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Familial myopathy with conspicuous depletion of mitochondria in muscle fibers: a morphologically distinct disease
Neuromusc. Disord., Vol. 5. No. 2, pp. 139 144, 1995 Elsevier Science Ltd Printed in Great Britain
Pergamon
0960-8966(94)00039-5
FAMILIAL MYOPATHY WITH CONSPICUOUS DEPLETION OF MITOCHONDRIA IN MUSCLE FIBERS: A MORPHOLOGICALLY DISTINCT DISEASE A N G E L A G E N G E , G E O R G E KARPATI,* D O U G L A S A R N O L D , ERIC A. S H O U B R I D G E and S T I R L I N G C A R P E N T E R The Neuromuscular Research Group of the Montreal NeurologicalInstitute and The Department of Neurology and Neurosurgery, McGill University, Montreal, Quebec. Canada (Received 5 February 1994; revised 26 April 1994; accepted 19 May 1994)
Abstract--Three patients (two of them siblings) presented with easy fatiguability and prominent postexercise pain. Muscle biopsy showed that large areas of about one third of the type II fibers were completely devoid of mitochondria. The remaining mitochondria were unusually large in size, but otherwise normal ultrastructurally. In two patients, 31p in vivo MRS showed low phosphocreatine (PCr), high ADP, low phosphorylation potential at rest and slow ADP and PCr recovery after aerobic exercise. This appears to be a pathologically unique form of metabolic myopathy. The cause of the focal mitochondrial depletion is not known. It should be distinguished from the mtDNA depletion syndrome in which muscle mitochondria are not reduced, but proliferate.
PATIENTS
INTRODUCTION The purpose of this paper is to describe a new form of familial myopathy with a mitochondrial abnormality. In this form there is microscopic evidence of severe depletion of mitochondria in a substantial proportion of muscle fibers, although the remaining mitochondria in these fiber segments become augmented in size. This is different from the focal reduction in mitochondria which has been observed in various pathological reactions of muscle fibers, such as targets [1], cores [2], Zdisc streaming [3], motheaten fibers [4] or maldistribution of mitochondria [5]. It must also be distinguished from congenital mitochondrial D N A depletion syndrome in which there is an actual excess of muscle fiber mitochondria although they contain much less D N A than normal [6].
Case l
*Author to whom correspondence should be addressed at: Montreal Neurological Institute, 3801 University St, Montreal. P.Q., Canada H3A 2B4.
This 18-yr-old female presented at the age of 15 yr with postexercise muscle pain without cramps. Exercise could be performed without difficulty, but hours later she experienced severe, persistent pain in the exercised muscle. Initially, the pain was only intermittent, but is now induced by even minimal exercise. There was one episode of pigmenturia, but myoglobinuria was never documented. A moderate degree of muscle weakness is present in the proximal limb distribution. All limb muscle groups demonstrate marked fatiguability. She is the sister of Case 2. Neither parent is clinically affected, although the father had mildly elevated creatine kinase (CK) values on several occasions. Two other siblings are unaffected. Lab data. Serum C K activity has been documented at 8000-10,000 U/1 during postexercise episodes of pain. When she is asymptomatic, (CK) activity is in the range of 1000 U/1. Immunologic blood tests were negative. Electromyography revealed abnormally small
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polyphasic motor unit potentials. Serum lactate was normal (0.9 mmol/1).
Case 2. This 15-yr-old male is the brother of Case 1. His symptoms started at age 12. Elevation of his serum CK values was documented during the investigation of his sister. He presented with the same clinical picture, but he is less affected than her. He also showed proximal muscle weakness and marked muscle fatiguability. There is no history of myoglobinuria. Lab data. Serum CK during episodes of pain was 1500-9000 U/1. Between these episodes CK has been documented at 900 U/1. Immunological blood tests were negative. E M G was normal. Serum lactate was normal (0.8 mmol/1).
