Effects of fasting, feeding and exercise on plasma acylcarnitines among subjects with CPT2D, VLCADD and LCHADD/TFPD

BACKGROUND

The plasma acylcarnitine profile is frequently used as a biochemical assessment for follow-up in diagnosed patients with fatty acid oxidation disorders (FAODs). Disease specific acylcarnitine species are elevated during metabolic decompensation but there is clinical and biochemical heterogeneity among patients and limited data on the utility of an acylcarnitine profile for routine clinical monitoring.

METHODS

We evaluated plasma acylcarnitine profiles from 30 diagnosed patients with long-chain FAODs (carnitine palmitoyltransferase-2 (CPT2), very long-chain acyl-CoA dehydrogenase (VLCAD), and long-chain 3-hydroxy acyl-CoA dehydrogenase or mitochondrial trifunctional protein (LCHAD/TFP) deficiencies) collected after an overnight fast, after feeding a controlled low-fat diet, and before and after moderate exercise. Our purpose was to describe the variability in this biomarker and how various physiologic states effect the acylcarnitine concentrations in circulation.

RESULTS

Disease specific acylcarnitine species were higher after an overnight fast and decreased by approximately 60% two hours after a controlled breakfast meal. Moderate-intensity exercise increased the acylcarnitine species but it varied by diagnosis. When analyzed for a genotype/phenotype correlation, the presence of the common LCHADD mutation (c.1528G > C) was associated with higher levels of 3-hydroxyacylcarnitines than in patients with other mutations.

CONCLUSIONS

We found that feeding consistently suppressed and that moderate intensity exercise increased disease specific acylcarnitine species, but the response to exercise was highly variable across subjects and diagnoses. The clinical utility of routine plasma acylcarnitine analysis for outpatient treatment monitoring remains questionable; however, if acylcarnitine profiles are measured in the clinical setting, standardized procedures are required for sample collection to be of value.

Hydroxylated Long-Chain Acylcarnitines are Biomarkers of Mitochondrial Myopathy

CONTEXT

Plasma acylcarnitines are biomarkers of β-oxidation and are useful in diagnosing several inborn errors of metabolism but have never been investigated systematically in patients with mitochondrial myopathy.

OBJECTIVE

We hypothesized that acylcarnitines can also be biomarkers of mitochondrial myopathy and sought to investigate the prevalence and pattern of elevated acylcarnitines.

DESIGN

This was a prospective cohort study of patients with confirmed mitochondrial myopathy followed at Copenhagen Neuromuscular Center, Rigshospitalet, Copenhagen, Denmark.

PATIENTS

We included 35 patients (44 ± 15 years, 15 women) with mitochondrial myopathy caused by single, large-scale deletions of mitochondrial DNA (n = 17), pathogenic variants in mitochondrial transfer RNA (n = 13), or in proteins of the respiratory chain complexes (n = 5).Concentrations of 35 acylcarnitines were measured using ultra-HPLC and tandem mass-spectrometry. Findings were compared with muscle mutation load in all patients and to respiratory chain activity in 26 patients.

MAIN OUTCOME MEASURES

Prevalence of elevated concentrations of acylcarnitines related to acyl-coenzyme A (CoA) dehydrogenases in patients with mitochondrial myopathy and relation to genotypes/phenotypes.

RESULTS

In total, 27 (77%) patients had elevated concentrations of acylcarnitines related to acyl-CoA dehydrogenases. Elevated concentrations of seven acylcarnitine species were more common in patients compared with a control cohort of >900 individuals, and a specific pattern involving hydroxylated long-chain acylcarnitines occurred in 22 (63%) patients. Severity of derangements was correlated with muscle mutation load and genotypes/phenotypes.

CONCLUSION

In conclusion, elevated concentrations of acylcarnitines is common in patients with mitochondrial myopathy and shows a specific pattern affecting hydroxylated long-chain acylcarnitines, which can have implications for future diagnostic workup of patients.

Metabolic Myopathies

PURPOSE OF REVIEW

Metabolic myopathies are genetic disorders that impair intermediary metabolism in skeletal muscle. Impairments in glycolysis/glycogenolysis (glycogen-storage disease), fatty acid transport and oxidation (fatty acid oxidation defects), and the mitochondrial respiratory chain (mitochondrial myopathies) represent the majority of known defects. The purpose of this review is to develop a diagnostic and treatment algorithm for the metabolic myopathies.

RECENT FINDINGS

The metabolic myopathies can present in the neonatal and infant period as part of more systemic involvement with hypotonia, hypoglycemia, and encephalopathy; however, most cases present in childhood or in adulthood with exercise intolerance (often with rhabdomyolysis) and weakness. The glycogen-storage diseases present during brief bouts of high-intensity exercise, whereas fatty acid oxidation defects and mitochondrial myopathies present during a long-duration/low-intensity endurance-type activity or during fasting or another metabolically stressful event (eg, surgery, fever). The clinical examination is often normal between acute events, and evaluation involves exercise testing, blood testing (creatine kinase, acylcarnitine profile, lactate, amino acids), urine organic acids (ketones, dicarboxylic acids, 3-methylglutaconic acid), muscle biopsy (histology, ultrastructure, enzyme testing), MRI/spectroscopy, and targeted or untargeted genetic testing.

SUMMARY

Accurate and early identification of metabolic myopathies can lead to therapeutic interventions with lifestyle and nutritional modification, cofactor treatment, and rapid treatment of rhabdomyolysis.

