Methioninemia and myopathy: a new disorder.

A 7 1/2-year-old girl with hypermethioninemia, myopathy, and mental deficiency (IQ = 65) is described. The increased methionine was not associated with deficiency of methionine adenosyltransferase, which was normal or increased in liver, muscle, erythrocytes, and cultured fibroblasts. Methionyl-tRNA synthetase in fibroblasts was normal. The hypermethioninemia and a concurrently increased blood S-adenosylmethionine declined on a diet low in methionine. There was a diffuse, symmetrical, moderate proximal muscle weakness, but muscle atrophy was not discernible, and the deep tendon reflexes were hypoactive but obtainable. Electromyographic abnormalities were not detected. Electron microscopy of muscle revealed 3 to 6 small myelin figures in the region of the I band in nearly every fiber, with occasional myelin figures at other sites also. These myelin figures were more numerous and smaller than those seen accompanying nonspecific myopathies and may reflect a more specific pathological change. Electron microscopy of liver revealed three nonspecific lesions in all hepatocytes: (1) numerous megamitochondria with crystalloid deposit in the matrix; (2) increased numbers of small vesicles of smooth endoplasmic reticulum; and (3) loss of plasma membrane microvilli, with extensive bleb formation and shedding of cytoplasm into Disse's space.