Mitochondrial genetics and
cardiological disorders The human mtDNA is a 16,569-bp, double-stranded, circular molecule containing 37 genes, 24 of which participate in the translation mechanism (2 rRNA’s – 22 tRNA´s). The 13 remaining genes left are responsible for the synthesis of respiratory chain subunits. Hence, among the approximately 900 genes that participate in the function of the organelle, only a few are localized in the mtDNA, whereas the remainder are in the nDNA. This explains why about 50% of adults and 80-90% of children, suspected to have a mitochondrial disease on the basis of biochemical and/or morphological features, Inhibitors,research,lifescience,medical remain Inhibitors,research,lifescience,medical genetically undiagnosed. Indeed, it is reasonable to believe that most mitochondrial diseases are scientific assays caused by undiscovered nuclear genes (12-14). On the other hand, mtDNA mutations, which were studied in greater details, obey to different genetic rules than those applied to “mendelian” disorders (15). First, mtDNA is maternally inherited as sperm mitochondria’s are eliminated early in embryogenesis. Hence, mtDNA
Inhibitors,research,lifescience,medical will only be transmitted through the maternal line. Second, there are multiple copies of mtDNA in each cell: homoplasmy refers to the situation in which all mtDNA copies are identical. If two or more sequence variants exist in a cell or individual, that condition Inhibitors,research,lifescience,medical is referred to as heteroplasmy. If deleteriously mutant (i.e, pathogenic) and normal mtDNA coexist in the same cell, the respiratory chain function will not be impaired
as long as there is sufficient normal mtDNA to overcome the effects of mutant DNA. If, however, the ratio of mutant to normal mtDNA exceeds a certain critical threshold, then the respiratory chain function will be impaired. The threshold at which symptoms will manifest depends on the tissue Inhibitors,research,lifescience,medical involved. Skeletal muscle (especially extraocular muscles) brain, heart, retina, renal tubular cells, and auditory cells of the organ of Corti are the most common tissues affected. Third, mitotic segregation of the multiple existing copies of mtDNA explains Entinostat why the level of mutant mtDNA can change during life (16); this may depend on the stage of embryonic development in which the original mutation occurs. Point mutations vs large rearrangements As a general rule, mtDNA can harbour two different types of genetic variants, point mutations or large-scale rearrangements, which can involve deletions, duplications, or both together. Point mutations are commonly maternally inherited and they may differ from non pathogenic polymorphisms since a single change of a nucleotide base (e.g. A to G in position 3243 commonly for MELAS) (17) produces subsequently modifications in the corresponding product leading to so defects in protein conformation.