You may never have heard of Pompe’s disease. It affects just 5,000 to 10,000 people in the United States, making it exceedingly rare and of little interest to the general public. But what it lacks in notoriety, it makes up for in personal devastation to those who have it.
|Robert Elmore nuzzles his son Grante ("Nikko"), who has lived twice as long as expected thanks to enzyme replacement therapy provided in a clinical trial.
Photos by Amy Snyder
Pompe (also known as acid maltase deficiency) is caused by a genetic deficiency of an enzyme that breaks down glycogen (stored sugar) inside muscle cells. In its severest form, it strikes during infancy, weakening the heart and the voluntary muscles, including those that control breathing. The disease can also manifest during childhood or adulthood, causing significant muscle weakness and respiratory problems.
Children and adults with the disease usually have a shortened life span, and most infants with the disease aren’t expected to live beyond 1 year of age.
But these grim prognoses could soon change, thanks to research led by Yuan-Tsong Chen, professor and chief of Medical Genetics in the Department of Pediatrics at Duke University in Durham, N.C., and director of the Institute of Biomedical Sciences, Academia Sinica, Taiwan. Through basic research supported by MDA and clinical trials supported by the biotech company Genzyme, Chen and his team at Duke have developed a way to supply the missing enzyme to people with Pompe disease.
In two trials, one completed in 2000 and the other last year, 11 babies have received this experimental treatment — called enzyme replacement therapy — and some are now healthy, walking toddlers.
A faulty enzyme, failing muscles
With currently available treatment, “There’s not much we can do for babies with Pompe disease,” Chen says.
Within weeks or months of birth, an infant with the disease can become too weak to suckle or breathe on its own. The muscular walls of the heart become enlarged, shrinking the heart’s inner chambers and reducing its pumping capacity, a condition known as hypertrophic cardiomyopathy. Most babies with the disease die from cardiac and respiratory failure within three to four months of diagnosis, Chen says.
This picture of infant-onset Pompe disease has changed little since it was first described by Dutch pathologist Joannes Pompe in the early 1930s. While studying at the University of Amsterdam, Pompe was asked to do an autopsy on a 7-month-old girl who had been admitted to the university hospital with difficulty breathing and had died three days later, apparently of pneumonia. Expecting to see her lungs filled with fluid, he was surprised to find that her heart had swollen to more than three times its normal size and the cells within it were filled with clumps of debris, which turned out to be glycogen.
It wasn’t until the 1960s that other researchers discovered the underlying basis of Pompe disease — a deficiency of the enzyme acid alpha-glucosidase (GAA), also called acid maltase. The deficiency can now be detected by blood tests that probe for GAA activity or for mutations in the gene encoding GAA, located on chromosome 17. It’s estimated that one in 85 to 100 people carries a mutation in a single copy of the gene; it takes mutations in both copies, one inherited from each parent, to cause the disease.
The severity of mutations in the GAA gene — that is, how much they alter the enzyme — determines, at least in part, the severity of the disease. Mutations that destroy the protein cause infant-onset Pompe disease, while mutations that leave some GAA intact tend to cause juvenile- and adult-onset forms of the disease.
The later-onset forms of Pompe are primarily “muscle diseases,” Chen says. Cardio-myopathy is mild in the juvenile form, and usually absent from the adult form. In children, the most common first symptom is delayed motor development; in adults, it’s difficulty walking. For both late-onset forms, respiratory weakness can be severe and often requires mechanical ventilation, Chen says.
|Working inside subcellular compartments called lysosomes, the enzyme acid alpha-glucosidase (GAA) breaks down glycogen (top). In Pompe disease, a deficiency of GAA causes glycogen to accumulate and rupture lysosomes (middle). In enzyme replacement therapy (ERT), intravenously injected GAA is taken up by lysosomes that have fused with the cell's outer surface, eventually making its way to glycogen-filled lysosomes (bottom).|
The lysosome connection
Chen has been studying glycogen storage disease, a category of diseases that includes Pompe, for more than 20 years. (Pompe disease is sometimes called glycogen storage disease type 2 or acid maltase deficiency; it’s one of 10 metabolic diseases of muscle covered by MDA’s program.)
Early in his career, he recognized that Pompe disease was going to be a tough nut to crack.
In most glycogen storage diseases, inadequate breakdown of glycogen leads to hypoglycemia, a drop in blood sugar levels that drains the body of energy. In these diseases, supplementing the diet with complex sugars like cornstarch can help maintain blood sugar levels and control symptoms.
But the symptoms of Pompe disease aren’t related to hypoglycemia; instead, they’re caused by the accumulation of glycogen itself. GAA is one of many enzymes found in lysosomes, compartments inside cells that “clean house” by trapping and degrading glycogen and other chemicals.
Without GAA, glycogen builds up inside lysosomes and ruptures them, an effect that’s especially damaging to muscle, which naturally makes large amounts of the energy-rich substance. (This makes Pompe disease a glycogen storage disease and a lysosomal storage disease, a category that includes Tay-Sachs and Niemann-Pick diseases.)
Chen has thus focused much of his research on how to deliver GAA to the lysosomes of people with Pompe disease. This is a tall order, one that might seem to parallel efforts at gene therapy and stem cell therapy for muscle disease, which haven’t yet shown success in the clinic.
|Yuan-Tsong Chen. Photos by Cramer Gallimore|
But Chen has been able to exploit a key feature of lysosomes: In their business of “housecleaning,” they fuse with the outer surface of the cell, allowing them to release their contents and take up substances from the outside. Because lysosomes take in other substances, Chen and others reasoned that intravenously delivered GAA might make its way into the lysosomes of muscle cells.
