Myoadenylate deaminase deficiency: absence of correlation with exercise intolerance in 452 muscle biopsies

Two Rare Cases of Long Surviving Riboflavin Transporter Deficiency with Co-Existing Adenosine Monophosphate Deaminase (AMP) Deficiency

(1) Background: Riboflavin transporter deficiency (RTD), formerly known as Brown−Vialetto−Van Laere syndrome, is a rare condition that causes a progressive neurological syndrome in early life with features of auditory and optic neuropathy, weakness of bulbar muscles and the diaphragm and sensorimotor neuropathy. Pathologic mutations in the genes that code for riboflavin transporters have been identified as the genetic basis of RTD, and the majority of the genetically confirmed cases are caused by mutations of SLC52A3, a riboflavin transporter 2 coding gene or compound mutations in SLC52A2, encoding riboflavin transporter 3. Fatality in childhood is common if the condition is left untreated, but survival into adulthood has been reported in cases treated with high-dose oral riboflavin. (2) Case summary: We report two long-term survivors of RTD type 2 due to compound heterozygous 185T> G and 1258G>A mutations in gene SLC2A2. They are two brothers in a family in which two female siblings died in childhood from a similar neurological disorder. Brother one, the older RTD survivor, is aged 71, and brother two is aged 58. Both have significant visual impairment from optic nerve atrophy and sensory ataxia. Their muscle biopsies showed decreased muscle adenosine monophosphate (AMP) deaminase activity. No AMPD1 mutation was detected through whole-genome sequencing. (3) Conclusion: Co-existing riboflavin transporter deficiency (RTD) type 2 and muscle AMP deaminase deficiency has not been previously reported. Apart from the possibility that there is a milder phenotype associated with these mutations in SLC2A2, AMP deaminase deficiency might have contributed to a survival benefit by preserving muscle function through accumulating intracellular AMP.

Secondary myoadenylate deaminase deficiency is not a common feature of inflammatory myopathies: A descriptive study

Background

Myoadenylate deaminase (MAD) deficiency is a form of metabolic myopathy, which generally causes only mild symptoms in the primary inherited form. Inflammatory myopathies are a group of autoimmune diseases which result in skeletal muscle weakness. In addition to inflammatory pathology, it has been speculated that non-inflammatory mechanisms, and possibly secondary MAD-deficiency, may potentially contribute to weakness in these conditions.

Methods

We investigated for an association between these two myopathic processes through two complementary methods. Firstly, muscle biopsy records in South Australia over a 17-year period were retrospectively reviewed for diagnosis of myositis or MAD-deficiency, as well as associated clinical features. Secondly, a prospective arm histochemically tested all incident biopsy specimens over a 12-month period for MAD-deficiency.

Results

In the retrospective arm, 30 MAD-deficient cases were identified (1.3% of all biopsies), with no significant difference observed in overall rates of myositis diagnosis between patients with intact and deficient MAD activity (21.3% vs 26.7%, P = 0.47). No cases of MAD-deficiency were detected in the prospective arm, despite 39 cases of myositis being identified over this period.

Conclusion

Secondary MAD deficiency is unlikely to be a major driver of symptoms in inflammatory myopathies.

Diagnostic Algorithm for Glycogenoses and Myoadenylate Deaminase Deficiency Based on Exercise Testing Parameters: A Prospective Study

Aim

Our aim was to evaluate the accuracy of aerobic exercise testing to diagnose metabolic myopathies.

Methods

From December 2008 to September 2012, all the consecutive patients that underwent both metabolic exercise testing and a muscle biopsy were prospectively enrolled. Subjects performed an incremental and maximal exercise testing on a cycle ergometer. Lactate, pyruvate, and ammonia concentrations were determined from venous blood samples drawn at rest, during exercise (50% predicted maximal power, peak exercise), and recovery (2, 5, 10, and 15 min). Biopsies from vastus lateralis or deltoid muscles were analysed using standard techniques (reference test). Myoadenylate deaminase (MAD) activity was determined using p-nitro blue tetrazolium staining in muscle cryostat sections. Glycogen storage was assessed using periodic acid-Schiff staining. The diagnostic accuracy of plasma metabolite levels to identify absent and decreased MAD activity was assessed using Receiver Operating Characteristic (ROC) curve analysis.

