MELAS syndrome: Clinical manifestations ...

YMGME-05922; No. of pages: 9; 4C: Molecular Genetics and Metabolism xxx (2015) xxx–xxx

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MELAS syndrome: Clinical manifestations, pathogenesis, and treatment options Ayman W. El-Hattab a, Adekunle M. Adesina b, Jeremy Jones c, Fernando Scaglia d,⁎ a

Division of Clinical Genetics and Metabolic Disorders, Department of Pediatrics, Tawam Hospital, Al-Ain, United Arab Emirates Department of Pathology and Immunology, Baylor College of Medicine, Houston, TX, USA. Singleton Department of Radiology, Texas Children's Hospital, Houston, TX, USA d Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, TX, USA b c

a r t i c l e

i n f o

Article history: Received 13 May 2015 Received in revised form 14 June 2015 Accepted 14 June 2015 Available online xxxx Keywords: Mitochondrial diseases Encephalomyopathy Lactic acidosis Nitric oxide deficiency Arginine Citrulline Angiopathy Endothelial dysfunction

a b s t r a c t Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome is one of the most frequent maternally inherited mitochondrial disorders. MELAS syndrome is a multi-organ disease with broad manifestations including stroke-like episodes, dementia, epilepsy, lactic acidemia, myopathy, recurrent headaches, hearing impairment, diabetes, and short stature. The most common mutation associated with MELAS syndrome is the m.3243ANG mutation in the MT-TL1 gene encoding the mitochondrial tRNALeu(UUR). The m.3243ANG mutation results in impaired mitochondrial translation and protein synthesis including the mitochondrial electron transport chain complex subunits leading to impaired mitochondrial energy production. The inability of dysfunctional mitochondria to generate sufficient energy to meet the needs of various organs results in the multi-organ dysfunction observed in MELAS syndrome. Energy deficiency can also stimulate mitochondrial proliferation in the smooth muscle and endothelial cells of small blood vessels leading to angiopathy and impaired blood perfusion in the microvasculature of several organs. These events will contribute to the complications observed in MELAS syndrome particularly the stroke-like episodes. In addition, nitric oxide deficiency occurs in MELAS syndrome and can contribute to its complications. There is no specific consensus approach for treating MELAS syndrome. Management is largely symptomatic and should involve a multidisciplinary team. Unblinded studies showed that L-arginine therapy improves stroke-like episode symptoms and decreases the frequency and severity of these episodes. Additionally, carnitine and coenzyme Q10 are commonly used in MELAS syndrome without proven efficacy. © 2015 Elsevier Inc. All rights reserved.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Clinical manifestations . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Neurological manifestations . . . . . . . . . . . . . . . . . . 2.2. Muscular manifestations . . . . . . . . . . . . . . . . . . . . 2.3. Lactic acidemia . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Cardiac manifestations . . . . . . . . . . . . . . . . . . . . . 2.5. Gastrointestinal manifestations . . . . . . . . . . . . . . . . . 2.6. Endocrine manifestations . . . . . . . . . . . . . . . . . . . 2.7. Renal, pulmonary, dermatological, and hematological manifestations Pathogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Molecular genetic defects . . . . . . . . . . . . . . . . . . . 3.2. Energy deficiency and angiopathy . . . . . . . . . . . . . . . . 3.3. Nitric oxide deficiency . . . . . . . . . . . . . . . . . . . . . 3.4. Pathogenesis of various complications . . . . . . . . . . . . . . 3.5. Phenotype variability . . . . . . . . . . . . . . . . . . . . . Management . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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⁎ Corresponding author at: Department of Molecular and Human Genetics, Baylor College of Medicine, One Baylor Plaza, MS BCM225, Houston, TX 77030, USA. E-mail address: [email protected] (F. Scaglia).

http://dx.doi.org/10.1016/j.ymgme.2015.06.004 1096-7192/© 2015 Elsevier Inc. All rights reserved.

Please cite this article as: A.W. El-Hattab, et al., MELAS syndrome: Clinical manifestations, pathogenesis, and treatment options, Mol. Genet. Metab. (2015), http://dx.doi.org/10.1016/j.ymgme.2015.06.004

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A.W. El-Hattab et al. / Molecular Genetics and Metabolism xxx (2015) xxx–xxx

4.1. Evaluation of multi-organ involvement 4.2. Management of complications . . . . 4.3. Medications to avoid . . . . . . . . 5. Conclusions . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . .

