Cerebrospinal fluid tau and ptau ₁₈₁ increase with cortical amyloid deposition in cognitively normal individuals: Implications for future clinical trials of Alzheimer's disease

Report26 November 2009Open Access Cerebrospinal fluid tau and ptau181 increase with cortical amyloid deposition in cognitively normal individuals: Implications for future clinical trials of Alzheimer's disease Anne M. Fagan Corresponding Author Anne M. Fagan [email protected] Department of Neurology, Washington University School of Medicine, St. Louis, MO, USA Alzheimer's Disease Research Center, Washington University School of Medicine, St. Louis, MO, USA Hope Center for Neurological Disorders, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author Mark A. Mintun Mark A. Mintun Alzheimer's Disease Research Center, Washington University School of Medicine, St. Louis, MO, USA Department of Radiology, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author Aarti R. Shah Aarti R. Shah Department of Neurology, Washington University School of Medicine, St. Louis, MO, USA Hope Center for Neurological Disorders, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author Patricia Aldea Patricia Aldea Department of Radiology, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author Catherine M. Roe Catherine M. Roe Department of Neurology, Washington University School of Medicine, St. Louis, MO, USA Alzheimer's Disease Research Center, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author Robert H. Mach Robert H. Mach Alzheimer's Disease Research Center, Washington University School of Medicine, St. Louis, MO, USA Department of Radiology, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author Daniel Marcus Daniel Marcus Department of Radiology, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author John C. Morris John C. Morris Department of Neurology, Washington University School of Medicine, St. Louis, MO, USA Alzheimer's Disease Research Center, Washington University School of Medicine, St. Louis, MO, USA Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author David M. Holtzman David M. Holtzman Department of Neurology, Washington University School of Medicine, St. Louis, MO, USA Alzheimer's Disease Research Center, Washington University School of Medicine, St. Louis, MO, USA Hope Center for Neurological Disorders, Washington University School of Medicine, St. Louis, MO, USA Department of Molecular Biology and Pharmacology, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author Anne M. Fagan Corresponding Author Anne M. Fagan [email protected] Department of Neurology, Washington University School of Medicine, St. Louis, MO, USA Alzheimer's Disease Research Center, Washington University School of Medicine, St. Louis, MO, USA Hope Center for Neurological Disorders, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author Mark A. Mintun Mark A. Mintun Alzheimer's Disease Research Center, Washington University School of Medicine, St. Louis, MO, USA Department of Radiology, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author Aarti R. Shah Aarti R. Shah Department of Neurology, Washington University School of Medicine, St. Louis, MO, USA Hope Center for Neurological Disorders, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author Patricia Aldea Patricia Aldea Department of Radiology, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author Catherine M. Roe Catherine M. Roe Department of Neurology, Washington University School of Medicine, St. Louis, MO, USA Alzheimer's Disease Research Center, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author Robert H. Mach Robert H. Mach Alzheimer's Disease Research Center, Washington University School of Medicine, St. Louis, MO, USA Department of Radiology, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author Daniel Marcus Daniel Marcus Department of Radiology, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author John C. Morris John C. Morris Department of Neurology, Washington University School of Medicine, St. Louis, MO, USA Alzheimer's Disease Research Center, Washington University School of