Species: Mouse
Genes: Trem2
Mutations: TREM2 R47H
Modification: Trem2: Knock-In
Disease Relevance: Alzheimer's Disease
Strain Name: Trem2^{em2 Bwef}
Genetic Background: C57BL/6N
Availability: Available through Christian Haass.

R47H is a rare variant in TREM2 that triples the risk of Alzheimer’s disease in heterozygous carriers. To create a mouse model carrying a single copy of R47H Trem2 under the control of its natural regulatory elements, CRISPR/Cas9 was used to introduce the R47H variant into the endogenous mouse Trem2 gene (Xiang et al., 2018). Similar to other Trem2 R47H knock-in lines (Trem2 R47H KI (JAX), Trem2 R47H KI (Lamb/Landreth)), these mice show decreased expression of Trem2. Lower Trem2 expression in R47H knock-in mice has been traced to aberrant splicing of the mutant allele, which introduces a premature stop codon and could promote nonsense-mediated decay (Xiang et al., 2018). The R47H mutation does not, however, induce mis-splicing or reduce expression of human TREM2 (Xiang et al., 2018; for discussion on extrapolating findings from R47H knock-in mice to humans, see Sept 2018 news).

In this model, CRISPR/Cas9 was used to introduce an R47H point mutation and three silent mutations into the mouse Trem2 gene.

Levels of Trem2 transcripts were reduced in the brains of R47H KI mice compared with wild-type mice, in a gene dose-dependent manner; in heterozygous mice, expression of the mutant allele was selectively reduced. There was a similar gene-dose-dependent reduction in the levels of TREM2 protein in microglia isolated from the brains of R47H KI mice compared with wild-type mice. Reductions in R47H mRNA and protein were confirmed in bone-marrow-derived macrophages from these mice.

Lower Trem2 expression in R47H KI mice was traced to aberrant splicing of the mutant allele, which introduced a premature stop codon. In cell-based assays, the R47H point mutation was sufficient to cause abnormal splicing, but two of the silent mutations (GA > TC) together also caused mis-splicing, and acted synergistically with the R47H mutation to alter splicing.

Modification Details

CRISPR/Cas9 was used to introduce an R47H point mutation arginine (CGC > histidine CAC) and three silent mutations (glycine: GGG > GGT; arginine: AGA > CGA; lysine: AAG > AAA) into the mouse Trem2 gene. The silent mutations were added to aid in genotyping and increase the efficiency of gene editing.

Phenotype Characterization

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References

Research Models Citations

1. Trem2 R47H KI (JAX)
2. Trem2 R47H KI (Lamb/Landreth)

News Citations

1. Model Morass? R47H Mutation Scuttles TREM2 Expression in Mice, Not People 13 Sep 2018

Paper Citations

1. Xiang X, Piers TM, Wefers B, Zhu K, Mallach A, Brunner B, Kleinberger G, Song W, Colonna M, Herms J, Wurst W, Pocock JM, Haass C. The Trem2 R47H Alzheimer's risk variant impairs splicing and reduces Trem2 mRNA and protein in mice but not in humans. Mol Neurodegener. 2018 Sep 6;13(1):49. PubMed.

Other Citations

1. Christian Haass

Further Reading