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1. Introduction Mitochondria are double membrane organelles found in all nucleated human cells and perform a variety of essential functions, including the generation of most cellular energy in the form of adenosine triphosphate (ATP). The inner mitochondrial membrane harbors the electron transport chain (ETC) complexes that transfer electrons, translocate protons, and produce ATP. Mitochondria contain extra-chromosomal DNA (mitochondrial DNA, mtDNA). However, only a very small proportion of mitochondrial proteins are encoded by that DNA; whereas the majority of mitochondrial proteins are encoded by nuclear DNA (nDNA). Mutations in mtDNA or mitochondria-related nDNA genes can result in mitochondrial dysfunction leading to mitochondrial diseases. Dysfunctional mitochondria are unable to generate sufficient ATP to meet the energy needs of various organs, particularly those with high energy demand, including the nervous system, skeletal and cardiac muscles, kidneys, liver, and endocrine systems. Some patients with mitochondrial diseases display a cluster of clinical features that fall into a discrete clinical syndrome. However, there is often considerable clinical variability, and many affected individuals do not fit into one particular syndrome [1]. Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome is one of the most frequent maternally inherited mitochondrial disorders which was first delineated in 1984 [2]. The molecular basis of MELAS syndrome was initially discovered in 1990 when adenine to guanine transition at position 3243 of mtDNA (m.3243ANG) in the MT-TL1 gene encoding tRNALeu(UUR) was found to be associated with this syndrome [3,4]. In 1992, clinical diagnostic criteria for MELAS syndrome were published indicating that the clinical diagnosis of this syndrome is based on the following three invariant criteria: 1) stroke-like episodes before age 40 years, 2) encephalopathy characterized by seizures and/or dementia, and 3) mitochondrial myopathy evident by lactic acidosis and/or ragged-red fibers (RRFs). The diagnosis is considered confirmed if there are also at least two of the following criteria: 1) normal early psychomotor development, 2) recurrent headaches, and 3) recurrent vomiting episodes [5]. More recently, the MELAS study group committee in Japan published other diagnostic criteria by which the diagnosis is considered definitive with at least two category A criteria (headaches with vomiting, seizures, hemiplegia, cortical blindness, and acute focal lesions in neuroimaging) and two category B criteria (high plasma or cerebrospinal fluid (CSF) lactate, mitochondrial abnormalities in muscle biopsy, and a MELASrelated gene mutation) [6]. The prevalence of MELAS syndrome has been estimated to be 0.2:100,000 in Japan [6]. Other mtDNA mutations were subsequently found to cause MELAS syndrome; however, the m.3243ANG remained the commonest universally. The m.3243ANG, which was subsequently found to be associated with other phenotypes that collectively constitute a wide spectrum ranging from MELAS syndrome at the severe end to asymptomatic carrier status, was found to be relatively common with a prevalence of 16–18:100,000 in Finland [7,8]. In this review, we summarize the clinical manifestations of MELAS syndrome along with its pathogenic mechanisms and management options.

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myopathy, recurrent headaches, hearing impairment, diabetes, and short stature. Childhood is the typical age of onset with 65–76% of affected individuals presenting at or before the age of 20 years. Only 5–8% of individuals present before the age of 2 years and 1–6% after the age of 40 years [6,9–11]. Individuals with MELAS syndrome frequently present with more than one initial clinical manifestation. Table 1 summarizes the initial manifestations in affected individuals [6,9,10]. Table 2 summarizes the clinical manifestations of MELAS syndrome organized according to their prevalence [6,9–11]. Below the manifestations of MELAS syndrome are presented according to the organ or system involved.

2.1. Neurological manifestations Stroke-like episodes are one of the cardinal features of MELAS syndrome that occur in 84–99% of affected individuals [6,9,10]. These episodes present clinically with partially reversible aphasia, cortical vision loss, motor weakness, headaches, altered mental status, and seizures with the eventual progressive accumulation of neurological deficits. The affected areas in neuroimaging do not correspond to classic vascular distribution (hence called “stroke-like”), are asymmetric, involve predominantly the temporal, parietal, and occipital lobes, and can be restricted to cortical areas or involve subcortical white matter [5,11] (Fig. 1). Brain magnetic resonance (MR) angiography is usually normal; whereas MR spectroscopy shows decreased N-acetylaspartate signals and accumulation of lactate [11]. The high ventricular lactate measured using MR spectroscopy was found to correlate with the degree of the neurological impairment in individuals with MELAS syndrome [12]. Dementia occurs in 40–90% of affected individuals [6,9,10]. Both the underlying neurological dysfunction and the accumulating cortical injuries due to stroke-like episodes contribute to the observed dementia which affects intelligence, language, perception, attention, and memory function [11]. Additionally, executive function deficits have been observed despite the relative sparing of the frontal lobe in neuroimaging suggesting an additional diffuse neurodegenerative process besides the damage caused by the stroke-like episodes [11]. Epilepsy is another common neurological manifestation occurring in 71–96% of individuals with MELAS syndrome [6,9,10]. Epilepsy in