Medicine, St. Louis, MO, USA Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author David M. Holtzman David M. Holtzman Department of Neurology, Washington University School of Medicine, St. Louis, MO, USA Alzheimer's Disease Research Center, Washington University School of Medicine, St. Louis, MO, USA Hope Center for Neurological Disorders, Washington University School of Medicine, St. Louis, MO, USA Department of Molecular Biology and Pharmacology, Washington University School of Medicine, St. Louis, MO, USA Search for more papers by this author Author Information Anne M. Fagan *,1,2,3, Mark A. Mintun2,4, Aarti R. Shah1,3, Patricia Aldea4, Catherine M. Roe1,2, Robert H. Mach2,4, Daniel Marcus4, John C. Morris1,2,5 and David M. Holtzman1,2,3,6 1Department of Neurology, Washington University School of Medicine, St. Louis, MO, USA 2Alzheimer's Disease Research Center, Washington University School of Medicine, St. Louis, MO, USA 3Hope Center for Neurological Disorders, Washington University School of Medicine, St. Louis, MO, USA 4Department of Radiology, Washington University School of Medicine, St. Louis, MO, USA 5Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, MO, USA 6Department of Molecular Biology and Pharmacology, Washington University School of Medicine, St. Louis, MO, USA *Tel: +1 341 362 3453; Fax: +1 341 362 2244 EMBO Mol Med (2009)1:371-380https://doi.org/10.1002/emmm.200900048 PDFDownload PDF of article text and main figures. ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InMendeleyWechatReddit Figures & Info Abstract Alzheimer's disease (AD) pathology is estimated to develop many years before detectable cognitive decline. Fluid and imaging biomarkers may identify people in early symptomatic and even preclinical stages, possibly when potential treatments can best preserve cognitive function. We previously reported that cerebrospinal fluid (CSF) levels of amyloid-β42 (Aβ42) serve as an excellent marker for brain amyloid as detected by the amyloid tracer, Pittsburgh compound B (PIB). Using data from 189 cognitively normal participants, we now report a positive linear relationship between CSF tau/ptau181 (primary constituents of neurofibrillary tangles) with the amount of cortical amyloid. We observe a strong inverse relationship of cortical PIB binding with CSF Aβ42 but not for plasma Aβ species. Some individuals have low CSF Aβ42 but no cortical PIB binding. Together, these data suggest that changes in brain Aβ42 metabolism and amyloid formation are early pathogenic events in AD, and that significant disruptions in CSF tau metabolism likely occur after Aβ42 initially aggregates and increases as amyloid accumulates. These findings have important implications for preclinical AD diagnosis and treatment. The paper explained PROBLEM: AD pathology is estimated to develop many years before detectable cognitive decline. Once symptoms are apparent, the brain has already experienced substantial neuronal and synaptic loss. Thus there is a great need to develop biomarkers that can identify people in the very earliest symptomatic and even ‘preclinical’ stages, prior to any cognitive impairment, when potential treatments will have the best opportunity to preserve cognitive function. RESULTS: We analysed CSF samples and in parallel determined the amount of cortical amyloid as evidenced by retention of the in vivo amyloid binding agent, PIB, in 189 cognitively normal research participants (age 43–89 years). We observed a positive linear relationship between the levels of CSF tau and ptau181 (primary constituents of neurofibrillary tangles) with the amount of cortical amyloid. We also observed a strong inverse relationship between cortical PIB binding and CSF Aβ42 (the primary constituent of amyloid plaques), but not plasma Aβ species, demonstrating that a low level of CSF Aβ42 is an excellent marker of brain amyloid even in the absence of cognitive symptoms. IMPACT: The data obtained shed light on the potential utility of PIB amyloid imaging and CSF Aβ42, tau and ptau181 as antecedent (‘preclinical’) biomarkers of AD and also provide insight into the normal time