Increased and early lipolysis in children with long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency during fast

Children with long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHAD) have a defect in the degradation of long-chain fatty acids and are at risk of hypoketotic hypoglycemia and insufficient energy production as well as accumulation of toxic fatty acid intermediates. Knowledge on substrate metabolism in children with LCHAD deficiency during fasting is limited. Treatment guidelines differ between centers, both as far as length of fasting periods and need for night feeds are concerned. To increase the understanding of fasting intolerance and improve treatment recommendations, children with LCHAD deficiency were investigated with stable isotope technique, microdialysis, and indirect calometry, in order to assess lipolysis and glucose production during 6 h of fasting. We found an early and increased lipolysis and accumulation of long chain acylcarnitines after 4 h of fasting, albeit no patients developed hypoglycemia. The rate of glycerol production, reflecting lipolysis, averaged 7.7 ± 1.6 µmol/kg/min, which is higher compared to that of peers. The rate of glucose production was normal for age; 19.6 ± 3.4 µmol/kg/min (3.5 ± 0.6 mg/kg/min). Resting energy expenditure was also normal, even though the respiratory quotient was increased indicating mainly glucose oxidation. The results show that lipolysis and accumulation of long chain acylcarnitines occurs before hypoglycemia in fasting children with LCHAD, which may indicate more limited fasting tolerance than previously suggested.

Mitochondrial myopathy associated with myasthenia gravis in a young man

An 18-year-old man presented with progressive weakness of proximal muscles with prominent diurnal variation for 3 months. He had bilateral ptosis since his childhood without diurnal variation or double vision. Neurological examination showed involvement of levator palpebrae superioris and lateral rectus muscles bilaterally. The plasma glucose after 75 gm glucose load was 302 mg/dL. The electrophysiological study revealed myopathic pattern and a decremental response in repetitive nerve stimulation. The plasma lactate was elevated and the muscle biopsy showed numerous ragged-red fibers. Serum acetylcholine receptor antibody assay was positive. We diagnosed myasthenia gravis with mitochondrial myopathy.

31P-MRS of skeletal muscle is not a sensitive diagnostic test for mitochondrial myopathy

Clinical phenotypes of persons with mitochondrial DNA (mtDNA) mutations vary considerably. Therefore, diagnosing mitochondrial myopathy (MM) patients can be challenging and warrants diagnostic guidelines. (31)phosphorous magnetic resonance spectroscopy ((31)P-MRS) have been included as a minor diagnostic criterion for MM but the diagnostic strength of this test has not been compared with that of other commonly used diagnostic procedures for MM. To investigate this, we studied seven patients with single, large-scale deletions-, nine with point mutations of mtDNA and 14 healthy subjects, who were investigated for the following: 1) (31)P-MRS of lower arm and leg muscles before and after exercise, 2) resting and peak-exercise induced increases of plasma lactate, 3) muscle morphology and -mitochondrial enzyme activity, 4) maximal oxygen uptake (VO(2max)), 5) venous oxygen desaturation during handgrip exercise and 6) a neurological examination. All MM patients had clinical symptoms of MM, > 2% ragged red fibers in muscle, and impaired oxygen desaturation during handgrip. Fourteen of 16 patients had impaired VO(2max), 10/16 had elevated resting plasma lactate, and 10/11 that were investigated had impaired citrate synthase-corrected complex I activity. Resting PCr/P(i) ratio and leg P(i) recovery were lower in MM patients vs. healthy subjects. PCr and ATP production after exercise were similar in patients and healthy subjects. Although the specificity for MM of some (31)P-MRS variables was as high as 100%, the sensitivity was low (0-63%) and the diagnostic strength of (31)P-MRS was inferior to the other diagnostic tests for MM. Thus, (31)P-MRS should not be a routine test for MM, but may be an important research tool.

Different pathophysiological mechanisms of intramitochondrial iron accumulation in acquired and congenital sideroblastic anemia caused by mitochondrial DNA deletion

Sideroblastic anemias (SA) are characterized by iron accumulation in the mitochondria of erythroblasts. Although we have evidence of mitochondrial gene alterations in sporadic congenital cases, the origin of acquired forms [refractory anemia with ring sideroblasts (RARS)], is still largely unknown. Here, we report the analysis of respiratory chain function in a patient with a large mitochondrial deletion and in patients with RARS. A young boy with SA showed symptoms typical of a mitochondrial disease with metabolic acidosis, muscle weakness and cerebral involvement. His bone marrow DNA was analyzed for the presence of mitochondrial deletions. We found a new mitochondrial (mt)DNA deletion spanning 3,614 bp and including all the mt genes encoding complex IV, plus ATPase 6 and 8, and several transfer (t)RNAs. All tissues analyzed (liver, skeletal muscle, brain, pancreas) showed a heteroplasmic distribution of this mutant DNA. Bone marrow homogenates were obtained from five patients with RARS and from three patients with normal bone marrow and respiratory chain function assayed by spectrophotometric analysis. Cytochrome c oxidase (CCO) activity was greatly reduced in the patient's bone marrow. In contrast, CCO activity and global respiratory chain function were conserved in patients with RARS. We conclude that deficient CCO activity secondary to mtDNA deletions is related to intramitochondrial iron accumulation, as in our patient or in those with Pearson's syndrome, whereas other mechanisms, e.g. nuclear DNA mutations, have to be proposed to be involved in the acquired forms of SA.

Long-term follow-up of neonatal mitochondrial cytopathies: a study of 57 patients

OBJECTIVES

We sought to determine the long-term clinical and biochemical outcome of newborns with mitochondrial cytopathies (MCs) and to identify possible prognostic factors that may modify the course of these diseases.

MATERIAL AND METHODS

Fifty-seven newborns with MCs were identified in a retrospective review (1983-2002). We defined 2 different outcome categories: clinical (neurologic, hepatic, myopathic, and multiorganic) and biochemical (lactate level normalization or initially normal remaining unchanged, decreased but not normalized, and persistently high). We used 2 different statistical approaches: (1) survival studies depending on the initial symptoms and lactate and enzymatic deficiencies using the Kaplan-Meier method; and (2) the same variables compared with different survival age groups and clinical and biochemical outcome categories using the chi2 test.

