Mitochondrial myopathies and the role of the pathologist in the molecular era

Cellular and Molecular Responses to Mitochondrial DNA Deletions in Kearns-Sayre Syndrome: Some Underlying Mechanisms

Kearns-Sayre syndrome (KSS) is a rare multisystem mitochondrial disorder. It is caused by mitochondrial DNA (mtDNA) rearrangements, mostly large-scale deletions of 1.1-10 kb. These deletions primarily affect energy supply through impaired oxidative phosphorylation and reduced ATP production. This impairment gives rise to dysfunction of several tissues, in particular those with high energy demand like brain and muscles. Over the past decades, changes in respiratory chain complexes and energy metabolism have been emphasized, whereas little attention has been paid to other reports on ROS overproduction, protein synthesis inhibition, myelin vacuolation, demyelination, autophagy, apoptosis, and involvement of lipid raft and oligodendrocytes in KSS. Therefore, this paper draws attention towards these relatively underemphasized findings that might further clarify the pathologic cascades following deletions in the mtDNA.

Unexplained dyspnea linked to mitochondrial myopathy following military deployment to Southwest Asia and Afghanistan

We identified a case of probable mitochondrial myopathy (MM) in a soldier with dyspnea and reduced exercise tolerance through cardiopulmonary exercise testing (CPET) following Southwest Asia (SWA) deployment. Muscle biopsy showed myopathic features. We compared demographic, occupational exposure, and clinical characteristics in symptomatic military deployers with and without probable MM diagnosed by CPET criteria. We evaluated 235 symptomatic military personnel who deployed to SWA and/or Afghanistan between 2010 and 2021. Of these, 168 underwent cycle ergometer maximal CPET with an indwelling arterial line. We defined probable MM based on five CPET criteria: arterial peak exercise lactate >12 mEq/L, anaerobic threshold (AT) ≤50%, maximum oxygen consumption (VO2max ) <95% predicted, oxygen (O2) pulse percent predicted (pp) at least 10% lower than heart rate pp, and elevated ventilatory equivalent for O2 at end exercise (VE/VO2 ≥ 40). We characterized demographics, smoking status/pack-years, spirometry, and deployment exposures, and used descriptive statistics to compare findings in those with and without probable MM. We found 9/168 (5.4%) deployers with probable MM. Compared to symptomatic deployers without probable MM, they were younger (p = 0.0025) and had lower mean BMI (p = 0.02). They had a higher mean forced expiratory volume (FEV1)pp (p = 0.02) and mean arterial oxygen partial pressure (PaO2) at maximum exercise (p = 0.0003). We found no significant differences in smoking status, deployment frequency/duration, or inhalational exposures. Our findings suggest that mitochondrial myopathy may be a cause of dyspnea and reduced exercise tolerance in a subset of previously deployed military personnel. CPET with arterial line may assist with MM diagnosis and management.

A Study of Histopathologic Evaluation and Clinical Correlation for Isolated Congenital Myogenic Ptosis and Aponeurotic Ptosis

PURPOSE

To evaluate light microscopy and transmission electron microscopy findings of levator muscle/aponeurosis materials and their correlation with clinical findings in isolated congenital myogenic and aponeurotic blepharoptosis.

METHODS

Demographic and clinical data were obtained from patients. Qualitative and quantitative evaluations for muscle fiber morphology were performed using light microscopy and transmission electron microscopy on tissue samples which were obtained from the most proximal part of the aponeurosis excised during levator muscle/aponeurosis resection surgery.

RESULTS

Seventeen (55%) of the cases were isolated congenital myogenic ptosis, and 14 (45%) were aponeurotic ptosis. Muscle bundle splitting, cytoplasmic loss, and centrally located nuclei were observed in both groups. Muscle tissue covered 25% of the sample in 67% of the cases, 50% in 11%, 75% in 11%, and 100% in 11% in the myogenic group. In the aponeurotic group, muscle tissue covered 25% of the sample in 44.5% of the cases, 50% in 11%, and 100% in 44.5% (χ, p = 0.52). Myofibrillar loss areas accompanied by Z-line disorganization which were occupied by degenerated organelles were present in both groups under transmission electron microscopy, and findings were not significantly different between groups (χ, p > 0.05). Mean mitochondrial diameter was significantly larger in aponeurotic ptosis (Mann-Whitney U, p = 0.047). No correlation was found between functional and microscopic parameters.

