Alongside this ray of hope comes news reminding the field how unruly these biomarkers can be when it comes to reproducing their measurements reliably across labs and protocols. Though CSF levels of Aβ42, tau, and phospho-tau can distinguish patients with AD from those without symptoms or with other types of dementia in the hands of several independent groups by now, absolute biomarker measurements differ significantly among labs, complicating data analysis in multicenter studies. A multi-institutional collaboration led by Marinus Blankenstein and Niek Verwey of VU University Medical Center, Amsterdam, has tried to address this issue more formally by comparing AD biomarker measurements made by 20 centers worldwide. The researchers sent three CSF samples—one with high tau and phospho-tau levels, one with low Aβ1-42, and one with a normal biomarker profile—to 13 labs in 2004. In 2008, they distributed the same samples to 18 labs, 11 of which had also participated in 2004.
The main gist of the findings, reported online 2 April in the Annals of Clinical Biochemistry, is that the sites differed considerably in their measurements of all three biomarkers tested—particularly Aβ1-42, which had more than 22 percent variability among centers. The team found substantial variation even when comparing biomarker measurements made by the same lab in 2004 and 2008. Variability must drop to less than 10 percent before scientists can reliably compare biomarker measurements from different sites to determine a reference range for AD that all centers can use, Verwey wrote in an e-mail to ARF (see full comment below).
For their part, Alzheimer’s Disease Neuroimaging Initiative (ADNI) scientists have analyzed CSF samples from several hundred subjects in the U.S. and Canada and defined, in a recent paper (Shaw et al., 2009 and ARF related news story), threshold values for CSF Aβ and tau, as well as standardized protocols for measuring protein levels and handling samples. (See also in-depth ARF ADNI series). Standardization may become even more crucial as new protein detection methods—for example, an ELISA that differentiates between oligomeric and monomeric forms of Aβ (Xia et al., 2009 and ARF related news story)—come onto the scene.
The kind of assay validation done by ADNI has not yet been done for blood Aβ measurements, as a recent study shows (Okereke et al., 2009). Researchers led by Francine Grodstein at Brigham and Women’s Hospital, Boston, spiked plasma samples with known amounts of Aβ. They sent the spiked samples to various U.S. labs that used five different protocols to measure plasma Aβ40 and Aβ42. Though assay reliability and Aβ stability after processing delay was encouraging, the study was disappointing in its recovery data. Recovery rates for Aβ40 ranged from -24.3 to 44.2 percent, and for Aβ42 from 17.1 to 60.7 percent. These findings make “comparisons of absolute Aβ values across studies inaccessible at this time,” wrote lead author Olivia Okereke, also of Brigham and Women’s Hospital, in an e-mail to ARF. “Clearly, an important next step for the field is assay standardization work, as has been accomplished recently for some of the lipid and inflammatory biomarkers in cardiovascular disease.”
While studies of the older CSF markers Aβ and tau proceed apace, scientists have identified what could turn out to be a new one. In this month’s Archives of Neurology, a team led by Greg Cole at the University of California, Los Angeles, has detected reduced levels of SORLA in CSF of AD patients.
SORLA, a transmembrane neuronal sorting protein that reduces Aβ production, emerged several years ago as a genetic risk factor for late-onset AD (Rogaeva et al., 2007 and ARF related news story). Scientists have detected decreased SORLA expression in LOAD brain tissue (Scherzer et al., 2004; Zhao et al., 2007), and a more recent study
has linked SORLA gene variants with reduced CSF Aβ42 in AD (Kölsch et al., 2008). For his part, Cole has shown that putting mice on a diet rich in omega-3 fatty acids leads to increased SORLA expression, suggesting that SORLA might be involved in mediating the Aβ-lowering effects of this special diet (Ma et al., 2007).
Whether this increase in SORLA expression occurs in people has not been tested. “But if you could see the levels of the protein in CSF, then you would be able to ask that question,” Cole told ARF. Given that SORLA gets cleaved near its membrane C-terminus, his team reasoned that the soluble N-terminal piece is secreted into the CSF and could be detected there. Analyzing postmortem human CSF samples, first author Qiu-Lan Ma and colleagues confirmed this hunch. They were able to detect SORLA in the CSF samples, and found that its expression was reduced in autopsy-confirmed AD cases. Furthermore, they showed that SORLA levels in CSF correlated strongly with Aβ42 concentrations.
“Taken together, these observations support the hypothesis that [SORLA] is directly involved in the pathogenesis of Alzheimer disease,” wrote Richard Mayeux, Columbia University, New York, and Peter St. George-Hyslop, University of Toronto, Canada, in an editorial accompanying the study. “More importantly, it is clear that a better understanding of subcellular trafficking of APP as well as the various functional roles of SORL1 may point to a novel therapeutic strategy that has not yet been considered.”—Esther Landhuis.
Bateman RJ, Siemens E, Mawuenyega KG, Wen G, Bronwing KR, Sigurdson WC, Yarasheski KE, Friedrich SW, DeMattos RB, May PC, Paul SM, Holtzman DM. A gamma-secretase inhibitor decreases amyloid-beta production in the central nervous system. Annals of Neurology, 2009 Apr 10. Abstract
Verwey NA, van der Flier WM, Blennow K, Clark C, Sokolow S, De Deyn PP, Galasko D, Hampel H, Hartmann T, Kapaki E, Lannfelt L, Mehta PD, Parnetti L, Petzold A, Pirttila T, Saleh L, Skinningsrud A, Swieten JC, Verbeek MM, Wiltfang J, Younkin S, Scheltens P, Blankenstein MA. A worldwide multicentre comparison of assays for cerebrospinal fluid biomarkers in Alzheimer's disease. Ann Clin Biochem. 2009 Apr 2. Abstract
Ma Q-L, Galasko DR, Ringman JM, Vinters HV, Edland SD, Pomakian J, Ubeda OJ, Rosario ER, Teter B, Frautschy SA, Cole GM. Reduction of SorLA/LR11, a Sorting Protein Limiting Beta-Amyloid Production, in Alzheimer Disease Cerebrospinal Fluid. Arch Neurol. Apr 2009;66(4):448-457. Abstract
Mayeux R, St. George-Hyslop P. Brain Traffic: Subcellular Transport of the Amyloid Precursor Protein. Arch. Neurol. Apr 2009;66(4):433-434. Abstract