Transcription-dependent heterochromatin at the Xist promoter shapes the random choice of the inactive X chromosome [Regulators_KD_CutnTag]

This is likely to occur during the initial phase of Xist upregulation, when Xist is frequently transcribed also in male cells and from both X chromosomes in females. A repressive function of Xist-dependent H3K9me3 accumulation is supported by our finding that premature, transient Xist overexpression primes an allele for future silencing and thereby skews the choice of the inactive X. Xist-dependent H3K9me3 recruitment does not require transcription of its antisense transcript Tsix, which can however enhance subsequent maintenance of the mark.

In addition, the X-linked Xist activator RNF12 counteracts H3K9me3 formation at the locus independently of its known target REX1. Our results thus suggest an important role of facultative heterochromatin formation in the choice process at the onset of XCI. Activation and repression mechanisms are intertwined, to ensure the silencing of one Xist allele and the activation of the other within each female cell.