Genetics: Of Muscles & Enzymes

The Bar Harbor elementary school, overlooking summer vacationers looking over the rockbound coast of Maine, seems an unlikely place to originate major medical news about some of the most baffling and intractable diseases of man. But last week the school was the classroom for a course in hereditary disease sponsored by the National Foundation-March of Dimes and attended by 100 research specialists from most of the top U.S. medical schools and research institutions. The results were highlighted by two significant reports: on dwarfism (see following story), and on the possible prevention of muscular dystrophy.

Muscular dystrophy is not a single disease but a group of hereditary disorders in which muscle fibers are damaged and eventually destroyed. At Bar Harbor last week an English exchange researcher at the Johns Hopkins University School of Medicine, Dr. Alan E. H. Emery, listed eight forms of the disease. Six are extremely rare. The other two are called the "Duchenne" type.* One of these, a less serious form, often affects adults. The other is the most common of all the dystrophies; it is also the most deadly. With rare exceptions it occurs only in boys, attacking them by age three or four and usually killing them before age 20. This type is transmitted by a defective gene on the female, or X, chromosome. There is no cure, and none in sight, for dystrophy of this type. But in many of these cases, Dr. Emery saw hope of prediction and of prevention through genetic counseling.

Carrier Mothers. Since boy victims of Duchenne dystrophy are severely crippled in their teens and dead at an early age, they do not reproduce. It is women who carry the curse, much as they carry that of hemophilia, without clearly falling victim to it themselves. Heredity decrees that half of a woman carrier's sons will be victims of the disease and half of her daughters will be carriers.

In its male victims, muscular dystrophy has long been known to cause a widespread upset in body chemistry, mainly in the enzyme system. In normal men and women, for example, the normal level of an enzyme called creatine kinase is up to 1.5 units per liter of blood. Early in life a boy with Duchenne dystrophy may have astronomically high levels, sometimes up to 1,000 units. Dr. Emery and his fellow workers at the Hopkins decided to check the creatine kinase level in mothers of normal boys, mothers of a single dystrophic boy (who might have produced a nonrecurring defective ovum and who might not be carriers) and mothers of two or more dystrophic boys who almost certainly would be carriers.