CONCLUSION

Decreased amount of striated muscle and the presence of fiber damage indicators were observed in both groups. Muscle fiber loss in myogenic ptosis may be a feature of muscle dysgenesis. Ultrastructural damage in aponeurotic ptosis may be explained with increased oxidative stress or long-term contractile stress. Further genetic and immunohistochemical studies will be helpful to further understand the pathogenesis of diseases.

Mitochondrial dysfunction in human skeletal muscle biopsies of lipid storage disorder

Mitochondria regulate the balance between lipid metabolism and storage in the skeletal muscle. Altered lipid transport, metabolism and storage influence the bioenergetics, redox status and insulin signalling, contributing to cardiac and neurological diseases. Lipid storage disorders (LSDs) are neurological disorders which entail intramuscular lipid accumulation and impaired mitochondrial bioenergetics in the skeletal muscle causing progressive myopathy with muscle weakness. However, the mitochondrial changes including molecular events associated with impaired lipid storage have not been completely understood in the human skeletal muscle. We carried out morphological and biochemical analysis of mitochondrial function in muscle biopsies of human subjects with LSDs (n = 7), compared to controls (n = 10). Routine histology, enzyme histochemistry and ultrastructural analysis indicated altered muscle cell morphology and mitochondrial structure. Protein profiling of the muscle mitochondria from LSD samples (n = 5) (vs. control, n = 5) by high-throughput mass spectrometric analysis revealed that impaired metabolic processes could contribute to mitochondrial dysfunction and ensuing myopathy in LSDs. We propose that impaired fatty acid and respiratory metabolism along with increased membrane permeability, elevated lipolysis and altered cristae entail mitochondrial dysfunction in LSDs. Some of these mechanisms were unique to LSD apart from others that were common to dystrophic and inflammatory muscle pathologies. Many differentially regulated mitochondrial proteins in LSD are linked with other human diseases, indicating that mitochondrial protection via targeted drugs could be a treatment modality in LSD and related metabolic diseases. Cover Image for this Issue: doi: 10.1111/jnc.14177.

Dai-Zong-Fang, A Traditional Chinese Herbal Formula, Ameliorates Insulin Resistance in db/db Mice

Intricate health problems, such as insulin resistance (IR) and its associated diseases, call for multi-targeted therapies with few side effects. Based on traditional Chinese medicine (TCM), Dai-Zong-Fang (DZF) is an herbal formula mainly composed of Rhizoma Coptidis (Huanglian) and Fructus Aurantii Immaturus (Zhishi), of which berberine and naringin are the main constituents. Though DZF has been clinically used for treatment of IR and metabolic syndrome for decades, its mechanism in vivo remains unknown. In the present study, we observed that both DZF and metformin, the first-line drug for type 2 diabetes, ameliorated insulin resistance with significant improvement of oral glucose tolerance test (OGTT) and homeostasis model assessment of IR (HOMA-IR) level in diabetic C57BL/Ksj-Lepr db^−/− (db/db) mice. Low-density lipoprotein cholesterol (LDL-C) and fatty acids (FAs) also decreased in the blood. Higher dose of DZF (1 g·kg⁻¹), but not metformin (0.25 g·kg⁻¹), alleviated hepatic steatosis with reduced liver weight and hepatic lipid accumulation and provided protection from hepatic injury with lower alanine aminotransferase and aspartate aminotransferase and increased hepatic superoxide dismutase activity in db/db mice. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) showed a decrease in FA synthase gene (Fasn) and an increase in FA oxidation gene Ppara expression. Western blot demonstrated that both DZF and metformin activated 5′ AMP-activated protein kinase (AMPK) but inhibited Notch intracellular domain (NICD) and Hairy/enhancer-of-split 1 (Hes1) of Notch signaling pathway in the liver. DZF also dramatically improved the ultrastructure of skeletal muscles, AMPK phosphorylation, and GLUT4 translocation. DZF also promoted FA transport and oxidation with Cd36 and Cpt1b up-regulation in the skeletal muscle. In conclusion, DZF improves insulin sensitivity by reducing hepatic lipids through AMPK activation and Notch signal pathway inhibition and enhancing energy metabolism in the skeletal muscle via AMPK. This study provides insights into the treatment of complex conditions, such as IR, where TCM herbal formulas exert multipronged effects through correlating pathways.