RESULTS

Thirty-three patients died (57.8%), 12 remain alive (21%), and 12 were lost in the follow-up; 6 of them are currently older than 4 years. Most of the patients manifested multiorganic disease (64.8%) and high lactate level (77.1%) over time. Children surviving to 2.5 to 3 years of age were more likely to survive for a long period of time. Initial neurologic and hepatic presentation increased the risk to develop neurologic disease and severe persistent hyperlactacidemia, respectively. Initial severe hyperlactacidemia and combined enzyme deficiencies were significant risk factors for higher mortality and multiorganic disorders. Two patients with exclusively myopathic outcome are alive and cognitively normal at 12 years of life.

CONCLUSIONS

Children with neonatal-onset MCs have very high mortality and poor prospects. However, some with life-threatening presentations may gradually improve, giving rise to less severe diseases. Those with exclusively myopathic symptoms have a better prognosis.

Spectrum of myopathic findings in 50 patients with the 3243A>G mutation in mitochondrial DNA

Myopathy is a typical clinical finding among patients with the 3243A>G mutation in mitochondrial DNA (mtDNA), but the variability in such findings has not been properly established. We have previously determined the prevalence of patients with 3243A>G in a defined population in northern Finland and characterized a group of patients who represent a good approximation to a population-based cohort. We report here on examinations performed on patients belonging to this cohort in order to determine the frequency of myopathy and to evaluate the clinical, histological, ultrastructural and single fibre mtDNA variability in muscle involvement. Fifty patients with 3243A>G underwent a thorough structured interview and clinical examination. Muscle histology, ultrastructure and single fibre analysis were examined in a subset of patients. A clinical diagnosis of myopathy was made in 50% of cases [95% confidence interval (CI), 36-64] and abnormalities in muscle histology were found in 72% (95% CI, 55-86). Moderate limb weakness leading to functional impairment was the most common myopathic sign, but mild weakness, ptosis and external ophthalmoplegia could also be found. The presence of intramitochondrial crystals and cytochrome c oxidase (COX)-negative fibres and variation in mitochondrial size and shape were more common in the muscles of the myopathic patients. Longitudinal variations in mutation heteroplasmy were examined in single muscle fibres from two severely affected patients. Although the total variation in mutation heteroplasmy along four ragged red fibres (RRFs) was small, the mutation heteroplasmy in five 10 microm segments was clearly lower (median 68%, range 64-74%) than that in the neighbouring segments. There were also segments with deviant mutation load in histologically normal fibres in one patient. The highest incidence of myopathy was in the fifth decade of life, but, apart from age, no other clinical variables such as gender, muscle heteroplasmy, physical inactivity or diabetes were associated with an increased risk of myopathy. The clinical presentation of myopathy is highly variable in patients with 3243A>G.

Stress lactate in mitochondrial myopathy under constant, unadjusted workload

As it is under debate if determination of lactate during cycle ergometry (lactate stress testing, LST) under a continuous, unadjusted, low workload is a valuable diagnostic tool for mitochondrial myopathy (MMP), the present study aimed to investigate how sensitive the LST is in a large cohort of patients with indications for MMP (MMP patients). Serum lactate was determined once before, three times during, and once after a 15-min, constant 30 W-workload on a bicycle ergometer in 115 healthy controls, 166 patients with neurological disorders other than MMP, and 291 MMP patients. Serum lactate's upper reference limit at rest, 5, 10, 15 min after starting, and 15 min after finishing the exercise was 2.0, 2.0, 2.1, 2.0 and 1.7 mmol/l, respectively. Resting lactate was increased in 75 MMP patients (26%). The specificity of resting lactate determination was 84%. The sensitivity of the LST was 66% and the specificity 84%. Among the 192 MMP patients with abnormal LST, 120 (63%) had a normal resting lactate. The LST is abnormal in two thirds of the MMP patients. The sensitivity of the LST is higher than that of resting lactate determination. The LST is a simple and cheap but effective and reliable screening method for detecting the impaired oxidative metabolism in MMP.

Increasing fat in the diet does not improve muscle performance in patients with mitochondrial myopathy due to complex I deficiency

Four myopathic patients with complex I deficiency followed diets containing 55 energy per cent (En%) as fat or 25 En% as fat, both for three weeks. Maximal workload and muscle force were not different on either diet. Exercise endurance time, oxygen consumption and lactate levels were also not different, but one patient had diminished endurance time on 25 En% as fat. Our observations do not support the use of increasing the fat in the diet of patients with mitochondrial complex I deficiency.

Abnormal blood lactate accumulation after exercise in patients with multiple mitochondrial DNA deletions and minor muscular symptoms

STUDY OBJECTIVES

Muscle is one of the most commonly affected organs in mitochondrial disorders, and the symptoms are often exercise related. The cardiopulmonary exercise test with the determination of lactic acid formation could give supplementary information about the exercise-induced metabolic stress and compensatory mechanisms used in these disorders. The aim of this study was to evaluate the exercise capacity and lactate kinetics related to exercise in subjects with two genetically characterized mitochondrial disorders (multiple mitochondrial DNA deletions with PEO, MELAS) compared with lactate kinetics in subjects with metabolic myopathy (McArdle's disease) and in the healthy controls.

DESIGN

The subjects were consecutive, co-operative patients of Department of Neurology of Helsinki University Hospital. Molecular genetic analyses were used for group classification of the mitochondrial myopathy.

STUDY SUBJECTS

The study groups consisted of 11 patients with multiple deletions (PEO) and five patients with a point mutation in the mitochondrial DNA (MELAS), four patients with a muscle phosphorylase enzyme deficiency (McArdle's disease) and 13 healthy controls. The clinical disease of the patients was relatively mild.

MEASUREMENTS AND RESULTS

A graded exercise test with ventilatory gas analyses and venous blood lactic acid analyses was performed. The main finding was the prolonged accumulation of blood lactate after the exercise in the PEO and MELAS groups compared with the controls. An overcompensation in ventilation was found in the MELAS and PEO group.