Building a better enzyme
Enzyme replacement therapy for Pompe disease wasn’t his idea, Chen acknowledges. In clinical trials in the 1970s, patients with the disease were given injections of GAA isolated from human placenta, but the treatment failed.
Later, Chen says, “We learned that in order for the enzyme to work, you need to make a special form of it that can be taken up by the cells in [voluntary] muscles and in the heart. The second critical issue is how to make sufficient quantities of the enzyme for a clinical trial.”
By the 1980s, scientists discovered that for efficient uptake by lysosomes in muscle cells, GAA and other enzymes must have a chemical “tag,” called mannose-6-phosphate (M6P). Human placenta had been a plentiful source of GAA, but it produces a version of the enzyme that has very little M6P.
In the 1990s, with advances in molecular biology and funds from MDA, Chen was able to engineer an M6P-laden version of the enzyme — called recombinant human GAA (rhGAA) — using a line of cells (CHO cells) to produce it in large amounts.
Armed with this new enzyme, Chen formed a collaboration with researchers from Tokyo, who were studying a strain of Japanese quail rendered flightless by naturally occurring Pompe’s disease. After three weeks of injections with rhGAA, the birds could fly.
Chen and his team were ready to test rhGAA in babies with Pompe disease, but first, they needed help from the biotech industry.
“In order to test the therapy in humans,” Chen explains, “we needed to make the enzyme in a GMP [good manufacturing practice] facility, we needed to have every single step documented, and we needed large bioreactors” — incubation chambers for growing the CHO cells that produce rhGAA. “These are things we’re not able to do in an academic research lab.”
A lifesaving treatment
Working first with Synpac, a pharmaceutical company based in Taiwan, and later with Genzyme, a Cambridge, Mass.-based company with a longstanding interest in lysosomal storage diseases, Chen began his first trial of enzyme replacement therapy for Pompe’s disease in 1999. The results were published in March 2001 in the journal Genetics in Medicine.
The three babies in the trial, who ranged from 2 months to 4 months old at its inception, had once been expected to die — but all of them are still alive. After about a year of twice-weekly intravenous infusions with rhGAA, all experienced significant reductions in heart size and improvements in cardiac function. Genzyme has continued to supply them with the treatment.
One baby has become “an essentially normal 3-year-old boy,” able to walk and to breathe on his own, Chen says. The other two, now 3 and 4 years old, require mechanical ventilation and haven’t developed normal motor skills, but they have normal cardiac function, he says.
In 2001, Genzyme and Duke scientists launched a second trial involving eight babies, ranging from 3 months to 14 months old. Five of the infants were studied at Duke and the others were studied at sites in Europe. Details of the results await publication, but Priya Kishnani, the trial’s lead investigator at Duke, presented some of her data at a scientific meeting in Dublin in September.
According to her report, all of the babies experienced significant reductions in heart size, two died from complications unrelated to the enzyme, and the remaining six were still alive after about a year of treatment. (For more about one of these toddlers, see “A Time to Celebrate.”)
“It’s such a fruitful experience to go from a diagnostic approach to a treatment approach for a disease that’s considered lethal,” Kishnani says. “By no means is this a cure; we don’t know the long-term benefits or side effects of the treatment. But there’s nothing else out there right now to change the natural course of this devastating disease.”
What the future holds
This year, Genzyme and the Duke team, led by Kishnani, will begin two larger trials of enzyme replacement therapy for Pompe disease.
The trials are a final step toward getting the treatment approved by the U.S. Food and Drug Administration, Chen says. One, already under way, is enrolling toddlers between 6 months and 3 years old and the other will enroll babies less than 6 months old. (For more information, contact Genzyme Medical Information at  745-4447.) Each trial will recruit up to 16 patients and will test a different version of rhGAA than that used in the previous trials.
Genzyme, which has made Pompe’s disease its largest research and development effort since its founding 21 years ago, now has an arsenal of rhGAA types. The company began testing enzyme replacement therapy for Pompe disease in 1998, through a joint venture with Pharming, a Dutch biotech company. Scientists from the two companies genetically engineered rabbits to produce rhGAA in their milk, and had begun testing this “transgenic” rhGAA in patients with the infantile and juvenile forms of Pompe disease. In 2001, Pharming went into receivership and Genzyme acquired the rights to the transgenic rhGAA.
Genzyme acquired another type of rhGAA, made in CHO cells like Chen’s, when it bought the Princeton, N.J., company Novazyme Pharmaceuticals.
Recently, Genzyme has developed a fourth version of rhGAA with “improved scalability,” meaning it can be produced in larger quantities than previous versions. This is the enzyme that Duke researchers will test in upcoming trials; once a sufficient amount of the enzyme is available, they hope to test it in adults with Pompe disease.
Looking to the more distant future, scientists at Genzyme and Duke are also investigating gene therapy for Pompe’s disease. One potential benefit of this approach is “a decreased need for frequent infusions of the enzyme. You could envision a gene therapy treatment that would only be required yearly,” says Andy Amalfitano, a co-investigator in the enzyme replacement therapy trials.
In fact, Amalfitano says, “Pompe disease may be one of the best diseases to consider treating by gene therapy... because we have an opportunity to treat every muscle in the body without inserting the [GAA] gene into every muscle.” A virus could be used to deliver GAA to the liver, which could then release the enzyme into the bloodstream, Amalfitano explains. In MDA-funded experiments at Duke, he’s used this approach to restore GAA activity to the muscles of mice and quail with Pompe disease.
Editor’s Note: Until now, MDA and Genzyme have made independent efforts to support the development of enzyme replacement therapy for Pompe’s disease. In November, MDA and Genzyme staff met at MDA’s National Headquarters in Tucson, Ariz., and discussed plans to collaborate on future research.