Genetic predisposition to high circulating levels of interleukin 6 and risk for Alzheimer's disease. Discovery and replication

IMPORTANCE: Aging is accompanied by immune dysregulation, which has been implicated in Alzheimer's disease (AD) pathogenesis. Individuals who are genetically predisposed to elevated levels of proinflammatory mediators might be at increased risk for AD. OBJECTIVE: To investigate whether genetic propensity for higher circulating levels of interleukin 6 (IL-6) is associated with AD risk. DESIGN: We analyzed data from the Hellenic Longitudinal Investigation of Aging and Diet (HELIAD). Mean follow-up was 2.9 (SD, 0.8) years. Baseline assessment was from 11/2009 to 11/2016, and cognitive follow-up from 01/2013 to 07/2019. Associations of interest were also examined in the UK Biobank (UKB) for replication purposes (mean follow-up was 12.9 (SD, 2.4) years; baseline assessment was from 12/2006 to 10/2010). SETTING: Population-based study. PARTICIPANTS: The HELIAD sample included 622 participants ≥65 years of age without baseline dementia or amnestic mild cognitive impairment (aMCI-the prodromal stage of AD). The UKB sample included 142,637 participants ≥60 years of age without prevalent dementia. EXPOSURES: Genetic predisposition to elevated circulating levels of IL-6 was estimated using a polygenic risk score (PRS), calculated based on the summary statistics of a current GWAS meta-analysis. MAIN OUTCOMES AND MEASURES: AD and MCI diagnoses were based on standard clinical criteria [HELIAD], or hospital records and death registry data [UKB]. Associations with AD or aMCI incidence [HELIAD] and AD incidence [UKB] were examined with Cox regression models. RESULTS: In HELIAD, mean age was 73.4 (SD, 5.0) years; 363 (58%) women. An increase in IL-6 PRS by 1 standard deviation unit (SDU) was associated with up to a 43% increase in the risk for incident AD/aMCI (HRGWAS significance threshold of 0.01, 1.43 [95%CI, 1.14 - 1.80]). In UKBB, mean age was 64.2 (SD, 2.8) years; 73,707 (52%) women. A 1 SDU increase in IL-6 PRS was associated with up to an 8% increase in the risk for incident AD (HRGWAS significance threshold of 0.2, 1.08 [95%CI, 1.04 - 1.12]). CONCLUSIONS AND RELEVANCE: Genetic predisposition to higher circulating levels of IL-6 was associated with an increased risk for AD, supporting the role of IL-6-related pathways in AD pathogenesis, and suggesting that genetic predisposition to proinflammatory states might trigger or accelerate AD-related neuropathology.