Table 1 Initial manifestations of MELAS syndrome. Frequency

Manifestations

N25%

Seizure Recurrent headaches Stroke-like episode Cortical vision loss Muscle weakness Recurrent vomiting Short stature Altered consciousness Impaired mentation Hearing impairment Diabetes Developmental delay Fever

10–24%

2. Clinical manifestations MELAS syndrome is a multi-organ disease with broad manifestations including stroke-like episodes, dementia, epilepsy, lactic acidemia,

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b10%



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Table 2 Overall manifestations of MELAS syndrome. Frequency

Manifestations

≥90%

Stroke-like episodes Dementia Epilepsy Lactic acidemia Ragged red fibers Exercise intolerance Hemiparesis Cortical vision loss Recurrent headaches Hearing impairment Muscle weakness Peripheral neuropathy Learning disability Memory impairment Recurrent vomiting Short stature Basal ganglia calcification Myoclonus Ataxia Episodic altered consciousness Gait disturbance Depression Anxiety Psychotic disorders Diabetes Optic atrophy Pigmentary retinopathy Progressive external ophthalmoplegia Motor developmental delay Cardiomyopathy Cardiac conduction abnormalities Nephropathy Vitiligo

75–89%

50–74%

25–49%

b25%

individuals with MELAS syndrome is heterogeneous. Despite the frequent focal nature of the brain insult by the stroke-like episodes, not only focal, but also primary generalized seizures can occur. Primary generalized seizures in MELAS syndrome can occur with normal neuroimaging or abnormalities including stroke-like episodes, white matter lesions, cortical atrophy, and corpus callosum agenesis or hypogenesis. Seizures can occur in MELAS syndrome as a manifestation of a strokelike episode or independently, and may even induce a stroke-like episode [13]. Recurrent headaches occur in 54–91% of individuals with MELAS syndrome [6,9–11]. Migrainous headaches in the form of recurrent attacks of severe pulsatile headaches with frequent vomiting are typical in individuals with MELAS syndrome and can precipitate stroke-like episodes [14]. On the other hand, these headache episodes are often more severe during the stroke-like episodes [10]. Hearing impairment occurs in 71–77% of individuals with MELAS syndrome [9–11]. Sensorineural hearing loss in MELAS syndrome is typically mild, insidiously progressive, and often an early clinical manifestation [11]. Peripheral neuropathy is another common manifestation of MELAS syndrome occurring in 22–77% of affected individuals. The neuropathy in MELAS syndrome is usually a chronic and progressive, sensorimotor, and distal polyneuropathy. Nerve conduction studies typically show an axonal or mixed axonal and demyelinating neuropathy [11,15,16]. Other neurological manifestations in MELAS syndrome include learning disability, memory impairment, myoclonus, ataxia, episodes of altered consciousness, basal ganglia calcifications in neuroimaging, and elevated protein in CSF analysis [9,10]. Ophthalmological complications include optic atrophy, pigmentary retinopathy, and ophthalmoplegia [9,10]. Psychiatric illnesses can occur in MELAS syndrome and include depression, bipolar disorder, anxiety, psychosis, and personality changes [17].

Fig. 1. Neuroimaging for a 9 year old girl with MELAS syndrome who presented with headache: A) axial CT image demonstrates basal ganglia calcification as well as diffuse decreased attenuation in the right occipital lobe involving both gray and white matter, B) sagittal reconstructed image reveals vermian atrophy, C) axial FLAIR image reveals increased signal in the cortex and subcortical white matter of the right occipital lobe as well as a smaller focus in the right lateral temporal lobe, D) axial diffusion weighted images demonstrated restriction in the subcortical white matter of the right occipital lobe, E) axial ADC (apparent diffusion coefficient) map confirms restricted diffusion, and F) axial post contrast T1-weighted image reveals increased sulcal enhancement, which may represent hyperemia or luxury perfusion. Six months later, the same girl presented with left sided weakness: G & H) FLAIR images demonstrate resolution of abnormal signal at the right occipital pole with evolving encephalomalacia in the lateral occipital lobe. New extensive cortical/subcortical signal abnormalities are appreciated in the right temporal lobe, bilateral frontal lobes, and bilateral parietal lobes.


