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

And in a rare occurrence in genetic research, a British-led international team's finding of a DNA region linked to depression has been replicated by another team from the United States who were studying an entirely separate group of people.

"What's remarkable is that both groups found exactly the same region in two separate studies," Pamela Madden, who led the US team at Washington University, said in a statement.

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 the other research group.    

The researchers said they believed many genes were involved in depression. These findings are unlikely to benefit patients immediately, with any new drugs developed from them likely to take another 10 to 15 years. However, they will help scientists understand what may be happening at the genetic and molecular levels in people with depression.   

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.  

"Normally in genetic studies of depression, replication of findings is very difficult and frequently takes years to emerge, if ever," said Breen, who gave a briefing in London about the work.

Major depression affects about 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. - Reuters