Neuronal Enhancers are Hotspots for DNA Single-Strand Break Repair [ChIP-seq]

Defects in DNA repair frequently lead to neurodevelopmental and neurodegenerative diseases, underscoring the particular importance of DNA repair in long-lived post-mitotic neurons. The cellular genome is subjected to a constant barrage of endogenous DNA damage, but surprisingly little is known about the identity of the lesion(s) that accumulate in neurons and whether they accrue throughout the genome or at specific loci. Here, we show that post-mitotic neurons accumulate unexpectedly high levels of DNA single-strand breakage at specific sites within the genome. Genome-wide mapping reveals that these single-strand breaks (SSBs) are located within enhancers at or near to CpG dinucleotides and sites of DNA demethylation, and are repaired by PARP1 and XRCC1-dependent mechanisms. Notably, deficiencies in XRCC1-dependent short-patch repair elevate the extent of DNA repair synthesis at neuronal enhancers, whereas deficiencies in long-patch repair reduce synthesis, suggesting that the high steady-state level of SSB repair in neuronal enhancers is sustained by both short-patch and long-patch processes. These data provide the first evidence of site- and cell type-specific SSB repair, revealing unexpected levels of localized and continuous DNA single-strand breakage in neurons. In addition, these data suggest an explanation for the neurodegenerative phenotypes that occur in patients with defective SSB repair.