CONCLUSIONS

The blood lactate accumulation after exercise occurs in patients with multiple mitochondrial DNA deletions or MELAS even in patients with only mild exercise intolerance. Cardiopulmonary exercise can be used in the diagnostic process of patients with mitochondrial myopathies.

Modified Exercise Test in Screening for Mitochondrial Myopathies – Adjustment of Workload in Relation to Muscle Strength

The aim of this study was to evaluate the usefulness of a modification of the bicycle ergometer test, the subanaerobic threshold exercise test (SATET), as a screening test for patients with mitochondrial myopathies. Since the original SATET is frequently found to be strenuous for weak patients, a new variable (relative muscle strength) was added to the workload formula. Plasma lactate levels were recorded at rest, then after 5 and 15 min of cycling on an ergometer, with constant workload. Nine patients with mitochondrial myopathy, 10 patients with other neuromuscular diseases and 9 healthy but sedentary volunteers undertook the test. An upper reference limit after exercise for plasma lactate was settled at 2.9 mmol/l. The modified SATET showed a sensitivity of 78% and a specificity compared to the healthy subjects of 100%. Compared to patients with other neuromuscular diseases, the specificity was lower (60%). All subjects completed the test without severe fatigue or pain.

Genotypes from patients indicate no paternal mitochondrial DNA contribution

A cornerstone of mitochondrial genetics, strict maternal inheritance, has been challenged recently by the study of a patient with mitochondrial myopathy due to a sporadic 2bp deletion. The mitochondrial DNA (mtDNA) harboring the mutation was paternal in origin, whereas the patient's blood was identical to the maternal genotype. To determine whether this is a common phenomenon, we studied mtDNA sequence variation between muscle and blood from 35 patients with sporadic mitochondrial myopathies, but detected no evidence of paternal mtDNA transmission. Our findings suggest that paternal transmission of mtDNA is rare and should not alter our genetic advice to families.

Lack of paternal inheritance of muscle mitochondrial DNA in sporadic mitochondrial myopathies

In 2002, paternal inheritance of muscle mitochondrial DNA (mtDNA) was reported in a patient with exercise intolerance and a mitochondrial DNA (mtDNA) mutation restricted to skeletal muscle. To evaluate whether paternal inheritance is a common phenomenon, we studied 10 sporadic patients with skeletal muscle-restricted mtDNA mutations: five harbored mtDNA point mutations in protein-coding genes and five had single mtDNA deletions. We performed haplotype analysis and direct sequencing of the hypervariable regions 1 and 2 of the D-loop in muscle and blood from the patients and, when available, in blood from their parents. We did not observe paternal inheritance in any of our patients.

Cycle ergometry is not a sensitive diagnostic test for mitochondrial myopathy

Cycle exercise has repeatedly been used to diagnose patients suspected of having mitochondrial myopathy (MM), in whom exercise intolerance and lactic acidosis are common. No standardized test, however, has been established. We evaluated the diagnostic value of incremental and constant workload (20 min at 65 % VO(2max)) cycle tests for the diagnosis of MM. Plasma lactate and oxidative capacity (VO(2) and workload) were measured in 15 well-characterized MM patients during cycling. Findings were compared with those in 10 myotonic dystrophy patients and 18 sedentary, healthy subjects. All MM patients had ragged red or COX-negative fibers on muscle biopsy. VO(2max) and maximal workload were lower in MM than in control groups (P < 0.02). Resting plasma lactate was higher in MM than in control groups (P < 0.005; sensitivity = 93 %; specificity = 85 %), while exercise-induced increases in plasma lactate were only higher during the constant workload protocol in MM patients vs. control groups (P < 0.05; sensitivity = 27 %; specificity = 86 %). The findings indicate that the diagnostic value of a constant workload protocol is superior to an incremental cycle test, but that the test is less sensitive for MM than simple testing of resting lactate and muscle morphology. Cycle testing of MM patients remains an important research tool, but should not be a standard diagnostic procedure for MM.

Mitochondriopathy as a differential diagnosis of amyotrophic lateral sclerosis

OBJECTIVES

To assess how often mitochondrial myopathy (MMP) mimics amyotrophic lateral sclerosis (ALS) and the phenotypic similarities and differences between these two disorders.

METHODS

Records of 123 MMP patients and 59 ALS patients, diagnosed during five years (1996-2000), were retrospectively evaluated.

RESULTS

Re-evaluation revealed that 8 patients, initially diagnosed as ALS had actually MMP (13.6%). Among the MMP patients, 6.5% were initially misdiagnosed as ALS. None of the MMP patients actually had ALS. Common features of ALS and MMP were weakness, wasting, upper motor neurone signs, bulbar abnormalities, and a neurogenic EMG. Features differentiating MMP from ALS were ptosis, sensory disturbances, multi-system involvement, slowly progressive disease course, abnormal lactate stress test, histological and biochemical abnormalities, and mtDNA analysis.

CONCLUSIONS

In a small number of cases MMP may clinically and electrophysiologically mimic ALS, particularly in the early stages of the disease. Patients with suspected ALS, but slow progression and multi-organ involvement, should undergo lactate stress testing and muscle biopsy. ALS should be diagnosed only if MMP has been excluded.

A forearm exercise screening test for mitochondrial myopathy

BACKGROUND

The authors hypothesized that impaired oxygen extraction in mitochondrial myopathy (MM) results in a high oxygen saturation in venous effluent blood from working muscle and that this phenomenon can be used as a diagnostic tool for MM.

METHODS

Twelve patients with MM, 10 patients with muscular dystrophy, and 12 healthy subjects were studied. All subjects performed intermittent static handgrip exercise (1/2 Hz) at 40% of maximal voluntary contraction (MVC) for 3 minutes. Cubital venous oxygen saturation and brachial artery flow were measured in the exercised arm.

