Life-long overexpression of S100β in Down’s syndrome: implications for Alzheimer pathogenesis

Chronic overexpression of the neurite growth-promoting factor S100β has been implicated in the pathogenesis of neuritic plaques in Alzheimer’s disease. Such plaques are virtually universal in middle-aged Down’s syndrome, making Down’s a natural model of Alzheimer’s disease. We determined numbers of astrocytes overexpressing S100β, and of neurons overexpressing β-amyloid precursor protein (β-APP), and assayed for neurofibrillary tangles in neocortex of 20 Down’s syndrome patients (17 weeks gestation to 68 years). Compared to controls, there were twice as many S100β-immunoreactive (S100β+) astrocytes in Down’s patients at all ages: fetal, young, and adult (p = 0.01, or better, in each age group). These were activated (i.e., enlarged), and intensely immunoreactive, even in the fetal group. There were no neurofibrillary changes in fetal or young Down’s patients. The numbers of S100β+ astrocytes in young and adult Down’s patients correlated with the numbers of neurons overexpressing β-APP (p < 0.05). Our findings are consistent with the idea that conditions—including Down’s syndrome—that promote chronic overexpression of S100β may confer increased risk for later development of Alzheimer’s disease.