This study suggests that CSF YKL-40, a novel inflammatory biomarker for AD, is increased in AD, and, together with Aβ42, will assist in prognosis of patients and clinical trial participants who are under examination for the preclinical and early clinical stages of AD.
Having identified CSF YKL-40 as a potential AD biomarker through non-biased proteomics, we verified this finding using a commercially available ELISA, and more importantly, validated the results in a much larger, independent cohort. By including very mildly impaired (CDR 0.5) individuals who may be classified at some other institutions as having MCI, or even “pre-MCI,” as some were insufficiently impaired to meet MCI criteria, this validation cohort revealed the promise of CSF YKL-40 as a biomarker for very early stage AD. By including individuals with FTLD and PSP, albeit in small numbers, we also demonstrated that CSF YKL-40 shows promise for distinguishing AD from PSP.
By including individuals who were cognitively normal at the time of CSF collection, but subsequently developed cognitive impairment, this validation cohort also revealed the potential utility of YKL-40, coupled with Aβ42, to predict cognitive decline. It has previously been shown that ratios of CSF tau/Aβ42 and p-tau181/Aβ42 can predict conversion from cognitively normal to cognitively impaired over a 2–4 year period (19
). Here we confirm those findings in a cohort of twice the size, and show that CSF YKL-40/Aβ42 has predictive value comparable to that of these best current CSF measures. This finding is particularly notable because, whereas CSF tau is derived principally from neurons, YKL-40 appears to be secreted predominantly from astrocytes. To our knowledge, YKL-40 is the first astrocyte-derived marker shown to be useful in such a way. CSF YKL-40/Aβ42 also showed promise in predicting progression of dementia from CDR 0.5 to CDR>0.5. However, tau/Aβ42 and p-tau181/Aβ42 appear to show greater utility for predicting progression.
We also evaluated plasma YKL-40 as a potential AD biomarker. While plasma YKL-40 levels displayed a pattern of elevation in the CDR 0.5 and 1 groups similar to that observed for CSF, and plasma and CSF levels were modestly correlated, plasma YKL-40 did not show similar prognostic utility. Whether this increase in plasma YKL-40 reflects passive or active export of central nervous system (CNS)-derived YKL-40 or coincident peripheral production in response to a systemic inflammatory signal is unclear. Similar coincident elevations of CSF and serum YKL-40 levels have been reported with aneurysmal subarachnoid hemorrhage (28
) and multiple sclerosis (12
). However, in the setting of CNS infection, CSF levels of YKL-40 appear to rise without a concomitant increase in serum levels (29
), suggesting that YKL-40 produced in the brain does not influence serum/plasma levels. Data to address the converse- whether YKL-40 produced in the periphery can influence CSF levels- have not yet been reported. This issue is important to assess in future studies because peripheral inflammatory and neoplastic conditions are not uncommon within populations most likely to be screened for AD.
To examine its role in AD and to identify potential sources of CSF YKL-40, we immunohistochemically double-labeled human AD brain tissue for YKL-40 and other cell-specific markers, and observed YKL-40 in a subset of plaque-associated astrocytes and in rare white matter neurons. These results should help to clarify the origins of CSF YKL-40, which have been controversial among the small number of relevant studies (29
). Additionally, the pattern of expression within a subset of plaque-associated astrocytes may account for the positive correlation we observe between CSF YKL-40 and cortical amyloid load (); as amyloid plaque burden increases, so does the amount of plaque associated-astrocyte activation, and likely, the amount of CSF YKL-40. It may also account for the lack of correlation we observe between CSF YKL-40 and CSF Aβ42, and for the relatively equal levels of CSF YKL-40 between CDR 0.5 and CDR 1 groups; once plaque formation commences, which is estimated to occur ~15 years prior to cognitive decline (4
), CSF Aβ42 remains at a low steady state (17
), so no correlation with YKL-40 would be expected. Likewise, amyloid burden appears close to its maximal extent once cognitive decline begins (17
), so plaque burden and CSF YKL-40 levels might be expected to be similar in CDR 0.5 and CDR 1 groups. More importantly, these results implicate YKL-40 in the astrocytic neuroinflammatory response to fibrillar Aβ deposition that appears to play a role in AD pathogenesis (36
What induces YKL-40 expression in the presence of AD pathology, and how increased YKL-40 expression may influence the disease process are unknown. In models of peripheral inflammation such as asthma and arthritis, tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) appear to stimulate YKL-40 synthesis in macrophages and chondrocytes (39
). Since TNF-α and IL-1β are implicated in AD neuroinflammation, it is reasonable to hypothesize that astrocytic expression of YKL-40 may be similarly induced. Given that TNF-α and IL-1β can cross the blood brain barrier, it is also reasonable to hypothesize that YKL-40 levels in plasma and CSF might be modulated by systemic or central inflammation. Defining the factors required to induce YKL-40 expression in astrocytes will be an important first step in understanding the role of YKL-40 in AD and, more generally, in the CNS.
