Carrying the Apoe4 allele is known to be linked to an increased risk of Alzheimer's disease and a steeper rate of memory decline in healthy adults, but what does the effect of the amount of accumulating amyloid plaque deposition in the brain have on memory and cognitive function?
The Australian Imaging Biomarkers and Lifestyle Study has now revealed their findings to answer this question. One hundred and forty one healthy and cognitively normal adults aged around their mid 70's underwent PET neuroimaging scans to assess their cerebral amyloid deposition, APOE genotyping and cognitive assessment. Eighteen months later their cognitive function was reassessed.
The results showed that in contrast to those found to have relatively little amyloid build up, those with a high amyloid load showed significantly greater decline (20%) in working memory, verbal and episodic memory at the 18 assessment. Those found to carry the Apoe4 allele showed a greater decline in visual memory, compared to non-carriers at the 18 month assessment. No interaction between Apoe4 and cerebral amyloid load was observed for any measure of cognitive function.
Yen Ying Lim MPsych from the University of Melbourne, suggests that plaque build up may be a more important factor in assessing people at risk of cognitive decline, but that testing for the genotyping for APOE4 is cheaper and easier to perform.
Plaque related memory decline can occur while people still have a normal memory and that these brain related changes can be detected and measured even while people are still healthy.
What does this mean?
There has been considerable debate about whether testing for the APOE genotype is appropriate or helpful in assessing those thought to be at risk of Alzheimer's disease.
Now while it is now relatively easy to perform this test and it is relatively inexpensive, testing is not recommended here in Australia for a variety of reasons including the fact that Alzheimer's is a multifactorial disease and it is well understood that carrying the APOE4 allele whilst increasing a person's relative risk is not a predictor of who will or won't develop the condition.
This study indicates than rather than focussing on genotype, other factors may prove to be more useful to determine who may be at risk of cognitive decline or memory problems at a time when they are still subjectively healthy.
The question still to be answered is whether the ability to reduce a person's amyloid load will make a significant difference to preventing future cognitive decline.
To floss or not to floss: that will be the question.
Y. Y. Lim, K. A. Ellis, R. H. Pietrzak, D. Ames, D. Darby, K. Harrington, R. N. Martins, C. L. Masters, C. Rowe, G. Savage, C. Szoeke, V. L. Villemagne, P. Maruff. Stronger effect of amyloid load than APOE genotype on cognitive decline in healthy older adults. Neurology, 2012; 79 (16): 1645 DOI: 10.1212/WNL.0b013e31826e9ae6