Objective. Single, large-scale deletions in mitochondrial DNA (mtDNA) are a common cause of mitochondrial disease. This study aimed to investigate the relationship between the genetic defect and molecular phenotype to improve understanding of pathogenic mechanisms associated with single, large-scale mtDNA deletions in skeletal muscle.
Methods. We investigated 23 muscle biopsies taken from adult patients (6 males/17 females with a mean age of 43y) with characterised single, large-scale mtDNA deletions. Mitochondrial respiratory chain deficiency in skeletal muscle biopsies was quantified by immunoreactivity levels for complex I and complex IV proteins. Single muscle fibres with varying degrees of deficiency were selected from six patient biopsies for determination of mtDNA deletion level and copy number by quantitative qPCR.
Results. We have defined three ‘classes’ of single, large-scale deletion with distinct patterns of mitochondrial deficiency, determined by the size and location of the deletion. Single fibre analyses showed that fibres with greater respiratory chain deficiency harboured higher levels of mtDNA deletion with an increase in total mtDNA copy number. For the first time, we have demonstrated that threshold levels for complex I and complex IV deficiency differ based on deletion class.
Interpretation. Combining genetic and immunofluorescent assays, we conclude that thresholds for complex I and complex IV deficiency are modulated by the deletion of complex-specific protein-encoding genes. Furthermore, removal of mt-tRNA genes impacts specific complexes only at high deletion levels, when complex-specific protein-encoding genes remain. These novel findings provide valuable insight into the pathogenic mechanisms associated with these mutations. This article is protected by copyright. All rights reserved.