[Paleopsych] Scientific American: Gene Doping

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Gene Doping
    June 21, 2004

    Gene therapy for restoring muscle lost to age or disease is poised to
    enter the clinic, but elite athletes are eyeing it to enhance
    performance Can it be long before gene doping changes the nature of

    By H. Lee Sweeney

    Athletes will be going to Athens next month to take part in a
    tradition begun in Greece more than 2,000 years ago. As the world's
    finest specimens of fitness test the extreme limits of human strength,
    speed and agility, some of them will probably also engage in a more
    recent, less inspiring Olympic tradition: using performance-enhancing
    drugs. Despite repeated scandals, doping has become irresistible to
    many athletes, if only to keep pace with competitors who are doing it.
    Where winning is paramount, athletes will seize any opportunity to
    gain an extra few split seconds of speed or a small boost in

    Sports authorities fear that a new form of doping will be undetectable
    and thus much less preventable. Treatments that regenerate muscle,
    increase its strength, and protect it from degradation will soon be
    entering human clinical trials for muscle-wasting disorders. Among
    these are therapies that give patients a synthetic gene, which can
    last for years, producing high amounts of naturally occurring
    muscle-building chemicals.

    This kind of gene therapy could transform the lives of the elderly and
    people with muscular dystrophy. Unfortunately, it is also a dream come
    true for an athlete bent on doping. The chemicals are
    indistinguishable from their natural counterparts and are only
    generated locally in the muscle tissue. Nothing enters the
    bloodstream, so officials will have nothing to detect in a blood or
    urine test. The World Anti-Doping Agency (WADA) has already asked
    scientists to help find ways to prevent gene therapy from becoming the
    newest means of doping. But as these treatments enter clinical trials
    and, eventually, widespread use, preventing athletes from gaining
    access to them could become impossible.

    Raising IGF-I allows us to break the connection between muscle use and
    its size.

    Is gene therapy going to form the basis of high-tech cheating in
    athletics? It is certainly possible. Will there be a time when gene
    therapy becomes so commonplace for disease that manipulating genes to
    enhance performance will become universally accepted? Perhaps. Either
    way, the world may be about to watch one of its last Olympic Games
    without genetically enhanced athletes.

    Loss Leads to Gain
    Research toward genetically enhancing muscle size and strength did not
    start out to serve the elite athlete. My own work began with observing
    members of my family, many of whom lived well into their 80s and 90s.
    Although they enjoyed generally good health, their quality of life
    suffered because of the weakness associated with aging. Both muscle
    strength and mass can decrease by as much as a third between the ages
    of 30 and 80.

    There are actually three types of muscle in the body: smooth muscle,
    lining internal cavities such as the digestive tract; cardiac muscle
    in the heart; and skeletal muscle, the type most of us think of when
    we think of muscle. Skeletal muscle constitutes the largest organ of
    the body, and it is this type--particularly the strongest so-called
    fast fibers--that declines with age. With this loss of strength,
    losing one's balance is more likely and catching oneself before
    falling becomes more difficult. Once a fall causes a hip fracture or
    other serious injury, mobility is gone completely.

    With gene therapy poised to become a viable medical treatment, gene
    doping cannot be far behind.

    Skeletal muscle loss occurs with age in all mammals and probably
    results from a cumulative failure to repair damage caused by normal
    use. Intriguingly, aging-related changes in skeletal muscle resemble
    the functional and physical changes seen in a suite of diseases
    collectively known as muscular dystrophy, albeit at a much slower

    In the most common and most severe version of MD--Duchenne muscular
    dystrophy--an inherited gene mutation results in the absence of a
    protein called dystrophin that protects muscle fibers from injury by
    the force they exert during regular movement. Muscles are good at
    repairing themselves, although their normal regenerative mechanisms
    cannot keep up with the excessive rate of damage in MD. In aging
    muscles the rate of damage may be normal, but the repair mechanisms
    become less responsive. As a result, in both aging and Duchenne MD,
    muscle fibers die and are replaced by infiltrating fibrous tissue and

    In contrast, the severe skeletal muscle loss experienced by astronauts
    in microgravity and by patients immobilized by disability appears to
    be caused by a total shutdown of muscles' repair and growth mechanism
    at the same time apoptosis, or programmed cell death, speeds up. This
    phenomenon, known as disuse atrophy, is still not fully understood but
    makes sense from an evolutionary perspective. Skeletal muscle is
    metabolically expensive to maintain, so keeping a tight relation
    between muscle size and its activity saves energy. Skeletal muscle is
    exquisitely tuned to changing functional demands. Just as it withers
    with disuse, it grows in size, or hypertrophies, in response to
    repeated exertions. The increased load triggers a number of signaling
    pathways that lead to the addition of new cellular components within
    individual muscle fibers, changes in fiber type and, in extreme
    conditions, addition of new muscle fibers.

