Inhibition of mammalian mtDNA transcription acts paradoxically to reverse diet-induced hepatosteatosis and obesity

The oxidative phosphorylation system1 in mammalian mitochondria plays a key role in transducing energy from ingested nutrients2. Mitochondrial metabolism is dynamic and can be reprogrammed to support both catabolic and anabolic reactions, depending on physiological demands or disease states. Rewiring of mitochondrial metabolism is intricately linked to metabolic diseases and promotes tumour growth3-5. Here, we demonstrate that oral treatment with an inhibitor of mitochondrial transcription (IMT)6 shifts whole-animal metabolism towards fatty acid oxidation, which, in turn, leads to rapid normalization of body weight, reversal of hepatosteatosis and restoration of normal glucose tolerance in male mice on a high-fat diet. Paradoxically, the IMT treatment causes a severe reduction of oxidative phosphorylation capacity concomitant with marked upregulation of fatty acid oxidation in the liver, as determined by proteomics and metabolomics analyses. The IMT treatment leads to a marked reduction of complex I, the main dehydrogenase feeding electrons into the ubiquinone (Q) pool, whereas the levels of electron transfer flavoprotein dehydrogenase and other dehydrogenases connected to the Q pool are increased. This rewiring of metabolism caused by reduced mtDNA expression in the liver provides a principle for drug treatment of obesity and obesity-related pathology.

PARKIN is not required to sustain OXPHOS function in adult mammalian tissues

Loss-of-function variants in the PRKN gene encoding the ubiquitin E3 ligase PARKIN cause autosomal recessive early-onset Parkinson's disease (PD). Extensive in vitro and in vivo studies have reported that PARKIN is involved in multiple pathways of mitochondrial quality control, including mitochondrial degradation and biogenesis. However, these findings are surrounded by substantial controversy due to conflicting experimental data. In addition, the existing PARKIN-deficient mouse models have failed to faithfully recapitulate PD phenotypes. Therefore, we have investigated the mitochondrial role of PARKIN during ageing and in response to stress by employing a series of conditional Parkin knockout mice. We report that PARKIN loss does not affect oxidative phosphorylation (OXPHOS) capacity and mitochondrial DNA (mtDNA) levels in the brain, heart, and skeletal muscle of aged mice. We also demonstrate that PARKIN deficiency does not exacerbate the brain defects and the pro-inflammatory phenotype observed in mice carrying high levels of mtDNA mutations. To rule out compensatory mechanisms activated during embryonic development of Parkin-deficient mice, we generated a mouse model where loss of PARKIN was induced in adult dopaminergic (DA) neurons. Surprisingly, also these mice did not show motor impairment or neurodegeneration, and no major transcriptional changes were found in isolated midbrain DA neurons. Finally, we report a patient with compound heterozygous PRKN pathogenic variants that lacks PARKIN and has developed PD. The PARKIN deficiency did not impair OXPHOS activities or induce mitochondrial pathology in skeletal muscle from the patient. Altogether, our results argue that PARKIN is dispensable for OXPHOS function in adult mammalian tissues.

Ataxia Syndrome With Hearing Loss and Nephronophthisis Associated With a Novel Homozygous Variant in XPNPEP3

Objectives

Biallelic variants in XPNPEP3 are associated with a rare mitochondrial syndrome characterized by nephronophthisis leading to kidney failure, essential tremor, hearing loss, seizures, and intellectual disability. Only 2 publications on this condition are available. We report a man with a complex ataxia syndrome, hearing loss, and kidney failure associated with a new biallelic variant in XPNPEP3.

Methods

Clinical evaluation, neuroimaging studies, a kidney biopsy, and whole genome sequencing (WGS) were applied. Since the phenotype was compatible with a mitochondrial disease, a muscle biopsy with morphological and mitochondrial biochemical investigations was performed.

Results

Axial ataxia, cerebellar atrophy, hearing loss, myopathy, ptosis, supranuclear palsy, and kidney failure because of nephronophthisis were the prominent features in this case. WGS revealed the novel biallelic variant c.766C>T (p.Gln256*) in XPNPEP3. A muscle biopsy revealed COX negative fibers, a few ragged red fibers, and ultrastructural mitochondrial changes. Enzyme activity in respiratory chain complex IV was reduced in muscle and fibroblasts.

Discussion

This is the first report of a slowly progressive cerebellar ataxia associated with a novel biallelic variant in XPNPEP3. Abnormalities typical for mitochondrial disease and the slow progression of kidney disease are also striking. Our report expands the spectrum of XPNPEP3-related diseases.

Human mitochondria require mtRF1 for translation termination at non-canonical stop codons

The mitochondrial translation machinery highly diverged from its bacterial counterpart. This includes deviation from the universal genetic code, with AGA and AGG codons lacking cognate tRNAs in human mitochondria. The locations of these codons at the end of COX1 and ND6 open reading frames, respectively, suggest they might function as stop codons. However, while the canonical stop codons UAA and UAG are known to be recognized by mtRF1a, the release mechanism at AGA and AGG codons remains a debated issue. Here, we show that upon the loss of another member of the mitochondrial release factor family, mtRF1, mitoribosomes accumulate specifically at AGA and AGG codons. Stalling of mitoribosomes alters COX1 transcript and protein levels, but not ND6 synthesis. In addition, using an in vitro reconstituted mitochondrial translation system, we demonstrate the specific peptide release activity of mtRF1 at the AGA and AGG codons. Together, our results reveal the role of mtRF1 in translation termination at non-canonical stop codons in mitochondria.

Case Report: A Novel Mutation in the Mitochondrial MT-ND5 Gene Is Associated With Leber Hereditary Optic Neuropathy (LHON)

Leber hereditary optic neuropathy (LHON) is a mitochondrial disease causing severe bilateral visual loss, typically in young adults. The disorder is commonly caused by one of three primary point mutations in mitochondrial DNA, but a number of other rare mutations causing or associated with the clinical syndrome of LHON have been reported. The mutations in LHON are almost exclusively located in genes encoding subunits of complex I in the mitochondrial respiratory chain. Here we report two patients, a mother and her son, with the typical LHON phenotype. Genetic investigations for the three common mutations were negative, instead we found a new and previously unreported mutation in mitochondrial DNA. This homoplasmic mutation, m.13345G>A, is located in the MT-ND5 gene, encoding a core subunit in complex I in the mitochondrial respiratory chain. Investigation of the patients mitochondrial respiratory chain in muscle found a mild defect in the combined activity of complex I+III. In the literature six other mutations in the MT-ND5 gene have been associated with LHON and by this report a new putative mutation in the MT-ND5 can be added.

Novel Mutation m.10372A>G in MT-ND3 Causing Sensorimotor Axonal Polyneuropathy

Objective

To investigate the pathogenicity of a novel MT-ND3 mutation identified in a patient with adult-onset sensorimotor axonal polyneuropathy and report the clinical, morphologic, and biochemical findings.

Methods

Clinical assessments and morphologic and biochemical investigations of skeletal muscle and cultured myoblasts from the patient were performed. Whole-genome sequencing (WGS) of DNA from skeletal muscle and Sanger sequencing of mitochondrial DNA (mtDNA) from both skeletal muscle and cultured myoblasts were performed. Heteroplasmic levels of mutated mtDNA in different tissues were quantified by last-cycle hot PCR.

Results

Muscle showed ragged red fibers, paracrystalline inclusions, a significant reduction in complex I (CI) respiratory chain (RC) activity, and decreased adenosine triphosphate (ATP) production for all substrates used by CI. Sanger sequencing of DNA from skeletal muscle detected a unique previously unreported heteroplasmic mutation in mtDNA encoded MT-ND3, coding for a subunit in CI. WGS confirmed the mtDNA mutation but did not detect any other mutation explaining the disease. Cultured myoblasts, however, did not carry the mutation, and RC activity measurements in myoblasts were normal.

