A clinically relevant mouse model of the Trem2*R47H Alzheimer’s disease risk variant

Abstract

AbstractBackgroundThe TREM2 (triggering receptor expressed on myeloid cells 2) locus harbors numerous relatively rare variants associated with Alzheimer’s disease risk. The TREM2*R47H allele is one of the strongest genetic risk factors for developing late‐onset Alzheimer’s disease; patients exhibit both earlier onset of symptoms and rapid cognitive decline. TREM2 is expressed by microglia in the brain and plays a key role in modulating microglial activity. TREM2 expression is increased in AD and is associated with recruitment of microglia to amyloid plaques. An experimental model is therefore critical to study mechanisms of late‐onset AD, and the MODEL‐AD set out to create a clinically relevant mouse model.MethodWe used CRISPR to engineer the R47H variant into the mouse Trem2 locus; this model is available as JAX #27918. This model, as well as two other CRISPR‐generated Trem2*R47H models, display a splicing event seen in mice, but not humans, that leads to a novel splice form with a deletion of 119bp at the 5’ end of exon 2 (Xiang et al, Molecular Neurodegeneration, 2018). This leads to a decrease in ∼50% of both transcript and protein levels in brain homogenate in the mouse, indicating that the model expresses mutant Trem2 protein, but is also a hypomorph. To generate a more clinically relevant model, we have modified the existing Trem2*R47H mouse allele by using CRISPR to humanize the exon 2 splice site. Expression in these models was compared to normal Trem2 levels using both RNA‐seq and Western blot.ResultThe new model, which we are calling Trem2*R47HHSS for humanized splice site (JAX #33781), showed normal levels of expression. Relative to B6J controls, this new model expresses normal levels of each of the four transcripts. No truncated transcript was detected. Protein levels were restored to wild‐type levels in the model with the humanized splice site.ConclusionWe expect that this model Trem2*R47H model will facilitate research into the mechanisms and pathways related to TREM2 genetic risk in late‐onset AD. For more information see www.model-ad.org.