Clinical manifestations

Presentation and course

Myoclonus epilepsy with ragged-red fibers (MERRF) is a multisystem disorder traditionally characterized by (1) myoclonus, (2) generalized epilepsy, (3) ataxia, and (4) myopathy with ragged-red fibers in the muscle biopsy (26).

Onset of the disease is usually in childhood and adolescence after normal psychomotor early development, but it is also possible in adulthood. About 80% of patients have a family history of mitochondrial encephalomyopathy, although not always full-blown MERRF (30). Oligosymptomatic and asymptomatic maternal relatives in MERRF pedigrees are frequently detected. Occasional patients that fulfill the clinical criteria for MERRF also may have additional neurologic manifestation such as migraine (70), hearing impairment, and peripheral neuropathy. Several individuals with depressive mood disorders have been reported.

Among non-neurologic manifestations, cardiac involvement and lipomatosis are frequent. Cardiopathy is common in MERRF and may require medical treatment; in a study of 18 patients with the m.8344A>G mutation, eight had cardiac involvement (dilated cardiopathy in four, Wolff-Parkinson-White syndrome in three, partial cardiac conduction block in one), and two died of heart failure (71). An Italian study showed that cardiac involvement was observed in 53% of 15 m.8344A>G MERRF patients, and a restrictive respiratory insufficiency requiring ventilatory support was observed in about half of the patients (10). Lipomatosis may be observed in adult MERRF patients, most commonly in the cervical region (14). Massive lipomas have also been reported.

Maternally inherited spinocerebellar degeneration, atypical Charcot-Marie-Tooth disease, and Leigh disease have been reported as unusual manifestations in a MERRF pedigree (31). One patient presented with parkinsonism without myoclonus, epilepsy, or ataxia (32), and a child presented with acute demyelination in the central and peripheral nervous system (24). A MERRF-resembling case of adolescent-onset and slowly progressive mitochondrial encephalomyopathy with epilepsy, ataxia, and dystonia in the absence of clinical myopathy, caused by a rare mutation in the MT-TW gene, was reported (72).

The largest cohort to date comprises 42 carriers of the m.8344A>G mutation (39). Clinical data were available for 39 subjects, and four of those were asymptomatic (10.3%). The mean age at onset of the 35 symptomatic patients was 30.1±18.4 years (range 0 to 66; 11/35 with clinical onset before the age of 16). The most frequent clinical manifestations were muscle weakness in 51.3%, exercise intolerance in 38.5%, seizures in 30.8%, hearing loss in 30.8%, multiple lipomatosis in 28.2%, ptosis/ophthalmoparesis in 25.6%, ataxia in 20.5%, and myoclonus in 20.5%. Moreover, the authors reviewed the literature and found 282 additional mutation carriers. Considering all 321 patients so far available at the mean age of approximately 35 years, the clinical picture was characterized by the following signs and symptoms: myoclonus, muscle weakness, ataxia (35% to 45% of patients), generalized seizures, hearing loss (25% to 34.9%), cognitive impairment, multiple lipomatosis, neuropathy, exercise intolerance (15% to 24.9%), increased creatine kinase levels, ptosis/ophthalmoparesis, optic atrophy, cardiomyopathy, muscle wasting, respiratory impairment, diabetes, muscle pain, tremor, and migraine (5% to 14.9%) (10). Taken together, the phenotype of m.8344A>G mutation carriers showed marked heterogeneity, reaching from asymptomatic to lethal multisystem diseases.

The same authors pointed out that the great majority of 8344A>G patients (approximately 80%) do not have myoclonus and that myoclonus, if present, is not inextricably linked to epilepsy, but to cerebellar ataxia (40). Hence, the term “myoclonic epilepsy” seems inadequate and potentially misleading, and the acronym MERRF could better be read as myoclonic encephalomyopathy with ragged-red fibers. In a clinical cross-sectional investigation on movement disorders on an Italian cohort of individuals with childhood-onset primary mitochondrial diseases, ataxia (pure cerebellar, sensitive, or spinocerebellar) was widely observed in both pediatric and adult patients. Not surprisingly, myoclonus was commonly observed in mitochondrial encephalopathies, mainly in MERRF, as predominant features at onset or appearing at follow-up in ataxic patients (46).

