Mitochondrial Depletion Syndrome Research Articles

Adult mitochondrial DNA depletion syndrome with mild manifestations

Mitochondrial DNA depletion syndrome (MDS) is usually a severe disorder of infancy or childhood, due to a reduced copy number of mtDNA molecules. MDS with only mild, nonspecific clinical manifestations and onset in adulthood has not been reported. A 47-year-old Caucasian female with short stature and a history of migraine, endometriosis, Crohn’s disease, C-cell carcinoma of the thyroid gland, and a family history positive for mitochondrial disorder (2 sisters, aunt, niece), developed day-time sleepiness, exercise intolerance, and myalgias in the lower-limb muscles since age 46y. She slept 9-10 hours during the night and 2 hours after lunch daily. Clinical exam revealed sore neck muscles, bilateral ptosis, and reduced Achilles tendon reflexes exclusively. Blood tests revealed hyperlipidemia exclusively. Nerve conduction studies, needle electromyography, and cerebral and spinal magnetic resonance imaging were noninformative. Muscle biopsy revealed detached lobulated fibers with subsarcolemmal accentuation of the NADH and SDH staining. Realtime polymerase chain reaction revealed depletion of the mtDNA down to 9% of normal. MDS may be associated with a mild phenotype in adults and may not significantly progress during the first year after onset. In an adult with hypersomnia, severe tiredness, exercise intolerance, and a family history positive for mitochondrial disorder, a MDS should be considered.

Transparent, a gene affecting stripe formation in Zebrafish, encodes the mitochondrial protein Mpv17 that is required for iridophore survival

SummaryIn the skin of adult zebrafish, three pigment cell types arrange into alternating horizontal stripes, melanophores in dark stripes, xanthophores in light interstripes and iridophores in both stripes and interstripes. The analysis of mutants and regeneration studies revealed that this pattern depends on interactions between melanophores and xanthophores; however, the role of iridophores in this process is less understood. We describe the adult viable and fertile mutant transparent (tra), which shows a loss or strong reduction of iridophores throughout larval and adult stages. In addition, in adults only the number of melanophores is strongly reduced, and stripes break up into spots. Stripes in the fins are normal. By cell transplantations we show that tra acts cell-autonomously in iridophores, whereas the reduction in melanophores in the body occurs secondarily as a consequence of iridophore loss. We conclude that differentiated iridophores are required for the accumulation and maintenance of melanophores during pigment pattern formation. The tra mutant phenotype is caused by a small deletion in mpv17, an ubiquituously expressed gene whose protein product, like its mammalian and yeast homologs, localizes to mitochondria. Iridophore death might be the result of mitochondrial dysfunction, consistent with the mitochondrial DNA depletion syndrome observed in mammalian mpv17 mutants. The specificity of the tra phenotype is most likely due to redundancy after gene multiplication, making this mutant a valuable model to understand the molecular function of Mpv17 in mitochondria.

Clinical, biochemical, cellular and molecular characterization of mitochondrial DNA depletion syndrome due to novel mutations in the MPV17 gene

Mitochondrial DNA (mtDNA) depletion syndromes (MDS) are severe autosomal recessive disorders associated with decreased mtDNA copy number in clinically affected tissues. The hepatocerebral form (mtDNA depletion in liver and brain) has been associated with mutations in the POLG, PEO1 (Twinkle), DGUOK and MPV17 genes, the latter encoding a mitochondrial inner membrane protein of unknown function. The aims of this study were to clarify further the clinical, biochemical, cellular and molecular genetic features associated with MDS due to MPV17 gene mutations. We identified 12 pathogenic mutations in the MPV17 gene, of which 11 are novel, in 17 patients from 12 families. All patients manifested liver disease. Poor feeding, hypoglycaemia, raised serum lactate, hypotonia and faltering growth were common presenting features. mtDNA depletion in liver was demonstrated in all seven cases where liver tissue was available. Mosaic mtDNA depletion was found in primary fibroblasts by PicoGreen staining. These results confirm that MPV17 mutations are an important cause of hepatocerebral mtDNA depletion syndrome, and provide the first demonstration of mosaic mtDNA depletion in human MPV17 mutant fibroblast cultures. We found that a severe clinical phenotype was associated with profound tissue-specific mtDNA depletion in liver, and, in some cases, mosaic mtDNA depletion in fibroblasts.

