Pathogenic mtDNA Mutations Research Articles

Mutational and clinical spectrum in Korean with Leber’s hereditary optic neuropathy

AbstractPurpose To investigate the spectrum of mutations of mitochondrial DNA (mtDNA) and clinical features in Korean with Leber’s hereditary optic neuropathy (LHON).Methods Total genomic DNA was isolated from the peripheral blood leukocytes of the patients who were suspicious of LHON. The presence of mutations were screened by direct sequencing were analyzed against mitochondrial reference sequence on genetic analyzer. The ophthalmologic examinations of patients were conducted, including visual acuity(VA), color vision, retinal nerve fiber layer(RNFL) thickness and visual field(VF).Results Two primary mutations (G1778A, T14484C) and five secondary mutations (T3394C, C3497T, G11696A, A11392C, T14502C) were detected in 34 of 164 unrelated patients. Among the 34 patients, primary mutation was detected in 21 patients, secondary mutation in 11 patients, and combined primary and secondary mutation in 2 patients. Compared the patients with primary mutation, patients with secondary mutation had increased proportion of woman. The VA was significantly better in secondary mutation than primary mutation. Also, the severity of the color vision deficiency was significantly mild in secondary mutation relative to primary mutation. No significant difference was found in the OCT parameters between the groups including temporal area. However, the visual field index and mean deviation of the visual field test were different significantly between groups.Conclusion Our results emphasize the possible heterogeneity of LHON‐associated mtDNA mutations and clinical phenotypes in Korean LHON patients. Candidate pathogenic mtDNA mutations including major primary mutations should be rigorously investigated.

Extensive pathogenicity of mitochondrial heteroplasmy in healthy human individuals.

A majority of mitochondrial DNA (mtDNA) mutations reported to be implicated in diseases are heteroplasmic, a status with coexisting mtDNA variants in a single cell. Quantifying the prevalence of mitochondrial heteroplasmy and its pathogenic effect in healthy individuals could further our understanding of its possible roles in various diseases. A total of 1,085 human individuals from 14 global populations have been sequenced by the 1000 Genomes Project to a mean coverage of ∼2,000× on mtDNA. Using a combination of stringent thresholds and a maximum-likelihood method to define heteroplasmy, we demonstrated that ∼90% of the individuals carry at least one heteroplasmy. At least 20% of individuals harbor heteroplasmies reported to be implicated in disease. Mitochondrial heteroplasmy tend to show high pathogenicity, and is significantly overrepresented in disease-associated loci. Consistent with their deleterious effect, heteroplasmies with derived allele frequency larger than 60% within an individual show a significant reduction in pathogenicity, indicating the action of purifying selection. Purifying selection on heteroplasmies can also be inferred from nonsynonymous and synonymous heteroplasmy comparison and the unfolded site frequency spectra for different functional sites in mtDNA. Nevertheless, in comparison with population polymorphic mtDNA mutations, the purifying selection is much less efficient in removing heteroplasmic mutations. The prevalence of mitochondrial heteroplasmy with high pathogenic potential in healthy individuals, along with the possibility of these mutations drifting to high frequency inside a subpopulation of cells across lifespan, emphasizes the importance of managing mitochondrial heteroplasmy to prevent disease progression.

Modeling pathogenic mtDNA mutations in bacteria for probing their functional consequences in complex I with special reference to proton pumping efficiency

Modeling pathogenic mtDNA mutations in bacteria for probing their functional consequences in complex I with special reference to proton pumping efficiency

Proteolytic Cleavage of Opa1 Stimulates Mitochondrial Inner Membrane Fusion and Couples Fusion to Oxidative Phosphorylation

Mitochondrial fusion is essential for maintenance of mitochondrial function. The mitofusin GTPases control mitochondrial outer membrane fusion, whereas the dynamin-related GTPase Opa1 mediates inner membrane fusion. We show that mitochondrial inner membrane fusion is tuned by the level of oxidative phosphorylation (OXPHOS), whereas outer membrane fusion is insensitive. Consequently, cells from patients with pathogenic mtDNA mutations show a selective defect in mitochondrial inner membrane fusion. In elucidating the molecular mechanism of OXPHOS-stimulated fusion, we uncover that real-time proteolytic processing of Opa1 stimulates mitochondrial inner membrane fusion. OXPHOS-stimulated mitochondrial fusion operates through Yme1L, which cleaves Opa1 more efficiently under high OXPHOS conditions. Engineered cleavage of Opa1 is sufficient to mediate inner membrane fusion, regardless of respiratory state. Proteolytic cleavage therefore stimulates the membrane fusion activity of Opa1, and this feature is exploited to dynamically couple mitochondrial fusion to cellular metabolism.

