Clinical heterogeneity and molecular mechanisms in inborn muscle AMP deaminase deficiency

Genetic Profile in Genes Associated with Sports Injuries in Elite Endurance Athletes

Injuries are a complex trait that can stem from the interaction of several genes. The aim of this research was to examine the relationship between muscle performance-related genes and overuse injury risk in elite endurance athletes, and to examine the feasibility of determining a total genotype score that significantly correlates with injury. A cohort of 100 elite endurance athletes (50 male and 50 female) was selected. AMPD1 (rs17602729), ACE (rs4646994), ACTN3 (rs1815739), CKM (rs8111989) and MLCK ([rs2849757] and [rs2700352]) polymorphisms were genotyped by using real-time polymerase chain reaction (real time-PCR). Injury characteristics during the athletic season were classified following the Consensus Statement for injuries evaluation. The mean total genotype score (TGS) in non-injured athletes (68.263±13.197 arbitrary units [a.u.]) was different from that of injured athletes (50.037±17.293 a.u., p<0.001). The distribution of allelic frequencies in the AMPD1 polymorphism was also different between non-injured and injured athletes (p<0.001). There was a TGS cut-off point (59.085 a.u.) to discriminate non-injured from injured athletes with an odds ratio of 7.400 (95% CI 2.548-21.495, p<0.001). TGS analysis appears to correlate with elite endurance athletes at higher risk for injury. Further study may help to develop this as one potential tool to help predict injury risk in this population.

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.

Genetic profile in genes associated with muscle injuries and injury etiology in professional soccer players

Many causes define injuries in professional soccer players. In recent years, the study of genetics in association with injuries has been of great interest. The purpose of this study was to examine the relationship between muscle injury-related genes, injury risk and injury etiology in professional soccer players. In a cross-sectional cohort study, one hundred and twenty-two male professional football players were recruited. AMPD1 (rs17602729), ACE (rs4646994), ACTN3 (rs1815739), CKM (rs8111989) and MLCK (rs2849757 and rs2700352) polymorphisms were genotyped by using Single Nucleotide Primer Extension (SNPE). The combined influence of the six polymorphisms studied was calculated using a total genotype score (TGS). A genotype score (GS) of 2 was assigned to the “protective” genotype for injuries, a GS of 1 was assigned to the heterozygous genotype while a GS of 0 was assigned to the “worst” genotype. Injury characteristics and etiology during the 2021/2022 season were classified following a Consensus Statement for injuries recording. The distribution of allelic frequencies in the AMPD1 and MLCK c.37885C&gt;A polymorphisms were different between non-injured and injured soccer players (p &lt; 0.001 and p = 0.003, respectively). The mean total genotype score (TGS) in non-injured soccer players (57.18 ± 14.43 arbitrary units [a.u.]) was different from that of injured soccer players (51.71 ± 12.82 a.u., p = 0.034). There was a TGS cut-off point (45.83 a.u.) to discriminate non-injured from injured soccer players. Players with a TGS beyond this cut-off had an odds ratio of 1.91 (95%CI: 1.14–2.91; p = 0.022) to suffer an injury when compared with players with lower TGS. In conclusion, TGS analysis in muscle injury-related genes presented a relationship with professional soccer players at increased risk of injury. Future studies will help to develop this TGS as a potential tool to predict injury risk and perform prevention methodology in this cohort of football players.

Skeletal muscle contraction kinetics and AMPK responses are modulated by the adenine nucleotide degrading enzyme AMPD1

AMP deaminase 1 (AMPD1; AMP → IMP + NH3) deficiency in skeletal muscle results in an inordinate accumulation of AMP during strenuous exercise, with some but not all studies reporting premature fatigue and reduced work capacity. To further explore these inconsistencies, we investigated the extent to which AMPD1 deficiency impacts skeletal muscle contractile function of different muscles and the [AMP]/AMPK responses to different intensities of fatiguing contractions. To reduce AMPD1 protein, we electroporated either an inhibitory AMPD1-specific miRNA encoding plasmid or a control plasmid, into contralateral EDL and SOL muscles of C57BL/6J mice (n = 48 males, 24 females). After 10 days, isolated muscles were assessed for isometric twitch, tetanic, and repeated fatiguing contraction characteristics using one of four (None, LOW, MOD, and HIGH) duty cycles. AMPD1 knockdown (∼35%) had no effect on twitch force or twitch contraction/relaxation kinetics. However, during maximal tetanic contractions, AMPD1 knockdown impaired both time-to-peak tension (TPT) and half-relaxation time (½ RT) in EDL, but not SOL muscle. In addition, AMPD1 knockdown in EDL exaggerated the AMP response to contractions at LOW (+100%) and MOD (+54%) duty cycles, but not at HIGH duty cycle. This accumulation of AMP was accompanied by increased AMPK phosphorylation (Thr-172; LOW +25%, MOD +34%) and downstream substrate phosphorylation (LOW +15%, MOD +17%). These responses to AMPD1 knockdown were not different between males and females. Our findings demonstrate that AMPD1 plays a role in maintaining skeletal muscle contractile function and regulating the energetic responses associated with repeated contractions in a muscle- but not sex-specific manner.NEW & NOTEWORTHY AMP deaminase 1 (AMPD1) deficiency has been associated with premature muscle fatigue and reduced work capacity, but this finding has been inconsistent. Herein, we report that although AMPD1 knockdown in mouse skeletal muscle does not change maximal isometric force, it negatively impacts muscle function by slowing contraction and relaxation kinetics in EDL muscle but not SOL muscle. Furthermore, AMPD1 knockdown differentially affects the [AMP]/AMPK responses to fatiguing contractions in an intensity-dependent manner in EDL muscle.

