PEO/CPEO and other Mitochondrial Myopathies
Definition
Mitochondrial myopathies and neuropathies (neuromyopathies) are heterogeneous group of disorders caused by dysfunction in mitochondria. Mitochondrial myopathies are progressive muscle conditions caused primarily by the impairment of oxidative phosphorylation (OXPHOS) in the mitochondria. This causes a deficit in energy production in the form of adenosine triphosphate (ATP), particularly in skeletal muscle. The diagnosis of mitochondrial myopathy is reliant on the combination of numerous techniques including traditional histochemical, immunohistochemical, and biochemical testing combined with the fast-emerging molecular genetic techniques, namely next-generation sequencing (NGS).
Mitochondrial genetics
Mitochondrial genome is 16.5-kB circular double stranded DNA that does not contain noncoding regions (introns). In some areas there is overlap of mitochondrial genes. There is a single promoter site. The transcription in mitochondria is polycistronic and mitochondrial genes are transcribed as two large RNAs. These subdivide into 13 mitochondrial mRNA (mt-mRNA) with are translated into 13 polypeptide proteins of the OXPHOS (respiratory chain complexes), 22 mitochondrial tRNA (mt-tRNA), and 2 mitochondrial rRNAs (mt-rRNA).
Primary mutations of the mtDNA include point mutations affecting protein coding regions of the genome and mt-tRNA genes which alter mitochondrial protein synthesis. They also include single, large-scale mtDNA deletions which can either be inherited or arise sporadically during embryogenesis. The pathogenicity of mtDNA mutations is further complicated by heteroplasmy (the state in which there is coexistence of both mutant and wild-type mtDNA in a given cell) and the threshold effect (when the proportion of mutant mtDNA exceeds a given limit and causes a biochemical defect in a cell)
The nDNA encodes 1171 known and 442 predicted mitochondrial proteins (MitoMiner v.Q2 2018), including OXPHOS subunits, assembly factors for OXPHOS complexes and proteins required for mtDNA maintenance. Mutations in the genes encoding the mtDNA maintenance machinery can lead to either mtDNA depletion or multiple mtDNA deletions.
The mitochondrial genome account for less than 5% of all mitochondrial proteins. The majority of mitochondrial proteins are encoded by the nuclear genome that are translated in the cytoplasm and subsequently are transported into the mitochondria. Also, the nucleus regulates the replication of mitochondrial genome.
Mitochondrial function is under the control of two genomes; the mitochondrial genome (mtDNA) and the nuclear genome (nDNA); as such, mitochondrial myopathy can be caused by pathogenic genetic variants in either of these genomes. This dual genetic control also means that mitochondrial disease is transmitted with the following inheritance patterns: maternal (mtDNA), X-linked, autosomal recessive, autosomal dominant. In addition, some relatively common mitochondrial myopathies occur de novo.
During fertilization, all the mitochondria are contributed by the mother. There are hundred of mitochondrial present in every cell in the body. Every mitochondria has several copies of mtDNA. Mutation is far more common to occur in mtDNA and these are more likely to manifest clinically than mutation in nDNA genes, because of the lack of introns in mtDNA and decreased DNA repair mechanisms in the mitochondrial genome. mtDNA mutations are randomly distributed in subsequent generation of somatic cells during mitosis and in germ cells during meiosis. Some cells will have few or no mutant genomes (normal homoplasty), some will have a mix of mutant and normal genomes (heteroplasmy), and some will have only mutant genomes (mutant homoplasty). Phenotypic expression depends on the burden of mutant genome. When the number of mitochondria bear sufficient mutated mtDNA, exceeding a certain threshold, mitochondrial function becomes impaired and patients manifest clinical symptoms and signs of disease (threshold effect). Some of the mutant mitochondria may borrow the mRNAs and tRNAs from neighboring normal mitochondria in a process called complementation. Thus, there can be a degree of normal translation of mtDNA-encoded proteins even in mitochondria harboring large DNA deletions.
Different organs have different susceptibility for mitochondrial abnormalities depending on their energy requirements. CNS has a constant demand for energy and is more severely impacted. The skeletal muscles have low energy demand at rest, but these demands drastically increase with exercise. This is the basis for exercise intolerance in many patients with mitochondrial myopathies.
Primary mutations of mtDNA can only be passed on by the mother. X-linked inheritance pattern affect men and women equally, while men are generally severely affected. Based on the degree of heteroplasmy, all the children of an affect mother may be affected to varying degrees.
95% of mitochondrial proteins are encoded by nDNA and can be inherited in an AD (e.g. CPEO) or AR (e.g. MNGIE) and even X-linked (e.g. some forms of Leigh syndrome) fashion. KSS is associated with large mtDNA deletions, but is sporadic in nature. Mutation in nDNA result in syndromes associated with depletion or multiple deletion of mtDNA. They can demonstrate AD or AR patterns. They can affect dNTPs (deoxyribonucleoside triphosphates) that serve as the building blocks for mtDNA. Replication of mtDNA requires POLG1, POLG2, PEO1, DNA2. Mutations in these genes result in mtDNA depletion and/or multiple mtDNA deletions that are found in various mitochondrial disorders (PEO, MNGIE, SANDO). POLG related disorders are due to mutations in the nuclear gene POLG and often result in multiple mtDNA deletions of different length. POLG related disorders may be AR or AD. A delicate balance of the 4 dNTPs (dATP, dGTP, dCTP, and dTTP) is crucial for mtDNA replication. Several nDNA encoded enzymes are key in preserving this balance (TK2, DGUOK, SUCLA2, SUCLG1, RRM2B, TYMP, and MPV17). Mutations in these genes also cause mtDNA depletion resulting in various mitochondrial disorders (mitochondrial depletion myopathy, MNGIE, Navajo neurohepatopathy). Furthermore, some nDNA genes are responsible for normal mitochondrial dynamics. For example, fusion of mitochondria is dependent on nDNA encoded profusion GTPases that are located in the mitochondrial membranes and MFN1 and MFN2.
Function of mitochondria:
Produces energy in the form of ATP for cellular function by converting fuel substrates (carbohydrates, lipids, and proteins).
Fatty acids are metabolized into molecules of acetyl CoA within mitochondria.
Amino acids are converted to pyruvate within mitochondria
Carbohydrates are metabolized to pyruvate in the cytoplasm and then transported into the mitochondria.
Pyruvate from both sources are converted to acetyl CoA.
Acetyl CoA then enters in the Krebs cycle from which electrons are generated.
Electrons derived from the Krebs cycle are shuttled to the respiratory chain and are processed through complexes 1-V to generate ATP molecules.
The respiratory chain complexes comprise 5 multienzyme complexes (I-V) present in the inner membrane of mitochondria.
Complex I (NADH:ubiquinone oxidoreductase) contains 45 polypeptide subunits, 7 are encoded by mtDNA
Complex II (SDH) contains 4 polypeptide subunits, all encoded by nDNA
Complex III (ubiquinol–cytochrome c oxidoreductase or bc1 complex) contains 11 polypeptide subunits, 1 encoded by mtDNA
Complex IV (cytochrome c oxidase) contains 13 subunits, 3 encoded by mtDNA
Complex V (ATPase synthetase) contains 19 subunits, 2 encoded by mtDNA.