Mitochondrial Sequence Changes in Keratoconus Patients

We investigated whether a group of patients with keratoconus (KTCN) harbor mutations in the mitochondrial genome. We sequenced the full mitochondrial genome in a group of Saudi patients with KTCN (n = 26) and 100 ethnically matched controls who had no KTCN by examination. A total of 10 KTCN patients (38.5%) had potentially pathogenic nonsynonymous mtDNA mutations. Of the nonsynonymous sequence changes detected, 4 (40%) were in Complex I, one was in the tRNA(Glutamine), one was in tRNA(Tryptophan), one was in tRNA(Asparagine), one was in tRNA(Histidine), and two were in the tRNA(Leucine2). One nonsynonymous sequence change was heteroplasmic, whereas all the remaining 9 were homoplasmic. These sequence changes were not detected in controls of similar ethnicity. Four sequence changes were novel (were not reported previously) and 5 were reported previously. Additionally, we detected 54 synonymous (does not result in an amino acid change) sequence changes with no pathologic significance. If our results are confirmed in a larger cohort and multiple ethnicities, then mtDNA mutation may be considered as a genetic risk factor contributing indirectly through the oxidative stress mechanism to the development and/or progression of KTCN.

Treatment strategies for inherited optic neuropathies: past, present and future

Bilateral visual loss secondary to inherited optic neuropathies is an important cause of registrable blindness among children and young adults. The two prototypal disorders seen in clinical practice are Leber hereditary optic neuropathy (LHON) and autosomal dominant optic atrophy (DOA). About 90% of LHON cases are due to one of three mitochondrial DNA (mtDNA) point mutations: m.3460G>A, m.11778G>A, and m.14484T>C, which affect critical complex I subunits of the mitochondrial respiratory chain. The majority of patients with DOA harbour pathogenic mutations within OPA1, a nuclear gene that codes for a multifunctional inner mitochondrial membrane protein. Despite their contrasting genetic basis, LHON and DOA share overlapping pathological and clinical features that serve to highlight the striking tissue-specific vulnerability of the retinal ganglion cell (RGC) layer to disturbed mitochondrial function. In addition to severe visual loss secondary to progressive optic nerve degeneration, a subgroup of patients will also develop a more aggressive syndromic phenotype marked by significant neurological deficits. The management of LHON and DOA remains largely supportive, but major advances in our understanding of the mechanisms underpinning RGC loss in these two disorders are paving the way for novel forms of treatment aimed at halting or reversing visual deterioration at different stages of the disease process. In addition to neuroprotective strategies for rescuing RGCs from irreversible cell death, innovative in vitro fertilisation techniques are providing the tantalising prospect of preventing the germline transmission of pathogenic mtDNA mutations, eradicating in so doing the risk of disease in future generations.

P32 Mutations in SPG7 cause chronic progressive external ophthalmoplegia through disordered mtDNA maintenance

P32 Mutations in SPG7 cause chronic progressive external ophthalmoplegia through disordered mtDNA maintenance

P31 Genotypic and phenotypic heterogeneity in adult-onset progressive external ophthalmoplegia (PEO) with mitochondrial DNA instability: a systematic review

P31 Genotypic and phenotypic heterogeneity in adult-onset progressive external ophthalmoplegia (PEO) with mitochondrial DNA instability: a systematic review

Exploring The Relationship Between Autophagy and Mitochondrial Dysfunction In a Mouse Model Of MDS