course of the pathophysiology of the disease as reflected in the CSF. These findings have important implications for preclinical AD diagnosis and treatment, and should aid in the design and evaluation of secondary prevention trials in AD. INTRODUCTION Alzheimer's disease (AD) is a progressive and fatal neurodegenerative disorder that currently affects ∼10.6 million people in the US and Europe, with projected estimates reaching 15.4 million by the year 2030 (Alzheimer's Association). Clinicopathological studies support the notion of a long ‘preclinical’ stage of the disease, with brain pathology (amyloid plaques and neurofibrillary tangles) estimated to begin ∼10–20 years prior to significant neuronal cell death and the consequent appearance of any behavioural signs or symptoms (Braak & Braak, 1997; Gomez-Isla et al, 1996; Hulette et al, 1998; Markesbery et al, 2006; Morris & Price, 2001; Price et al, 2001). Fluid and imaging biomarkers of this pathology are currently being sought in order to confirm early diagnoses and, importantly, to identify individuals in the preclinical stage so emerging therapies ultimately have a chance to preserve normal brain function (Craig-Schapiro et al, 2008). We recently reported an inverse relationship between cortical amyloid deposits, as viewed by positron emission tomography (PET) imaging with the amyloid binding agent, Pittsburgh compound B (PIB) and the amount of cerebrospinal fluid (CSF) amyloid-β42 (Aβ42), the primary constituent of brain amyloid plaques (Fagan et al, 2006, 2007). Individuals with cortical amyloid (as detected by PET PIB) had low CSF Aβ42 whereas those without cortical amyloid had high CSF Aβ42. This relationship was observed independent of clinical status; several cognitively normal individuals had a CSF Aβ42/PIB profile indistinguishable from that of other individuals diagnosed clinically with early stage dementia of the Alzheimer type (DAT). These observations are consistent with the idea of preclinical AD and suggest that these measures may have clinical utility as antecedent biomarkers of the disease. We have now obtained CSF and PIB data from 189 cognitively normal individuals ranging in age from 43 to 89 years. We explored the relationship between in vivo brain amyloid and CSF markers of proteins present in neurons and constituents of neurofibrillary tangles (tau and ptau181), as well as Aβ species in plasma, and investigated whether the CSF Aβ42/PIB relationship remains robust in this large cohort of non-demented individuals. The data we obtained shed further light on the potential utility of these measures as antecedent biomarkers of AD, and also provide insight into the normal time course of the pathophysiology of the disease as reflected in CSF, information that should aid in the design and evaluation of secondary prevention trials. RESULTS One hundred and eighty-nine research participants with a clinical dementia rating of 0 (CDR 0, indicating cognitively normal) (Morris, 1993) met the selection criterion of having a PIB scan within 2 years of CSF collection by lumbar puncture (LP). Combined PIB and biomarker data from 25 of these participants have been reported by us in previous studies (Fagan et al, 2006, 2009), whereas the remaining 164 are unique to the present study. The demographic characteristics of the present cohort are similar to what we have previously published with the exception of age (Table 1). By design, we have included a wide range of ages in the present study (43–89 years, normally distributed) so as to better capture the various biomarker correlations during the potential preclinical stage of AD (which is estimated to begin 10–20 years prior to cognitive symptoms). Therefore, the mean age of our cohort is younger (64.7 ± 10.4) than what we have reported on previously (71.41 ± 8.62; Fagan et al, 2009). In keeping with this age difference, CSF Aβ42 levels in the present study (652 ± 235 pg/ml) are higher than that reported by us in our studies of older individuals (572 ± 208 pg/ml) and, as expected, CSF tau (285 ± 151 pg/ml vs. 334 ± 180 pg/ml), and ptau181 (52 ± 23 pg/ml vs. 61 ± 