Detecting Alzheimer’s and Parkinson’s diseases as early as possible is critical. But while doctors know that the conditions can start 15 to 20 years before the symptoms appear, there aren’t many reliable ways of pinpointing exactly when that occurs. Now, scientists led by Dr. Ildefonso Rodriguez-Leyva at Central Hospital in University of San Luis Potosi in Mexico report that the skin may hold the clue to such early detection.

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In a study that will be presented in April at the American Academy of Neurology’s annual meeting in Washington, D.C., Rodriguez-Leyva found that compared to healthy patients and those with age-related dementia, patients with Alzheimer’s and Parkinson’s diseases had seven times higher levels of an altered form of a protein called tau in skin biopsies, and Parkinson’s patients also showed seven to eight times greater levels of a harmful version of another protein known as alpha-synuclein. Researchers aren’t sure what alpha-synuclein’s role is in the brain, but in Parkinson’s patients, it tends to clump into harmful aggregates that interrupt normal nerve function. Tau is involved in the brain decline associated with Alzheimer’s; as nerve cells die, the normally aligned molecules of tau, which function like railroad tracks to transport nutrients, collapse, twisting into unorganized masses of tangled protein.

“This skin test opens the possibility to see abnormal proteins in the skin before central nervous system symptoms — cognitive or motor deficits — appear,” Rodriguez-Leyva says.

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Rodriguez-Leyva turned to the skin to look for signs of the altered brain proteins since the skin and brain share a common embryonic origin; while everyone makes the two proteins, those who go on to develop Alzheimer’s or Parkinson’s seem to be especially vulnerable to having them fold in abnormal ways and stick together in damaging masses in the brain. If there were genetic signals dictating these sticky forms of the proteins, he speculated, then those signals might be detectable in the skin as well. “The ectoderm originates the nervous tissue and the skin,” he writes in an email to TIME discussing the study. “Our idea is that they have a similar program of protein expression. Therefore the skin could reflect events taking place in the nervous system.”

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The study involved only a few dozen patients — 20 with Alzheimer’s, 16 Parkinson’s patients and 17 with age-related dementia, who were compared to 12 healthy controls — so more work needs to be done to confirm the findings. But the results hint that it may be possible to detect these neurodegenerative conditions sooner, and it also provides drug developers with more confidence that targeting abnormal forms of tau and alpha-synuclein may lead to effective treatments.

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