This paper is very interesting, and in keeping with our recent findings using amyloid imaging (see Rentz et al., 2010). I think the take-home message is that cognitive reserve does provide a protective performance effect in the earliest stages of the disease and may mask evidence of pathology. This means that people with higher reserve tend to maintain normal cognitive performance for longer. Unfortunately, if you were attempting to find people with early memory deficits for treatment trials, you would overlook these individuals, as they would continue to perform above cut-offs, despite the disease process. The failure to account for cognitive reserve may be one factor in the negative findings in some studies that could not find an association between Aβ deposition and performance. The strength and value of this study by Yaffe et al. is its nine-year longitudinal follow-up and the large number of participating subjects.

View all comments by Dorene Rentz

In this paper, Yaffe et al. describe the results of a prospective study in which 997 persons underwent baseline blood draws and were followed for nine years with serial cognitive assessments using the 3MS. This is an important area of study, as Aβ is thought to play a critical role in driving the AD process in the central nervous system. As the authors point out, however, the relationship between plasma Aβ levels and AD risk is not clear, with prior studies yielding contradictory results; some studies indicate that high and others indicate that low Aβ42 levels put one at risk for cognitive decline and AD. The current study addresses this issue using the largest cohort and the longest follow-up I am aware of. Also of interest is the ethnicity (about 50 percent African-American) and wide range of educational levels represented in the study population.

Dividing the population into tertiles based on the plasma Aβ42/40 ratios, the authors showed that persons (mean age of 74) with the lowest ratios had the most cognitive decline in the subsequent nine years as assessed with the...  Read more

In this paper, Yaffe et al. describe the results of a prospective study in which 997 persons underwent baseline blood draws and were followed for nine years with serial cognitive assessments using the 3MS. This is an important area of study, as Aβ is thought to play a critical role in driving the AD process in the central nervous system. As the authors point out, however, the relationship between plasma Aβ levels and AD risk is not clear, with prior studies yielding contradictory results; some studies indicate that high and others indicate that low Aβ42 levels put one at risk for cognitive decline and AD. The current study addresses this issue using the largest cohort and the longest follow-up I am aware of. Also of interest is the ethnicity (about 50 percent African-American) and wide range of educational levels represented in the study population.

Dividing the population into tertiles based on the plasma Aβ42/40 ratios, the authors showed that persons (mean age of 74) with the lowest ratios had the most cognitive decline in the subsequent nine years as assessed with the 3MS. A limitation was that the tertiles differed in such a way that persons in the lowest Aβ42/40 tertile were more likely black, diabetic, had lower literacy, and to have an ApoE4 allele—all things that also contribute to dementia and AD risk. When these variables were controlled for statistically, the effect was diminished but nonetheless remained. The correlation between low plasma Aβ42/40 and AD risk might be interpreted as a shift in equilibrium towards deposition of Aβ in the brain, as has been postulated to explain the decrease in Aβ42 levels in the CSF. This is consistent with some prior studies, including one by our group, looking at persons at risk for familial Alzheimer’s disease (FAD). In our study (involving the smallest number of subjects with the shortest duration of follow-up!), we found that asymptomatic FAD mutation carriers had high levels of plasma Aβ42, while mildly symptomatic mutation carriers (Clinical Dementia Rating Scale score of 0.5) had lower levels (1). This suggests that whether high or low levels of plasma Aβ42 are associated with subsequent cognitive decline may depend on exactly when during the risk period the levels are measured. What continues to be lacking, and is not addressed by the current article, is a mechanistic explanation linking plasma and CSF Aβ42 levels, as they do not typically correlate well.

Yaffe et al. also demonstrated that “cognitive reserve,” as indexed by literacy and level of education, modified the relationship such that the most cognitive decline occurred in the low reserve, lowest Aβ tertile group. This may be interpreted as indicating that persons with higher reserve are less prone to the toxic effects of Aβ42, though this is far from conclusive.

As in previous studies, levels of plasma Aβ42 and Aβ40 were highly variable, and there was a great degree of overlap among the tertiles. Also, the size of the effect of the Aβ42/40 ratio on the outcome variable was mild; a three-point difference (on a 100-point scale) in some analyses over a nine-year period. Therefore, though these results are interesting, they bring into question the utility of plasma amyloid measures as clinically relevant predictors of cognitive decline.

