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Germ-line deletions of mtDNA in mitochondrial myopathy
J Poulton1, M E Deadman, S Ramacharan
1Department of Paediatrics, University of Oxford, John Radcliffe Hospital, Headington, England.
Abstract:
mtDNA encodes subunits of the electron transport chain and is exclusively maternally inherited in mammals. It has been suggested that mtDNA might be the site of some of the mutations causing a group of human disorders called the "mitochondrial myopathies," because these may both be (1) accompanied by defects in the electron transport chain and (2) display a maternal pattern of inheritance. However, all of the deletions and duplications of mtDNA which occur in these patients have been sporadic, apart from families in whom affected members all carry different deletions suggesting a mutant autosomal dominantly inherited nuclear gene with de novo deletions in each individual. We present the first evidence for the presence of deleted mtDNAs in the germ line in these disorders. The patient carries a higher level of deleted mtDNAs than do his relatives, corresponding to severity of symptoms and consistent with a predicted dosage effect. "Selfishness" of deleted mtDNAs is probably one of the factors over and above random segregation of a small number of "founder" mtDNAs (the bottleneck hypothesis) which may be invoked to explain the usual distribution of mtDNAs in different tissues of patients with mtDNA deletions.
Insights
This study provides the first evidence of deleted mitochondrial DNAs (mtDNAs) in the germ line of patients with mitochondrial myopathies. Higher levels of deleted mtDNAs correlate with symptom severity, suggesting a dosage effect and potential "selfishness" of these deletions.
Area of Science:
- Genetics
- Molecular Biology
- Cell Biology
Background:
- Mitochondrial DNA (mtDNA) encodes essential electron transport chain subunits and is maternally inherited.
- Mitochondrial myopathies are human disorders potentially linked to mtDNA mutations, exhibiting electron transport chain defects and maternal inheritance.
- Previous studies found sporadic mtDNA deletions/duplications in patients, with rare exceptions suggesting nuclear gene involvement.
Purpose of the Study:
- To investigate the presence of deleted mtDNAs in the germ line of individuals with mitochondrial myopathies.
- To explore the correlation between deleted mtDNA levels, disease severity, and inheritance patterns.
Main Methods:
- Analysis of mtDNA deletions in patients and their relatives.
- Quantification of deleted mtDNA levels across different tissues and the germ line.
- Comparison of deleted mtDNA levels with clinical symptom severity.
Main Results:
- First evidence presented for deleted mtDNAs within the germ line of patients with mitochondrial myopathies.
- The patient exhibited higher levels of deleted mtDNAs compared to relatives.
- Increased deleted mtDNA levels correlated with greater symptom severity, supporting a dosage effect.
Conclusions:
- Deleted mtDNAs can be present in the germ line, contributing to mitochondrial myopathies.
- The 'selfishness' of deleted mtDNAs, beyond random segregation (bottleneck hypothesis), likely influences their distribution and impact.
- Findings suggest a complex interplay between mtDNA deletions, nuclear factors, and disease manifestation.