The German mitoNET registry described the clinical features of 34 m.8344A>G MERRF patients (02). The canonical featured seizures, myoclonus, cerebellar ataxia, and ragged-red fibers that are traditionally associated with MERRF, and they occurred in 61%, 59%, 70%, and 63% of the patients, respectively. In contrast, other features such as hearing impairment were even more frequently present (72%), as well as migraine (52%), psychiatric disorders (54%), respiratory dysfunction (45%), gastrointestinal symptoms (38%), dysarthria (36%), and dysphagia (35%). In the German cohort, the association between myoclonus and ataxia in adult subjects was not significant, but a trend was observed (p=0.0699).

Prognosis and complications

MERRF is a chronic condition, which is slowly progressive. The disease gradually progresses over years. The age of death has ranged from seven to 79 years of age (30). The major complications are seizures and, less commonly, blindness and cardiac failure. Chronic pancreatitis has been reported in one MERRF patient (68). Overall, the exact prognosis is difficult to ascertain, given the wide heterogeneity of phenotypic spectrum.

Clinical vignette

The woman was normal at birth with normal early development. At 10 years of age she developed involuntary jerking of her head and limbs; soon thereafter, drop attacks began. Anticonvulsant therapy was initiated. She showed progressive gait ataxia, proximal limb weakness, and a decline in her school performance. By the age of 13 years, she had a marked intellectual decline, inability to stand, limb weakness with diffuse muscle wasting, and hearing loss. Serum lactate and pyruvate were elevated. A muscle biopsy showed many ragged-red fibers. Her brother and a sister were similarly affected. Her 22-year-old sister was clinically normal. Her mother, at 33 years of age, noted difficulty climbing stairs and lifting objects. At 38 years of age, she was given the diagnosis of limb-girdle muscular dystrophy. At 46 years of age serum lactate and pyruvate were elevated, and her muscle biopsy showed numerous ragged-red fibers. Mitochondrial DNA analysis revealed the presence of the m.8344A>G in the MT-TK gene.

Biological basis

Etiology and pathogenesis

An A-to-G transition mutation at nucleotide 8344 (m.8344A> G) in the MT-TK gene encoding tRNALys has been found in about 80% of MERRF patients tested (59; 30). A second point mutation was found in the MT-TK at nucleotide 8356 (m.8356T> C) in one pedigree with typical MERRF (60) and in another with a syndrome in which MERRF overlapped with MELAS (77; 78). Two additional mutations in the MT-TK gene have been identified in MERRF patients, one at nucleotide 8363 in two Japanese families (52) and the other at nucleotide 8361 (57). Thus, the mitochondrial MT-TK gene is clearly a hot spot for mutations causing MERRF.

However, MERRF and MERRF-like phenotypes have been associated with other possible or definite pathogenic alterations in different mitochondrial genes. For a complete list of genes see Table 1. Interestingly, in a unique family, a MERRF-MELAS overlap syndrome was found to be due to double mutations in mtDNA: the MERRF mutation at nucleotide 8356 and the MELAS mutation at nucleotide 3243 (50).

Table 1. Mutations in mtDNA Genes Associated with MERRF

In addition, MERRF-like phenotypes were described with mutations in MT-TF (m.611G> A) (37), in MT-TI (m.4284 G> A) (28), and also in association with the classical MELAS mutation, m.3243A>G in MT-TL1 (08). In a patient with MERRF plus pigmentary retinopathy, a MT-TP mutation was identified (05). A single patient with MERRF was found to have multiple mitochondrial DNA deletions (06). In addition to the m.8356T> C mutation, point mutations at nucleotide 7512 in MT-TS1 (49), nucleotide 12147 in MT-TH (45), nucleotide 12207 in MT-TS2 (73), nucleotide 13042 in ND5 encoding subunit 5 of complex I (48), and nucleotide 5521 in the MT-TW (29) have also been associated with MERRF/MELAS overlap syndromes. In a unique family, a MERRF-MELAS overlap syndrome was found to be due to double mutations in mtDNA: the MERRF mutation at nucleotide 8356 and the MELAS mutation at nucleotide 3243 (50). MERRF and Kearns-Sayre syndrome overlap syndromes have been described in single patients with point mutation in MT-TK (51) and in MT-TL1 (m.3291T> C) (22). A unique patient with an overlap MERRF-NARP syndrome harbored the mutation m.12300G>A in the mitochondrial MT-TL2 gene (44). Shih-Jie Chou demonstrated that cardiomyocyte-like cells or neural progenitor cells differentiated from MERRF-iPSCs (induced pluripotent stem cells) harboring the A8344G mutation contain mitochondria with an abnormal ultrastructure, produce increased ROS levels, and express upregulated antioxidant genes.