Case report: perioperative management of caesarean section for a parturient with mitochondrial myopathy

Background

Mitochondrial myopathies represent a group of disorders caused by mitochondrial defects that disrupt energy production. Most patients have issues from infancy to early childhood. Pregnancy in women with mitochondrial myopathy is uncommon and the management for these parturients is full of challenges.

Case presentation

A 36-year-old woman with mitochondrial myopathy was scheduled for caesarean section under a combined spinal-epidural anaesthesia and multi-model analgesia. The parturient was safe and the delivery was performed safely and smoothly, but there were some complications after surgery due to the complex condition of the patient. After consultation with and treatment from multiple disciplines, both the parturient and neonate were well and discharged.

Conclusion

It is important that patients with mitochondrial diseases are comprehensively assessed and monitored perioperatively.

Myopathology of Adult and Paediatric Mitochondrial Diseases

Mitochondria are dynamic organelles ubiquitously present in nucleated eukaryotic cells, subserving multiple metabolic functions, including cellular ATP generation by oxidative phosphorylation (OXPHOS). The OXPHOS machinery comprises five transmembrane respiratory chain enzyme complexes (RC). Defective OXPHOS gives rise to mitochondrial diseases (mtD). The incredible phenotypic and genetic diversity of mtD can be attributed at least in part to the RC dual genetic control (nuclear DNA (nDNA) and mitochondrial DNA (mtDNA)) and the complex interaction between the two genomes. Despite the increasing use of next-generation-sequencing (NGS) and various omics platforms in unravelling novel mtD genes and pathomechanisms, current clinical practice for investigating mtD essentially involves a multipronged approach including clinical assessment, metabolic screening, imaging, pathological, biochemical and functional testing to guide molecular genetic analysis. This review addresses the broad muscle pathology landscape including genotype–phenotype correlations in adult and paediatric mtD, the role of immunodiagnostics in understanding some of the pathomechanisms underpinning the canonical features of mtD, and recent diagnostic advances in the field.

Clinical Neuropathology practice guide 3-2014: combined nerve and muscle biopsy in the diagnostic workup of neuropathy - the Bordeaux experience

Simultaneous combined superficial peroneal nerve and peroneous brevis muscle biopsy, via the same cutaneous incision, allows examination of several tissue specimens and significantly improves the diagnosis of systemic diseases with peripheral nerve involvement. Vasculitides are certainly the most frequently diagnosed on neuro-muscular biopsies, but this procedure is also well advised to asses a diagnosis of sarcoidosis or amyloidosis. More occasionally, combined nerve and muscle biopsy may reveal an unpredicted diagnosis of cholesterol embolism, intra-vascular lymphoma, or enables complementary diagnosis investigations on mitochondrial cytopathy or storage disease.

Insulin resistance and impaired mitochondrial function in obese adolescent girls

BACKGROUND

Mitochondrial dysfunction plays a role in the development of muscle insulin resistance (IR) and the accumulation of intramyocellular lipid (IMCL) in skeletal muscle that can, in turn, interfere with insulin signaling. The purpose of this study was to assess mitochondrial function (MF) and IMCL in obese adolescent girls with and without IR to determine whether: (1) Girls with IR have impaired MF, and (2) impaired MF in girls with IR is related to higher IMCL.

METHODS

We examined 22 obese girls aged 13-21 years old for IR [defined as a homeostasis model assessment of insulin resistance (HOMA-IR) value >4. Phosphorus magnetic resonance spectroscopy (31P-MRS) and proton magnetic resonance spectroscopy (1H-MRS), respectively, were used to determine MF and IMCL of the soleus muscle along with magnetic resonance imaging (MRI) measures of visceral, subcutaneous, and total adipose tissue (VAT, SAT, and TAT) in girls with HOMA-IR >4 (insulin-resistant group) versus HOMA-IR ≤ 4 (insulin-sensitive group). Serum lipids and waist-to-hip ratio (W/H) were also measured.

RESULTS

Girls with IR (n=8) did not differ from the insulin-sensitive group (n=14) for age, bone age, weight, VAT, SAT, TAT, or IMCL. However, the insulin-resistant group had higher W/H. Additionally the insulin-resistance group had a lower log rate of postexercise phosphocreatine (PCr) recovery (ViPCr) and a higher log PCr recovery constant (tau), indicative of impaired MF.

CONCLUSIONS

Obese girls with increased IR have impaired mitochondrial function. This association is not mediated by alterations in IMCL or adipose tissue. Further studies are necessary to determine whether there is a causal relation between impaired mitochondrial function and IR in obesity and mediators of such a relationship.

