The mutant mitochondrial genes in mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) were selectively amplified through generations.

Abstract

Mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) is one of the clinical phenotypes of mitochondrial myopathies characterized by stroke-like episodes (Pavlakis et al 1984). For the cytogenetic analyses, we isolated respiratory-deficient and normal myogenic cell lines from muscle of a patient with MELAS by transfecting replication origin-defective SV40 DNA (Nakamigawa et al 1988; Shimoizumi et al t989). By sequencing mitochondriat DNA (mtDNA) of these two clones, we identified a transition of A to G in the mitochondrial tRNA-Leu(UUR) gene at Cambridge nucleotide number 3243 only in the mtDNA of the respiratorydeficient clone, and concluded that this base transition was associated with MELAS (Kobayashi et al 1990, 1991). Another research group reported the same point mutation by analysing mtDNA of muscle tissues of patients (Goto et al 1990). This indicates that MELAS is associated with mutation in mtDNA like other mitochondrial myopathies, such as Kearns-Sayre syndrome or progressive external ophthalmoplegia in which deletions of mtDNA were detected (Holt et al 1988, 1989; Moraes et al 1989), and myoclonus epilepsy associated with ragged-red fibres in which a base transition in mitochondrial tRNA-Lys gene was reported (Shoffner et al 1990; Yoneda et al 1990). Being different from the nuclearly coded gene mutations, there should exist certain unknown cytogenetic processes for a mutation in one mtDNA to become dominant and produce the patient. To analyse the processes of amplification and distribution of the mutant mtDNA in MELAS, the relative proportion of the mutant mtDNA was quantitatively studied in patients and family members as well as in various tissues. We used PCR-amplified mtDNA fragments after assessing the validity of the method in the quantification of the mutant mtDNA by using normal and the mutant mtDNA cloned from respiratory-deficient and normal myogenic cells.