Moreover, Wu and colleagues successfully established the iPSCs-derived neural progenitor cells and cortical-like neurons of patients with MERRF syndrome that retained the heteroplasmy of the m.8344A > G mutation starting from the patients’ skin fibroblasts (75). MERRF neural cells exhibited impaired mitochondrial bioenergetic function, elevated ROS levels, and imbalanced expression of antioxidant enzymes. Furthermore, neural cells were characterized by neural immaturity and synaptic protein loss leading to the impairment of neuronal activity and plasticity. In vivo electrophysiological recordings found that neurons harboring a high level of the m.8344A > G mutation exhibited impairment of the spontaneous and evoked potential-stimulated neuronal activities. These in vitro-modelling approach, which exploit PSCs capacity to differentiate into many cell types, permits both detailed investigation of cellular pathomechanisms and validation of promising treatment options.

The origin of the mitochondrial DNA mutations is uncertain. The MERRF mutations are maternally transmitted. Clinical expression of the mutation depends on three factors: (1) mitochondrial DNA heteroplasmy, (2) mitochondrial DNA tissue distribution, and (3) tissue threshold. Silvestri and colleagues found that there was no clear correlation between the percentage of the m.8344A> G mutation in muscle and clinical severity (59). However, the amount of mutation in muscle might not reflect the amount in the central nervous system, to which most clinical symptoms are related. In one of the very few postmortem examinations, mutation loads in 43 different tissue samples of a 16-year-old MERRF patient varied merely between 89% and 100% (09). However, mtDNA copy numbers were increased three- to seven-fold in predominantly affected brain areas and in skeletal muscle while normal in unaffected tissues. Brinckmann and colleagues concluded that "futile" stimulation of mtDNA replication per se or a secondary failure to increase the mitochondrial mass may contribute to the regionalized pathology seen in MERRF syndrome.

Analyses of respiratory chain enzymes in muscle biopsies have demonstrated variable or no defects of respiratory chain complexes. Cytochrome c oxidase activity seems to be most consistently decreased. Skeletal muscle biopsies demonstrate mitochondrial abnormalities. Modified Gomori trichrome stain shows ragged-red fibers in over 90% of patients with myoclonus epilepsy with ragged-red fibers (30); the succinate dehydrogenase histochemical stain reveals fibers with darker than normal reaction and represents a more sensitive indicator of mitochondrial proliferation. Ultrastructural studies have confirmed the increased number of mitochondria, as well as morphologically abnormal mitochondria, which sometimes contain paracrystalline inclusions. Cytochrome c oxidase stain demonstrates individual muscle fibers lacking histochemical activity. Pathological studies have shown neuronal loss in the dentate nucleus, inferior olivary nucleus, degeneration of the posterior columns of the spinal cord, and diffuse gliosis of the cerebellar white matter and of the brain (26). It is unclear how the mitochondrial DNA point mutations cause the MERRF clinical phenotype. Chomyn and colleagues, using the rho-0 cell line repopulated with mitochondria harboring the m.8344A> G mutation, found that high proportions of the mutation correlated with decreased protein synthesis, decreased oxygen consumption, and cytochrome c oxidase deficiency (13). The polypeptides containing higher numbers of lysine residues were more severely affected by the mutation, suggesting that the tRNA mutation directly inhibits protein synthesis. Similarly, cultured myotubules containing greater than 85% mutant mitochondrial DNA showed a decrease in translation, affecting proteins that contained larger numbers of lysine residues more severely (07). Decreased levels of tRNALys and aminoacylated tRNALys (transfer RNA with covalently bound lysine) have been observed in cells harboring the m.8344A> G mutation (23). The mutation appears to be functionally recessive, because only about 15% wild-type mitochondrial DNA was needed to restore translation and cytochrome c oxidase activities to near-normal levels. Masucci and colleagues confirmed the decreased protein synthesis and oxygen consumption in rho-0 cell repopulated with the m.8344A> G mutation and found similar results with the MERRF-8356 mutation (43). They also identified aberrant mitochondrial protein in both mutant cell lines, which they attributed to ribosomal frameshifting. Studies of in vitro transcribed tRNALys mutants showed that the mutations associated with MERRF had no effect on lysylation efficiency whereas the two mutations associated with encephalomyopathies without typical MERRF features (G8313A and G8328A) severely impaired lysylation (61). Yasukawa and colleagues observed that the m.8344A> G mutation caused a lack of modification in tRNALys leading to impaired protein synthesis (76). The m.8344A> G-induced protein synthesis defects not only reduce oxygen consumption and ATP synthesis but may also increase oxidative stress (74). Human cybrid cells harboring either the classical MERRF or MELAS mutation are more prone to undergo apoptosis then their wild-type counterpart when challenged with various apoptotic inducers, such as staurosporine. This may be mediated by perturbed calcium homeostasis induced by mitochondrial dysfunction (56).