27 pg/ml) levels are lower (Fagan et al, 2009). The absolute plasma Aβ values cannot be compared between our various studies because different methods were used to measure these analytes. Table 1. Participant demographic characteristics, psychometric performance and biomarker values CDR 0 participants N 189 Age at LP (year) 64.7 (10.4) Age range (year) 43–89 M/F (%F) 62/127 (67%) APOE ε4–/ε4+ (% ε4+) 125/64 (34%) Selective remembering (possible score, 0–48) 31.2 (6.3) Animal naming 21.6 (5.3) Trailmaking A (# of connections/s) 0.916 (0.303) Trailmaking B (# of connections/s) 0.384 (0.151) CSF Aβ38 1228 (514) CSF Aβ40 8958 (4464) CSF Aβ42 652 (235) CSF tau 285 (151) CSF ptau181 52 (23) Plasma Aβ1–40 217 (60) Plasma Aβx–40 37 (11) Plasma Aβ1–42 193 (44) Plasma Aβx–42 25 (9) PIB MCBP 0.0931 (0.213) Fluid psychometric and PET PIB (MCBP) values are represented as means (standard deviations). CSF and plasma values are in pg/ml. PIB MCBP values are in arbitrary units (generated by Logan graphical analyses). APOE, apolipoprotein E; CDR, clinical dementia rating; CSF, cerebrospinal fluid; LP, lumbar puncture; MCBP, mean cortical binding potential; PIB, Pittsburgh compound B. Given the wide range of ages in the present cohort, we first investigated whether any of the biomarker measures correlated with age at the time of LP. As shown in Fig. 1, positive correlations were observed between age and cortical amyloid (represented by mean cortical PIB binding potential (MCBP); Mintun et al, 2006), CSF tau and ptau181 and plasma Aβ1–40. An inverse correlation was observed between age and CSF Aβ42, and no relationships between age and CSF Aβ38 or Aβ40 were found. Due to these age effects, all subsequent analyses were corrected for age. Figure 1. Cortical amyloid as detected by PET PIB and fluid biomarkers in CDR 0 participants (n = 189) as a function of age. Levels of A.. cortical amyloid are positively correlated with age in this CDR 0 cohort, B.. The level of CSF Aβ38 is not correlated with age, C.. nor is CSF Aβ40. D.. CSF Aβ42 is negatively correlated with age. E.. Positive correlations with age are observed for CSF tau, F.. CSF ptau181 and G.. plasma Aβ1–40. H.. Plasma Aβ1–42 is not correlated with age in this cohort. Download figure Download PowerPoint We next investigated whether the inverse relationship between CSF Aβ42 and cortical PIB binding that we had reported previously in a mixed cohort of mildly demented and non-demented individuals was observed in this cohort of cognitively normal individuals. Overall, 29 participants in this cohort had MCBP values greater than or equal to 0.18 whereas 160 participants had MCBPs below 0.18 (Fig 2A). In individuals with MCBP values greater than 0.18, PIB retention is visualized in the neocortex and appears qualitatively greater than background levels. We continued to observe a robust and linear relationship between CSF Aβ42 and cortical amyloid in this group of cognitively normal individuals (Fig 2B). Every participant with high PIB binding had CSF Aβ42 values <582 pg/ml; 86% had CSF Aβ42 values <500 pg/ml. A large majority (84%) of participants with low PIB binding had CSF Aβ42 values >500 pg/ml (Fig 2A). Consistent with our previous findings (Fagan et al, 2006, 2007), many of the CDR 0 participants within this broad age range had little or no cortical amyloid and high mean CSF Aβ42 levels (≥500 pg/ml) (Fig 2B). Twenty-five of the 189 CDR 0 participants displayed the typical AD biomarker phenotype in relation to Aβ, with high PIB binding and low CSF Aβ42 (Fig 2B). In many cases their PET PIB scans were indistinguishable from demented individuals with DAT (CDR > 0) (Fig 2C). In contrast to CSF Aβ42, CSF Aβ40 was not related to the presence or amount of cortical amyloid in these individuals (Fig 2D). Similarly, levels of CSF Aβ38 were not correlated with cortical amyloid load (Fig 2E), but the ratio of CSF Aβ38/Aβ42 was positively correlated with amyloid load (Fig 2F), likely due to the drop in CSF Aβ42 with amyloid deposition. Figure 2. Cortical amyloid as detected by PET PIB and its relationship to CSF Aβ in CDR 0 participants (n = 