Suppression of DNA double-strand break formation by DNA polymerase β in active DNA demethylation is required for development of hippocampal pyramidal neurons

Genome stability is essential for brain development and function, as de novo mutations during neuronal development cause psychiatric disorders. However, the contribution of DNA repair to genome stability in neurons remains elusive. Here, we demonstrate that the base excision repair protein DNA polymerase β (Polb) is involved in hippocampal pyramidal neuron differentiation via a TET-mediated active DNA demethylation during early postnatal stages using Nex-Cre/Polb^fl/fl mice of either sex, in which forebrain postmitotic excitatory neurons lack Polb expression. Polb deficiency induced extensive DNA double-strand breaks (DSBs) in hippocampal pyramidal neurons, but not dentate gyrus granule cells, and to a lesser extent in neocortical neurons, during a period in which decreased levels of 5-methylcytosine and 5-hydroxymethylcytosine were observed in genomic DNA. Inhibition of the hydroxylation of 5-methylcytosine by expression of microRNAs miR-29a/b-1 diminished DSB formation. Conversely, its induction by TET1 catalytic domain overexpression increased DSBs in neocortical neurons. Furthermore, the damaged hippocampal neurons exhibited aberrant neuronal gene expression profiles and dendrite formation, but not apoptosis. Comprehensive behavioral analyses revealed impaired spatial reference memory and contextual fear memory in adulthood. Thus, Polb maintains genome stability in the active DNA demethylation that occurs during early postnatal neuronal development, thereby contributing to differentiation and subsequent learning and memory.