RESULTS

Exercise-induced venous oxygen desaturation was smaller in patients with MM (Delta - 7 +/- 5%) than in subjects with muscular dystrophy (Delta - 38 +/- 2%; p = 0.00001) and healthy subjects (Delta - 43 +/- 2%; p = 0.0000002). MVC and exercise blood flow were similar in patients with MM (18 +/- 3 kg; 436 +/- 65 mL/min) and patients with muscular dystrophy (15 +/- 3 kg; 460 +/- 85 mL/min), but were higher in healthy subjects (32 +/- 4 kg; 630 +/- 58 mL/min; p < 0.03). In seven patients with MM and seven patients with McArdle disease, studied with a slightly different protocol, exercise-induced oxygen desaturation was also impaired in MM (Delta - +/- 5%) compared with McArdle disease (Delta - 26 +/- 3%; p = 0.007).

CONCLUSION

Oxygen desaturation in venous blood from exercising muscle is markedly lower in patients with mitochondrial myopathy than in subjects with other muscle diseases and healthy subjects, suggesting that a forearm exercise test can be a diagnostic screening tool for mitochondrial myopathy.

Lactate stress testing in 155 patients with mitochondriopathy

OBJECTIVE

Few data are available about the diagnostic yield of the lactate stress test (LST) in a large group of patients with mitochondriopathy (MCP).

METHODS

Serum lactate was determined once before, three times during, and once after a 15-minute, constant 30W workload on a bicycle in 62 controls, aged 17 to 84 years, 155 patients with MCP, aged 17 to 87 years, and 31 patients with neurological disorders other than MCP.

RESULTS

Lactate's upper reference limits at rest, 5, 10, 15 minutes after starting, and 15 minutes after finishing the exercise were 2.0, 2.1, 2.1, 2.1 and 1.8 mmol/l respectively. The test was regarded abnormal if more than two of the five lactate values exceeded the cut-off levels. Among the 103 patients with abnormal LST, 64 (62 %) had normal resting lactate. The sensitivity of the test was 67% and the specificity 94%.

CONCLUSION

The LST proved to have a high sensitivity and specificity in the detection of patients with MCP, being thus a simple but powerful tool to assess the impaired oxidative metabolism in MCP patients.

Recognition of mitochondrial DNA deletion syndrome with non-neuromuscular multisystemic manifestation

PURPOSE

To correlate the molecular characteristics of the mtDNA deletions with clinical phenotypes.

METHODS

Southern analysis and polymerase chain reaction (PCR)/DNA sequencing were used to determine the size and location of deletions in 16 patients with mtDNA deletion syndrome. An additional 48 reported cases from the literature were also included in the statistical analysis.

RESULTS

The common 5-kb deletion is found in eight of nine patients with Kearns-Sayre syndrome (KSS), mitochondrial myopathies (MM), or progressive external ophthalmoplegia (PEO). The rare/novel deletions were found in six of seven patients with extra-neuromuscular multisystemic manifestations and infantile/early childhood onset.

CONCLUSIONS

Patients with mtDNA deletion syndrome who manifest non-neuromuscular multisystemic disorders at a very young age usually harbor mutant mtDNA with novel or rare deletions in every tissue analyzed. For this group of patients, it is possible to use the less invasive blood specimens instead of muscle biopsies for molecular diagnosis. Overwhelmingly, the common 5-kb deletion is mostly seen in the muscle specimens of patients with KSS and age of onset after the second decade of life.

Diagnostic yield of the lactate stress test in respiratory chain disorders under absolute and relative workload

Usually, the lactate stress test is carried under a constant, low workload maintained for 15 min, although there are indications that the lactate response to exercise is a function of the relative workload, and that a workload for only 5 min does not decrease its sensitivity. Thus, this study compared the diagnostic yield of the lactate stress test when carried out under a constant workload of 30 W (LSTA) and under a workload of 30% of maximum (LSTR), and when the workload was maintained for 5 and 15 min. In 16 patients with respiratory chain disorders, 12 women, four men, aged 27--88 years, the LSTA and LSTR were carried out on 2 different days, within 48 h. For both tests serum lactate was determined before, during and after a 15-min workload on a bicycle. Upper reference limits at rest, 5, 10, 15 min after starting, and 15 min after finishing the exercise were 1.9, 2.0, 2.0, 2.0, and 1.6 mmol/l for the LSTA, and 2.3, 3.0, 3.2, 3.4 and 2.7 mmol/l for the LSTR. The sensitivity was 88% for the LSTA and 63% for the LSTR. The specificity of the LSTA was 94%. The diagnostic yield of both tests was similar when the workload was maintained for 5 or 15 min. In conclusion, the LSTA should be preferred to the LSTR. Three lactate determinations during 15 min have no advantage over a single lactate determination after a 5 min workload of 30 W.

Cytochrome-c detection: a diagnostic marker for myocardial infarction

Following a myocardial infarction (MI) cells die or are damaged and their contents leak into the blood circulation, resulting in elevated serum levels of various enzymes, proteins, and organic molecules. Over the past few decades, it has become standard practice to employ the detection of these elevated substances as markers for the confirmation of MIs and to monitor MI patients' response to treatment. Although it has previously been shown that cytochrome-c, a small respiratory protein, is among those elevated, the lack of a suitable detection system has prevented its routine use in the diagnosis of MIs. We present a preliminary study in which chemiluminescence was employed to detect elevated levels of cytochrome-c in the serum of MI patients. The technique, which is specific for c-type proteins, is approx 30 times more sensitive than the traditional Coomassie blue stain and can detect as little as 0.03 microg of protein. It also has potential for diagnostic use in other diseases that are characterized by mitochondrial damage.

