Valproic acid triggers acute rhabdomyolysis in a patient with carnitine palmitoyltransferase type II deficiency

Detection of Early Onset Carnitine Palmitoyltransferase II Deficiency by Newborn Screening: Should CPT II Deficiency Be a Primary Disease Target?

Early-onset carnitine palmitoyltransferase II deficiency (CPT II deficiency) (OMIM 600650) can result in severe outcomes, which are often fatal in the neonatal to infantile period. CPT II deficiency is a primary target in the Maritime Newborn Screening Program. We report a case of neonatal-onset CPT II deficiency identified through expanded newborn screening with tandem mass spectrometry. Identification through newborn screening led to early treatment interventions, avoidance of metabolic decompensation, and a better clinical outcome. Newborn screening for CPT II deficiency is highly sensitive and specific with no false positives identified. The only screen positive case detected identified a true positive case. This experience illustrates the importance of newborn screening for CPT II deficiency and demonstrates why reconsideration should be taken to add this disease as a primary newborn screening target.

Valproate attenuates hypertonic glycerol-induced rhabdomyolysis and acute kidney injury

BACKGROUND AND AIM

The current study investigated the effects of treatment with 300 mg/kg valproic acid on rhabdomyolysis and acute kidney injury induced by intramuscular injection of hypertonic glycerol in rats.

METHODS

Four groups of male wistar rats: control and hypertonic glycerol, valproic acid and valproic acid + hypertonic glycerol treated groups were used. Blood urea nitrogen, serum creatinine, creatinine kinase (CK) and CK MB, myoglobin and renal reduced glutathione (GSH) levels were measured. Histopathological examination of the kidneys was carried out to evaluate the degree of renal injury in each group. The expression of interleukin-1 beta "IL-1β" in renal tissue was detected using immunohistochemistry.

RESULTS

Hypertonic glycerol administration led to severe renal tubular damage with a significant elevation of blood urea nitrogen, serum creatinine, creatinine kinase, CK MB and myoglobin levels and overexpression of IL-1β compared to control group. Valproic acid administration attenuated both the muscle injury and the acute kidney injury induced by hypertonic glycerol, estimated through a significant reduction of creatinine kinase, myoglobin, and serum creatinine. Valproic acid administration caused a significant increase in GSH in the hypertonic glycerol + valproic acid group compared to the hypertonic glycerol group. A significant decrease in tubular necrosis grade, and expression of IL-1β in hypertonic glycerol + valproic acid group compared to the hypertonic glycerol group was observed.

CONCLUSION

This study demonstrates, for the first time to the best of our knowledge, that valproic acid could ameliorate the rhabdomyolysis and the related acute kidney injury in rats.

Effect of Levetiracetam Usage on Serum Creatine Phosphokinase Concentration in Patients with Epilepsy

Background:

Levetiracetam (LEV) is a widely used antiepileptic drug (AED) in the treatment of various type of seizures, including generalized epileptic seizure as well as focal seizures, and it is generally well tolerated. Common side effects of LEV are somnolence, asthenia, dizziness, mood changes, kidney dysfunction, minor infections, and thrombocytopenia. Recently, increased creatine phosphokinase (CPK) concentration or rhabdomyolysis after LEV administration has been reported. The goal of the study was to evaluate clinical risk factors associated with increased CPK concentration or rhabdomyolysis in LEV administration.

Materials and Methods:

One hundred and sixty children were enrolled. The risk factors were retrospectively analyzed.

Results:

Among the 160 patients, 84 (52.5%) were boys and 76 (47.5%) were girls, and the mean age was 85.95 ± 49.03 months (9–188 months). Of the 160 patients, 66 (41.3%) were treated with monotherapy, and 94 (58.8%) with polytherapy. We detected increased CPK concentration or rhabdomyolysis in three patients (1.9%). The CPK values of these three patients were 943, 1504, and 5046, respectively. No significant differences were observed in the serum CPK concentration between the patients treated with LEV.

