Mitochondrial DNA deletions and cytochrome c oxidase deficiency in muscle fibres

Abstract

We have studied cytochrome c oxidase (COX) deficient muscle fibre segments in 6 patients with mitochondrial myopathy and deletions of mitochondrial DNA (mtDNA). The distribution of transcripts of normal and mutated mtDNA in skeletal muscle sections was studied by in situ hybridization. The results were compared with the enzyme histochemical activity of COX and the immunohistochemical distribution of mtDNA encoded and nuclear DNA encoded subunits of COX. In all cases a proportion of the muscle fibres (< 1–30% of the fibres in cross-sections) and had low COX activity and high activity of succinate dehydrogenase (COX deficient muscle fibres). Transcripts of normal and deleted mtDNA showed the same distribution within the tissue as the corresponding mtDNA, indicating that the deleted mtDNA is transcribed. The COX deficient muscle fibres showed accumulation of transcripts of deleted mtDNA, which had a similar distribution as the accumulated mitochondria within these fibres. With few exceptions, there was a low level of transcripts of normal mtDNA in these COX deficient fibres. Immunohistochemical analysis revealed low levels of immunoreactive material using antiserum to the mtDNA encoded subunits II/III as well as the nuclear DNA encoded subunit IV of COX in all COX deficient muscle fibres. The fraction of deleted mtDNA in muscle ranged from 43 to 87%. There was no correlation between the proportion of COX deficient muscle fibres and the fraction of deleted mtDNA. In 2 cases the deletion did not involve any COX gene. One of these cases had 87% deleted mtDNA but less than 1% COX deficient muscle fibres. This case had several muscle fibres with accumulation of transcripts of deleted mtDNA concomitant with normal COX activity. The other of these 2 cases had 10% COX deficient muscle fibres. Our results show that the proportion of COX deficient muscle fibres is not correlated to the fraction of deleted mtDNA in muscle tissue and that a high proportion of COX deficient fibres may be present even if COX subunit genes are not affected by the deletion. Reduced enzyme histochemical activity of COX in muscle fibre segments is correlated to low levels of normal mtDNA in these fibre segments.