Conclusions

We report a case with adult-onset sensorimotor axonal polyneuropathy caused by a novel mtDNA mutation in MT-ND3. Loss of heteroplasmy in blood, cultured fibroblasts and myoblasts from the patient, and normal measurement of RC activity of the myoblasts support pathogenicity of the mutation. These findings highlight the importance of mitochondrial investigations in patients presenting with seemingly idiopathic polyneuropathy, especially if muscle also is affected.

Severe congenital lactic acidosis and hypertrophic cardiomyopathy caused by an intronic variant in NDUFB7

Mutations in structural subunits and assembly factors of complex I of the oxidative phosphorylation system constitute the most common cause of mitochondrial respiratory chain defects. Such mutations can present a wide range of clinical manifestations, varying from mild deficiencies to severe, lethal disorders. We describe a patient presenting intrauterine growth restriction and anemia, which displayed postpartum hypertrophic cardiomyopathy, lactic acidosis, encephalopathy, and a severe complex I defect with fatal outcome. Whole genome sequencing revealed an intronic biallelic mutation in the NDUFB7 gene (c.113-10C>G) and splicing pattern alterations in NDUFB7 messenger RNA were confirmed by RNA Sequencing. The detected variant resulted in a significant reduction of the NDUFB7 protein and reduced complex I activity. Complementation studies with expression of wild-type NDUFB7 in patient fibroblasts normalized complex I function. Here we report a case with a primary complex I defect due to a homozygous mutation in an intron region of the NDUFB7 gene.

The one-carbon pool controls mitochondrial energy metabolism via complex I and iron-sulfur clusters

Induction of the one-carbon cycle is an early hallmark of mitochondrial dysfunction and cancer metabolism. Vital intermediary steps are localized to mitochondria, but it remains unclear how one-carbon availability connects to mitochondrial function. Here, we show that the one-carbon metabolite and methyl group donor S-adenosylmethionine (SAM) is pivotal for energy metabolism. A gradual decline in mitochondrial SAM (mitoSAM) causes hierarchical defects in fly and mouse, comprising loss of mitoSAM-dependent metabolites and impaired assembly of the oxidative phosphorylation system. Complex I stability and iron-sulfur cluster biosynthesis are directly controlled by mitoSAM levels, while other protein targets are predominantly methylated outside of the organelle before import. The mitoSAM pool follows its cytosolic production, establishing mitochondria as responsive receivers of one-carbon units. Thus, we demonstrate that cellular methylation potential is required for energy metabolism, with direct relevance for pathophysiology, aging, and cancer.

Clinical Presentation, Genetic Etiology, and Coenzyme Q10 Levels in 55 Children with Combined Enzyme Deficiencies of the Mitochondrial Respiratory Chain

OBJECTIVES

To evaluate the clinical symptoms and biochemical findings and establish the genetic etiology in a cohort of pediatric patients with combined deficiencies of the mitochondrial respiratory chain complexes.

STUDY DESIGN

Clinical and biochemical data were collected from 55 children. All patients were subjected to sequence analysis of the entire mitochondrial genome, except when the causative mutations had been identified based on the clinical picture. Whole exome sequencing/whole genome sequencing (WES/WGS) was performed in 32 patients.

RESULTS

Onset of disease was generally early in life (median age, 6 weeks). The most common symptoms were muscle weakness, hypotonia, and developmental delay/intellectual disability. Nonneurologic symptoms were frequent. Disease causing mutations were found in 20 different nuclear genes, and 7 patients had mutations in mitochondrial DNA. Causative variants were found in 18 of the 32 patients subjected to WES/WGS. Interestingly, many patients had low levels of coenzyme Q10 in muscle, irrespective of genetic cause.

CONCLUSIONS

Children with combined enzyme defects display a diversity of clinical symptoms with varying age of presentation. We established the genetic diagnosis in 35 of the 55 patients (64%). The high diagnostic yield was achieved by the introduction of massive parallel sequencing, which also revealed novel genes and enabled elucidation of new disease mechanisms.

Defects of mitochondrial RNA turnover lead to the accumulation of double-stranded RNA in vivo

The RNA helicase SUV3 and the polynucleotide phosphorylase PNPase are involved in the degradation of mitochondrial mRNAs but their roles in vivo are not fully understood. Additionally, upstream processes, such as transcript maturation, have been linked to some of these factors, suggesting either dual roles or tightly interconnected mechanisms of mitochondrial RNA metabolism. To get a better understanding of the turn-over of mitochondrial RNAs in vivo, we manipulated the mitochondrial mRNA degrading complex in Drosophila melanogaster models and studied the molecular consequences. Additionally, we investigated if and how these factors interact with the mitochondrial poly(A) polymerase, MTPAP, as well as with the mitochondrial mRNA stabilising factor, LRPPRC. Our results demonstrate a tight interdependency of mitochondrial mRNA stability, polyadenylation and the removal of antisense RNA. Furthermore, disruption of degradation, as well as polyadenylation, leads to the accumulation of double-stranded RNAs, and their escape out into the cytoplasm is associated with an altered immune-response in flies. Together our results suggest a highly organised and inter-dependable regulation of mitochondrial RNA metabolism with far reaching consequences on cellular physiology.

Although a number of factors have been implemented in the turnover of mitochondrial (mt) DNA-derived transcripts, their exact functions and interplay with one another is not entirely clear. Several of these factors have been proposed to co-ordinately regulate both transcript maturation, as well as degradation, but the order of events during mitochondrial RNA turnover is less well understood. Using a range of different genetically modified Drosophila melanogaster models, we studied the involvement of the RNA helicase SUV3, the polynucleotide phosphorylase PNPase, the leucine-rich pentatricopeptide repeat motif-containing protein LRPPRC, and the mitochondrial RNA poly(A) polymerase MTPAP, in stabilisation, polyadenylation, and degradation of mitochondrial transcripts. Our results show a tight collaborative activity of these factors in vivo and reveal a clear hierarchical order of events leading to mitochondrial mRNA maturation. Furthermore, we demonstrate that the loss of SUV3, PNPase, or MTPAP leads to the accumulation of mitochondrial-derived antisense RNA in the cytoplasm of cells, which is associated with an altered immune-response in flies.

TEFM regulates both transcription elongation and RNA processing in mitochondria

Regulation of replication and expression of mitochondrial DNA (mtDNA) is essential for cellular energy conversion via oxidative phosphorylation. The mitochondrial transcription elongation factor (TEFM) has been proposed to regulate the switch between transcription termination for replication primer formation and processive, near genome‐length transcription for mtDNA gene expression. Here, we report that Tefm is essential for mouse embryogenesis and that levels of promoter‐distal mitochondrial transcripts are drastically reduced in conditional Tefm‐knockout hearts. In contrast, the promoter‐proximal transcripts are much increased in Tefm knockout mice, but they mostly terminate before the region where the switch from transcription to replication occurs, and consequently, de novo mtDNA replication is profoundly reduced. Unexpectedly, deep sequencing of RNA from Tefm knockouts revealed accumulation of unprocessed transcripts in addition to defective transcription elongation. Furthermore, a proximity‐labeling (BioID) assay showed that TEFM interacts with multiple RNA processing factors. Our data demonstrate that TEFM acts as a general transcription elongation factor, necessary for both gene transcription and replication primer formation, and loss of TEFM affects RNA processing in mammalian mitochondria.