    To be able to influence muscle growth, scientists are piecing together
    the molecular details of how muscle is naturally built and lost.
    Unlike the typical cell whose membrane contains liquid cytoplasm and a
    single nucleus, muscle cells are actually long cylinders, with
    multiple nuclei, and cytoplasm consisting of still more long tiny
    fibers called myofibrils. These myofibrils, in turn, are made of
    stacks of contractile units called sarcomeres. Collectively, their
    shortening produces muscle contractions, but the force they generate
    can damage the muscle fiber unless it is channeled outward.
    Dystrophin, the protein missing in Duchenne muscular dystrophy
    patients, conducts this energy across the muscle cell's membrane,
    protecting the fiber.

    Yet even with dystrophin's buffering, muscle fibers are still injured
    by normal use. In fact, that is believed to be one way that exercise
    builds muscle mass and strength. microscopic tears in the fibers
    caused by the exertion set off a chemical alarm that triggers tissue
    regeneration, which in muscle does not mean production of new muscle
    fibers but rather repairing the outer membrane of existing fibers and
    plumping their interior with new myofibrils. Manufacturing this new
    protein requires activation of the relevant genes within the muscle
    cell's nuclei, and when the demand for myofibrils is great, additional
    nuclei are needed to bolster the muscle cell's manufacturing capacity.

    Local satellite cells residing outside the muscle fibers answer this
    call. First these muscle-specific stem cells proliferate by normal
    cell division, then some of their progeny fuse with the muscle fiber,
    contributing their nuclei to the cell. Both progrowth and antigrowth
    factors are involved in regulating this process. Satellite cells
    respond to insulinlike growth factor I, or IGF-I, by undergoing a
    greater number of cell divisions, whereas a different
    growth-regulating factor, myostatin, inhibits their proliferation.

    With these mechanisms in mind, about seven years ago my group at the
    University of Pennsylvania, in collaboration with Nadia Rosenthal and
    her colleagues at Harvard University, began to assess the possibility
    of using IGF-I to alter muscle function. We knew that if we injected
    the IGF-I protein alone, it would dissipate within hours. But once a
    gene enters a cell, it should keep functioning for the life of that
    cell, and muscle fibers are very long-lived. A single dose of the
    IGF-I gene in elderly humans would probably last for the rest of their
    lives. So we turned our attention to finding a way to deliver the
    IGF-I gene directly to muscle tissue.

    Donning New Genes
    Then as now, a major obstacle to successful gene therapy was the
    difficulty of getting a chosen gene into the desired tissue. Like many
    other researchers, we selected a virus as our delivery vehicle, or
    vector, because viruses are skilled at smuggling genes into cells.
    They survive and propagate by tricking the cells of a host organism
    into bringing the virus inside, rather like a biological Trojan horse.
    Once within the nucleus of a host cell, the virus uses the cellular
    machinery to replicate its genes and produce proteins. Gene therapists
    capitalize on this ability by loading a synthetic gene into the virus
    and removing any genes the virus could use to cause disease or to
    replicate itself. We selected a tiny virus called adeno-associated
    virus (AAV) as our vector, in part because it infects human muscle
    readily but does not cause any known disease.

    We modified it with a synthetic gene that would produce IGF-I only in
    skeletal muscle and began by trying it out in normal mice. After
    injecting this AAV-IGF-I combination into young mice, we saw that the
    muscles' overall size and the rate at which they grew were 15 to 30
    percent greater than normal, even though the mice were sedentary.
    Further, when we injected the gene into the muscles of middle-aged
    mice and then allowed them to reach old age, their muscles did not get
    any weaker.