189). A.. A high percentage (84%) of participants with low PIB values (MCBP < 0.18) had high CSF Aβ42 levels (mean (SD) = 705 pg/ml (211)) whereas the vast majority of participants (86%) in the cohort who had high PIB binding (MCBP ≥ 0.18) had low CSF Aβ42 (mean (SD) = 362 pg/ml (115)). Horizontal lines represent the group means, and these means are statistically different from each other (asterisk, p < 0.0001). B.. Relationship between CSF Aβ42 levels and cortical amyloid. Most participants had low MCBP values. The vast majority (86%) of participants with MCBPs ≥ 0.18 had low CSF Aβ42 levels. These CDR 0 participants are hypothesized to have preclinical AD. The box outlined by dashed lines identifies the 28 individuals who have low cortical PIB binding (MCBP < 0.18) with low CSF Aβ42. There is a linear relationship between CSF Aβ42 and the amount of cortical amyloid although CSF Aβ42 appears to drop and then stay low as the amyloid load increases. C.. MRI (left) and PET PIB (right) images of a representative low PIB (MCBP = 0.0270) CDR 0 participant (top panel), a high PIB (MCBP = 0.7790) CDR 0 participant (middle panel), and a high PIB (MCBP = 0.7812) CDR > 0 participant (bottom panel). The amount of cortical PIB binding (yellow-red corresponds to high binding) in the high PIB CDR 0 participant and the high PIB CDR > 0 participant is comparable, whereas there is only background PIB binding (green) in white matter tracks in the low PIB CDR 0 participant. D,E.. No relationship between CSF Aβ40 (D) and CSF Aβ38 (E) levels and cortical amyloid was observed in this cognitively normal cohort (r = −0.0287, p = 0.6963; r = 0.06851, p = 0.3515, respectively). F.. A negative correlation was found between cortical amyloid and the CSF Aβ38/Aβ42 ratio. All Pearson correlation coefficients are corrected for age. n.s., not significant. Download figure Download PowerPoint Twenty-eight CDR 0 individuals showed a mismatch, appearing in the ‘lower quadrant’ of Fig 2B (whose boundaries are indicated by the square outlined in a dashed line in Fig 2B) in that they had little or no cortical PIB binding (MCPB < 0.18) but had low CSF Aβ42 (<500 pg/ml). The mean interval between LP and PIB scans for individuals in this quadrant did not differ statistically from the mean intervals of those participants in the other quadrants (i.e. low PIB/high CSF Aβ42 and high PIB/low CSF Aβ42) (p = 0.4693). In addition, this low PIB/low CSF Aβ42 group (‘lower quadrant’, n = 28) did not differ from the low PIB/high CSF Aβ42 group (‘upper quadrant’, n = 132) in the frequency of the ε4 allele of APOE (39% vs. 27%, respectively, p = 0.2049), nor did it differ from the high PIB/low CSF Aβ42 (‘PIB-positive quadrant’) group (39% vs. 62%, respectively), but it did approach statistical significance (p = 0.0854). The ε4+ frequency in the high PIB/low CSF Aβ42 (‘PIB-positive quadrant’) group did, however, differ significantly from that of the low PIB/high CSF Aβ42 (‘upper quadrant’) group (p = 0.0003). We did not observe any significant associations between quadrant membership and performance on any of the psychometric tests when adjusted for age (Selective Reminding, p = 0.2486; Animal Naming, p = 0.1209; Trailmaking A, p = 0.8561; Trailmaking B, p = 0.2817). The groups also did not differ in the percentage of self-reported presence or absence of heart disease, diabetes, history of stroke and/or TIAs, prior head trauma (with loss of consciousness) or NSAID use (all p > 0.05, Fisher's exact test). However, the low PIB/low CSF Aβ42 group (‘lower quadrant’) had a greater frequency of reported hypertension than the low PIB/high CSF Aβ42 group (‘upper quadrant’) (50% vs. 29.6%, respectively, p = 0.0469) and a greater frequency of arthritis than the low PIB/high CSF Aβ42 group (14.3% vs. 3.03%, respectively, p = 0.0323). The biological significance of these findings, if any, remains unclear but warrants further investigation. Overall, longitudinal PIB follow-up of the participants in this lower quadrant will be required to understand whether their low CSF Aβ42 represents Aβ aggregation in diffuse (PIB-negative) plaques, oligomeric