Modulation of mtDNA copy number ameliorates the pathological consequences of a heteroplasmic mtDNA mutation in the mouse

Heteroplasmic mtDNA mutations typically act in a recessive way and cause mitochondrial disease only if present above a certain threshold level. We have experimentally investigated to what extent the absolute levels of wild-type (WT) mtDNA influence disease manifestations by manipulating TFAM levels in mice with a heteroplasmic mtDNA mutation in the tRNA^Ala gene. Increase of total mtDNA levels ameliorated pathology in multiple tissues, although the levels of heteroplasmy remained the same. A reduction in mtDNA levels worsened the phenotype in postmitotic tissues, such as heart, whereas there was an unexpected beneficial effect in rapidly proliferating tissues, such as colon, because of enhanced clonal expansion and selective elimination of mutated mtDNA. The absolute levels of WT mtDNA are thus an important determinant of the pathological manifestations, suggesting that pharmacological or gene therapy approaches to selectively increase mtDNA copy number provide a potential treatment strategy for human mtDNA mutation disease.

Mutations in the mitochondrial tryptophanyl-tRNA synthetase cause growth retardation and progressive leukoencephalopathy

Background

Mutations in mitochondrial aminoacyl tRNA synthetases form a subgroup of mitochondrial disorders often only perturbing brain function by affecting mitochondrial translation. Here we report two siblings with mitochondrial disease, due to compound heterozygous mutations in the mitochondrial tryptophanyl‐tRNA synthetase (WARS2) gene, presenting with severe neurological symptoms but normal mitochondrial function in skeletal muscle biopsies and cultured skin fibroblasts.

Methods

Whole exome sequencing on genomic DNA samples from both subjects and their parents identified two compound heterozygous variants c.833T>G (p.Val278Gly) and c.938A>T (p.Lys313Met) in the WARS2 gene as potential disease‐causing variants. We generated patient‐derived neuroepithelial stem cells and modeled the disease in yeast and Drosophila melanogaster to confirm pathogenicity.

Results

Biochemical analysis of patient‐derived neuroepithelial stem cells revealed a mild combined complex I and IV defect, while modeling the disease in yeast demonstrated that the reported aminoacylation defect severely affects respiration and viability. Furthermore, silencing of wild type WARS2 in Drosophila melanogaster showed that a partial defect in aminoacylation is enough to cause lethality.

Conclusions

Our results establish the identified WARS2 variants as disease‐causing and highlight the benefit of including human neuronal models, when investigating mutations specifically affecting the nervous system.

DARS2 protects against neuroinflammation and apoptotic neuronal loss, but is dispensable for myelin producing cells

Although mitochondria are ubiquitous, each mitochondrial disease has surprisingly distinctly different pattern of tissue and organ involvement. Congruently, mutations in genes encoding for different mitochondrial tRNA synthetases result in the development of a very flamboyant group of diseases. Mutations in some of these genes, including aspartyl-tRNA synthetase (DARS2), lead to the onset of a white matter disease-leukoencephalopathy with brainstem and spinal cord involvement, and lactate elevation (LBSL) characterized by progressive spastic ataxia and characteristic leukoencephalopathy signature with multiple long-tract involvements. Puzzled by the white matter disease phenotypes caused by DARS2 deficiency when numerous other mutations in the genes encoding proteins involved in mitochondrial translation have a detrimental effect predominantly on neurons, we generated transgenic mice in which DARS2 was specifically depleted in forebrain-hippocampal neurons or myelin-producing cells. Our results now provide the first evidence that loss of DARS2 in adult neurons leads to strong mitochondrial dysfunction and progressive loss of cells. In contrast, myelin-producing cells seem to be resistant to cell death induced by DARS2 depletion despite robust respiratory chain deficiency arguing that LBSL might originate from the primary neuronal and axonal defect. Remarkably, our results also suggest a role for early neuroinflammation in the disease progression, highlighting the possibility for therapeutic interventions of this process.

A multi-systemic mitochondrial disorder due to a dominant p.Y955H disease variant in DNA polymerase gamma

Mutations in the mitochondrial DNA polymerase, POLG, are associated with a variety of clinical presentations, ranging from early onset fatal brain disease in Alpers syndrome to chronic progressive external ophthalmoplegia. The majority of mutations are linked with disturbances of mitochondrial DNA (mtDNA) integrity and maintenance. On a molecular level, depending on their location within the enzyme, mutations either lead to mtDNA depletion or the accumulation of multiple mtDNA deletions, and in some cases these molecular changes can be correlated to the clinical presentation. We identified a patient with a dominant p.Y955H mutation in POLG, presenting with a severe, early-onset multi-systemic mitochondrial disease with bilateral sensorineural hearing loss, cataract, myopathy, and liver failure. Using a combination of disease models of Drosophila melanogaster and in vitro biochemistry analysis, we compare the molecular consequences of the p.Y955H mutation to the well-documented p.Y955C mutation. We demonstrate that both mutations affect mtDNA replication and display a dominant negative effect, with the p.Y955H allele resulting in a more severe polymerase dysfunction.

Respiratory chain complex III deficiency due to mutated BCS1L: a novel phenotype with encephalomyopathy, partially phenocopied in a Bcs1l mutant mouse model

BACKGROUND

Mitochondrial diseases due to defective respiratory chain complex III (CIII) are relatively uncommon. The assembly of the eleven-subunit CIII is completed by the insertion of the Rieske iron-sulfur protein, a process for which BCS1L protein is indispensable. Mutations in the BCS1L gene constitute the most common diagnosed cause of CIII deficiency, and the phenotypic spectrum arising from mutations in this gene is wide.

RESULTS

A case of CIII deficiency was investigated in depth to assess respiratory chain function and assembly, and brain, skeletal muscle and liver histology. Exome sequencing was performed to search for the causative mutation(s). The patient's platelets and muscle mitochondria showed respiration defects and defective assembly of CIII was detected in fibroblast mitochondria. The patient was compound heterozygous for two novel mutations in BCS1L, c.306A > T and c.399delA. In the cerebral cortex a specific pattern of astrogliosis and widespread loss of microglia was observed. Further analysis showed loss of Kupffer cells in the liver. These changes were not found in infants suffering from GRACILE syndrome, the most severe BCS1L-related disorder causing early postnatal mortality, but were partially corroborated in a knock-in mouse model of BCS1L deficiency.

CONCLUSIONS

We describe two novel compound heterozygous mutations in BCS1L causing CIII deficiency. The pathogenicity of one of the mutations was unexpected and points to the importance of combining next generation sequencing with a biochemical approach when investigating these patients. We further show novel manifestations in brain, skeletal muscle and liver, including abnormality in specialized resident macrophages (microglia and Kupffer cells). These novel phenotypes forward our understanding of CIII deficiencies caused by BCS1L mutations.

Reduced metabolism in the hypothalamus of the anorectic anx/anx mouse

The anorectic anx/anx mouse exhibits a mitochondrial complex I dysfunction that is related to aberrant expression of hypothalamic neuropeptides and transmitters regulating food intake. Hypothalamic activity, i.e. neuronal firing and transmitter release, is dependent on glucose utilization and energy metabolism. To better understand the role of hypothalamic activity in anorexia, we assessed carbohydrate and high-energy phosphate metabolism, in vivo and in vitro, in the anx/anx hypothalamus. In the fasted state, hypothalamic glucose uptake in the anx/anx mouse was reduced by ~50% of that seen in wild-type (wt) mice (P < 0.05). Under basal conditions, anx/anx hypothalamus ATP and glucose 6-P contents were similar to those in wt hypothalamus, whereas phosphocreatine was elevated (~2-fold; P < 0.001) and lactate was reduced (~35%; P < 0.001). The anx/anx hypothalamus had elevated total AMPK (~25%; P < 0.05) and GLUT4 (~60%; P < 0.01) protein contents, whereas GLUT1 and GLUT3 were similar to that of wt hypothalamus. Interestingly, the activation state of AMPK (ratio of phosphorylated AMPK/total AMPK) was significantly decreased in hypothalamus of the anx/anx mouse (~60% of that in wt; P < 0.05). Finally, during metabolic stress (ischemia), accumulation of lactate (measure of glycolysis) and IMP and AMP (breakdown products of ATP) were ~50% lower in anx/anx vs wt hypothalamus. These data demonstrate that carbohydrate and high-energy phosphate utilization in the anx/anx hypothalamus are diminished under basal and stress conditions. The decrease in hypothalamic metabolism may contribute to the anorectic behavior of the anx/anx mouse, i.e. its inability to regulate food intake in accordance with energy status.

