The gene APP (amyloid precursor protein) makes a chemical called amyloid-beta which sticks together in the brain and blocks neurons from communicating with each other, according to the scientists.
In a study, scientists screened the DNA of almost two thousand people and found those with a mutated form of the gene were less likely to develop dementia.
The nuerologists concluded that mimicking this variant with a drug could stop Alzheimer's in its tracks, reported the Daily Mail.
The onset of the disease has been linked to such clumps which form when fragments of the protein gather together and cause the classic symptoms such as memory loss and speech problems.
Dr Kari Stefansson, chief executive of Icelandic company deCODE Genetics in Reykjavik, and co-scientists said the rare mutation results in a 40 per cent reduction in the formation of these harmful 'plaques'.
The researchers whose findings are published in Nature also found dementia free elderly people with the variant have better cognitive function between the age of 80 and 100 than those with the normal version.
More From This Section
Mutations in the APP gene have already been implicated in early-onset Alzheimer's that runs in families, but had not been linked to the common form of the disease that occurs in later life.
The research supports previous ideas that interfering with the gene, which can be achieved with existing drugs, may prevent dementia, Stefansson said.
Despite almost forty years of research, scientists have made little progress in developing drugs that inhibit interactions between proteins.
This is, in part, because the drug molecules are many times smaller than the proteins, so even if they can attach themselves to the larger molecules they are too small to prevent other proteins binding elsewhere.
"The prevalence of dementia in the Western world in people over the age of 60 has been estimated to be greater than five per cent, about two thirds of which are due to Alzheimer's disease, said Stefansson.
"The age-specific prevalence of Alzheimer's disease nearly doubles every five years after age 65, leading to a prevalence of greater than 25 per cent in those over the age of 90. We found a coding mutation in the APP gene that protects against Alzheimer's disease and cognitive decline in the elderly without Alzheimer's disease," Stefansson said.