References:
1. Ringman JM, Younkin SG, Pratico D, et al. Biochemical markers in persons with preclinical familial Alzheimer disease. Neurology 2008;71:85-92. Abstract

View all comments by John Ringman

The Yaffe paper is indeed very interesting. Finding a diagnostic and/or prognostic plasma biomarker is the real Holy Grail. It's still unclear, however, what alterations in plasma Aβ levels/ratios reflect. Dr. Zetterberg mentioned in the accompanying article that it would be great if we knew whether there was a relationship between plasma Aβ levels and PIB retention in the brain. Our own work in this area (Fagan et al., 2009; Figure 3) has demonstrated no relationship between plasma Aβ (Aβ1-40, Aβx-40, Aβ1-42, Aβx-42) levels and mean cortical PIB binding potential in a large cohort (n = 189) of cognitively normal individuals despite a strong relationship between PIB binding and CSF Aβ42 in this same cohort (as we and now several other groups have shown). Thus, the plasma Aβ story remains elusive...and warranting ongoing study.

References:
Fagan AM, Mintun MA, Shah AR, Aldea P, Roe CM, Mach RH, Marcus D, Morris JC, Holtzman DM. Cerebrospinal fluid tau and ptau181 increase with cortical amyloid deposition in cognitively normal individuals: Implications for future clinical trials of Alzheimer’s disease. EMBO Mol Med, 2009, 1:371-80. Abstract

View all comments by Anne Fagan

Our article (Ray et al., 2007) gained a lot of attention, but it was very early days and we had to work with what was available. Our samples were from multiple centers, and the cases and controls were not perfectly matched for each. There was also a difference in age between cases and controls, and the analytical platform we had used was a somewhat moving target, because the manufacturer (RayBiotech) made several changes to the array during the time we used it. Nevertheless, I think several of the markers we identified have biological relevance in AD and brain aging, and we are pursuing some of them successfully (e.g., MCSF). I would also draw attention to work from our lab that has been overlooked (Britschgi et al., 2011). We used an independent set of samples, a different analytical platform, and an innovative new approach to predict pathological parameters in AD using plasma markers as variables. Several models we developed reproduced six proteins out of the 18-protein Ray signature....  Read more

Our article (Ray et al., 2007) gained a lot of attention, but it was very early days and we had to work with what was available. Our samples were from multiple centers, and the cases and controls were not perfectly matched for each. There was also a difference in age between cases and controls, and the analytical platform we had used was a somewhat moving target, because the manufacturer (RayBiotech) made several changes to the array during the time we used it. Nevertheless, I think several of the markers we identified have biological relevance in AD and brain aging, and we are pursuing some of them successfully (e.g., MCSF). I would also draw attention to work from our lab that has been overlooked (Britschgi et al., 2011). We used an independent set of samples, a different analytical platform, and an innovative new approach to predict pathological parameters in AD using plasma markers as variables. Several models we developed reproduced six proteins out of the 18-protein Ray signature. Most consistently, we found changes in MCSF, GCSF and IL-3.

Still, I think even now there are major challenges to find markers that will hold up in multiple studies across different centers and become clinical tools. It took maybe 10 years to achieve clinical utility with CSF Aβ and tau ELISAs, and I think it will take as long with any other protein-based assay (one at a time).

The main problem is that protein measurements are extremely difficult to standardize, and multiplex assays are notoriously inexact. Major problems with current assays are that the reagents (antibodies, standards) are "research use only" and not clinical grade. They may, therefore, change from batch to batch, leading to variations in sensitivity and absolute concentrations for a given protein. Another, more trivial problem is that assays (e.g., ELISAs, Luminex) from different manufacturers may detect different isoforms of the same protein, active versus pre-proteins, or post-translationally modified proteins versus unmodified, leading sometimes to completely opposite results between groups.

I think we are at a similar stage in this field as genetics was with SNP studies 10 years ago. Geneticists produced lists of more than 100 genes with linkage to AD, of which most did not hold up in the much larger GWAS. This showed that sample size is key. However, even if thousands of blood samples will be analyzed, we will still have the problem that the protein assays and sample collection are not standardized.

Our lab continues to develop and use protein screens, and we currently measure more than 600 proteins in blood plasma or CSF using antibody-based microarrays. We know these arrays produce false-positive and -negative results, but we run several hundred samples in one batch to reduce variability. We have identified several interesting new proteins and pathways that we are now validating in biological assays and animal models of AD. I think we will have to go this hard way and link biology to any of the proteins that come out of screens before they are worth the effort to produce a clinical-grade assay.

View all comments by Tony Wyss-Coray