P4HA1 and Alzheimer disease: As shown in Fig. 3D, comparison of 1,838 keratinocyte DEGs and 115 gut microbiota-related genes ("gutgenes") identified seven key genes (AKR1C2, GALE, GGH, NR4A1, P4HA1, PLA2G4B, TYMS), which are both linked to gut microbial metabolites and differentially expressed in AD keratinocytes.