The common cause of MERRF, m.8344A> G mutation, is also a common cause of multiple symmetric lipomatosis (47). The Nation-wide Italian Collaborative Network of Mitochondrial Diseases analyzed the incidence and characteristics of lipomas among an Italian cohort of patients with mitochondrial diseases (47). A total of 22 patients (1.7% of the cohort) with lipomas have been identified. In about 18%, multiple systemic lipomatosis was the only clinical manifestation, whereas 54% of patients showed a classical MERRF syndrome, and 86% had mutations in the mtDNA tRNA lysine. Finally, in patients with MERRF, lipomas have been shown to harbor a deletion in chromosome 6 (6q24) in addition to the m.8344A> G mutation (34).

Epidemiology

MERRF patients have been found worldwide. The disorder has no known ethnic predilection and affect both males and females equally. Onset usually occurs in childhood or early adulthood. Three epidemiological studies in northern European countries have estimated prevalence of the m.8344A>G mutation in affected adulthood to be 1 in 5000, and nuclear mutations were responsible for clinically overt adult mitochondrial disease in 2.9 per 100,000 adults (12; 17; 55; 27).

Prevention

Current strategies for preventing the transmission of mitochondrial disease to offspring include techniques known as mitochondrial replacement and mitochondrial gene editing. However, these techniques raise several ethical concerns, as well as technical and safety problems. Thus, to date, there are no reliable means to prevent this genetic disorder.

Differential diagnosis

The differential diagnosis includes other primary mitochondrial diseases (ie, POLG1 related-ataxia, MERRF-MELAS overlap syndrome), degenerative hereditary cerebellar ataxia (36) or syndromes characterized by myoclonus epilepsy and ataxia including Unverricht-Lundborg disease, Lafora body disease, neuronal ceroid lipofuscinosis, and sialidosis (03; 42). Eberhardt and Topka provide a comprehensive review of genetic and nongenetic causes of myoclonic disorders (21).

Diagnostic workup

Screening patients for MERRF should begin with routine blood tests, including complete blood count, serum electrolytes, liver function tests, blood urea nitrogen, creatinine, lactate, and pyruvate. These tests may reveal kidney or liver dysfunction. Lactate and pyruvate at rest might be elevated in MERRF patients and may increase dramatically after moderate exercise. Blood leukocyte and urine sediment DNA should be screened for a mitochondrial DNA point mutation because identification of a mitochondrial DNA mutation will obviate the need for a costly and invasive muscle biopsy.

ECG may reveal pre-excitation or partial cardiac conduction block. Echocardiogram may demonstrate cardiomyopathy. Lumbar puncture may show elevated cerebrospinal fluid protein, but no patients have had cerebrospinal fluid protein greater than 100 mg/dL. Electromyography and nerve conduction studies are typically consistent with a myogenic process, although a neuropathy may also be present. Typically, there are decreased amplitudes of compound muscle or nerve action potentials indicating axonal degeneration (15). Electroencephalography may show atypical generalized spike and wave discharges, with abnormal background slowing; focal epileptiform discharges may also be seen (62). As for any epilepsy secondary to mitochondrial disease, in MERRF patients there is an increased propensity for seizures arising from the posterior cerebral regions (25). Somatosensory evoked responses may show giant cortical evoked responses (62). Brain imaging with CT or MRI may show basal ganglia calcification and atrophy (04). In the Italian cohort, MRI was normal in 58% of the cases; the remaining patients presented cortical/subcortical atrophy, white matter abnormalities, periaqueductal lesions, cysts or vacuolated lesions, brainstem atrophy, and stroke-like lesions. In the German cohort, cerebral atrophy, cerebellar atrophy, or both was present at brain MRI in 43%. In two further patients, brain white matter abnormalities were noted. No definitive data on brain MRI spectroscopy are available from the two cohorts.