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2.2. Muscular manifestations Myopathy is a cardinal manifestation of MELAS syndrome. Exercise intolerance and muscle weakness occurs in 73–100% and 42–89% of affected individuals, respectively [6,9–11]. Motor developmental delay was reported in 23% of affected children [18]. Histologic examination of muscle tissue from individuals with MELAS syndrome shows scattered vacuolated muscle fibers with clear surrounding rim using hematoxylin and eosin (H&E) staining. Using the Gomori trichrome stain, the RRFs can be seen which represent mitochondrial proliferation below the plasma membrane of the muscular fibers causing the contour of the muscle fiber to become irregular. These proliferated mitochondria in RRFs also stain strongly with the succinate dehydrogenase (SDH) stain giving the appearance of ragged blue fibers. Although RRFs are present in many other mitochondrial diseases e.g., MERRF (myoclonic epilepsy with ragged red fibers), most of the RRFs in MELAS stain positively with the cytochrome c oxidase (COX) histochemical stain unlike other mitochondrial diseases where RRFs do not react with COX. In MELAS syndrome, the COX stain of muscle tissue can be decreased, normal, or increased which may reflect variable m.3243ANG heteroplasmy in different muscle fibers (Fig. 2). Another characteristic feature in MELAS syndrome is the excessive mitochondrial proliferation observed in smooth muscle and endothelial cells in intramuscular blood vessels revealed with the SDH stain and called strong SDH reactive blood vessels (SSVs). Biochemical analysis of respiratory chain enzymes in muscle extracts usually shows multiple partial defects, especially involving complex I and complex IV [10,11]. 2.3. Lactic acidemia Lactic acidemia is a cardinal sign that is present in 94% of affected individuals. CSF lactate is also elevated in the majority of individuals with MELAS syndrome [9,10]. Lactic acidemia is not specific for MELAS syndrome as it can occur in other mitochondrial diseases, metabolic diseases, and systemic illness. On the other hand, lactate level can be normal in a minority of individuals with MELAS syndrome [9,10]. 2.4. Cardiac manifestations Cardiomyopathy occurs in 18–30% of individuals with MELAS syndrome [6,9,10]. Both dilated and hypertrophic cardiomyopathies have been observed in MELAS syndrome, however, the more typical is a non-obstructive concentric hypertrophy [11]. Cardiac conduction abnormalities including Wolff–Parkinson–White syndrome has been reported in 13–27% of individuals with MELAS syndrome [9,10,18,19]. 2.5. Gastrointestinal manifestations Gastrointestinal complications are common in individuals with MELAS syndrome and occur in 64–77% of affected individuals [9–11]. Recurrent or cyclic vomiting is the commonest observed gastrointestinal complaint in MELAS syndrome. Diarrhea, constipation, gastric dysmotility, intestinal pseudo-obstruction, and recurrent pancreatitis have also been reported in MELAS syndrome [20]. Failure to thrive has been reported in 28% of children with MELAS syndrome [6]. 2.6. Endocrine manifestations Diabetes occurs in 21–33% of individuals with MELAS syndrome [6, 11]. Diabetes in MELAS syndrome manifests at the average age of 38 years and can be type 1 or type 2 in nature. Individuals with type 2 diabetes can initially be treated by diet or sulfonylurea. Significant insulinopenia can develop and affected individuals may require insulin treatment [21]. Individuals with MELAS syndrome are typically shorter than their unaffected family members. Short stature has been reported in 33–82% of affected individuals [6,9–11]. Growth hormone deficiency

Fig. 2. Muscle histopathologic changes in individuals with MELAS syndrome: A–C show ragged red fiber with modified Gomori trichrome, NADH tetrazolium reductase, and cytochrome c oxidase histochemistry, respectively. The arrow identifies the ragged red fiber. The images are at 400× magnification each.

has occasionally been found in individuals with MELAS syndrome [22]. Hypothyroidism, hypogonadotropic hypogonadism, and hypoparathyroidism have also been reported in individuals with MELAS syndrome [23–25].