forms prior to substantial fibrillar (PIB-positive) Aβ deposition or simply reflects the low end of the normal spectrum of CSF Aβ42 levels. It is interesting to note that one of the individuals in this quadrant (having no cortical PIB binding but low CSF Aβ42) has come to autopsy 2 years after LP and PIB testing. This participant was CDR 0 at the time of LP and PIB (6 months apart) but received a CDR rating of 0.5 (very mild dementia) just prior to death. Subsequent histological analysis of the brain revealed abundant diffuse but few neuritic (amyloid) plaques, (Cairns et al, 2009) consistent with the first proposed hypothesis. Despite the strong relationship between PIB binding and CSF Aβ42, we observed no relationship between cortical amyloid load and plasma levels of Aβ42, Aβx–42, Aβ40 or Aβx–40 (Fig. 3). Our previous study reported the same results in a much smaller, clinically mixed cohort. Furthermore, for the present study we used the xMAP plasma kit (Inno-Bia Plasma Aβ Forms Multiplex Assay) which generates reliable values in the lower pg/ml range required for plasma measures (Blennow et al, 2009; Lachno et al, 2009). We obtained reliable values (with low coefficients of variability) for all but five samples; these five had very low levels of Aβx–42 that were below the level of detection so they were assigned a value of 0 pg/ml. Figure 3. Cortical amyloid as detected by PET PIB and its relationship to plasma Aβ42 and Aβ40 species in CDR 0 participants (n = 189). No relationship was observed between mean cortical PIB binding and plasma A.. Aβ1–40 (r = −0.0724, p = 0.3234), B.. Aβ x–40 (r = 0.04583, p = 0.5323), C.. Aβ1–42 (r = −0.1015, p = 0.1658) or D.. Aβx–42 (r = −0.03869, p = 0.5981). Five participants had levels of plasma Aβx–42 below the level of detection so they are represented as having 0 pg/ml. All Pearson correlation coefficients are corrected for age. n.s., not significant. Download figure Download PowerPoint Importantly, analysis of this CDR 0 cohort revealed a novel pattern of increases in CSF tau (and ptau181) with increasing cortical amyloid deposition (Fig 4A,B). Elevations in CSF tau in general did not appear to occur substantially in participants with an MCBP less than 0.5 but did increase in many, but not all, participants with binding potentials 0.5 and greater. Regression analyses correcting for age revealed a linear relationship between CSF tau (and ptau181) and PIB binding (Fig 4A,B). In addition, the ratios of CSF tau/Aβ42 and ptau181/Aβ42 also increased linearly with amyloid deposition and the correlations were particularly robust (Fig 4C,D). Similar to what we observed for CSF tau and ptau181, the ratios of tau and ptau181 to CSF Aβ42 were generally not elevated until substantial PIB binding values were reached. Figure 4. Cortical amyloid as detected by PET PIB and its relationship to CSF tau and ptau181 and the ratios of CSF tau/Aβ42 and ptau181/Aβ42 in CDR 0 participants (n = 189). A linear relationship is observed between the amount of cortical amyloid and A.. the levels of CSF tau B.. the levels of CSF ptau181 C.. the ratios of CSF tau/Aβ42 and D.. the ratios of the ptau181/Aβ42. The correlations between the CSF tau(s)/Aβ42 ratios and MCBP remain significant even when the statistical outlier (high PIB, high ratio) is omitted from the analysis (tau/Aβ42, r = 0.74227, p < 0.0001; ptau181/Aβ42, r = 0.73510, p < 0.0001). All Pearson correlation coefficients are corrected for age. Download figure Download PowerPoint All participants in this cognitively normal cohort were administered a common battery of psychometric tests, including Selective Reminding (a measure of episodic memory) (Grober et al, 1988), Animal Naming (assesses semantic memory) (Goodglass & Kaplan, 1983) and a speeded visuospatial test with two parts: Trailmakings A and B (Armitage, 1946). None of the biomarker measures showed significant associations with performance on any of the psychometric tests, with the exception of negative correlations between Trailmaking A and CSF Aβ38 (r = −0.14621, p = 0.0464) and plasma Aβ1–40 (r = −0.24069, p = 0.0009). Due