Blood tests in development for Alzheimer’s disease rely on Aβ and phosphorylated tau, markers of its hallmark pathologies. While powerful, these markers are only accurate in about 90 percent of cases. Now, a new study suggests that a broad protein panel can do better, and allow clinicians to determine disease stage. In the May 25 Alzheimer’s and Dementia, researchers led by Nancy Ip at the Hong Kong University of Science and Technology debuted a 19-protein panel that detected clinically diagnosed AD in two small cohorts with 97 percent accuracy. The panel includes several proteins that change with disease stage, allowing it to distinguish mild from severe dementia. Ip believes this set of diverse proteins might hold clues to underlying biological processes and mechanisms of disease as well.
- A panel of 19 plasma proteins distinguishes AD from controls with 97 percent accuracy.
- It also differentiates mild and severe dementia.
- It is not yet clear if the panel is specific for AD.
Commenters agreed the panel has potential. “This is a very nice, first-step study that should be followed up in larger, independent samples,” Sid O’Bryant at the University of North Texas Health Science Center in Fort Worth wrote to Alzforum (full comment below). Pieter Jelle Visser and Betty Tijms at Amsterdam University Medical Center suggested comparing the panel to traditional CSF and PET biomarkers of AD, as well as testing its ability to detect preclinical disease. “A number of validation studies would be needed in order to further understand the clinical utility of this panel,” they wrote to Alzforum (full comment below).
Several groups have attempted to define plasma biomarkers of AD over the years, but these typically have not held up in replication studies (e.g., Mar 2014 news; Feb 2016 news). Among those efforts, a panel of 18 proteins identified by researchers in Tony Wyss-Coray’s lab at Stanford University and a set of four proteins found by William Hu and colleagues at Emory University, Atlanta, garnered some attention, but neither finding has been reproduced (Oct 2007 news; Aug 2012 news).
To get more robust results, Ip turned to a relatively new methodology, proximity extension assay (PEA). In this approach, plasma proteins are detected by pairs of antibodies that recognize different epitopes on the same protein. Each antibody is linked to a complementary DNA strand, one being longer than the other. When the two antibodies bind their target protein, the strands hybridize, and DNA polymerase elongates the short strand, completing a unique “bar code” for each protein target. Lastly, a quantitative PCR step amplifies that signal (Lundberg et al., 2011). This assay has the advantage of being able to detect proteins across a broad concentration range, from 0.01 pg/mL to 1 mg/mL, Ip noted. This is important because many proteins are present in the blood in very tiny quantities, and older methods such as ELISAs cannot detect them. PEA is also suitable for high-throughput studies, assessing roughly 100 different proteins at a time.
With this method, first author Yuanbing Jiang profiled 1,160 proteins in the blood of 106 Alzheimer’s patients and 74 age-matched controls. All participants were seen at the Hong Kong clinic, and AD was diagnosed by clinical criteria. Excluding the known AD biomarkers—Aβ42/Aβ40, total tau, and NfL—the authors found 429 plasma proteins that were more, or less, abundant between AD patients and controls. Many of these differences were in line with previous work, with this set including three proteins from Wyss-Coray’s panel and one from Hu’s, and showing good agreement with a recent BioFINDER plasma protein study (Whelan et al., 2019).
The authors clustered these 429 proteins into 19 distinct biological processes, for example cell adhesion, extracellular matrix disassembly, and apoptosis, and then selected the protein in each group that changed most dramatically in AD. These 19 “hub proteins” formed the new diagnostic panel.
The panel detected AD status in the cohort with an area under the curve (AUC) of 0.98. In an independent cohort of 36 AD patients and 61 controls, the panel identified AD with an AUC of 0.97. Notably, this accuracy is higher than that of plasma Aβ42/Aβ40, total tau, and NfL, which have a combined AUC of 87 percent, and about equal to the accuracy of plasma p-tau181 and p-tau217, which have AUCs of 0.98 and 0.96, respectively.
Commenters said the data are valuable, but cautioned that “overfitting,” i.e., assessing a diagnostic in the same cohort used to determine it, can result in misleadingly high accuracy. They also noted that the replication cohort was small, and only 16 of the 19 proteins repeated in this group. “The AUCs … are impressive but maybe slightly over-optimistic, and warrant further validation in biomarker-verified AD,” Sebastian Palmqvist at Lund University, Sweden, wrote to Alzforum, expressing the common view (full comment below).
Others pointed out that because AD status in these cohorts was determined clinically, it is unclear if the panel is specific for Alzheimer’s or detects dementia more generally. John Ringman at the University of Southern California, Los Angeles, noted that if the 19-protein panel picks up non-AD forms of dementia as well, that may explain why it appeared to have higher diagnostic accuracy than traditional AD biomarkers (comment below).
However, because the protein set reflects broader biology than do the classic AD biomarkers, it may convey additional information than just the presence or absence of disease. Ip and colleagues found that seven of the 19 proteins changed with disease stage, allowing the panel to separate mild from severe dementia. Three of these proteins, NELL1, centrin 2, and keratin 14, were altered even in people with relatively little cognitive impairment, and were even worse in severe dementia. NELL1 affects cell growth, centrin 2 is a structural protein, and keratin 14 is part of the extracellular matrix. Three others, the tyrosine kinase LYN, protein kinase C theta, and the receptor LIF-R, were altered in people with moderate cognitive impairment, and did not further change in severe disease. The seventh protein, kallikrein-related peptidase 4, was only altered in people with severe cognitive impairment. “Dysregulation of different tissues, cells, and biological processes may be involved during distinct stages of the disease,” Ip told Alzforum.
Others agreed that the use of multiple proteins could have added value. “Biomarker panels that reflect multiple aspects of AD pathophysiology have the potential to expand our understanding of AD heterogeneity and staging,” Nick Seyfried and Erik Johnson at Emory University wrote to Alzforum (full comment below).
In future work, Ip will investigate whether the panel can distinguish subtypes of AD, and whether it differentiates AD from other neurodegenerative diseases such as Parkinson’s. She will also conduct longitudinal studies to determine how these plasma proteins change with aging and disease progression.—Madolyn Bowman Rogers
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