CLPP coordinates mitoribosomal assembly through the regulation of ERAL1 levels

Despite being one of the most studied proteases in bacteria, very little is known about the role of ClpXP in mitochondria. We now present evidence that mammalian CLPP has an essential role in determining the rate of mitochondrial protein synthesis by regulating the level of mitoribosome assembly. Through a proteomic approach and the use of a catalytically inactive CLPP, we produced the first comprehensive list of possible mammalian ClpXP substrates involved in the regulation of mitochondrial translation, oxidative phosphorylation, and a number of metabolic pathways. We further show that the defect in mitoribosomal assembly is a consequence of the accumulation of ERAL1, a putative 12S rRNA chaperone, and novel ClpXP substrate. The presented data suggest that the timely removal of ERAL1 from the small ribosomal subunit is essential for the efficient maturation of the mitoribosome and a normal rate of mitochondrial translation.

Mitochondrial Polyadenylation Is a One-Step Process Required for mRNA Integrity and tRNA Maturation

Polyadenylation has well characterised roles in RNA turnover and translation in a variety of biological systems. While polyadenylation on mitochondrial transcripts has been suggested to be a two-step process required to complete translational stop codons, its involvement in mitochondrial RNA turnover is less well understood. We studied knockdown and knockout models of the mitochondrial poly(A) polymerase (MTPAP) in Drosophila melanogaster and demonstrate that polyadenylation of mitochondrial mRNAs is exclusively performed by MTPAP. Further, our results show that mitochondrial polyadenylation does not regulate mRNA stability but protects the 3' terminal integrity, and that despite a lack of functioning 3' ends, these trimmed transcripts are translated, suggesting that polyadenylation is not required for mitochondrial translation. Additionally, loss of MTPAP leads to reduced steady-state levels and disturbed maturation of tRNA^Cys, indicating that polyadenylation in mitochondria might be important for the stability and maturation of specific tRNAs.

The polyadenylation of cellular RNAs is a well-studied signal for gene expression, with a defined function in either RNA turnover or translation, in the majority of systems. In mammalian mitochondria the role of polyadenylation is less clear, and can to date only be attributed to completing the translational stop signal on several mitochondrial transcripts. Previous work though demonstrated that mitochondrial polyadenylation requires a certain length and shortening of the poly(A) tail signal has detrimental effects on mitochondrial function. In this study we deleted the mitochondrial polymerase responsible for polyadenylation in the fly, Drosophila melanogaster, and demonstrate that the mitochondrial poly(A) tail is essential for preserving the 3’ ends of mitochondrial transcripts, with no other enzyme capable of completing stop signals. Our study also shows that polyadenylation does not regulate transcript stability nor is it required for translation, but might be involved in the maturation of certain mitochondrial tRNAs. We therefore conclude that besides completing translational stop signals, mitochondrial polyadenylation protects the 3’ termini from degradation.

Biotin and Thiamine Responsive Basal Ganglia Disease--A vital differential diagnosis in infants with severe encephalopathy

We report two siblings of Swedish origin with infantile Biotin and Thiamine Responsive Basal Ganglia Disease (BTRBG).

CASE REPORT

Initial symptoms were in both cases lethargia, with reduced contact and poor feeding from the age of 5 weeks. Magnetic resonance imaging showed altered signal in the basal ganglia, along with grey and white matter abnormalities. The diagnosis BTRBG was not recognized in the first sibling who died at the age of 8 weeks. The second sibling was started on biotin and thiamine immediately upon development of symptoms, leading to clinical improvement and partial reversion of the magnetic resonance imaging findings. Genetic analysis of the SLC19A3 gene identified two mutations, c.74dupT and c.1403delA, carried in compound heterozygous form in both boys, each inherited from one parent.

COMMENTS

The first mutation has previously been described in children with BTRBG, and the second mutation is novel. Although the clinical picture in BTRGB is very severe it is also rather unspecific and the diagnosis may be missed.

CONCLUSION

This report highlights the importance of considering biotin and thiamine treatment also in a European infant born to non-consanguineous parents, who presents with symptoms of acute/subacute encephalopathy.

Cyclophilin D, a target for counteracting skeletal muscle dysfunction in mitochondrial myopathy

Muscle weakness and exercise intolerance are hallmark symptoms in mitochondrial disorders. Little is known about the mechanisms leading to impaired skeletal muscle function and ultimately muscle weakness in these patients. In a mouse model of lethal mitochondrial myopathy, the muscle-specific Tfam knock-out (KO) mouse, we previously demonstrated an excessive mitochondrial Ca(2+) uptake in isolated muscle fibers that could be inhibited by the cyclophilin D (CypD) inhibitor, cyclosporine A (CsA). Here we show that the Tfam KO mice have increased CypD levels, and we demonstrate that this increase is a common feature in patients with mitochondrial myopathy. We tested the effect of CsA treatment on Tfam KO mice during the transition from a mild to terminal myopathy. CsA treatment counteracted the development of muscle weakness and improved muscle fiber Ca(2+) handling. Importantly, CsA treatment prolonged the lifespan of these muscle-specific Tfam KO mice. These results demonstrate that CsA treatment is an efficient therapeutic strategy to slow the development of severe mitochondrial myopathy.

Intra-mitochondrial Methylation Deficiency Due to Mutations in SLC25A26

S-adenosylmethionine (SAM) is the predominant methyl group donor and has a large spectrum of target substrates. As such, it is essential for nearly all biological methylation reactions. SAM is synthesized by methionine adenosyltransferase from methionine and ATP in the cytoplasm and subsequently distributed throughout the different cellular compartments, including mitochondria, where methylation is mostly required for nucleic-acid modifications and respiratory-chain function. We report a syndrome in three families affected by reduced intra-mitochondrial methylation caused by recessive mutations in the gene encoding the only known mitochondrial SAM transporter, SLC25A26. Clinical findings ranged from neonatal mortality resulting from respiratory insufficiency and hydrops to childhood acute episodes of cardiopulmonary failure and slowly progressive muscle weakness. We show that SLC25A26 mutations cause various mitochondrial defects, including those affecting RNA stability, protein modification, mitochondrial translation, and the biosynthesis of CoQ10 and lipoic acid.

SUV3 helicase is required for correct processing of mitochondrial transcripts

Mitochondrial gene expression is largely regulated by post-transcriptional mechanisms that control the amount and translation of each mitochondrial mRNA. Despite its importance for mitochondrial function, the mechanisms and proteins involved in mRNA turnover are still not fully characterized. Studies in yeast and human cell lines have indicated that the mitochondrial helicase SUV3, together with the polynucleotide phosphorylase, PNPase, composes the mitochondrial degradosome. To further investigate the in vivo function of SUV3 we disrupted the homolog of SUV3 in Drosophila melanogaster (Dm). Loss of dmsuv3 led to the accumulation of mitochondrial mRNAs, without increasing rRNA levels, de novo transcription or decay intermediates. Furthermore, we observed a severe decrease in mitochondrial tRNAs accompanied by an accumulation of unprocessed precursor transcripts. These processing defects lead to reduced mitochondrial translation and a severe respiratory chain complex deficiency, resulting in a pupal lethal phenotype. In summary, our results propose that SUV3 is predominantly required for the processing of mitochondrial polycistronic transcripts in metazoan and that this function is independent of PNPase.

