FOXD3 acts as a repressor of repeat elements in a heterochromatin-mediated pathway

Repeat element transcription plays a vital role in early embryonic development. Upregulation of repeats such as MERVL characterises the 2 cell-like population in a mouse embryonic stem cell culture. Repeat element sequences contain binding sites for numerous transcription factors. We identify a the forkhead domain transcription factor FOXD3 as a regulator of repeat element transcription. FOXD3 binds to and recruits the histone methyl transferase Suv39h1 to MERVL and major satellite repeats in mouse embryonic stem cells, consequentially repressing the transcription of these repeats by establishment of the H3K9me3 heterochromatin modification. Notably, depletion of Foxd3 leads to the de-pression of MERVL and major satellite repeats and in turn the activation of a subset of genes expressed in the 2 cell state, shifting the balance between the stem cell and 2 cell-like population in culture. Thus FOXD3 acts as a negative regulator of repeat transcription, ascribing a novel function to this transcription factor. Total RNA Seq analysis of Foxd3 conditional KO mouse ES cells without and with tamoxifen induced depletion of Foxd3