MRI of the brain in childhood-onset mitochondrial disorders with central nervous system involvement

We retrospectively studied the brain MRIs of 66 pediatric patients with mitochondrial disorder with central nervous system involvement. Forty-one patients had an identified genetic etiology. A predominance of cerebrocortical lesions was mainly seen in patients with MELAS and Alpers syndrome. Basal ganglia were predominantly affected in patients with Leigh syndrome. All patients with leukoencephalopathy had pathological spectroscopy. Cerebrocortical atrophy with agenesis/atrophy of the corpus callosum was seen in patients with congenital lactic acidosis with or without pyruvate dehydrogenase complex deficiency. The diagnostic approach used in our study — from the neuroanatomical/neurofunctional lesion to disease identification — assists the physician in the use of brain neuroimaging early in the diagnostic work-up of suspected mitochondrial disorders.

Coenzyme Q10 deficiency in mitochondrial DNA depletion syndromes

We evaluated coenzyme Q10 (CoQ) levels in patients studied under suspicion of mitochondrial DNA depletion syndromes (MDS) (n=39). CoQ levels were quantified by HPLC, and the percentage of mtDNA depletion by quantitative real-time PCR. A high percentage of MDS patients presented with CoQ deficiency as compared to other mitochondrial patients (Mann–Whitney-U test: p=0.001). Our findings suggest that MDS are frequently associated with CoQ deficiency, as a possible secondary consequence of disease pathophysiology. Assessment of muscle CoQ status seems advisable in MDS patients since the possibility of CoQ supplementation may then be considered as a candidate therapy.

MPV17, DGUOK and Mitochondrial Depletion Disorders

Mutations in DGUOK, a gene encoding a protein catalyst of the mitochondrial salvage pathway, and MPV17, a gene encoding an inner mitochondrial membrane protein of unknown function, have been shown to cause the hepatocerebral form of mitochondrial DNA depletion syndrome. MPV17 has been postulated to function as a nucleotide transporter. HeLa cell culture models were established using siRNA to model the mutations found in these patients. Following qPCR analysis, it was found that mtDNA copy number modestly decreased in both MPV17 and DGUOK siRNA‐treated cells cultured in nutrient replete medium. Surprisingly, MPV17 siRNA‐treated cells cultured in nutrient deplete medium exhibited a modest increase in mtDNA copy number. Individual nucleotide supplementation experiments were conducted, and the results indicate that pyrimidine nucleotide availability increases mtDNA copy number in MPV17 siRNA‐treated cells as much as 10‐fold.

Gray matter atrophy distinguishes between Parkinson disease motor subtypes

To assess differences in gray matter (GM) atrophy between 2 Parkinson disease (PD) subtypes: the tremor dominant (TD) subtype and the postural instability gait difficulty (PIGD) subtype. Patients were classified as belonging to the predominately PIGD (n = 30) or predominately TD (n = 29) subtype. Voxel-based morphometry was used to compare GM in these 2 subtypes and to evaluate correlations between predefined regions of interest and the degree of symptoms. In the regions where GM atrophy was associated with symptoms, the relationship between GM volumes and functional connectivity was examined. GM was reduced in the predominately PIGD group, compared with the predominately TD group, in areas that involve motor, cognitive, limbic, and associative functions (p < 0.05, false discovery rate corrected). Lower GM volumes in the pre-supplementary motor area (SMA) and in the primary motor area were associated with increased severity of PIGD symptoms (r = -0.42, p < 0.001; r = -0.38, p < 0.003, respectively). Higher GM volumes within the pre-SMA were associated with stronger functional connectivity between the pre-SMA and the putamen (r = 0.415, p < 0.025) in the patients with predominately PIGD. In patients with PD, PIGD symptoms are apparently associated with GM atrophy in motor-related regions and decreased functional connectivity. GM degeneration and a related decrease in spontaneous coactivation between cortical and subcortical motor-planning areas may partially account for the unique clinical characteristics of a subset of patients with PD.

