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A mutation in the tRNA(Leu)(UUR) gene associated with the MELAS subgroup of mitochondrial encephalomyopathies
1Division of Ultrastructural Research, National Center of Neurology and Psychiatry, Tokyo, Japan.
Abstract:
Mitochondrial encephalomyopathies are usually divided into three distinct clinical subgroups: (1) mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS); (2) myoclonus epilepsy associated with ragged-red fibres (MERRF); and (3) chronic progressive external ophthalmoplegia (CPEO) including Kearns-Sayre syndrome. Large deletions of human mitochondrial DNA and a transition mutation at the mitochondrial transfer RNALys gene give rise to CPEO including Kearns-Sayre syndrome and MERRF, respectively. Here we report an A-to-G transition mutation at nucleotide pair 3,243 in the dihydrouridine loop of mitochondrial tRNA(Leu)(UUR) that is specific to patients with MELAS. Because this mutation creates an ApaI restriction site, we could perform a simple molecular diagnostic test for the disease. The mutation was present in 26 out of 31 independent MELAS patients and 1 out of 29 CPEO patients, but absent in the 5 MERRF and 50 controls tested. Southern blot analysis confirmed that the mutant DNA always coexists with the wild-type DNA (heteroplasmy).
Insights
Researchers identified a specific A-to-G mutation in mitochondrial tRNA(Leu)(UUR) linked to mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS). This discovery enables a simple molecular diagnostic test for MELAS.
Area of Science:
- Genetics
- Neurology
- Mitochondrial Diseases
Background:
- Mitochondrial encephalomyopathies present as distinct clinical subgroups, including MELAS, MERRF, and CPEO.
- Known genetic causes for CPEO and MERRF involve large deletions and tRNA Lys gene mutations, respectively.
- A specific mutation associated with MELAS remained unidentified.
Purpose of the Study:
- To identify the specific genetic mutation responsible for MELAS.
- To develop a molecular diagnostic test for MELAS.
Main Methods:
- DNA sequencing to detect mutations in mitochondrial tRNA genes.
- ApaI restriction enzyme digestion to identify the specific A-to-G transition at nucleotide 3,243.
- Southern blot analysis to confirm heteroplasmy.
Main Results:
- An A-to-G transition mutation at nucleotide 3,243 in the mitochondrial tRNA(Leu)(UUR) gene was identified.
- This mutation was present in 26/31 MELAS patients and 1/29 CPEO patients, but not in MERRF patients or controls.
- The mutation creates an ApaI restriction site, facilitating molecular diagnosis.
- Southern blot confirmed the coexistence of mutant and wild-type mitochondrial DNA (heteroplasmy).
Conclusions:
- The identified A-to-G mutation at np 3,243 in tRNA(Leu)(UUR) is specific to MELAS.
- This mutation provides a reliable molecular marker for MELAS diagnosis.
- The findings contribute to understanding the genetic basis of mitochondrial encephalomyopathies.