A reaction-diffusion framework for de novo Polycomb spreading

Eukaryotic organisms utilize post-translational modifications in chromatin to sustain consistent patterns of gene silencing. Isolated trimethylation at histone H3 lysine 27 (H3K27me3) is established by Polycomb Repressive Complex 2 (PRC2) and spreads throughout the genome during embryogenesis. This process is crucial for cell type specification; however, we lack quantitative prediction capabilities for the de novo establishment of Polycomb states.

Quantifying H3K27 methylation kinetics in vivo poses challenges, and the intricate network of molecular interactions affecting Polycomb states further complicates the matter. In their study, researchers utilized the Drosophila embryonic system to measure H3K27me3 dynamics via ChIP-seq and determine the rate at which the modification spreads along chromatin within living organisms.

To explain the observed rate mechanistically, they developed a reaction-diffusion framework that models PRC2-mediated establishment of gene silencing states from scratch. This reaction-diffusion system aligns with experimental findings in both wild-type and mutant embryos, indicating that PRC2 can diffuse along chromatin in a linear (1D) manner.

The diffusion rate is accelerated by Polycomb Repressive Complex 1 (PRC1). By defining a minimal set of parameters governing Polycomb dynamics in vivo, the team provides evidence that the early embryo generates a "super-charged" environment conducive to epigenetic modification.