Genetic profiles to identify talents in elite endurance athletes and professional football players

The genetic profile that is needed to identify talents has been studied extensively in recent years. The main objective of this investigation was to approach, for the first time, the study of genetic variants in several polygenic profiles and their role in elite endurance and professional football performance by comparing the allelic and genotypic frequencies to the non-athlete population. In this study, genotypic and allelic frequencies were determined in 452 subjects: 292 professional athletes (160 elite endurance athletes and 132 professional football players) and 160 non-athlete subjects. Genotyping of polymorphisms in liver metabolisers (CYP2D6, GSTM1, GSTP and GSTT), iron metabolism and energy efficiency (HFE, AMPD1 and PGC1a), cardiorespiratory fitness (ACE, NOS3, ADRA2A, ADRB2 and BDKRB2) and muscle injuries (ACE, ACTN3, AMPD1, CKM and MLCK) was performed by Polymerase Chain Reaction-Single Nucleotide Primer Extension (PCR-SNPE). The combination of the polymorphisms for the “optimal” polygenic profile was quantified using the genotype score (GS) and total genotype score (TGS). Statistical differences were found in the genetic distributions between professional athletes and the non-athlete population in liver metabolism, iron metabolism and energy efficiency, and muscle injuries (p<0.001). The binary logistic regression model showed a favourable OR (odds ratio) of being a professional athlete against a non-athlete in liver metabolism (OR: 1.96; 95% CI: 1.28–3.01; p = 0.002), iron metabolism and energy efficiency (OR: 2.21; 95% CI: 1.42–3.43; p < 0.001), and muscle injuries (OR: 2.70; 95% CI: 1.75–4.16; p < 0.001) in the polymorphisms studied. Genetic distribution in professional athletes as regards endurance (professional cyclists and elite runners) and professional football players shows genetic selection in these sports disciplines.

Genotype scores in energy and iron-metabolising genes are higher in elite endurance athletes than in nonathlete controls

Information about the association of energy and iron-metabolising genes with endurance performance is scarce. The objective of this investigation was to compare the frequencies of polymorphic variations of genes involved in energy generation and iron metabolism in elite endurance athletes versus nonathlete controls. Genotype frequencies in 123 male elite endurance athletes (75 professional road cyclists and 48 elite endurance runners) and 122 male nonathlete participants were compared by assessing 4 genetic polymorphisms: AMPD1 c.34C/T (rs17602729), PPARGC1A c.1444G/A (rs8192678) HFE_H63D c.187C/G (rs1799945) and HFE_C282Y c.845G/A (rs1800562). A weighted genotype score (w-TGS; from 0 to 100 arbitrary units (a.u.)) was calculated by assigning a corresponding weight to each polymorphism. In the nonathlete population, the mean w-TGS value was lower (39.962 ± 14.654 a.u.) than in the group of elite endurance athletes (53.344 ± 17.053 a.u). The binary logistic regression analysis showed that participants with a w-TGS > 38.975 a.u had an odds ratio of 1.481 (95% confidence interval: 1.244-1.762; p < 0.001) for achieving elite athlete status. The genotypic distribution of polymorphic variations involved in energy generation and iron metabolism was different in elite endurance athletes vs. controls. Thus, an optimal genetic profile in these genes might contribute to physical endurance in athlete status. Novelty Genetic profile in energy generation and iron-metabolising genes in elite endurance athletes is different than that of nonathletes. There is an implication of an "optimal" genetic profile in the selected genes favouring endurance sporting performance.