Thymidine Kinase 2 Deficiency-Induced mtDNA Depletion in Mouse Liver Leads to Defect β-Oxidation

Thymidine kinase 2 (TK2) deficiency in humans causes mitochondrial DNA (mtDNA) depletion syndrome. To study the molecular mechanisms underlying the disease and search for treatment options, we previously generated and described a TK2 deficient mouse strain (TK2−/−) that progressively loses its mtDNA. The TK2−/− mouse model displays symptoms similar to humans harboring TK2 deficient infantile fatal encephalomyopathy. Here, we have studied the TK2−/− mouse model to clarify the pathological role of progressive mtDNA depletion in liver for the severe outcome of TK2 deficiency. We observed that a gradual depletion of mtDNA in the liver of the TK2−/− mice was accompanied by increasingly hypertrophic mitochondria and accumulation of fat vesicles in the liver cells. The levels of cholesterol and nonesterified fatty acids were elevated and there was accumulation of long chain acylcarnitines in plasma of the TK2−/− mice. In mice with hepatic mtDNA levels below 20%, the blood sugar and the ketone levels dropped. These mice also exhibited reduced mitochondrial β-oxidation due to decreased transport of long chain acylcarnitines into the mitochondria. The gradual loss of mtDNA in the liver of the TK2−/− mice causes impaired mitochondrial function that leads to defect β-oxidation and, as a result, insufficient production of ketone bodies and glucose. This study provides insight into the mechanism of encephalomyopathy caused by TK2 deficiency-induced mtDNA depletion that may be used to explore novel therapeutic strategies.

Prospective study of POLG mutations presenting in children with intractable epilepsy: Prevalence and clinical features

PurposeTo assess the frequency and clinical features of childhood-onset intractable epilepsy caused by the most common mutations in the POLG gene, which encodes the catalytic subunit of mitochondrial DNA polymerase gamma.MethodsChildren presenting with nonsyndromic intractable epilepsy of unknown etiology but without documented liver dysfunction at presentation were eligible for this prospective, population-based study. Blood samples were analyzed for the three most common POLG mutations. If any of the three tested mutations were found, all the exons and the exon–intron boundaries of the POLG gene were sequenced. In addition, we retrospectively reviewed the notes of patients presenting with intractable epilepsy in which we had found POLG mutations. All available clinical data were collected by questionnaire and by reviewing the medical records.Key FindingsWe analyzed 213 blood DNA samples from patients fulfilling the inclusion criteria of the prospective study. Among these, five patients (2.3%) were found with one of the three common POLG mutations as homozygous or compound heterozygous states. In addition, three patients were retrospectively identified. Seven of the eight patients had either raised cerebrospinal fluid (CSF) lactate (n = 3) or brain magnetic resonance imaging (MRI) changes (n = 4) at presentation with intractable epilepsy. Three patients later developed liver dysfunction, progressing to fatal liver failure in two without previous treatment with sodium valproate (VPA). Furthermore, it is worth mentioning that one patient presented first with an autism spectrum disorder before seizures emerged.SignificanceMutations in POLG are an important cause of early and juvenile onset nonsyndromic intractable epilepsy with highly variable associated manifestations including autistic features. This study emphasizes that genetic testing for POLG mutations in patients with nonsyndromic intractable epilepsies is very important for clinical diagnostics, genetic counseling, and treatment decisions because of the increased risk for VPA-induced liver failure in patients with POLG mutations. We recommend POLG gene testing for patients with intractable seizures and at least one elevated CSF lactate or suggestive brain MRI changes (predominantly abnormal T2-weighted thalamic signal) with or without status epilepticus, epilepsia partialis continua, or liver manifestations typical for Alpers disease, especially when the disease course is progressive.

Mitochondrial DNA depletion syndromes: review and updates of genetic basis, manifestations, and therapeutic options.

