Effects of AMPD1 common mutation on the metabolic-chronotropic relationship: Insights from patients with myoadenylate deaminase deficiency

Integrated Functions of Cardiac Energetics, Mechanics, and Purine Nucleotide Metabolism

Purine nucleotides play central roles in energy metabolism in the heart. Most fundamentally, the free energy of hydrolysis of the adenine nucleotide adenosine triphosphate (ATP) provides the thermodynamic driving force for numerous cellular processes including the actin-myosin crossbridge cycle. Perturbations to ATP supply and/or demand in the myocardium lead to changes in the homeostatic balance between purine nucleotide synthesis, degradation, and salvage, potentially affecting myocardial energetics and, consequently, myocardial mechanics. Indeed, both acute myocardial ischemia and decompensatory remodeling of the myocardium in heart failure are associated with depletion of myocardial adenine nucleotides and with impaired myocardial mechanical function. Yet there remain gaps in the understanding of mechanistic links between adenine nucleotide degradation and contractile dysfunction in heart disease. The scope of this article is to: (i) review current knowledge of the pathways of purine nucleotide depletion and salvage in acute ischemia and in chronic heart disease; (ii) review hypothesized mechanisms linking myocardial mechanics and energetics with myocardial adenine nucleotide regulation; and (iii) highlight potential targets for treating myocardial metabolic and mechanical dysfunction associated with these pathways. It is hypothesized that an imbalance in the degradation, salvage, and synthesis of adenine nucleotides leads to a net loss of adenine nucleotides in both acute ischemia and under chronic high-demand conditions associated with the development of heart failure. This reduction in adenine nucleotide levels results in reduced myocardial ATP and increased myocardial inorganic phosphate. Both of these changes have the potential to directly impact tension development and mechanical work at the cellular level. © 2024 American Physiological Society. Compr Physiol 14:5345-5369, 2024.

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.

The ratio of maximal handgrip force and maximal cycloergometry power as a diagnostic tool to screen for metabolic myopathies

Metabolic myopathies comprise a diverse group of inborn errors of intermediary metabolism affecting skeletal muscle, and often present clinically as an inability to perform normal exercise. Our aim was to use the maximal mechanical performances achieved during two functional tests, isometric handgrip test and cycloergometer, to identify metabolic myopathies among patients consulting for exercise-induced myalgia. Eighty-three patients with exercise-induced myalgia and intolerance were evaluated, with twenty-three of them having a metabolic myopathy (McArdle, n = 9; complete myoadenylate deaminase deficiency, n = 10; respiratory chain deficiency, n = 4) and sixty patients with non-metabolic myalgia. In all patients, maximal power (MP) was determined during a progressive exercise test on a cycloergometer and maximal voluntary contraction force (MVC) was assessed using a handgrip dynamometer. The ratio between percent-predicted values for MVC and MP was calculated for each subject (MVC%pred:MP%pred ratio). In patients with metabolic myopathy, the MVC%pred:MP%pred ratio was significantly higher compared to non-metabolic myalgia (1.54 ± 0.62 vs. 0.92 ± 0.25; p < 0.0001). ROC analysis of MVC%pred:MP%pred ratio showed AUC of 0.843 (0.758–0.927, 95% CI) for differentiating metabolic myopathies against non-metabolic myalgia. The optimum cutoff was taken as 1.30 (se = 69.6%, sp = 96.7%), with a corresponding diagnostic odd ratio of 66.3 (12.5–350.7, 95% CI). For a pretest probability of 15% in our tertiary reference center, the posttest probability for metabolic myopathy is 78.6% when MVC%pred:MP%pred ratio is above 1.3. In conclusion, the MVC%pred:MP%pred ratio is appropriate as a screening test to distinguish metabolic myopathies from non-metabolic myalgia.

