PARIS, Nov 06, 2012 - Researchers said Tuesday they had seen the earliest-ever warning signs of Alzheimer's Disease - among a high-risk group of 20-somethings - in the ongoing quest for early detection and prevention.

A major problem in the search for a cure for this debilitating form of dementia is that symptoms appear years after irreversible brain decay has already set in.

For the study, a team of scientists from the United States and Colombia tested 18- to 26-year-old members of an extended Colombian family that share a common ancestor and a genetic predisposition to develop an inherited form of Alzheimer's.

One in three members of the clan carry a gene mutation that will lead to a rare form of the disease which hits people in their 40s, unlike the common variant which presents much later.

A brain scan comparison found that individuals who carry the errant gene have less grey matter in certain areas of the brain than those who don't, scientists wrote in The Lancet medical journal.

They also found that those with the mutation had higher levels in their cerebrospinal fluid of a protein called amyloid beta, implicated in the plaque build-ups found on the brains of Alzheimer's sufferers.

The findings “suggest that neurodegenerative changes occur more than 20 years before symptom onset and somewhat earlier than was suggested by findings from previous MRI studies,” Nick Fox of the University College London's Dementia Research Centre said in a comment on the study.

Alzheimer's disease causes two-thirds of dementia cases - attacking one in 200 people - the rate is increasing as the world's population ages.

Trial participants, 20 with the fateful gene mutation and 24 without, were not told whether they had it or not. All had normal cognitive abilities at the time of the study.

“The findings... could ultimately lead to improved early detection and better clinical trials of preventative treatments,” The Lancet said in a statement.

But the outcome also raises questions about scientists' understanding of how Alzheimer's progresses.

“These findings... raise new questions about the earliest brain changes involved in the predisposition to Alzheimer's and the extent to which they could be targeted by future prevention therapies,” research leader Eric Reiman from the Banner Alzheimer's Institute in Arizona said.

Scientists still do not know quite what to make of the plaques and tangles that German doctor Alois Alzheimer first spotted in the brain of a dementia patient who died in 1906.

They disagree on the respective roles of beta amyloid plaque build-ups and of a protein called tau which forms tangles inside the brain cells.

Most test therapies have targeted beta amyloids, but some now suggest it is actually tau killing the brain cells.

According to Fox, the new study questioned existing models of Alzheimer's Disease “on several fronts”.

Among other things, “neurodegeneration would seem to be occurring in advance of evidence of plaque deposition”, widely thought to cause the brain damage.

Fox said the results should be treated with caution as the trial sample was small and the outcome may not apply to the much more common sporadic, late-onset form of Alzheimer's.

Alzheimer's Disease International (ADI) projects the number of people with dementia will rise from 35.6 million in 2010 to 65.7 million by 2030 and 115.4 million by 2050.