Lack of IL-6 production during exercise in patients with mitochondrial myopathy

In the present study we investigated the possibility that exercise-induced increases in plasma levels of interleukin (IL)-6 are associated with plasma lactate levels. Patients with mitochondrial myopathy (MM) are characterised by high levels of plasma lactate. In this study, seven patients with MM underwent an ergometer cycle test for 25 min without treatment. They were then treated with dichloroacetate (DCA) for 15 days. DCA inhibits pyruvate-dehydrogenase-kinase, thereby increasing the activity of the pyruvate-dehydrogenase complex. The same exercise test was repeated on the last day of treatment. DCA lowered the plasma lactate and increased plasma IL-6 concentrations at rest. IL-6 increased in response to exercise only during DCA treatment. Furthermore, plasma IL-6 was negatively correlated to plasma lactate at rest (r = -0.786, P = 0.05). Given that IL-6 is a cytokine with growth-promoting potential, the results of this study suggest that high lactate production contributes to the decreased muscle function observed in MM patients by inhibiting the production of IL-6.

Lactate-stress testing in 54 patients with mitochondriopathy

Since there are only few data available about the lactate stress test in a group of patients with mitochondrial myopathy, we investigated the sensitivity of this test in a larger cohort of such patients. Serum lactate was determined before, during and after a 15 minute, constant 30 W workload on a bicycle ergometer in 47 controls, aged 15 to 72 years and 54 patients with mitochondrial myopathy, aged 15 to 74 years. Lactate's upper reference limits at rest, 5, 10, 15 minutes after starting, and 15 minutes after finishing the exercise were 2.0, 2.1, 2.1, 2.1 and 1.8 mmol/l respectively. The sensitivity of the lactate-stress test was 69%. The lactate-stress test complements electrophysiological and bioptical findings and proved to be helpful in diagnosing mitochondrial myopathy.

Catalytic activities of mitochondrial ATP synthase in patients with mitochondrial DNA T8993G mutation in the ATPase 6 gene encoding subunit a

We investigated the biochemical phenotype of the mtDNA T8993G point mutation in the ATPase 6 gene, associated with neurogenic muscle weakness, ataxia, and retinitis pigmentosa (NARP), in three patients from two unrelated families. All three carried >80% mutant genome in platelets and were manifesting clinically various degrees of the NARP phenotype. Coupled submitochondrial particles prepared from platelets capable of succinate-sustained ATP synthesis were studied using very sensitive and rapid luminometric and fluorescence methods. A sharp decrease (>95%) in the succinate-sustained ATP synthesis rate of the particles was found, but both the ATP hydrolysis rate and ATP-driven proton translocation (when the protons flow from the matrix to the cytosol) were minimally affected. The T8993G mutation changes the highly conserved residue Leu(156) to Arg in the ATPase 6 subunit (subunit a). This subunit, together with subunit c, is thought to cooperatively catalyze proton translocation and rotate, one with respect to the other, during the catalytic cycle of the F(1)F(0) complex. Our results suggest that the T8993G mutation induces a structural defect in human F(1)F(0)-ATPase that causes a severe impairment of ATP synthesis. This is possibly due to a defect in either the vectorial proton transport from the cytosol to the mitochondrial matrix or the coupling of proton flow through F(0) to ATP synthesis in F(1). Whatever mechanism is involved, this leads to impaired ATP synthesis. On the other hand, ATP hydrolysis that involves proton flow from the matrix to the cytosol is essentially unaffected.

Effects of aerobic training on lactate and catecholaminergic exercise responses in mitochondrial myopathies

The aim of this study was to evaluate the effects of an aerobic training program on the metabolic and sympathetic responses to exercise in 12 patients with mitochondrial myopathies. A 10-week course of aerobic training, consisting of supervised exercise every other day on an electrically braked pedal-rate bicycle ergometer was prescribed to each patient and four healthy controls. Venous lactate, epinephrine (EP) and norepinephrine (NEP) levels were assessed at baseline and after the aerobic training by means of constant-workload exercise performed at near lactate threshold (LT). In patients, a decrease in exercise peak values, significant for lactate (-38.6%, P < 0.01) but not for catecholamines (EP: -26.0%, NEP: -22.1%) was observed after training, findings confirmed by the lactate/EP and lactate/NEP area ratios. The results show that lactate accumulation during exercise is decreased after aerobic training in mitochondrial myopathies and that the effect is partially dissociated from the catecholaminergic response. This in turn suggests that the lactate decrease can be explained, at least in part, by the improved muscle oxidative metabolism consequent to the proposed training program.

Carnitine levels in patients with skeletal myopathy due to anorexia nervosa before and after refeeding

OBJECTIVE

To assess the role of carnitine in the skeletal myopathy present in anorexia nervosa.

METHOD

Serum levels of free and total carnitine were measured in a group of severely underweight women with anorexia nervosa and skeletal myopathy before and after an inpatient refeeding program.

RESULTS

Carnitine levels were within the reference range before refeeding and remained unchanged despite significant weight gain in all the subjects.

CONCLUSION

These findings suggest that carnitine plays no part in the muscle weakness seen in severe anorexia nervosa.

The relationship of plasma catecholamine and lactate during anaerobic threshold exercise in mitochondrial myopathies

Sympathetic system activation is considered one of the main factors influencing lactate production during exercise in normal individuals. In order to assess the role of such activation in mitochondrial myopathies, we compared blood catecholamine levels to those of lactate during an intermittent exercise performed at workloads near anaerobic lactate threshold. Following an initial increment, the patients (n = 10) exhibited a steady-state blood lactate shifted right relative to controls (n = 7), the peaks being respectively 665 +/- 29% and 322 +/- 11% of baseline. Plasma catecholamine increase in mitochondrial myopathies was 272 +/- 21% for norepinephrine and 261 +/- 18% for epinephrine, not statistically different from controls. Lactate/norepinephrine and lactate/epinephrine area ratios were significantly higher in the subjects than in controls (2.36 versus 1.48 and 2.40 versus 1.57, respectively). The study shows that the abnormal lactate production in mitochondrial myopathies is independent of the catecholaminergic response at the transition from aerobic to anaerobic exercise.