Results

The study involved 51 patients. Omitting patients with glycogenoses (n = 3), MAD staining was absent in 5, decreased in 6, and normal in 37 subjects. Lactate/pyruvate at the 10th minute of recovery provided the greatest area under the ROC curves (AUC, 0.893 ± 0.067) to differentiate Abnormal from Normal MAD activity. The lactate/rest ratio at the 10th minute of recovery from exercise displayed the best AUC (1.0) for discriminating between Decreased and Absent MAD activities. The resulting decision tree achieved a diagnostic accuracy of 86.3%.

Conclusion

The present algorithm provides a non-invasive test to accurately predict absent and decreased MAD activity, facilitating the selection of patients for muscle biopsy and target appropriate histochemical analysis.

Muscle histopathology and physiology in chronic fatigue syndrome

Chronic fatigue syndrome (CFS) is characterized by fatigue at rest which is made worse by exercise. Previous biopsy studies on small numbers of CFS patients have shown a range of morphological changes to which have been attributed fatigue and myalgia. We have now studied 108 patients with CFS or muscle pain and 22 normal volunteers by light and electron microscopy. There was no consistent correlation between symptoms and changes in fibre type prevalence, fibre size, degenerative or regenerative features, glycogen depletion, or mitochondrial abnormalities. Physiological contractile properties of quadriceps (maximal isometric force generation, frequency: force characteristics and relaxation rate) were also examined before and for up to 48 hours after a symptom-limited incremental cycle ergometer exercise test in 12 CFS patients and 12 normal volunteers. Voluntary and stimulated force characteristics were normal at rest and during recovery. Exercise duration was similar in the two groups although CFS patients had higher perceived exertion scores in relation to heart rate during exercise, indicating a reduced effort sensation threshold. On physiological and pathological grounds it is clear that CFS is not a myopathy. Psychological/psychiatric factors appear to be of greater importance in this condition.

Skeletal muscle contractile performance and ADP accumulation in adenylate kinase-deficient mice

The production of AMP by adenylate kinase (AK) and subsequent deamination by AMP deaminase limits ADP accumulation during conditions of high-energy demand in skeletal muscle. The goal of this study was to investigate the consequences of AK deficiency (-/-) on adenine nucleotide management and whole muscle function at high-energy demands. To do this, we examined isometric tetanic contractile performance of the gastrocnemius-plantaris-soleus (GPS) muscle group in situ in AK1(-/-) mice and wild-type (WT) controls over a range of contraction frequencies (30-120 tetani/min). We found that AK1(-/-) muscle exhibited a diminished inosine 5'-monophosphate formation rate (14% of WT) and an inordinate accumulation of ADP ( approximately 1.5 mM) at the highest energy demands, compared with WT controls. AK-deficient muscle exhibited similar initial contractile performance (521 +/- 9 and 521 +/- 10 g tension in WT and AK1(-/-) muscle, respectively), followed by a significant slowing of relaxation kinetics at the highest energy demands relative to WT controls. This is consistent with a depressed capacity to sequester calcium in the presence of high ADP. However, the overall pattern of fatigue in AK1(-/-) mice was similar to WT control muscle. Our findings directly demonstrate the importance of AMP formation and subsequent deamination in limiting ADP accumulation. Whole muscle contractile performance was, however, remarkably tolerant of ADP accumulation markedly in excess of what normally occurs in skeletal muscle.

Intolérance à l’effort et rhabdomyolyses d’effort : étiologies et démarche diagnostique

Exercise intolerance, sometimes complicated by rhabdomyolysis is a frequent complaint of patients consulting in neuromuscular centers. It may be the main clinical manifestation of many metabolic myopathies and muscular dystrophies. The first step of diagnosis relies on the performance of in vivo metabolism investigations: forearm or bicycle ergometer exercise tests, phosphorus nuclear magnetic resonance spectroscopy. A few enzymatic defects may be directly assessed on blood samplings, in particular carnitine palmitoyltransferase deficiency; but muscle biopsy is necessary in most cases in order to precise the etiology. When CK levels are elevated at rest, a muscle CT scan should be performed in order to detect muscles fatty replacement suggestive of a muscular dystrophy; this diagnosis will be confirmed with immunohistochemical and western-blot analysis of muscle proteins. We present a description of the main metabolic myopathies manifesting by exercise intolerance with an overview of clinical and laboratory evaluation leading to diagnosis. Differential diagnosis are also discussed.

