PFKM gene defect and glycogen storage disease GSDVII with misleading enzyme histochemistry

Objective: To elaborate the diagnostic methods used as “gold standard” in one of the most common glycogen storage diseases (GSDs), Tarui disease (GSDVII). Methods: Two siblings with disease suggestive of GSD underwent thorough clinical analysis, including muscle biopsy, muscle MRI, exercise tests, laboratory examinations, and wholeexome sequencing (WES). Results: Both siblings had juvenile-onset exercise intolerance with cramping and infrequent myoglobinuria. Muscle biopsy showed extralysosomal glycogen accumulation, but because of normal phosphofructokinase histochemistry, GSDVII was thought to be excluded. However, WES revealed a causative homozygous PFKM gene defect, R39Q, in both siblings, establishing the diagnosis of GSDVII, which was confirmed by very low residual phosphofructo-1-kinase (PFK) enzyme activity in biochemical studies. Conclusions: We suggest that in patients with suspicion of GSD and extralysosomal glycogen accumulation, biochemical activity assay of PFK followed by molecular genetics should be performed even when enzyme histochemistry is normal. Neurol Genet 2015;1:e7; doi: 10.1212/ NXG.0000000000000007