Comparative transcriptome analysis of skeletal muscle in ADSSL1 myopathy

ADSSL1 myopathy was recently identified as the cause of muscular disorders in Korean patients with distal myopathy. We generated transcriptome profiles of muscles from control subjects and patients with ADSSL1 myopathy. In the present study, RNA sequencing was conducted with seven vastus lateralis muscle samples from four patients with ADSSL1 myopathy and three control subjects. The hierarchical clustering result revealed a separation between myopathy and control groups. A total of 1,260 transcripts were significantly differentially expressed (|fold change| ≥ 2, p < 0.05), with 740 upregulated transcripts and 520 downregulated transcripts in myopathy group. Eighteen transcripts that mapped to purine metabolism pathway were significantly differentially expressed between the two groups, with ten downregulated transcripts and eight upregulated transcripts in myopathy group. In particular, three genes involved in purine nucleotide cycle (ADSSL1, ADSL, and AMPD1) were significantly downregulated in myopathy group. Ten transcripts in glycolysis/gluconeogenesis pathway were also significantly differentially expressed. This is the first study on the altered expression of transcripts in muscle tissues from patients with ADSSL1 myopathy. Our results provide new insights into the pathogenesis of ADSSL1 myopathy.

Knockout of human muscle genes revealed by large scale whole-exome studies

Large scale whole-exome sequence studies have revealed that a number of individuals from different populations have predicted loss-of-function of different genes due to nonsense, frameshift, or canonical splice-site mutations. Surprisingly, many of these mutations do not apparently show the deleterious phenotypic consequences expected from gene knockout. These homozygous null mutations, when confirmed, can provide insight into human gene function and suggest novel approaches to correct gene dysfunction, as the lack of the expected disease phenotype may reflect the existence of modifier genes that reveal potential therapeutic targets. Human knockouts complement the information derived from mouse knockouts, which are not always good models of human disease. We have examined human knockout datasets searching for genes expressed exclusively or predominantly in striated muscle. A number of well-known muscle genes was found in one or more datasets, including genes coding for sarcomeric myosins, components of the sarcomeric cytoskeleton, sarcoplasmic reticulum and plasma membrane, and enzymes involved in muscle metabolism. The surprising absence of phenotype in some of these human knockouts is critically discussed, focusing on the comparison with the corresponding mouse knockouts.

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.

A compendium of inborn errors of metabolism mapped onto the human metabolic network

Inborn errors of metabolism (IEMs) are hereditary metabolic defects, which are encountered in almost all major metabolic pathways occurring in man. Many IEMs are screened for in neonates through metabolomic analysis of dried blood spot samples. To enable the mapping of these metabolomic data onto the published human metabolic reconstruction, we added missing reactions and pathways involved in acylcarnitine (AC) and fatty acid oxidation (FAO) metabolism. Using literary data, we reconstructed an AC/FAO module consisting of 352 reactions and 139 metabolites. When this module was combined with the human metabolic reconstruction, the synthesis of 39 acylcarnitines and 22 amino acids, which are routinely measured, was captured and 235 distinct IEMs could be mapped. We collected phenotypic and clinical features for each IEM enabling comprehensive classification. We found that carbohydrate, amino acid, and lipid metabolism were most affected by the IEMs, while the brain was the most commonly affected organ. Furthermore, we analyzed the IEMs in the context of metabolic network topology to gain insight into common features between metabolically connected IEMs. While many known examples were identified, we discovered some surprising IEM pairs that shared reactions as well as clinical features but not necessarily causal genes. Moreover, we could also re-confirm that acetyl-CoA acts as a central metabolite. This network based analysis leads to further insight of hot spots in human metabolism with respect to IEMs. The presented comprehensive knowledge base of IEMs will provide a valuable tool in studying metabolic changes involved in inherited metabolic diseases.

Genes, physical fitness and ageing

Persons aged 80 years and older are the fastest growing segment of the population. As more individuals live longer, we should try to understand the mechanisms involved in healthy ageing and preserving functional independence in later life. In elderly people, functional independence is directly dependent on physical fitness, and ageing is inevitably associated with the declining functions of systems and organs (heart, lungs, blood vessels, skeletal muscles) that determine physical fitness. Thus, age-related diminished physical fitness contributes to the development of sarcopenia, frailty or disability, all of which severely deteriorate independent living and thus quality of life. Ageing is a complex process involving many variables that interact with one another, including - besides lifestyle factors or chronic diseases - genetics. Thus, several studies have examined the contribution of genetic endowment to a decline in physical fitness and subsequent loss of independence in later life. In this review, we compile information, including data from heritability, candidate-gene association, linkage and genome-wide association studies, on genetic factors that could influence physical fitness in the elderly.

Polymorphisms influencing muscle phenotypes in North-African and Spanish populations

AIM

The purpose of this study was to determine the allelic and genotypic frequency distribution of the C34T mutation in the muscle isoform of the adenosine monophosphate deaminase 1 (AMPD1) gene and of the missense substitution K153R in the myostatin (GDF8) gene in one Spanish and two North African populations.

