A team of scientists has located one or more genes that could contribute to "susceptibility" to the severe depression afflicting as much as 2 per cent of the population.

University of Rochester researcher Dr. Lowell R. Weitkamp said the finding is the "strongest evidence to date" in a long search to pinpoint a genetic basis for this major form of mental illness.

Weitkamp and Canadian colleagues at the University of Toronto believe that the finding will contribute not only to improved understanding of depression but also could provide a marker to predict who is likely to incur the disease.

Weitkamp said that the research should be "confirmed and extended" before clinical use is possible but that he hopes it could be "applied more broadly" within "a couple of years."

Since environmental and genetic factors are thought to contribute to depression, he emphasized that the finding would help identify those who are most susceptible to the disease but may not necessarily get it.

Weitkamp said the research also may help determine the genetic relationship of depressive illness to other disorders, such as alcoholism, that are found with "increased frequency" in relatives of patients with depression.

The study is published today in the New England Journal of Medicine.

An editorial in the same issue said the study represented "major progress in understanding the genetics of depressive disorders" and a practical "possibility" of early detection of those most susceptible to such illness. "This finding will certainly elevate the mood of researchers who study depression," it concluded.

Depression includes a range of mood disorders affecting physical and mental well-being, and its victims are more likely to commit suicide than those who do not suffer from the disease. Some patients experience a form of the illness with wide mood swings known as manic-depression.

Doctors have long suspected a genetic basis for depression because it often runs in families. In twins, it more commonly strikes both members of identical pairs rather than fraternal or non-identical twins.

The study reports that one or more genes at a specific point on the sixth of the 23 chromosome pairs in humans may be associated with depression. This is on or near a region that codes for so-called HLA, or human leukocyte antigen, a protein found on the surface of most cells and involved in the body's immune mechanisms.

There are dozens of types of HLA, which vary in individuals and can be detected through blood tests. HLA changes also may contribute to susceptibility to other diseases, Weitkamp said, but the link between HLA and depression is not yet understood. The researchers suggested that immune defects may be involved.

In the new study, parents and children in 20 families with diagnosed cases of depression were analyzed by HLA types. The study showed that the disease has an inherited basis linked with the HLA gene region.

Weitkamp noted that some earlier studies also suggested such a link but that others had failed to find one. Earlier studies also have suggested that depression may be linked with the "X" sex chromosome.

In a widely publicized study that originated in Nature Magazine in 1979, a California researcher claimed to have identified a brain protein as evidence of a gene for depression. Weitkamp said that research did not precisely locate the gene but that this protein may be involved.