Associations of Cerebrospinal Fluid Orexin-A, Alzheimer Disease Biomarkers, and Cognitive Performance

Objective: Cerebrospinal fluid (CSF) orexin-A has been suggested to be a biomarker of Alzheimer disease (AD). In both cognitively unimpaired healthy older adults and individuals with symptomatic AD, CSF orexin-A is positively associated with CSF Aβ42, p-tau181, and total tau (t-tau) concentrations. However, a recent systematic review and meta-analysis did not support differences in orexin-A between AD and controls. In this study, we tested the association between CSF orexin-A concentrations, AD biomarkers, and cognitive performance in older adults with and without symptomatic AD. Methods: Two hundred and seventy community-dwelling older adults underwent standardized cognitive assessments, sleep monitoring with a single-channel electroencephalography test, one night of home sleep apnea testing, biofluid and imaging AD biomarker measurement within 1 year of sleep monitoring, and APOE genotyping. Plasma and CSF AD biomarkers were measured by immunoassay or mass spectrometry. CSF orexin-A was measured by radioimmunoassay. Results: CSF orexin-A levels did not differ by amyloid positivity, cognitive status, or AD stage. However, CSF AD biomarkers (Aβ40, Aβ42, and t-tau) were positively associated with CSF orexin-A levels even after correction for multiple comparisons. CSF orexin-A was not associated with any measure of cognitive performance. Interpretation: This study showed that CSF orexin-A is associated with multiple CSF AD biomarkers, but not with AD pathology or cognitive performance. We hypothesize that this is due to similar mechanisms of production/release of these proteins with sleep–wake activity. Future studies measuring other forms of orexin peptides, such as orexin-B, may provide evidence for orexin as a marker for AD.