Conclusion:

We detected that LEV may cause increased CPK concentration or rhabdomyolysis. When treating patients with LEV, clinicians should closely monitor serum CPK level. To the best of our knowledge, this is the first study of elevated CPK concentration or rhabdomyolysis associated with LEV therapy in children.

Understanding the role of OXPHOS dysfunction in the pathogenesis of ECHS1 deficiency

Mitochondria provide the main source of energy for eukaryotic cells, oxidizing fatty acids and sugars to generate ATP. Mitochondrial fatty acid β-oxidation (FAO) and oxidative phosphorylation (OXPHOS) are two key pathways involved in this process. Disruption of FAO can cause human disease, with patients commonly presenting with liver failure, hypoketotic glycaemia and rhabdomyolysis. However, patients with deficiencies in the FAO enzyme short-chain enoyl-CoA hydratase 1 (ECHS1) are typically diagnosed with Leigh syndrome, a lethal form of subacute necrotizing encephalomyelopathy that is normally associated with OXPHOS dysfunction. Furthermore, some ECHS1-deficient patients also exhibit secondary OXPHOS defects. This sequela of FAO disorders has long been thought to be caused by the accumulation of inhibitory fatty acid intermediates. However, new evidence suggests that the mechanisms involved are more complex, and that disruption of OXPHOS protein complex biogenesis and/or stability is also involved. In this review, we examine the clinical, biochemical and genetic features of all ECHS1-deficient patients described to date. In particular, we consider the secondary OXPHOS defects associated with ECHS1 deficiency and discuss their possible contribution to disease pathogenesis.

Rhabdomyolysis caused by carnitine palmitoyltransferase 2 deficiency: A case report and systematic review of the literature

Carnitine palmitoyltransferase 2 deficiency is an inherited metabolic disorder involving a deficiency in a mitochondrial enzyme necessary for long chain fatty acid oxidation, and therefore decreased utilisation of fatty acids. The adult form of this condition leads to recurrent rhabdomyolysis triggered by exercise, fasting and infection. It is a very rare condition with only a few hundred reported cases worldwide. Here we present a case of severe rhabdomyolysis in the context of carnitine palmitoyltransferase 2 deficiency in which major organ involvement was avoided, and organ support was not needed. This prompted us to perform a systematic review of the existing case reports in the literature to ascertain the most frequent patterns of organ involvement and assess the outcomes that are seen in these patients. Our findings suggest that these patients most frequently develop isolated renal failure, often requiring renal replacement therapy; however, the outcomes following this are very good, supporting the early involvement of intensive care teams.

Rhabdomyolysis and Hepatotoxicity From Valproic Acid: Case Reports

Valproic acid (VPA) has been widely used more frequently as its approved indications have been expanded. More and more case reports on rare toxicities have been published in the literature (ie, hepatotoxicities, hyperammonemic encephalopathy, coagulation disorders, pancreatitis, thrombocytopenia). In spite of the long history of VPA, there is a lack of awareness of VPA toxicities among clinicians. We present two cases of a 44-year-old African American female and a 60-year-old Hispanic male taking chronic VPA therapy for psychiatric disorders admitted to the hospital with a combination of hepatotoxicities and acute kidney injury-associated rhabdomyolysis. In both cases, home VPA therapy was continued during hospitalization. Consequently, the female patient deceased and the male patient survived and discharged with continuation of his chronic VPA therapy. In cases of surviving patients, resumption of maintenance VPA upon discharge should be held and alternative therapy should be considered.

