Comprehensive mapping of 5-hydroxymethylcytosine epigenetic dynamics in axon regeneration

In contrast to central nervous system neurons, dorsal root ganglia (DRG) neurons can switch to a regenerative state after peripheral axotomy. In a screen for chromatin regulators of the regenerative responses in this conditioning lesion paradigm, we identified Tet methylcytosine dioxygenase 3 (Tet3) as upregulated in DRG neurons, along with increased 5-hydroxymethylcytosine (5 hmC). We generated genome-wide 5 hmC maps in adult DRG, which revealed that peripheral and central axotomy (leading to no regenerative effect) triggered differential 5 hmC changes that are associated with distinct signaling pathways. 5 hmC was altered in a large set of regeneration-associated genes (RAGs), including well-known RAGs, such as <i>Atf3, Bdnf</i>, and <i>Smad1</i>, that regulate axon growth potential of DRG neurons, thus supporting its role for RAG regulation. Our analyses also predicted HIF-1, STAT, and IRF as potential transcription factors that may collaborate with Tet3 for 5 hmC modifications. Intriguingly, central axotomy resulted in widespread 5 hmC modifications that had little overlap with those of peripheral axotomy, thus potentially constituting a roadblock for regeneration. Our study revealed 5 hmC dynamics as a previously unrecognized epigenetic mechanism underlying the divergent responses after axonal injury.