Although myelodysplastic syndrome (MDS) is uncommon in children, with an annual estimated incidence of 0.5 to 4 cases per million children compared to 20 to 40 cases per million in adults, the incidence of MDS appears to be increasing in the United States with the improvements in survival rates of childhood cancers, at least in part because the intensive chemo- and radiotherapy regimens that cure pediatric patients with cancer may also cause therapy-related MDS. Unlike in other blood-related cancers, the disease-causing mutations in pediatric MDS and t-MDS/AML are unknown. Nevertheless, recent studies have suggested that the mitochondrial dysfunction associated with MDS results from the inability of hematopoietic stem/progenitor cells to appropriately cope with mitochondrial damage andmitochondrial dysfunction appears to be an early event in the development of t-MDS/AML.Autophagy is a catabolic process involved in the sequestration and breakdown of cellular components, including organelles such as mitochondria and long-lived or misfolded proteins. By sequestering and degrading depolarized fragments of mitochondria that result from cycles of mitochondrial fusion and fission, autophagy plays a critical role in the maintenance of healthy pools of mitochondria. Given the importance of autophagy in mitochondrial quality control and increasing evidence that mitochondrial dysfunction plays a role in the pathogenesis of MDS, we sought to explore the role of autophagy in the pathogenesis of MDS associated with mitochondrial dysfunction using the previously established PolgAmt/mt mouse model (a.k.a. mtDNA mutator mice).The mtDNA mutator mice express a proof-reading-deficient mutant of mtDNA polymerase (PolgA-D257A), which results in increased mitochondrial mutation frequencies in multiple tissues, and a decline in respiratory function of mitochondria-encoded complexes. The delayed onset of an MDS-like phenotype in these mice provides a window of time to interrogate the role of autophagy in the progression of disease symptoms. Disruption of autophagy in erythroid progenitors did not cause a significant decrease in hemoglobin or red blood cell count in PolgAwt/wt mice, however, it accelerated the onset of anemia and morbidity significantly in PolgAmt/mt mice, suggesting that autophagy delays the onset of symptoms associated with pathologic mtDNA mutations. Flow cytometric evaluation of peripheral blood of PolgAmt/mt mice and wild-type littermates revealed an unexpected defect in RBC maturation in PolgAmt/mt animals, which became apparent with age or following serial phlebotomy. The defect was similar to that observed in autophagy-defective Ulk1-deficient mice suggesting that pathogenic mtDNA mutations may cause an age-dependent delay in autophagy. Transformed PolgAmt/mt murine embryonic fibroblasts (MEFs) with mitochondrial dysfunction also showed a defect in basal LC3 conversion and p62 accumulation, consistent with impaired flux through the autophagy pathway. Together, our data indicates that although autophagy initially combats accumulation of dysfunctional mitochondria resulting from an increased mtDNA mutation burden, the latter eventually leads to a decline in basal autophagy and impaired clearance of mitochondria, which contributes to the decline in mitochondrial function and progression of MDS-like disease in mice. Disclosures:No relevant conflicts of interest to declare.

Mitochondrial DNA Mutation Load

Mitochondrial DNA Mutation Load

Specific elimination of mutant mitochondrial genomes in patient-derived cells by mitoTALENs

Mitochondrial diseases are commonly caused by mutations in the mitochondrial DNA (mtDNA), which in most cases co–exists with the wild–type (mtDNA heteroplasmy). We have engineered TAL–effector nucleases (TALENs) to localize to mitochondria and cleave different classes of pathogenic mtDNA mutations. MitoTALEN expression led to permanent reductions in deletion or point mutant mtDNA in patient–derived cells, raising the possibility that they can be curative to some of these diseases.

Clinical Significance in Oral Cavity Squamous Cell Carcinoma of Pathogenic Somatic Mitochondrial Mutations

Somatic mutations affecting the mitochondrial DNA (mtDNA) have been frequently observed in human cancers and proposed as important oncological biomarkers. However, the clinical significance of mtDNA mutations in cancer remains unclear. This study was therefore performed to explore the possible clinical use in assessing oral squamous cell carcinoma (OSCC) of pathogenic mtDNA mutations. The entire mitochondrial genome of 300 OSCC with their matched control DNAs was screened by direct sequencing and criteria were set to define a pathogenic somatic mutation. The patients' TP53 R72P genotypes were determined by polymerase chain reaction-restriction fragment length polymorphism. The relationships between pathogenic somatic mutations, clinicopathogical features, TP53 R72P genotype and clinical prognosis were analyzed. Overall, 645 somatic mtDNA mutations were identified and 91 of these mutations were defined as pathogenic. About one quarter (74/300) of the OSCC tumor samples contained pathogenic mutations. Individuals with the TP53 R allele had a higher frequency of pathogenic somatic mutation than those with the PP genotype. Kaplan-Meier analysis indicated that TP53 R allele patients with pathogenic somatic mutations demonstrated a significant association with a poorer disease-free survival than other individuals (HR = 1.71; 95% CI, 1.15–2.57; p = 0.009) and this phenomenon still existed after adjusting for mtDNA haplogroup, tumor stage with treatment regimens, differentiation and age at diagnosis (HR = 1.59; 95% CI, 1.06–2.40; p = 0.03). Subgroup analyses showed that this phenomenon was limited to patients who received adjuvant radiotherapy/chemo-radiotherapy after surgery. The results strongly indicated that pathogenic mtDNA mutations are a potential prognostic marker for OSCCs. Furthermore, functional mitochondria may play an active role in cancer development and the patient's response to radiotherapy/chemo-radiotherapy.