Mice lacking TR4 nuclear receptor develop mitochondrial myopathy with deficiency in complex I

The estimated incidence of mitochondrial diseases in humans is approximately 1:5000 to 1:10,000, whereas the molecular mechanisms for more than 50% of human mitochondrial disease cases still remain unclear. Here we report that mice lacking testicular nuclear receptor 4 (TR4(-/-)) suffered mitochondrial myopathy, and histological examination of TR4(-/-) soleus muscle revealed abnormal mitochondrial accumulation. In addition, increased serum lactate levels, decreased mitochondrial ATP production, and decreased electron transport chain complex I activity were found in TR4(-/-) mice. Restoration of TR4 into TR4(-/-) myoblasts rescued mitochondrial ATP generation capacity and complex I activity. Further real-time PCR quantification and promoter studies found TR4 could modulate complex I activity via transcriptionally regulating the complex I assembly factor NDUFAF1, and restoration of NDUFAF1 level in TR4(-/-) myoblasts increased mitochondrial ATP generation capacity and complex I activity. Together, these results suggest that TR4 plays vital roles in mitochondrial function, which may help us to better understand the pathogenesis of mitochondrial myopathy, and targeting TR4 via its ligands/activators may allow us to develop better therapeutic approaches.

Current molecular diagnostic algorithm for mitochondrial disorders

Mitochondrial respiratory chain disorders (RCD) are a group of genetically and clinically heterogeneous diseases, due in part to the biochemical complexity of mitochondrial respiration and the fact that two genomes, one mitochondrial and one nuclear, encode the components of the respiratory chain. Because of the large number of genes involved, attempts to classify mitochondrial RCD incorporate clinical, biochemical, and histological criteria, in addition to DNA-based molecular diagnostic testing. While molecular testing is widely viewed as definitive, confirmation of the diagnosis by molecular methods often remains a challenge because of the large number of genes, the two genome complexity and the varying proportions of pathogenic mitochondrial DNA (mtDNA) molecules in a patient, a concept termed heteroplasmy. The selection of genes to be analyzed depends on the family history and clinical, biochemical, histopathological, and imaging results, as well as the availability of different tissues for analysis. Screening of common point mutations and large deletions in mtDNA is typically the first step. In cases where tissue-specific, recognizable clinical syndromes or characteristic RC complex deficiencies and histochemical abnormalities are observed, direct sequencing of the specific causative nuclear gene(s) can be performed on white blood cell DNA. Measurement of mtDNA content in affected tissues such as muscle and liver allows screening for mtDNA depletion syndromes. The ever-expanding list of known disease-causing genes will undoubtedly improve diagnostic accuracy and genetic counseling.

Morphological findings of extraocular myopathy with chronic progressive external ophthalmoplegia

Mitochondrial diseases are a large group of disorders resulting from mutations of nuclear DNA (nDNA) and mitochondrial DNA (mtDNA). Patients present clinically with multiple manifestations, including myopathies and multiple system disorders. Establishing a specific diagnosis often requires extensive clinical and laboratory evaluation. In this study of 2 adult patients with presumptive mitochondrial disease, the authors have identified distinctive morphological changes in medial rectus muscle biopsies that confirm the diagnosis of chronic progressive external ophthalmoplegia (CPEO). These findings demonstrate the usefulness of electron microscopy using medial rectus muscle in the diagnosis of adult patients with a slowly progressive course of mild skeletal weakness and CPEO.

Reactive oxygen species act remotely to cause synapse loss in a Drosophila model of developmental mitochondrial encephalopathy

Mitochondrial dysfunction is a hallmark of many neurodegenerative diseases, yet its precise role in disease pathology remains unclear. To examine this link directly, we subtly perturbed electron transport chain function in the Drosophila retina, creating a model of Leigh Syndrome, an early-onset neurodegenerative disorder. Using mutations that affect mitochondrial complex II, we demonstrate that mild disruptions of mitochondrial function have no effect on the initial stages of photoreceptor development, but cause degeneration of their synapses and cell bodies in late pupal and adult animals. In this model, synapse loss is caused by reactive oxygen species (ROS) production, not energy depletion, as ATP levels are normal in mutant photoreceptors, and both pharmacological and targeted genetic manipulations that reduce ROS levels prevent synapse degeneration. Intriguingly, these manipulations of ROS uncouple synaptic effects from degenerative changes in the cell body, suggesting that mitochondrial dysfunction activates two genetically separable processes, one that induces morphological changes in the cell body, and another that causes synapse loss. Finally, by blocking mitochondrial trafficking into the axon using a mutation affecting a mitochondrial transport complex, we find that ROS action restricted to the cell body is sufficient to cause synaptic degeneration, demonstrating that ROS need not act locally at the synapse. Thus, alterations in electron transport chain function explain many of the neurodegenerative changes seen in both early- and late-onset disorders.

