Image caption The woman had some rare genetic changes

The entire DNA sequence of a woman who lived to 115 has been pieced together by scientists.

The woman, who was the oldest in the world at the time of her death, had the mind of someone decades younger and no signs of dementia, say Dutch experts.

The study, reported at a scientific conference in Canada, suggests she had genes that protected against dementia.

Further work could give clues to why some people are born with genes for a long life, says a UK scientist.

It is more than 10 years since the first draft of the human genetic code was revealed.

Sequencing the genome of the world's oldest woman is an important starting point Dr Jeffrey Barrett, Sanger Centre

Since then, perhaps a few hundred individuals have had their genes mapped in full, as the technology to "read" DNA gets better and cheaper.

The woman, whose identity is being kept secret, and is known only as W115, is the oldest person to have her genes mapped.

She donated her body to medical science, allowing doctors to study her brain and other organs, as well as her entire genetic code.

Dr Henne Holstege, of the Department of Clinical Genetics at the VU University Medical Center in Amsterdam, says she appeared to have some rare genetic changes in her DNA.

It is not yet clear what role they carry out, but it appears there is something in her genes that protects against dementia and other diseases of later life.

Dr Holstege told the BBC: "We know that she's special, we know that her brain had absolutely no signs of Alzheimer's.

"There must be something in her body that is protective against dementia.

"We think that there are genes that may ensure a long life and be protective against Alzheimer's."

Proof of principle

W115 was born prematurely and was not expected to survive.

But she lived a long and healthy life, and entered a care home at the age of 105.

She eventually died from a stomach tumour, having been treated for breast cancer at the age of 100.

A test of her mental skills at the age of 113 showed she had the performance of a woman aged 60-75 years.

At post-mortem examination, doctors found no evidence of dementia or the furring of the arteries seen in heart disease.

They are making her gene sequence available to other researchers, to further the cause of science.

The work, which has yet to be published, was presented at the American Society of Human Genetics annual meeting in Montreal, Canada.

Commenting on the study, Dr Jeffrey Barrett, of the Wellcome Trust Sanger Institute in Cambridge, UK, said it was an important proof of principle.

He told the BBC: "Sequencing the genome of the world's oldest woman is an important starting point to understand how DNA variation relates to the process of having a long, healthy life.

"But in order to really understand the underlying biology of living a long, healthy life, we will need to look at the DNA sequence of hundreds or thousands of people."