Lactate stress test in the diagnosis of mitochondrial myopathy

The aim of the study was to determine the sensitivity and specificity of the lactate stress test in the detection of mitochondrial myopathies. Thirty one healthy subjects, 10 patients with non-mitochondrial myopathy and 26 patients with mitochondrial myopathy underwent lactate stress testing at a standardized workload of 30 W during 15 min on a bicycle ergometer. Lactate was determined before the exercise (R1), 5, 10, 15 min after starting the exercise (S5, S10, S15) and 15 min after finishing the exercise (R2). A result was interpreted as pathologic if more than two of the five lactate values were above the corresponding upper reference limits. The upper reference limits for the venous lactate at R1, S5, S10, S15 and R2 were 1.9, 2.0, 2.1, 2.0 and 1.7 mmol/l respectively. The lactate stress test was pathologic in 1/10 of the non-mitochondrial myopathies and in 18/26 of the mitochondrial myopathies. The sensitivity of the lactate stress test was 69%. The specificity of the test was 90%. In conclusion, the lactate stress test proved to be helpful for evaluating the integrity of the oxidative metabolism in the majority of patients with mitochondrial myopathy.

Simultaneous analysis of plasma free fatty acids and their 3-hydroxy analogs in fatty acid beta-oxidation disorders

We present a new derivatization procedure for the simultaneous gas chromatographic-mass spectrometric analysis of free fatty acids and 3-hydroxyfatty acids in plasma. Derivatization of target compounds involved trifluoroacetylation of hydroxyl groups and tert-butyldimethylsilylation of the carboxyl groups. This new derivatization procedure had the advantage of allowing the complete baseline separation of free fatty acids and 3-hydroxyfatty acids while the superior gas chromatographic and mass spectrometric properties of tert-butyldimethylsilyl derivatives remained unchanged, permitting a sensitive analysis of the target compounds. Thirty-nine plasma samples from control subjects and patients with known defects of mitochondrial fatty acid beta-oxidation were analyzed. A characteristic increase of long-chain 3-hydroxyfatty acids was observed for all of the long-chain 3-hydroxyacyl-CoA dehydrogenase-deficient and mitochondrial trifunctional protein-deficient plasma samples. For medium-chain acyl-CoA dehydrogenase deficiency and very-long-chain acyl-CoA dehydrogenase deficiency, decenoic and tetradecenoic acids, respectively, were the main abnormal fatty acids, whereas the multiple acyl-CoA dehydrogenase-deficient patients showed variable increases of these unusual intermediates. The results showed that this selective and sensitive method is a powerful tool in the diagnosis and monitoring of mitochondrial fatty acid beta-oxidation disorders.

Analytical derivatizations of volatile and hydrophilic carbonyls from aqueous matrix onto a solid phase of a polystyrene-divinylbenzene macroreticular resin

Extraction and derivatization of carbonyls to benzyloximes, pentafluorobenzyloximes or 2,4-dinitrophenylhydrazones is simplified and reaction times are substantially reduced by simultaneous sorption and derivatization from aqueous solution onto a solid phase. In this reaction a macroreticular polystyrene-divinylbenzene resin acts as a sorbent and catalyst to allow simultaneous extraction and derivatization of hydrophilic and lipophilic aldehydes and ketones from simple as well as complex matrices including plasma. Conversion to the 2,4-dinitrophenylhydrazones or pentafluorobenzyloximation at ambient temperature requires 10 and 20 min, respectively. These reaction conditions correspond to at least a 6-fold reduction in reaction times for derivatization of the reactive aldelhydes and a 36-72-fold reduction for preparation of derivatives for the slower reacting ketones.

Muscle fatigue, lactate, and pyruvate in mitochondrial myopathy with progressive external ophthalmoplegia

We studied muscle fatigue and serum lactate and pyruvate levels in 20 patients with mitochondrial myopathy with progressive external ophthalmoplegia (PEO). Fatigue was assessed in the adductor pollicis muscle (AP) using a low-intensity exercise protocol (20 min). Forces (TFs) and relaxation times of ulnar nerve evoked twitches, compound muscle action potentials (CMAPs), and maximal voluntary contractions (MVCs) were monitored. Serum lactate and pyruvate levels were independently measured at rest and after exercise on a bicycle (15 min, 30 W). Most patients showed abnormal fatigue of the AP with a reduction of TFs and MVCs and normal CMAPs. The reduced TFs were significantly correlated with lactate levels at rest (r= - 0.60, P<0.05) and less so with those after exercise (r=- 0.47,P<0.05). Pyruvate levels revealed a similar correlation although they were widely scattered. We conclude that abnormal fatigue in PEO is metabolic, is localized beyond the muscle fiber membrane, and involves the electrome-chanical coupling and the contractile apparatus. Serum lactate levels at rest are good predictors of fatigue in PEO.

Mitochondrial disease. Pulmonary function, exercise performance, and blood lactate levels

Mitochondrial diseases are a heterogeneous group of disorders in which it has been suggested that genetic defects in oxidative phosphorylation lead to specific alterations in exercise performance and lactate metabolism during exercise. To investigate this possibility, we evaluated pulmonary function tests, incremental exercise testing, and serial blood lactate levels in a group of subjects with mitochondrial disease (M) and compared them with a group of patients with nonmitochondrial (N) myopathies and healthy subjects (H). The two groups were demographically comparable and had no significant differences in pulmonary function. Both groups showed similar degrees of reduced exercise tolerance compared with a group of healthy subjects (M: 61.08% predicted VO2max +/- 19.58 SD, n = 13; N: 62.14 +/- 28.89, n = 7; H: 115.17 +/- 19.35, n = 12; p < 0.001). The mitochondrial disease group more frequently showed abnormalities in cardiac response to exercise than did the nonmitochondrial myopathy subjects (M: 12/13, N: 3/7, H: 3/12, p = 0.002). Minute ventilation greater than predicted occurred with similar frequency in both groups. Although resting lactate level was increased in some subjects with mitochondrial myopathy compared with disease controls, there were no differences between groups for peak venous lactate level normalized for oxygen uptake or the rate of lactate clearance. These findings, while confirming the presence of some specific abnormalities in mitochondrial disease, are against the notion that exercise limitation in this condition directly results from specific abnormalities in oxidative metabolism.