Rescue of primary ubiquinone deficiency due to a novel COQ7 defect using 2,4-dihydroxybensoic acid

Background

Coenzyme Q is an essential mitochondrial electron carrier, redox cofactor and a potent antioxidant in the majority of cellular membranes. Coenzyme Q deficiency has been associated with a range of metabolic diseases, as well as with some drug treatments and ageing.

Methods

We used whole exome sequencing (WES) to investigate patients with inherited metabolic diseases and applied a novel ultra-pressure liquid chromatography—mass spectrometry approach to measure coenzyme Q in patient samples.

Results

We identified a homozygous missense mutation in the COQ7 gene in a patient with complex mitochondrial deficiency, resulting in severely reduced coenzyme Q levels We demonstrate that the coenzyme Q analogue 2,4-dihydroxybensoic acid (2,4DHB) was able to specifically bypass the COQ7 deficiency, increase cellular coenzyme Q levels and rescue the biochemical defect in patient fibroblasts.

Conclusion

We report the first patient with primary coenzyme Q deficiency due to a homozygous COQ7 mutation and a potentially beneficial treatment using 2,4DHB.

Rapid pulsed whole genome sequencing for comprehensive acute diagnostics of inborn errors of metabolism

Background

Massively parallel DNA sequencing (MPS) has the potential to revolutionize diagnostics, in particular for monogenic disorders. Inborn errors of metabolism (IEM) constitute a large group of monogenic disorders with highly variable clinical presentation, often with acute, nonspecific initial symptoms. In many cases irreversible damage can be reduced by initiation of specific treatment, provided that a correct molecular diagnosis can be rapidly obtained. MPS thus has the potential to significantly improve both diagnostics and outcome for affected patients in this highly specialized area of medicine.

Results

We have developed a conceptually novel approach for acute MPS, by analysing pulsed whole genome sequence data in real time, using automated analysis combined with data reduction and parallelization. We applied this novel methodology to an in-house developed customized work flow enabling clinical-grade analysis of all IEM with a known genetic basis, represented by a database containing 474 disease genes which is continuously updated. As proof-of-concept, two patients were retrospectively analysed in whom diagnostics had previously been performed by conventional methods. The correct disease-causing mutations were identified and presented to the clinical team after 15 and 18 hours from start of sequencing, respectively. With this information available, correct treatment would have been possible significantly sooner, likely improving outcome.

Conclusions

We have adapted MPS to fit into the dynamic, multidisciplinary work-flow of acute metabolic medicine. As the extent of irreversible damage in patients with IEM often correlates with timing and accuracy of management in early, critical disease stages, our novel methodology is predicted to improve patient outcome. All procedures have been designed such that they can be implemented in any technical setting and to any genetic disease area. The strategy conforms to international guidelines for clinical MPS, as only validated disease genes are investigated and as clinical specialists take responsibility for translation of results. As follow-up in patients without any known IEM, filters can be lifted and the full genome investigated, after genetic counselling and informed consent.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1090) contains supplementary material, which is available to authorized users.

Loss of UCP2 attenuates mitochondrial dysfunction without altering ROS production and uncoupling activity

Although mitochondrial dysfunction is often accompanied by excessive reactive oxygen species (ROS) production, we previously showed that an increase in random somatic mtDNA mutations does not result in increased oxidative stress. Normal levels of ROS and oxidative stress could also be a result of an active compensatory mechanism such as a mild increase in proton leak. Uncoupling protein 2 (UCP2) was proposed to play such a role in many physiological situations. However, we show that upregulation of UCP2 in mtDNA mutator mice is not associated with altered proton leak kinetics or ROS production, challenging the current view on the role of UCP2 in energy metabolism. Instead, our results argue that high UCP2 levels allow better utilization of fatty acid oxidation resulting in a beneficial effect on mitochondrial function in heart, postponing systemic lactic acidosis and resulting in longer lifespan in these mice. This study proposes a novel mechanism for an adaptive response to mitochondrial cardiomyopathy that links changes in metabolism to amelioration of respiratory chain deficiency and longer lifespan.

Partial tetrasomy 14 associated with multiple malformations

We report on an 8-year-old female patient with multiple malformations including bilateral cleft lip and palate, coloboma, and craniosynostosis. She presented with severe intellectual disability, seizures, and gastrointestinal dysfunction. Mitochondrial investigations in a muscle biopsy revealed reduced activity in complex I of the mitochondrial respiratory chain. Chromosome analysis and fluorescent in situ hybridization (FISH) studies showed an isodicentric marker chromosome 14 that was identified in all cells analyzed in peripheral blood lymphocytes and cultured fibroblasts. Parental chromosome studies were normal. To further characterize the marker chromosome and determine its origin, we performed array-based comparative genomic hybridization (CGH) and polymorphic marker analysis with quantitative fluorescent PCR (QF-PCR). The combined results from cytogenetic and array-CGH analyses showed tetrasomy 14p13q13.1 and results from the QF-PCR point to formation of the marker chromosome in the maternal meiosis. Isodicentric chromosomes involving partial 14q have previously been reported in four cases; however, this is the first patient with tetrasomy 14p13q13.1 in non-mosaic form surviving beyond infancy.

MTERF1 binds mtDNA to prevent transcriptional interference at the light-strand promoter but is dispensable for rRNA gene transcription regulation

Mitochondrial transcription termination factor 1, MTERF1, has been reported to couple rRNA gene transcription initiation with termination and is therefore thought to be a key regulator of mammalian mitochondrial ribosome biogenesis. The prevailing model is based on a series of observations published over the last two decades, but no in vivo evidence exists to show that MTERF1 regulates transcription of the heavy-strand region of mtDNA containing the rRNA genes. Here, we demonstrate that knockout of Mterf1 in mice has no effect on mitochondrial rRNA levels or mitochondrial translation. Instead, loss of Mterf1 influences transcription initiation at the light-strand promoter, resulting in a decrease of de novo transcription manifested as reduced 7S RNA levels. Based on these observations, we suggest that MTERF1 does not regulate heavy-strand transcription, but rather acts to block transcription on the opposite strand of mtDNA to prevent transcription interference at the light-strand promoter.

MTERF3 regulates mitochondrial ribosome biogenesis in invertebrates and mammals

Regulation of mitochondrial DNA (mtDNA) expression is critical for the control of oxidative phosphorylation in response to physiological demand, and this regulation is often impaired in disease and aging. We have previously shown that mitochondrial transcription termination factor 3 (MTERF3) is a key regulator that represses mtDNA transcription in the mouse, but its molecular mode of action has remained elusive. Based on the hypothesis that key regulatory mechanisms for mtDNA expression are conserved in metazoans, we analyzed Mterf3 knockout and knockdown flies. We demonstrate here that decreased expression of MTERF3 not only leads to activation of mtDNA transcription, but also impairs assembly of the large mitochondrial ribosomal subunit. This novel function of MTERF3 in mitochondrial ribosomal biogenesis is conserved in the mouse, thus we identify a novel and unexpected role for MTERF3 in coordinating the crosstalk between transcription and translation for the regulation of mammalian mtDNA gene expression.

