Amyloid Burden in Cognitively Normal Elderly: What Does It Mean?
About one-third of cognitively normal elderly demonstrate an elevated load of beta-amyloid,* a pathologic hallmark of Alzheimer disease, in the brain, according to a newly published study from the Mayo Clinic. These data support previous observations, in which the PET-imaged brains of about a third of elderly, nondemented subjects will exhibit an abnormal accumulation of the AD-associated protein.
However, the Mayo investigators went a bit further by examining the potential association between the load of beta-amyloid, as measured with an established radiolabeled tracer (Pittsburgh compound B, or PiB) on PET images, and cognitive performance on various memory, language, attention, and visuospatial tests. As well, a relationship among brain amyloid, cognitive performance, and APOEe4 status (a well-known genetic marker for AD) was examined. What the researchers found was that poorer cognitive performance was associated with greater amyloid deposition, and that this relationship was more robust in APOEe4 carriers. Conversely the association between amyloid load and relative cognitive impairment was much weaker (ie, only “modest”) in subjects who did not carry an APOEe4 allele, suggesting (the authors concluded) “that APOE isoforms modulate the harmful effects of [beta-amyloid] on cognitive function.”
What any of this information means practically is very murky,
however. Will some of these cognitively normal subjects with heavier
amyloid burdens (and who perform less well on cognitive tests) develop
AD—that is, if they live long enough? Is AD more likely in these
subjects if they’re APOE e4 carriers? We don’t know, and obviously further longitudinal work is necessary. The Mayo authors do imply that follow-up is ongoing.
In an accompanying editorial, Buchman and Bennett commended the Mayo investigators for the size of the study (a highly respectable 408 subjects [with a median age of about 80 years]) and their “important contribution to our understanding of AD, illustrating that even among persons without dementia or [mild cognitive impairment], amyloid deposition is associated with very mild symptoms, especially among carriers of the APOE e4 allele.” But “[w]hether…amyloid imaging agents will have clinical utility remains to be determined,” the editorialists appropriately caution. They advise about the lack of data concerning the prognostic value of amyloid retention (presumably in any subjects, whether demented or not) and how amyloid retention may change over time. And they add that the utility of amyloid-imaging agents “will remain low in the absence of an effective amyloid modulating agent.” In other words, what’s the point of knowing the amyloid burden if you can’t do anything to lighten the load?
A related issue, however, not explored by the editorial authors, is whether even removing amyloid in the context of cognitive impairment, is beneficial or, in fact, does more harm. Existing AD trials suggest that amyloid-modulating agents (eg, bapineuzumab) can cause brain edema—presumably due to the removal of vascular amyloid—and that they do so without improving cognition to any substantial degree overall.
Intervening earlier with anti-amyloid drugs in less cognitively impaired subjects, as proposed by some industry investigators, is a tricky move: obviously primum non nocere in persons with only mild cognitive impairment and certainly in individuals with no practical cognitive problems (regardless of their amyloid burden).
PET = positron emission tomography.
* Defined by a “global cortical PiB retention ratio” of greater than 1.50.
Photograph: Atrophied brain from person with AD from National Institute on Alcohol Abuse and Alcoholism.