Three sets of laboratory experiments investigating the effects of interfering with myostatin, a protein that limits muscle growth, have shown that this approach may have to be individualized with respect to different types and stages of muscular dystrophy, and that some myostatin suppression strategies may be better than others.
The findings come on the heels of the announcement by Wyeth Pharmaceuticals (Madison, N.J.) earlier this month that it will not continue development of MYO-029, an antibody (immune-system protein) that blocks myostatin, for muscular dystrophy. (See “Wyeth won’t continue.”)
In the March issue of Muscle & Nerve, Tejvir Khurana at the University of Pennsylvania, with MDA-supported Sasha Bogdanovich at that institution and Elizabeth McNally at the University of Chicago, announced that blocking myostatin with a Wyeth-supplied myostatin antibody in mice with type 2C limb-girdle muscular dystrophy (LGMD2C) improved some aspects of muscle health but failed to improve others. (Khurana and McNally have MDA research grants but were not specifically funded for this work.)
The investigators say they observed an “uncoupling” of effects on muscle physiology and effects on muscle-fiber appearance and structure in these LGMD2C-affected mice, which, like humans with this disease, lack the muscle protein gamma-sarcoglycan.
The treated mice received intraperitoneal (abdominal) injections of mouse myostatin antibodies weekly for three months starting at the age of 4 weeks. Untreated mice were injected with saline (salt solution).
The mice treated with the myostatin antibodies showed increases in muscle bulk, body weight and muscle-fiber size, as well as improvement in their ability to stay on a rotating rod. However, their muscle tissue looked the same as it did in the untreated mice, and there was no reduction in the level of serum creatine kinase, an enzyme that leaks out of damaged fibers.
The average number of fibers in a leg muscle remained relatively constant, leading the researchers to conclude that any increase in muscle bulk was due to enlargement of individual fibers rather than generation of new fibers.
They note that previous studies have shown more benefit from myostatin blocking in dystrophin-deficient mice with Duchenne muscular dystrophy (DMD) and in mice with early-stage type 2F limb-girdle MD (LGMD2F) resulting from a deficiency of delta-sarcoglycan, than it has in mice with late-stage LGMD2F or merosin-deficient congenital MD.
They say that, because of differences in study designs, it isn’t possible to make direct comparisons of these results, but that it’s likely the benefits of myostatin blocking are limited by the age at which treatment is started and the natural history and severity of the disease being treated.
In a different set of experiments, reported online March 11 in Proceedings of the National Academy of Sciences, Brian Kaspar at Nationwide Children’s Hospital Research Institute in Columbus, Ohio, and colleagues describe the benefits in DMD-affected mice of a gene-therapy approach to inhibition of myostatin. (MDA is supporting Kaspar and Jerry Mendell, also on this study team and also at Nationwide, for other types of muscle-directed gene therapy.)
First, these investigators injected genes for the protein follistatin inside an adeno-associated viral shell into upper and lower leg muscles in 3-week old mice with DMD. Follistatin is known to inhibit myostatin activity. The mice, divided into high-dose and low-dose treatment groups and an untreated (control) group, were then observed for five months.
The mice treated with follistatin genes developed larger bodies and larger, heavier muscles, with the high-dose group showing the greatest effects. Follistatin was detected in the bloodstream of low- and high-dose-treated mice, and it affected muscles far from the injection sites.
The investigators observed an increase in the size of muscle fibers in mice receiving the gene therapy but not in fiber numbers.
Both groups of treated mice showed reduced levels of creatine kinase, indicating less leakiness of muscle-fiber membranes compared to control mice. The researchers speculate that the treated fibers became less susceptible to damage.
The investigators then injected 7-month-old DMD-affected mice with follistatin genes in viral shells. These older mice showed increases in strength about two months after the injections, which persisted for the more than 18 months during which the mice were evaluated.
At the end of the study, the treated mice had substantially fewer groups of dead muscle fibers, fewer inflammatory cells in their muscles, and less scar tissue than did untreated mice, and their muscle fibers were larger in diameter than those of the control group.
The improvements were sustained and well tolerated over more than two years.
The investigators note that the follistatin gene transfer was beneficial in aged dystrophin-deficient mice even after they had undergone multiple rounds of muscle degeneration and regeneration, implying that this type of therapy could have potential for treating older DMD patients.
They note that follistatin not only suppresses myostatin but also affects various cell signaling pathways and reduces inflammation. They conclude that “the striking ability of follistatin to provide gross and functional long-term improvement to dystrophic muscles in aged animals warrants its consideration for clinical develoment to treat musculoskeletal diseases, including older DMD patients.”
New results from the University of Michigan reveal a previously unrecognized downside of myostatin blocking.
John Faulkner and colleagues, who published their results Jan. 8 in Proceedings of the National Academy of Sciences, have found that mice bred to lack myostatin from birth have tendons that are 14 times stiffer than tendons in mice that produce myostatin.
Tendons attach muscles to bone, and their flexibility plays a role in protecting muscle fibers from contraction-associated injuries. Muscle fibers in boys with DMD are particularly susceptible to this type of injury.
It isn’t yet known whether myostatin blocking has the same effect on human tendons as it does on mouse tendons, or whether blocking myostatin months to years after birth would be different from stopping its production before birth. However, the findings are a caveat about strategies to block myostatin as a treatment for muscular dystrophy.