By David N. Leff
"They look like little Arnold Schwarzenegger mice," said molecular biologist Michael Rudnicki.
He was describing a strain of rodent called the mdx mouse, which pinch-hits as an animal model for human Duchenne muscular dystrophy (DMD). When a small boy with this X-linked disease reaches the age of about three, his peripheral skeletal muscles — notably, legs and thighs — begin to swell up like the musculature of a bodybuilder, a circus strong man, or a Schwarzenegger.
But, unlike the awesome biceps on such figures, the imposing, fleshy calves of a DMD child contain fat deposits and connective tissue. (See BioWorld Today, Dec. 2, 1996, p. 1.)
Early childhood difficulties in walking spread upward from wasting leg muscles to the musculature of diaphragm, heart, jaw and arms, so that by age 12, a DMD victim is usually wheelchair-bound. Heart or lung failure brings on death by about age 20.
This creeping catastrophe afflicts one in 3,500 male infants born in the U.S., with over 1,000 new cases reported each year. Its perpetrator is the dystrophin protein — or, rather, its mutated absence. It's expressed by a DNA sequence, which at 2.3 million base pairs is the largest pathogenic gene in the human genome.
A moderately faithful animal mimic of human DMD — that muscle-bound mdx mouse — surfaced 18 years ago, in Scotland. It has served ever since as a living template for experimental treatments to abate or cure DMD, but so far with no therapeutic payoffs, Rudnicki told BioWorld Today. He is an associate professor of pathology and molecular medicine at McMaster University's Institute for Molecular Biology and Biotechnology, in Hamilton, Ontario.
While muscular dystrophy apparently limits its tissue attacks to skeletal muscle, its death sentence is most often executed by cardiac muscle.
"So, really, the heart now is a major problem for life span in DMD patients," Rudnicki observed. Seeking to explicate this skeletal-cardiac molecular mechanism, in 1992 he knocked out a different muscle protein, called myoD, from mouse genome.
"The transcription factor myoD," Rudnicki said, "is a master regulator of muscle development. And the role that it plays in muscle regeneration, as we showed in 1996, is what myoD does for satellite cells — the stem cells of adult skeletal muscle. In the absence of myoD, in response to trauma, satellite cells will undergo self-renewal, rather than proliferate, and then differentiate to repair the damage."
He said myoD is "strictly involved in skeletal muscle development and satellite function. It has nothing whatever to do with heart development."
Rudnicki interbred his myoD knockouts with the classic mdx dystrophin-minus mouse model. He reported the upshot of this mdx/myoD cross animal in the current Proceedings of the National Academy of Sciences (PNAS), dated Jan. 5, 1999. The paper's title is "Severe cardiomyopathy in mice lacking dystrophin and MyoD."
Unexpected Heart Problems Turn Up
"Lacking myoD, our combined knockout mice were unable efficiently to repair experimental muscle trauma," Rudnicki said. "And, with this dystrophic profile, they became very sick, and died after one year of age. We made the somewhat surprising observation that the mdx/myoD compound mutant mice had an accentuated cardiomyopathic phenotype. Their hearts showed markedly increased areas of scarring, necrosis, and cellular hypertrophy. We didn't see full-blown enlargement or dilated cardiopathology, such as occurs in Duchenne or Beckers [a milder form of DMD] muscular dystrophy patients. And it may be that the reason why [the mice] are dying is because of their cardiac problems."
Reiterating that "myoD is not involved in heart development whatsoever," Rudnicki suggested that "the simplest interpretation is that this heart failure is a result of increased peripheral resistance in blood flow. And what that means in human terms is that, when you lose fibers in skeletal muscle, and because you're more inactive, your heart has to work harder to move the blood through your body. In those mdx model mice, their heart is working harder as well; it's more prone to this sort of cardiac-muscle damage, analogous to what happens to their diaphragm with heavy breathing and their masseter [jaw] muscles, with vigorous chewing. So, if you extend this to humans in a very speculative way, the conclusion is just that amelioration of skeletal muscle dystrophy in patients would delay the onset of cardiac problems.
"Regeneration of heart tissue is biologically more difficult than renewal of skeletal muscle," Rudnicki said. "Skeletal muscle already has a built-in program for regeneration. It has satellite cells that can be activated to replace muscle fibers that are lost. The heart, however, doesn't appear to regenerate. It apparently doesn't have a population of stem cells. So, if one can devise therapies that improve the ability of muscles to regenerate, and reintroduce dystrophin using, let's say, gutted adenovirus vectors or stem cell therapies, then the cardiac myopathy problems should be delayed."
Delaying Tactics: Drugs Or Gene Therapy
"They will probably still occur eventually," Rudnicki went on, "as they do in Beckers patients, but they will not be as acute as they are now. Apparently, many DMD patients are using delaying therapies so they can make it into their 30s, but are still dying because of cardiac problems. This provides us a ray of hope that if one can ameliorate the skeletal muscle degeneration, that the heart problem will take care of itself. That would require therapy in the longer term, like perhaps gene therapy or small-molecule drugs. But I think it encouraging from the point of view that previously the basis of the cardiomyopathy really wasn't understood, so if you could cure the skeletal muscle of dystrophin patients, they would still die of cardiomyopathy, potentially.
"It's thought that both issues were independent of one another," he concluded, "but, if one can do something to attenuate skeletal muscle wastage, then perhaps the interpretation of our experiment is that the heart problem would be delayed." *
Targets Age-Related Macular Degeneration