetic abnormalities of the mitochondrial replication and translation machineries, and of the metabolic pathways controlling the nucleotide supply to organelles, cells and tissues. Another relevant contribution has been given to the molecular dissection of coenzyme Q deficiency, a clinically heterogeneous, potentially treatable condition, thanks to the biochemical and genetic characterization of the first defects in coenzyme Q biosynthesis. Finally, the genetic determinants controlling the penetrance of mitochondrial disorders, as well as the role of mitochondrial dysfunction in neurodegenerative conditions such as Parkinson's and Huntington's diseases, have been investigated in both patients and animal models.

SUMMARY

The dual genetic contribution controlling mitochondrial biogenesis, and the intricacy and universality of the metabolic pathways operating in the mitochondrion explain the complexity of what is now known as 'mitochondrial medicine'.