Stem Cell Activity-Coupled Suppression of Endogenous Retrovirus Governs Adult Tissue Regeneration

Mammalian retrotransposons constitute 40% of the genome. During tissue regeneration, adult stem cells coordinately repress retrotransposons and activate lineage genes, but how this coordination is controlled is poorly understood. Here, we observed that dynamic expression of histone methyltransferase SETDB1 (a retrotransposon repressor) closely mirrors stem cell activities in the murine skin. SETDB1 ablation leads to reactivation of endogenous retroviruses (ERVs, a type of retrotransposon) and assembly of viral-like particles, resulting in hair loss and stem cell exhaustion that is reversible by antiviral drugs. Mechanistically, at least two molecularly and spatially distinct pathways are responsible: antiviral defense mediated by hair follicle stem cells and progenitors, and antiviral independent response due to replication stress in transient amplifying cells. ERV reactivation is promoted by DNA demethylase TET-mediated hydroxymethylation and recapitulated by ablating cell fate transcription factors. Together, we demonstrated ERV silencing is coupled with stem cell activity and essential for adult hair regeneration.