to the number of statistical tests conducted overall, however, some statistically significant differences could be due to chance. DISCUSSION The data presented here are part of an ongoing longitudinal study investigating fluid and imaging measures as possible antecedent (preclinical) biomarkers of AD. Importantly, they shed light on what may be the pathophysiology of the earliest events in the disease process and their relationship with CSF biomarkers. Consistent with our previous, smaller studies which included both non-demented and demented individuals (Fagan et al, 2006, 2007), we observed a robust relationship between cortical PIB binding and levels of CSF Aβ42 but not CSF Aβ40 (or Aβ38) in this large cohort of cognitively normal participants. While this relationship is linear, visual inspection of the graphs gives the impression of CSF Aβ42 levels dropping early in the disease process and staying low as the amount of cortical amyloid increases. The lack of correlation we observed between CSF Aβ38 and the amount of cortical amyloid is consistent with other studies suggesting this Aβ species does not change with dementia status (Mehta & Pirttila, 2005; Welge et al, 2009). However, these studies suggested that the ratio of Aβ38 to Aβ42, as opposed to Aβ38 alone, may have better specificity for distinguishing AD from controls or other non-AD dementias, although not all studies have reported this (Schoonenboom et al, 2005). The positive correlation we observed between cortical amyloid and the ratio of Aβ38 to Aβ42 is likely driven by the drop in Aβ42, as suggested by others (Mehta & Pirttila, 2005). We were also now able to measure, with great sensitivity and reliability, a number of Aβ species in plasma, including Aβ1–40, Aβx–40, Aβ1–42 and Aβx–42. However, the level of these species did not in any way relate to the presence or amount of amyloid in the brain. Previous studies investigating the utility of plasma Aβ species have reported mixed results. While plasma Aβ42 has been reported to be neither specific nor sensitive for a clinical diagnosis of mild cognitive impairment (MCI) or AD (Fukumoto et al, 2003), nor in predicting the probability of progression from MCI to AD (Hansson et al, 2008), other studies have reported that the ratio of plasma Aβ42/Aβ40 may be useful as an antecedent marker for identifying risk for developing cognitive impairment in cognitively normal elders (Graff-Radford et al, 2007). Others have reported alterations in the direction of change in plasma Aβ species over the course of the disease (Schupf et al, 2008). Using the same protocol as we used in the present study, a very recent study reported significant decreases in the levels of plasma Aβ42 and the Aβ42/Aβ40 ratio in those who were classified as having AD or MCI that would progress to AD (with subjects preselected based on clinical and CSF biomarker profiles) compared to those who did not have the ‘abnormal’ profile (Lewczuk et al, 2009). However, this decrease was not particularly robust, on the order of 10–15%, with great overlap between the groups. Thus, while the mechanism(s) underlying potential changes in Aβ metabolism in plasma is still unclear, our cross-sectional data indicate that Aβ42 levels in plasma do not reflect the amount of amyloid in the brain in cognitively normal individuals (nor are they related to the level of Aβ42 in the CSF, data not shown). In this large cohort we now observed a new grouping of participants; those who had low CSF Aβ42 levels in the absence of cortical PIB binding. It is unlikely that this is simply an APOE effect (Sunderland et al, 2004) since the frequency of the ε4 allele did not differ between the low PIB groups with low versus high CSF Aβ42. Instead, these data suggest that CSF Aβ42 may drop prior to amyloid becoming detectable by PIB, or this drop may reflect the presence of diffuse plaques and/or oligomeric Aβ species, consistent with the participant in the lower (low PIB/low CSF Aβ42) quadrant who has come to autopsy with abundant diffuse, but few amyloid (neuritic), plaques (Cairns et al, 2009). However, we cannot rule out the possib