The researchers discovered a specific mutation in the probands' APP gene which probably protects them against contracting Alzheimer's disease. They were able to demonstrate in an in vitro laboratory experiment that the APP protein altered by the mutation is less easily cleaved by the enzyme beta secretase. This cleavage gives rise to pathogenic fragments that lead to the typical protein deposits in the brains of sufferers of the disease.

The gene variant apparently not only protects against Alzheimer's disease but also slows the natural deterioration of mental faculties in old age, as the geneticists reported on the website of the science journal "Nature". They therefore consider it possible that the Alzheimer-type dementia could also be an extreme form of age-related cognitive functions subsides.

Literature:
Þorlákur Jonsson et al. (2012): A mutation in APP protects against Alzheimer's disease and age-related cognitive decline, Nature.
Abstract: http://www.nature.com/