2.7. Renal, pulmonary, dermatological, and hematological manifestations Renal manifestations of MELAS syndrome include Fanconi proximal tubulopathy, proteinuria, and focal segmental glomerulosclerosis [26]. Pulmonary hypertension has been rarely reported in individuals with MELAS syndrome [27]. Dermatological complaints, including vitiligo and diffuse erythema with reticular pigmentation, are infrequent



manifestations in MELAS syndrome [28,29]. Chronic anemia has been reported in individuals with MELAS syndrome [30]. 3. Pathogenesis The pathogenesis of MELAS syndrome is not fully understood. The observed phenotype of MELAS syndrome can be explained by several interacting mechanisms including impaired mitochondrial energy production, microvasculature angiopathy, and nitric oxide (NO) deficiency (Fig. 3). 3.1. Molecular genetic defects The m.3243ANG mutation in the MT-TL1 gene encoding tRNALeu(UUR) is found in 80% of individuals with MELAS syndrome. Additional mutations (e.g., m.3271TNC and m.3252ANG) in the MT-TL1 gene can also cause MELAS syndrome. Rarely, mutations in other mitochondrial genes have been reported to cause MELAS syndrome including MT-TL2 encoding tRNALeu(CUN), MT-TK encoding tRNALys, MT-TH encoding tRNAHis, MT-TQ encoding tRNAGln, MT-TF encoding tRNAPhe, MT-TV encoding tRNAVal, MT-ND1, MT-ND4, MT-ND5, and MT-ND6 encoding subunits of complex I, MT-CO2 and MT-CO3 encoding subunits of complex IV, and MT-CYB encoding a subunit of complex III [10,31]. In addition, mutations in the nuclear gene POLG encoding the mitochondrial DNA polymerase gamma have been associated with a MELAS-like phenotype [32]. 3.2. Energy deficiency and angiopathy The m.3243ANG mutation results in impaired mitochondrial translation that leads to decreased mitochondrial protein synthesis affecting the ETC complex subunits. Decreased synthesis of ETC complexes results in impaired mitochondrial energy production [33,34]. The inability of dysfunctional mitochondria to generate sufficient ATP to meet the energy needs of various organs results in the multi-organ dysfunction observed in MELAS syndrome (Fig. 3). Energy deficiency can also stimulate mitochondrial proliferation. Angiopathy due to mitochondrial proliferation in smooth muscle and endothelial cells of small blood vessels occurs in MELAS syndrome and leads to impaired blood perfusion in microvasculature contributing significantly to the complications observed in MELAS syndrome particularly stroke-like episodes [11,35,36] (Fig. 3). 3.3. Nitric oxide deficiency In addition to energy depletion there has been growing evidence that NO deficiency occurs in MELAS syndrome and can contribute significantly to its complications [37–39]. NO is formed from arginine via the enzyme nitric oxide synthase (NOS), which catalyzes the conversion of arginine to citrulline. Citrulline can be converted to arginine via argininosuccinate synthase and argininosuccinate lyase. Therefore, both citrulline and arginine are considered as NO donors. Citrulline is a nonessential amino acid for which the main source is the de novo synthesis in small intestine enterocytes through a number of mitochondrial enzymes [40]. NO produced by vascular endothelium plays a major role in vascular smooth muscle relaxation that is needed to maintain the patency of small blood vessels [41,42]. Therefore, NO deficiency in MELAS syndrome can result in impaired blood perfusion in the microvasculature of different organs that can contribute to the pathogenesis of several complications [38,39]. NO deficiency in MELAS syndrome is believed to be multifactorial in origin (Fig. 3). Mitochondrial proliferation in vascular endothelial cells can result in impaired normal endothelial function (endothelial dysfunction), and impaired endothelial NO synthesis can reflect one aspect of endothelial dysfunction. Decreased availability of NO precursors, arginine and citrulline, may have a major

