London, Sep 7 (IANS): Scientists have developed a "gene signature" that could be used to predict the onset of diseases such as Alzheimer's years in advance.

The study aimed to define a set of genes associated with "healthy ageing" in 65-year-olds.

We use birth year or chronological age to judge everything from insurance premiums to whether we will get a medical procedure or not.

"Most people accept that all 60 year olds are not the same but there has been no reliable test for underlying 'biological age'," said lead study author James Timmons from King's College London.

Such a molecular profile could be useful for distinguishing people at earlier risk of age-related diseases.

"Our discovery provides the first robust molecular 'signature' of biological age in humans and should be able to transform the way that 'age' is used to make medical decisions," Timmons reported.

The researchers analysed the RNA of healthy 65-year-old people and used the information to develop a signature of 150 RNA genes that indicated "healthy ageing".

The signature was found to be a reliable predictor for risk of age-related disease when studying RNA from tissues including human muscle, brain and skin.

With this RNA signature, they developed a "healthy age gene score" which they used to test and compare the RNA profiles of different individuals.

A greater score was associated with better health in both men and women.

The researchers then studied RNA from healthy 70-year-old participants and analysed follow-up health data over two decades.

Despite all participants being born within a year of each other, their RNA at around 70 years of age demonstrated a very wide distribution in "healthy age gene score".

"This variation was shown to link to long-term health. A greater gene score was also associated with better cognitive health and renal function across a 12 year span -- both important determinants of mortality," the authors noted.

In particular, patients diagnosed with Alzheimer's disease had an altered "healthy ageing" RNA signature in their blood.

The "healthy age gene score" could be integrated to help decide which middle-aged people could be offered entry into a preventative clinical trial many years before the clinical expression of Alzheimer's, the authors suggested.

The paper was published in the open access journal Genome Biology.