Mitochondrial DNA (mtDNA) depletion syndromes (MDS) are a genetically and clinically heterogeneous group of autosomal recessive disorders that are characterized by a severe reduction in mtDNA content leading to impaired energy production in affected tissues and organs. MDS are due to defects in mtDNA maintenance caused by mutations in nuclear genes that function in either mitochondrial nucleotide synthesis (TK2, SUCLA2, SUCLG1, RRM2B, DGUOK, and TYMP) or mtDNA replication (POLG and C10orf2). MDS are phenotypically heterogeneous and usually classified as myopathic, encephalomyopathic, hepatocerebral or neurogastrointestinal. Myopathic MDS, caused by mutations in TK2, usually present before the age of 2years with hypotonia and muscle weakness. Encephalomyopathic MDS, caused by mutations in SUCLA2, SUCLG1, or RRM2B, typically present during infancy with hypotonia and pronounced neurological features. Hepatocerebral MDS, caused by mutations in DGUOK, MPV17, POLG, or C10orf2, commonly have an early-onset liver dysfunction and neurological involvement. Finally, TYMP mutations have been associated with mitochondrial neurogastrointestinal encephalopathy (MNGIE) disease that typically presents before the age of 20years with progressive gastrointestinal dysmotility and peripheral neuropathy. Overall, MDS are severe disorders with poor prognosis in the majority of affected individuals. No efficacious therapy is available for any of these disorders. Affected individuals should have a comprehensive evaluation to assess the degree of involvement of different systems. Treatment is directed mainly toward providing symptomatic management. Nutritional modulation and cofactor supplementation may be beneficial. Liver transplantation remains controversial. Finally, stem cell transplantation in MNGIE disease shows promising results.

Gene Expression Deregulation in Postnatal Skeletal Muscle of TK2 Deficient Mice Reveals a Lower Pool of Proliferating Myogenic Progenitor Cells

Loss of thymidine kinase 2 (TK2) causes a heterogeneous myopathic form of mitochondrial DNA (mtDNA) depletion syndrome (MDS) in humans that predominantly affects skeletal muscle tissue. In mice, TK2 deficiency also affects several tissues in addition to skeletal muscle, including brain, heart, adipose tissue, kidneys and causes death about 3 weeks after birth. We analysed skeletal muscle and heart muscle tissues of Tk2 knockout mice at postnatal development phase and observed that TK2 deficient pups grew slower and their skeletal muscles appeared significantly underdeveloped, whereas heart was close to normal in size. Both tissues showed mtDNA depletion and mitochondria with altered ultrastructure, as revealed by transmission electron microscopy. Gene expression microarray analysis showed a strong down-regulation of genes involved in cell cycle and cell proliferation in both tissues, suggesting a lower pool of undifferentiated proliferating cells. Analysis of isolated primary myoblasts from Tk2 knockout mice showed slow proliferation, less ability to differentiate and signs of premature senescence, even in absence of mtDNA depletion. Our data demonstrate that TK2 deficiency disturbs myogenic progenitor cells function in postnatal skeletal muscle and we propose this as one of the causes of underdeveloped phenotype and myopathic characteristic of the TK2 deficient mice, in addition to the progressive mtDNA depletion, mitochondrial damage and respiratory chain deficiency in post-mitotic differentiated tissue.

TK2 mutation presenting as indolent myopathy

Recessive mutations in the TK2 gene typically cause fatal infantile mitochondrial DNA (mtDNA) depletion syndromes (MDS).(1-3) However, the progression of weakness may vary,(4) as shown by recently described adult patients with late-onset myopathy.(5,6) To date, only 5 adult patients with TK2-related MDS have been reported. Herein, we describe a man who had several unusual features. Clinically, he was weak as a child but sought medical attention as an adult. At the molecular level, multiple mtDNA deletions in muscle were more prominent than mtDNA depletion.