METHOD

One sample of 98 individuals was genotyped from the South of Spain (Alpujarra) and two samples from Morocco (77 Berbers and 78 Arabs).

RESULTS

The frequency of the AMPD1 C34T mutation was lower in Berbers (0.071) compared with the Alpujarra cohort (0.153, p = 0.018). The GDF8 K153R substitution showed little variability among the three cohorts.

CONCLUSIONS

Studies with larger cohorts and other ethnic groups are needed to corroborate that there does not exist any major variability in the genotype distribution of genes associated with muscle phenotypes in the South-Eastern Mediterranean area.

Malignant hyperthermia: human stress triggering

Letter to the Editor concerns the question of a discussion of awake porcine malignant hyperthermia that erroneously omits the awake human stress reaction of malignant hyperthermia.

AMPD1 gene mutations are associated with obesity and diabetes in Polish patients with cardiovascular diseases

Previous studies showed an association of the common functional polymorphism (C34T, Gln12Stop) in the adenosine monophosphate deaminase-1 (AMPD1) gene with survival in heart failure (HF) and/or coronary artery disease (CAD). The aim of the study was to search for other mutations in selected regions of the AMPD1 gene in Polish CAD and HF patients, and to analyze their associations with obesity and diabetes. Exons 2, 3, 5, and 7 of AMPD1 were scanned for mutations in 97 patients with CAD without HF (CAD+ HF−), 104 patients with HF (HF+), and 200 newborns from North-Western Poland using denaturing high-performance liquid chromatography (DHPLC), polymerase chain reaction–restriction fragment length polymorphism (PCR-RFLP), and direct sequencing. Frequencies of AMPD1 C34T mutation, as well as novel A99G, G512A, IVS4-6delT, and C784T sequence alterations, were similar in the three groups, but 860T mutated allele was less frequent in the combined CAD+ HF− and HF+ groups than in the controls (1.7% vs. 4.3%, p = 0.040). Heterozygous 34CT genotype was associated with lower (odds ratio [OR] = 0.32, 95% confidence interval [CI] = 0.13–0.81) and 860AT with higher (OR = 13.7, 95%CI = 1.6–118) prevalence of diabetes or hyperglycemia in relation to wild-type homozygotes. Abdominal obesity was more frequent in 860AT patients than in wild-type homozygotes and 34CT heterozygotes (86% vs. 40% vs. 29%, p < 0.05). Nine genes containing polymorphisms linked with AMPD1 C34T mutation were found in the HapMap database. AMPD1 C34T nonsense mutation is associated with reduced prevalence of diabetes and obesity in patients with CAD or HF, but A860T substitution seems to exert opposite metabolic effects and should always be accounted for in the studies of the AMPD1 genotype.

Comparative gene expression profiling between human cultured myotubes and skeletal muscle tissue

Background

A high-sensitivity DNA microarray platform requiring nanograms of RNA input facilitates the application of transcriptome analysis to individual skeletal muscle (SM) tissue samples. Culturing myotubes from SM-biopsies enables investigating transcriptional defects and assaying therapeutic strategies. This study compares the transcriptome of aneurally cultured human SM cells versus that of tissue biopsies.

Results

We used the Illumina expression BeadChips to determine the transcriptomic differences between tissue and cultured SM samples from five individuals. Changes in the expression of several genes were confirmed by QuantiGene Plex assay or reverse transcription real-time PCR. In cultured myotubes compared to the tissue, 1216 genes were regulated: 583 down and 633 up. Gene ontology analysis showed that downregulated genes were mainly associated with cytoplasm, particularly mitochondria, and involved in metabolism and the muscle-system/contraction process. Upregulated genes were predominantly related to cytoplasm, endoplasmic reticulum, and extracellular matrix. The most significantly regulated pathway was mitochondrial dysfunction. Apoptosis genes were also modulated. Among the most downregulated genes detected in this study were genes encoding metabolic proteins AMPD1, PYGM, CPT1B and UCP3, muscle-system proteins TMOD4, MYBPC1, MYOZ1 and XIRP2, the proteolytic CAPN3 and the myogenic regulator MYF6. Coordinated reduced expression of five members of the GIMAP gene family, which form a cluster on chromosome 7, was shown, and the GIMAP4-reduction was validated. Within the most upregulated group were genes encoding senescence/apoptosis-related proteins CDKN1A and KIAA1199 and potential regulatory factors HIF1A, TOP2A and CCDC80.

Conclusions

Cultured muscle cells display reductive metabolic and muscle-system transcriptome adaptations as observed in muscle atrophy and they activate tissue-remodeling and senescence/apoptosis processes.