Cytochrome complex I is the first enzyme of the respiratory chain. It oxidizes NADH, which is generated through the Krebs cycle in the mitochondrial matrix, and uses the two electrons to reduce ubiquinone to ubiquinol. Ubiquinol is reoxidized by the cytochrome III complex and transfers electrons to reduce molecular oxygen to water at cytochrome complex IV. The redox energy released during this process is used to transfer protons from the mitochondrial matrix to the periplasmic space that generates proton-motive force across the inner mitochondrial membrane at cytochrome complexes I, III, and IV. Complex V uses this proton-motive force to produce ATP from ADP and inorganic phosphate. This entire process constitutes OXPHOS. Because complex I is the major entry point for electrons to the respiratory chain and is suggested as the rate-limiting step in overall respiration, it plays a central role in energy metabolism.
Mitochondrial disorders Classification:
Biochemical defects - metabolic function affected
Transport
Substrate utilization
Krebs cycle
Oxidation/phosphorylation coupling
Respiratory chain defects
Genetic defects
mtDNA
nDNA affecting directly or indirectly the respiratory chain complex
nDNA affecting mtDNA maintenance or mitochondrial dynamics.
Patients with mitochondrial myopathies have diverse clinical phenotypes, some features may be similar to other myopathies and others are more specific for patients with mitochondrial disease. In virtually all patients with mitochondrial myopathy, there is potential involvement of other systems which may well be the prominent and life-threatening feature—for example, cardiomyopathy, epilepsy, or stroke-like episodes. Indeed the most frequent presentation of mitochondrial myopathy is a proximal myopathy in combination with other symptoms which is often the clue to likely mitochondrial involvement. Therefore, t is essential that they are considered when evaluating all patients.
Eyes: Ptosis, external ophthalmoplegia, and optic atrophy.
Nervous system: Seizures, tremor, developmental delay, deafness, strokelike episodes, ataxia, and peripheral neuropathy.
Heart: Cardiomyopathy.
Kidneys: Fanconi syndrome, kidney failure, and nephrotic syndrome.
Liver: Liver failure and hepatic steatosis.
Pancreas: Diabetes.
Gastrointestinal: Gut dysmotility, intestinal pseudoobstruction, and chronic diarrhea.
Skeletal muscle: Muscle weakness, fatigue, exercise intolerance, and cramps. Respiratory failure due to diaphragmatic dysfunction requiring ventilatory support.
Diagnostic work-up
The diagnosis of mitochondrial myopathies involves a multidisciplinary approach. History and physical examination are crucial for recognizing that mitochondrial myopathy is a potential diagnosis but also to suggest the most appropriate diagnostic studies. The diagnostic investigations include histological and immunohistochemical studies, enzymatic analysis of the OXPHOS complexes, and the genetic analysis of the mtDNA. Additionally, if a nuclear genetic diagnosis is suspected, a targeted nDNA sequencing approach may be used. If no pathogenic mutation is identified, whole genome and whole exome screens are now commonly used to search for potential genetic diagnoses, with new disease genes constantly being identified. By integrating the information from these diagnostic tests, it allows for a diagnosis in the majority of patients.
Labs: CK, lactic acid, pyruvate in serum. CSF lactate, urine amino acids. Bicycle ergometry, low levels of workload, forearm exercise test.
Lactate–Pyruvate: Lactate is a product of anaerobic glucose metabolism which accumulates when the metabolism is impaired, causing a shift in the oxidized-to-reduced NAD+/NADH ratio within the mitochondria—indicated by the blood lactate–pyruvate ratio. The determination of lactate concentrations at rest or following exercise has become a diagnostic tool for mitochondrial myopathy. However, many patients do present with consistently normal lactate–pyruvate ratio especially in adults.
Exercise Test. The use of exercise testing, usually by bicycle or treadmill, is used for research and as a clinical diagnostic test for mitochondrial myopathy. Exercise intolerance during clinical observation can be demonstrated through taking venous blood sampling both during and after exercise. The test highlights a potential increase in concentration of muscle metabolites (lactate and pyruvate) in venous blood supply (measured as systemic arteriovenous oxygen (a-vO2)) during post-exercise recovery and a slow clearance of the accumulated plasma lactate. The aerobic forearm test is also used as a screening tool for mitochondrial myopathy, coupled with the venous oxygen saturation measurements. In a patient with mitochondrial myopathy, the combined test will reveal a decrease in oxygen desaturation in the venous blood as mitochondrial dysfunction in skeletal muscle results in its inability to extract oxygen from blood.
Forearm exercise test can be performed instead of bicycle ergometry testing. The patient is instructed to open and close their hand (about once every 2 seconds at 40% of maximal voluntary contraction for 3 minutes). Use dynamometer to assess maximal and 40% maximal contraction prior to the exercise. A butterfly needle is place in the antecubital fossa and venous oxygen and lactate can be measured at baseline and each minute during and immediately following exercise. Patients with mitochondrial myopathies and exercise intolerance often demonstrate excessive and prolonged lactate production and paradoxically increased venous oxygen saturation. The range of elevated venous PO2 during forearm exercise in mitochondrial myopathy patients (32-82 mm Hg) correlates closely with the severity of oxidative impairment as assessed during cycle exercise. The forearm exercise test is an easy screening test that sensitively detects impaired oxygen use and accurately assesses the severity of oxidative-impairment in patients with mitochondrial myopathy and exercise intolerance.
Fibroblast Growth Factor 21: Fibroblast growth factor 21 (FGF-21), which has a regulatory role in lipid metabolism, can also be used as a blood serum biomarker for mitochondrial diseases. FGF-21 levels are seen to be raised in patients who have muscle involvement, and thus, in cases with suspected mitochondrial myopathies, the noninvasive measurement of the growth factor acts as a helpful and sensitive first-line investigation in the diagnostic process. Furthermore, FGF-21 specificity for mitochondrial disease has been determined to be above 90%
Growth Differentiation Factor 15: GDF-15, which is a member of the transforming growth factor β superfamily, is shown to be significantly higher in the serum of patients with mitochondrial disease . The serum marker has been seen to be associated with disease severity and so is deemed the most useful biomarker for mitochondrial diseases
EDx: NARP is axonal neuropathy. MNGIE is demyelinating in nature. EMG
ECG, EEG, MRI of brain, MRS of brain. MRI of skeletal muscle. MRS of skeletal muscle with P and H compounds permits the analysis of ATP, creatine phosphate, inorganic phosphate, and pH in muscle and brain. In mitochondrial disorders there is a rapid fall in levels of creatine phosphate and an abnormal accumulation of inorganic phosphates in tissues with exercise. In addition, there is delay in recovery of phosphocreatine levels to normal after exercise.
