* Two separate studies find same genetic link

* Scientists hope findings will lead to better treatments

* Depression a common and costly disorder worldwide

LONDON- Scientists say they have discovered the first solid evidence that variations in some peoples' genes may cause depression -- one of the world's most common and costly mental illnesses.

And in a rare occurrence in genetic research, the findings by a British-led international team have been replicated at the same time by another group from Washington University who were studying an entirely separate group of people.

The researchers said they hoped the findings would bring scientists closer to developing more effective treatments for patients with depression, since currently available medicines for depression only work in around half of patients.

"These findings ... will help us track down specific genes that are altered in people with this disease," said Gerome Breen of King's College London's Institute of Psychiatry, who led one of the studies.

He added, however, that any new drugs developed from these findings would be unlikely to be ready for treating patients for another 10 to 15 years.

The first study analysed more 800 families with recurrent depression, while the second examined depression and heavy smoking in a series of families from Australia and Finland.

Both studies were published in the American Journal of Psychiatry on Monday and both teams reported a strong link between depression and genetic variations in a region called chromosome 3p25-26.

"What is remarkable is that two different data sets, gathered for different purposes and studied in different ways found exactly the same region," said Breen, who gave a briefing in London about the work. "Normally in genetic studies of depression, replication of findings is very difficult and frequently takes years to emerge, if ever.

Major depression affects around 20 percent of people at some point in their lives. Severe and recurring depression affects up to 4 percent of people and is notoriously hard to treat.

The World Health Organisation has forecast that depression will rival heart disease as the health disorder with the highest disease burden in the world by 2020.

According to a 2006 study, depression is responsible for 100 million lost working days a year in England and Wales alone at a cost of 9 billion pounds ($14.6 billion).

Studies of families with depression have indicated that the disorder has a genetic link and scientists think around 40 percent of the risk of developing it is contributed by genes, with the rest down to environmental and other external factors.

"We are just beginning to make our way through the maze of influences on depression and this is an important step toward understanding what may be happening at the genetic and molecular levels," Michele Pergadia, who worked on Washington University study, said in a statement about the findings.

Breen's team is now conducting detailed gene sequencing studies in 40 of the families involved in the first study to try to find specific genes and variations that show a link.