Inborn errors of purine and pyrimidine metabolism

Genetic disorders of purine and pyrimidine (PP) metabolism are under-reported and infrequently mentioned in the general literature, as well as in reviews dedicated to other inborn errors of metabolism. Owing to limited awareness, relatively recent recognition, as well as considerable phenotypic variation, these disorders may often be misdiagnosed or remain undiagnosed. Disorders that arise as a result of dysfunction in PP metabolism represent some of the most challenging diagnostic problems in medicine. In addition to their low prevalence rates, they also present with extremely variable signs and symptoms. They may affect any system in a variety of manners, and often mimic other, more recognizable disorders. The diagnostic problem is compounded by the fact that some biochemically affected patients are symptom-free. Rapidly evolving laboratory techniques such as high-performance liquid chromatography coupled to tandem mass spectrometry are now well established as the preferred method for detection for these defects, but currently the most important step in diagnosis consists of suspecting the disorder. Diagnosis is vital because genetic counselling can be provided and, in some cases, specific treatment can be offered that may slow or even reverse clinical symptoms. If undiagnosed, these disorders can be devastating to patients and their families, resulting in early death or institutionalization for the rest of patient's life. This article describes the current state of knowledge about inborn errors of purine and pyrimidine metabolism, focusing on the varying clinical presentations, the laboratory findings and discusses indications for selective screening for these disorders.

AMP deaminase deficiency is associated with lower sprint cycling performance in healthy subjects

AMP deaminase (AMPD) deficiency is an inherited disorder of skeletal muscle found in ∼2% of the Caucasian population. Although most AMPD-deficient individuals are asymptomatic, a small subset has exercise-related cramping and pain without any other identifiable neuromuscular complications. This heterogeneity has raised doubts about the physiological significance of AMPD in skeletal muscle, despite evidence for disrupted adenine nucleotide catabolism during exercise in deficient individuals. Previous studies have evaluated the effect of AMPD deficiency on exercise performance with mixed results. This study was designed to circumvent the perceived limitations in previous reports by measuring exercise performance during a 30-s Wingate test in 139 healthy, physically active subjects of both sexes, with different AMPD1 genotypes, including 12 AMPD-deficient subjects. Three of the deficient subjects were compound heterozygotes characterized by the common c.34C>T mutation in one allele and a newly discovered AMPD1 mutation, c.404delT, in the other. While there was no significant difference in peak power across AMPD1 genotypes, statistical analysis revealed a faster power decrease in the AMPD-deficient group and a difference in mean power across the genotypes ( P = 0.0035). This divergence was most striking at 15 s of the 30-s cycling. Assessed by the fatigue index, the decrease in power output at 15 s of exercise was significantly greater in the deficient group compared with the other genotypes ( P = 0.0006). The approximate 10% lower mean power in healthy AMPD-deficient subjects during a 30-s Wingate cycling test reveals a functional role for the AMPD1 enzyme in sprint exercise.

Neuromuscular implications in left ventricular hypertrabeculation/noncompaction

This review focuses on recent advances in the association between left ventricular hypertrabeculation/noncompaction (LVHT), a form of unclassified cardiomyopathy, and neuromuscular disorders (NMD). So far, LVHT has been found in single patients with dystrophinopathy, dystrobrevinopathy, laminopathy, zaspopathy, myotonic dystrophy, infantile glycogenosis type II (Pompe's disease), myoadenylate-deaminase deficiency, mitochondriopathy, Barth syndrome, Friedreich ataxia, and Charcot-Marie-Tooth disease. Most frequently LVHT is found in patients with Barth syndrome and mitochondrial disorders. The prevalence of LVHT in NMD patients is not known. On the contrary, NMD can be detected in up to four fifths of the patients with LVHT. Because LVHT is associated with an increased risk of rhythm abnormalities and heart failure, it is essential to detect LVHT as soon as possible. Because of adequate therapeutic options, all patients with NMD should undergo a comprehensive cardiological examination as soon as their neurological diagnosis is established. In reverse, all patients with LVHT should undergo a comprehensive neurological investigation following the detection of LVHT.

Ecto- and cytosolic 5′-nucleotidases in normal and AMP deaminase-deficient human skeletal muscle