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contribution in impaired NO production. Low plasma citrulline may result from decreased citrulline synthesis in the mitochondria of enterocytes due to mitochondrial dysfunction [37,43]. Most of the citrulline flux is directed toward arginine synthesis; therefore, lower citrulline availability can result in decreased de novo arginine synthesis and lower intracellular arginine availability [37–39]. Decreased NO production can also result from impaired NOS activity due to reactive oxygen species (ROS) overproduction (oxidative stress) resulting from the ETC impairment [44,45]. Oxidative stress due to mitochondrial dysfunction may also result in increased asymmetric dimethylarginine (ADMA), which is an endogenous inhibitor of NOS [37,39,46]. In addition to impaired NO production, postproduction NO sequestration can contribute to the NO deficiency in MELAS syndrome. Mitochondrial proliferation in endothelial cells in MELAS syndrome can be associated with increased COX activity, which can react with and thus sequester NO [11,43]. In addition, oxidative stress can result in decreased NO availability by shunting NO into reactive nitrogen species (RNS) formation [47]. 3.4. Pathogenesis of various complications During early stages of stroke-like episodes in MELAS syndrome, SPECT (single photon emission computed tomography) scanning studies have demonstrated hypoperfusion in the affected regions, indicating that these episodes are due to ischemic insults [48]. It is believed that these ischemic insults result from impaired perfusion in cerebral microvasculature due to the angiopathy and NO deficiency that occur in MELAS syndrome [11,37–39,43,48]. The inability of dysfunctional mitochondria to generate sufficient ATP to meet the energy needs of muscle tissue can explain the observed myopathy in MELAS syndrome. NO deficiency may also play a significant role in the myopathic manifestations of this syndrome. Normally, endothelial cells release basal and stimulated NO. During physical activity increased muscular blood flow stimulates endothelial NO production that contributes significantly to exercise-induced hyperemia in muscular tissue [40]. Decreased NO availability can potentially lead to impaired muscle exercise-induced hyperemia and thus contribute to the exercise intolerance reported in individuals with MELAS syndrome. In addition, NO deficiency may result in decreased basal muscular perfusion, leading to limited availability of nutrients such as amino acids, and thus decreased muscle protein synthesis that may contribute to the myopathy and muscle wasting observed in MELAS syndrome [39]. Lactic acidemia in MELAS syndrome results from the inability of dysfunctional mitochondria to adequately oxidize glucose, leading to the accumulation of pyruvate and shunting of pyruvate to lactate [11]. Moreover, hypoperfusion may result in lactic acidosis due to decreased oxygen delivery to peripheral tissues and a shift to anaerobic glycolysis. NO deficiency in MELAS syndrome can result in decreased blood perfusion and therefore may aggravate lactic acidosis [39]. Diabetes develops in MELAS syndrome due to multiple defects in insulin and glucose metabolism including insulin deficiency, increased gluconeogenesis, and insulin resistance [49]. In pancreatic β-cells, an ATP sensitive potassium channel is required for insulin release. Decreased ATP synthesis as a result of mitochondrial dysfunction can result in impaired insulin secretion and insulinopenia [11,21]. Two factors may contribute to increasing gluconeogenesis in MELAS syndrome: insulin resistance, which results in reduced insulin inhibition of hepatic gluconeogenesis, and lactic acidemia, which can fuel gluconeogenesis in the liver [49]. The etiology of insulin resistance in MELAS syndrome is likely to be multifactorial. First, decreased glucose oxidative capacity due to the mitochondrial dysfunction can result in impaired insulin responsiveness. Second, through enhancing endothelial NOS (eNOS) activity, insulin can induce vasodilation that results in an adequate delivery of glucose and insulin to muscle tissue [50]. Therefore, NO deficiency in MELAS syndrome may lead to impaired insulin-mediated


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Fig. 3. Pathogenesis of MELAS syndrome (NO: nitric oxide, NOS: nitric oxide synthase, ADMA: asymmetric dimethylarginine, RNS: reactive nitrogen species).

vasodilation, resulting in impaired insulin delivery and contributing to insulin resistance [49]. Finally, increased ROS production due to mitochondrial dysfunction can play a role in impaired insulin responsiveness and the development of insulin resistance [51]. Renal Fanconi syndrome is believed to result from impaired activity of ATP-dependent sodium–potassium pumps that are needed in the tubular reabsorption process. Growth failure may reflect the effects of a chronic systemic state of energy deficiency [11].

accounting for the clinical diversity seen in individuals harboring this mutation [1]. Interestingly, it has been demonstrated that the muscle m.3243ANG heteroplasmy correlates with maximum oxygen uptake and workload, resting plasma lactate, and muscle morphology abnormalities in individuals with MELAS syndrome indicating that the threshold of muscle mutation load at which oxidative impairment occurs is about 50% [55]. 4. Management

3.5. Phenotype variability The m.3243ANG mutation is associated with variable phenotypes that collectively constitute a wide spectrum, ranging from MELAS syndrome at the severe end in only ~ 10% of individuals with the m.3243ANG mutation to asymptomatic carrier status in another ~10% of m.3243ANG mutation carriers. Between these two extremes intermediate phenotypes exist including single organ involvement (e.g., cardiomyopathy or DM) and multi-organ involvement with various combinations of symptoms (e.g., myopathy, diabetes, and deafness) that occasionally constitute distinctive syndromes including maternally inherited deafness and diabetes (MIDD) and progressive external ophthalmoplegia (PEO) [11,52]. Other mitochondrial syndromes including Leigh syndrome and MERRF have been associated with the m.3243ANG mutation as well [53,54]. The extreme variability in phenotypes associated with the m.3243ANG mutation is a common observation in many mtDNArelated mitochondrial diseases. Similarly to the majority of mtDNA mutations, the m.3243ANG is a heteroplasmic mutation, i.e., present in some copies of mtDNA, and therefore the cells harbor a mixture of mutant and normal mtDNA. During cell division, mutant mtDNAs are distributed randomly among daughter cells. Therefore, the percentage of mutant mtDNAs differs in different tissues and organs within the same individual. These tissues and organs have different thresholds in heteroplasmy percentage before clinical phenotypes manifest thus

