LATE-ONSET MUSCLE WEAKNESS IN PARTIAL PHOSPHOFRUCTOKINASE DEFICIENCY - A UNIQUE MYOPATHY WITH VACUOLES, ABNORMAL MITOCHONDRIA, AND ABSENCE OF THE COMMON EXON-5 INTRON-5 JUNCTION POINT MUTATION

Three patients (ages 51, 59, and 79) from two generations of an Ashken azi Jewish family had partial (33% activity) phosphofructokinase (PFK) deficiency that presented with fixed muscle weakness after the age of 50 years. MR spectroscopy revealed accumulation of phosphomonoesters during exercise. Muscle biopsy showed a vacuolar myopathy with increas ed autophagic activity and several ragged-red and cytochrome c oxidase -negative fibers. The older patient, age 79 at biopsy, had several nec rotic fibers. Electron microscopy revealed subsarcolemmal and intermyo fibrillar glycogen accumulation and proliferation of mitochondria with paracrystalline inclusions, probably related to reduced availability of energy due to impaired glycolysis. The common point mutation of exo n 5/intron 5 junction seen in Jewish Ashkenazi patients with PFK defic iency was excluded. We conclude that late-onset fixed muscle weakness occurs in partial PFK deficiency and it may represent the end result o f continuing episodes of muscle fiber destruction. Partial enzyme defi ciency in two successive generations suggests a unique molecular mecha nism.