Mitochondrial Fatty Acid Oxidation Disorders Associated with Short-Chain Enoyl-CoA Hydratase (ECHS1) Deficiency

Mitochondrial fatty acid β-oxidation (FAO) is the primary pathway for fatty acid metabolism in humans, performing a key role in liver, heart and skeletal muscle energy homeostasis. FAO is particularly important during times of fasting when glucose supply is limited, providing energy for many organs and tissues, including the heart, liver and brain. Deficiencies in FAO can cause life-threatening metabolic disorders in early childhood that present with liver dysfunction, hypoglycemia, dilated hypertrophic cardiomyopathy and Reye-like Syndrome. Alternatively, FAO defects can also cause ‘milder’ adult-onset disease with exercise-induced myopathy and rhabdomyolysis. Short-chain enoyl-CoA hydratase (ECHS1) is a key FAO enzyme involved in the metabolism of fatty acyl-CoA esters. ECHS1 deficiency (ECHS1D) also causes human disease; however, the clinical manifestation is unlike most other FAO disorders. ECHS1D patients commonly present with Leigh syndrome, a lethal form of subacute necrotizing encephalomyelopathy traditionally associated with defects in oxidative phosphorylation (OXPHOS). In this article, we review the clinical, biochemical and genetic features of the ESHS1D patients described to date, and discuss the significance of the secondary OXPHOS defects associated with ECHS1D and their contribution to overall disease pathogenesis.

Lipid Storage Myopathy with Ketonuria: A Case of Fatty Acid Oxidation-Related Myopathy and Encephalopathy due to Multiple Acyl-CoA Dehydrogenase Deficiency

Encephalopathy and Myopathy in children of varying ages can be due to variety of causes including Mitochondrial diseases, metabolic diseases like renal tubular acidosis, storage diseases as well as fatty acid oxidation (FAO) disorders. FAO related disorders have variable clinical presentation and manifest in different ages. They may present with hypoglycemia, effort intolerance, multi organ involvement with or without ketonuria. High degree of suspicion and appropriate investigations are mandatory for diagnosis. Here we describe an 11 Year old boy, born to non – consanguineous parents. Presented with exertion induced muscle pain and fatigue of 1year duration, which slowly progressed to severe weakness and vomiting. His reflexes were retained. Therefore metabolic vs inflammatory muscle diseases were considered. Patient had ketonuria with elevated blood levels of medium chain acyl carnitine and long chain acyl carnitine suggestive of MADD. Urine organic acid assessment showed elevated excretion of 2-hydroxyglutarate (2HG), adipate and arabitol. Muscle biopsy showed multiple fine vacuoles on Eosin- hematoxylin stained preparation. Modified Gomori - trichrome stain showed vacuolated fibers with red granular material consistent with ragged red fibers. Oil Red O stains showed vacuolated fibers with ‘oil red O’ positive material suggesting lipid storage. Above combination of features is consistent of MADD. Genetic evaluation is not done due to financial constraint. Patient was started on high dose riboflavin and carnitine, with which the child became near normal. Our patient is a case of MADD presenting as Reye’s syndrome like features and showed excellent response to riboflavin, carnitine, dietary and life style changes. High degree of suspicion is lifesaving.

Sodium Valproate Exacerbating an Underlying Disorder of Fatty Acid Metabolism

We describe a 29-year-old female who presented with rhabdomyolysis shortly after starting a course of sodium valproate. A thorough investigation revealed a likely mitochondrial origin inducing this susceptibility. An underlying mitochondrial disorder should be considered in all patients who present with undifferentiated disease whilst taking sodium valproate.

Diagnostic evaluation of rhabdomyolysis

Rhabdomyolysis is characterized by severe acute muscle injury resulting in muscle pain, weakness, and/or swelling with release of myofiber contents into the bloodstream. Symptoms develop over hours to days after an inciting factor and may be associated with dark pigmentation of the urine. Serum creatine kinase and urine myoglobin levels are markedly elevated. Clinical examination, history, laboratory studies, muscle biopsy, and genetic testing are useful tools for diagnosis of rhabdomyolysis, and they can help differentiate acquired from inherited causes of rhabdomyolysis. Acquired causes include substance abuse, medication or toxic exposures, electrolyte abnormalities, endocrine disturbances, and autoimmune myopathies. Inherited predisposition to rhabdomyolysis can occur with disorders of glycogen metabolism, fatty acid β-oxidation, and mitochondrial oxidative phosphorylation. Less common inherited causes of rhabdomyolysis include structural myopathies, channelopathies, and sickle-cell disease. This review focuses on the differentiation of acquired and inherited causes of rhabdomyolysis and proposes a practical diagnostic algorithm. Muscle Nerve 51: 793-810, 2015.

