A workshop entitled “Mitochondrial DNA Mutations and Cardiomyopathy, Heart Failure, and Ischemic Heart Disease,” chaired by Dr David A. Clayton and sponsored by the Division of Heart and Vascular Diseases, National Heart, Lung, and Blood Institute, was convened on May 15 and 16, 1995, in Bethesda, Md.
There is now convincing evidence that proper functioning of human mitochondrial DNA (mtDNA) is critical to normal cellular metabolism and that mutations in mtDNA can result in severe disease phenotypes. It is also clear that cardiac dysfunction is one of the most important problems in human health and is a condition that presents not only in mature adults but in infants as well. Therefore it is appropriate and timely to address the role of mtDNA mutation in heart disease and then to focus on appropriate strategies to elucidate the physiological cause-and-effect relationship between alterations in this genome and pathological phenotypes. The purposes of this workshop were: to review the current state of knowledge about mtDNA mutations; to provide a forum for dialogue between investigators interested specifically in the heart with those whose work is mainly in other organs/tissues/cells; to allow the clinical investigators to become aware of current knowledge concerning mtDNA and the powerful techniques available to study mitochondrial function; and to make recommendations concerning the direction of future research on the mtDNA mutations to improve prevention, diagnosis, and treatment of heart failure and ischemic heart disease. The workshop, attended by about 50 people, was very well received.
The Clinical Perspective
Evidence that mutations in mtDNA are associated with heart disease in humans is now established. Patients with defects in mitochondrial genes encoding tRNAs frequently manifest dilated cardiomyopathy or hypertrophic cardiomyopathy in association with diseases affecting other organ systems such as mitochondrial encephalopathy, lactic acidosis, stroke-like episodes (MELAS), and mitochondrial encephalopathy, ragged-red fibers (MERRF). In addition, an accumulation of deleted forms of mtDNA in the myocardium frequently results in cardiac conduction block, an example of which is the Kearns-Sayre syndrome. There is virtually no doubt that cardiac disease in these patients is caused by the mtDNA mutations, but the pathophysiological events that give rise to specific forms of heart disease in association with different mtDNA mutations remain obscure. There is now a consensus view that abnormalities in mitochondrial structure and function, and mutations in mtDNA are associated with widely prevalent forms of cardiac disease including ischemic heart disease, idiopathic dilated cardiomyopathy, hypertrophic cardiomyopathy, and cardiomyopathy of aging. In these common conditions, however, strict causal relationships between abnormalities in mtDNA and/or abnormalities in mitochondrial biogenesis and, in turn, cardiac dysfunction, are not fully elucidated.
Germline mtDNA mutations associated with mitochondrial myopathy and cardiomyopathy can either present as isolated heart and/or muscle disease or as heart-muscle disease together with other symptoms. Heart and muscle pathology are commonly associated with mitochondrial protein synthesis–related mutations in mtDNA. Protein synthesis defects can result from either mtDNA sequence rearrangements or from base substitutions in mtDNA-encoded tRNA genes, and perhaps other mutations yet to be defined. Both types of mutations are associated with proliferation of abnormal subsarcolemmal mitochondria in skeletal muscle in association with the degeneration of the muscle fibers and precipitation of mitochondrial creatine kinase in intramitochondrial paracrystalline arrays. Very severe protein synthesis mutations associated with mtDNA rearrangements or base substitution mutations are frequently cases in isolation, while milder protein synthesis mutations have been associated with maternally transmitted diseases.
Somatic mtDNA mutations also have been reported to accumulate in the hearts of patients with ischemic heart disease caused by coronary artery atherosclerotic plaques. Patients who have experienced long-term chronic ischemia and reperfusion have been found to accumulate significantly more mtDNA rearrangements in their hearts than age-matched control subjects. It is hypothesized that this increase in mtDNA mutation results from production of oxygen radicals generated by mitochondria during cyclic ischemia and reperfusion.
Cardiomyopathy also may be associated with bioenergetic defects caused by mutations in nuclear-encoded oxidative phosphorylation subunits or in nuclear genes that control the integrity, replication, and/or expression of mtDNA. Excellent progress is being made in defining transcription factors that regulate critical nuclear genes involved in these processes. This offers great promise in leading to an understanding of mitochondrial biogenesis at the level of nuclear gene expression.
Despite the cumulative circumstantial evidence, there is still a compelling need to resolve the issue of causation of heart disease caused by mtDNA mutations. From a clinical standpoint, it seems most important to determine whether the greater abundance of deleted and mutated forms of mtDNA in hearts of patients with ischemic heart disease, dilated cardiomyopathy, or the cardiomyopathy of aging carries pathophysiological significance in a subset of patients with these disorders. A direct and satisfying answer to this question seems unlikely to emerge from additional observational studies in human populations or from descriptive analysis of existing animal models. New animal models in which the mitochondrial genotype in the heart is manipulated experimentally, or carefully selected tissue culture systems, are needed to provide rigorous and unambiguous tests of causation.
Recommended Research Initiatives
A national effort to develop animal models for this purpose would be based most rationally on fundamental research in three areas: gene discovery, mitochondrial biogenesis, and gene transfer techniques.
Additional research is needed to identify genes important for mtDNA replication or repair, segregation of mitochondrial genomes during cell division, and control of mtDNA copy numbers. Such studies are likely to be pursued most productively with the use of a combination of yeast and animal cell systems, nonmammalian organisms with special advantages of genetics or ease of experimental manipulation (eg, Drosophila, Xenopus), or linkage analysis in the rare human kindreds in which a propensity to develop mtDNA abnormalities is inherited in a mendelian pattern. A special emphasis should be given to development of a mouse model for human heart disease. This is because the mouse is the industry standard for mammalian development, mouse genetics is well developed, and our knowledge of mouse mtDNA structure and function is as advanced as that of human. In fact, human and mouse mtDNAs are the most extensively studied of metazoans, and it is clear that the systems share common features of organization, mode of DNA replication, genetic content, and mechanism of gene expression.
Control of Mitochondrial Biogenesis
Regulatory proteins and signal transduction pathways that control expression of nuclear genes encoding mitochondrial proteins should be defined. Very recent progress in this area attests to its importance and experimental tractability. Molecular mechanisms that coordinate expression of nuclear and mitochondrial genes should be elucidated. Increased knowledge of regulatory controls governing mitochondrial biogenesis may facilitate the development of clinically relevant animal models. In addition, this information ultimately may promote development of countermeasures to lessen the clinical impact of mtDNA mutations.
Gene Transfer Techniques
The development of methods to effect transfer of recombinant DNA or RNA to the mitochondrial matrix of oocytes or embryonic stem cells would provide the most direct approach to generation of animal models of human mitochondrial disease. Other approaches using strategies for conditional, cardiac-specific disruption of nuclear genes, or forced expression of dominant negative nuclear transgenes also hold promise for generations of mitochondrial mutations in laboratory animals in which the cardiac phenotype can be defined in physiological, structural, and biochemical terms.
Other items that require elucidation include: the mechanisms of mitochondrial transformation and repair; the regulation of mitochondrial fission and fusion (if nuclear genes are involved in this they need to be identified); the role of mitochondria in the generation of oxygen free radicals and whether mitochondria with mutated mtDNA are a site of greater free radical production; differences between mitochondria in dividing and nondividing cells; what controls division and turnover of mitochondria; and the reason that some mitochondrial mutations and deletions give rise to different heart disorders and some have no effect.
- Copyright © 1995 by American Heart Association