Macrophagic myofasciitis in children is a localized reaction to vaccination

Macrophagic myofasciitis is a novel, "inflammatory myopathy" described after a variety of vaccinations, almost exclusively in adults. We examined the relevance of histological findings of this myopathy to the clinical presentation in pediatric patients. Muscle biopsies from 8 children (7 months to 6 years old) with histological features of macrophagic myofasciitis were reviewed and correlated with the clinical manifestations. Patients underwent quadriceps muscle biopsy for suspected mitochondrial disease (4 patients), spinal muscular atrophy (2 patients), myoglobinuria (1 patient), and hypotonia with motor delay (1 patient). All biopsies showed identical granulomas composed of periodic acid-Schiff-positive and CD68-positive macrophages. Characteristic aluminum hydroxide crystals were identified by electron microscopy in 2 cases. The biopsy established diagnoses other than macrophagic myofasciitis in 5 patients: spinal muscular atrophy (2), Duchenne muscular dystrophy (1), phospho-glycerate kinase deficiency (1), and cytochrome c oxidase deficiency (1). Three children with manifestations and/or a family history of mitochondrial disease had otherwise morphologically normal muscle. All children had routine vaccinations between 2 months and 1 year before the biopsy, with up to 11 intramuscular injections, including the biopsy sites. There was no correlation between histological findings of macrophagic myofasciitis in biopsies and the clinical symptoms. We believe that macrophagic myofasciitis represents a localized histological hallmark of previous immunization with the aluminum hydroxide adjuvants contained in vaccines, rather than a primary or distinct inflammatory muscle disease.

Light and electron microscopy characteristics of the muscle of patients with SURF1 gene mutations associated with Leigh disease

AIMS

Leigh syndrome (LS) is characterised by almost identical brain changes despite considerable causal heterogeneity. SURF1 gene mutations are among the most frequent causes of LS. Although deficiency of cytochrome c oxidase (COX) is a typical feature of the muscle in SURF1-deficient LS, other abnormalities have been rarely described. The aim of the present work is to assess the skeletal muscle morphology coexisting with SURF1 mutations from our own research and in the literature.

METHODS

Muscle samples from 21 patients who fulfilled the criteria of LS and SURF1 mutations (14 homozygotes and 7 heterozygotes of c.841delCT) were examined by light and electron microscopy.

RESULTS

Diffuse decreased activity or total deficit of COX was revealed histochemically in all examined muscles. No ragged red fibres (RRFs) were seen. Lipid accumulation and fibre size variability were found in 14 and 9 specimens, respectively. Ultrastructural assessment showed several mitochondrial abnormalities, lipid deposits, myofibrillar disorganisation and other minor changes. In five cases no ultrastructural changes were found. Apart from slight correlation between lipid accumulation shown by histochemical and ultrastructural techniques, no other correlations were revealed between parameters investigated, especially between severity of morphological changes and the patient's age at the biopsy.

CONCLUSION

Histological and histochemical features of muscle of genetically homogenous SURF1-deficient LS were reproducible in detection of COX deficit. Minor muscle changes were not commonly present. Also, ultrastructural abnormalities were not a consistent feature. It should be emphasised that SURF1-deficient muscle assessed in the light and electron microscopy panel may be interpreted as normal if COX staining is not employed.

Electron microscopic findings in levator muscle biopsies of patients with isolated congenital or acquired ptosis

OBJECTIVE

Systemic mitochondriopathies as chronic progressive external ophthalmoplegia (CPEO) are frequently associated with ptosis. We investigated whether mitochondrial abnormalities in the levator muscle are also found in patients with isolated congenital or acquired ptosis showing no other signs of mitochondrial cytopathy.

METHODS

Biopsies of levator muscle were taken during surgery from 24 patients with isolated congenital (group 1) or early-onset acquired ptosis (group 2). All patients were given a thorough clinical examination before and after surgery. Ultrathin muscle sections were examined by transmission electron microscopy. The findings were compared with biopsies from five patients with CPEO (positive control) and two patients with traumatic ptosis or pseudoptosis (negative control).