Altered dopamine metabolism and increased vulnerability to MPTP in mice with partial deficiency of mitochondrial complex I in dopamine neurons

A variety of observations support the hypothesis that deficiency of complex I [reduced nicotinamide-adenine dinucleotide (NADH):ubiquinone oxidoreductase] of the mitochondrial respiratory chain plays a role in the pathophysiology of Parkinson's disease (PD). However, recent data from a study using mice with knockout of the complex I subunit NADH:ubiquinone oxidoreductase iron-sulfur protein 4 (Ndufs4) has challenged this concept as these mice show degeneration of non-dopamine neurons. In addition, primary dopamine (DA) neurons derived from such mice, reported to lack complex I activity, remain sensitive to toxins believed to act through inhibition of complex I. We tissue-specifically disrupted the Ndufs4 gene in mouse heart and found an apparent severe deficiency of complex I activity in disrupted mitochondria, whereas oxidation of substrates that result in entry of electrons at the level of complex I was only mildly reduced in intact isolated heart mitochondria. Further analyses of detergent-solubilized mitochondria showed the mutant complex I to be unstable but capable of forming supercomplexes with complex I enzyme activity. The loss of Ndufs4 thus causes only a mild complex I deficiency in vivo. We proceeded to disrupt Ndufs4 in midbrain DA neurons and found no overt neurodegeneration, no loss of striatal innervation and no symptoms of Parkinsonism in tissue-specific knockout animals. However, DA homeostasis was abnormal with impaired DA release and increased levels of DA metabolites. Furthermore, Ndufs4 DA neuron knockouts were more vulnerable to the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Taken together, these findings lend in vivo support to the hypothesis that complex I deficiency can contribute to the pathophysiology of PD.

LRPPRC is necessary for polyadenylation and coordination of translation of mitochondrial mRNAs

Regulation of mtDNA expression is critical for maintaining cellular energy homeostasis and may, in principle, occur at many different levels. The leucine-rich pentatricopeptide repeat containing (LRPPRC) protein regulates mitochondrial mRNA stability and an amino-acid substitution of this protein causes the French-Canadian type of Leigh syndrome (LSFC), a neurodegenerative disorder characterized by complex IV deficiency. We have generated conditional Lrpprc knockout mice and show here that the gene is essential for embryonic development. Tissue-specific disruption of Lrpprc in heart causes mitochondrial cardiomyopathy with drastic reduction in steady-state levels of most mitochondrial mRNAs. LRPPRC forms an RNA-dependent protein complex that is necessary for maintaining a pool of non-translated mRNAs in mammalian mitochondria. Loss of LRPPRC does not only decrease mRNA stability, but also leads to loss of mRNA polyadenylation and the appearance of aberrant mitochondrial translation. The translation pattern without the presence of LRPPRC is misregulated with excessive translation of some transcripts and no translation of others. Our findings point to the existence of an elaborate machinery that regulates mammalian mtDNA expression at the post-transcriptional level.

Complete Deletion of a POLG1 Allele in a Patient with Alpers Syndrome

Mutations in the gene encoding the catalytic subunit of polymerase γ (POLG1) are a major cause of human mitochondrial disease. More than 150 different point mutations in the gene have been reported to be disease causing, resulting in a large range of clinical symptoms. Depending on the mutation or combination of mutations, disease onset can occur in early infancy or late in adult life. Here, we describe the use of multiplex ligation-dependent probe amplification (MLPA) analysis to detect deletions within POLG1, which could otherwise go undetected by solely sequencing of the gene. We present a case where an entire POLG1 allele is deleted, with a known pathogenic mutation (W748S) on the remaining allele. The deletion was found in a boy with Alpers syndrome, presenting at 18 months of age with slightly retarded motor development, balance problems, and seizures. Administration of valproic acid (VPA) led to rapidly progressive fatal liver failure in our patient, and we would like to highlight the need to carry out complete POLG1 gene analysis before administration of VPA in cases of pediatric seizure disorders of unknown origin. Debut and severity of the disease in this patient was unique when compared to homozygous or heterozygous patients with the W748S mutation, leading to the conclusion that gene dosage plays a role in the clinical phenotype of this disease.

The bicoid stability factor controls polyadenylation and expression of specific mitochondrial mRNAs in Drosophila melanogaster

The bicoid stability factor (BSF) of Drosophila melanogaster has been reported to be present in the cytoplasm, where it stabilizes the maternally contributed bicoid mRNA and binds mRNAs expressed from early zygotic genes. BSF may also have other roles, as it is ubiquitously expressed and essential for survival of adult flies. We have performed immunofluorescence and cell fractionation analyses and show here that BSF is mainly a mitochondrial protein. We studied two independent RNAi knockdown fly lines and report that reduced BSF protein levels lead to a severe respiratory deficiency and delayed development at the late larvae stage. Ubiquitous knockdown of BSF results in a severe reduction of the polyadenylation tail lengths of specific mitochondrial mRNAs, accompanied by an enrichment of unprocessed polycistronic RNA intermediates. Furthermore, we observed a significant reduction in mRNA steady state levels, despite increased de novo transcription. Surprisingly, mitochondrial de novo translation is increased and abnormal mitochondrial translation products are present in knockdown flies, suggesting that BSF also has a role in coordinating the mitochondrial translation in addition to its role in mRNA maturation and stability. We thus report a novel function of BSF in flies and demonstrate that it has an important intra-mitochondrial role, which is essential for maintaining mtDNA gene expression and oxidative phosphorylation.

The majority of the cellular energy currency ATP is formed in a tubular network, termed mitochondria, present within virtually all eukaryotic cells. The mitochondria are unique among cellular organelles in that they contain their own genome, which encodes critical proteins necessary for cellular energy production. However, the vast majority of mitochondrial proteins are encoded in the nucleus and imported into mitochondria. Gene expression thus needs to be coordinated between the two genomes to ensure efficient mitochondrial function and sufficient adaptation to different physiological demands. The regulation of the mitochondrial genome is poorly understood, with many of the basic regulators not yet being characterized. We used RNAi in the fruit fly to study the in vivo function of the bicoid stability factor (BSF), previously thought to be a cytoplasmic and nuclear protein important for fly development. We show here that BSF is mainly localized to mitochondria, where it is essential for mtDNA gene expression, regulating the polyadenylation and maturation of specific mRNAs. Furthermore, BSF coordinates the translation and assembly of mitochondrial peptides in the inner mitochondrial membrane.

Maintenance of respiratory chain function in mouse hearts with severely impaired mtDNA transcription

The basal mitochondrial transcription machinery is essential for biogenesis of the respiratory chain and consists of mitochondrial RNA polymerase, mitochondrial transcription factor A (TFAM) and mitochondrial transcription factor B2. This triad of proteins is sufficient and necessary for mtDNA transcription initiation. Abolished mtDNA transcription caused by tissue-specific knockout of TFAM in the mouse heart leads to early onset of a severe mitochondrial cardiomyopathy with lethality within the first post-natal weeks. Here, we describe a mouse model expressing human TFAM instead of the endogenous mouse TFAM in heart. These rescue mice have severe reduction in mtDNA transcription initiation, but, surprisingly, are healthy at the age of 52 weeks with near-normal steady-state levels of transcripts. In addition, we demonstrate that heavy-strand mtDNA transcription normally terminates at the termination-associated sequence in the control region. This termination is abolished in rescue animals resulting in heavy (H)-strand transcription of the entire control region. In conclusion, we demonstrate here the existence of an unexpected mtDNA transcript stabilization mechanism that almost completely compensates for the severely reduced transcription initiation in rescue hearts. Future elucidation of the underlying molecular mechanism may provide a novel pathway to treat mitochondrial dysfunction in human pathology.

Two novel mutations in thymidine kinase-2 cause early onset fatal encephalomyopathy and severe mtDNA depletion

Deficiency of thymidine kinase-2 (TK2) has been described in children with early onset fatal skeletal myopathy. TK2 is a mitochondrial deoxyribonucleoside kinase required for the phosphorylation of deoxycytidine and deoxythymidine and hence is vital for the maintenance of a balanced mitochondrial dNTP pool in post-mitotic tissues. We describe a patient with two novel TK2 mutations, which caused disease onset shortly after birth and death at the age of three months. One mutation (219insCG) generated an early stop codon, thus preventing the synthesis of a functional protein. The second mutation (R130W) resulted in an amino acid substitution, which caused a severe reduction (<3%) of TK2 enzyme activity. These two novel TK2 mutations cause an extremely severe phenotype with overwhelming central nervous system symptoms not commonly seen in patients with TK2-deficiency. We conclude that the severe clinical presentation in this patient was due to a virtual lack of mitochondrial TK2 activity.