Case 3 This 27-yr-old female is unrelated to the other patients. Symptoms started at the age of 20 with postexercise muscle pain without cramps. Her symptoms continue to be intermittent although occurring with progressively less intense exercise. She has no demonstrable muscle weakness and, although there is muscle fatiguability, it is less pronounced than the previous case. Historically, there have been two episodes of myoglobinuria. Family history is negative. Lab data. Serum C K activity between episodes of pain was 700-900 U/1. The immunological work-up, E M G and mtDNA analysis were unrevealing. Serum lactate was 0.5 mmol/l (normal <1.2). RESULTS
31p in vivo M R S ofgastrocnemius Two sets of data were obtained from each patient using a 1.5 Tesla Phillips Gyroscan. The coil was placed over the gastrocnemius and the first 3~p spectrum was obtained with the patient at rest. In the first two cases the phosphocreatine and the phosphorylation potentials were low and the adenosine-diphosphate was elevated. The third case had a normal resting spectrum. Next, the patient's leg was exercised using a pedal and a blood pressure cuff was inflated around the thigh to produce an ischemic exercise effort. Postexercise spectra
were obtained. Phosphecreatine recovery halftime was diminished in Case 2 (0.28; normal range 3.18-7.34). In both Case 1 and 2 the ADP recovery half-time was increased. (0.51 and 0.49, respectively; normal range 0~).39). These results indicate that oxidative phosphorylation was impaired at rest and its return to the resting state postexercise was also impaired. Case 3 showed normal recovery, pH i was normal in all patients at rest and during exercise.
Muscle biopsy findings Histochemistry. The findings in the three biopsies were essentially similar. With NADHtetrozolium reductase, succinate dehydrogenase and cytochrome oxidase reactions, 30-40% of muscle fibers contained extensive areas sometimes sparing the periphery of fibers that were devoid of enzyme activity, indicating an absence of mitochondria in these regions (Figs 1-4). There were no areas of increased staining to indicate an excessive number of mitochondria in other regions. The percentage of crosssectional area of muscle fibers devoid of mitochondria at a given cross-sectional level varied about 5-100%. Fibers with a near total absence of mitochondria at a given level are illustrated in Fig. 2. On longitudinal sections, areas without mitochondrial enzyme activity could extend up to a millimeter (Fig. 4). The changes predominantly affected type II fibers. About one-third of the type II fibers were atrophic although polygonal (Fig. 5). Not all atrophic fibers showed mitochondrial depletion, but most fibers with mitochondrial depletion were about one-third smaller in calibre than those without mitochondrial depletion. It was striking that no myofibrillar abnormalities were visible in the affected fibers (Fig. 5). Modified Gomori trichrome and immunostaining for class 1 and class 2 major histocompatibility antigens were normal. There were no inflammatory infiltrates. Dystrophin immunostaining was normal. Resin histology and electron microscopy. On semithin sectors muscle fibers could be found in which the normal pattern of intramyofibrillar mitochondria in the I band was missing. Instead these fibers showed very few but unusually large dark mitochondria. The largest seen was 2 sarcomeres in length. By electron microscopy a few of the abnormal fibers were found (Figs 6-7). The most abnormal con-
Familial Myopathy and Mitochondrial Depletion
141
Fig. 1. Many muscle fibers of patient 1 have large irregular areas where diformazan is lacking (examples are marked by arrows). NADH-tetrazolium reductase, x 350. Fig. 2. All type 2 and several type 1 fibers of patient 1 show large areas without cytochrome oxidase activity. A few fibers (asterisks) are totally or almost totally devoid of the enzyme activity. Cytochrome oxidase, x 350. Fig. 3. In several muscle fibers of patient 3, there is marked depletion of mitochondria (arrows), while the remaining mitochondria appear larger than those in the unaffected fibers. Succinic dehydrogenase, x 520. Fig. 4 On longitudinal sections, the mitochondria-depleted segments extend up to 1 mm (arrows). Patient 1 NADH tetrazolium reductase, x 350. tained very few mitochondria, less than onetenth of the normal number but all mitochondria in them were unusually large, about 0.7-0.8 gm in their smallest diameter (Fig. 7). They were elongated in the long axis of the fibers, without any particular relationship to the I bands (Fig. 6). On high power the internal structure o f these mitochondria appeared normal, except that matrix granules may have been slightly increased per unit area (Fig. 8). The myofibrils in these fibers tended to be smaller than normal and triads were sometimes in tandem arrangement (Fig. 7). Otherwise the muscle fibers appeared normal. Mitochondrial DNA analysis. Southern blot analysis showed no evidence for a m t D N A deletion or duplication, nor for m t D N A depletion. The t R N A ~eu and t R N A lys genes in m t D N A were sequenced to look for mutations associated with . M E L A S (and associated
phenotypes) and M E R R F . All sequences were identical with the Cambridge consensus sequence [7].