Muscle Biopsy: Skeletal muscle is commonly affected in mitochondrial disease and is the most frequently biopsied tissue, although the increasing use of genetic tests is likely to reduce the need for muscle biopsies in the future. Patients with mitochondrial myopathy may show histochemical alterations in their skeletal muscle which indicate mitochondrial dysfunction, although in some patients, the muscle biopsy can appear normal. These mitochondrial abnormalities can be identified using several routine histological and immunological studies. One example detected in some patients is the staining of skeletal muscle cryosections with the modified Gomori Trichrome, highlighting the presence of ragged-red fibers (RRF). These fibers can also be detected using succinate dehydrogenase (SDH, complex II) histochemistry, which detects the mitochondrial aggregates in the subsarcolemmal region of the fiber due to mitochondrial proliferation that occurs as a result of mitochondrial OXPHOS dysfunction. Another important diagnostic feature of mitochondrial myopathy is the presence of cytochrome c oxidase (COX, complex IV)-negative fibers as detected by the sequential COX/SDH histochemistry. A mosaic pattern is commonly observed with COX-negative fibers appearing blue, among the normal COX-positive fibers which appear brown. Furthermore, COX deficiency is segmental along the length of the muscle fiber. The mosaic pattern is due to different levels of mutational heteroplasmy with high mutation load leading to respiratory chain deficiency. Mutation load is known to increase in cells throughout life, a process termed clonal expansion, which means that healthy aged individuals accumulate a low frequency of COX-negative fibers. However, a suggestive diagnosis of mitochondrial myopathy is only made when individuals harbor COX-negative fibers at a frequency of > 5%. While COX/SDH fails to provide information on complex I deficiency, a more recently established quadruple immunofluorescent assay has been shown to effectively identify isolated complex I downregulation in skeletal muscle biopsies of patients for diagnostic purposes. These patients had a confirmed genetic diagnosis in a nDNA-encoded subunits of complex I or assembly factors, or mutations in the mtDNA complex I subunits, with some patients having mitochondrial myopathy. Other pathological features which may be seen in skeletal muscle are nonspecific. These include neurogenic atrophy, internal nuclei, abnormal variation in fiber size, and accumulations of glycogen or lipid.
RRF on modified Gomori Trichome Stain. An increase number of lipid droplets is also present within the abnormal muscle fibers.
Oxidative stains (NADH, SDH, and COX).
Ragged blue fibers on NADH and SDH
Mitochondrial myopathies which are not associated with mt-tRNA mutations have no RRF and normal SDH and NADH staining.
COX stain fibers that have subunits encoded by mtDNA. It can also highlight the subsarcolemmal accumulations of mitochondria.
COX staining in combination of SDH is valuable. When SDH intensely stain muscle fibers which are not stained by COX, it implies abnormality in mtDNA. This is because SDH component of complex II is entirely coded by nDNA, while 3 of 13 subunits of complex IV (COX) are encoded by mtDNA.
EM shows increased number of normal-appearing mitochondria, enlarged mitochondria with abnormal cristae, and mitochondria with paracrystalline inclusions.
Biochemical Studies: Spectrophotometric evaluation of individual respiratory chain complex activities is an important approach to the biochemical investigation and diagnosis of mitochondrial myopathies. It can be performed in either fresh or frozen muscle homogenate; however, the latter is more common in diagnostic centers due to cross-continental or national referral of patients. Each complex can be analyzed in isolation following the oxidation/reduction of specific substrates or substrate analogs. The spectrophotometric enzyme assays are as follows: NADH:ubiquinone oxidoreductase for complex I, succinate:cytochrome c oxidase oxidoreductase for complex II, ubiquinol cytochrome c oxidoreductase for complex III, cytochrome c oxidase for complex IV. The measurement of complex V (oligomycin-sensitive ATP synthase) is more challenging and requires the use of fresh material and is often measured in cultured skin fibroblasts. Bernier et al. recommended 20–30% of normal complex activities as a criterion for the diagnosis of mitochondrial myopathies. However, a normal respiratory chain enzyme activity does not exclude the diagnosis of mitochondrial myopathies as a small percentage of respiratory chain deficiency cells may not be detectable by enzyme measurements on tissue homogenates. Another biochemical study which proves to be a helpful step in the diagnosis of mitochondrial myopathy is the blue native acrylamide page (BN-PAGE) assay, which is used to assess the relative abundance of fully assembled respiratory chain enzyme complexes. Similar to the spectrophotometric assay, deficiency of the complexes can be seen as an isolated single complex or a combined deficiency.
Molecular Genetics Studies: While histochemical and biochemical studies pave the way for appropriate molecular genetic testing, the diagnostic procedure in mitochondrial diseases has shifted towards the “genetic first approach” because of the advances made with next-generation sequencing (NGS). The application of NGS techniques including targeted multi-gene panels of candidate genes, unbiased exome sequencing and whole genome sequencing, has revolutionized the diagnostic approach, replacing the sequential method of sequencing candidate genes through Sanger sequencing as the first diagnostic approach. The high-throughput analysis of many genes has led to an increase in the pace of diagnosis and reducing costs. Moreover, unlike Sanger sequencing, NGS can be used for determination of mtDNA heteroplasmy and deletions, although real-time PCR, pyrosequencing, and long-range PCR are still commonly used. The identification of a causative molecular defect facilitates a diagnosis of mitochondrial myopathy in the patient and family members, permitting disease management, genetic counselling, and a variety of reproductive options.
Molecular Studies to Identify Causative Genes Mutations causing mitochondrial myopathy can be found in either the mtDNA or nDNA. For mitochondrial disease in general, pathogenic mutations have been reported in all 37 mtDNA genes and more than 254 nDNA-encoded genes. A search of OMIM, ClinVar, and MitoMap yielded 31 mtDNA genes and 29 nDNA genes that are associated with mitochondrial myopathy at time of writing. However, with new disease genes being identified at a rapid rate, this is likely to be an underestimate and the muscle is often involved in many of the more systemic mitochondrial diseases.
When clinical history shows evidence of a maternally inherited disorder, or a typical mitochondrial myopathy clinical phenotype, the entire mitochondrial genome is typically analyzed. NGS allows for deep coverage across mtDNA and thus detection of low levels of heteroplasmy, point mutations, and breakpoints of single, large-scale mtDNA deletions. Analysis can be undertaken from mtDNA extracted from blood, skin biopsy, or muscle. However, possible mitotic segregation or indeed a loss of mtDNA mutation from mitotic tissues means that if an mtDNA mutation is strongly suspected, then analysis of muscle mtDNA is recommended.
When the compilation of various information from the clinical history and histochemical and biochemical tests indicate that the pathogenic mutation is located in the nDNA, whole exome sequencing (WES), whole genome sequencing (WGS) or a targeted multigene panel of candidate genes can be employed to identify the causative gene using DNA extracted from blood. Due to the large number of nuclear-encoded mitochondrial genes the benefits of NGS are obvious.
In the case of novel mutations that have not been previously reported, functional tests must be undertaken to confirm the pathogenicity of the defect. These typically include looking at pathogenicity predictions based on protein structure and “rescue” experiments where patient cells are transfected with a wild-type copy of the suspected gene and this is shown to rescue the disease phenotype.
Additional Molecular Studies of mtDNA. For some patients with mitochondrial myopathy, it is helpful to determine mtDNA copy number in muscle tissue using real-time PCR. If the mtDNA content is depleted, it indicates that the defect has occurred in a nuclear gene responsible for mtDNA replication and/or maintenance of the deoxynucleotide pools and thus may be helpful for targeted nuclear genetic testing if not already completed.
