Mitochondrial disease

Mitochondrial disease is a group of genetic disorders caused by dysfunction of the mitochondria, the organelles present in nearly every human cell except red blood cells that convert food into the cellular energy molecule ATP. The disorders arise from inherited or acquired mutations affecting either the small separate genome carried within the mitochondria themselves or the much larger set of genes in the cell's nucleus that encode mitochondrial proteins. Mitochondrial DNA is inherited only from the mother, and because each cell contains many mitochondria and many copies of mitochondrial DNA, a defective copy can be unevenly distributed among a person's cells during development, a phenomenon called heteroplasmy that helps explain why the same mutation can produce very different severity in different people. The group includes recognized conditions such as Leber's hereditary optic neuropathy, Leigh syndrome, MELAS syndrome, and MERRF syndrome, and a subset marked mainly by muscle weakness are known as mitochondrial myopathies. In the United States, roughly one in 4,000 children develops a mitochondrial disease by age ten, amounting to as many as 4,000 affected births a year, and the first disease-causing mutation in mitochondrial DNA was identified in 1988.