Background: An increase in YKL-40 levels seems to correlate with disease severity and poor prognosis in many diseases, including several neurodegenerative diseases such as multiple sclerosis, amyotrophic lateral sclerosis, and Alzheimer's disease (AD). Specifically, YKL-40 protein is increased in mild AD with respect to controls, both in cerebrospinal fluid (CSF) and plasma.
Objective: We hypothesize that subjects in the preclinical (Pre-AD) and prodromal (Prod-AD) stage of AD could already present an increase in CSF YKL-40 levels with respect to healthy controls and idiopathic REM sleep behavior disorder (iRBD) subjects, included as a control group of a distinct neurological disease.
Methods: We measured CSF YKL-40 levels using a commercial ELISA kit in a cohort of 95 subjects, consisting of controls (n = 43), Pre-AD (n = 18), Prod-AD (n = 22), and iRBD (n = 12) subjects. We explored for possible correlations of YKL-40 levels with demographic characteristics, a wide battery of neuropsychological tests, and the AD CSF biomarkers: amyloid-β42 (Aβ42), total-tau protein (t-tau), and phosphorylated-tau protein (p-tau).
Results: We detected statistically significant differences between Prod-AD patients and controls. YKL-40 levels showed a significant correlation with t-tau and p-tau levels in the predementia AD continuum and the Pre-AD group. We also observed significant correlations with the MMSE, FCSRT, and M@T tests within the AD continuum, but not in iRBD subjects.
Conclusion: Our data suggest that CSF YKL-40 levels, although not useful as a diagnostic marker for Prod-AD, may be a valuable marker to detect early physiopathological changes potentially linked with the neurodegenerative process.
Keywords: Alzheimer's disease; YKL-40; amyloid-$\beta_{42}$; cerebrospinal fluid; correlation; neuropsychological tests; preclinical; tau protein.