Finally, muscle biopsy can be performed to confirm the diagnosis. Ragged-red fibers on modified Gomori trichrome stain are the hallmark histological feature and a defining criterion. In addition, a mosaic pattern of cytochrome c oxidase (COX) deficient fibers is typically seen; however, at least one MERRF patient with the m.8344A>G mutation lacked ragged-red and COX-deficient fibers (41). Mitochondrial enzyme activities can be measured in whole muscle homogenate or in isolated mitochondria and usually demonstrate multiple respiratory chain defects, particularly in complex IV (35). Muscle mitochondrial DNA should be screened for the mutations associated with MERRF.

Management

The seizures of MERRF can be treated with conventional anticonvulsant therapy but are usually refractory. Thus, multi-anticonvulsant therapies may be needed. An international consortium of experts published a consensus on safe medication use in patients with a primary mitochondrial disease, including antiepileptic drugs (20). In this scenario, valproic acid should be avoided in mitochondrial disease, mainly in patients with POLG mutations because it could be toxic and cause liver failure. There are no controlled studies to compare efficacy of different antiepileptic regimens. Myoclonus in MERRF improved in individuals treated with levetiracetam (16; 38). A double-blind placebo-controlled study to evaluate efficacy and safety of vatiquinone for treating mitochondrial disease in participants with refractory epilepsy, including MERRF patients, is currently running (no more recruiting). More information can be accessed at the following website: https://www.clinicaltrials.gov/search?cond=Mitochondrial%20Diseases.

Aerobic exercise is helpful in MERRF and other mitochondrial diseases (64).

No treatment for primary mitochondrial diseases is available so far, beside idebenone in Leber optic atrophy. Coenzyme Q10 (200 to 1000 mg/day), L-carnitine (up to 2 g/day), riboflavin and other mitochondrial vitamins have been used to improve mitochondrial function without clear benefit. In MERRF fibroblasts and cybrids, supplemental coenzyme Q10 showed partial efficacy in restoring respiratory chain activity, mitochondrial protein expression, increased oxidative stress, and increased mitophagy (18).

Aerobic exercise is helpful in MERRF and other mitochondrial diseases (64).

With recent advances in molecular technologies, the understanding of the pathological mechanism of a growing list of mitochondrial disorders has been expanded. Consequently, there is an increasing number of clinical trials in mitochondrial disorders, including MERRF, aiming for more specific, molecular-targeted and effective therapies (01). For a detailed list please visit: https://www.clinicaltrials.gov/ct2/results?cond=Mitochondrial+Diseases&term=&cntry=&state=&city=&dist=.

Special considerations

Pregnancy

Women with the mitochondrial DNA mutations associated with MERRF have borne children but are at risk of passing the mutation to their progeny. Prenatal testing of the fetus may be useful; however, there are no published data. There is one report of uneven distribution of the m.8344A>G mutation in dizygotic twins (53). According to Chinnery and colleagues, high levels of m.8344A>G mutation in mothers' blood were associated with an increased incidence of affected offspring. However, they cautioned against using this information to predict absolute risks to women considering pregnancy (11).

Anesthesia

There are no conclusive data regarding management of MERRF during anesthesia. In general anesthesia, drugs that lower seizure threshold should be avoided; anesthetic drugs, such as succinylcholine and nondepolarizing muscle relaxants, should also be avoided (54). Mitochondrial patients may have increased levels of lactic acid during period of physiological and\or medical stress. Therefore, preoperatory fasting could be risky, and increment of lactic acid should be prevented, avoiding hypoglycemia, hypoxia, and hypotension. More information may be obtained in the consensus on safe medication use in patients with a primary mitochondrial disease, including some anesthetics (20).