DISCUSSION These three patients appear to have a particular syndrome, characterized by severe postexercise muscle pain with markedly elevated serum C K activity and mild but progressive proximal limb muscle weakness, as well as unusual findings on muscle biopsy. The reduced number of mitochondria would suggest that the involved muscle fibers suffer from an impaired supply of ATP. M R S confirmed this in our patients by showing evidence of impaired oxidative phosphorylation in two, but there was no evidence of acidosis [8-10]. However, muscle pain from impaired energy
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Fig. 5. The myofibrillar ATPase reaction (pH 4.6 preincubation), shows no pallor or irregularity of the activity, despite mitochondrial depletion in many fibers. A few small-diameter type 2B (intermediate) fibers are present, x 350.
metabolism, whether from impaired oxidative phosphorylation, as in electron transport chain defects or glycolytic problems, tends to occur during exercise and not after it. The mechanism of the muscle postexercise pain, thus, remains
obscure. The mild weakness is presumably related to the fiber atrophy, and that in turn may be related to the tendency of smallness of myofibrils in involved fibers. The findings are different from known lesions
Fig. 6 This electron micrograph shows on the left an abnormal fiber from the biopsy of patient 2. Mitochondria are much fewer than normal, but they are elongated in the long axis of the fiber. Myofibrils appear somewhat small compared to those in the normal fiber on the right. × 7500.
Familial Myopathy and Mitochondrial Depletion
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Fig. 7. This electron micrograph shows part of an abnormal muscle fiber from patient 2. The mitochondria are far fewer than normal but their girth is increased. Stacking of SR cisterns and T-tubules can be seen (arrows) × 10,000.
Fig. 8. The individual enlarged mitochondria, like this one, show normal internal structure with very numerous matrix granules, x 50,000.
with focal lack of mitochondria in muscle fibers such as central cores [1], multicores [2], target fibers of denervation [3] or motheaten changes [4] commonly occurring in reinnervated fibers. It is also clearly different from fibers with maldistribution of mitochondria (i.e. trabecular fibers in limb girdle dystrophy) [11] or fibers at the edge of fascicles in dermatomyositis [12] since no significant areas of increased mitochondrial density are seen. In some of those conditions (i.e. targets and some cores) there is often streaming of the Z disc in the affected segments and there are no consistent changes in the size of the mitochondria. The most characteristic finding in the welldescribed mitochondrial myopathies is the presence of ragged red fibers, showing a significant increase of the number of mitochondria [13]. The increase may result from an impaired function, since these mitochondria generally have defects in their electron transport chain and
oxidative phosphorylation [14]. In two of the present cases impairment of mitochondrial function could also be documented by in vivo MRS, but instead of proliferation mitochondria showed depletion. Since the total m t D N A did not show a significant decrease, we assume that some of the remaining mitochondria contained more than normal DNA. The volume of the enlarged mitochondria was estimated to be about 20-fold above normal mitochondrial volume. A corresponding increase of m t D N A in these organelles is probable. In other words, m t D N A copy member per unit mitochondrial volume is constant. Mitochondrial depletion can conceivably be due to extramitochondrial cause(s), which may impair energy metabolism. In this event the mitochondrial depletion could be contributing very little to the signs and symptoms. Another possible cause of mitochondrial depletion is an impairment of normal mitochondrial prolifera-
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tion to keep pace with the normal turnover. Almost nothing is known about the cellular or molecular factors that control mitochondrial proliferation in the normal state [15]. In any event the basic abnormality in the present cases appears to be different from all other mitochondrial myopathies. REFERENCES
1. Engel A G, Banker B Q. Basic reactions of muscle. Myology. New York: McGraw-Hill, 1986:866-868 2. Engel A G, Banker B Q. Basic reactions of muscle. Myology. New York: McGraw-Hill: 1986:863-866 3. Carpenter S, Karpati G. Organelles and their reactions. Pathology o f Skeletal Muscle. New York: Churchill Livingston, 1984: 225. 4. Carpenter S, Karpati G. Diseases of skeletal muscles. Pathology of Skeletal Muscle. New York: Churchill Livingston, 1984: 467. 5. Karpati G, Arnold D, Matthews P M, et al. Correlative multidisciplinary approach to the study of mitochondrlal encephalomyopathies. Rev Neurol 1991; 146: 455-461. 6. Moraes C T, Shanske S, Tritschler H-J, et aL mtDNA depletion with variable expression: a novel genetic abnormality in mitochondrial diseases. Am J Hum Genet 1991; 8: 492-501.
