# APOE genotype and sex drive microbiome divergence after microbiome standardization in APOE-humanized mice

## TL;DR

This study shows how APOE genotype and sex influence gut microbiome changes over time in mice, revealing potential therapeutic targets for APOE-related diseases.

## Contribution

First data on inherited microbiome divergence driven by APOE genotype and identification of genotype-specific bacterial species.

## Key findings

- APOE2 females and males showed increased anti-inflammatory bacteria and decreased Turicibacteraceae at 6 months.
- Microbiome divergence in homozygous mice was significant based on sex and APOE genotype.
- Standardized microbiomes in APOE-humanized mice revealed inherited divergence at species and strain levels.

## Abstract

The APOE4 allele is the greatest known genetic factor for sporadic or late-onset Alzheimer’s Disease (LOAD). Gut microbiome (GMB) dysbiosis can lead to poorer outcomes in disease. The intersection of sex, APOE genotype, inflammation, and gut microbiota is incompletely understood. Previous studies in humans and humanized APOE mice have demonstrated APOE-genotype-specific differences in the GMB. However, most of these studies were unable to resolve bacteria to the species level. It remains unclear how GMB changes with age and sex in the context of APOE genotype. In this study, humanized male mice with either APOE 2, 3, or 4 genotype were bred with the same two C57BL/6J sisters to standardize microbiomes across lines and monitor divergence based on APOE allele. Stool samples were collected at breeder set up and from the heterozygous (F1) and homozygous (F2) generations at wean and 6 months old. Stool was assessed via shallow shotgun sequencing to enable species and strain-level taxonomic resolution. The heterozygous pups’ microbiome resembled each other at wean across all genotypes. However, the heterozygous pups and their homozygous offspring continued to diverge, particularly the APOE2 females. In homozygous mice, the GMB demonstrated significant divergence at 6 months of age based on sex and APOE genotype. In comparison to their APOE3 and APOE4 counterparts, APOE2 females and males demonstrated an increased quantity of bacteria associated with anti-inflammatory profiles, including in the Lachnospiraceae family (Lachnospiraceae bacterium UBA3401) and decreased quantities in the Turicibacteraceae family (higher levels are associated with LOAD).

The APOE4 allele is implicated as a significant risk factor for many diseases, including cardiovascular disease (responsible for more deaths than any other disease) and sporadic or late-onset Alzheimer’s Disease (accounts for an estimated 60%–80% of all dementia cases). It is known that the gut microbiome (GMB) is affected by different genotypes and disease states. Mouse model studies have environmental and genetic controls, allowing a specific gene to be studied. This study aims at discovering key GMB species differences allowing for future therapeutic targets. The GMB of the experimental mice was standardized, and genotype and sex-specific divergence was observed with species and even strain level taxonomic resolution. Reported here are the first data demonstrating GMB divergence over time driven by APOE genotype from an inherited source and the first data to identify APOE genotype-specific bacteria species that may serve as therapeutic targets in APOE-driven disease.