RESULTS

The mean levator function equalled 7.3 mm (range 4-10 mm) in group 1 and 12.8 mm (range 9-15 mm) in group 2. Eight out of 11 patients in group 1 and eight out of 13 patients in group 2 were found to have mitochondrial alterations such as megamitochondria, mitochondrial matrix alterations and abnormal cristae, similar to CPEO. Within group 1 and 2, no significant clinical differences were found between patients with and without mitochondrial abnormalities.

CONCLUSION

Mitochondrial alterations were found in a surprisingly large proportion of levator biopsies from patients with isolated congenital or early-onset acquired ptosis. There was no statistically significant correlation between mitochondrial alterations and levator function. Our findings suggest that the ultrastructural assessment of mitochondria in the eyelid muscle is a valuable tool, and may guide further biochemical and mutation screening tests that will help to understand the etiopathology of this disease.

Investigation of children for mitochondriopathy confirms need for strict patient selection, improved morphological criteria, and better laboratory methods

We studied muscle biopsies of 103 pediatric patients in whom clinical suspicion for disorder of energy metabolism was highest in 13 patients, intermediate in 8 patients, and lowest in 82 patients. Electron transport complex (ETC) enzyme activity measurements were available in 96 of 103 patients. Most children with unclassified encephalopathy before biopsy had negative or equivocal morphological and biochemical evaluation for disorder of energy metabolism (72/85). The incidence of ETC abnormality and morphological abnormality in muscle from 39 patients with clinical encephalomyopathy (groups I, II, and III) was 20% and 38%, respectively. In 21 children with high or intermediate clinical suspicion of mitochondriopathy, light microscopy was confirmative in 12, ultrastructure was confirmative in 15, and major ETC abnormality was present in only 4 (29%) of 14. In 82 children with lower clinical suspicion of mitochondriopathy, morphological criteria at both the light and electron microscopic level were absent, and major abnormality of ETC activity was uncommon, in 9 (11%) of 82. Partial reductions of ETC activity occurred in 15 (18%) of 82, but are of uncertain significance. Ragged blue fibers were more prevalent in infants with mitochondriopathy than ragged red fibers. Increase of large, but not small, subsarcolemmal mitochondrial aggregates based on succinate dehydrogenase histochemistry is a useful indicator for mitochondriopathy. Thus, a distinction should be made between small aggregates (normal) and large aggregates. Using strict criteria to define pathological mitochondria, we concluded that electron microscopy is a powerful tool in the diagnosis of mitochondriopathy mainly when clinical suspicion is high. We found no consistent difference in the frequency of mitochondrial "proliferation" as currently defined or in citrate synthase activity in any group. Better patient selection in infants and children and better methods for investigation of mitochondriopathy are needed.

Pathology of mitochondrial encephalomyopathies

Muscle biopsy provides the best tissue to confirm a mitochondrial cytopathy. Histochemical features often correlate with specific syndromes and facilitate the selection of biochemical and genetic studies. Ragged-red fibres nearly always indicate a combination defect of respiratory complexes I and IV. Increased punctate lipid within myofibers is a regular feature of Kearns-Sayre and PEO, but not of MELAS and MERRF. Total deficiency of succinate dehydrogenase indicates a severe defect in Complex II; total absence of cytochrome-c-oxidase activity in all myofibres correlates with a severe deficiency of Complex IV or of coenzyme-Q10. The selective loss of cytochrome-c-oxidase activity in scattered myofibers, particularly if accompanied by strong succinate dehydrogenase staining in these same fibres, is good evidence of mitochondrial cytopathy and often of a significant mtDNA mutation, though not specific for Complex IV disorders. Glycogen may be excessive in ragged-red zones. Ultrastructure provides morphological evidence of mitochondrial cytopathy, in axons and endothelial cells as well as myocytes. Abnormal axonal mitochondria may contribute to neurogenic atrophy of muscle, a secondary chronic feature. Quantitative determinations of respiratory chain enzyme complexes, with citrate synthase as an internal control, confirm the histochemical impressions or may be the only evidence of mitochondrial disease. Biological and technical artifacts may yield falsely low enzymatic activities. Genetic studies screen common point mutations in mtDNA. The brain exhibits characteristic histopathological alterations in mitochondrial diseases. Skin biopsy is useful for mitochondrial ultrastructure in smooth erector pili muscles and axons; skin fibroblasts may be grown in culture. Mitochondrial alterations occur in many nonmitochondrial diseases and also may be induced by drugs and toxins.

