LONDON — Signs of Alzheimer’s disease can be found in people with a rare, inherited form of the illness at least 20 years before the onset of symptoms, according to new studies that may guide earlier preventive treatment.

In a study of 44 adults in Colombia between the ages of 18 and 26, people with the presenilin 1 gene mutation, a predictor of the disease, had higher levels of a protein called amyloid beta, which lead to plaques in the brain that characterize the ailment, in their cerebrospinal fluid, according to a study published Nov. 6 in the Lancet Neurology journal. Another study by the same scientists showed that amyloid plaques begin to accumulate in the brains of people with the mutation when they’re in their late 20s.

People with the presenilin 1 mutation typically begin to show Alzheimer’s symptoms at the age of 45, so the studies showed that signs of the disease are evident at least 20 years before that, according to the researchers. That’s earlier than identified in any previous study of families with rare inherited forms of Alzheimer’s, the authors said.

The 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,” Eric Reiman, executive director of the Banner Alzheimer’s Institute in Phoenix, Ariz., and one of the study authors, said in an emailed statement.

The results should be treated with caution given the small number of participants and the findings observed at a single point in time, Nick Fox, professor of clinical neurology at University College London, said in a comment accompanying the article.

Still, “these findings add to accumulating evidence that Alzheimer’s disease is characterized by a long presymptomatic period of slowly progressive changes that can be identified and tracked using imaging and fluid biomarkers, thereby opening up a therapeutic window for early intervention,” Fox said.

The research is based on what is referred to as “the amyloid hypothesis,” which focuses on the amyloid plaques in the brain. Some scientists argue that tau proteins that form tangles in the brain should also be targeted in treatments.

Multiple strategies are needed to tackle Alzheimer’s, according to the Chicago-based Alzheimer’s Association, which supports research of amyloid-, tau- and inflammation-based approaches, among others.