In skeletal muscle, adenosine monophosphate (AMP) is mainly deaminated by AMP deaminase. However, the C34T mutation in the AMPD1 gene severely reduces AMP deaminase activity. Alternatively, intracellular AMP is dephosphorylated to adenosine via cytosolic AMP 5'-nucleotidase (cN-I). In individuals with a homozygous C34T mutation, cN-I might be a more important pathway for AMP removal. We determined activities of AMP deaminase, cN-I, total cytosolic 5'-nucleotidase (total cN), ecto-5'-nucleotidase (ectoN) and whole homogenate 5'-nucleotidase activity in skeletal muscle biopsies from patients with different AMPD1 genotypes [homozygotes for C34T mutation (TT); heterozygotes for C34T mutation (CT); and homozygotes for wild type (CC): diseased controls CC; and normal controls CC]. AMP deaminase activity showed genotype-dependent differences. Total cN activity in normal controls accounted for 57+/-22% of whole homogenate 5'-nucleotidase activity and was not significantly different from the other groups. A weak inverse correlation was found between AMP deaminase and cN-I activities (r2=0.18, p<0.01). There were no significant differences between different groups in the activities of cN-I, whole homogenate 5'-nucleotidase and ectoN, or in cN-I expression on Western blots. No correlation for age, fibre type distribution and AMPD1 genotype was found for whole homogenate nucleotidase, total cN and cN-I using multiple linear regression analysis. There was no gender-specific difference in the activities of whole homogenate nucleotidase, total cN and cN-I. The results indicate no changes in the relative expression or catalytic behaviour of cN-I in AMP deaminase-deficient human skeletal muscle, but suggest that increased turnover of AMP by cN-I in working skeletal muscle is due to higher substrate availability of AMP.

Frequency of the C34T mutation of the AMPD1 gene in world-class endurance athletes: does this mutation impair performance?

The C34T mutation in the gene encoding for the skeletal muscle-specific isoform of AMP deaminase (AMPD1) is a common mutation among Caucasians (i.e., one of five individuals) that can impair exercise capacity. The purpose of this study was twofold. First, we determined the frequency distribution of the C34T mutation in a group of top-level Caucasian (Spanish) male endurance athletes (cyclists and runners, n = 104). This group was compared with randomly selected Caucasian (Spanish) healthy (asymptomatic) nonathletes (n = 100). The second aim of this study was to compare common laboratory indexes of endurance performance (maximal oxygen uptake or ventilatory thresholds) within the group of athletes depending on their C34T AMPD1 genotype. The frequency of the mutant T allele was lower (P < 0.05) in the group of athletes (4.3%) compared with controls (8.5%). On the other hand, indexes of endurance performance did not differ (P > 0.05) between athlete carriers or noncarriers of the C34T mutation (e.g., maximal oxygen uptake 72.3 +/- 4.6 vs. 73.5 +/- 5.9 ml.kg(-1).min(-1), respectively). In conclusion, although the frequency distribution of the mutant T allele of the AMPD1 genotype is lower in Caucasian elite endurance athletes than in controls, the C34T mutation does not significantly impair endurance performance once the elite-level status has been reached in sports.

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.

Association of medically unexplained fatigue with ACE insertion/deletion polymorphism in Gulf War veterans

Genes associated with muscle metabolism and physical endurance were evaluated for variants that may contribute to the etiology of medically unexplained severe and chronic fatigue. Subjects included 49 Gulf War veterans and 61 nonveterans with chronic fatigue syndrome (CFS) or idiopathic chronic fatigue (ICF) and 30 veterans and 45 nonveterans who served as healthy controls. Increased risk for CFS/ICF was associated with alterations of the insertion/deletion (I/D) polymorphism in the angiotensin-converting enzyme gene within the Gulf War veteran sample only. The I allele frequency was decreased in affected versus unaffected veterans (0.15 versus 0.48; odds ratio [OR], 5.08; 95% confidence interval [CI], 1.97-13.35; P < 0.0001). Correspondingly, the II genotype was decreased fourfold in affected veterans (0.08 versus 0.35; OR = 5.87; 95% CI: 1.21-28.36; P = 0.02), and the DD genotype was increased twofold (0.78 versus 0.39; OR, 5.4; 95% CI, 1.6-18.4; P = 0.007). Veterans with the DD genotype were eight times more likely to develop CFS/ICF than were those with the II genotype (OR, 8.30; 95% CI, 1.50-56.09; P = 0.009).

Associations between cardiorespiratory responses to exercise and the C34T AMPD1 gene polymorphism in the HERITAGE Family Study

The associations of the C34T polymorphism of the adenosine monophosphate deaminase 1 (AMPD1) gene with cardiorespiratory phenotypes were tested during cycling exercise at absolute and relative power outputs progressing to exhaustion before and after endurance training for 20 wk in the HERITAGE Family Study cohort (n = 779). Since no blacks were mutant homozygotes (TT), only whites were considered for analysis (400 normal homozygotes, CC; 97 heterozygotes, CT; and 6 TT). For sedentary state, cycling at the absolute power output of 50 W resulted in a higher rating of perceived exertion in TT (P < 0.0001). At the relative intensity of 60% of Vo(2 max), stroke volume was lower in TT (P < 0.05). Maximal values for power output, systolic blood pressure, heart rate, Vco(2), and respiratory exchange ratio were lower in TT (P < 0.05). The cardiorespiratory training response at 50 W and at 60% of Vo(2 max) was similar across C34T-AMPD1 genotypes. However, the maximal values for ventilation, Vo(2), and Vco(2) during exercise increased less in TT (P < 0.01). The results indicate that subjects with the TT genotype at the C34T AMPD1 gene have diminished exercise capacity and cardiorespiratory responses to exercise in the sedentary state. Furthermore, the training response of ventilatory phenotypes during maximal exercise is more limited in TT.

