The human mitochondrial tRNA mutations are associated with a variety of diseases including mitochondrial myopathies. However, it is understood that defects in the mitochondrial DNA (mtDNA) have been associated with these diseases, and studies have been able to assign biochemical defects. One of these defects has to do with the decreased energy available for cell processes. As muscles are stained with Gömöri trichrome, characteristic ragged red fibers are visible under the microscope. This appearance is due to the accumulation of abnormal mitochondria below the plasma membrane of the muscle fiber. These may extend throughout the muscle fiber as the disease severity increases. The mitochondrial aggregates cause the contour of the muscle fiber to become irregular, leading to the "ragged" appearance. Diagnosis
The diagnosis varies from individual to individual. Each is evaluated and diagnosed according to age, clinical phenotype, and pressed inheritance pattern. If the individual has been experiencing myoclonus, the doctor will run a series of genetic studies to determine if it is a mitochondrial disorder.The molecular genetic studies are run to identify the reason of for the mutations underlying the mitochondrial dysfunction. This approach will avoid the need for a muscle biopsy or an exhaustive metabolic evaluation. After sequencing the mitochondrial genomes, four points mutations in the genome can be identified which are associated with MERRF: A8344G, T8356C, G8361A, and G8363A.
Epidemiology
This disease is endemic in Portuguese locations Póvoa de Varzim and Vila do Conde (Caxinas), with more than 1000 affected people, coming from about 500 families, where 70% of the people develop the illness. ll the analysed Portuguese families presented the same haplotype (haplotype I) associated with the Met 30 mutation. In northern Sweden, more specifically  Skellefteå (it is locally called "Skelleftesjukan", the Skellefteå disease), 1.5% of the population has the mutated gene. There are many other populations in the world who exhibit the illness after having developed it independently. References
External links

GeneReviews/NIH/NCBI/UW entry on Familial Transthyretin Amyloidosis