Much of the energy contracting muscles need comes from cellular structures known as mitochondria (the singular is "mitochondrion"), where the process known as oxidative metabolism -- an oxygen-requiring process used by almost all our cells to extract energy from nutrient molecules -- takes place.

by Salvatore DiMauro

The life of a doctor and clinical scientist is influenced as much by uniquely interesting patients as by professional mentors (in my case, Professors Massimiliano Aloisi in Italy and Lewis P. Rowland in the United States) or by clever students (too many to be listed here).

Two such patients stand out in my memory, both because of their unusual backgrounds and because they drew my interest to mitochondrial biology and mitochondrial diseases. I saw them with Dr. Rowland at the University of Pennsylvania, when I was a young MDA research grantee.

One patient, Abraham, was an Amish farmer from Lancaster County, Pa., whose eyelids had been increasingly drooping in the past few years, and who had noticed difficulty baling hay, a task he had performed easily all his life. I remember his big round beard, his wide hat and how he untied the tassels of his blue shirt to show us the incipient wasting of his once powerful shoulder muscles.

The second patient, Nabilia, a 22-year-old Jordanian woman, was already somewhat of a minor medical celebrity when she joined us at Penn, having been diagnosed at the American University of Beirut as the second patient with "Luft disease." This condition, described almost 10 years earlier by two distinguished Swedish scientists, Rolf Luft, an endocrinologist, and Lars Ernster, a biochemist, was then the only biochemically defined mitochondrial disease, sort of the flagship of a fledgling group of disorders soon to be ascribed to mitochondrial dysfunction.

I remember Nabilia sitting contentedly for hours in the cold room of the Clinical Research Center, her nightgown drenched with perspiration despite the cold, her bright face flushed as if she had just run several miles. She had the same condition of seemingly uncontrolled excessive metabolism previously described by Luft in a young Swedish woman. Both women had normal thyroid function (abnormalities of the thyroid being a reason for excessive metabolic activity). Their problem resided in the mitochondria of their skeletal muscles.

The muscle biopsies from both Abraham and Nabilia showed abundant "ragged red fibers" when treated with a special staining technique. As documented by W. King Engel (who also came up with the picturesque descriptive term), what makes these fibers appear both red and ragged is a massive proliferation of mitochondria, a sort of SOS message saying "red alert: critical energy shortage; more mitochondria needed." In this sense, ragged red fibers (or RRF, in our pervasively acronymic medical language) are a telltale sign of mitochondrial disease.

GETTING HOOKED

Having thus established that our two patients had mitochondrial myopathies, I remember asking Dr. Rowland, "How come that at Penn, which has one of the best mitochondrial centers in the world, patients like these are not studied in more detail?" To which Dr. Rowland, in his typical matter-of-fact way, answered, "Right; why don't you do it?"

So started my initiation into the difficult art of isolating functional intact mitochondria, first from rat muscle, then from human biopsies. I still remember the excitement of witnessing the changing slope of tracings from oxygen electrodes attached to my first decently isolated human mitochondria.

I was hooked on mitochondria, these fascinating intracellular organelles (miniature "organs"), which started off as independent, free-floating, bacteria-like organisms many millions of years ago, and then took up permanent residence in nucleated cells like the ones that make up all human tissues. To these cells, which were not capable of utilizing oxygen for metabolism, mitochondria brought a gift as precious as the fire that Prometheus gave humankind in the Greek myth: oxidative metabolism.

Because they started off as independent organisms, mitochondria have their own DNA (mtDNA), a small, circular molecule that was ignored by clinical scientists until 1988, when the first mutations in this type of DNA were discovered by the late Anita Harding and her colleagues at Queen Square in London, and by Doug Wallace and co-workers at Emory University in Atlanta.

To make things even more interesting, the rules by which mitochondrial DNA mutations are transmitted differ from the "classical" rules of Mendelian genetics. First, mtDNA (and most mtDNA mutations) are transmitted by a maternal form of inheritance, because only women pass on their mtDNA to the next generation; men do not. Second, because there are hundreds or thousands of mitochondria (and mtDNA) in each cell, the relative proportion of mutant mtDNAs transmitted from mother to child becomes an important factor in determining the type and severity of the disease associated with that particular mutation.

While the discovery of over 50 harmful mtDNA mutations in the past 10 years has opened up a new chapter in medicine (which, perhaps with a touch of justifiable pride, Professor Luft has called "mitochondrial medicine"), a lot of work remains to be done. Important as mtDNA has turned out to be in human medicine, we should not forget that it encodes only 13 of the many hundreds of proteins that make up mitochondria. The rest are encoded by DNA in the cell's nucleus. Yet, compared to the rapid pace with which we have discovered mtDNA mutations, our knowledge of nuclear mutations that affect mitochondria has lagged behind.

This is clearly one of the new hot areas of research, made even more exciting by recent unexpected discoveries that well-known and relatively common neurological diseases, such as Friedreich's ataxia, some forms of hereditary spastic paraplegia and Wilson's disease, are due to mutations in nuclear-encoded mitochondrial proteins.

THERAPIES STILL LACKING

So, what happened to Abraham and Nabilia? Abraham returned to Lancaster County with a diagnosis of "mitochondrial myopathy" and no specific therapy (nowadays, he would receive a molecular diagnosis but therapy would not be more specific or effective). Nabilia returned to her village in Jordan with a battery-operated fan, which, I hope, made the rest of her short life less miserable.

Are we doing any better with therapy today? Honestly, not much. The interval between biochemical and molecular understanding of a disease and development of a rational therapy is, unfortunately, long. Yet, knowledge of a disease's cause is the indispensable starting point, and all that we are learning about mitochondrial diseases will undoubtedly pay off in terms of treatment. At the very least, we can today offer solid genetic counseling, no small accomplishment for diseases that can be caused by defects in two different sets of genes.

For a few mitochondrial diseases, such as primary carnitine deficiency or primary coenzyme Q10 deficiency, effective therapy is provided by replacing the defective compound. Both carnitine and coenzyme Q10 are of some nonspecific benefit in patients with other mitochondrial diseases, especially defects of the biochemical pathway known as the respiratory chain.

For diseases due to nuclear gene defects that affect mitochondria, there is, of course, the hope of gene therapy. Gene therapy, however, is more problematic when dealing with thousands of mitochondrial genes, some of which are mutated while others are not. In this case, attempts are under way to shift the balance in favor of the nonmutated genes, with the hope that even minor shifts could result in major clinical benefits. Our group, among others, is directing an effort toward developing therapeutic strategies, and MDA is still supporting our work, just as it did when I first got hooked on mitochondria.

Salvatore DiMauro, M.D., graduated from the University of Padova in Italy with a specialization in neurology in 1967. He came to the University of Pennsylvania in 1968 and began receiving MDA research grants in 1969. DiMauro has served as an MDA scientific adviser and has written hundreds of articles and book chapters on muscle disorders. Since 1991, he has been the Lucy G. Moses Professor of Neurology at Columbia University.