Transgenic dCas9-DNMT3A mouse lines enable ex vivo and in vivo methylation editing with locus-dependent transcriptional effects

The number of epigenome-wide association studies linking CpG DNA methylation with disease, traits and exposures, continues to rise. Despite the rapid development of epigenome editing tools, establishing causation remains challenging, particularly in vivo. In this study, we developed and characterized three Cre-dependent CRISPR-based mouse lines that enable locus-specific DNA methylation deposition by either constitutive or inducible dCas9-DNMT3A expression. We demonstrate robust highly locus-specific DNA methylation deposition at MHC class II (H2-Ab1) and interleukin 6 (Il6) genes in bone marrow-derived myeloid cells ex vivo. Moreover, neuron-specific methylation targeting resulted in reduced cannabinoid receptor 1 (Cnr1) expression in striatal neurons in vivo. Notably, we demonstrate that the causal effect of DNA methylation on gene expression is locus-dependent, reinforcing the necessity of such editing tools for detailed understanding of the role of DNA methylation and for addressing the causality of disease-associated CpGs.