Splice mutations preserve myophosphorylase activity that ameliorates the phenotype in McArdle disease.
Over 100 mutations in the myophosphorylase gene, which cause McArdle disease, are known. All these mutations have resulted in a complete block of muscle glycogenolysis, and accordingly, no genotype-phenotype correlation has been identified in this condition. We evaluated physiologic and genetic features of two patients with a variant form of McArdle disease, associated with unusually high exercise capacity. Physiologic findings were compared to those in 47 patients with typical McArdle disease, and 17 healthy subjects. Subjects performed an ischaemic forearm exercise test to assess lactate and ammonia production. Peak oxidative capacity (VO2max) and cardiac output were determined, using cycle ergometry as the exercise modality. The two patients with atypical McArdle disease carried common mutations on one allele (R50X and G205S), and novel splice mutations in introns 3 [IVS3-26A>G (c.425-26A>G)] and 5 [IVS5-601G>A (c.856-601G>A)] on the other allele. Plasma lactate after ischaemic exercise decreased in all typical McArdle patients, but increased in the two atypical McArdle patients (10% of that in healthy subjects). Peak workload and oxidative capacity were 2-fold higher in patients with atypical McArdle disease compared to typical McArdle patients. Oxygen uptake, relative to cardiac output, was severely impaired in the 47 patients with typical McArdle disease, and partially normalized in the milder affected McArdle patients. These findings identify the first distinct genotype-phenotype relationship in McArdle disease, and indicate that minimal myophosphorylase activity ameliorates the typical McArdle disease phenotype by augmenting muscle oxidative capacity. The milder form of McArdle disease provides important clues to the level of functional myophosphorylase needed to support muscle oxidative metabolism.
Department of Neurology 2082, University of Copenhagen, Rigshospitalet, Blegdamsvej 9, Copenhagen, Denmark. vissing@rh.dk
John
J
Vissing
Morten
M
Duno
Marianne
M
Schwartz
Ronald G
RG
Haller
D000328:Adult; D002302:Cardiac Output / Q000502:physiology; D005080:Exercise Test / Q000379:methods; D005260:Female; D005542:Forearm / Q000503:physiopathology; D024982:Glycogen Phosphorylase, Muscle Form / Q000172:deficiency / Q000235:genetics* / Q000378:metabolism; D006012:Glycogen Storage Disease Type V / Q000201:enzymology / Q000235:genetics* / Q000503:physiopathology; D006801:Humans; D019344:Lactic Acid / Q000097:blood; D008297:Male; D018482:Muscle, Skeletal / Q000201:enzymology; D009154:Mutation; D010101:Oxygen Consumption / Q000502:physiology; D010641:Phenotype; D022821:RNA Splice Sites / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction / Q000379:methods
D022821:RNA Splice Sites; D019344:Lactic Acid; D024982:Glycogen Phosphorylase, Muscle Form