A California geneticist claims to have identified the gene in humans that could be a cause of mental depression, chronic alcoholism and even multiple sclerosis.

Writing in the upcoming issue of the prestigious British journal, Nature, Dr. David E. Comings of the City of Hope Medical Center in Duarte, Calif., said he has isolated and identified in people who died of suicide and alcoholism an abnormal gene that was more than twice as prevalent in their brain cells as it was in the brains of people who died of stroke, heart disease, cancer or accidents.

"Schizophrenia and depression are psychoses that each afflict one percent of the population," Comings said last night in a telephone interview from his home in Duarte, about 30 miles northeast of Low Angeles. "We claim to have found what appears to be the first biochemical evidence of one of those psychoses."

While the finding of that evidence is a basic medical ovservation, it is nowhere near a cure for depression or any of the other two diseases linked with the gane. But at the very least, it is a first major step toward improving the understanding and treatment of depressionn -- the leading cause of suicide.

Comings calls the newly discovered gene Pc Duarte, saying it is a mutant gene that is genetically different from any other gene found in the brain. He said he found the gene in the gray and white matter of the brain, in men and in women and in whites and blacks.

The California researcher identified the gene as a cytoseol protein, which he found only in the brain. He said this makes it impossible to test for in infants before they are born. Some other genetic defects can be found before birth by tapping the amniotic fluid in a mother's womb.

"We have only identified this gene in the brains of dead people," Comings said. "We don't know yet if we can devise a test to identify the gene in people still living. One would have to devise such a test to treat people for depression born of this gene."

Comings said he identified the abnormal gene after a fruitless search for the gene responsible for a rare neurologic disease known as Huntington Disease, a degenerative ailment of the central nervous system. Huntington Disease is always fatal. It is perhaps best known as the lisease that claimed the life of folk singer Wooldy Guthrie.

Separating genes from the brains of dead victims of Huntington Disease, Comings said he found no signs of an abnormal gene that behaved like it might be a cause of the rare brain disorder. But in looking at the brains of stroke and heart attack victims to be matched against those dead of Huntington Disease, Comings discovered the mutant gene he called Pc 1 Duarte in about 30 percent of the victims of stroke and heart attack and more than 50 percent of the victims of multiple sclerosis.

"We looked for this gene in schizo-phrenics and couldn't find it," Comings said. "Then a doctor in St. Louis donated to us 28 brains of people who had a medical record of depression and who'd committed suicide. We found the gene in 65 percent of the brains."

Just as important, Comings found a double dose of the abnormal gene in 20 percent of the St. Louis suicides, meaning they had received the gene from both their mothers and fathers. Only 2 1/2 percent of the stroke and heart attack victims showed evidence of a double dose of the brain gene.

"This told us that if you had the mutant gene," Comings said, "your risk of being depressive runs much greater than if you don't have it."

Comings conceded his finding is not conclusive evidence that the gene causes social depression but said it is almoslt surely involved. "Either this gene is the only gene involved," he said, "or it is one of the genes involved."

Later work with the brains of dead alcoholics showed almost the same frequency of the abnormal gene as it existed in depressions, which Comings said strongly suggests that the gene is a cause of chronic alcholism.

"The preliminary evidence suggesting a relationship with chronic alcoholism could finally explain some of the genetic aspects of this disorder," Comings wrote in Nature, "and its association with affective illness."

Far more puzzling is the connection between the gene and multiple sclerosis, said Comings: "We don't pretend to understand the connection."

Still to be identified is the environmental "trigger" that acts together with the gene to produce depression. Since 30 percent of "normal" people possess the gene, Comings said, there must be another mechanism such as a virus, that allows the gene to infect the brain and produce the disease