7. Andersen S, Bankier A T, Barrell B G, et al. Sequence and organization of the human mitochondrial genome. Nature 1981; 290: 457-465. 8. Argov Z, Bank W J, Marls J, Peterson P, Chance B. Bioenergetic heterogenicity of human mitochondrial myopathies: phosphorus magnetic resonance spectroscopy study. Neurology 1987; 37: 257-262. 9. Arnold D L, Taylor D J, Radda G K. Investigation of human mitochondrial myopathies by phosphorus magnetic resonance spectroscopy. Ann Neurol 1985; 18: 189-196. 10. Matthews P M, Allaire C, Shoubridge E A, et al. In vivo muscle magnetic resonance spectroscopy in the clinical investigation of mitochondrlal disease. Neurology 1991; 41:114-120. 11. Bethlem J, Van Wijingaarden G K, De Jong J. The incidence of lobulated fibers in the facioscapulohumeral type of muscular dystrophy and the limb-girdle syndrome. J Neurol Sci 1973; 18: 351-358. 12. Carpenter S, Karpati G, Rothman S, Watters G. The childhood type of dermatomyositis. Neurology 1976; 26: 952-962. 13. Carpenter S, Karpati G. Diseases of skeletal muscle. Pathology o f Skeletal Muscle. New York: Churchill Livingston: 1984; 603-607. 14. Morgan-Hughes J A. The mitochondrial myopathies. In Engel A G, Banker B Q, eds. Myology. New York: McGraw-Hill, 1986: 1709-1793. 15. Karpati G, Shoubridge E A. Mitrochondrial Encephalomyopathies due to electron transport chain defects. In Appel S, ed. Current Neurology. 1993: 133-166.
Pergamon
0960-8966(94)00039-5
FAMILIAL MYOPATHY WITH CONSPICUOUS DEPLETION OF MITOCHONDRIA IN MUSCLE FIBERS: A MORPHOLOGICALLY DISTINCT DISEASE A N G E L A G E N G E , G E O R G E KARPATI,* D O U G L A S A R N O L D , ERIC A. S H O U B R I D G E and S T I R L I N G C A R P E N T E R The Neuromuscular Research Group of the Montreal NeurologicalInstitute and The Department of Neurology and Neurosurgery, McGill University, Montreal, Quebec. Canada (Received 5 February 1994; revised 26 April 1994; accepted 19 May 1994)
Abstract--Three patients (two of them siblings) presented with easy fatiguability and prominent postexercise pain. Muscle biopsy showed that large areas of about one third of the type II fibers were completely devoid of mitochondria. The remaining mitochondria were unusually large in size, but otherwise normal ultrastructurally. In two patients, 31p in vivo MRS showed low phosphocreatine (PCr), high ADP, low phosphorylation potential at rest and slow ADP and PCr recovery after aerobic exercise. This appears to be a pathologically unique form of metabolic myopathy. The cause of the focal mitochondrial depletion is not known. It should be distinguished from the mtDNA depletion syndrome in which muscle mitochondria are not reduced, but proliferate.
PATIENTS
INTRODUCTION The purpose of this paper is to describe a new form of familial myopathy with a mitochondrial abnormality. In this form there is microscopic evidence of severe depletion of mitochondria in a substantial proportion of muscle fibers, although the remaining mitochondria in these fiber segments become augmented in size. This is different from the focal reduction in mitochondria which has been observed in various pathological reactions of muscle fibers, such as targets [1], cores [2], Zdisc streaming [3], motheaten fibers [4] or maldistribution of mitochondria [5]. It must also be distinguished from congenital mitochondrial D N A depletion syndrome in which there is an actual excess of muscle fiber mitochondria although they contain much less D N A than normal [6].