Study of brain structures on MRI of Parkinson's

Event Abstract Back to Event Study of brain structures on MRI of Parkinson's Yazid CHERFA1* and Assia CHERFA1 1 Saad Dahlab University, LARIM Laboratory, Algeria Problem Statement: The aim of our work is to track the different kind of deformations, that has suffered the parkinsonian brain, at the level of brain tissues (WM, GM, CSF). At first, after a step of pretreatments, we segment the three matters of healthy and parkinsonian brains using the FCM algorithm, and then we will compare the results. Then we segment the ventricles by approach regions and we refine the result using deformable models. Approach: Pretreatment: Before segmentation, we must apply a pretreatment phase, including: • Noise filtering: is to filter the image to reduce noise in areas where it operates, but at the same time to avoid the smoothing of edges. The main goal pursued by the anisotropic diffusion Perona and Malik the result using deformable models. • Skull stretching: The brain is extracted by a robust method, using morphological operations. This eliminates the radiometric classes that do not interest us (the skin, air, bone, fat). Hough. Segmentation: After pretreatment, we segment the brain structures by using FCM algorithm, and we calculate surface of the segmented structures. A registration between healthy and Parkinsonian segmented brains allows comparing the surfaces of cerebral tissues. Results: The surfaces of the MB and MG a Parkinsonian brain are lower than those of a healthy brain. These results are satisfactory and confirm the interpretation of the physician, namely atrophy in the white matter and gray matter, which involves the deformation of the cerebrospinal fluid and internal structures. Conclusions/Recommendations: We have segmented the structures of a healthy brain, and those of a Parkinsonian brain, and we have compared them. Segmentation of brain tissue shows degeneration of white and gray matter. Our results are satisfactory and confirm the interpretation of the doctor. However, this must be confirmed by the study of MRIs of the same subject Parkinson, taken at different times. Keywords: MRI, segmentation, Spinal Cord, FCM, Parkinson Disease Conference: 4th Conference of the Mediterrarnean Neuroscience Society, Istanbul, Turkey, 30 Sep - 3 Oct, 2012. Presentation Type: Poster Presentation Topic: Abstracts Citation: CHERFA Y and CHERFA A (2013). Study of brain structures on MRI of Parkinson's. Conference Abstract: 4th Conference of the Mediterrarnean Neuroscience Society. doi: 10.3389/conf.fnhum.2013.210.00017 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 04 Apr 2013; Published Online: 11 Apr 2013. * Correspondence: Prof. Yazid CHERFA, Saad Dahlab University, LARIM Laboratory, BLIDA, Algeria, cherfa_yazid@yahoo.fr Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Yazid CHERFA Assia CHERFA Google Yazid CHERFA Assia CHERFA Google Scholar Yazid CHERFA Assia CHERFA PubMed Yazid CHERFA Assia CHERFA Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Exome sequencing reveals a homozygous mutation in TWINKLE as the cause of multisystemic failure including renal tubulopathy in three siblings

Three deceased infants from a Pakistani consanguineous family presented with a similar phenotype of cholestatic liver disease, hypotonia, severe failure to thrive, recurrent vomiting, renal tubulopathy, and a progressive neurodegenerative course. Mitochondrial DNA depletion syndrome was considered in view of multisystem involvement. Exome sequencing, revealed a homozygous novel mutation c.1183T>C (p.F395L) in exon 1 of the C10orf2 TWINKLE gene. The hepatocerebral phenotype is well recognized in association with recessive mutations involving the C10orf2 TWINKLE gene. The feature of renal tubulopathy adds to the multisystemic presentation in our patients and further demonstrates an expansion of the phenotype in mitochondrial DNA depletion syndrome associated with TWINKLE gene mutations. The absence of features of an epileptic encephalopathy appears to be of added interest.

Gray and white matter degeneration revealed by diffusion in an Alzheimer mouse model

In patients with Alzheimer's disease (AD) the severity of white matter degeneration correlates with the clinical symptoms of the disease. In this study, we performed diffusion-tensor magnetic resonance imaging at ultra-high field in a mouse model for AD (APPswe/PS1dE9) in combination with a voxel-based approach and tractography to detect changes in water diffusivity in white and gray matter, because these reflect structural alterations in neural tissue. We found substantial changes in water diffusion parallel and perpendicular to axonal tracts in several white matter regions like corpus callosum and fimbria of the hippocampus, that match with previous findings of axonal disconnection and myelin degradation in AD patients. Moreover, we found a significant increase in diffusivity in specific hippocampal subregions, which is supported by neuronal loss as visualized with Klüver-Barrera staining. This work demonstrates the potential of ultra-high field diffusion-tensor magnetic resonance imaging as a noninvasive modality to describe white and gray matter structural changes in mouse models for neurodegenerative disorders, and provides valuable knowledge to assess future AD prevention strategies in translational research.