Short-term dichloroacetate treatment improves indices of cerebral metabolism in patients with mitochondrial disorders

We performed a short-term, double-blind, placebo-controlled, crossover trial of sodium dichloroacetate (DCA) therapy in 11 patients affected by various primary mitochondrial disorders. Independent measures of oxidative metabolism (venous blood metabolites, exercise testing, phosphorus magnetic resonance [MR] spectroscopy of muscle, and proton MR spectroscopy of brain) were used in order to monitor metabolic responses to the drug. One week of DCA treatment produced significant decreases (p < 0.05) in blood lactate, pyruvate, and alanine at rest and after bicycle exercise. Proton MR spectra collected from a supraventricular volume of interest in brain of seven of 11 patients also showed significant changes. Brain lactate/creatine ratio decreased by 42% during DCA treatment (p < 0.05). Brain choline/creatine ratio (which is low in patients with myelinopathies) increased by 18% (p < 0.01) after therapy. N-Acetylaspartate/creatine ratio (an index of neuronal damage or loss) increased by 8% after treatment (p < 0.05). Proton MR spectra collected in two of 11 patients from a volume of interest including the basal ganglia showed similar results (decrease of 36.6% in lactate/creatine; increases of 16% in choline/creatine and 4.5% in N-acetylaspartate/creatine). Phosphorus MR spectroscopy of muscle and self-assessed clinical disability were unchanged. Our study indicates that short-term DCA treatment not only lowers blood lactate but also improves indices of both brain oxidative metabolism and neuronal and glial density or function.

Determination of the blood lactate:pyruvate ratio as a noninvasive test for the diagnosis of zidovudine myopathy

OBJECTIVE

To evaluate the use of the lactate: pyruvate ratio as a test for the detection of zidovudine myopathy.

METHODS

Twenty consecutive human immunodeficiency virus-infected patients with muscle involvement and 20 without muscle involvement were studied prospectively. Blood lactate and pyruvate levels and serum creatine kinase levels were tested, muscle involvement was assessed both clinically and electrophysiologically, and muscle biopsy was performed in patients with myopathy.

RESULTS

Nine patients had biopsy-proven zidovudine myopathy. All 9 had a high lactate:pyruvate ratio, with elevations on 2 of 2 determinations in 6 patients and on 1 of 2 in 3 patients. Two of 11 patients with other myopathies and 2 of 20 patients without myopathy had a high lactate:pyruvate ratio on 1 of 2 determinations.

CONCLUSION

The lactate:pyruvate ratio, when determined repeatedly, is a sensitive test for detecting mitochondrial muscular toxicity of zidovudine.

Skeletal muscle mitochondrial myopathy as a cause of exercise intolerance in a horse

Although exertional myopathies are commonly recognized in horses, specific etiologies have not been identified. This is the first report in the horse of a deficiency of Complex I respiratory chain enzyme associated with profound exercise intolerance. Physical examination, routine blood tests, endoscopy, and ultrasonograms of the heart and iliac arteries were unremarkable. With slow, incremental exercise (speeds 1.5-7 m/s), the Arabian mare showed a marked lactic acidosis, increased mixed venous PVO2, and little change in oxygen consumption. Muscle biopsies contained large accumulations of mitochondria with bizarre cristae formations. Biochemical analyses revealed a very low activity of the first enzyme complex in the mitochondrial respiratory chain (NADH CoQ reductase). The exercise intolerance and muscle stiffness in this horse were attributed to a profound lactic acidosis resulting from impaired oxidative energy metabolism during exercise.

Plasma carnitine insufficiency and effectiveness of L-carnitine therapy in patients with mitochondrial myopathy

Plasma carnitine "insufficiency," (plasma esterified carnitine to free carnitine ratio above 0.25) was found in 21 of 48 (43.8%) patients with mitochondrial myopathy, of whom 4 also showed both total and free carnitine deficiencies in plasma. In addition, plasma levels of SCAC and LCAC were higher in patients with mitochondrial myopathy than in controls (P < 0.001 and P < 0.01, respectively). Patients diagnosed as having plasma carnitine insufficiency or deficiency were treated with L-carnitine (50-200 mg/kg per day in four daily doses). Muscle weakness improved in 19 of 20 patients, failure to thrive in 4 of 8, encephalopathy in 1 of 9, and cardiomyopathy in 8 of 8 patients. Plasma carnitine "insufficiency" provides an additional clue to the diagnosis of mitochondrial myopathy and an indication for L-carnitine therapy.

Oral glucose lactate stimulation test in mitochondrial disease

We selected 23 patients with neurologic diseases, including 15 boys and 8 girls from 1 month to 10 years of age, who were divided into mitochondrial and nonmitochondrial disease groups. All patients were required to fast and rest for at least 4-10 hours before the test. Glucose was administered orally using a 50% glucose:water solution at a dose of 1.75 mg/kg. Blood samples then were drawn from a retained heparinized venous line at 0, 30, 60, 120, and 180 min and tested for lactate and glucose levels. Of the mitochondrial disease group, 10 of 11 patients had an upward sloping curve of lactate metabolism; the one who had a flat line response was a patient who suffered from a free-fatty-acid metabolic defect presenting with a recurrent Reye-like syndrome. There was a relatively flat line response in the nonmitochondrial disease group except in a patient with alternating hemiplegia whose symptoms responded well to flunarizine therapy. A significant increase in blood lactate content at 60 min after glucose loading occurred in the mitochondrial disease group, in contrast to that of the nonmitochondrial group. Sensitivity and specificity of a 5 mg/dl increase in blood lactate concentration at 60 min was 72.7% vs. 91.7% Moreover, all 4 patients whose blood lactate increased by 13 mg/dl at 60 min were in the mitochondrial disease group.