Hierarchical chromatin polyvalency governs robust gene regulation and organogenesis

Precise temporal control of gene expression is fundamental for embryonic development, but the epigenetic and chromatin basis governing transcriptional timing remains poorly understood. The bivalency model proposed that coexistence of H3K4me3 and H3K27me3 marks a poised state ready for activation, yet the model has been challenged because H3K27me3 depletion does not rapidly activate genes, suggesting H3K27me3 is not responsible for silencing.

Through temporal epigenomic profiling of post-implantation mouse embryos and use of the protein degradation tag (dTAG) system, the study demonstrates that H2Aub, not H3K27me3, functions as the major repressor. It further reveals a hierarchical repression architecture in which H2Aub is responsible for transcriptional silencing, while H3K27me3 and H3K9me3 serve to reinforce the silencing state in post-implantation embryos.

Functionally, disruption of this H2Aub-centered hierarchy perturbs temporal control of polyvalent gene activation, leading to severe organogenesis defects. Mechanistically, acute loss of H2Aub disrupts the retinoic acid–FGF signaling pathway, causing somitogenesis arrest.

Together, the findings establish the chromatin polyvalency model as a multi-layered, hierarchical repression mechanism that governs temporal control of gene expression during embryogenesis.