Cause Behind Rare Form of Epilepsy Is Discovered : Medicine: It is the first time that a biological and genetic mechanism for such a brain disorder has been worked out in detail.

A genetic mutation in small, energy-producing bodies called mitochondria is the cause of a rare form of epilepsy called myoclonic epilepsy, a Georgia scientist has found.

The discovery marks the first time that a biological and genetic mechanism for such a brain disorder has been worked out in detail, geneticist Douglas Wallace of Emory University said in a telephone interview Friday.

This discovery “is very exciting . . . because it opens the door to the possibility that there are other mutational events that cause other forms of epilepsy,” said geneticist Carl Merrill of the National Institute of Mental Health.

Added neurologist Salvatore DiMauro of Columbia-Presbyterian Medical Center in New York City: “The real impact (of the discovery) has to do with seizures. For the first time, we get a genetic, biochemical clue to one form of seizures. It may open the door to understanding the mechanism of other seizures.”

The incidence of myoclonic epilepsy in the general population is not clear, in part because the disorder has a broad spectrum of symptoms. In the least-severe cases, the only effects may be mild aberrations in brain electrical patterns. The most severe forms are characterized by uncontrolled jerking, hearing loss, muscle disease, mental impairment and heart problems. The severity generally increases with age.

In most other types of epilepsy, the primary symptoms are seizures.

Wallace is an expert on mitochondria, which provide the energy that powers cells throughout the body. The mitochondria are unusual in that they are the only location other than the nuclei of cells where genetic information is stored. They contain the genes that serve as a blueprint for 13 proteins that are involved in the production of energy.

These genes are passed directly from mothers to children without any genetic input by fathers, and are thus changed only by random mutations. If a mother has a genetic defect in the mitochondria, each of her children will therefore inherit it.

By studying variations in mitochondria genes, Wallace and others have shown that all living people are descended from one woman who lived in Africa 100,000 to 200,000 years ago and that 95% of American Indians are descended from a small band of pioneers who crossed the Bering Strait from Asia more than 15,000 years ago. (Please see adjacent story.)

Wallace studied individuals with myoclonic epilepsy in three families and found, in each case, that the afflicted individuals had a specific defect in the production of proteins by the mitochondria. Because of that defect, the mitochondria were not able to produce as much energy as normal, and the function of the cells was impaired. Cells that require a lot of energy, such as brain cells, are more severely impaired than those that require less, such as skin cells.

“I liken this to a metropolitan brown-out,” Wallace said. “If part of the grid of power stations is down, the lights may go dim in various parts of the city,” while some systems that require large amounts of energy, such as computers, may be shut down completely.

Wallace’s new results, announced this week at a meeting at the Jackson Laboratories in Bar Harbor, Me., represent the second genetic defect discovered in mitochondria. He had previously found that an eye disease called Leber’s hereditary optic neuropathy is caused by a mitochondrial defect.

The new discovery “raises the question of whether mitochondria shouldn’t be looked at more carefully,” Merrill said.