Mutations in human DNA polymerase confer unique mechanisms of catalytic deficiency that mirror the disease severity in mitochondrial disorder patients

Human mitochondrial DNA polymerase γ (pol γ) is solely responsible for the replication and repair of the mitochondrial genome. Unsurprisingly, alterations in pol γ activity have been associated with mitochondrial diseases such as Alpers syndrome and progressive external ophthalmoplegia. Thus far, predicting the severity of mitochondrial disease based the magnitude of deficiency in pol γ activity has been difficult. In order to understand the relationship between disease severity in patients and enzymatic defects in vitro, we characterized the molecular mechanisms of four pol γ mutations, A957P, A957S, R1096C and R1096H, which have been found in patients suffering from aggressive Alpers syndrome to mild progressive external ophthalmoplegia. The A957P mutant showed the most striking deficiencies in the incorporation efficiency of a correct deoxyribonucleotide triphosphate (dNTP) relative to wild-type pol γ, with less, but still significant incorporation efficiency defects seen in R1096H and R1096C, and only a small decrease in incorporation efficiency observed for A957S. Importantly, this trend matches the disease severity observed in patients very well (approximated as A957P ≫ R1096C ≥ R1096H ≫ A957S, from most severe disease to least severe). Further, the A957P mutation conferred a two orders of magnitude loss of fidelity relative to wild-type pol γ, indicating that a buildup of mitochondrial genomic mutations may contribute to the death in infancy seen with these patients. We conclude that characterizing the unique molecular mechanisms of pol γ deficiency for physiologically important mutant enzymes is important for understanding mitochondrial disease and for predicting disease severity.

Valproic acid-induced idiosyncratic liver injury in 4 cases

Children with refractory epilepsy who suffered from severe liver function impairment during valproic acid (VPA) treatment at routine dosage were studied. The clinical manifestations and therapeutic approaches were investigated in order to improve its diagnosis and management. Clinical information as well as features and management of 4 inpatients who were suffered from intractable epilepsy with severe liver function impairment induced by VPA since 2006 were collected and analyzed, including age of onset of epilepsy, VPA using age and the time when liver injury occurred, clinical manifestations, auxiliary examinations and management. Among the 4 cases, three were male and one was female. The admitted age ranged from 1 - 9 years and 1 month. The course of disease was 25 d - 6 months. They manifested as refractory epilepsy of epilepsia partialis continua which was difficult to control. After using VPA for 62 d (50 - 76 d), all developed severe impairment of liver synthetic function which was not related to the concentration of VPA. One was diagnosed with Alpers syndrome, two were suspicious of Alpers syndrome, and the other was diagnosed gliocytoma after brain biopsy. VPA was stopped immediately and symptomatic therapies were used. Other than that, intravenous injection of L-carnitine in 3 cases recovered the liver function. VPA-associated severe hepatotoxicity can manifest first as impaired liver synthetic function. Besides alanin transaminase and aspartate transaminase, the liver synthetic function test is more important than monitoring of liver enzymatic functions in monitoring for the hepatotoxicity. Intravenous injection of L-carnitine in early stage showed good treatment effect.

The Channel-Forming Sym1 Protein Is Transported by the TIM23 Complex in a Presequence-Independent Manner

The majority of multispanning inner mitochondrial membrane proteins utilize internal targeting signals, which direct them to the carrier translocase (TIM22 complex), for their import. MPV17 and its Saccharomyces cerevisiae orthologue Sym1 are multispanning inner membrane proteins of unknown function with an amino-terminal presequence that suggests they may be targeted to the mitochondria. Mutations affecting MPV17 are associated with mitochondrial DNA depletion syndrome (MDDS). Reconstitution of purified Sym1 into planar lipid bilayers and electrophysiological measurements have demonstrated that Sym1 forms a membrane pore. To address the biogenesis of Sym1, which oligomerizes in the inner mitochondrial membrane, we studied its import and assembly pathway. Sym1 forms a transport intermediate at the translocase of the outer membrane (TOM) complex. Surprisingly, Sym1 was not transported into mitochondria by an amino-terminal signal, and in contrast to what has been observed in carrier proteins, Sym1 transport and assembly into the inner membrane were independent of small translocase of mitochondrial inner membrane (TIM) and TIM22 complexes. Instead, Sym1 required the presequence of translocase for its biogenesis. Our analyses have revealed a novel transport mechanism for a polytopic membrane protein in which internal signals direct the precursor into the inner membrane via the TIM23 complex, indicating a presequence-independent function of this translocase.

