The other DNA:
research on mitochondrial diseases
By ERIC A. SCHON and SALVATORE DIMAUROMITOCHONDRIA-- BACTERIUM-SIZED organelles residing in most of our cells--convert fuel from food into the body's most biologically useful form of energy, adenosine triphosphate or ATP. Mitochondria are the only non-nuclear constituents of the cell with their own DNA (mtDNA) and machinery for synthesizing RNA and proteins. This remarkable capability reflects their descent from bacteria. The endosymbiont hypothesis, first proposed by Lynn Margulies and now widely accepted, states that early in evolution, an energy-poor cell engulfed a bacterium with far more efficient energy-producing machinery and ultimately co-opted its functions; over time, the bacteria evolved into mitochondria. (Chloroplasts, too, are endosymbionts, though they lack separate DNA.) Endosymbiosis was probably critical for the development of large multicellular organisms, including us.
Only in the past seven years, with advances in cellular and molecular biology, have we appreciated the complexity of genetic mechanisms and clinical presentations in mitochondrial disorders. The history of mitochondrial disease goes back to the early 1960s, however, when Lars Ernster and Rolf Luft in Stockholm described a patient who ate voraciously yet stayed thin, sweating profusely even in winter. Ernster and Luft implicated a defect in mitochondrial energy metabolism and showed that this patient's muscle mitochondria could make only a fraction of the energy they should normally produce; the unconverted fuel was diverted into heat production. The exact cause of Luft disease (perhaps the rarest condition known: only one other patient has been found) remains unknown, but these investigators broke new ground by linking mitochondrial function defects to human disease.
The diagnosis of mitochondrial diseases long relied on evidence of massive mitochondrial proliferation in muscle, giving rise to bizarre structures called ragged-red fibers. In the late 1980s, however, two reports introduced molecular genetics as a diagnostic tool for these diseases.(1) One group found large numbers of mitochondria with smaller, deleted, non-functional genomes arising spontaneously in patients with eye-muscle paralysis. Another group described a maternally inherited point mutation in patients with Leber's hereditary optic neuropathy, a rare cause of blindness in young adults. Almost 50 other disease-causing mutations have since been described, demonstrating key concepts in mitochondrial (as opposed to nuclear, or mendelian) genetics.
Mitochondrial genetics is population genetics. Each cell contains only one nucleus but hundreds or even thousands of mitochondria and mtDNAs (which, as Alexander Tzagoloff of Columbia's biological sciences department discovered, have their own unique genetic code). Tissues with high demands for energy, such as muscle, heart, brain, and eye, are particularly vulnerable to mitochondrial defects. Second, at fertilization all mitochondria in the zygote come from the oocyte; thus, both mtDNA and most mtDNA-related diseases are maternally inherited. Third, patients with mitochondrial genome defects usually harbor a mixture of normal and mutant mtDNAs, a condition known as heteroplasmy. A tissue with 20 percent mutant mtDNAs, unsurprisingly, suffers different effects than one with 90 percent. Moreover, the proportion of mutated mtDNAs can vary both in space (among tissues) and in time (over the patient's lifespan); one mutation may cause two diseases, or a patient may have totally different diseases early and later in life.
For example, large numbers of mtDNA deletions in heart, muscle, and brain become fatal in young adulthood (Kearns-Sayre syndrome), while large numbers of the same deletion in blood cause fatal anemia in infancy (Pearson syndrome). A few children with Pearson syndrome have survived after blood transfusions, only to develop Kearns-Sayre syndrome later, as mutated mtDNAs increased in other organs while declining in blood.
Besides maternally inherited and sporadic mtDNA mutations, there are also mendelian-inherited errors in mtDNA. The discovery of two disorders has highlighted this seemingly paradoxical situation. In one, mtDNA deletions arise in muscle because of a defect in a nuclear gene that probably increases mtDNA's proclivity to suffer deletions. The second defect, which our group discovered, is a quantitative error in the mtDNAs present in specific tissues: "mtDNA depletion." Again, the defect is probably in a nuclear gene, most likely controlling mtDNA replication and, ultimately, the number of mitochondrial genomes in different tissues.
Environmental toxins also can deplete mtDNA. Zidovudine (azidothymidine or AZT), a drug used to treat AIDS, produces severe muscle weakness and ragged-red fibers in about 10 percent of patients. Our group discovered that AZT depletes mtDNA in these patients, probably by "molecular mimicry"--unwittingly inhibiting the mitochondrial DNA replicating machinery along with HIV's replication apparatus. Other AIDS drugs that rely on the same principle, such as didanosine and zalcitabine, can have a similar effect.
ALL THE DISEASES mentioned so far are rare. Are more common disorders also associated with mtDNA mutations? The answer is a qualified yes. Diabetes is unusually frequent in mitochondrial diseases, and about 2 percent of patients with adult-onset (type II) diabetes have known mutations in mtDNA, often with deafness.
Another may be the most prevalent disease of all: aging. The deleted mtDNAs that are present in massive amounts in Kearns-Sayre and Pearson syndromes are also present in much smaller amounts in the normal elderly; such deletions increase exponentially in long-lived tissues, such as muscle and brain. The biological significance of this finding is still unclear; we do not know whether a few deleted mtDNAs in muscle fibers or neurons can cause mitochondrial dysfunction. Nevertheless, many symptoms of mitochondrial diseases, such as muscle weakness, diabetes, vision loss, hearing loss, and dementia, are also the hallmarks of aging. Mutated mtDNAs may also have roles in the progressive symptoms of late-onset neurodegenerative diseases, such as Parkinson's and Alzheimer's diseases. The "mitochondrial theory of aging," proposed many years ago, now has the beginnings of experimental support.
The high mutation rate of mtDNA not only is important in aging but has anthropologic and forensic ramifications. Analysis of mtDNA mutations in isolated ethnic groups has shown that each group contains a stereotypical set of naturally occurring mutations not associated with disease ("neutral polymorphisms"). This discovery has opened up the new field of molecular anthropology, allowing researchers to track the migrations, languages, and origins of peoples. One well-known result of such investigations has been the identification of the "mitochondrial Eve," a hypothetical woman (or, more accurately, group of women) who lived about 200,000 years ago in Africa: the mothers of us all, as everyone today carries the progeny of their mitochondrial DNA. A second result has been the use of mtDNA polymorphisms to trace the origin of the waves of migration of peoples from Asia across the Bering land bridge to populate North and South America-and be mislabeled "Native" Americans. And thanks to mtDNA analysis, we now know that the Tyrolean Iceman was not a hoax, but an authentic ice-age European. A more pointed application of mtDNA analysis is in forensic investigations: mtDNA analyses contributed heavily to the recent confirmation that the remains found in a shallow grave in Yekaterinburg, Russia, were indeed those of the Romanovs, and that Anna Anderson was not the still missing Princess Anastasia.
In biology and anthropology, forensics and medicine, mitochondria--long called the powerhouses of the cell--have now powered their way to some of the most intriguing realms of research.
Schon EA, Hirano M, and DiMauro S (1994). Mitochondrial encephalomyopathies: clinical and molecular analysis. J. Bioenerg. Biomemb. 26, 291-299.
ERIC A. SCHON is associate professor of genetics and development (in neurology) at Columbia. SALVATORE DeMAURO is the Lucy G. Moses Professor of Neurology at Columbia. They have been collaborating on the study of the genetics of human mitochondrial function and dysfunction for the past 10 years.
PHOTO & ART CREDITS: photomicrographs, Eduardo Bonilla, Columbia University; illustration, Amy Pollack