A new disease-related mutation for mitochondrial encephalopathy lactic acidosis and strokelike episodes (MELAS) syndrome affects the ND4 subunit of the respiratory complex I

The molecular lesions in two patients exhibiting classical clinical manifestations of MELAS (mitochondrial encephalopathy, lactic acidosis, and strokelike episodes) syndrome have been investigated. A recently reported disease-related A→G base substitution at nt 3243 of the mtDNA, in the DHU loop of tRNA^Leu, was detected by restriction-enzyme analysis of the relevant PCR-amplified segment of the mtDNA of one patient but was not observed, by either restriction-enzyme analysis or nucleotide sequencing, in the other. To define the molecular lesion in the patient who does not have the A→G base substitution at nt 3243, the total mitochondrial genome of the patient has been sequenced. An A→G base substitution at nt 11084, leading to a Thr-to-Ala amino acid replacement in the ND4 subunit of the respiratory complex I, is suggested to be a disease-related mutation.