Myotonic dystrophy: a short gene at best.

For years, neurologists have noticed a strange phenomenon: Patients afflicted by myotonic muscular dystrophy, a muscle-stiffening disorder, often have children with a more severe form of the disease. And their children's children, in turn, are usually affected even more severely, and at a younger age.

What causes this distressing genetic generation gap? Last week, three groups of researchers studying myotonic dystrophy came closer to answering that question in simultaneous findings that they say should allow for better screening tests for the inherited disease.

The three groups of geneticists found that people with myotonic dystrophy have extra bits of DNA at a specific spot on the long arm of chromosome 19. Moreover, they discovered that those with the worst symptoms have the most extra DNA.

In the Feb. 6 NATURE, the researchers speculate that the extra DNA pieces somehow disrupt an as-yet-unidentified gene, one possibly involved in controlling muscle tone. The three teams -- led by Duncan J. Shaw of the University of Wales College of Medicine in Cardiff, Keith Johnson of Charing Cross and Westminster Medical School in London and Pieter J. de Jong of the Lawrence Livermore (Calif.) National Laboratory -- are now attempting to isolate and characterize the gene.

Myotonic dustrophy, the most common form of muscular dustrophy affecting adults, strikes roughly one in every 8,000 persons worldwide. Symptoms of the disorder -- which usually emerges in adolescence or early adulthood -- include muscle spasms and wasting, particularly in the head and neck. While mildly affected people may simply have difficulty unclenching a fist, those with more severe forms of myotonic dustrophy cannot walk and have difficulty swallowing. The disorder's other symptoms include cataracts, premature balding, shrunken ovaries or testicles, and mental retardation.

To uncover the genetic defect, the three research groups used enzymes to chop DNA taken from myotonic dystrophy patients into tiny pieces. After sorting the pieces by size on a gel, all three groups found that myotonic patients had longer DNA pieces than did healthy individuals. They also found that the affected children of people with the disorder had even longer pieces tha their parents.

Myotonic dystrophy is only the third genetic disease that scientists have associated with longer-than-normal DNA segments. Last year, U.S. and Dutch researchers found that people with fragile X syndrome -- the most common inherited cause of mental retardation -- bear repetitive DNA segments in a gene they named FMR-1, for fragile X mental retardation-1 (SN: 6/8/91, p. 359). Such enlarged genetic segments have also been discovered in spinal-bulbar muscular atrophy, a rare inherited muscle-wasting syndrome.

Myotonic dystrophy may be an example of "genetic imprinting," in which the same gene produces a different effect, depending on which parent provides the gene (SN: 5/20/89, p. 312). Mothers with myotonic dustrophy tend to have babies severely affected by the disease from birth.

The new finding should "permit new approaches to understanding the molecular pathology [of myotonic dystrophy]," Johnson says. He adds that some medical laboratories can now use the same technique as his research team to tell whether the children of myotonic dystrophy patients will also suffer from the disease, and if so, how severely it may affect them.

The anticipated discovery of the gene disrupted in myotonic of the gene also lead to a treatment for the disorder, says Leon Charash, chairman of the medical advisory committee of the Muscular Dystrophy Association. He predicts that researchers will identify the gene underlying myotonic dystrophy "in the very, very near future . . . possibly within the next six months."