Genetic insights into the association between inflammatory bowel disease and Alzheimer's disease

Myeloid cells, including monocytes, macrophages, and microglia, play major roles in innate and adaptive immune responses. Alzheimer's disease (AD) and inflammatory bowel disease (IBD) susceptibility loci are both enriched for genes expressed in myeloid cells, so we assessed whether these myeloid pathways may be shared. Leveraging genome-wide association study results, we investigated the causal effect of IBD (including ulcerative colitis and Crohn's disease) variants on AD and its endophenotypes. Microglia and monocyte expression Quantitative Trait Locus (eQTLs) were used to examine the functional consequences of IBD and AD variants. Our results revealed that the sets of genes and pathways implicated in AD and IBD susceptibility are largely distinct. Specifically, AD loci were enriched for microglial eQTLs, while IBD loci were enriched for monocyte eQTLs. Nonetheless, genetically determined IBD was associated with a modest protective effect against AD (p < 0.03), whereas CD susceptibility was linked to a modest increase in amyloid accumulation (β = 7.14, p = 0.02) and AD risk. UC susceptibility, on the other hand, was associated with increased TDP-43 deposition (β = 7.58, p value = 6.11 × 10^-4). Thus, the relationship between gastrointestinal inflammatory diseases and AD is complex, but there is evidence for a modest role of IBD susceptibility on AD risk that could yield valuable therapeutic insights.