Sex difference in CHI3L1 expression levels in human brain aging and in Alzheimer's disease

Several genetic sexual dimorphisms have been identified in animal and human brains, which may form a neural basis for sex-specific predisposition to neurological diseases. In the last years, clinical studies have observed that Alzheimer's disease (AD) disproportionately affects women compared with men. Chitinase-3-Like 1 protein (CHI3L1) has been frequently investigated in body fluids as a surrogate marker of neuroinflammation in AD and other neurological disorders. Nevertheless, the sex-related differences in CHI3L1 expression in the human brain has not yet been investigated. Here we aimed to evaluate the specificity of increase of CHI3L1 in five brain regions (cerebellum, dorsolateral prefrontal cortex, prefrontal cortex, hippocampus, and visual cortex) of male and female controls during normal aging, as well as in AD patients. We selected ten microarray datasets from NCBI, representing normal aging (n = 1290) and AD (n = 992), and stratified the brain specimens according to age, gender and brain region. The expression levels of CHI3L1 were correlated with age and gender. Female control brain specimens showed higher CHI3L1 expression than male brains. The expression differences between men and women were most obvious in older subjects. The expression analysis of CHI3L1 in the different brain regions of AD subjects also showed sex differences; females with AD had greater expression in the cerebellum than males. Notably, sex-associated CHI3L1 expression differences in hippocampus disappeared in AD. These findings demonstrate that the expression of CHI3L1 in the brains of cognitively unimpaired subjects and AD patients is closely linked to age and sex, which was most obvious in the cerebellum. Further studies are needed to confirm our results.