Abstract
Background: Apolipoprotein E (APOE) alleles impact pathogenesis and risk for multiple human diseases, making them primary targets for disease treatment and prevention. Previously, we and others reported an association between APOE alleles and the gut microbiome. Here, we tested whether these results are confirmed by using mice that were maintained under ideal conditions for microbiome analyses. Methods: To model human APOE alleles, this study used APOE targeted replacement (TR) mice on a C57Bl/6 background. To minimize genetic drift, APOE3 mice were crossed to APOE2 or APOE4 mice prior to the study, and the resulting heterozygous progeny crossed further to generate the study mice. To maximize environmental homogeneity, mice with mixed genotypes were housed together and used bedding from the cages was mixed and added back as a portion of new bedding. Fecal samples were obtained from mice at three-, five- and seven-months of age, and microbiota analyzed by 16S ribosomal RNA gene amplicon sequencing. APOE2/E2 and APOE2/E3 mice were categorized as APOE2, APOE3/E4 and APOE4/E4 mice were categorized as APOE4, and APOE3/E3 mice were categorized as APOE3. Linear discriminant analysis of Effect Size (LefSe) identified taxa associated with APOE status, depicted as cladograms to show phylogenetic relatedness. The influence of APOE status was tested onalpha-diversity (Shannon H index) and beta-diversity (principal coordinate analyses and PERMANOVA). Individual taxa associated with APOE status were identified by classical univariate analysis. Whether findings in the APOE mice were replicated in humans was evaluated by using published microbiome genome wide association data. Results: Cladograms revealed robust differences with APOE in male mice and limited differences in female mice. The richness and evenness (alpha-diversity) and microbial community composition (beta-diversity) of the fecal microbiome was robustly associated with APOE status in male but not female mice. Classical univariate analysis revealed individual taxa that were significantly increased or decreased with APOE, illustrating a stepwise APOE2-APOE3-APOE4 pattern of association. The Clostridia class, Clostridiales order, Ruminococacceae family and related genera increased with APOE2 status. The Erysipelotrichia phylogenetic branch increased with APOE4 status, a finding that extended to humans.Conclusions: In this study wherein mice were maintained in an ideal fashion for microbiome studies, gut microbiome profiles were strongly and significantly associated with APOE status in male APOE-TR mice. Erysipelotrichia in particular appears to increase with APOE4 in both mice and humans. Further evaluation of these findings in humans, as well as studies evaluating the impact of the APOE-associated microbiota on disease-relevant phenotypes, will be necessary to determine if alterations in the gut microbiome represents a novel mechanism whereby APOE alleles impact disease.
Gut microbiome modulates response to anti–PD-1 immunotherapy in melanoma patients