A case of rhabdomyolysis in which levetiracetam was suspected as the cause

Several studies have reported rhabdomyolysis induced by various drugs but not by the antiepileptic drug levetiracetam. We present a case of suspected levetiracetam-induced rhabdomyolysis. A 29-year-old woman was hospitalized for generalized tonic–clonic seizure and given levetiracetam for the first time. One day after starting levetiracetam, she developed myalgia, particularly backache, and weakness in both lower limbs. Based on her clinical symptoms and blood test results indicating hyperCKemia, our diagnosis was levetiracetam-induced rhabdomyolysis. Withdrawal of levetiracetam immediately improved the clinical symptoms and hyperCKemia. This first report of suspected levetiracetam-induced rhabdomyolysis provides important information for treating patients early in levetiracetam administration.

Functional analysis of iPSC-derived myocytes from a patient with carnitine palmitoyltransferase II deficiency

INTRODUCTION

Carnitine palmitoyltransferase II (CPT II) deficiency is an inherited disorder involving β-oxidation of long-chain fatty acids (FAO), which leads to rhabdomyolysis and subsequent acute renal failure. The detailed mechanisms of disease pathogenesis remain unknown; however, the availability of relevant human cell types for investigation, such as skeletal muscle cells, is limited, and the development of novel disease models is required.

METHODS

We generated human induced pluripotent stem cells (hiPSCs) from skin fibroblasts of a Japanese patient with CPT II deficiency. Mature myocytes were differentiated from the patient-derived hiPSCs by introducing myogenic differentiation 1 (MYOD1), the master transcriptional regulator of myocyte differentiation. Using an in vitro acylcarnitine profiling assay, we investigated the effects of a hypolipidemic drug, bezafibrate, and heat stress on mitochondrial FAO in CPT II-deficient myocytes and controls.

RESULTS

CPT II-deficient myocytes accumulated more palmitoylcarnitine (C16) than did control myocytes. Heat stress, induced by incubation at 38°C, leads to a robust increase of C16 in CPT II-deficient myocytes, but not in controls. Bezafibrate reduced the amount of C16 in control and CPT II-deficient myocytes.

DISCUSSION

In this study, we induced differentiation of CPT II-deficient hiPSCs into mature myocytes in a highly efficient and reproducible manner and recapitulated some aspects of the disease phenotypes of CPT II deficiency in the myocyte disease models. This approach addresses the challenges of modeling the abnormality of FAO in CPT II deficiency using iPSC technology and has the potential to revolutionize translational research in this field.

Drug-induced rhabdomyolysis: from systems pharmacology analysis to biochemical flux

The goal of this study was to integrate systems pharmacology and biochemical flux to delineate drug-induced rhabdomyolysis by leveraging prior knowledge and publicly accessible data. A list of 211 rhabdomyolysis-inducing drugs (RIDs) was compiled and curated from multiple sources. Extended pharmacological network analysis revealed that the intermediators directly interacting with the pharmacological targets of RIDs were significantly enriched with functions such as regulation of cell cycle, apoptosis, and ubiquitin-mediated proteolysis. A total of 78 intermediators were shown to be significantly connected to at least five RIDs, including estrogen receptor 1 (ESR1), synuclein gamma (SNCG), and janus kinase 2 (JAK2). Transcriptomic analysis of RIDs profiled in Connectivity Map on the global scale revealed that multiple pathways are perturbed by RIDs, including ErbB signaling and lipid metabolism pathways, and that carnitine palmitoyl transferase 2 (CPT2) was in the top 1 percent of the most differentially perturbed genes. CPT2 was downregulated by nine drugs that perturbed the genes significantly enriched in oxidative phosphorylation and energy-metabolism pathways. With statins as the use case, biochemical pathway analysis on the local scale implicated a role for CPT2 in statin-induced perturbation of energy homeostasis, which is in agreement with reports of statin-CPT2 interaction. Considering the complexity of human biology, an integrative multiple-approach analysis composed of a biochemical flux network, pharmacological on- and off-target networks, and transcriptomic signature is important for understanding drug safety and for providing insight into clinical gene-drug interactions.