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.

Phenotype modulators in myophosphorylase deficiency

Myophosphorylase deficiency is characterized by exercise intolerance, muscle cramps, and recurrent myoglobinuria. Some patients are severely affected, whereas others are minimally affected or asymptomatic. The molecular basis of the disease has been elucidated but does not provide an explanation for the clinical variability. In a large cohort of patients with myophosphorylase deficiency, we tested the hypothesis that polymorphic variants in either myoadenylate deaminase (MADA) or angiotensin-converting enzyme (ACE) could act as modulators of phenotype expression. Forty-seven patients were evaluated. Clinical severity was assessed according to a severity scale of four grades. MADA activity was studied by histochemical and biochemical analysis of muscle, and the Q12X mutation in the adenine monophosphate deaminase 1 gene (AMPD1) and the insertion/deletion polymorphism in the ACE gene were assessed genetically. A complete MADA defect together with the Q12X mutation was detected in one severely affected patient. Eleven patients were heterozygous for the Q12X mutation. There was no association between clinical grading and MADA status. In contrast, we found a highly significant (p < 0.01) association between ACE genotype and clinical severity, with strong correlation between severe phenotype and number of D alleles. We show that ACE insertion/deletion polymorphism may play a significant role as phenotype modulator in McArdle's disease.

A G468-T AMPD1 mutant allele contributes to the high incidence of myoadenylate deaminase deficiency in the Caucasian population

Myoadenylate deaminase deficiency is the most common metabolic disorder of skeletal muscle in the Caucasian population, affecting approximately 2% of all individuals. Although most deficient subjects are asymptomatic, some suffer from exercise-induced myalgia suggesting a causal relationship between a lack of enzyme activity and muscle function. In addition, carriers of this derangement in purine nucleotide catabolism may have an adaptive advantage related to clinical outcome in heart disease. The molecular basis of myoadenylate deaminase deficiency in Caucasians has been attributed to a single mutant allele characterized by double C to T transitions at nucleotides +34 and +143 in mRNA encoded by the AMPD1 gene. Polymerase chain reaction-based strategies have been developed to specifically identify this common mutant allele and are considered highly sensitive. Consequently, some laboratories preferentially use this technique over other available diagnostic tests for myoadenylate deaminase deficiency. We previously identified a G468-T mutation in one symptomatic patient who was only heterozygous for the common AMPD1 mutant allele. In this report, nine additional individuals with this compound heterozygous genotype are revealed in a survey of 48 patients with documented deficiency of skeletal muscle adenosine monophosphate deaminase and exercise-induced myalgia. Western blot analysis of leftover biopsy material from one of these individuals does not detect any immunoreactive myoadenylate deaminase polypeptide. Baculoviral expression of the G468-T mutant allele produces a Q156H substitution enzyme exhibiting labile catalytic activity. These combined results demonstrate that the G468-T transversion is dysfunctional and further indicate that AMPD1 alleles harboring this mutation contribute to the high incidence of partial and complete myoadenylate deaminase deficiency in the Caucasian population. Consequently, genetic tests for abnormal AMPD1 expression designed to diagnose patients with metabolic myopathy, and to evaluate genetic markers for clinical outcome in heart disease should not be based solely on the detection of a single mutant allele.

Myoadenylate deaminase deficiency does not affect muscle anaplerosis during exhaustive exercise in humans

1. Myoadenylate deaminase (AMPD) deficiency is present in 1--2 % of the population. In theory, this deficiency may alter exercise energy metabolism by impairing the purine nucleotide cycle (PNC) and reducing tricarboxylic acid (TCA) cycle anaplerosis. The role of the PNC in TCA cycle anaplerosis is still a debated issue in physiology. Using patients with the AMPD1 mutation will allow a human 'knockout' approach to answering this question. 2. Muscle AMPD activity and genotype (whole blood AMPD1 analysis) was used to classify participants into three groups: n = 3 with absence of AMPD activity and -/- AMPD1 genotype (homozygous); n = 4 with less than 50 % normal AMPD activity and +/- genotype (heterozygous) and n = 12 with normal AMPD activity and +/+ genotype (control). Biopsies were taken from the vastus lateralis muscle before and after incremental cycle ergometry exercise to exhaustion. The muscle biopsies were analysed for AMPD activity, purine nucleotides/nucleosides and bases, creatine, phosphocreatine, amino acids, and the TCA cycle intermediates malate, citrate and fumarate. 3. Time to exhaustion on the cycle ergometer was not different between groups. Muscle adenosine monophosphate increased significantly with exercise for homozygous subjects as compared with the other groups (P < 0.05). Inosine monophosphate increased significantly after exercise for control (P < 0.05) but not for the homozygous subjects. There were no other between-group differences for any other measured variables. 4. In summary, complete and partial muscle AMPD deficiency did not affect TCA cycle anaplerosis, phosphocreatine hydrolysis, energy charge or exercise performance.