Case l
*Author to whom correspondence should be addressed at: Montreal Neurological Institute, 3801 University St, Montreal. P.Q., Canada H3A 2B4.
This 18-yr-old female presented at the age of 15 yr with postexercise muscle pain without cramps. Exercise could be performed without difficulty, but hours later she experienced severe, persistent pain in the exercised muscle. Initially, the pain was only intermittent, but is now induced by even minimal exercise. There was one episode of pigmenturia, but myoglobinuria was never documented. A moderate degree of muscle weakness is present in the proximal limb distribution. All limb muscle groups demonstrate marked fatiguability. She is the sister of Case 2. Neither parent is clinically affected, although the father had mildly elevated creatine kinase (CK) values on several occasions. Two other siblings are unaffected. Lab data. Serum C K activity has been documented at 8000-10,000 U/1 during postexercise episodes of pain. When she is asymptomatic, (CK) activity is in the range of 1000 U/1. Immunologic blood tests were negative. Electromyography revealed abnormally small
139
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A. Genge et aL
polyphasic motor unit potentials. Serum lactate was normal (0.9 mmol/1).
Case 2. This 15-yr-old male is the brother of Case 1. His symptoms started at age 12. Elevation of his serum CK values was documented during the investigation of his sister. He presented with the same clinical picture, but he is less affected than her. He also showed proximal muscle weakness and marked muscle fatiguability. There is no history of myoglobinuria. Lab data. Serum CK during episodes of pain was 1500-9000 U/1. Between these episodes CK has been documented at 900 U/1. Immunological blood tests were negative. E M G was normal. Serum lactate was normal (0.8 mmol/1).
Case 3 This 27-yr-old female is unrelated to the other patients. Symptoms started at the age of 20 with postexercise muscle pain without cramps. Her symptoms continue to be intermittent although occurring with progressively less intense exercise. She has no demonstrable muscle weakness and, although there is muscle fatiguability, it is less pronounced than the previous case. Historically, there have been two episodes of myoglobinuria. Family history is negative. Lab data. Serum C K activity between episodes of pain was 700-900 U/1. The immunological work-up, E M G and mtDNA analysis were unrevealing. Serum lactate was 0.5 mmol/l (normal <1.2). RESULTS
31p in vivo M R S ofgastrocnemius Two sets of data were obtained from each patient using a 1.5 Tesla Phillips Gyroscan. The coil was placed over the gastrocnemius and the first 3~p spectrum was obtained with the patient at rest. In the first two cases the phosphocreatine and the phosphorylation potentials were low and the adenosine-diphosphate was elevated. The third case had a normal resting spectrum. Next, the patient's leg was exercised using a pedal and a blood pressure cuff was inflated around the thigh to produce an ischemic exercise effort. Postexercise spectra
were obtained. Phosphecreatine recovery halftime was diminished in Case 2 (0.28; normal range 3.18-7.34). In both Case 1 and 2 the ADP recovery half-time was increased. (0.51 and 0.49, respectively; normal range 0~).39). These results indicate that oxidative phosphorylation was impaired at rest and its return to the resting state postexercise was also impaired. Case 3 showed normal recovery, pH i was normal in all patients at rest and during exercise.