Myotonia congenita and myoadenylate deaminase deficiency: case report

Approximately 1-2% of the population has a deficiency of the enzyme myoadenylate deaminase. Early reports suggested that patients with myoadenylate deaminase deficiency had various forms of myalgia, and exercise intolerance. However, a deficiency of the enzyme has been described in many conditions, including myopathies, neuropathies, and motor neuron disease. We report a patient with clinical diagnosis of myotonia congenita and absent myoadenylate deaminase reaction on the muscle biopsy. This is the first description of myoadenilate deaminase deficiency with myotonia congenita. Myoadenylate deaminase deficiency is the most common enzymatic deficit of muscle, and the association with other neuromuscular diseases is coincidental.

Myoadenylate deaminase deficiency. A common inherited defect with heterogeneous clinical presentation

Myoadenylate deaminase deficiency is a clinically heterogeneous metabolic disorder that is commonly diagnosed in a variety of neurologic settings. Although the molecular basis for this purine nucleotide catabolic derangement may typically be attributed to the inheritance of a single prevalent mutant allele, the clinical spectrum in the absence of other definable abnormalities can range from asymptomatic to mild exercise-induced myalgia. Moreover, myoadenylate deaminase deficiency is also found associated with other definable neuromuscular disorders. The myoadenylate deaminase deficiency in these latter cases may, in part, be precipitated by pathologic change or act synergistically in combination with another metabolic disease.

Enzymes of the purine nucleotide cycle in muscle of patients with exercise intolerance

The activities of adenylosuccinate synthetase, adenylosuccinate lyase, and adenosine monophosphate deaminase were measured in muscle from patients suffering from fatigue and cramps following exercise. Results denote the existence of secondary deficiencies of adenylosuccinate synthetase and/or adenylosuccinate lyase in subjects with congenital or acquired myopathies. They also suggest that searches are warranted for primary deficiencies of adenylosuccinate synthetase as a cause of exercise intolerance.

Clinical heterogeneity and molecular mechanisms in inborn muscle AMP deaminase deficiency

Lack of the muscle-specific isoform of AMP deaminase (myoadenylate deaminase deficiency) can cause a metabolic myopathy, with exercise-induced muscle symptoms such as early fatigue, cramps and/or myalgia. It is the most common muscle enzyme defect in man, found in about 2-3% of all muscle biopsies. The genetic basis of the inherited defect is the nonsense mutation C34-T in the AMPD1 gene encoding myoadenylate deaminase. The mutation results in a premature stop of the enzyme synthesis. In a healthy German population, the frequency of the mutant allele was 0.1, and 1% of this population is expected to be homozygous for the mutation. In people with muscle symptoms, the allele frequency was significantly higher (0.145). The correlation between allele frequency and muscle symptoms underscores the clinical significance of this defect. However, the vast majority of homozygous subjects do not develop a metabolic myopathy. This clinical heterogeneity may be due to molecular genetic factors such as alternative splicing of the exon harbouring the mutation, or due to metabolic conditions such as pathways compensating for the defect. The real basis for the high percentage of asymptomatic homozygous subjects remains to be revealed.

Molecular biology of AMP deaminase deficiency

In man, there are at least four isoforms of adenosine monophosphate deaminase (AMPD): myoadenylate deaminase in skeletal muscle, the L isoform in liver, and the E1 and E2 isoforms in erythrocytes. Myoadenylate deaminase is encoded by the AMPD1 gene located on chromosome 1 p13-p21, the L isoform by the AMPD2 gene, and both isoforms in erythrocytes by the AMPD3 gene. Myoadenylate deaminase deficiency is found in 2-3% of all muscle biopsies. The inborn type of myoadenylate deaminase deficiency is caused by a single mutant allele harbouring two mutations: C34-->T (Gln-->Stop) and C143-->T (Pro-48-->Leu). Population studies revealed a frequency of the mutant allele of 0.12 in Caucasian Americans and Germans. The C34-->T mutation is located in exon 2, which is alternatively spliced in part of the AMPD1 transcript in human muscle. Since the second mutation does not affect enzyme function, alternatively spliced mRNA encodes a catalytically active enzyme. Only one patient with a disorder linked to liver AMPD has been described so far. In this patient the decreased inhibition of this enzyme by GTP resulted in uric acid overproduction and gout. A complete lack of erythroyte AMPD activity is found in asymptomatic subjects. The molecular basis of both disorders is not yet known.

