A tandem duplication in the D-loop of human mitochondrial DNA is associated with deletions in mitochondrial myopathies.

About 40 per cent of patients with mitochondrial myopathies have two populations of mitochondrial DNA (mtDNA) in muscle, one of which is deleted. All patients with single mtDNA deletions and neurological disease are sporadic cases, suggesting that deletions arise as fresh mutational events. We have detected a low abundance heteroplasmic tandem duplication involving the displacement loop of mtDNA in 18 of 58 patients with deletions and 5/5 of their mothers, but not in normal subjects. The location of the duplication to a region that controls both replication and transcription of mtDNA could explain features suggesting mild mitochondrial dysfunction in the muscle biopsies of three patients' mothers, and a predisposition to deletion.