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Mitochondrial DNA and RNA processing in MELAS
P Kaufmann1, Y Koga, S Shanske
1H. Houston Merritt Clinical Research Center for Muscular Dystrophy and Related Disorders, New York, NY, USA.
Abstract:
Mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), a maternally inherited disorder, is usually associated with a point mutation in mitochondrial DNA (mtDNA) at position 3,243 in the tRNA Leu(UUR) gene. To further study the pathogenesis of MELAS, we analyzed tissues from 8 MELAS-3,243 patients. Southern blot analysis showed an increase in the ratio of mtDNA to nuclear DNA in almost all tissues examined, implying that mitochondrial proliferation is ubiquitous and is not confined to ragged-red fibers in muscle. By northern blot analysis, we demonstrated increased steady-state levels of RNA 19, a polycistronic transcript corresponding to the 16S rRNA + tRNA Leu(UUR) + ND1 genes (which are contiguous in the mtDNA) in heart, kidney, and muscle. These results provide further evidence that altered mitochondrial nucleic acid metabolism may have pathogenic significance in MELAS.
Insights
Mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) involves increased mitochondrial DNA and altered RNA metabolism. These changes, observed in patient tissues, suggest a key role in the disorder's pathogenesis.
Area of Science:
- Genetics
- Mitochondrial Biology
- Neurology
Background:
- Mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) is a maternally inherited disorder.
- It is typically linked to a specific point mutation (m.3243A>G) in the mitochondrial DNA (mtDNA) tRNA Leu(UUR) gene.
Purpose of the Study:
- To investigate the pathogenesis of MELAS by analyzing tissues from patients with the 3,243 mtDNA mutation.
- To examine mitochondrial DNA and RNA alterations in various tissues affected by MELAS.
Main Methods:
- Southern blot analysis to assess the ratio of mitochondrial DNA (mtDNA) to nuclear DNA.
- Northern blot analysis to quantify steady-state levels of specific polycistronic transcripts.
Main Results:
- Increased mtDNA to nuclear DNA ratio observed in most tissues from MELAS patients, indicating widespread mitochondrial proliferation.
- Elevated levels of RNA 19, a transcript encompassing 16S rRNA, tRNA Leu(UUR), and ND1 genes, found in heart, kidney, and muscle tissues.
- Mitochondrial proliferation is not limited to ragged-red fibers in muscle tissue.
Conclusions:
- Altered mitochondrial nucleic acid metabolism, including increased mtDNA and specific transcript levels, plays a significant role in MELAS pathogenesis.
- The findings support the hypothesis that disruptions in mitochondrial gene expression contribute to the multi-systemic manifestations of MELAS.