Monozygotic twins concordant for probable Alzheimer disease and increased platelet membrane fluidity

This report describes monozygotic twins who were concordant for probable Alzheimer disease, as defined by currently-accepted clinical criteria. Monozygosity was established by blood typing. Their ages of symptomatic onset were 57 and 66 yr, and the times from onset to institutionalization were 8 and 2 yr, respectively. These results suggest that age at onset and rate of progression are clinical features that can be affected by random processes or exposure to environmental factors. The platelet membrane fluidity of both twins was abnormally increased, and the respective values were identical within experimental limits. This result is consistent with published data suggesting that increased platelet membrane fluidity is associated with a clinically distinct subtype of Alzheimer disease and that this platelet membrane characteristic may be genetically determined.