Tight coupling between glucose and mitochondrial metabolism in clonal beta-cells is required for robust insulin secretion

The biochemical mechanisms underlying glucose-stimulated insulin secretion from pancreatic beta-cells are not completely understood. To identify metabolic disturbances in beta-cells that impair glucose-stimulated insulin secretion, we compared two INS-1-derived clonal beta-cell lines, which are glucose-responsive (832/13 cells) or glucose-unresponsive (832/2 cells). To this end, we analyzed a number of parameters in glycolytic and mitochondrial metabolism, including mRNA expression of genes involved in cellular energy metabolism. We found that despite a marked impairment of glucose-stimulated insulin secretion, 832/2 cells exhibited a higher rate of glycolysis. Still, no glucose-induced increases in respiratory rate, ATP production, or respiratory chain complex I, III, and IV activities were seen in the 832/2 cells. Instead, 832/2 cells, which expressed lactate dehydrogenase A, released lactate regardless of ambient glucose concentrations. In contrast, the glucose-responsive 832/13 line lacked lactate dehydrogenase and did not produce lactate. Accordingly, in 832/2 cells mRNA expression of genes for glycolytic enzymes were up-regulated, whereas mitochondria-related genes were down-regulated. This could account for a Warburg-like effect in the 832/2 cell clone, lacking in 832/13 cells as well as primary beta-cells. In human islets, mRNA expression of genes such as lactate dehydrogenase A and hexokinase I correlated positively with HbA(1c) levels, reflecting perturbed long term glucose homeostasis, whereas that of Slc2a2 (glucose transporter 2) correlated negatively with HbA(1c) and thus better metabolic control. We conclude that tight metabolic regulation enhancing mitochondrial metabolism and restricting glycolysis in 832/13 cells is required for clonal beta-cells to secrete insulin robustly in response to glucose. Moreover, a similar expression pattern of genes controlling glycolytic and mitochondrial metabolism in clonal beta-cells and human islets was observed, suggesting that a similar prioritization of mitochondrial metabolism is required in healthy human beta-cells. The 832 beta-cell lines may be helpful tools to resolve metabolic perturbations occurring in Type 2 diabetes.

AGC1 deficiency associated with global cerebral hypomyelination

The mitochondrial aspartate-glutamate carrier isoform 1 (AGC1), specific to neurons and muscle, supplies aspartate to the cytosol and, as a component of the malate-aspartate shuttle, enables mitochondrial oxidation of cytosolic NADH, thought to be important in providing energy for neurons in the central nervous system. We describe AGC1 deficiency, a novel syndrome characterized by arrested psychomotor development, hypotonia, and seizures in a child with a homozygous missense mutation in the solute carrier family 25, member 12, gene SLC25A12, which encodes the AGC1 protein. Functional analysis of the mutant AGC1 protein showed abolished activity. The child had global hypomyelination in the cerebral hemispheres, suggesting that impaired efflux of aspartate from neuronal mitochondria prevents normal myelin formation.

Sensory ataxic neuropathy in golden retriever dogs is caused by a deletion in the mitochondrial tRNATyr gene

Sensory ataxic neuropathy (SAN) is a recently identified neurological disorder in golden retrievers. Pedigree analysis revealed that all affected dogs belong to one maternal lineage, and a statistical analysis showed that the disorder has a mitochondrial origin. A one base pair deletion in the mitochondrial tRNA(Tyr) gene was identified at position 5304 in affected dogs after re-sequencing the complete mitochondrial genome of seven individuals. The deletion was not found among dogs representing 18 different breeds or in six wolves, ruling out this as a common polymorphism. The mutation could be traced back to a common ancestor of all affected dogs that lived in the 1970s. We used a quantitative oligonucleotide ligation assay to establish the degree of heteroplasmy in blood and tissue samples from affected dogs and controls. Affected dogs and their first to fourth degree relatives had 0-11% wild-type (wt) sequence, while more distant relatives ranged between 5% and 60% wt sequence and all unrelated golden retrievers had 100% wt sequence. Northern blot analysis showed that tRNA(Tyr) had a 10-fold lower steady-state level in affected dogs compared with controls. Four out of five affected dogs showed decreases in mitochondrial ATP production rates and respiratory chain enzyme activities together with morphological alterations in muscle tissue, resembling the changes reported in human mitochondrial pathology. Altogether, these results provide conclusive evidence that the deletion in the mitochondrial tRNA(Tyr) gene is the causative mutation for SAN.

Glare from outdoor high mast lighting: effects on visual acuity and contrast sensitivity in comparative studies of different floodlighting systems

Glare effects on visual acuity and contrast sensitivity were studied at outdoor timber terminal work with conventional far-reaching floodlighting and with a new type of oblique floodlighting with an asymmetrical light distribution. The binocular work visual acuity of 19 volunteers was measured at 40, 55 and 80 per cent contrast and the contrast sensitivity of 14 volunteers at spatial frequencies of 0.5, 1, 2, 4, 8 and 16 cycles/degree. With the light source 15 degrees from the centre of the visual fields the visual acuity as well as the contrast sensitivity was consistently lower with the conventional type than the new type of floodlight . The results thus indicate that not only the vision of details is impaired by glare but also the contrast perception of sparse patterns. In standardized rating scales the timber terminal workers scored the difference in glare between the two floodlight systems as "great"--"very great".

Methylation of 12S rRNA is necessary for in vivo stability of the small subunit of the mammalian mitochondrial ribosome

The 3' end of the rRNA of the small ribosomal subunit contains two extremely highly conserved dimethylated adenines. This modification and the responsible methyltransferases are present in all three domains of life, but its function has remained elusive. We have disrupted the mouse Tfb1m gene encoding a mitochondrial protein homologous to bacterial dimethyltransferases and demonstrate here that loss of TFB1M is embryonic lethal. Disruption of Tfb1m in heart leads to complete loss of adenine dimethylation of the rRNA of the small mitochondrial ribosomal subunit, impaired assembly of the mitochondrial ribosome, and abolished mitochondrial translation. In addition, we present biochemical evidence that TFB1M does not activate or repress transcription in the presence of TFB2M. Our results thus show that TFB1M is a nonredundant dimethyltransferase in mammalian mitochondria. In addition, we provide a possible explanation for the universal conservation of adenine dimethylation of rRNA by showing a critical role in ribosome maintenance.

MtDNA mutations are a common cause of severe disease phenotypes in children with Leigh syndrome

Leigh syndrome is a common clinical manifestation in children with mitochondrial disease and other types of inborn errors of metabolism. We characterised clinical symptoms, prognosis, respiratory chain function and performed extensive genetic analysis of 25 Swedish children suffering from Leigh syndrome with the aim to obtain insights into the molecular pathophysiology and to provide a rationale for genetic counselling. We reviewed the clinical history of all patients and used muscle biopsies in order to perform molecular, biochemical and genetic investigations, including sequencing the entire mitochondrial DNA (mtDNA), the mitochondrial DNA polymerase (POLGA) gene and the surfeit locus protein 1 (SURF1) gene. Respiratory chain enzyme activity measurements identified five patients with isolated complex I deficiency and five with combined enzyme deficiencies. No patient presented with isolated complex IV deficiency. Seven patients had a decreased ATP production rate. Extensive sequence analysis identified eight patients with pathogenic mtDNA mutations and one patient with mutations in POLGA. Mutations of mtDNA are a common cause of LS and mtDNA analysis should always be included in the diagnosis of LS patients, whereas SURF1 mutations are not a common cause of LS in Sweden. Unexpectedly, age of onset, clinical symptoms and prognosis did not reveal any clear differences in LS patients with mtDNA or nuclear DNA mutations.

