By David N. Leff
A rare and devastating disease, with fewer than 100 known cases the world over, has just taken three giant steps toward unmasking its secrets.
The malady is known as MNGIE, because its full name is mitochondrial neurogastrointestinal encephalomyopathy. That means it savages nerves, guts, brain and muscle.
Neurogeneticist Michio Hirano lists the three basic findings that he and his colleagues at New York's Columbia University College of Physicians and Surgeons have just announced.
"Number one," he told BioWorld Today, "we have found the cause of MNGIE. Number two, this is an 'experiment of nature,' which will allow us to understand better the function of its defective enzyme. And number three, it is likely to lead to therapy for this so-far untreatable disease."
Hirano is senior author of a report on his new data in today's issue of Science, dated Jan. 29, 1999. Its title is: "Thymidine phosphorylase [TP] gene mutations in MNGIE, a human mitochondrial disorder."
Mitochondria are free-floating organelles inhabiting the cytoplasm of every mammalian cell. There's evidence that early in evolution, bacteria that had invaded cells dropped their pathogenic proclivities and took up housekeeping as permanent symbiotic subcellular entities. As such, they continue to sport their own genomic DNA, a mere 16,569 base pairs long, encoding 13 polypeptides.
When those genes go wrong, their mutations cause a number of mitochondrial diseases. (See BioWorld Today, Aug. 27, 1997, p. 1.)
"In the case of MNGIE," Hirano explained, "a defective TP enzyme, which is encoded in the cell's nuclear DNA, leads to deletions of mitochondrial DNA. Those mutations lead to an abnormal protein, which is TP, and the enzymatic activity of that protein is defective. That in turn leads to deletions of mitochondrial DNA, which is not part of the TP gene. In other words, the nuclear factor, which then causes abnormalities of mitochondrial DNA, represents a breakdown of intergenomic communication between the cell's nucleus and its organelle."
MNGIE first came to light in 1987, when a German neuropathologist described its protean symptoms.
"We started taking an interest in the disorder in 1989," Hirano recalled, "when we saw our first patient. Our research group has had a long-standing interest in mitochondrial diseases, so we paid early attention to this new one."
MNGIE's most striking visible symptom, he said, is that, for no known reason, these patients "are extremely thin. One we saw was 5 feet 8 inches tall, and weighed only 85 pounds. They have eye-muscle abnormalities - droopy eyelids (ptosis) and impaired ocular movements. Another feature is prominent gastrointestinal dysmotility. That causes frequent diarrhea, and often pseudo-obstruction of the gut. MNGIE's other salient feature is peripheral neuropathy, which is marked by numbness and tingling of hands or feet, with progressive weakness in limb muscles."
The disease afflicts young adults rather than children, and brings on certain death between the ages of about 20 and 50. It's entirely inherited.
"The pattern of inheritance," Hirano explained, "is autosomal recessive, meaning that a low percentage of the population carries mutations in the [TP] gene. If one carrier happens to have a child with another carrier, that offspring would have a 25 percent chance of developing MNGIE."
To pin down MNGIE's genetic whys and wherefores, Hirano and his co-authors collected DNA samples of blood or muscle tissue from 12 patients in North America, Europe and Asia.
"Because our group here at Columbia is well-known for research on mitochondrial diseases," he said, "people from around the world referred samples to us. As a clinician, I've seen quite a few patients myself, made several trips to places to examine these individuals.
Hunting, And Finding, The MNGIE Gene
"We started our gene hunt," Hirano went on, "with a traditional linkage-analysis approach, and identified the locus for MNGIE on the long arm of chromosome 22. Then, we did positional cloning, to identify candidate TP genes. The third gene that we sequenced happened to be the TP gene."
The co-authors identified 10 mutations in all 12 of the probands they examined. These correlated with a biochemical defect, showing that mutation causes a severe decrease in the activity of the enzyme. "All of the patients we've tested so far," Hirano said, "had less than five percent of the normal TP activity. The main function of the normal TP enzyme seems to be the breakdown or catabolism of the nucleoside thymidine into the ribose sugar and the thymine base. We've actually measured levels of thymine and thymidine in the plasma from MNGIE patients, and found that their thymidine levels are increased about seven-fold. So the enzyme defect causes an accumulation of that nucleoside. And there have been tissue-culture studies showing that thymidine by itself can be toxic to cells."
Besides its day job of breaking down thymidine, TP has also been described by oncological researchers - before they knew that the TP gene had been identified and sequenced - as a platelet-derived endothelial cell growth factor. "It may have some angiogenic properties," Hirano observed. "Some people in the cancer field have been studying this enzyme, because it seems to be increased in tumors, and may be a factor in producing blood vessels to feed those tumors. Then, they independently cloned the gene, and found out it had exactly the same sequence as TP. So, the two fields merged.
"At the time," Hirano went on, "Amgen Inc. [of Thousand Oaks, Calif.] had constructed a human recombinant form of this TP protein, which they produced for a while. Then, they apparently lost interest in it from a carcinogenic point of view, and turned it over to other researchers. I wanted to treat MNGIE patients with that product, but they referred me to someone else."
He and his co-authors are now "exploring in a very preliminary way the possibility of enzyme-replacement therapy for MNGIE patients. We'd need to get an IND [investigational new drug application], and various approvals from institutions. In fact, now that this recombinant human protein is available, it would certainly be a rational and feasible enzyme-replacement treatment. An example of that is Genzyme Corp.'s recombinant Ceredase for Gaucher's disease."
Genzyme, of Cambridge, Mass., markets Ceredase and Cerezyme for Gaucher's disease.
Hirano concluded with "a speculative leap. In terms of broader clinical significance, we know that deletions of mitochondrial DNA accumulate with aging. I certainly don't think MNGIE is a model for aging, but perhaps it would be a key to understanding the phenomenon." n