    To further evaluate this approach and its safety, Rosenthal created
    mice genetically engineered to overproduce IGF-I throughout their
    skeletal muscle. Encouragingly, they developed normally except for
    having skeletal muscles that ranged from 20 to 50 percent larger than
    those of regular mice. As these transgenic mice aged, their muscles
    retained a regenerative capacity typical of younger animals. Equally
    important, their IGF-I levels were elevated only in the muscles, not
    in the bloodstream, an important distinction because high circulating
    levels of IGF-I can cause cardiac problems and increase cancer risk.
    Subsequent experiments showed that IGF-I overproduction hastens muscle
    repair, even in mice with a severe form of muscular dystrophy.

    Raising local IGF-I production allows us to achieve a central goal of
    gene therapy to combat muscle-wasting diseases: breaking the close
    connection between muscle use and its size. Simulating the results of
    muscle exercise in this manner also has obvious appeal to the elite
    athlete. Indeed, the rate of muscle growth in young sedentary animals
    suggested that this treatment could also be used to genetically
    enhance performance of healthy muscle. Recently my laboratory worked
    with an exercise physiology group headed by Roger P. Farrar of the
    University of Texas at Austin to test this theory.

    We injected AAV-IGF-I into the muscle in just one leg of each of our
    lab rats and then subjected the animals to an eight-week
    weight-training protocol. At the end of the training, the
    AAV-IGF-I-injected muscles had gained nearly twice as much strength as
    the uninjected legs in the same animals. After training stopped, the
    injected muscles lost strength much more slowly than the unenhanced
    muscle. Even in sedentary rats, AAV-IGF-I provided a 15 percent
    strength increase, similar to what we saw in the earlier mouse

    We plan to continue our studies of IGF-I gene therapy in dogs because
    the golden retriever breed is susceptible to a particularly severe
    form of muscular dystrophy. We will also do parallel studies in
    healthy dogs to further test the effects and safety of inducing IGF-I
    overproduction. It is a potent growth and signaling factor, to which
    tumors also respond.

    Safety concerns as well as unresolved questions about whether it is
    better to deliver AAV in humans through the bloodstream or by direct
    injection into muscle mean that approved gene therapy treatments using
    AAV-IGF-I could be as much as a decade away. In the shorter term,
    human trials of gene transfer to replace the dystrophin gene are
    already in planning stages, and the Muscular Dystrophy Association
    will soon begin a clinical trial of IGF-I injections to treat myotonic
    dystrophy, a condition that causes prolonged muscle contraction and,
    hence, damage.

    A still more immediate approach to driving muscle hypertrophy may come
    from drugs designed to block myostatin. Precisely how myostatin
    inhibition builds muscle is still unclear, but myostatin seems to
    limit muscle growth throughout embryonic development and adult life.
    It acts as a brake on normal muscle growth and possibly as a promoter
    of atrophy when functional demands on muscle decrease. Experiments on
    genetically engineered mice indicate that the absence of this
    antigrowth factor results in considerably larger muscles because of
    both muscle fiber hypertrophy and hyperplasia, an excessive number of
    muscle fibers.

    Making Muscle and More
    Pharmaceutical and biotechnology companies are working on a variety of
    myostatin inhibitors. Initially, the possibility of producing meatier
    food animals piqued commercial interest. Nature has already provided
    examples of the effects of myostatin blockade in the Belgian Blue and
    Piedmontese cattle breeds, both of which have an inherited genetic
    mutation that produces a truncated, ineffective version of myostatin.
    These cattle are often called double-muscled, and their exaggerated
    musculature is all the more impressive because an absence of myostatin
    also interferes with fat deposition, giving the animals a lean,
    sculpted appearance.

    The first myostatin-blocking drugs to have been developed are
    antibodies against myostatin, one of which may soon undergo clinical
    testing in muscular dystrophy patients. A different approach mimics
    the cattle mutation by creating a smaller version of myostatin, which
    lacks the normal molecule's signaling ability while retaining the
    structures that dock near satellite cells. This smaller protein, or
    peptide, essentially caps those docking locations, preventing
    myostatin from attaching to them. Injecting the peptide into mice
    produces skeletal muscle hypertrophy, and my colleagues and I will be
    attempting to create the same effect in our dog models by transferring
    a synthetic gene for the peptide.