Real-time PCR and long-range PCR may be used if a single, large-scale mtDNA deletion or multiple mtDNA deletions are suspected. Single, large-scale mtDNA deletions are most reliably detected in muscle and through determining the size of deletion and heteroplasmy provides important information as regards to disease severity and progression. In comparison, the presence of multiple mtDNA deletions indicates a genetic defect in a nuclear gene involved in mtDNA maintenance and provides guidance for nDNA sequencing.
There are currently no effective or disease-modifying treatments available for the vast majority of patients with mitochondrial myopathies. Existing therapeutic options focus on the symptomatic management of disease manifestations.
The management is tailored to the individual patient according to their specific needs and requires the input of many different healthcare professionals such as neurologists, endocrinologists, cardiologists, dieticians, speech and language therapists, and physiotherapists.
Treatment guidelines are in place to help provide for the management of patients in a clinical setting (for further information, see the guidelines available at: http://www.newcastle-mitochondria.com/clinical-professional-home-page/clinical-publications/clinical-guidelines/).
An increasing number of clinical trials, usually designed to be double blinded and placebo controlled, have investigated the therapeutic effects of various vitamins, cofactors, and nutritional supplements, though often these trials have failed to show definitive beneficial primary and secondary outcomes (referred to in this review). Moreover, new molecular and cellular strategies are being proposed that act on a molecular or cellular level, for example restriction endonucleases technologies.
Mitochondrial cocktail: Co-Q10, 200 mg p.o. twice daily, folic acid 1 mg p.o. daily, riboflavin 400 mg p.o. daily, thiamine to 50 mg p.o. daily, vitamin D 200 units p.o. daily. Ubiquinone and Ni cotinamide riboside (mitochondrial myopathy) may also be added.
New molecular strategies, such as mitochondrially targeted nuclease technology demonstrates programmable nuclease-based technology that clinical manipulaties mtDNA heteroplasmy may be on the horizon. Recent developments in mtZFNs (mitochondrial zinc-finger protein nuclease) and mtTALENs (mitochondrial transcription-activator-like effector nuclease) technologies that can potentially cause beneficial heteroplasmic shifts in cell lines harboring varying pathogenic mtDNA mutations offer hope for the future. These technologies act on a molecular level to manipulate the mitochondrial genome and viewed as therapeutic options for mitochondrial diseases. This manipulation of mtDNA is achieved with the use of restriction endonucleases, engineered specifically for the mitochondria. The approaches are designed to recognize and bind to specific mtDNA sequences, induce a double-strand break, and initiate targeted degradation of the mutant mtDNA in a heteroplasmic population. One such approach involves the use of mitochondrially targeted zinc finger nucleases (mtZFN), a heterodimer nuclease consisting of a DNA binding site (tandem repeat of zinc fingers binding three bases each), and a DNA cleavage domain (a Fok1 endonuclease). A further development to mtZFN is mitochondrially targeted transcription activator like effectors (TALE) fused with a Fok1 nuclease (together abbreviated to mtTALENs). Studies have shown the ability of both mtZFNs and mtTALENs to selectively eliminate mutant mtDNA in cell lines harboring a number of pathogenic mutations, including the mtDNA point mutations m.8993T > G (associated with neuropathy, ataxia, and retinitis pigmentosa (NARP)), m.8344A > G (associated with myoclonic epilepsy with ragged-red fibers (MERRF)), m.13513A > G MT-ND5 mutation (associated with MELAS and Leigh syndrome (LS)) and also the “common deletion” m.8483-1345del4977 (associated with CPEO and Kearns–Sayre syndrome (KSS)). These studies showed that through the targeted elimination of mutated mtDNA, a beneficial shift in heteroplasmy and improvement in the biochemical deficit can be achieved. However, despite the encouraging findings, these techniques are limited by their lack of efficacy in the delivery mechanism to affected tissues. Once this issue has been resolved, gene-editing techniques could offer a potential treatment option for patients with primary mutations in the mitochondrial genome.
Ubiquinone and Ubiquinone Analog CoQ10, an electron transport chain component involved in shuttling electrons from complex I and complex II to complex III via the quinone pool, has been shown to be an effective treatment option in patients who harbor a rare congenital CoQ10 deficiency. The compound has been evaluated in a phase 2 clinical trial which included adult patients with either myoclonic epilepsy, lactic acidosis, and stroke-like episodes (MELAS), Leber’s hereditary optic neuropathy (LHON), or CPEO. The study reported minor improvements in the aerobic capacity and post-exercise lactate levels, but there were no improvement in clinical measures such as strength or resting lactate. A phase 3 trial has also been completed which aimed to show that oral CoQ10 was a safe effective treatment for children with inherited mitochondrial disease caused by defects in specific respiratory chain complexes or mtDNA mutations (Clinical Trial identifier: NCT00432744). No results have been reported as of yet.
Idebenone, a synthetic quinone analog of CoQ10, may have the potential to restore cellular ATP generation. It also acts as an antioxidant by protecting the lipid membrane and mitochondria from oxidative damage. Idebenone was used in a randomized controlled trial (Clinical Trial identifier: NCT00747487) which showed an improvement in secondary outcome measures in LHON patients with discordant visual acuities. Following this, LHON became the first mitochondrial disease for which a treatment has been approved by the European Medicine Agency. Idebenone was also investigated in a phase 2 clinical trial (Clinical Trial identifier: NCT00887562) at two different doses of either 900 mg or 2400 mg in MELAS patients. On the completion of the trial, the results reached no statistical significance.
Nicotinamide riboside (NR), a form of vitamin B3 and a natural precursor of NAD+, has been shown to be a promising treatment strategy for mitochondrial myopathy. NR increases the levels of NAD+ which has been shown to induce mitochondrial biogenesis, thus increasing oxidative ATP production capacity. A study by Khan et al. , investigated the effect of NR using Deletor mice which have a mutation in the mtDNA Twinkle (TWNK) helicase. Multiple mtDNA deletions accumulate in the skeletal muscle from these mice causing COX-negative fibers and eventually myopathy after the age of 12 months. The study administered NR at a dose of 400 mg/kg/day to all control, presymptomatic, and postsymptomatic mice for a period of 4 months, a regime which had already proven to increase levels of NAD+ in skeletal muscle of wild-type mice. The findings showed that NR prevented both the development and progression of mitochondrial myopathy. The treatment also resulted in a significant induction of mitochondrial biogenesis and oxidative metabolism. The increased biogenesis resulted in a delayed development of morphological hallmarks, ultrastructural abnormalities, and the prevention of mtDNA deletion accumulation in skeletal muscle. A more recent clinical trial is underway investigating NR supplementation in patients with mitochondrial disease to induce mitochondrial biogenesis (Clinical Trial identifier: NCT03432871) but results are yet to be confirmed at the completion of the trial. Further research is also underway to improve the uptake of NR into patient blood stream by providing patients with a potential modified release oral supplement or intravenous version of NR.
