Single cell analysis reveals dynamics of transposable element transcription following epigenetic de-repression

Spontaneous or pharmacological loss of epigenetic repression exposes thousands of promoters encoded in transposable elements (TEs) for pervasive transcriptional activation. How TE responses differ between epigenetically relaxed cancer cells and what factors govern such variation remains however largely unknown. By quantifying TE transcription initiation at single cell and locus resolution in epigenetically targeted cancer cells, we characterize specific groups of co-regulated loci that drive over ten-fold variation in TE load per single cell. Such variable activity patterns are largely linked to cell cycle stages, stress response signatures, and immune pathways. Furthermore, cells with high levels of specific transcription factors show increased TE expression, while within such cells, multi-copy families are differentially regulated in response to local sequence divergence of binding sites and the locus repressive or active chromosomal contexts. Our data thereby implicates the regulation of potent promoters within TEs as an underestimated source of transcriptional heterogeneity following epigenetic therapy.