    Myostatin-blocking therapies also have obvious appeal to healthy
    people seeking rapid muscle growth. Although systemic drugs cannot
    target specific muscles, as gene transfer can, drugs have the benefit
    of easy delivery, and they can immediately be discontinued if a
    problem arises. On the other hand, such drugs would be relatively easy
    for sport regulatory agencies to detect with a blood test.

    But what if athletes were to use a gene therapy approach similar to
    our AAV-IGF-I strategy? The product of the gene would be found just in
    the muscle, not in the blood or urine, and would be identical to its
    natural counterpart. Only a muscle biopsy could test for the presence
    of a particular synthetic gene or of a vector. But in the case of AAV,
    many people may be naturally infected with this harmless virus, so the
    test would not be conclusive for doping. Moreover, because most
    athletes would be unwilling to undergo an invasive biopsy before a
    competition, this type of genetic enhancement would remain virtually

    And what of the safety of rapidly increasing muscle mass by 20 to 40
    percent? Could an athlete sporting genetically inflated musculature
    exert enough force to snap his or her own bones or tendons? Probably
    not. We worry more about building muscle in elderly patients with
    bones weakened by osteoporosis. In a healthy young person, muscle
    growth occurring over weeks or months would give supporting skeletal
    elements time to grow to meet their new demands.

    This safety question, however, is just one of the many that need
    further study in animals before these treatments can even be
    considered for mere enhancement of healthy humans. Nevertheless, with
    gene therapy poised to finally become a viable medical treatment, gene
    doping cannot be far behind, and overall muscle enlargement is but one
    way that it could be used. In sports such as sprinting, tweaking genes
    to convert muscle fibers to the fast type might also be desirable. For
    a marathoner, boosting endurance might be paramount.

    Muscle is most likely to be the first tissue subject to genetic
    enhancement, but others could eventually follow. For example,
    endurance is also affected by the amount of oxygen reaching muscles.
    Erythropoietin is a naturally occurring protein that spurs development
    of oxygen-carrying red blood cells. Its synthetic form, a drug called
    Epoietin, or simply EPO, was developed to treat anemia but has been
    widely abused by athletes--most publicly by cyclists in the 1998 Tour
    de France. An entire team was excluded from that race when their EPO
    use was uncovered, yet EPO abuse in sports continues.

    Gene transfer to raise erythropoietin production has already been
    tried in animals, with results that illustrate the potential dangers
    of prematurely attempting such enhancements in humans. In 1997 and
    1998 scientists tried transferring synthetic erythropoietin genes into
    monkeys and baboons. In both experiments, the animals' red blood cell
    counts nearly doubled within 10 weeks, producing blood so thick that
    it had to be regularly diluted to keep their hearts from failing.

    The technology necessary to abuse gene transfer is certainly not yet
    within reach of the average athlete. Still, officials in the athletic
    community fear that just as technically skilled individuals have
    turned to the manufacture and sale of so-called designer steroids,
    someday soon a market in genetic enhancement may emerge. Policing such
    abuse will be much harder than monitoring drug use, because detection
    will be difficult.

    It is also likely, however, that in the decades to come, some of these
    gene therapies will be proved safe and will become available to the
    general population. If the time does arrive when genetic enhancement
    is widely used to improve quality of life, society's ethical stance on
    manipulating our genes will probably be much different than it is
    today. Sports authorities already acknowledge that muscle-regenerating
    therapies may be useful in helping athletes to recover from injuries.

    So will we one day be engineering superathletes or simply bettering
    the health of the entire population with gene transfer? Even in its
    infancy, this technology clearly has tremendous potential to change
    both sports and our society. The ethical issues surrounding genetic
    enhancement are many and complex. But for once, we have time to
    discuss and debate them before the ability to use this power is upon

    H. LEE SWEENEY is professor and chairman of physiology at the
    University of Pennsylvania School of Medicine. He is a member of the
    Board of Scientific Councilors for the National Institute of Arthritis
    and Musculoskeletal Diseases, scientific director for Parent Project
    Muscular Dystrophy, and a member of the Muscular Dystrophy
    Association's Translational Research Advisory Council. His research
    ranges from basic investigation of structures that allow cells to move
    and generate force, particularly the myosin family of molecular
    motors, to translating insights about muscle cell design and behavior
    into gene therapy interventions for diseases, including Duchenne
    muscular dystrophy. He took part in a 2002 symposium on the prospect
    of gene doping organized by the World Anti-Doping Agency.

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