A NAD+ precursor compound named acipimox has also been tested in an interventional clinical trial study which recruited patients with type 2 diabetes mellitus (Clinical Trial identifier: NCT00943059). Findings from the trial showed that a 2-week treatment with 250 mg of acipimox three times daily resulted in an increase in skeletal muscle mitochondrial oxidative capacity and an improvement in the ATP production. Though the researchers believe further insight need to be given into the safety and efficacy of the compound as a potential treatment option. An upcoming trial for acipimox is soon to start in Newcastle, UK.
Bezafibrate is a pan-PPAR agonist that stimulates PGC1α, a coactivator responsible for the induction of mitochondrial biogenesis through its interaction with a number of transcription factors. Preclinical studies undertaken on various mouse models have reported varied findings as to whether bezafibrate could be a potential therapeutic strategy for mitochondrial myopathies. One study on Deletor mice showed that bezafibrate treatment, starting at the time of disease manifestation (12 months of age), resulted in a milder progression of myopathy and reduced COX-negative fibers and mtDNA deletions in skeletal muscle. In contrast, studies using the Surf1-KO mice (a model of early-onset partial COX-deficiency) and the COX15−/− mice showed no change in COX-activity, percentage COX-deficient fibers, and, importantly, no induction of mitochondrial biogenesis. Rather, the studies showed a suppression with no increase in the PPARs or PGC1α expression following treatment. However, both studies did highlight the adverse effects of the drug on mice, mainly severe lipid metabolism side effects and hepatomegaly. Despite these findings, two clinical trials are currently ongoing for bezafibrate treatment. The first is a randomized placebo-controlled trial aiming to assess the safety and efficacy of bezafibrate in mitochondrial myopathy patients (EudraCT number: 2012-002692-34). The trial is administrating bezafibrate orally at a dose of 200 mg in patients aged 2 to 50 years. The second trial, which is a phase 2, open labelled feasibility study is aiming to provide proof of the effects of bezafibrate on six patients with the m.3243A > G mutation (Clinical Trial number: NCT02398201). The results of these clinical trials are not yet published.
RTA-408 is a synthetic triterpenoid compound and a potent activator of nuclear factor erythroid 2-related factor 2 (Nrf2), working to increase the cellular antioxidant response. Nrf2 is a transcriptional target of PGC1α and Nrf2 which promotes mitochondrial biogenesis. Therefore, RTA-408 has the potential to improve muscle function, oxidative phosphorylation, antioxidant capacity, and mitochondrial efficiency in patients with mitochondrial myopathy. The drug form of RTA-408 is called omaveloxolone which has been investigated in a recently completed phase 2, clinical trial (in a total of 53 patients with mitochondrial myopathy (Clinical Trial number: NCT02255422—known as the MOTOR trial); however, the results are not yet published.
Ketogenic diets, which consist of a low carbohydrate and high lipid content which helps the lipid utilization by the mitochondria, have been proposed as a possible treatment for mitochondrial myopathies. The diet was found to simulate mitochondrial oxidative metabolism in Deletor mice, reducing the amount of COX-negative fibers, preventing mitochondrial ultrastructural abnormalities in the skeletal muscle and inducing mitochondrial biogenesis. Another preclinical trial found that a ketogenic culture medium killed cybrid cell lines carrying mtDNA mutation derived from a heteroplasmic patient with a single, large-scale mtDNA deletion. A reduction in the mtDNA deletion load was detected in the heteroplasmic cells line indicative of a heteroplasmic shift. Furthermore, Ahola et al. tested a ketogenic diet called the modified Atkins Diet (mAD) in five patients with mitochondrial myopathy with either single or multiple mtDNA deletions. Results of the pilot study showed no induction of mitochondrial biogenesis. A 2-year follow-up of the patients revealed an improvement in muscle strength, showing a potential activation of muscle regeneration. Despite the positive outcomes, further work is warranted to determine whether ketogenic diets can have a therapeutic effect on patients with mitochondrial myopathy.
Targeting the Regulation of Lipid Dynamics Elamipretide (previously Bendavia), is a member of the Szeto-schiller (SS) family. The drug targets the mitochondrial intermembrane lipid cardiolipin, working to stabilize the lipid structure. Promising findings from preclinical trials have shown that the compound led to an increase in the OXPHOS efficiency through a reduction of ROS generation and so ultimately an increase in ATP production. The drug has been assessed in mitochondrial myopathy patients (Clinical Trial identifier: NCT02367014, known as MMPOWER) and recently published data from the trial showed that patients had improved exercise intolerance and walking distance after the administration of the drug at the highest dose. These findings have led to an ongoing extension trial (Clinical Trial identifier: NCT02976038, MMPOWER2).
Nutritional Supplementations. Another small molecule being investigated for treatment of mitochondrial myopathy is KH176, a vitamin E derivative which acts a potent ROS scavenger. An initial dose–dependent study conducted by Koene et al. showed that the molecule was tolerated at a dose of 800 mg in male participants, causing clinically relevant changes to cardiac electrophysiology tests. A phase 2 clinical trial is currently being undertaken to investigate this further (Clinical trial identifier: NCT02909400).
Exercise. It is well documented that exercise programs (either aerobic, endurance, or resistance) can provide a safe therapeutic option to patients with mitochondrial myopathy, benefiting the biochemical (increasing phosphocreatine synthesis and mitochondrial enzymes) and clinical (work capacity, fatigue, quality of life, and strength) end points. Exercise has been shown to promote mitochondrial biogenesis through activation of PGC1α, AMPK, P38Ɣ, MAPK, and RCG-1β. However, it is not yet clear if exercise is having an effect on the underlying pathogenesis of mitochondrial myopathy or reversing the deconditioning of the muscle. A phase 2 exercise trial consisting of 50 patients with mitochondrial myopathy is currently ongoing (Clinical Trial identifier: NCT00457314) and results are yet to be confirmed. It should, however, be remembered that the benefits of exercise will be limited to those patients that are physically able; thus, development of exercise-mimetic drugs is desirable. Physiotherapy is recommended to patients with mitochondrial myopathy.
Deoxypyramidine bypass therapy has been trialed in mitochondrial depletion syndromes, such as those caused by a mutation in thymidine kinase 2 (TK2). Garone et al. investigated the effect of dCMP and dTMP supplementation in a TK2 mouse model. This supplementation bypassed the TK2 defect, increasing dTTP levels, and ameliorated biochemical abnormalities in these mice.
Reproductive Options
In addition to a wide variety of treatment options, reproductive options are also available. Patients who have been diagnosed with a pathogenic mutation in either nDNA or mtDNA known to cause mitochondrial disease have a number of reproductive options that aim to reduce the risk of transmitting the disease to their offspring. One option is oocyte or sperm donation (depending upon which parent is affected) which can prevent the inheritance of mitochondrial disease. An important consideration for some, however, is that the mother or father will not be genetically related to the offspring. Therefore, an alternative is prenatal diagnosis and preimplantation genetic diagnosis (PGD). These options have been used successfully to prevent transmission of nDNA and mtDNA mutations associated with mitochondrial disease. For the latter, the unique features of mtDNA mean that they will not be suitable for women who are homoplasmic for an mtDNA mutation or produce oocytes with high a mutation load.