Clinical spectrum, morbidity, and mortality in 113 pediatric patients with mitochondrial disease

OBJECTIVES

The aim of this study was to elucidate the frequency of major clinical manifestations in children with mitochondrial disease and establish their clinical course, prognosis, and rates of survival depending on their clinical features.

METHODS

We performed a retrospective review of the medical records of 400 patients who were referred for evaluation of mitochondrial disease. By use of the modified Walker criteria, only patients who were assigned a definite diagnosis were included in the study.

RESULTS

A total of 113 pediatric patients with mitochondrial disease were identified. A total of 102 (90%) patients underwent a muscle biopsy as part of the diagnostic workup. A significant respiratory chain (RC) defect, according to the diagnostic criteria, was found in 71% of the patients who were evaluated. In this cohort, complex I deficiency (32%) and combined complex I, III, and IV deficiencies (26%) were the most common causes of RC defects, followed by complex IV (19%), complex III (16%), and complex II deficiencies (7%). Pathogenic mitochondrial DNA abnormalities were found in 11.5% of the patients. A substantial fraction (40%) of patients with mitochondrial disorders exhibited cardiac disease, diagnosed by Doppler echocardiography; however, the majority (60%) of patients had predominant neuromuscular manifestations. No correlation between the type of RC defect and the clinical presentation was observed. Overall, the mean age at presentation was 40 months. However, the mean age at presentation was 33 months in the cardiac group and 44 months in the noncardiac group. Twenty-six (58%) patients in the cardiac group exhibited hypertrophic cardiomyopathy, 29% had dilated cardiomyopathy, and the remainder (13%) had left ventricular noncompaction. Patients with cardiomyopathy had an 18% survival rate at 16 years of age. Patients with neuromuscular features but no cardiomyopathy had a 95% survival at the same age.

CONCLUSIONS

This study gives strong support to the view that in patients with RC defects, cardiomyopathy is more common than previously thought and tends to follow a different and more severe clinical course. Although with a greater frequency than previously reported, mitochondrial DNA mutations were found in a minority of patients, emphasizing that most mitochondrial disorders of childhood follow a Mendelian pattern of inheritance.

The Cytochromebc1Complex: Function in the Context of Structure

The bc1 complexes are intrinsic membrane proteins that catalyze the oxidation of ubihydroquinone and the reduction of cytochrome c in mitochondrial respiratory chains and bacterial photosynthetic and respiratory chains. The bc1 complex operates through a Q-cycle mechanism that couples electron transfer to generation of the proton gradient that drives ATP synthesis. Genetic defects leading to mutations in proteins of the respiratory chain, including the subunits of the bc1 complex, result in mitochondrial myopathies, many of which are a direct result of dysfunction at catalytic sites. Some myopathies, especially those in the cytochrome b subunit, exacerbate free-radical damage by enhancing superoxide production at the ubihydroquinone oxidation site. This bypass reaction appears to be an unavoidable feature of the reaction mechanism. Cellular aging is largely attributable to damage to DNA and proteins from the reactive oxygen species arising from superoxide and is a major contributing factor in many diseases of old age. An understanding of the mechanism of the bc1 complex is therefore central to our understanding of the aging process. In addition, a wide range of inhibitors that mimic the quinone substrates are finding important applications in clinical therapy and agronomy. Recent structural studies have shown how many of these inhibitors bind, and have provided important clues to the mechanism of action and the basis of resistance through mutation. This paper reviews recent advances in our understanding of the mechanism of the bc1 complex and their relation to these physiologically important issues in the context of the structural information available.

The utility of muscle biopsy

Despite major advances in molecular genetics, histopathologic evaluation of muscle biopsy specimens continues to provide important diagnostic information in patients with suspected muscle diseases and in patients with vasculitic neuropathies. Muscle biopsy specimens are used in diagnosing many inherited as well as inflammatory and toxic myopathies. Furthermore, the study of muscle histopathology can also enhance our understanding of disease pathogenesis.

