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Molecular pathology of MELAS and MERRF. The relationship between mutation load and clinical phenotypes
P F Chinnery1, N Howell, R N Lightowlers
1Department of Neurology, University of Newcastle upon Tyne, UK.
Abstract:
Many patients with inherited mitochondrial encephalopathies have one of two pathogenic mutations of mitochondrial DNA (mtDNA): A3243G or A8344G. Individuals who harbour these mutations carry both mutant and wild-type alleles within each cell (heteroplasmy). Despite clear evidence of a direct relationship between the level of mutation and mitochondrial respiratory chain function in vitro, it has been more difficult to demonstrate a clear correlation between clinical phenotype and the level of mutant mtDNA in vivo. To address this issue, we identified 245 individuals who carry either the A3243G or A8344G mutations, and studied the relationship between the incidence of specific clinical features and the level of mutant mtDNA in blood (for A3243G, n = 73; for A8344G, n = 25) and/or skeletal muscle (for A3234G, n = 111; for A8344G, n = 55). Within this study group, the frequency of key clinical features was significantly different for individuals harbouring the A3243G and A8344G mutations. For both mutations, there was a correlation between the frequency of the more common clinical features and the level of mutant mtDNA in muscle. In contrast, we did not observe a correlation between the frequency of clinical features and the level of mutant mtDNA in blood. Therefore, measurement of the level of the A3243G and A8344G mutations in muscle will allow the identification of individuals who are at risk of developing specific complications, thus improving the prognostic advice that can be given to patients and family members who carry these mutations.
Insights
Mitochondrial DNA (mtDNA) mutations A3243G and A8344G cause inherited encephalopathies. Muscle mtDNA levels correlate with clinical features, aiding prognosis for patients with these mutations.
Area of Science:
- Genetics
- Neurology
- Mitochondrial Biology
Background:
- Inherited mitochondrial encephalopathies are often caused by specific mitochondrial DNA (mtDNA) mutations, notably A3243G and A8344G.
- Individuals with these mutations exhibit heteroplasmy, possessing both mutant and wild-type mtDNA alleles.
- While in vitro studies show a link between mutation load and respiratory chain function, in vivo correlations with clinical outcomes are less clear.
Purpose of the Study:
- To investigate the relationship between mutant mtDNA levels and clinical phenotypes in patients with A3243G or A8344G mutations.
- To determine if mutant mtDNA levels in blood or skeletal muscle are better indicators of clinical features.
Main Methods:
- Studied 245 individuals with either the A3243G or A8344G mtDNA mutations.
- Assessed the correlation between specific clinical features and mutant mtDNA levels in blood and/or skeletal muscle samples.
- Compared the frequency of clinical features between individuals with the A3243G and A8344G mutations.
Main Results:
- Significant differences in key clinical features were observed between individuals with A3243G and A8344G mutations.
- A correlation was found between the level of mutant mtDNA in muscle and the frequency of common clinical features for both mutations.
- No significant correlation was observed between clinical features and mutant mtDNA levels in blood.
Conclusions:
- Measurement of mutant mtDNA levels in skeletal muscle is a valuable tool for predicting clinical complications in patients with A3243G and A8344G mutations.
- This approach can improve prognostic accuracy and genetic counseling for affected individuals and families.
- Muscle biopsy is a more reliable indicator than blood for assessing genotype-phenotype correlations in these mitochondrial disorders.