Researchers have taken a major step towards understanding the causes of Alzheimer's disease with the largest study yet into the genetics of the disorder.

Findings from the international team suggest at least 20 genes play a role in the common late-onset form of Alzheimer's, more than double the number scientists had previously identified.

The work gives researchers an unprecedented view of the biological pathways that drive the neurodegenerative disorder, and raises the prospect of a test that could determine a person's susceptibility to the disease. Such a test could be helpful in the future if preventative drugs become available.

Researchers led by Philippe Amouyel at the Pasteur Institute in Lille used genetic information from more than 74,000 Alzheimer's patients and healthy controls to find regions of DNA that were more common in people who had the disease.

The scientists found many genes already implicated in the disease, including APOE4, which is strongly linked to late-onset Alzheimer's. But 11 of the gene regions had never before been linked to the disorder.

Alzheimer's disease is the most common form of dementia and affects around 500,000 people in Britain. An irreversible degenerative disorder, the condition takes hold when areas of damage and tangles form in the brain that cause nerve cells to die off. The disease causes memory loss and confusion and ultimately leaves patients needing full-time care. One in 14 people over the age of 65 are affected.

The findings reveal a complex disease that is driven by changes in inflammatory responses, the immune system, the way proteins are handled in the brain and how neurons talk to one another.

Among the most intriguing results from the study published in Nature Genetics is the discovery of a risk-raising gene involved in the immune system which is already thought to put people at greater risk of multiple sclerosis and Parkinson's disease. "This helps us understand the pathophysiology of the disease," said Amouyel.

"If we are able to develop preventative treatments for Alzheimer's disease, they would need to be used very early on," said Amouyel. "This could help us identify people who are more prone to the disease by estimating their individual risk."

James Pickett, the head of research at the Alzheimer's Society, said the work opened up new avenues to explore in the search for treatments for the condition.

"This truly global effort has doubled the number of genes linked to Alzheimer's and showed what can be achieved when researchers collaborate. We now need continued global investment into dementia research to understand exactly how these genes affect the disease process," he said.