Probing striated muscle mitochondrial phenotype in neuromuscular disorders

Multisystemic disorders with predominantly neurologic manifestations often present with mitochondrial abnormalities in striated muscle biopsies. Decreased respiratory complex activities and abnormalities in mitochondrial structure and DNA constitute the spectrum of mitochondrial changes used as diagnostic and prognostic indicators in patients with neuromuscular disorders. This study assessed mitochondrial defects present in a cohort of 154 young patients to determine diagnostic efficiency and probe the relationship of mitochondrial to clinical phenotype. Striated muscle biopsies were analyzed for mitochondrial structure and number, levels of enzyme activities of complex I-V and citrate synthase, mitochondrial DNA and specific mitochondrial DNA deletions, and presence of 15 pathogenic mitochondrial DNA point mutations. Reduced complex I, III, IV, and V activities were the most ubiquitous finding, with complex III most commonly affected. Mitochondrial structural defects (39%) included changes in mitochondria sizes/shapes and number and aberrant cristae formation. Mitochondrial DNA deletions were evident in 15 patients, three displayed mitochondrial DNA depletion, and only two harbored pathogenic point mutations. Reductions in specific enzyme activities may be the most sensitive diagnostic indicator, whereas defects in ultrastructure and mitochondrial DNA integrity were frequently accompanied by the full spectrum of mitochondrial abnormalities. Some phenotypes displayed specific mitochondrial abnormalities; however, most clinical phenotypes displayed little specificity with regard to mitochondrial phenotype.

Dysfunction of mitochondria in human skeletal muscle in type 2 diabetes

Skeletal muscle is strongly dependent on oxidative phosphorylation for energy production. Because the insulin resistance of skeletal muscle in type 2 diabetes and obesity entails dysregulation of the oxidation of both carbohydrate and lipid fuels, the current study was undertaken to examine the potential contribution of perturbation of mitochondrial function. Vastus lateralis muscle was obtained by percutaneous biopsy during fasting conditions from lean (n = 10) and obese (n = 10) nondiabetic volunteers and from volunteers with type 2 diabetes (n = 10). The activity of rotenone-sensitive NADH:O(2) oxidoreductase, reflecting the overall activity of the respiratory chain, was measured in a mitochondrial fraction by a novel method based on providing access for NADH to intact mitochondria via alamethicin, a channel-forming antibiotic. Creatine kinase and citrate synthase activities were measured as markers of myocyte and mitochondria content, respectively. Activity of rotenone-sensitive NADH:O(2) oxidoreductase was normalized to creatine kinase activity, as was citrate synthase activity. NADH:O(2) oxidoreductase activity was lowest in type 2 diabetic subjects and highest in the lean volunteers (lean 0.95 +/- 0.17, obese 0.76 +/- 0.30, type 2 diabetes 0.56 +/- 0.14 units/mU creatine kinase; P < 0.005). Also, citrate synthase activity was reduced in type 2 diabetic patients (lean 3.10 +/- 0.74, obese 3.24 +/- 0.82, type 2 diabetes 2.48 +/- 0.47 units/mU creatine kinase; P < 0.005). As measured by electron microscopy, skeletal muscle mitochondria were smaller in type 2 diabetic and obese subjects than in muscle from lean volunteers (P < 0.01). We conclude that there is an impaired bioenergetic capacity of skeletal muscle mitochondria in type 2 diabetes, with some impairment also present in obesity.

Mitochondrial dysfunction in patients with hypotonia, epilepsy, autism, and developmental delay: HEADD syndrome

A group of 12 children clinically presenting with hypotonia, intractable epilepsy, autism, and developmental delay, who did not fall into previously described categories of mitochondrial encephalomyopathy, were evaluated for mitochondrial respiratory enzyme activity levels, mitochondrial DNA, and mitochondrial structural abnormalities. Reduced levels in specific respiratory activities were found solely in enzymes with subunits encoded by mitochondrial DNA in seven of eight biopsied skeletal muscle specimens evaluated. Five cases exhibited increased levels of large-scale mitochondrial DNA deletions, whereas pathogenic point mutations previously described in association with mitochondrial encephalomyopathies were not found. Mitochondrial structural abnormalities were present in three of four patients examined. Our findings suggest that mitochondrial dysfunction, including extensive abnormalities in specific enzyme activities, mitochondrial structure, and mitochondrial DNA integrity, may be present in children with a clinical constellation including hypotonia, epileptic seizures, autism, and developmental delay. The acronym HEADD is presented here to facilitate pursuit of mitochondrial defects in patients with this clinical constellation after other causes have been excluded.