Abstract 2009: Clinical value of pathogenic mitochondrial mutations in oral cavity squamous cell carcinoma.

Abstract Mitochondria appear to have a central role in two of the basic cancer cellular hallmarks made by Hanahan and Weinberg: reprogramming of energy metabolism and glucose-shortage induced apoptosis. Somatic mutations in mitochondrial DNA (mtDNA) have been frequently observed in human cancers and proposed as important oncological biomarkers. However, the clinical significance of mtDNA mutations in cancer remains unclear. In the present study, we screened the entire mitochondrial genome of 300 oral cavity squamous cell carcinomas (OSCC) with their matched control tissues by direct sequencing. We found that 240 (80%) OSCC tumor tissues had one or more somatic mutations and 85 % (203/240) of somatic mutation occurs in the control region (D-loop region) of mitochondrial genome, 47.1% (113/240) in the coding region, 13.8% (33/240) in rRNA and 8.3% (20/240) cases with in tRNA. Overall, 645 somatic mutations at 329 unique nucleotide positions were identified and 355 (55.0%) of 645 somatic mutations were located in the D-loop. The relative mutation frequency (mutations/nucleotides) in the D-loop was 15.6-fold higher than in the other regions (355/1121 versus 290/15448). The highest mutation was in the D310 polycytidine stretch region, where 165 (25.6%, 165/645) mutations were found. Among the 10 somatic mtDNA hot mutation sites, 6 are known mono-/di-nucleotide repeat length polymorphic sites, namely nucleotide position 310-317 (D310, also known as hypervariable region 1), 514-523 (CA repeat), 568-576 (C stretch 1), 956-965 (12S RNA), 5895-5899 (C stretch 2) and 16180-16195 (hypervariable region 2. We found that the mutation frequency at these 6 mono-/di-nucleotide repeat length polymorphic sites was strongly associated with the number of the repeat length and status of heteroplasmy. In the total 645 mutations, most common mutation type was base substitution (58.3%, 376/645), and transition mutation (G:C > A:T and A:T > G:C) accounted for the most mutations (94.7%, 356/376). The total and pathogenic mutation load for mitochondrial respiratory complexes was 1.10% and 0.76%, respectively. No significant association was found between somatic mtDNA mutations in whole mitochondrial genome and the clinicopathological features. However, we found that individuals with p53 R allele had higher frequency of pathogenic somatic mtDNA mutation than those with PP genotype. The Kaplan-Meier plot analysis indicated that pathogenic somatic mtDNA mutations was associated with poor disease-free survival (p = 0.02) and this phenomenon still existed after adjusting for tumor stage, differentiation and age at diagnosis (hazard ratio = 1.557; 95% confidence interval = 1.058 - 2.292); while, there was no difference in overall survival. Taken together, these data suggest that pathogenic somatic mtDNA mutations may play important roles in the aggressiveness of OSCCs. Citation Format: Ling-Ling Hsieh, Chih-Hsiung Lai, Shiang-Fu Huang, Chun-Ta Liao, I-How Chen, Hung-Ming Wang. Clinical value of pathogenic mitochondrial mutations in oral cavity squamous cell carcinoma. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2009. doi:10.1158/1538-7445.AM2013-2009

Preventing the transmission of pathogenic mitochondrial DNA mutations: can we achieve long-term benefits from germ-line gene transfer?