The most recent reproductive option that may allow some women to have a genetically related child while reducing the risk of mitochondrial disease is mitochondrial donation (also known as mitochondrial replacement therapy). The novel IVF treatment involves transferring the mother’s nDNA from an affected egg into an enucleated egg from an unaffected donor, resulting in an embryo containing nDNA from both parents but mostly wild-type mtDNA from the healthy donor and a much lower risk of mitochondrial disease. The technique can be performed either before or after fertilization. Before fertilization, maternal spindle transfer (MST) or polar body transfer (PBT) can be used, while in fertilized eggs pronuclear transfer (PNT) can be used [71, 72]. Preclinical research studies have been important in confirming the potential of these techniques to reduce the risk of mitochondrial disease [73–75] and addressing the safety and efficacy of mitochondrial donation before clinical implementation [76].
Mitochondrial donation was approved for clinical use in the UK following an extensive policy process that led to the mitochondrial donation regulations being passed into law in March 2015 [77]. This legislative change allows the Human Fertilizations and Embryology Authority (HFEA) to issue licences to fertility centers who want to offer mitochondrial donation as a novel IVF treatment to reduce the risk of mitochondrial disease. To ensure strict regulation, license applications must be approved on a case-by-case basis for every patient and will only be considered when certain criteria are fulfilled as outlined by the HFEA. As this option is only applicable for mitochondrial diseases caused by mtDNA mutations, it could potentially benefit approximately 150 women each year in the UK [78].
My Lab work-up
I usually check CK, serum lactate, pyruvate, total and free plasma carnitine, acylcarnitine/free carnitine ratio, amino acids, urine organic acids. GDF-15 (growth differentiating factor – 15 from Mayo). EMG/NCS, ophthalmologic evaluation, GI (if MNGIE suspected), PFT, ECG, echocardiogram. In my opinion EMG, genetics and muscle biopsy have been helpful.
Mitochondrial Focused Nuclear Gene Panel | Test catalog for genetic & genomic testing | GeneDx
CPEO
Progressive external ophthalmoplegia (PEO), marked by progressive bilateral ptosis and diffuse reduction in ocular motility, represents a finding of mitochondrial myopathy rather than a true diagnosis. PEO often occurs with other systemic features of mitochondrial dysfunction that can cause significant morbidity and mortality. It represents a descriptive clinical finding rather than a diagnosis. When it occurs in isolation, it is often referred to as chronic progressive external ophthalmoplegia (CPEO). PEO often co-occurs with other symptoms of mitochondrial dysfunction (CPEO plus syndrome) or as one feature of a defined systemic mitochondrial cytopathy. It can be part of a multisystem disease such as Kearns–Sayre syndrome and is commonly associated with proximal myopathy.
Patients with PEO may have ptosis and ophthalmoparesis with or without extremity weakness, diplopia, and dysphagia. They lack pigmentary retinopathy, cardiac conduction defects, endocrinopathy and other systemic manifestations as seen in Kearns Sayre syndrome (KSS). Some cases of PEO are sporadic and associated with single large deletions of mtDNA, representing a partial expression of KSS. There are autosomal and recessive forms as well as maternally (mitochondrial) inherited forms of PEO.
PEO spectrum disorders (e.g., CPEO, CPEO plus syndromes, KSS)
Clinical Features
The progressive ptosis is typically the most noticeable feature, often leading to the gradual acquisition of a chin-up compensatory head position.
Ptosis almost always presents prior to or simultaneous with ophthalmoparesis, although rare cases of PEO without ptosis exist.
Ptosis is usually symmetric, it may present in an asymmetric fashion, however.
A hallmark exam feature of PEO related ptosis is poor function of the levator palpebrae superioris (LPS) muscle. Whereas the normal excursion of the superior eyelid from maximal downgaze to upgaze with stabilization of the brow is ≥12 mm, PEO patients often have less than 8–10 mm of function.
Early on, examination reveals slowed and incomplete, omnidirectional saccades. Late in the disease course, ophthalmoplegia may become complete.
Retinopathy and optic neuropathy can occur. Pigmentary retinopathy is a hallmark feature of Kearns-Sayre syndrome (KSS), which includes PEO .
CPEO patients (who do not fit the diagnostic criteria of KSS) may also demonstrate pigmentary retinopathy. Often described as a "salt and pepper" retinopathy, there is a speckled pattern of retinal pigment epithelium (RPE) clumping alternating with areas devoid of normal RPE. Unlike retinitis pigmentosa demonstrating "bone spicule"-like clumps of RPE in the mid-periphery of the retina, PEO-related RPE changes are most prominent in the macula, peripapillary retina, and the equatorial region. The pigmentary retinopathy only rarely causes severe vision loss, and most patients retain normal or near normal acuity and visual fields.
Proximal myopathy, sensory ataxic neuropathy, cardiomyopathy and parkinsonism.
Differential diagnosis
PEO spectrum disorders (e.g., CPEO, CPEO plus syndromes, KSS)
MG. Single-fiber EMG does not contribute diagnostically, since both MG and CPEO have been shown to demonstrate abnormal jitter.
Agerelated levator dehiscence causing ptosis and sagging eye syndrome (SES)
OPMD
Wernicke’s encephalopathy
Thyroid-associated ophthalmopathy (TAO)
Progressive supranuclear palsy (PSP)
Miller Fisher syndrome
Congenital fibrosis of the extraocular muscles (CFEOMs), along with Duane retraction syndrome and Mobius syndrome, belong to the large group of disorders known as congenital cranial dysinnervation syndromes.
PEO may occur in association with chronic (>10 years) nucleoside reverse transcriptase inhibitor use for human immunodeficiency virus (HIV)
HMG-CoA reductase inhibitors (statins) for hyperlipidemia have established systemic myopathic toxicity. In a 2008 study, 256 reports of ptosis, diplopia, and/or ophthalmoplegia associated with statin use were investigated. Sixty-two cases of symptom resolution with drug cessation (positive dechallenge) and 14 cases of symptom recurrence with reinitiation of the drug (positive re-challenge) led to a World Health Organization criteria-based conclusion that statin drugs possibly cause ophthalmoparesis.
Laboratory Features
Serum CK, lactate, CSF lactate can be normal or elevated. CSF protein may be increased. ECG does not demonstrates cardiac conduction defects. NCS are normal. EMG is usually normal, although myopathic MUP may be seen in weak extremities.
MRI: MRI of orbit with contrast can show abnormal bright signal seen within EOMs on T1WI on high-resolution MRI in the absence of orbital or EOM inflammmation, may be clinically useful in distinguishing CPEO from other conditions causing severe ophthalmoplegia. MRI orbits can show slender EOM, but not in all cases.
Molecular genetics and pathogenesis
40% to 70% of PEO have a single arge mtDNA deletions similar to KSS. These cases are sporadic and not genetically inherited. Point mutations have been identified. within various mitochondrial tRNAs (Leu, Ile, Asn, Tryp) genes in several kinships with maternal inheritance of PEO. Autosomal dominant PEP is usually associated with mutiple mtDNA deletions and is genetically heterogeneous. Genes identified in autosomal dominant PEOs. PEO (PEOA: PEOA1 due to mutation in POLG1 (polymerase gamma 1 gene); PEOA1 caused by mutation in ANT1 gene that encodes for adenine nucleotide translocator (ANTI; PEOA3 caused by mutations in twinkle gene (C10orf2).