Synergistic heterozygosity: disease resulting from multiple partial defects in one or more metabolic pathways

Inborn errors of metabolism show considerable variation in the severity of symptoms. This is often ascribed to the differential effects of specific mutations on gene/enzyme function; however, such genotype/phenotype correlations are usually imprecise. In addition, in some patients with clinical and biochemical findings consistent with a defect in a particular metabolic pathway, it is ultimately impossible to arrive at a precise enzymatic diagnosis. In this situation, we have increasingly been identifying concurrent partial defects in more than one pathway, or at multiple steps in one pathway. In this study, we present the clinical, biochemical, and molecular findings from several patients showing multiple partial defects in energy metabolism. These patients show clinical symptoms consistent with a defect in the affected pathways even though they do not have a complete deficiency in any one enzyme. We hypothesize that such patients are exhibiting clinically significant reductions in energy metabolism related to the compound effects of these partial defects, a phenomenon we term "synergistic heterozygosity." Based on the frequencies of known disorders of energy metabolism, we propose that this may represent a previously unrecognized, relatively common mechanism of disease of potentially great clinical relevance.

Chronic fatigue syndrome: a matter of enzyme deficiencies?

The etiology of chronic fatigue syndrome (CFS) remains an enigma. But literature concerning chronic fatigue which does not focus on CFS points to all sorts of enzyme deficiencies as possible causes. The deficiencies are probably dismissed as causes of CFS because other characteristic symptoms are lacking in CFS patients. But these symptoms are often also lacking in patients with a deficiency. Symptom patterns in enzyme deficiencies are extremely variable. Therefore, patients with CFS should be screened systematically for enzyme deficiencies.

The molecular diagnosis of metabolic myopathies

The metabolic myopathies are distinguished by extensive clinical and genetic heterogeneity within and between individual disorders. There are a number of explanations for the variability observed that go beyond single gene mutations or degrees of heteroplasmy in the case of mitochondrial DNA mutations. Some of the contributing factors include protein subunit interactions, tissue-specificity, modifying genetic factors, and environmental triggers. Advances in the molecular analysis of metabolic myopathies during the last decade have not only improved the diagnosis of individual disorders but also helped to characterize the contributing factors that make these disorders so complex.

Genetic and other determinants of AMP deaminase activity in healthy adult skeletal muscle

AMPD1 genotype, relative fiber type composition, training status, and gender were evaluated as contributing factors to the reported variation in AMP deaminase enzyme activity in healthy skeletal muscle. Multifactorial correlative analyses demonstrate that AMPD1 genotype has the greatest effect on enzyme activity. An AMPD1 mutant allele frequency of 13.7 and a 1.7% incidence of enzyme deficiency was found across 175 healthy subjects. Homozygotes for the AMPD1 normal allele have high enzyme activities, and heterozygotes display intermediate activities. When examined according to genotype, other factors were found to affect variability as follows: AMP deaminase activity in homozygotes for the normal allele exhibits a negative correlation with the relative percentage of type I fibers and training status. Conversely, residual AMP deaminase activity in homozygotes for the mutant allele displays a positive correlation with the relative percentage of type I fibers. Opposing correlations in different homozygous AMPD1 genotypes are likely due to relative fiber-type differences in the expression of AMPD1 and AMPD3 isoforms. Gender also contributes to variation in total skeletal muscle AMP deaminase activity, with normal homozygous and heterozygous women showing only 85-88% of the levels observed in genotype-matched men.

Genetic characteristics of myoadenylate deaminase deficiency

Two types of myoadenylate deaminase (MAD) deficiency have been described, primary or inherited, and secondary or acquired MAD deficiency. In this study, we investigated whether secondary MAD deficiency is indeed acquired or merely coincidental. We demonstrated the same underlying molecular defect, a C34T transition, in both types of deficiency. Furthermore, the same frequency of the mutant MAD allele was found in the general population as in patients with neuromuscular complaints. We therefore conclude that in the Dutch population, secondary MAD deficiency is merely a "coincidental" finding, and that MAD deficiency is a harmless genetic variant.