Unsolved issues related to human mitochondrial diseases

Special emphasis is made on heteroplasmy, a specific feature of mitochondrial diseases, defined as the coexistence within the cell of mutant and wild type mitochondrial DNA molecules. Its existence permits unequal segregation during mitoses of the mitochondrial DNA populations and consequently heterogeneous tissue distribution of the mutation load. The observed tissue distributions of recurrent human mitochondrial DNA deleterious mutations are diverse but reproducible for a given mutation demonstrating that the segregation is not a random process. Its extent and mechanisms remain essentially unknown despite recent advances obtained in animal models.