Under the Microscope/Prof William Reville: Most people had eagerly awaited the start of the Olympic Games. However, in recent times the pleasure we can take from watching top athletes has been significantly diminished by revelations of widespread use of performance-enhancing drugs in sport.
The latest opportunity for athletes to cheat comes in the form of genetic manipulation, a practice described popularly as gene doping. It is not known if gene doping is already in practice, but if not, it is pretty certain that it soon will be used. The science behind gene doping is described by H.L. Sweeney in Scientific American, July 2004.
We have three types of muscles - skeletal, cardiac, and smooth muscle. Cardiac muscle is found in the heart and smooth muscle powers involuntary movements in various parts of the body, e.g. the gut. Skeletal muscle is attached to our skeleton and allows us to move around and to maintain body posture. Individual muscles are elongated organs attached through tendons at either end to bone. Muscles can actively shorten and pull on the bones thereby causing movement. In terms of mass, skeletal muscle is our biggest tissue, comprising over one-third of adult body weight.
Like all tissues, muscle is composed of cells. Each cell is a long cylindrical structure and can stretch the entire length of the muscle. The muscle cell is filled with long thin myofibrils each about one millionth of a metre wide, that run the length of the cell. The myofibril is composed of overlapping arrays of protein filaments arranged in units called sarcomeres lined up in series along the length of the myofibril. Sarcomeres can shorten when the filaments slide over each other, thereby causing the myofibril to contract and the cell to shorten. This causes the muscle to shorten and to effect skeletal movement. The muscle reverts to rest length when the filaments slide in the opposite direction.
Muscle contraction damages the cells a little. This damage must be repaired and the cell has mechanisms to effect this. When a muscle is exercised intensely it responds by growing bigger as the repair mechanism lays down extra myofibrils in the cells. Conversely, an under-used muscle gets smaller through the loss of some myofibrils from the cells.
In various muscle wasting diseases (dystrophies), repair mechanisms are deficient and contraction damage is not repaired. Consequently, many muscle cells die and are replaced by fatty fibrous tissue. A similar process occurs in muscles of the elderly. The most serious human dystrophy is Duchenne muscular dystrophy, carried by females but expressed only in males and usually fatal by the late teens.
Muscle tissue also contains stem cells. These are undifferentiated cells - they are not long cylinders filled with myofibrils like your typical muscle cell. When there is heavy demand for muscle growth, the muscle calls on the stem cells to help. Myofibrils are made from proteins and instructions for the construction of proteins reside in genes in the nucleus of the cell. When the muscle cell is very actively increasing in size it needs extra nuclei to direct the synthesis of the many new proteins required to make new myofibrils. In these circumstances, the stem cells are activated. They fuse with the muscle cells and donate their nuclei to the muscle cells. Stem cells are naturally activated by a protein growth factor called IGF-1. Another growth factor called myostatin inhibits stem cell activation, and so, the balance between IGF-1 and myostatin determines the net result.
Artificial manipulations to increase activity of IGF-1 or inhibit activity of myostatin would augment muscle size and strength. Introduction of the gene for IGF-1 into skeletal muscle enhances activity of the growth factor, increasing muscle size and strength. This approach has been used successfully by Sweeney and his group in experimental mice, using a tiny virus called adeno-associated virus (AAV) to carry the IGF-1 gene into the muscle where it is incorporated into the cell's nuclear genetic material. Injecting this AAV IGF-1 combination into young mice increased muscle size and rate of growth by 30 per cent greater than normal. When the same procedure was carried out with middle-aged mice, the muscles did not get any weaker with subsequent ageing.
Further studies have shown that enhanced IGF-1 production speeds up muscle repair even in mice with severe muscular dystrophy. Also, the increased IGF levels are evident only in muscle, not in blood, which is important because high circulating levels of IGF-1 can cause heart problems and increase cancer risk. This type of gene therapy has obvious potential for combating muscular dystrophies and for minimising debilitating loss of muscle in the elderly. It can also enhance athletic performance. Using the above proach, it will be possible to introduce genes into muscle to make it bigger or faster or more enduring, and many of these interventions will not alter blood chemistry and therefore will be undetectable using blood or urine tests.
Muscle gene therapy will undoubtedly be developed for medical purposes. Once the techniques are reliably developed there will be great temptation to use them to enhance athletic performance. This issue bristles with ethical implications but there is time to adequately debate them before the technology comes on stream. William Reville is associate professor of
biochemistry and director of microscopy at University College Cork