Principles Governing DNA Methylation during Neuronal Lineage and Subtype Specification

Although comprehensively described during early neuronal development, the role of DNA methylation/demethylation in neuronal lineage and subtype specification is not well understood. By studying two distinct neuronal progenitors as they differentiate to principal neurons in mouse hippocampus and striatum, we uncovered several principles governing neuronal DNA methylation during brain development. (1) The program consists of three stages: an initial genome-wide methylation during progenitor proliferation is followed by loss of methylation during the transition of regional progenitors to "young" hippocampal/striatal neurons, which is then reversed by gain in methylation during maturation to subtype-specific neurons. (2) At the first two stages, gain and loss of methylation are limited to CpGs, whereas during the third maturation stage, methylation also occurs at non-CpG sites in both lineages. (3) Methylation/demethylation, similar to transcription, are initially highly similar in the two lineages, whereas diversification in methylation and transcription during maturation creates subtype-specific methylation differences. (4) Initially, methylation targets all genomic locations, whereas later, during early and late differentiation, the preferred targets are intronic/intergenic sequences with enhancer-like activity. (5) Differentially methylated genes are enriched in sequential neurodevelopmental functions (such as progenitor proliferation, migration, neuritogenesis, and synaptic transmission); upregulated genes represent current and consecutive stage-specific functions, and downregulated genes represent preceding functions that are no longer required. The main conclusion of our work is that the neuronal methylation/demethylation program is predominantly developmental with minimal lineage specificity, except in the final stage of development when neuron subtype-specific differences also emerge.

Our work is the first to describe a set of relatively simple rules that govern DNA methylation and demethylation in neuronal development in vivo. By dividing neurodevelopment to three major stages and applying rules to each of them, we created a matrix that comprehensively describes DNA methylation/demethylation events in two neuronal lineages, with a total of 10 cell types spanning the entire neurodevelopment. Beyond increasing our understanding of the epigenetic regulation of normal development, our work will be useful in deciphering how environmental perturbations, such as gestational toxins, drugs, stress, infection, and offspring neglect/maltreatment, interfere with the developmental methylation program.