Mitochondrial medicine is one of the few areas of genetic disease where germ-line transfer is being actively pursued as a treatment option. All of the germ-line transfer methods currently under development involve some carry-over of the maternal mitochondrial DNA (mtDNA) heteroplasmy, potentially delivering the pathogenic mutation to the offspring. Rapid changes in mtDNA heteroplasmy have been observed within a single generation, and so any ‘leakage’ of mutant mtDNA could lead to mtDNA disease in future generations, compromising the reproductive health of the first generation, and leading to repeated interventions in subsequent generations. To determine whether this is a real concern, we developed a model of mtDNA heteroplasmy inheritance by studying 87 mother–child pairs, and predicted the likely outcome of different levels of ‘mutant mtDNA leakage’ on subsequent maternal generations. This showed that, for a clinical threshold of 60%, reducing the proportion of mutant mtDNA to <5% dramatically reduces the chance of disease recurrence in subsequent generations, but transmitting >5% mutant mtDNA was associated with a significant chance of disease recurrence. Mutations with a lower clinical threshold were associated with a higher risk of recurrence. Our findings provide reassurance that, at least from an mtDNA perspective, methods currently under development have the potential to effectively eradicate pathogenic mtDNA mutations from subsequent generations.

Somatic complex I disruptive mitochondrial DNA mutations are modifiers of tumorigenesis that correlate with low genomic instability in pituitary adenomas

Mitochondrial DNA (mtDNA) mutations leading to the disruption of respiratory complex I (CI) have been shown to exhibit anti-tumorigenic effects, at variance with those impairing only the function but not the assembly of the complex, which appear to contribute positively to cancer development. Owing to the challenges in the analysis of the multi-copy mitochondrial genome, it is yet to be determined whether tumour-associated mtDNA lesions occur as somatic modifying factors or as germ-line predisposing elements. Here we investigated the whole mitochondrial genome sequence of 20 pituitary adenomas with oncocytic phenotype and identified pathogenic and/or novel mtDNA mutations in 60% of the cases. Using highly sensitive techniques, namely fluorescent PCR and allele-specific locked nucleic acid quantitative PCR, we identified the most likely somatic nature of these mutations in our sample set, since none of the mutations was detected in the corresponding blood tissue of the patients analysed. Furthermore, we have subjected a series of 48 pituitary adenomas to a high-resolution array comparative genomic hybridization analysis, which revealed that CI disruptive mutations, and the oncocytic phenotype, significantly correlate with low number of chromosomal aberrations in the nuclear genome. We conclude that CI disruptive mutations in pituitary adenomas are somatic modifiers of tumorigenesis most likely contributing not only to the development of oncocytic change, but also to a less aggressive tumour phenotype, as indicated by a stable karyotype.

Regulation of mitochondrial FoF1ATPase activity by Sirt3-catalyzed deacetylation and its deficiency in human cells harboring 4977 bp deletion of mitochondrial DNA

Sirt3, a mitochondrial NAD+-dependent deacetylase, is regarded as a potential regulator in cellular metabolism. However, the role of Sirt3 in the regulation of mitochondrial FoF1ATPase and the linkage to mitochondrial diseases is unclear. In this study, we demonstrated a role of Sirt3 in the regulation of FoF1ATPase activity in human cells. Knockdown of Sirt3 in 143B cells by shRNA transfection caused increased acetylation levels of the α and OSCP subunits of FoF1ATPase. We showed that Sirt3 physically interacted with the OSCP and led to its subsequent deacetylation. By incubation of mitochondria with the purified Sirt3 protein, Sirt3 could regulate FoF1ATPase activity through its deacetylase activity. Moreover, suppression of Sirt3 reduced the FoF1ATPase activity, consequently decreased the intracellular ATP level, diminished the capacity of mitochondrial respiration, and compromised metabolic adaptability of 143B cells to the use of galactose as the energy source. In human cells harboring ≅85% of mtDNA with 4977bp deletion, we showed that oxidative stress induced a reduction of Sirt3 expression, and an increased acetylation of the OSCP subunit of FoF1ATPase. Importantly, the expression of Sirt3 was also decreased in the skin fibroblasts from patients with CPEO syndrome. We further demonstrated that oxidative stress induced by 5–10μM of menadione impaired the Sirt3-mediated deacetylation and activation on FoF1ATPase activity through decreasing the protein level of Sirt3. Our findings suggest that increased intracellular ROS levels might modulate the expression of Sirt3 which deacetylates and activates FoF1ATPase in human cells with mitochondrial dysfunction caused by a pathogenic mtDNA mutation.