There is no specific consensus approach for treating individuals with MELAS syndrome. Management is largely symptomatic and should involve a multidisciplinary team that may include a neurologist, cardiologist, endocrinologist, audiologist, ophthalmologist, physical and occupational therapists, psychologist, and social worker. 4.1. Evaluation of multi-organ involvement Upon diagnosis, a comprehensive evaluation for the multi-organ involvement is needed. Affected individuals should also be followed at regular intervals to monitor progression and screen for potential complications. Performing the following evaluations has been recommended at time of diagnosis and as needed during regular follow up visits: comprehensive neurological examination with cognitive assessments, brain MRI, audiologic and ophthalmologic examinations, growth assessment, echocardiogram and electrocardiogram, screening for hypothyroidism, and screening for diabetes by fasting blood glucose and glucose tolerance test [10]. 4.2. Management of complications Sensorineural hearing loss has been successfully treated with cochlear implants [56]. Seizures respond to traditional anticonvulsant therapy and standard analgesics can be used for migraine headaches.



Cardiac manifestations can also benefit from standard pharmacologic therapy. Diabetes can be managed by dietary modifications with or without oral hypoglycemic agents. However, insulin therapy is often required [10]. Nutrition support is needed for children failing to thrive and rehabilitation with physical and occupational therapy is needed after stroke-like episodes. Psychiatric evaluation and treatment are needed for individuals with psychiatric manifestations. Regular exercise can improve exercise capacity in individuals with MELAS syndrome and other mitochondrial myopathies. Endurance training can induce mitochondrial biogenesis whereas resistance training can induce transferring normal mitochondrial templates from satellite cells to mature muscle that may lower the mutation heteroplasmy [57]. Several supplementations, including antioxidants and cofactors, are being used in MELAS syndrome based on limited clinical trials [58]. Unblinded studies have showed that L-arginine therapy can be beneficial in stroke-like episode treatment and prevention. Intravenous L-arginine infusion during the acute phase has been shown to improve stroke-like episode symptoms whereas oral L-arginine supplementation during the interictal phase has been shown to decrease the frequency and severity of stroke-like episodes [48,59]. The suggested intravenous L-arginine dose is 0.5 g/kg for children or 10 g/m2 body surface area for adults with a similar daily dose to be given orally in three divided doses during the interictal phase [58]. The therapeutic effect of arginine in stroke-like episodes in MELAS syndrome is proposed to be due to increase NO availability leading to improve intracerebral vasodilation and blood flow. This potential mechanism has been supported by the demonstration that arginine supplementation to subjects with MELAS syndrome results in increased NO production and improved flow-mediated dilation (FMD), which is considered a measure of NO synthesized by endothelial cells in response to re-perfusion [37,60]. Although the clinical effects of citrulline administration in MELAS syndrome have not been studied, a stable isotope study demonstrated that citrulline supplementation to subjects with MELAS syndrome resulted in increased NO production. Interestingly, citrulline supplementation was found to induce a greater increase in the NO synthesis rate than that associated with arginine supplementation, indicating that citrulline is a more effective NO precursor than arginine. Therefore, citrulline may have a better therapeutic effect than arginine [37]. Additionally, increasing NO availability with arginine or citrulline supplementation will potentially improve perfusion in all microvasculature compartments. Therefore, the effect of arginine and citrulline supplementation may not be limited to improving stroke-like episodes, but may also lead to improvements in other clinical features of MELAS syndrome, including muscle weakness, exercise intolerance, and lactic acidosis. Interestingly, arginine and citrulline supplementation has been reported to result in a reduction in plasma alanine and lactate concentrations, suggesting that such supplementation may improve lactic acidemia in MELAS syndrome by increasing NO production and improving perfusion and oxygen delivery [37,61]. Additional clinical studies assessing the clinical effects of citrulline and arginine supplementations on different manifestations of MELAS syndrome are needed to determine their potential therapeutic utility in this syndrome. Coenzyme Q10 (CoQ10) facilitates electron transfer from complexes I and II to complex III of ETC and stabilizes the ETC complexes by providing protective antioxidant effects. Some studies showed beneficial effects on muscle weakness, fatigability, and lactate level for CoQ10 in individuals with MELAS syndrome [62,63]. The recommended doses are 5–10 mg/kg/day for children and 200–400 mg/day for adults [58]. Creatine, which is mainly stored in muscle, heart, and brain, is metabolized to phosphocreatine which is an essential phosphate donor for ATP regeneration in muscle and brain. Creatine monohydrate supplementation was shown to increase the strength of high-intensity anaerobic and aerobic activities in individuals with MELAS syndrome and other mitochondrial cytopathies [64]. The recommended doses are 100 mg/kg/day for children and 2–5 g/day for adults [58].