Defining the targets of YKL-40 in the brain is also critically important for understanding its role in AD. In the periphery, YKL-40 can reportedly stimulate connective tissue cell growth (41
); modulate the effects of inflammatory cytokines in fibroblasts (43
); bind collagen and influence its fibrillogenesis (44
); stimulate endothelial cell migration (45
); modulate vascular smooth muscle cell adhesion and migration (46
); support antigen-induced Th2 inflammatory responses (47
); and stimulate alveolar macrophages to release metalloproteinases and proinflammatory and fibrogenic chemokines (40
). In the brain, YKL-40 is reported to release extracellular matrix-bound bFGF (29
). Clearly, further study of YKL-40 in AD and, more generally, within the CNS and periphery, is warranted to define its pathophysiological role(s).
This study identifies YKL-40 as a novel astrocyte-derived CSF biomarker that can distinguish groups of AD and control subjects and predict risk of developing dementia among cognitively normal subjects. Nevertheless, like all AD biomarker candidates to date, YKL-40 is likely to have less value when applied in isolation, and, alone, will be insufficient to provide definitive information for an individual patient. While significant differences in mean CSF and plasma YKL-40 levels exist between CDR 0 and CDR 0.5, and CDR 0 and CDR 1 groups, the ranges of YKL-40 values among the groups show considerable overlap. This overlap may stem from several sources. The greatest contribution is likely due to the inclusion of individuals with asymptomatic (preclinical) AD pathology in the CDR 0 group; AD neuropathology is present in ~25% of non-demented individuals age ≥75 years (48
). It is also possible that different alleles of the CHI3L1
gene may influence baseline or reactive levels of YKL-40 protein expression, or that members of this cohort may be afflicted by other diseases that affect CSF YKL-40 levels. For example, elevated CSF YKL-40 has been reported in the setting of other CNS pathologies (12
); however, most of these conditions would be easily distinguishable from early AD on the basis of clinical assessment. It is important to note that the overlap observed for CSF YKL-40 is comparable to that seen for the best biomarkers identified to date, CSF Aβ42 and CSF tau () (50
). The best use of YKL-40 may be in a panel of biomarkers that provide complementary information to guide diagnosis, prognosis, clinical trial design, and treatment decisions. Indeed, in other work stemming from this 2-D DIGE study, stepwise logistic regression analyses indicate that YKL-40, as part of a panel with other CSF biomarkers, contributes additional sensitivity and specificity for discriminating mildly demented individuals from cognitively normal individuals (Perrin RJ, Craig-Schapiro R, Holtzman DM et al. 2010, in preparation). Additionally, YKL-40 may confer specificity to a panel by distinguishing PSP or other illnesses from AD, as our early results suggest. It will be of interest in future studies to confirm these results and to evaluate CSF YKL-40 levels in the setting of additional dementing conditions. Perhaps more importantly, YKL-40, for its own part, might contribute diagnostic sensitivity for early cognitive impairment, prognostic information for risk of cognitive decline in normal and very mildly impaired individuals, and, more fundamentally, a direct estimate of neuroinflammation, which tau and Aβ42 do not provide.