Exercise efficiency impairment in metabolic myopathies

Metabolic myopathies are muscle disorders caused by a biochemical defect of the skeletal muscle energy system resulting in exercise intolerance. The primary aim of this research was to evaluate the oxygen cost (∆V’O₂/∆Work-Rate) during incremental exercise in patients with metabolic myopathies as compared with patients with non-metabolic myalgia and healthy subjects. The study groups consisted of eight patients with muscle glycogenoses (one Tarui and seven McArdle diseases), seven patients with a complete and twenty-two patients with a partial myoadenylate deaminase (MAD) deficiency in muscle biopsy, five patients with a respiratory chain deficiency, seventy-three patients with exercise intolerance and normal muscle biopsy (non-metabolic myalgia), and twenty-eight healthy controls. The subjects underwent a cardiopulmonary exercise test (CPX Medgraphics) performed on a bicycle ergometer. Pulmonary V’O₂ was measured breath-by-breath throughout the incremental test. The ∆V’O₂/∆Work-Rate slope for exercise was determined by linear regression analysis. Lower oxygen consumption (peak percent of predicted, mean ± SD; p < 0.04, one-way ANOVA) was seen in patients with glycogenoses (62.8 ± 10.2%) and respiratory chain defects (70.8 ± 23.3%) compared to patients with non-metabolic myalgia (100.0 ± 15.9%) and control subjects (106.4 ± 23.5%). ∆V’O₂/∆Work-Rate slope (mLO₂.min⁻¹.W⁻¹) was increased in patients with MAD absent (12.6 ± 1.5), MAD decreased (11.3 ± 1.1), glycogenoses (14.0 ± 2.5), respiratory chain defects (13.1 ± 1.2), and patients with non-metabolic myalgia (11.3 ± 1.3) compared with control subjects (10.2 ± 0.7; p < 0.001, one-way ANOVA). In conclusion, patients with metabolic myopathies display an increased oxygen cost during exercise and therefore can perform less work for a given VO₂ consumption during daily life-submaximal exercises.

Exercise testing-based algorithms to diagnose McArdle disease and MAD defects

OBJECTIVE

As exercise intolerance and exercise-induced myalgia are commonly encountered in metabolic myopathies, functional screening tests are commonly used during the diagnostic work-up. Our objective was to evaluate the accuracy of isometric handgrip test (IHT) and progressive cycle ergometer test (PCET) to identify McArdle disease and myoadenylate deaminase (MAD) deficiency and to propose diagnostic algorithms using exercise-induced lactate and ammonia variations.

METHODS

A prospective sample of 46 patients underwent an IHT and a PCET as part of their exercise-induced myalgia and intolerance evaluation. The two diagnostics tests were compared against the results of muscle biopsy and/or the presence of mutations in PYGM. A total of 6 patients had McArdle disease, 5 a complete MAD deficiency (MAD absent), 12 a partial MAD deficiency, and 23 patients had normal muscle biopsy and acylcarnitine profile (disease control).

RESULTS

The two functional tests could diagnose all McArdle patients with statistical significance, combining a low lactate variation (IHT: <1 mmol/L, AUC = 0.963, P < .0001; PCET: <1 mmol/L, AUC = 0.990, P < .0001) and a large ammonia variation (IHT: >100 μmol/L, AUC = 0.944, P = .0005; PCET: >20 μmol/L, AUC = 1). PCET was superior to IHT for MAD absent diagnosis, combining very low ammonia variation (<10 μmol/L, AUC = 0.910, P < .0001) and moderate lactate variation (>1 mmol/L).

CONCLUSIONS

PCET-based decision tree was more accurate than IHT, with respective generalized squared correlations of 0.796 vs 0.668. IHT and PCET are both interesting diagnostic tools to identify McArdle disease, whereas cycle ergometer exercise is more efficient to diagnose complete MAD deficiency.

Identification of Two Novel Single Nucleotide Polymorphisms in the Promoter Region of the Pig AMP Deaminase 1 Gene Associated with Carcass Traits

The AMP deaminase 1 (AMPD1) gene plays an important role in purine nucleotide interconversion and energy metabolism. In this study, two novel single nucleotide polymorphisms (SNPs) (g.-626 G > A and g.-566 A>G) were detected in the proximal promoter region of the AMPD1 gene. The Chinese indigenous pig breeds (Meishan and Tongcheng) had higher frequencies of the G and A alleles than Western meat-type breeds (Landrace and Large White) at the g.-626 G > A and g.-566 A>G loci. The transcriptional activity of the AMPD1 promoter carrying the haplotype H1 (A^-626G^-566) was significantly (p < 0.05) higher than that of the haplotype H2 (G^-626A^-566). In addition, pigs with the haplotype combination H1H1 had significantly (p < 0.05) higher mRNA expression levels of the AMPD1 gene than those with haplotype combinations H1H2 and H2H2 in two different skeletal muscles. Association analyses revealed that the pigs with the haplotype combination H1H1 had significantly higher lean meat percentage values but lower average backfat thickness (ABT, cm), buttock fat thickness (cm), and thorax-waist fat thickness (cm) values than the pigs with the haplotype combinations H1H2 and H2H2. These findings demonstrate that the two novel SNPs in the promoter region of the AMPD1 gene have significant associations with pig fat deposition traits.