Blood Marker Shows Alzheimer's Changes in Down Syndrome Two Decades Early

A new study finds that serum beta-synuclein, a protein marking the loss of connections between nerve cells, differs in adults with Down syndrome from people without the condition an estimated 21 years before the average age of a prodromal Alzheimer's diagnosis in the cohort studied, earlier than three other blood markers already used to track the disease.

A study published in Alzheimer's & Dementia on September 21, 2026, by researchers at the Sant Pau Research Institute in Barcelona, Martin Luther University in Germany, and the German Center for Neurodegenerative Diseases reports that serum beta-synuclein, a protein found at the junctions where nerve cells communicate, is measurably higher in adults with Down syndrome long before the first symptoms of Alzheimer's disease appear. Based on participants' ages, the researchers estimated the marker begins to diverge from levels seen in people without Down syndrome roughly 21 years before the average age at which prodromal Alzheimer's disease was diagnosed in the cohort, and more than seven years earlier than changes in three other blood markers already used in Alzheimer's research: pTau217, neurofilament light, and glial fibrillary acidic protein.

The cross-sectional study measured serum beta-synuclein using immunoprecipitation-mass spectrometry in 131 adults: 88 with Down syndrome, spanning the Alzheimer's continuum from no symptoms (35 people) through prodromal disease (18 people) to dementia (35 people), and 43 cognitively unimpaired adults without Down syndrome. Levels of the protein were significantly higher in people with Down syndrome overall (p less than 0.001), and higher readings tracked with thinning of brain regions vulnerable to Alzheimer's disease, lower metabolic activity in those regions, and weaker performance on tests of episodic memory (all p less than 0.01).

The finding extends earlier reporting in this section that serum beta-synuclein also rises in two unrelated amyloid diseases outside classic Alzheimer's, a pattern that raised questions about how specific the marker really is. This latest study does not resolve that question, but it does something the earlier work could not: it places beta-synuclein on a timeline, ahead of the three other blood markers, within a population known to develop Alzheimer's pathology at a predictable age because of an extra copy of chromosome 21.

Because people with Down syndrome carry three copies of chromosome 21 rather than two, the researchers note that part of the signal could reflect effects of the extra chromosome rather than Alzheimer's disease specifically. The study compared different people at a single point in time rather than following the same individuals over years, so it cannot show that a rising level in one person actually predicts later decline.

The research team says the next step is to follow the same individuals over years to test whether rising beta-synuclein predicts who goes on to develop symptoms, rather than simply differing between groups at one point in time.