MTERF3 is a negative regulator of mammalian mtDNA transcription

Regulation of mammalian mtDNA gene expression is critical for altering oxidative phosphorylation capacity in response to physiological demands and disease processes. The basal machinery for initiation of mtDNA transcription has been molecularly defined, but the mechanisms regulating its activity are poorly understood. In this study, we show that MTERF3 is a negative regulator of mtDNA transcription initiation. The MTERF3 gene is essential because homozygous knockout mouse embryos die in midgestation. Tissue-specific inactivation of MTERF3 in the heart causes aberrant mtDNA transcription and severe respiratory chain deficiency. MTERF3 binds the mtDNA promoter region and depletion of MTERF3 increases transcription initiation on both mtDNA strands. This increased transcription initiation leads to decreased expression of critical promoter-distal tRNA genes, which is possibly explained by transcriptional collision on the circular mtDNA molecule. To our knowledge, MTERF3 is the first example of a mitochondrial protein that acts as a specific repressor of mammalian mtDNA transcription initiation in vivo.

Effects of palmitate on Ca(2+) handling in adult control and ob/ob cardiomyocytes: impact of mitochondrial reactive oxygen species

Obesity and insulin resistance are associated with enhanced fatty acid utilization, which may play a central role in diabetic cardiomyopathy. We now assess the effect of the saturated fatty acid palmitate (1.2 mmol/l) on Ca(2+) handling, cell shortening, and mitochondrial production of reactive oxygen species (ROS) in freshly isolated ventricular cardiomyocytes from normal (wild-type) and obese, insulin-resistant ob/ob mice. Cardiomyocytes were electrically stimulated at 1 Hz, and the signal of fluorescent indicators was measured with confocal microscopy. Palmitate decreased the amplitude of cytosolic Ca(2+) transients (measured with fluo-3), the sarcoplasmic reticulum Ca(2+) load, and cell shortening by approximately 20% in wild-type cardiomyocytes; these decreases were prevented by the general antioxidant N-acetylcysteine. In contrast, palmitate accelerated Ca(2+) transients and increased cell shortening in ob/ob cardiomyocytes. Application of palmitate rapidly dissipated the mitochondrial membrane potential (measured with tetra-methyl rhodamine-ethyl ester) and increased the mitochondrial ROS production (measured with MitoSOX Red) in wild-type but not in ob/ob cardiomyocytes. In conclusion, increased saturated fatty acid levels impair cellular Ca(2+) handling and contraction in a ROS-dependent manner in normal cardiomyocytes. Conversely, high fatty acid levels may be vital to sustain cardiac Ca(2+) handling and contraction in obesity and insulin-resistant conditions.

Respiratory chain dysfunction in skeletal muscle does not cause insulin resistance

Insulin resistance in skeletal muscle is a characteristic feature of diabetes mellitus type 2 (DM2). Several lines of circumstantial evidence suggest that reduced mitochondrial oxidative phosphorylation capacity in skeletal muscle is a primary defect causing insulin resistance and subsequent development of DM2. We have now experimentally tested this hypothesis by characterizing glucose homeostasis in tissue-specific knockout mice with progressive respiratory chain dysfunction selectively in skeletal muscle. Surprisingly, these knockout mice are not diabetic and have an increased peripheral glucose disposal when subjected to a glucose tolerance test. Studies of isolated skeletal muscle from knockout animals show an increased basal glucose uptake and a normal increase of glucose uptake in response to insulin. In summary, our findings indicate that mitochondrial dysfunction in skeletal muscle is not a primary etiological event in DM2.

Secondary metabolic effects in complex I deficiency

The objective of this study was to investigate clinical, biochemical, and genetic features in 7 probands (a total of 11 patients) with nicotine-amide adenine dinucleotide (NADH) dehydrogenase (complex I) deficiency. We screened the mitochondrial DNA for mutations and found pathogenic mutations in complex I genes (mitochondrial NADH dehydrogenase subunit (MTND) genes) in three probands. The 10191T>C mutation in MTND3 and the 14487T>C mutation in MTND6 were present in two probands with Leigh's-like and Leigh's syndrome, respectively. Four siblings with a syndrome consisting of encephalomyopathy with hearing impairment, optic nerve atrophy, and cardiac involvement had the 11778G>A mutation in MTND4, previously associated with Leber hereditary optic neuropathy. These findings demonstrate that mutations in MTND genes are relatively frequent in patients with complex I deficiency. Biochemical measurements of respiratory chain function in muscle mitochondria showed that all patients had a moderate decrease of the mitochondrial adenosine triphosphate production rate. Interestingly, the complex I deficiency caused secondary metabolic alterations with decreased oxaloacetate-induced inhibition of succinate dehydrogenase (complex II) and excretion of Krebs cycle intermediates in the urine. Our results thus suggest that altered regulation of metabolism may play an important role in the pathogenesis of complex I deficiency.

Somatic mtDNA mutations cause aging phenotypes without affecting reactive oxygen species production

The mitochondrial theory of aging proposes that reactive oxygen species (ROS) generated inside the cell will lead, with time, to increasing amounts of oxidative damage to various cell components. The main site for ROS production is the respiratory chain inside the mitochondria and accumulation of mtDNA mutations, and impaired respiratory chain function have been associated with degenerative diseases and aging. The theory predicts that impaired respiratory chain function will augment ROS production and thereby increase the rate of mtDNA mutation accumulation, which, in turn, will further compromise respiratory chain function. Previously, we reported that mice expressing an error-prone version of the catalytic subunit of mtDNA polymerase accumulate a substantial burden of somatic mtDNA mutations, associated with premature aging phenotypes and reduced lifespan. Here we show that these mtDNA mutator mice accumulate mtDNA mutations in an approximately linear manner. The amount of ROS produced was normal, and no increased sensitivity to oxidative stress-induced cell death was observed in mouse embryonic fibroblasts from mtDNA mutator mice, despite the presence of a severe respiratory chain dysfunction. Expression levels of antioxidant defense enzymes, protein carbonylation levels, and aconitase enzyme activity measurements indicated no or only minor oxidative stress in tissues from mtDNA mutator mice. The premature aging phenotypes in mtDNA mutator mice are thus not generated by a vicious cycle of massively increased oxidative stress accompanied by exponential accumulation of mtDNA mutations. We propose instead that respiratory chain dysfunction per se is the primary inducer of premature aging in mtDNA mutator mice.

SOD2 overexpression: enhanced mitochondrial tolerance but absence of effect on UCP activity

We have created P1 artificial chromosome transgenic mice expressing the human mitochondrial superoxide dismutase 2 (SOD2) and thus generated mice with a physiologically controlled augmentation of SOD2 expression leading to increased SOD2 enzyme activities and lowered superoxide levels. In the transgenic mice, effects on mitochondrial function such as enhanced oxidative capacity and greater resistance against inducers of mitochondrial permeability were observed. Superoxide in the mitochondrial matrix has been proposed to activate uncoupling proteins (UCPs), thus providing a feedback mechanism that will lower respiratory chain superoxide production by increasing a proton leak across the inner mitochondrial membrane. However, UCP1 and UCP3 activities and mitochondrial ATP production rates were not altered in isolated mitochondria from SOD2 transgenic mice, despite lowered superoxide levels. Globally, the transgenic mice displayed normal resting metabolic rates, indicating an absence of effect on any UCP activities, and normal oxygen consumption responses after norepinephrine injection. These results strongly suggest that endogenously generated matrix superoxide does not regulate UCP activity and in vivo energy expenditure.