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Additionally, a randomized, double-blind, placebo-controlled study showed that a combination therapy including creatine monohydrate, CoQ10, and lipoic acid resulted in improved muscle strength and lower plasma lactate in individuals with MELAS syndrome and other mitochondrial cytopathies [65]. CoQ10 does not cross the blood–brain barrier; therefore, it may have limited effect on the central nervous system. Idebenone is a CoQ10 analog that can cross the blood–brain barrier and has been shown to improve neurological complications in some case reports [66,67]. L-Carnitine is required for long-chain fatty acid transportation to the mitochondrial matrix where it undergoes β-oxidation. Secondary carnitine deficiency can be rarely observed in MELAS syndrome [58, 68]. Carnitine supplementation can potentially enhance β-oxidation and replenish the intracellular pools of coenzyme A [58]. The carnitine can be given at doses of 3 g daily in three divided doses for adults and 100 mg/kg/day for children in three divided doses [58].

4.3. Medications to avoid Valproic acid should be avoided in the treatment of seizure because of its deleterious effects on mitochondrial function. Clinically, valproic acid may result in worsening or triggering seizures in individuals with MELAS syndrome [69,70]. Other antiepileptic drugs which may affect the mitochondrial metabolism, include phenobarbital, carbamazepine, phenytoin, oxcarbazepine, ethosuximide, zonisamide, topiramate, gabapentin and vigabatrin [71]. Metformin should be avoided in individuals with MELAS syndrome because of its propensity to cause lactic acidosis [11]. Dichloroacetate, which reduces lactate by activating the pyruvate dehydrogenase enzyme, should be avoided in MELAS syndrome. A study evaluating the effect of dichloroacetate in individuals with MELAS syndrome was terminated because of onset or worsening of peripheral neuropathy indicating that dichloroacetate can be associated with peripheral nerve toxicity [72]. Individuals with MELAS syndrome should also avoid agents with potential mitochondrial toxicity including aminoglycosides, linezolid, and alcohol [10]. Cigarette smoke contains hundreds of compounds many of which can accumulate in mitochondria and disturb the function of the ETC including phenolic compounds, aldehydes, heavy metals, carbon monoxide, nicotine, and aromatic compounds [73]. Therefore, smoking can aggravate mitochondrial dysfunction in individuals with mitochondrial diseases. 5. Conclusions MELAS syndrome, which is a frequent maternally inherited mitochondrial disorder, is a multi-organ disease with broad manifestations including stroke-like episodes, dementia, epilepsy, lactic acidemia, and myopathy. The m.3243ANG mutation in the MT-TL1 gene occurs in 80% of individuals with MELAS syndrome. Several mechanisms can interact to result in the multi-organ phenotype of MELAS syndrome including impaired mitochondrial energy production, microvasculature angiopathy, and NO deficiency. Management of MELAS syndrome is largely symptomatic and should involve a multidisciplinary team. Several supplementations, including antioxidants and cofactors, are being used in MELAS syndrome based on limited clinical trials. Valproic acid, metformin, and dichloroacetate should be avoided in individuals with MELAS syndrome. References [1] A.W. El-Hattab, F. Scaglia, Mitochondrial disorders, in: B. Lee, F. Scaglia (Eds.), Inborn Errors of Metabolism: From Neonatal Screening to Metabolic Pathways, Oxford University Press, New York, NY, USA 2015, pp. 180–202. [2] S.G. Pavlakis, P.C. Phillips, S. DiMauro, D.C. De Vivo, L.P. Rowland, Mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes: a distinctive clinical syndrome, Ann. Neurol. 16 (1984) 481–488. [3] Y. Goto, I. Nonaka, S. Horai, A mutation in the tRNA(Leu)(UUR) gene associated with the MELAS subgroup of mitochondrial encephalomyopathies, Nature 348 (1990) 651–653.


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