Mitochondria are cytoplasmic organelles whose major function is to generate ATP by the process of oxidative phosphorylation under aerobic conditions. This process is mediated by the respiratory electron transport chain (ETC) multiprotein enzyme complexes I–V and the two electron carriers, coenzyme Q (CoQ) and cytochrome c. Other cellular processes to which mitochondria make a major contribution include apoptosis (programmed cell death) and additional cell type–specific functions (Table 85e-1). The efficiency of the mitochondrial ETC in ATP production is a major determinant of overall body energy balance and thermogenesis. In addition, mitochondria are the predominant source of reactive oxygen species (ROS), whose rate of production also relates to the coupling of ATP production to oxygen consumption. Given the centrality of oxidative phosphorylation to the normal activities of almost all cells, it is not surprising that mitochondrial dysfunction can affect almost any organ system (Fig. 85e-1). Thus, physicians in many disciplines might encounter patients with mitochondrial diseases and should be aware of their existence and characteristics.
TABLE 85e-1Functions of Mitochondria ||Download (.pdf) TABLE 85e-1Functions of Mitochondria
|All Cells and Tissues |
|Oxidative phosphorylation |
|Apoptosis (programmed cell death) |
|Tissue- or Cell-Specific |
|Cholesterol metabolism |
|Amino and organic acid metabolism |
|Fatty acid beta oxidation |
|Sex steroid synthesis |
|Heme synthesis |
|Hepatic ammonia detoxification |
|Neurotransmitter metabolism |
Dual genetic control and multiple organ system manifestations of mitochondrial disease. (Reproduced with permission from DR Johns: Mitochondrial DNA and disease. N Engl J Med 333:638, 1995.)
The integrated activity of an estimated 1500 gene products is required for normal mitochondrial biogenesis, function, and integrity. Almost all of these are encoded by nuclear genes and thus follow the rules and patterns of nuclear genomic inheritance (Chap. 84). These nuclear-encoded proteins are synthesized in the cell cytoplasm and imported to their location of activity within the mitochondria through a complex biochemical process. In addition, the mitochondria contain their own small genome consisting of numerous copies (polyploidy) per mitochondrion of a circular, double-strand mitochondrial DNA (mtDNA) molecule comprising 16,569 nucleotides. This mtDNA sequence (also known as the “mitogenome”) might represent the remnants of endosymbiotic prokaryotes from which mitochondria are thought to have originated. The mtDNA sequence contains a total of 37 genes, of which 13 encode mitochondrial protein components of the ETC (Fig. 85e-2). The remaining 22 tRNA- and 2 rRNA-encoding genes are dedicated to the process of translating the 13 mtDNA-encoded proteins. This dual nuclear and mitochondrial genetic control of mitochondrial function results in unique and diagnostically challenging patterns of inheritance. The current chapter focuses on heritable traits and diseases related to the mtDNA component of the dual genetic control of mitochondrial function. The reader is referred to Chaps. 84 and 462e for consideration of mitochondrial disease originating from mutations in the ...