Systems pharmacology modeling: an approach to improving drug safety

Advances in systems biology in conjunction with the expansion in knowledge of drug effects and diseases present an unprecedented opportunity to extend traditional pharmacokinetic and pharmacodynamic modeling/analysis to conduct systems pharmacology modeling. Many drugs that cause liver injury and myopathies have been studied extensively. Mitochondrion-centric systems pharmacology modeling is important since drug toxicity across a large number of pharmacological classes converges to mitochondrial injury and death. Approaches to systems pharmacology modeling of drug effects need to consider drug exposure, organelle and cellular phenotypes across all key cell types of human organs, organ-specific clinical biomarkers/phenotypes, gene-drug interaction and immune responses. Systems modeling approaches, that leverage the knowledge base constructed from curating a selected list of drugs across a wide range of pharmacological classes, will provide a critically needed blueprint for making informed decisions to reduce the rate of attrition for drugs in development and increase the number of drugs with an acceptable benefit/risk ratio.

Central role of mitochondria in drug-induced liver injury

A frequent mechanism for drug-induced liver injury (DILI) is the formation of reactive metabolites that trigger hepatitis through direct toxicity or immune reactions. Both events cause mitochondrial membrane disruption. Genetic or acquired factors predispose to metabolite-mediated hepatitis by increasing the formation of the reactive metabolite, decreasing its detoxification, or by the presence of critical human leukocyte antigen molecule(s). In other instances, the parent drug itself triggers mitochondrial membrane disruption or inhibits mitochondrial function through different mechanisms. Drugs can sequester coenzyme A or can inhibit mitochondrial β-oxidation enzymes, the transfer of electrons along the respiratory chain, or adenosine triphosphate (ATP) synthase. Drugs can also destroy mitochondrial DNA, inhibit its replication, decrease mitochondrial transcripts, or hamper mitochondrial protein synthesis. Quite often, a single drug has many different effects on mitochondrial function. A severe impairment of oxidative phosphorylation decreases hepatic ATP, leading to cell dysfunction or necrosis; it can also secondarily inhibit ß-oxidation, thus causing steatosis, and can also inhibit pyruvate catabolism, leading to lactic acidosis. A severe impairment of β-oxidation can cause a fatty liver; further, decreased gluconeogenesis and increased utilization of glucose to compensate for the inability to oxidize fatty acids, together with the mitochondrial toxicity of accumulated free fatty acids and lipid peroxidation products, may impair energy production, possibly leading to coma and death. Susceptibility to parent drug-mediated mitochondrial dysfunction can be increased by factors impairing the removal of the toxic parent compound or by the presence of other medical condition(s) impairing mitochondrial function. New drug molecules should be screened for possible mitochondrial effects.

