McArdle Disease Treatment by Ketogenic Diet

McArdle disease (myophosphorylase deficiency, glycogen storage disease type 5, GSD5, OMIM # 232600) is an inherited metabolic disorder of skeletal muscle. Affected patients suffer from genetically determined lack of the enzyme muscle glycogen phosphorylase, which is essential for glycogen metabolism. The condition is caused by homozygous or compound heterozygous mutations in the muscle glycogen phosphorylase gene (PYGM) located at chromosome 11q13. Many pathogenic mutations have been identified in the gene, which spans 20 exons, and many are population specific. The most common mutation in Northern Europe and North America is a nonsense mutation at Arg50stop (R50X) in exon 1 (previously referred to as R49X). A second frequent mutation in this population, and in Spanish patients, is Gly205Ser (G205S). McArdle disease is a rare disorder with an estimated incidence of 1:100,000.

Complete absence of muscle phosphorylase results in the inability to mobilize muscle glycogen stores, which are normally required as substrate for energy generation during anaerobic metabolism, which occurs during start of exercise and high-intensity efforts. In affected people, symptoms of fatigue and discomfort therefore occur within minutes of initiating any activity and during strenuous activity such as lifting heavy weights or walking uphill. If the activity is continued despite symptoms, a severe cramp (which is called a contracture in GSD5, because the muscle contraction is not caused by neural stimulation) occurs, which leads to muscle damage. If the damage is substantial, acute rhabdomyolysis may occur, which in turn can result in dark brown/black discoloration of urine (myoglobinuria). When rhabdomyolysis is severe, myoglobinuria can lead to acute renal failure, requiring treatment with dialysis.

In patients with GSD5, aerobic metabolism is limited and varies as a function of the availability of alternative fuels as a function of exercise and diet. The second wind phenomenon is illustrative. The phenomenon is characterized by the ability to increase work output after about 7-8 minutes of exercise. The second wind occurs as a consequence of increased availability and metabolism of alternative fuel substrates, preferentially glucose supplied from the liver, but also free fatty acids metabolized through oxidative phosphorylation and ketones produced by the liver. Despite these compensatory fuels, which can substitute for the absent glycogen breakdown in muscle, the capacity for oxidative phosphorylation is impaired in GSD5, because of an almost complete absence of pyruvate, a by-product of glycolysis.

Reduced oxidative phosphorylation in untrained patients with GSD5 in turn reduces oxygen consumption to approximately 35% of normal and there is a disproportionate increase in heart rate during exercise in patients with GSD5 compared with healthy controls. Thus, unconditioned people with GSD5 have very limited exercise capacity, which affects quality of life.

Most patients present in the second or third decade, although symptoms are often reported retrospectively from childhood. With advancing age a 20-25% proportion of patients develop fixed muscle weakness predominantly affecting the shoulder girdle. No clear cut genotype-phenotype correlation has been found to explain the clinical variation in severity observed even within families, but the influence of polymorphisms in other genes has been hypothesized.

Currently, there is no treatment for the condition. There have been a small number of randomized controlled treatment trials, however the largest number of participants in any previous study was 19.

Taking glucose prior to exercise alleviates muscle symptoms by inducing a 'second wind' at the onset of exercise, but has detrimental effects on weight if used too frequently. A Cochrane systematic review of training in GSD5 identified a few non-randomized trials of aerobic training or dietary manipulation either with supplements such as creatine or with shift towards lipid sources, which showed no harmful effect and suggest benefit over a number of months however long-tern results and confirmation on larger cohorts are warranted.

In spite of these indications, controlled training and dietary habits are seldom followed by patients, who experience significant limitations in activity of daily living and restriction in their participation.

A key limitation to exercise in GSD5 is the bottleneck in fuel flow through the Tri Carboxylic Acid (TCA) cycle, which is imposed by the minimal supply of glucosyl units from muscle glycogen and thus glycolytic flux to feed the TCA cycle.

Dietary manipulation has been identified since the eighties as a potential strategy to improve functioning in GSD5. In spite of initial indications for high protein regimens, later experimental comparison of high protein vs high carbohydrate diets indicated a superiority for the latter. Particular interest was also focussed on diets with predominant lipid energy source (ketogenic or low carbohydrate ketogenic LCKD) with the assumption that ketones are easily taken up by mitochondria and can substitute for the missing acyl-CoA moieties not provided by the staggering glycolysis blocked upstream for the inaccessibility of muscle glycogen. LCKD has a long history as a therapeutic strategy for several conditions (epilepsy, PDH defect, GLUT1 defect) with a good record of safety and efficacy and a poorer record of tolerability. Isolated experiences of LCKD have been carried out in GSD5 patients (maximum 4 patients) with promising results.