Uncovering Disease Mechanisms in a Novel Mouse Model Expressing Humanized APOEε4 and Trem2*R47H

Abstract

Late-onset Alzheimer’s disease (AD; LOAD) is the most common human neurodegenerative disease, however, the availability and efficacy of disease-modifying interventions is severely lacking. Despite exceptional efforts to understand disease progression via legacy amyloidogenic transgene mouse models, focus on disease translation with innovative mouse strains that better model the complexity of human AD is required to accelerate the development of future treatment modalities. LOAD within the human population is a polygenic and environmentally influenced disease with many risk factors acting in concert to produce disease processes parallel to those often muted by the early and aggressive aggregate formation in popular mouse strains. In addition to extracellular deposits of amyloid plaques and inclusions of the microtubule-associated protein tau, AD is also defined by synaptic/neuronal loss, vascular deficits, and neuroinflammation. These underlying processes need to be better defined, how the disease progresses with age, and compared to human-relevant outcomes. To create more translatable mouse models, MODEL-AD (Model Organism Development and Evaluation for Late-onset AD) groups are identifying and integrating disease-relevant, humanized gene sequences from public databases beginning with APOEε4 and Trem2*R47H, two of the most powerful risk factors present in human LOAD populations. Mice expressing endogenous, humanized APOEε4 and Trem2*R47H gene sequences were extensively aged and assayed using a multi-disciplined phenotyping approach associated with and relative to human AD pathology. Robust analytical pipelines measured behavioral, transcriptomic, metabolic, and neuropathological phenotypes in cross-sectional cohorts for progression of disease hallmarks at all life stages. In vivo PET/MRI neuroimaging revealed regional alterations in glycolytic metabolism and vascular perfusion. Transcriptional profiling by RNA-Seq of brain hemispheres identified sex and age as the main sources of variation between genotypes including age-specific enrichment of AD-related processes. Similarly, age was the strongest determinant of behavioral change. In the absence of mouse amyloid plaque formation, many of the hallmarks of AD were not observed in this strain. However, as a sensitized baseline model with many additional alleles and environmental modifications already appended, the dataset from this initial MODEL-AD strain serves an important role in establishing the individual effects and interaction between two strong genetic risk factors for LOAD in a mouse host.

A Trem2*R47H mouse model without cryptic splicing drives age- and disease-dependent tissue damage and synaptic loss in response to plaques

“…The R47H missense mutation in TREM2 directly implicates microglial biology in LOAD, and elucidating the underlying mechanisms depends on experimental models that recapitulate the human function. Results of previous studies of mice with the TREM2 R47H missense mutation introduced via CRISPR suggested that it acts as a near-complete loss of function, recapitulating phenotypes seen in Trem2 KO mice (Cheng-Hathaway et al, 2018;Kotredes et al, 2021). However, subsequent analyses of Trem2 expression and splicing in these models identified the unexpected generation of a cryptic splice site and subsequent loss of Trem2 expression due to the synonymous codon changes introduced as part of the CRISPR repair template (Xiang et al, 2018).…”

Plcg2M28Linteracts with high fat-high sugar diet to accelerate Alzheimer’s disease-relevant phenotypes in mice

“…The copyright holder for this preprint this version posted February 12, 2022. ; https://doi.org/10.1101/2022.02.11.480032 doi: bioRxiv preprint homozygous for APOE4 and Trem2 R47H (termed LOAD1) [10]. Mice were aged to 24 months and data showed age but not genotype was the major factor driving changes in LOAD1 mice [10].…”

“…Two centers, one from Indiana University (IU), The Jackson Laboratory (JAX), University of Pittsburg (Pitt) and Sage Bionetworks, and the other from University California Irvine (UCI) have focused on incorporating genetic risk factors into C57BL/6J (B6) mice and assaying human-relevant phenotypes. Initially, the IU/JAX/Pitt group created mice double homozygous for APOE4 and Trem2 R47H (termed LOAD1) [10]. Mice were aged to 24 months and data showed age but not genotype was the major factor driving changes in LOAD1 mice [10].…”

“…Initially, the IU/JAX/Pitt group created mice double homozygous for APOE4 and Trem2 R47H (termed LOAD1) [10]. Mice were aged to 24 months and data showed age but not genotype was the major factor driving changes in LOAD1 mice [10].…”

“…Therefore, these mice provided a sensitized background for assessing additional genetic and environmental risk factors [10]. To further sensitize LOAD1 mice, additional LOAD genetic risk factors were added via CRISPR/CAS9.…”