Mitochondria are intracellular organelles that play a critical role in cellular metabolism. Mitochondria contain the electron transport chain, which transfers electrons to oxygen by means of a process called oxidative phosphorylation. This process releases energy for the production of adenosine triphosphate (ATP) by forming a pH and electrical gradient (called the chemiosmotic gradient) across the inner mitochondrial membrane. In addition to oxidative phosphorylation, the mitochondria fulfill a number of other functions, including the following:
- Make ATP for cellular energy
- Metabolize fats, carbohydrates, and amino acids
- Interconvert carbohydrates, fats, and amino acids
- Synthesize some proteins
- Reproduce themselves (replicate)
- Participate in apoptosis
- Make free radicals
Of these functions apoptosis is particularly important in development and disease. However, human disease may result from impairment of any of these functions.
Mitochondria are inherited from the mother, but not from the father. In the process of egg formation, there is thought to be a "bottleneck" in mitochondrial number, such that the unfertilized egg may have as few as 1,000 mitochondria. This number increases 100-fold after the ovum is fertilized. The mitochondria contain their own DNA, mitochondrial or mtDNA, and during development there may be selective amplification of some of these mtDNA molecules, leading to increases or decreases in the presence of mutated mtDNAs.
The Importance of the Electron Transport Chain
The origins of mitochondria are unknown, but the likely explanation, called the endosymbiont hypothesis, holds that they arose as free-living bacteria that colonized proto-eukaryotic cells, thereby establishing a symbiotic relationship. Primitive eukaryotic cells with intracellular mitochondria capable of metabolizing oxygen would have had an advantage in an oxygen-rich environment. The electron transport chain produces far more energy for each molecule of glucose consumed than is produced by anaerobic respiration. The oxidative phosphorylation process conducted by the mitochondria produces thirty-eight molecules of ATP, compared to two molecules of ATP produced by anaerobic glycolysis. Oxidative phosphorylation allows the conversion of toxic oxygen to water, a protective biological advantage.
A disadvantage of oxidative phosphorylation, however, is the formation of reactive oxygen species, such as singlet oxygen and hydroxyl radicals, which damage such cellular components as lipids, proteins, and DNA. A normally functioning electron transport chain produces reactive oxygen species from about 2 percent of the electrons that it transports. In disease states and in aging, larger quantities of reactive oxygen species are generated, and this may be a significant factor in cellular deterioration as well as a major contributor to the aging process.
Mitochondrial Genes and Disease
Mitochondrial DNA encodes approximately 3 percent of mitochondrial proteins. The relative contribution of the mitochondrial and nuclear genomes in coding for electron transport chain subunits is detailed in Figure 1. Human mtDNA contains 16,569 nucleotide bases and encodes thirteen polypeptides of the electron transport chain, twenty-two transfer RNAs (tRNAs) and two ribosomal RNAs (rRNAs). In addition, mtDNA has a control region (termed the D-loop), which contains considerable genetic variation. The D-loop forms the basis of forensic medicine DNA identification and has been very useful in the molecular anthropological study of human origins.
In 1988 the first human disease associated with mtDNA deletions was reported. These patients suffered from muscle and brain diseases with ragged red fiber muscle disease (myopathy), with or without progressive neurological deterioration. Ragged red fibers are muscle fibers, that have a disorganized structure and an excess of abnormal mitochondria and that stain red when treated with a histochemical stain called modified Gomori trichrome (Figure 2). In 1988 Kearns-Sayre syndrome, which primarily affects the muscles, heart, and brain, was found to be due to mtDNA deletions or duplications. About the same time, the maternally inherited disorder Leber's hereditary optic atrophy was traced to point mutations in mitochondrial DNA encoding subunits of complex I of the electron transport chain.
Table 1
| Organ or System Diseased | Symptoms | |
| brain | stroke, seizures, dementia, ataxia, developmental delay | |
| muscle | weakness, pain, fatigue | |
| nerve | neuropathy | |
| heart | cardiomyopathy, heart failure, heart block, arrhythmia | |
| pancreas | diabetes, pancreatitis | |
| eye | retinopathy, optic neuropathy | |
| hearing | sensorineural deafness | |
| kidney | renal failure | |
| GI system | diarrhea, pseudo-obstruction, dysmotility | |
Table 1.
Mitochondrial diseases tend to affect multiple organ systems. The cells and organs most severely affected are those most heavily dependent on ATP, such as those listed in Table 1. Patients will frequently have multiple symptoms or signs, a circumstance that often causes confusion in diagnosis and treatment.
One of the more common presentations of mitochondrial disease in infants and young children is Leigh's disease, first described by the pathologist Dennis Leigh in 1951. This progressive disease primarily affects the brain, with episodic deterioration that is often triggered by mild viral illnesses. Other organ systems are often involved, and there is often high blood or brain lactic acid as a result of a failure in oxidative metabolism (lactic acid is formed from glucose in the absence of oxygen). Figure 1 details the sites of metabolic defect and the percentages of cases affected in cases of Leigh's syndrome. Complex I and IV defects are autosomal recessive diseases, with the culprit genes residing on the nuclear chromosomes. Complex V mutations are mtDNA inherited, and another 25 percent of cases are X-linked, due to pyruvate dehydrogenase deficiency (another mitochondrial enzyme, not shown in Figure 1).
One of the most common mtDNA diseases seen is due to a single point mutation at position 3,243, with an adenine to guanine mutation in a tRNA leucine gene. Patients with this mutation may have phenotypes ranging from asymptomatic (that is, having no visible effects) to diabetes mellitus (with or without deafness). It is estimated that 1 to 2 percent of all diabetics have the A3243G mutation as the cause, affecting 200,000 people in the United States alone. The most severe phenotype to occur from this mutation has been given the acronym MELAS, for mitochondrial encephalomyopathy, with lactic acidosis and stroke-like episodes. The variability of disease phenotype or heterogeneity of disease due to mtDNA mutations arises in part because of variations in the amount of mutated mtDNA within different tissues. This mixture of wild type and mutant DNA within a cell is called heteroplasmy. In many mtDNA diseases, heteroplasmy changes over time, so that there is an increase in mutant DNA in nondividing cells and tissues such as muscle, heart, and brain, with a decrease over time in rapidly dividing tissues such as bone marrow.
Bibliography
Johns, D. R. "Mitochondrial DNA and Disease." New England Journal of Medicine333, no. 10 (1995): 638-644.
Raha, S., and B. H. Robinson. "Mitochondria, Oxygen Free Radicals, and Apoptosis." American Journal of Medical Genetics 106, no. 1 (2001): 62-70.
Wallace, D. C. "Mitochondrial DNA in Aging and Disease." Scientific American 277, no. 2 (1997): 40-47.
—Richard Haas
