APOEe4.Trem2*R47H Mice Show Changes in Alzheimer’s Disease-Relevant Processes in the Absence of Amyloid Plaques

Abstract

Late-onset Alzheimer’s disease (LOAD) is the most common human neurodegenerative disease. Legacy amyloidogenic mouse models have been useful for understanding disease progression, however in the face of failing human trials more focus on disease translation with new mouse strains that better model human Alzheimer’s disease (AD) is required. MODEL-AD (Model Organism Development and Evaluation for Late-onset AD) groups are identifying and integrating disease-relevant, humanized gene sequences from public databases to create more translatable mouse models for therapy development. Mice expressing strong genetic risk factors for LOAD, APOEe4 and Trem2*R47H, were extensively aged and assayed using a multi-disciplined phenotyping approach associated with and relative to human AD pathology. Behavioral, transcriptomic, metabolic, and neuropathological assays identified sex and age as the main sources of variation between genotypes including age-specific enrichment of AD-related processes in the absence of mouse amyloid plaque formation. These data provide an important, baseline understanding of the individual effects and interaction between two strong genetic risk factors for LOAD. These two alleles together form a sensitized, background strain (B6.APOE4.Trem2*R47H, which we have termed ‘LOAD1’) necessary to examine how important underlying risk factors interact with any subsequent genetic or environmental cues to drive pathology.

In vivovalidation of late-onset Alzheimer’s disease genetic risk factors

“…In this study, we have focused on introducing coding variants on a LOAD1 background (20), aged the mice to middle age (12 months), and characterized the animals using a gene expression panel developed for rapid comparison to recent human study results (21). In future work we will extend our approach to model candidate noncoding variants at LOAD genetic loci without strong candidate coding SNPs, humanizing loci and regulatory regions when necessary (Benzow K, et al, this issue).…”

“…Prioritization and construction of the APOE and TREM2 variants in the LOAD1 strain were previously discussed (20). Late-onset variants were selected based on human genetic association, predicted pathogenicity, conservation with mouse homolog, and allele frequency.…”

“…33 Subsequently, we added gene annotation of the human APOE gene into the mouse gene annotation file. Additionally, we have also introduced annotation for novel Trem2 isoform in mouse gene annotation file (GTF file), that is identical to primary transcript but truncated exon2 by 119 bp from its start position (20). Afterward, a STAR index was built for this chimeric mouse genome sequence for alignment, then STAR aligner output coordinate-sorted BAM files for each sample mapped to the chimeric mouse genome using this index.…”

“…Variants were introduced using a knock-in approach to avoid known issues with transgenic models (11,(17)(18)(19). To potentially enhance disease-relevant outcomes, variants were created on a more LOAD-susceptible genetic background expressing humanized APOE with the ε4 variant and the R47H mutation in Trem2, two of the strongest genetic risk factors for LOAD (20). The effects of each variant were assessed by gene expression changes in aging male and female brains using a newly developed transcriptomics panel (21), representing key LOAD-associated changes in clinical AD samples (22).…”

Genetically modified mice for research on human diseases: A triumph for Biotechnology or a work in progress?

“…However, there are now some mouse models of late onset AD which may move the therapeutic field onto newer treatment strategies (355). Some of these new models show aspects of both neuropathological and neurobehavioural phenotypes of AD (356,357), while others have shown only the neuropathological phenotype (358), and will require an evaluation of the behavioural phenotype, which will lead to the next generation of transgenic mouse models.…”