Mitochondrial involvement in drug-induced liver injury

Mitochondrial dysfunction is a major mechanism of liver injury. A parent drug or its reactive metabolite can trigger outer mitochondrial membrane permeabilization or rupture due to mitochondrial permeability transition. The latter can severely deplete ATP and cause liver cell necrosis, or it can instead lead to apoptosis by releasing cytochrome c, which activates caspases in the cytosol. Necrosis and apoptosis can trigger cytolytic hepatitis resulting in lethal fulminant hepatitis in some patients. Other drugs severely inhibit mitochondrial function and trigger extensive microvesicular steatosis, hypoglycaemia, coma, and death. Milder and more prolonged forms of drug-induced mitochondrial dysfunction can also cause macrovacuolar steatosis. Although this is a benign liver lesion in the short-term, it can progress to steatohepatitis and then to cirrhosis. Patient susceptibility to drug-induced mitochondrial dysfunction and liver injury can sometimes be explained by genetic or acquired variations in drug metabolism and/or elimination that increase the concentration of the toxic species (parent drug or metabolite). Susceptibility may also be increased by the presence of another condition, which also impairs mitochondrial function, such as an inborn mitochondrial cytopathy, beta-oxidation defect, certain viral infections, pregnancy, or the obesity-associated metabolic syndrome. Liver injury due to mitochondrial dysfunction can have important consequences for pharmaceutical companies. It has led to the interruption of clinical trials, the recall of several drugs after marketing, or the introduction of severe black box warnings by drug agencies. Pharmaceutical companies should systematically investigate mitochondrial effects during lead selection or preclinical safety studies.

Valproic acid metabolism and its effects on mitochondrial fatty acid oxidation: a review

Valproic acid (VPA; 2-n-propylpentanoic acid) is widely used as a major drug in the treatment of epilepsy and in the control of several types of seizures. Being a simple fatty acid, VPA is a substrate for the fatty acid beta-oxidation (FAO) pathway, which takes place primarily in mitochondria. The toxicity of valproate has long been considered to be due primarily to its interference with mitochondrial beta-oxidation. The metabolism of the drug, its effects on enzymes of FAO and their cofactors such as CoA and/or carnitine will be reviewed. The cumulative consequences of VPA therapy in inborn errors of metabolism (IEMs) and the importance of recognizing an underlying IEM in cases of VPA-induced steatosis and acute liver toxicity are two different concepts that will be emphasized.

Mutations of carnitine palmitoyltransferase II (CPT II) in Japanese patients with CPT II deficiency

Carnitine palmitoyltransferase II (CPT II) deficiency is an inherited disorder involving beta-oxidation of long-chain fatty acids. CPT II deficiency is a wide-spectrum disorder that includes a lethal neonatal form, an infantile form, and an adult-onset form. However, the ethnic characteristics and the relationship between genotype and clinical manifestation are not well understood. We investigated three non-consanguineous Japanese patients with CPT II deficiency and examined cell lines from 4 unrelated patients and 50 healthy donors. The CPT 2 gene was typed by direct DNA sequencing of polymerase chain reaction-amplified gene products. Case 1 (infantile form) was heterozygous for a phenylalanine to tyrosine substitution at position 383 (p.F383Y) and a novel valine to leucine substitution at 605 (p.V605L). Cases 2, 4, and 5 (infantile form) and case 3 (adult-onset form) were heterozygous for a single mutation at F383Y. Case 6 (adult-onset form) was compound heterozygous at the CPT 2 locus, with deletion of cytosine and thymine at residue 408, resulting in a stop signal at 420 (p.Y408fsX420), and an arginine to cysteine substitution at position 631 (p.R631C). Case 7 (adult-onset form) was homozygous for the p.F383Y mutation. In conclusion, we identified p.F383Y mutations in six of seven patients with CPT II deficiency and two novel variants of the coding gene: p.Y408fsX420 and p.V605L. These mutations differ from those in Caucasian patients, who commonly harbor p.S113L, p.P50H, and p.Q413fsX449 mutations; therefore, our data and those of other Japanese groups suggest that the p.F383Y mutation is significant in Japanese patients with CPT II deficiency.