Maternal uniparental disomy of chromosome 2 in a patient with a DGUOK mutation associated with hepatocerebral mitochondrial DNA depletion syndrome

We report maternal uniparental disomy of chromosome 2 (matUPD2) in a 9-month-old girl presenting with hepatocerebral mitochondrial DNA depletion syndrome. This patient was homozygous for the c.352C>T (p.Arg118Cys) mutation in DGUOK gene. The proband's mother was heterozygous for the mutation was absent in DNA of the father. For proband, the absence of paternal contribution at the DGUOK locus prompted us to exclude intragenic DGUOK deletion of the paternal allele with Multiplex ligation-dependent probe amplification (MLPA) analysis. We also excluded non-paternity by studying various markers at different loci. Then we performed an analysis of copy number variations and absence of heterozygosity (AOH) on the proband DNA using high resolution oligonucleotides microarray. Several large regions of AOH with no copy number change were detected on chromosome 2 and one of these AOH regions encompassed DGUOK gene. These results were confirmed with haplotype analysis using polymorphic markers. Informative SNPs and microsatellites markers spanning the whole chromosome 2 showed a matUPD2 with heterodisomy and isodisomy regions, the absence of paternal allele and presence of two maternal alleles, with only one maternal allele on the region of DGUOK locus in 2p13.1. This is the first demonstration of matUPD2 with segmental isodisomy at 2p13.1 locus in hepatocerebral mitochondrial DNA depletion syndrome. The identification of UPD2 will impact genetic counseling for the proband's parents. Because the recurrence risk for UPD2 is very low, the risk for disease in further offspring for this couple is negligible.

Functional outcome of a novel SLC29A3 mutation identified in a patient with H syndrome

The H syndrome (OMIM 612391) is an autosomal recessive disorder characterized by hyperpigmentation, hypertrichosis, histiocytosis and short stature. It is caused by mutations in the SLC29A3 gene, which encodes for the equilibrative nucleoside transporter 3 protein (ENT3), of still uncertain subcellular localisation. Here we report a new case of H syndrome with the novel mutation c.243delA, which has been concomitantly described by others [A. Bolze, A. Abhyankar, A.V. Grant, B. Patel, R. Yadav, M. Byun, D. Caillez, J.F. Emile, M. Pastor-Anglada, L. Abel, A. Puel, R. Govindarajan, L. de Pontual, J.L. Casanova, A mild form of SLC29A3 disorder: a frameshift deletion leads to the paradoxical translation of an otherwise noncoding mRNA splice variant, PLoS ONE 7 (2012) e29708]. Patient-derived primary skin fibroblasts and B-lymphoblastoid cell lines (B-LCL) were obtained and, although no differences were found in mRNA levels of ENT3, a significant increase in plasma membrane equilibrative transport activity was found in fibroblasts from the patient.Loss of function of key proteins implicated in nucleoside metabolism can lead to mitochondrial DNA (mtDNA) depletion syndromes (MDS). Measurement of respiratory chain complex activity revealed that mitochondrial function was unaltered. Neither fibroblasts nor B-LCL showed mtDNA depletion when compared with controls. Fibroblasts and B-LCL from the patient were not particularly protected when mitochondrial damage was induced using nucleoside-derived drugs susceptible to being transported by ENT3. Analysis of mtDNA amounts in tissues obtained at autopsy proved inconclusive with respect to mitochondrial involvement in the pathogenesis of this syndrome. Overall, the data do not support the inclusion of H syndrome among the MDS and these findings are compatible with its recent inclusion among the lysosomal storage diseases.