Muscle biopsy findings Histochemistry. The findings in the three biopsies were essentially similar. With NADHtetrozolium reductase, succinate dehydrogenase and cytochrome oxidase reactions, 30-40% of muscle fibers contained extensive areas sometimes sparing the periphery of fibers that were devoid of enzyme activity, indicating an absence of mitochondria in these regions (Figs 1-4). There were no areas of increased staining to indicate an excessive number of mitochondria in other regions. The percentage of crosssectional area of muscle fibers devoid of mitochondria at a given cross-sectional level varied about 5-100%. Fibers with a near total absence of mitochondria at a given level are illustrated in Fig. 2. On longitudinal sections, areas without mitochondrial enzyme activity could extend up to a millimeter (Fig. 4). The changes predominantly affected type II fibers. About one-third of the type II fibers were atrophic although polygonal (Fig. 5). Not all atrophic fibers showed mitochondrial depletion, but most fibers with mitochondrial depletion were about one-third smaller in calibre than those without mitochondrial depletion. It was striking that no myofibrillar abnormalities were visible in the affected fibers (Fig. 5). Modified Gomori trichrome and immunostaining for class 1 and class 2 major histocompatibility antigens were normal. There were no inflammatory infiltrates. Dystrophin immunostaining was normal. Resin histology and electron microscopy. On semithin sectors muscle fibers could be found in which the normal pattern of intramyofibrillar mitochondria in the I band was missing. Instead these fibers showed very few but unusually large dark mitochondria. The largest seen was 2 sarcomeres in length. By electron microscopy a few of the abnormal fibers were found (Figs 6-7). The most abnormal con-
Familial Myopathy and Mitochondrial Depletion
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Fig. 1. Many muscle fibers of patient 1 have large irregular areas where diformazan is lacking (examples are marked by arrows). NADH-tetrazolium reductase, x 350. Fig. 2. All type 2 and several type 1 fibers of patient 1 show large areas without cytochrome oxidase activity. A few fibers (asterisks) are totally or almost totally devoid of the enzyme activity. Cytochrome oxidase, x 350. Fig. 3. In several muscle fibers of patient 3, there is marked depletion of mitochondria (arrows), while the remaining mitochondria appear larger than those in the unaffected fibers. Succinic dehydrogenase, x 520. Fig. 4 On longitudinal sections, the mitochondria-depleted segments extend up to 1 mm (arrows). Patient 1 NADH tetrazolium reductase, x 350. tained very few mitochondria, less than onetenth of the normal number but all mitochondria in them were unusually large, about 0.7-0.8 gm in their smallest diameter (Fig. 7). They were elongated in the long axis of the fibers, without any particular relationship to the I bands (Fig. 6). On high power the internal structure o f these mitochondria appeared normal, except that matrix granules may have been slightly increased per unit area (Fig. 8). The myofibrils in these fibers tended to be smaller than normal and triads were sometimes in tandem arrangement (Fig. 7). Otherwise the muscle fibers appeared normal. Mitochondrial DNA analysis. Southern blot analysis showed no evidence for a m t D N A deletion or duplication, nor for m t D N A depletion. The t R N A ~eu and t R N A lys genes in m t D N A were sequenced to look for mutations associated with . M E L A S (and associated
phenotypes) and M E R R F . All sequences were identical with the Cambridge consensus sequence [7].
DISCUSSION These three patients appear to have a particular syndrome, characterized by severe postexercise muscle pain with markedly elevated serum C K activity and mild but progressive proximal limb muscle weakness, as well as unusual findings on muscle biopsy. The reduced number of mitochondria would suggest that the involved muscle fibers suffer from an impaired supply of ATP. M R S confirmed this in our patients by showing evidence of impaired oxidative phosphorylation in two, but there was no evidence of acidosis [8-10]. However, muscle pain from impaired energy
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Fig. 5. The myofibrillar ATPase reaction (pH 4.6 preincubation), shows no pallor or irregularity of the activity, despite mitochondrial depletion in many fibers. A few small-diameter type 2B (intermediate) fibers are present, x 350.
metabolism, whether from impaired oxidative phosphorylation, as in electron transport chain defects or glycolytic problems, tends to occur during exercise and not after it. The mechanism of the muscle postexercise pain, thus, remains
obscure. The mild weakness is presumably related to the fiber atrophy, and that in turn may be related to the tendency of smallness of myofibrils in involved fibers. The findings are different from known lesions
Fig. 6 This electron micrograph shows on the left an abnormal fiber from the biopsy of patient 2. Mitochondria are much fewer than normal, but they are elongated in the long axis of the fiber. Myofibrils appear somewhat small compared to those in the normal fiber on the right. × 7500.
Familial Myopathy and Mitochondrial Depletion
143
Fig. 7. This electron micrograph shows part of an abnormal muscle fiber from patient 2. The mitochondria are far fewer than normal but their girth is increased. Stacking of SR cisterns and T-tubules can be seen (arrows) × 10,000.
