In 1984, two years before mutations in the X-chromosome gene for the muscle protein now known as dystrophin were identified as the cause of Duchenne MD, researchers at the University of California at Berkeley and an agricultural center in Scotland announced they’d found an X-linked muscular dystrophy in mice.
(Doctors had long known that DMD was a genetic disease and that, because of its inheritance pattern, the gene involved had to be on the X chromosome.)
These new mice, which the researchers dubbed mdx, had an X-chromosome disease, high blood levels of the enzyme creatine kinase, muscles that looked dystrophic under the microscope, and no abnormalities in their brains or spinal cords. They shared all these characteristics with boys with human DMD.
There was one problem: The mice had only mild, if any, observable weakness, in contrast to the obvious, severe weakness that human patients experienced.
In 1986, MDA-supported investigators identified mutations in the dystrophin gene as the cause of DMD in humans, and in 1989, the muscle abnormalities in the mdx mouse were likewise found to be caused by a dystrophin mutation.
The type of mutation in the mdx mouse is a premature stop codon, a genetic error that stops the synthesis of a protein (dystrophin in this case) before a functional molecule is made.
It’s been estimated that about 15 percent of the human DMD population has this type of mutation. The stop codon in the mdx mouse comes so early that the mice have virtually no dystrophin in their muscles, so their relatively normal strength was — and is — surprising.
In fact, some investigators have made this difference in the way humans and mice respond to dystrophin deficiency the basis of their inquiries. Zeroing in on how the mouse maintains its relatively normal muscle health, they reason, could provide clues for treatment strategies in people.
Today, the mdx mouse remains the most widely used DMD model, despite some investigators’ reservations. And, for the most part, results of treatment trials conducted in these mice have been fairly predictive of results in humans, with the caveat that few medications have actually moved from mouse to human trials.
Many scientists temper their enthusiasm for the mdx model by thinking back some 15 years, when experiments showed this mouse accepted transplants of dystrophin-carrying, immature muscle cells (myoblasts). Several subsequent tests of myoblast transfer in boys with DMD found that few, if any, of the new cells survived, and no one derived any benefit.
But there are success stories as well. Prednisone, a corticosteroid now recommended for use in treating DMD by the American Academy of Neurology, improved strength in mdx mice at a dose of 1 milligram per kilogram per day, but weakened the mice at 5- and 10-milligram per kilo dosages. The recommended dose for boys with DMD is fairly close to the mouse dose — 0.75 milligrams per kilo per day.
When mdx mice were given compounds to block myostatin, a natural body protein that limits muscle growth, they got big and strong in much the same way as a child did who was born with a severe myostatin deficiency.
A clinical trial of a myostatin blocker is now under way in Becker muscular dystrophy (which also results from dystrophin gene mutations) and two other types of MD, as are trials in DMD of pentoxifylline, which may decrease inflammation and scar tissue formation, and oxatomide, an antihistamine, both of which improved strength in the mdx mouse.
Meanwhile, the biotechnology company PTC Therapeutics, with support from MDA, will soon test a compound called PTC124 in boys with DMD. This experimental medication is designed to allow cells to "read through" premature stop codons and make full-length dystrophin. That trial is going forward because the drug was found safe and effective at increasing dystrophin levels in mdx mice.
Investigators, including Gregory Cox of the Jackson Laboratory in Bar Harbor, Maine, have high hopes for PTC124. "They seem, for the subset of patients with stop codons, to have huge potential," Cox says of this type of drug. "They have a real, mechanistic reason to work — and the mdx mouse was the best model to test them in."