Life is the interplay between structure and energy, yet the role of energy deficiency in human disease has been poorly explored by modern medicine. Since the mitochondria use oxidative phosphorylation (OXPHOS) to convert dietary calories into usable energy, generating reactive oxygen species (ROS) as a toxic by-product, I hypothesize that mitochondrial dysfunction plays a central role in a wide range of age-related disorders and various forms of cancer. Because mitochondrial DNA (mtDNA) is present in thousands of copies per cell and encodes essential genes for energy production, I propose that the delayed-onset and progressive course of the age-related diseases results from the accumulation of somatic mutations in the mtDNAs of post-mitotic tissues. The tissue-specific manifestations of these diseases may result from the varying energetic roles and needs of the different tissues. The variation in the individual and regional predisposition to degenerative diseases and cancer may result from the interaction of modern dietary caloric intake and ancient mitochondrial genetic polymorphisms. Therefore the mitochondria provide a direct link between our environment and our genes and the mtDNA variants that permitted our forbears to energetically adapt to their ancestral homes are influencing our health today. Keywords mitochondria; reactive oxygen species; human origins; diabetes; neurodegenerative diseases; aging; mtDNA: mitochondrial DNA; Mitochondrion (s), mitochondria (pl): cellular organelle of endosymbiotic origin that resides in the cytosol of most nucleated (eukaryotic) cells and which produces energy by oxidizing organic acids and fats with oxygen by the process oxidative phosphorylation (OXPHOS) and generates oxygen radicals (reactive oxygen species, ROS) as a toxic by-product; ROS: reactive oxygen species, oxygen radicals; OXPHOS: oxidative phosphorylation; Mitochondrial DNA (mtDNA): the portion of the mitochondrial genome that currently resides in the matrix of the mitochondrion, as a circular DNA molecule containing the mitochondrial rRNA genes, tRNA genes, and 13 subunits of the mitochondrial oxidative phosphorylation (OXPHOS) enzyme complexes; CR: mtDNA control region; TCA: mitochondrial tricarboxylic acid cycle; SDH: succinate dehydrogenase; COX: cytochrome c oxidase, complex IV; ETC: mitochondrial electron transport chain, a part of the OXPHOS system; ANT: adenine nucleotide translocator; Unc 1,2,3: uncoupling proteins 1,2,3; Reactive oxygen species (ROS): primarily superoxide anion (O 2• − ), hydrogen peroxide (H 2 O 2 ), and hydroxyl radical ( • OH), commonly referred to as oxygen radicals; generated as a toxic by-product of oxidative energy production by OXPHOS damage the mitochondrial and cellular DNA, proteins, lipids, and other molecules causing oxidative stress; Oxidative phosphorylation (OXPHOS): the process by which the mitochondrion generates energy through oxidation of organic acids and fats with oxygen to create a capacitor [electron chemical gradient (ΔP = ΔΨ + ΔpH)] across the mito...

Over the past 10 years, mitochondrial defects have been implicated in a wide variety of degenerative diseases, aging, and cancer. Studies on patients with these diseases have revealed much about the complexities of mitochondrial genetics, which involves an interplay between mutations in the mitochondrial and nuclear genomes. However, the pathophysiology of mitochondrial diseases has remained perplexing. The essential role of mitochondrial oxidative phosphorylation in cellular energy production, the generation of reactive oxygen species, and the initiation of apoptosis has suggested a number of novel mechanisms for mitochondrial pathology. The importance and interrelationship of these functions are now being studied in mouse models of mitochondrial disease.

Contrary to conventional wisdom, functional mitochondria are essential for the cancer cell. Although mutations in mitochondrial genes are common in cancer cells, they do not inactivate mitochondrial energy metabolism but rather alter the mitochondrial bioenergetic and biosynthetic state. These states communicate with the nucleus through mitochondrial ‘retrograde signalling’ to modulate signal transduction pathways, transcriptional circuits and chromatin structure to meet the perceived mitochondrial and nuclear requirements of the cancer cell. Cancer cells then reprogramme adjacent stromal cells to optimize the cancer cell environment. These alterations activate out-of-context programmes that are important in development, stress response, wound healing and nutritional status.

To determine the role of reactive oxygen species in mammalian longevity, we generated transgenic mice that overexpress human catalase localized to the peroxisome, the nucleus, or mitochondria (MCAT). Median and maximum life spans were maximally increased (averages of 5 months and 5.5 months, respectively) in MCAT animals. Cardiac pathology and cataract development were delayed, oxidative damage was reduced, H2O2 production and H2O2-induced aconitase inactivation were attenuated, and the development of mitochondrial deletions was reduced. These results support the free radical theory of aging and reinforce the importance of mitochondria as a source of these radicals.