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Previous studies have suggested that some patients with large-scale mitochondrial DNA (mtDNA) deletions also presented a heteroplasmic 260 bp tandem duplication in the mtDNA D-loop region. Such duplications were observed not only in patients with mitochondrial pathology but also in aged subjects. However, the percentage of duplicated mtDNA did not exceed a few per cent of the total mtDNA, except in one example where it reached 30%. We report here another type of 200 bp duplication in the mtDNA D-loop region that, instead of being associated with a large-scale deletion, is correlated to the presence of a point mutation in the cytochrome  b gene. The 200 bp duplication concerned up to 95% of the total mtDNA of some muscle mitochondria and was absent from the patient lymphocyte DNA. The percentages of the 200 bp duplication and that of the cytochrome  b  mutation were relatively close in whole muscle as well as in single muscle fibres, suggesting a correlation between the mutation and the duplication. This duplication could also be detected by PCR in two other patients with mitochondrial disorders but without known deletion or mtDNA mutation. These data suggest that the accumulation of these small duplications in the mtDNA D-loop could be indicative of the presence of other defects of the mtDNA which would damage the respiratory chain function. These deficiencies would induce the generation of small duplications in the D-loop.

Co-Existence of High Levels of a Cytochrome b Mutation and of a Tandem 200 bp Duplication in the D-loop of Muscle Human Mitochondrial DNA