The other genome.
The study of mitochondrial gene mutations presents investigators with new technical problems not inherent to the study of nuclear gene mutations. This genome is thought to be derived from an evolutionarily ancient organism that parasitized primitive cells, conferring on them enhanced oxidative capacity and the capability of making profitable use of atmospheric oxygen, a fairly toxic substance. The structure of the present day human mitochondrial genome reflects its unusual origin. The mitochondrial genome is a small (16.5 kb) circular DNA encoding only 13 proteins, 2 rRNAs, and a set of tRNAs. All proteins encoded by the mitochondrial genome are components of the mitochondrial electron transport chain, the energy-transducing, oxidative apparatus of the cell. Unlike nuclear genes, which exist in pairs by virtue of their location on paired chromosomes, mitochondrial genes exist in numerous copies per cell, with each mitochondrion containing several copies of the genome and each cell containing many mitochondria. In the case of a nuclear gene, only a limited number of combinations of mutated genes are possible: a situation in which both are wild type, a situation in which one is mutated and one is wild type, or a situation in which both are mutated. This binary type of arithmetic gives rise to the well-known patterns of Mendelian inheritance with recessive and dominant traits. In contrast, mitochondrial gene mutations can exist in a continuous spectrum ranging from none of the copies of a given mitochondrial gene being mutated to all of the copies being mutated. The existence of more than one genotype is termed heteroplasmy. The degree of heteroplasmy correlates with phenotype, and there appears to be a threshold of mutational burden below which the cell retains enough mitochondrial capacity to function normally and no abnormality of phenotype results. Further complicating the situation is the fact that an individual's mitochondrial genotype may change over time. This occurs because mitochondria replicate (through fission), and there is some evidence that defective mitochondria may replicate more quickly than healthy (mutation-free) mitochondria (4). Consequently, the mitochondrial genetics of disease may represent a kind of Darwinian population genetics within the cell. Thus, quick, accurate detection and quantification of mitochondrial gene mutations is critical. This need points up the importance of the findings of Chen et al. (3) who present a simple, rapid, and sensitive method for the detection of heteroplasmic mutations.
Traditional arguments have held that hereditary disorders involving mitochondrial gene mutations should be readily identifiable through analysis of pedigree of information because mtDNA is inherited exclusively maternally with no paternal contribution. A disorder derived from mtDNA should be transmitted vertically through a family in a situation resembling autosomal dominant inheritance with both genders expressing the phenotype, but there should be no instances of father-to-child transmission. Indeed, David C. Wallace (5,6) recognized the genetically unusual situation in LHON long before it was understood that this disorder is mitochondrially inherited (7). This type of pedigree analysis has led to the correct identification of other mtDNA-derived disorders such as myoclonus-epilepsy, ragged red fiber disease, and the syndrome of myopathy, encephalopathy, lactic acidosis, and stroke (8).
The emerging challenge in the field is the identification of a role for mitochondrial genes in disorders that occur sporadically and without a clear pattern of maternal inheritance. A theoretical argument was advanced a decade ago that some disorders resulting from mtDNA mutations might appear sporadically in a population and that mitochondrial genetics might offer a general approach to the problem of sporadic disease (9). In support of this argument, a specific mitochondrial electron transport chain defect (complex I, NADH:ubiquinone oxidoreductase) was identified in platelets of patients with sporadic Parkinson disease (PD), suggesting that PD is in fact a systemic disorder and not confined to a small portion of the brain (9). This same defect has been identified in PD brain and other tissues (10). This is not unlike the situation with LHON in which there is a widespread biochemical and genetic lesion but pathology is typically confined to the optic nerve. Further importance is lent to the finding of complex I deficiency in PD by the fact that complex I is the target enzyme of the PD-causing neurotoxin, methylphenyltetrahydropyridine (11).
The origin of the complex I defect in PD has been investigated through cybrid technology (12). This technique involves creation of culturable human cell lines that have been depleted of their own endogenous mtDNA by prolonged culture in the presence of ethidium bromide, which concentrates in mitochondria and binds to mtDNA. This ultimately leads to formation of a cell line that lacks mtDNA and which is designated [[rho].sup.0]. These cells can be repopulated with mtDNA of the investigator's choosing by fusion of these cells with enucleated cytoplasts or with platelets (which lack a nucleus but contain mitochondria and mtDNA). The resulting cytoplasmic hybrid (cybrid) can be passed in culture. Cell lines repopulated with PD mtDNA can be compared to lines derived from the same [[rho].sup.0] cells but repopulated with mtDNA from controls. Any differences likely arise from mtDNA. PD cybrid lines express complex I deficiency, indicating that this biochemical lesion is probably genetic in origin and arises from mtDNA (12). In addition to a loss of complex I activity, these cells also demonstrate other pathogenic features typical of PD, including increased production of oxygen radicals, up-regulation of oxygen radical defense enzymes, tendency toward apoptotic cell death, and perturbed calcium metabolism (13). These results suggest that mtDNA is pathogenically important in PD and that studies are needed of abnormalities of mtDNA sequences in PD.