[Medically induced myopathia]

Muscular side effects of various anesthetics, analgetics, antibiotics, antihistaminic drugs, antiretrovirals, cardiotropics, immunosuppressants, lipid-lowering drugs, psychotropic drugs, anticancer drugs, and other substances are more frequent than assumed and are easily overlooked. Clinically, muscular side effects manifest as fatigue, myalgias, persistent or transient weakness, stiffness, intolerance to exercise, psychomotor slowing, muscle cramps, wasting, dyspnea, dysphagia, fasciculations, reduced tendon reflexes, impaired consciousness, myoglobinuria, renal failure, or hyperthermia. Diagnosis of these drug-induced myopathies is based on history, clinical neurologic examination, blood work, urine analysis, repetitive stimulation, electromyography, and muscle biopsy. A drug which induces muscular side effects should never be given again. Particularly in patients suffering from primary myopathy, myotoxic drugs should be applied with caution. The drugs which most frequently induce muscular side effects are steroids, statins, fibrates, antiretrovirals, immunosuppressants, colchicine, amiodarone, and anticancer drugs. Many drugs exhibit their myotoxic potential only in combination with other drugs or premorbid pathologic myogenic conditions.

Carnitine palmitoyltransferase II deficiency: a clinical, biochemical, and molecular review

Congenital deficiency of carnitine palmitoyltransferase (CPT) II has been known for at least 30 years now, and its phenotypic variability remains fascinating. Three distinct clinical entities have been described, the adult, the infantile, and the perinatal, all with an autosomal recessive inheritance pattern. The adult CPT II clinical phenotype is somewhat benign and requires additional external triggers such as high-intensity exercise before the predominantly myopathic symptoms are elicited. The perinatal and infantile forms involve multiple organ systems. The perinatal disease is the most severe form and is invariably fatal. The introduction of mass spectrometry to analyze blood acylcarnitine profiles has revolutionized the diagnosis of fatty acid oxidation disorders including CPT II deficiency. Its use in expanded neonatal screening programs has made presymptomatic diagnosis a reality. An increasing number of mutations are being identified in the CPT II gene with a distinct genotype-phenotype correlation in most cases. However, clinical variability in some patients suggests additional genetic or environmental modifiers. Herein, we present a new case of lethal perinatal CPT II deficiency with a rare missense mutation, R296Q (907G>A) associated with a previously described 25-bp deletion on the second allele. We review the clinical features, the diagnostic protocol including expanded neonatal screening, the treatment, and the biochemical and molecular basis of CPT II deficiency.

Iatrogenic and toxic myopathies

There has been increasing awareness of the adverse effects of therapeutic agents and exogenous toxins on the structure and function of muscle. The resulting clinical syndrome varies from one characterized by muscle pain to profound myalgia, paralysis, and myoglobinuria. Because toxic myopathies are potentially reversible, their prompt recognition may reduce their damaging effects or prevent a fatal outcome. Interest in the toxic myopathies, however, derives not only from their clinical importance but also from the fact that they serve as useful experimental models in muscle research. Morphological and biochemical studies have increased our understanding of the basic cellular mechanisms of myotoxicity. Toxins may produce, for instance, necrotizing, lysosomal-related, inflammatory, anti-microtubular, mitochondrial, hypokalemia-related, or protein synthesis-related muscle damage.

Metabolic and drug-induced muscle disorders

PURPOSE OF REVIEW

The inherited disorders of muscle metabolism affect both substrate utilization and the final intramitochondrial oxidation through the Krebs cycle and the respiratory chain. Almost every step of these complex biochemical pathways can be affected by inborn errors, whose expression depends on peculiar tissue-specific or systemic gene expression. This review updates current knowledge in this broad field.

RECENT FINDINGS

New inherited defects are still being discovered, such as the beta-enolase deficiency in glycogenosis type XIII and mutations in the gene encoding an esterase/lipase/thioesterase protein in Chanarin-Dorfman syndrome, a multisystem triglyceride storage disease.

SUMMARY

Therapeutic approaches to the metabolic myopathies are still lagging behind, although remarkable observations have been made on the rare coenzyme Q10 deficiency syndrome. However, transgenic animal models may offer the opportunity both to investigate muscle pathogenesis and explore therapeutic targets. Finally, human myotoxicity may provide novel paradigms for naturally occurring muscle disorders.