Mitochondria, oxidative stress and neurodegeneration

Mitochondria are involved in ATP supply to cells through oxidative phosphorylation (OXPHOS), synthesis of key molecules and response to oxidative stress, as well as in apoptosis. They contain many redox enzymes and naturally occurring inefficiencies of oxidative phosphorylation generate reactive oxygen species (ROS).CNS functions depend heavily on efficient mitochondrial function, since brain tissue has a high energy demand. Mutations in mitochondrial DNA (mtDNA), generation and presence of ROS and environmental factors may contribute to energy failure and lead to neurodegenerative diseases. Many rare metabolic disorders have been associated with mitochondrial dysfunction. More than 300 pathogenic mtDNA mutations involve proteins that regulate OXPHOS and mitochondrial structural integrity, and have also been described in neurodegenerative diseases with autosomal inheritance. Mitochondria may have an important role in ageing-related neurodegenerative disorders like Parkinson's disease (PD), Alzheimer's disease (AD), Huntington's disease (HD) and amyotrophic lateral sclerosis (ALS). In primary mitochondrial and neurodegenerative disorders, there is strong evidence that mitochondrial dysfunction occurs early and has a primary role in pathogenesis. In the present review, we discuss several mitochondrial diseases as models of neurodegeneration.

Adult-Onset Progressive Spastic Paraparesis Due to a Novel, Heteroplasmic Mitochondrial tRNAGlu (m.14685A&gt;G) Gene Mutation (IN7-1.010)

Objective: We report a 45 year old lady with a five year history of progressive spastic paraparesis and ataxia. Background There was visual impairment – cataracts aged 8 and primary photo–receptor retinopathy, bilateral sensory–neural deafness as a teenager and stroke–like episodes as an adult. She was initially believed to have Usher9s syndrome and then congenital rubella following evidence of bilateral basal ganglia calcification on MRI brain. Family history was unremarkable, a male sibling had spina–bifida whilst her maternal grandmother had multiple sclerosis. Design/Methods: Investigations revealed elevated plasma lactate, with no evidence of CSF abnormalities, or CSF rubella RNA and rubella specific intrathecal antibodies. Brain MRI demonstrated progressive atrophy of the cerebral hemispheres and spinal cord, and the existing basal ganglia and thalamic calcifications. Muscle biopsy revealed numerous COX-deficient fibres, some of which showed “ragged-red” changes. Results: Having excluded common mtDNA mutations, sequencing of the entire mitochondrial genome was undertaken, identifying a novel, heteroplasmic m.14685G>A tRNAGlu (MTTE) variant affecting an evolutionary conserved nucleotide within the TψC arm of the mt-tRNAGlu molecule. Single fibre PCR analysis revealed segregation of m.14685G>A mutation with the COX deficiency, confirming pathogenicity of this novel mt-tRNA mutation. She died of broncho-pneumonia. Autopsy revealed generalised cerebral atrophy with dense calcification of the lentiform nuclei with neuronal loss and gliosis. Mutation analysis of post-mortem skeletal and cardiac muscle tissues confirmed the m.14685G>A MTTE gene mutation at intermediate levels of heteroplasmy (42% and 68% respectively) whilst genetic analysis of the family indicated that the mutation had arisen sporadically. Conclusions: This is an unusual presentation of adult–onset spastic paraparesis related to mitochondrial dysfunction, due to a novel, pathogenic mtDNA mutation. Mitochondrial diseases are an important differential of Usher9s syndrome which can manifest into adulthood, and of perinatal infections (TORCH complex) which can involve the eye and CNS with focal cerebral calcification. Disclosure: Dr. Gnanapavan has nothing to disclose. Dr. Alston has nothing to disclose. Dr. Lax has nothing to disclose. Dr. Yarham has nothing to disclose. Dr. Everett has nothing to disclose. Dr. Hewazy has nothing to disclose. Dr. O9Donovan has nothing to disclose. Dr. Dean has nothing to disclose. Dr. Taylor has nothing to disclose.