Inheritance of mtDNA mutation is clearly one route to human disease. Another approach has been suggested by Wallace (14), who has stressed the role of acquired mtDNA mutation. The intramitochondrial location of mtDNA and its lack of a protective histone coating make mtDNA susceptible to oxidative damage and mutation. It is possible that damage acquired over a lifetime could ultimately lead to loss of adequate mitochondrial function and cell death. This is a particularly appealing mechanism in the case of age-associated neurodegenerative disorders such as PD. Careful studies of mtDNA sequence may distinguish between these two possibilities or may show that both are important.
There is now abundant evidence that defective mitochondria can play a critical role in the initiation of apoptosis through release of cytochrome c and perhaps other factors as well (15, 16). This link between mitochondrial dysfunction and programmed cell death again emphasizes the need to understand the origin of disease-associated mitochondrial dysfunction. If it is indeed primary, pharmacologic interventions aimed either at repairing mitochondrial dysfunction or at breaking the signaling link between mitochondria and apoptosis should be useful.
Mitochondrial genes, acting either alone or in concert with nuclear genes and/or environmental factors, may be of great importance in the pathogenesis of relatively common disorders. Thorough investigations of sequence changes in mtDNA will be critical in proving or rejecting this hypothesis and may bring about a new appreciation of this emerging field of human genetic disease.
(1.) Wallace DC, Singh G, Lott MT, Hodge JA, Schurr TG, Lezza AMS, et al. Mitochondrial DNA mutation associated with Leber's hereditary optic neuropathy. Science 1988;242:1427-30.
(2.) Parker WD, Oley CA, Parks JK. Deficient NADH:coenzyme Q oxidoreductase in Leber's hereditary optic neuropathy. N Engl J Med 1989;320:1331-3.
(3.) Chen T-J, Boles RG, Wong L-JC. Detection of mitochondrial DNA mutations by temporal temperature gradient gel electrophoresis. Clin Chem 1999;45: 1162-7.
(4.) Poulton J, Morten K. Noninvasive diagnosis of the MELAS syndrome from blood DNA [Letter]. Ann Neurol 1993;34:116.
(5.) Wallace DC. A new manifestation of Leber's disease and a new explanation for the agency responsible for its unusual pattern of inheritance. Brain 1970;93:121-32.
(6.) Wallace DC. Leber's optic atrophy: a possible example of vertical transmission of a slow virus in man. Aust Ann Med 1970;3:259-62.
(7.) Howell N, McCullough D. An example of Leber hereditary optic neuropathy not involving a mutation in the mitochondrial ND4 gene. Am J Hum Genet 1990;47:629-34.
(8.) Shoffner JIM, Lott MT, Lezza AMS, Seibel P, Ballinger SW, Wallace DC. Myoclonic epilepsy and ragged-red fiber disease (MERRF) is associated with a mitochondrial DNA tRNA(+lys) mutation. Cell 1990;61:931-7.
(9.) Parker WD, Boyson SJ, Parks JK. Abnormalities of the electron transport chain in idiopathic Parkinson's disease. Ann Neurol 1989;26:719-23.
(10.) Schapira AHV, Cooper JIM, Dexter D, Clark JB, Jenner P, Marsden CD. Mitochondrial complex I deficiency in Parkinson's disease. J Neurochem 1990; 54:823-7.
(11.) Vyas I, Heikkila RE, Nicklas WJ. Studies on the neurotoxicity of 1-methyl-4phenyl-1,2,5,6-tetrahydropyridine: inhibition of NAD-linked substrate oxidation by its metabolite, 1-methyl-4-pyridinium. J Neurochem 1986; 46: 1501-7.
(12.) Swerdlow RH, Parks JK, Miller SW, Tuttle JB, Trimmer PA, Sheehan JP, et al. The origin and functional consequences of the complex I defect in Parkinson's disease. Ann Neurol 1996;40:663-71.
(13.) Parker WD, Swerdlow RH. Mitochondrial dysfunction in idiopathic Parkinson's disease. Am J Hum Genet 1998;66:758-62.
(14.) Wall ace DC. Mitochondrial genetics: a paradigm for aging and degenerative diseases? Science 1992;256:628-32.
(15.) Liu X, Kim CN, Yang J, Jemmerson R, Wang X. Induction of apoptotic program in cell-free extracts: requirement for dATP and cytochrome c. Cell 1996;86:147-57.
(16.) Zamzami N, Hirsch T, Dallaporta B, Petit PX, Kroemer G. Mitochondrial implication in accidental and programmed cell death: apoptosis and necrosis. J Bioenerg Biomembr 1997; 29:185-93.
W. Davis Parker, Jr.
Departments of Neurology and Pediatrics
and the Center for the
Study of Neurodegenerative Disorders
University of Virginia School of Medicine
Charlottesville, VA 22901
|Printer friendly Cite/link Email Feedback|
|Author:||Parker, W. Davis, Jr.|
|Date:||Aug 1, 1999|
|Next Article:||Biochemical markers in the management of patients with metastatic bone disease.|