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© Bethany Clarke/The TimesChildren taking part in activities at a residential weight-loss camp near Leeds
Two children considered to be at risk of abuse because they are severely obese have been removed from the protection register after scientists discovered that they carry a newly identified genetic abnormality that explains their weight.

Evidence from a ground-breaking study has convinced social workers that the children's obesity was not caused by parental neglect or deliberate overfeeding but by a missing segment of DNA. The cases of another two children on the at-risk register have also been placed under review, after research showed them to have the same genetic deletion.

The children's weights were in the top 1 to 2 per cent for their age. An 8-year-old child in this category would weigh at least 11st (70kg), and a 10-year-old child at least 15st (95kg). Scientists behind the research told The Times that while genetic mutations that cause obesity are very rare, they may be more common among children who are extremely obese and who attract the attention of social services.

"We found five children with this deletion, and when we got in touch with their families with the results we found four had been put on the at-risk register," said Sadaf Farooqi, of the University of Cambridge. "We're at various stages of discussions with social services. Two of the children have already been taken off the register, and we're optimistic about the other two cases."

She said the genetic diagnosis had come as a great relief to the families affected. "It has been an emotional rollercoaster for them and a very stigmatising experience," she said.

The discovery comes amid growing concern about childhood obesity, and increasing pressure on social services to intervene to protect severely overweight children. Surveys have suggested that obesity is a factor in about 20 child-protection cases each year. In 2007 an girl aged 8 was taken into care by Cumbria County Council as she was dangerously overweight.

As genetic factors can trigger extreme early-onset obesity, these should always be excluded before social services act, Dr Farooqi said. She said, however, that genetic mutations were not a significant factor in the obesity epidemic: the newly discovered abnormality was present in just two of more than 7,000 healthy people in the study's control group.

While common genetic variations affect people's tendency to put on weight, rising obesity in recent decades have been driven by changes in diet and lifestyle, she added. The study, published in the journal Nature, is the first to identify a link between obesity and a genetic variation in which segments of DNA are deleted or duplicated.

The researchers, led by Dr Farooqi and Matt Hurles, of the Wellcome Trust Sanger Institute,compared the DNA of 300 children who developed severe obesity before the age of 10 with that of more than 7,000 apparently healthy volunteers. The most common deletion, of a part of chromosome 16, was found in five obese children, including the four on social service registers. Follow-up studies have identified the same mutation in some of the children's parents, who are also obese.

The missing segment of DNA includes a gene called SH2B1, which has previously been linked to weight regulation and blood sugar control.