Quantification of tRNA3243(Leu) point mutation of mitochondrial DNA in MELAS patients and its effects on mitochondrial transcription.

The MELAS syndrome is a mitochondrial encephalomyopathy associated with a point mutation at nucleotide 3243 of mitochondrial DNA (mtDNA). The same mutation has also been found in patients with maternally inherited diabetes mellitus. The mutation occurs within a sequence needed for termination of mitochondrial transcription downstream of the ribosomal RNA (rRNA) genes, thus possibly reducing rRNA synthesis in relation to more distal transcripts. This study presents a family in which maternally transmitted diabetes and MELAS syndrome overlap, and a suggestive correlation between the amount of mutant mtDNA and clinical symptoms is observed. Mutant mtDNA was quantified in several tissues of a newborn infant and the highest amount of mutant mtDNA was found in the placenta, which is promising for the development of genetic counselling in MELAS. The consequences of the MELAS mutation were further studied in cultured clonal myoblasts. We found that the myoblasts with 93% of mutant mtDNA terminate the mitochondrial transcription, resulting in a steady-state amount of 16S rRNA 45 times as high as the more distal transcripts. However, myoblasts with a deletion of mtDNA not involving the transcription termination site had 120 times as much 16S rRNA as the distal transcripts. In both the MELAS myoblasts and in those with a deletion of mtDNA the amount of 16S rRNA increased as the mutant mtDNA increased, suggesting that the production of ribosomal RNAs is a response to the translational defect caused by the mutation. We present evidence here that the MELAS mutation causes a defect in transcription termination, thus leading to no absolute deficiency of ribosomal RNAs, but to a reduced capacity to compensate the defective translation.