ANT is the most abundant mitochondrial protein and transports ATP across the mitochondrial inner membrane, while twinkle and POLG are involved in mtDNA replication. Less common, PEO has been associated with mutations in POLG2, TK2, OPA1, DGOUK, and RRM2B.
OPA1 gene mutations can also cause a subset of CPEO plus syndrome with optic atrophy .
Autosomal recessive form of PEO (PEOB) is also commonly caused by mutations in POLG1 gene. It has been implicated with Alper syndrome, which causes a clinical triad of psychomotor retardation, intractable epilepsy, and liver failure in infants and young children. Less common mutations have been identified in TK2, DGOUK, RRM2B, MPV17, DNA2, and SPG7 genes.
CPEO case Template:
CPEO (chronic progressive external ophthalmoplegia), bilateral [H49.43]
The vast majority of CPEO patients have clinically evident weakness of limb muscles or facial muscles. Late onset CPEO (defined as symptom onset after age 20) suffer from dysphagia. Manometry in CPEO patients has shown abnormal contractions with decreased amplitude and duration predominately in the proximal esophagus [Domenis DR, Okubo PM, Sobreira C, Dantas RO. Esophageal contractions in patients with chronic progressive external ophthalmoplegia. Dig Dis Sci. 2011;56(8):2343–8.] Patients with CPEO or KSS also demonstrated cricopharyngeal achalasia on esophageal videofluoroscopy and manometry [Kornblum C, Broicher R, Walther E, et al. Cricopharyngeal achalasia is a common cause of dysphagia in patients with mtDNA deletions. Neurology. 2001;56(10):1409–12.]
Other associated features include migraine, fatigue, neuropsychiatric disorders, hypothyroidism, diabetes and optic atrophy. (5 %) [12]. PEO-related syndromes can have overlapping features including CPEO and Kearns-Sayre-syndrome (KSS) and exhibit ataxia, cardiomyopathy, cardiac conduction abnormalities, pigmentary retinopathy, and short stature PEO plus - KSS). Autosomal dominant PEP is usually associated with mutiple mtDNA deletions and is genetically heterogeneous. Genes identified in autosomal dominant PEOs include mutation in POLG1, ANT1, twinkle gene (C10orf2). Less common, mutations POLG2, TK2, OPA1, DGOUK, and RRM2B. MPV17, DNA2, and SPG7 genes.
RECOMMENDATIONS:
ECG and echocardiogram every six to 12 months to monitor cardiac conduction and contractility. Yearly evaluation by neurologist, audiologist, ophthalmologist, and endocrinologist; referral to other specialists (e.g., gastroenterologist, pulmonologist) based on symptom occurrence. Significant PEO-associated dysphagia should prompt referral to a gastroenterologist because cricopharyngeal myotomy for achalasia could be beneficial. Evaluation by neuroophthalmologist and/or retinal specialist for CPEO, pigmentary retinopathy with appropriate surveillance testing (e.g., electroretinography, optical coherence tomography, visual fields.) Agents/circumstances to avoid: Drugs potentially toxic to mitochondria, including chloramphenicol, aminoglycosides, linezolide, valproic acid, and nucleoside reverse transcriptase inhibitors. Volatile anesthetic hypersensitivity may occur. Avoid prolonged propofol (>30-60 minutes.)
MERRF (myoclonic epilepsy and ragged red fibers)
Myoclonus, generalized seizures (myoclonic and tonic-clonic), ataxia, dementia, SNHL, optic atrophy, and progressive muscular weakness developing in childhood or adult life. The myoclonus is stimulus sensitive, but can be present at rest. The seizures may be photosensitive. Patients are often misdiagnosed as having juvenile myoclonic epilepsy (JME).
Muscle weakness and atrophy can be generalized, but there is a predilection for involvement of proximal arm and leg muscles.
Generalized sensorimotor PN and pes cavus deformities may be appreciated.
MERFF unlike KSS does not present with ptosis, ophthalmoparesis, and pigmentary retinopathy.
CM with conduction block or heart failure may also be seen in MERRF, in cases that present early.
MERFF can also be complicated with ventilatory muscle weakness and associated with life-threatening hypoventilation in the setting of surgery, sedation, or intercurrent infection.
Some patients present with multiple symmetric lipomatosis.
Laboratory features:
CK is normal or mildly elevated.
Serum lactate is normal or elevated.
EEG: generalized slowing of background activity and burst of spikes and slow waves.
MRI or CT of brain reveals cerebral and cerebellar atrophy.
EDX:
NCS may demonstrate reduced SNAP amplitudes - axonopathy.
EMG is usually normal except for myopathic motor units seen in weak muscles.
Histopathology:
RRF, COX-negative and increased SDH staining.
Neuronal loss and gliosis of dentate nuclei, GP, red nuclei, substantia nigra, inferior olivary nuclei, optic nerves, and cerebellar cortex. Demyelination and gliosis of corticospinal and spinothalamic tracts and posterior columns - autopsy.
Molecular genetics and pathogenesis:
Maternally inherited. 80% caused by point mutations at nucleotide position 8344 of the mitochondrial genome that results in an A to G transition in the tRNA gene (MTTK). There is phenotypic pleiotropy with this gene as patients with PEO, Leigh syndrome, or multiple symmetric lipomatosis can present with this specific mutation.
Mutations in other locations of tRNA gene (positions 8356, 8366) and mutations in tRNA (MTTL1) can also present with MERRF. The latter mutation can be seen in MELAS. MTTH, MTTF and MTTS1 are also associated with MERRF.
MERRF phenotype can also be seen in multiple mtDNA deletions caused by mutations in POLG1 and MTND5.
Treatment:
No specific treatment
Treat myoclonus with clonazepam and seizures with AEDs.
Creatine monohydrate 5-10 g/day may be tried as a slight benefit was reported ; although evidence is weak.
Special care in patients with mitochondrial myopathies is needed as they can develop marked alveolar hypoventilation in response to sedating medications and anesthetic agents.
MELAS (Mitochondrial myopathy lactic acidosis and stroke like episodes)
Muscle weakness, high lactate levels in serum or CSF, and stroke-like episodes.
Onset in 1st year of life in <10%; 60-80% develop symptoms and signs by age of 15 years. It can rarely present as late as eight decade.
Recurrent episodes of migraine-type headaches with nausea and vomiting. Stroke like episodes hemiparesis, hemianopsia, language deficits or cortical blindness. These are not long-lasting episodes and most recover. These episodes may be provoked by exercise or intercurrent infection. Progressive dementia may occur.
Proximal muscle weakness and complain of easy fatigue and myalgias with exercise.
Short-statured.
Some may develop myoclonus, seizures, or ataxia and thus overlap with MERRF.
Ptosis, ophthalmoparesis, retinitis pigmentosa and/or cardiomyopathy occur in less than 10% of patients.
Rare individuals with the most common A3243G mutation in the MTTL1 gene manifest with only diabetes mellitus and/or deafness.