Fig. 8. The individual enlarged mitochondria, like this one, show normal internal structure with very numerous matrix granules, x 50,000.
with focal lack of mitochondria in muscle fibers such as central cores [1], multicores [2], target fibers of denervation [3] or motheaten changes [4] commonly occurring in reinnervated fibers. It is also clearly different from fibers with maldistribution of mitochondria (i.e. trabecular fibers in limb girdle dystrophy) [11] or fibers at the edge of fascicles in dermatomyositis [12] since no significant areas of increased mitochondrial density are seen. In some of those conditions (i.e. targets and some cores) there is often streaming of the Z disc in the affected segments and there are no consistent changes in the size of the mitochondria. The most characteristic finding in the welldescribed mitochondrial myopathies is the presence of ragged red fibers, showing a significant increase of the number of mitochondria [13]. The increase may result from an impaired function, since these mitochondria generally have defects in their electron transport chain and
oxidative phosphorylation [14]. In two of the present cases impairment of mitochondrial function could also be documented by in vivo MRS, but instead of proliferation mitochondria showed depletion. Since the total m t D N A did not show a significant decrease, we assume that some of the remaining mitochondria contained more than normal DNA. The volume of the enlarged mitochondria was estimated to be about 20-fold above normal mitochondrial volume. A corresponding increase of m t D N A in these organelles is probable. In other words, m t D N A copy member per unit mitochondrial volume is constant. Mitochondrial depletion can conceivably be due to extramitochondrial cause(s), which may impair energy metabolism. In this event the mitochondrial depletion could be contributing very little to the signs and symptoms. Another possible cause of mitochondrial depletion is an impairment of normal mitochondrial prolifera-
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A. Genge et al.
tion to keep pace with the normal turnover. Almost nothing is known about the cellular or molecular factors that control mitochondrial proliferation in the normal state [15]. In any event the basic abnormality in the present cases appears to be different from all other mitochondrial myopathies. REFERENCES
1. Engel A G, Banker B Q. Basic reactions of muscle. Myology. New York: McGraw-Hill, 1986:866-868 2. Engel A G, Banker B Q. Basic reactions of muscle. Myology. New York: McGraw-Hill: 1986:863-866 3. Carpenter S, Karpati G. Organelles and their reactions. Pathology o f Skeletal Muscle. New York: Churchill Livingston, 1984: 225. 4. Carpenter S, Karpati G. Diseases of skeletal muscles. Pathology of Skeletal Muscle. New York: Churchill Livingston, 1984: 467. 5. Karpati G, Arnold D, Matthews P M, et al. Correlative multidisciplinary approach to the study of mitochondrlal encephalomyopathies. Rev Neurol 1991; 146: 455-461. 6. Moraes C T, Shanske S, Tritschler H-J, et aL mtDNA depletion with variable expression: a novel genetic abnormality in mitochondrial diseases. Am J Hum Genet 1991; 8: 492-501.
7. Andersen S, Bankier A T, Barrell B G, et al. Sequence and organization of the human mitochondrial genome. Nature 1981; 290: 457-465. 8. Argov Z, Bank W J, Marls J, Peterson P, Chance B. Bioenergetic heterogenicity of human mitochondrial myopathies: phosphorus magnetic resonance spectroscopy study. Neurology 1987; 37: 257-262. 9. Arnold D L, Taylor D J, Radda G K. Investigation of human mitochondrial myopathies by phosphorus magnetic resonance spectroscopy. Ann Neurol 1985; 18: 189-196. 10. Matthews P M, Allaire C, Shoubridge E A, et al. In vivo muscle magnetic resonance spectroscopy in the clinical investigation of mitochondrlal disease. Neurology 1991; 41:114-120. 11. Bethlem J, Van Wijingaarden G K, De Jong J. The incidence of lobulated fibers in the facioscapulohumeral type of muscular dystrophy and the limb-girdle syndrome. J Neurol Sci 1973; 18: 351-358. 12. Carpenter S, Karpati G, Rothman S, Watters G. The childhood type of dermatomyositis. Neurology 1976; 26: 952-962. 13. Carpenter S, Karpati G. Diseases of skeletal muscle. Pathology o f Skeletal Muscle. New York: Churchill Livingston: 1984; 603-607. 14. Morgan-Hughes J A. The mitochondrial myopathies. In Engel A G, Banker B Q, eds. Myology. New York: McGraw-Hill, 1986: 1709-1793. 15. Karpati G, Shoubridge E A. Mitrochondrial Encephalomyopathies due to electron transport chain defects. In Appel S, ed. Current Neurology. 1993: 133-166.