CLINICAL AND MORPHOLOGIC FEATURES OF A MYOPATHY ASSOCIATED WITH A POINT MUTATION IN THE MITOCHONDRIAL TRNA(PRO) GENE

We studied a 9-year-old girl with progressive weakness of her extremit ies for two years. Her neurologic evaluation showed weakness of proxim al muscles but no ophthalmoparesis. With the exception of elevated ser um lactic acid, the general blood screen, EMG, nerve conduction veloci ty tests, and ECG were normal. Light and electron microscopy of a musc le biopsy showed proliferation of mitochondria containing disorganized cristae. Activities of respiratory chain enzymes containing mitochond rial DNA (mtDNA)-encoded subunits were significantly impaired in muscl e homogenates. A G-->A transition at position 15990 previously detecte d in this patient's muscle was not present in peripheral blood cells o f her mother or sister. However, the patient's WBCs appeared to contai n a very small percentage of mutant mtDNAs, indicating that the mutati on may have originated during early embryogenesis.