Laboratory features:
CK is normal or elevated.
Serum and CSF lactate is elevated.
EEG may show epileptiform activity
EDX:
NCS is normal
EMG may reveal myopathic MUAPs.
MRI of brain reveal cortical atrophy and high T2 signal and FLAIR abnormalities in the cerebral cortex, BG, and thalamus.
MRS of acute cortical lesions reveal severely elevated lactate levels and reduced concentrations of NAA, glutamate, and myo-inositol.
MRS of skeletal muscle may show reduced phosphocreatine level, and elevated concentration of inorganic phosphate and free adenosine-5'-diphosphate and an abnormally low phosphorylation potential.
Histopathology:
Similar to MERRF but with more variability in COX staining. Arterioles are also strongly SDH reactive and an increased number of mitochondria are found in the muscle wall of the arterioles.
Mitochondrial enzyme analysis of muscle tissue reveals reduced activities of complexes I, III, IV, and V.
Molecular Genetics and Pathogenesis:
Maternally inheritied. 70% of cases are caused by mtDNA mutation, an A to G substitution, at nucleotide position 3243 in the gene MTTL1 encoding tRNA. Mutations have been also identified at positions 3252, 3260, 3271, 3291 in the tRNA gene as well as in MTTV, MTTK, MTTF, MTTS1, MTND1, MTND4, MTND5, and MTCYB.
Treatment:
No specific therapy other than treatment for seizures and myoclonus.
No benefit form CoQ, DCA
Creatine monohydrate 5-10 g/day modestly improved in a few patients with MELAS.
L-arginine ?
KSS (Kearns-Sayre Syndrome)
Clinical triad of PEO, retinitis pigmentosa, CM with onset <20 years.
Short stature, proximal muscle weakness, SNHL, dementia, ataxia, depressed ventilatory drive, and multiple endocrinopathies (DM, hypothyroidism, hypoparathyroidism, delayed secondary sexual characteristics).
Affected individuals are very sensitive to anesthetic agents and sedatives as these can provoke ventilatory failure.
Laboratory features:
Serum CK is normal
Serum lactate and pyruvate may be elevated.
CSF protein may be increased.
ECG reveals conduction defects.
EDX: normal to axonal sensory or sensorimotor polyneuropathy. EMG may show normal insertional and spontaneous activity, but may reveal myopathic MUAPs in weak muscles.
Histopathology
RRF, little variability in COX unlike MERRF and MELAS.
Spongy degeneration of cerebral white matter - autopsy.
Molecular genetics and pathogenesis
Single large mtDNA deletions (1.3 to 8.8 kB) can be demonstrated in most patients with KSS.
More than 150 different mtDNA deletions have been associated with KSS. A deletion of 4977 base pairs known as m.8470_13446del4977 is encountered most frequently. The same mutation and numerous other types of deletions of varying length have been identified in Pearson syndrome (sideroblastic anemia and exocrine pancreas dysfunction) and progressive external ophthalmoplegia (PEO).
Autosomal recessive cardiomyopathy and ophthalmoplegia (ARCO).
Mitochondrial depletion myopathy - TK2, DGOUK
Some forms of PEO, MNGIE, MIRAS, OA1, and Navajo neurohepatopathy are associated with MDS.
TYMP, POLG1, POLG2, twinkle (C10orf2), MPV17, RRM2B, OPA1, SUCLG1, SUCLA2.
POLG1 related mitochondrial disorders: PEO1 - AR, MGNIE, SANDO, MIRAS, MERRF.
MGNIE (Mitochondrial neurogastrointestinal encephalomyopathy)
AR mitochondrial disorder due to mutations in TYMP, the gene encoding thymidine phosphorylase enzyme. The activity of thymidine phosphorylase enzyme is reduced resulting in elevated plasma concentrations of thymidine.
POLG1 mutations may cause MNGIE-like syndrome without leukoencephalopathy and normal thymidine plasma levels
Sensorimotor PN, leukoencephalopathy on MRI of brain, RRF on muscle biopsy, and chronic intestinal pseudo-obstruction.
Onset <20 years of age
Progressive with severe disability or death by 3rd or 4th decade of life.
Distal greater than proximal muscle weakness and atrophy, stocking-glove distribution of sensory loss, and reduced MSR throughout.
Ptosis and EOM weakness.
Despite of leukoencephalopathy on MRI, most affected patients have little in the way of CNS symptoms or signs. Some have mental retardation. Others have retinitis pigmentosa, SNHL, facial weakness, hoarseness, or dysarthria.
Laboratory features:
Serum CK can be normal or mildly elevated.
Serum lactate, pyruvate, and CSF protein is elevated.
Thymidine phosphorylase activity is decreased (<10%) in leukocytes and platelets. Thymidine levels are increased in plasma and deoxyuridine is increased in serum and plasma in those cases caused by mutation in the TYMP gene that encodes for thymidine phosphorylase.
Plasma thymidine levels greater than 3 umol/L and deoxyuridine concentration greater than 5 umol/L can be sufficient to confirm the diagnosis.
MRI of brain shows leukoencephalopathy in cerebral and cerebellar white matter.
Dilatation and dysmotility of esophagus, stomach, and small intestine.
ECG has shown conduction defects but many remain asymptomatic.
EDX:
Motor and sensory NCV may be slow to within the demyelination range, while F-wave latencies are usually markedly prolonged.
Other cases are more suggestive of axonopathy with reduced SNAP and CMAP amplitudes.
EMG shows fibrillation potentials and psw. Recruitment of MUAPs can be decreased in distal muscles. Quantitative EMG of proximal muscle show myopathic motor units.
SANDO (sensory ataxia, neuropathy, dysarthria/dysphagia, and ophthalmoplegia)
POLG1, C10orf2
MIRAS (mitochondrial recessive ataxic syndrome) - POLG1
NARP (neuropathy ataxia and retinitis pigmentosa)
Navajo Neurohepatopathy
Optic atrophy 1 (OA1) syndrome
Leigh Syndrome
Focal Mitochondrial depletion
Mitochondrial myopathies associated with exercise intolerance/recurrent myoglobinuria
CMT2A, CMT2K, CMT4.
Summary
Mitochondrial myopathies are progressive myopathies caused by the impairment of oxidative phosphorylation (OXPHOS). Alongside the traditional histochemical, immunohistochemical, and biochemical assays, the diagnosis of mitochondrial myopathies has been revolutionized by the introduction of NGS. NGS allows for a high-throughput screening of mtDNA and nDNA. However, there are currently no effective or disease-modifying treatments available for mitochondrial myopathies to halt the progression of the disease. Instead, existing therapeutic options have been focusing on the symptomatic management of disease manifestations. The development of large cohorts of patients with mitochondrial disease is enabling extensive studies to investigate the therapeutic effects of a variety of compounds shown to be of potential value in animal models. New molecular strategies, namely mtZFNs and mtTALENs, that cause beneficial heteroplasmic shifts in cell lines harboring varying pathogenic mtDNA mutations offer hope for the future. Moreover, recent developments in the reproductive options for patients with mitochondrial myopathies mean that for most families, the possibility of preventing transmission of the mutation to the next generation is now possible.