MERRF and Kearns-Sayre Overlap Syndrome Due to the Mitochondrial DNA M.3291T&gt;C Mutation (IN7-1.008)

Objective: To report a patient with a complex phenotype of myoclonus epilepsy with ragged-red fibers (MERRF) syndrome and Kearns–Sayre syndrome (KSS), who harbored a m.3291T>C mutation in the tRNALeu(UUR) gene. This mutation has never been reported in association with a MERRF/KSS syndrome. Background Individuals who harbor a pathogenic mtDNA mutation can manifest overlapping features of typical mitochondrial syndromes. MERRF is a multisystem disorder characterized by myoclonic seizures, cerebellar ataxia, mitochondrial myopathy, and ragged-red fibers (RRF) on muscle biopsy, commonly caused by the m.8344A>G mutation in the tRNALys gene. KSS is a mitochondrial disorder characterized by onset before 20 years of age of progressive external ophthalmoplegia or pigmentary retinopathy, together with at least one of a triad of cerebellar ataxia, heart block, and elevated cerebrospinal fluid protein. Typically, KSS is due to single, large-scale deletions of mtDNA. Design/Methods: Clinical data were collected. Muscle histochemical and biochemical analysis was performed. Muscle DNA was screened for mtDNA large-scale rearrangements and point mutations. Mutation level was quantified by restriction fragment length polymorphism (RFLP) analysis. Results: A 48-year-old man presented with progressive myoclonus epilepsy, cerebellar ataxia, hearing loss, myopathic weakness, ophthalmoparesis, pigmentary retinopathy, and bifascicular heart block. Muscle biopsy demonstrated multiple RRF. Biochemical analysis of respiratory chain enzymes was normal. Genetic tests were negative for the common MERRF mtDNA mutations, and no deletions in mitochondrial DNA were detected. Direct sequencing mtDNA revealed a T-to-C transition at nucleotide position 3291 in the tRNALeu(UUR) gene. PCR RFLP showed 92% mutant genome in muscle. Conclusions: This is the first report associating a m.3291T>C mutation in the tRNALeu(UUR) gene with an adult-onset MERRF/KSS overlap. Our data reinforce the well-established concept that, in mtDNA-related disorders, the same genetic abnormality can be associated with a wide spectrum of phenotypes, and the observation that tRNALeu(UUR) is a hotspot for mtDNA mutations. Supported by: The National Institutes of Health (NIH HD32062) and by the Marriott Mitochondrial Disorders Clinical Research Fund (MMDCRF). M.H. was supported by NIH grants (R01HD57543, R01HD056103, and RCNS070232) and by the Muscular Dystrophy Association. Disclosure: Dr. Emmanuele has nothing to disclose. Dr. Silvers has nothing to disclose. Dr. Sotiriou has nothing to disclose. Dr. Tanji has nothing to disclose. Dr. DiMauro has nothing to disclose. Dr. Hirano has received personal compensation for activities with Athena Diagnostics as a speaker.Dr. Hirano has received personal compensation in an editorial capacity for Current Neurology and Neuroscience Reports.Dr. Hirano has received research support from Santhera Pharmaceutical and Edison Pharmaceuticals.

Mitochondrial DNA variant associated with Leber hereditary optic neuropathy and high-altitude Tibetans

The distinction between mild pathogenic mtDNA mutations and population polymorphisms can be ambiguous because both are homoplasmic, alter conserved functions, and correlate with disease. One possible explanation for this ambiguity is that the same variant may have different consequences in different contexts. The NADH dehydrogenase subunit 1 (ND1) nucleotide 3394 T > C (Y30H) variant is such a case. This variant has been associated with Leber hereditary optic neuropathy and it reduces complex I activity and cellular respiration between 7% and 28% on the Asian B4c and F1 haplogroup backgrounds. However, complex I activity between B4c and F1 mtDNAs, which harbor the common 3394T allele, can also differ by 30%. In Asia, the 3394C variant is most commonly associated with the M9 haplogroup, which is rare at low elevations but increases in frequency with elevation to an average of 25% of the Tibetan mtDNAs (odds ratio = 23.7). In high-altitude Tibetan and Indian populations, the 3394C variant occurs on five different macrohaplogroup M haplogroup backgrounds and is enriched on the M9 background in Tibet and the C4a4 background on the Indian Deccan Plateau (odds ratio = 21.9). When present on the M9 background, the 3394C variant is associated with a complex I activity that is equal to or higher than that of the 3394T variant on the B4c and F1 backgrounds. Hence, the 3394C variant can either be deleterious or beneficial depending on its haplogroup and environmental context. Thus, this mtDNA variant fulfills the criteria for a common variant that predisposes to a "complex" disease.