Scientists have identified the first gene for obesity in the general population in a discovery that could help to revolutionise the treatment of a disorder that affects an increasing number of children.

The gene is involved in appetite stimulation and the researchers believe it could form the basis of a test that could identify youngsters who carry a genetic predisposition to put on weight. About one in five British children are overweight due to a combination of lack of exercise and high-calorie diets. Some of them are likely to develop clinical obesity in later life, which raises the risk of heart disease, diabetes and cancer.

Scientists believe that an accurate test to identify a genetic tendency to become obese in adulthood will enable doctors to target those children most at risk so that their diet and lifestyle can be changed.

Philippe Froguel, professor of genomic medicine at Imperial College London, said that a genetic test to look for an inherited predisposition would become an essential tool in the fight against the growing problem of obesity. "Obesity is like many diseases. It has a genetic basis with about 50 per cent of the risk of obesity being driven by the genes," Professor Froguel said. "I think for obesity, it [a test] will be important in children. We need to have some way to test them, to see which of them will be at high risk of obesity or diabetes."

The gene identified by Professor Froguel's team lies on chromosome 10 - one of 23 pairs that constitute the human genome. Called GAD2, it works by speeding up the production of a chemical transmitter in the brain, called GABA, which is involved in appetite stimulation.

The researchers believe that people who carry a more active form of the GAD2 gene build up a larger than normal quantity of GABA in the brain, which stimulates the appetite to a point where they overeat. "The discovery that this one gene plays a role in determining whether someone is likely to overeat could be crucial in understanding the continued rise in obesity rates around the world," Professor Froguel said.

Professor Froguel analysed the genetic makeup of 576 people from obese families and compared their DNA with that taken from 646 people of normal weight. The study is published in the Public Library of Science Biology.

He identified variations in the gene that seemed to be significantly more common in obese subjects, and which could partly account for their condition. "One in 10 obese people are probably obese because of this gene," he said.

One form of the GAD2 gene appeared to protect some people against obesity, while another form increased the risk of the disorder. "Genetic factors alone cannot explain the rapid rise in obesity rates, but they may provide clues to preventative and therapeutic approaches," he said. "Having identified this gene, it may be possible to develop a screening programme to identify those who may be at risk of becoming obese later in life, and take effective preventative measures."

About 20 genes may be involved in causing common obesity - as opposed to a strictly inherited form of the disorder which runs in families and is due to a wholly genetic imbalance.

Scientists working on the human genome at the Wellcome Trust Sanger Institute in Cambridge will announce today that they passed a landmark by sequencing the two billionth letter of the genetic code of man.

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