Plasma β-amyloid and duration of Alzheimer's disease in adults with Down syndrome

Objectives: To investigate the relation of plasma levels of A beta peptides (A beta 1-40 and A beta 1-42) and apolipoprotein E (APOE) genotype to dementia status, and the duration of Alzheimer's disease (AD) in adults with Down syndrome (DS). Methods: Adults with DS were recruited from community settings and followed up for a mean period of 6.7 years. Plasma levels A beta 1-40 and A beta 1-42 and APOE genotype were determined at the last visit. Results: There were 83 nondemented participants and 44 participants with prevalent AD. Overall, plasma levels of A beta 1-42, A beta 1-40 and the ratio A beta 1-42/A beta 1-40 did not differ significantly between the adults with DS. Among demented participants, the mean level of A beta 1-40 was significantly lower (157.0 vs. 195.3) and the ratio of A beta 1-42/A beta 1-40 was significantly higher (0.28 vs. 0.16) in those with more than 4 years duration of dementia than in those with 4 or fewer years' duration of dementia. This pattern was generally similar in those with and without an APOE epsilon 4 allele. Conclusions: There is an association between plasma A beta peptide levels and the duration of AD in older persons with DS. The predictive and diagnostic roles of A beta 1-42 and A beta 1-40 measurements for AD, however, remain controversial. Change in A beta peptide levels with onset of AD and with the duration of dementia may account for a lack of difference between prevalent cases and nondemented individuals and for variation in the predictive power of A beta peptide levels. Copyright (C) 2009 John Wiley & Sons, Ltd.