AMP deaminase in skeletal muscle of healthy males quantitatively determined by new assay

A new, sensitive assay for the quantitative determination of AMP deaminase activity in human skeletal muscle is presented. The method is based on the determination of the direct product of the AMP deaminase reaction, the formed IMP, by high performance liquid chromatography (HPLC). In order to evaluate the relationship between AMP deaminase activity on the one hand and the contractile and metabolic characteristics of the muscle and the physical performance on the other, muscle biopsies were taken from 20 male subjects. The subjects also performed a 30 s sprint test on a cycle ergometer. The inter-individual variation in AMP deaminase activity was large, ranging from 5.4 to 27.4 microkat g-1 dry muscle. AMP deaminase was positively correlated with phosphofructokinase (PFK), the marker of the glycolytic capacity of the muscle, but there was no correlation with enzymes of oxidative metabolism, such as 3-hydroxyacyl-CoA dehydrogenase and citrate synthase, or with the activity of myokinase and lactate dehydrogenase. There was no significant correlation between AMP deaminase activity and the proportion of the different muscle fibre types. A weak positive correlation was found between the sprint performance and the AMP deaminase activity. In conclusion, the HPLC assay was found to be a fast, sensitive and reliable method for the quantitative determination of AMP deaminase activity in muscle. A direct relationship between AMP deaminase activity on the one hand and glycolytic capacity and sprint performance on the other was found. However, no relationship to oxidative capacity or contractile properties was found.

The purine nucleotide cycle and its molecular defects

Three enzymes of purine metabolism, adenylosuccinate synthetase, adenylosuccinate lyase and AMP deaminase, have been proposed to form a functional unit, termed the purine nucleotide cycle. This cycle converts AMP into IMP and reconverts IMP into AMP via adenylosuccinate, thereby producing NH3 and forming fumarate from aspartate. In muscle, the purine nucleotide cycle has been shown to function during intense exercise; the metabolic flux through the cycle has been proposed to play a role in the regeneration of ATP by pulling the adenylate kinase reaction in the direction of formation of ATP, and by providing Krebs cycle intermediates. In kidney, the purine nucleotide cycle was shown to account for the release of NH3 under the normal acid-base status, but not under acidotic conditions. In brain, the purine nucleotide cycle might function under conditions that induce a loss of ATP, and thereby contribute to its recovery. There is no evidence that the purine nucleotide cycle operates in liver. Deficiency of muscle AMP deaminase is an apparently frequent disorder, which might affect approximately 2% of the general population. The observation that it can be found in clinically asymptomatic individuals suggests, paradoxically, that the ATP-regenerating function which has been attributed to the purine nucleotide cycle is not essential for muscle function. Further work should be aimed at identifying the conditions under which AMP deaminase deficiency becomes symptomatic. Adenylosuccinate lyase deficiency provokes psychomotor retardation, often accompanied by autistic features. Its clinical heterogeneity justifies systematic screening in patients with unexplained mental deficiency. Additional studies are required to determine the mechanisms whereby this enzyme defect results in psychomotor retardation.

Rhabdomyolysis in childhood. A primer on normal muscle function and selected metabolic myopathies characterized by disordered energy production

Patients with rhabdomyolysis present an important clinical problem. In acute episodes immediate treatment may be necessary to prevent significant morbidity and mortality. Evaluation of affected patients necessitates an understanding of basic muscle pathophysiology and of the variety of disturbances that can interfere with muscle energy metabolism. The physician must then pursue a systematic stepwise evaluation (Table 6) that includes obtaining relevant history and laboratory studies, as well as arranging for appropriate provocative testing and muscle biopsy. Once the diagnosis is established, patient and family counseling is necessary, particularly in genetic disorders. Unfortunately, specific therapies have not proven entirely successful, and treatment generally has been directed at reducing the severity of rhabdomyolytic episodes.

Type 2a fibre rhabdomyolysis in myoadenylate deaminase deficiency

A 31-year-old woman developed an acute, potentially fatal rhabdomyolysis of undetermined origin. Muscle biopsy revealed selective lysis involving exclusively type 2a fibers. Myoadenylate-deaminase (MAD) deficiency was proven by a negative histochemical reaction as well as by an enzymatic biochemical determination. The significantly greater energetic dependence of type 2a fibres on MAD explains their selective damage. The patient's mother also suffers from a similar muscle disease of still unclarified origin.