Mechanistic basis of lineage restriction

Lineage restriction is the biological phenomenon where cells lose developmental potency during differentiation for all except their adopted lineages. Lineage restriction is foundational to multicellularity as it secures the functional identities of the myriad cell types in the body. As yet, the mechanisms of lineage restriction remain enigmatic. By developing Potency-Seq to systematically map transcriptional potency of genes across the genome, we uncovered a link between the restriction of developmental potency of cells and the occlusion of transcriptional potency of their genomes. We further showed that, strikingly, genes can undergo irreversible occlusion of their transcriptional potency simply by chromatinization into the nucleosomal form with unmodified histones. These findings led to a comprehensive mechanistic account of lineage restriction as driven by gene occlusion. Specifically, naive pluripotent stem cells at the onset of development possess the unique capacity to erase occlusion globally to reset full transcriptional potency of the genome, which in turn establishes full developmental potency of the cells. Such deocclusion capacity is abolished when cells progress from naive to primed pluripotency. From this stage onward, lineage-inappropriate genes can undergo nucleosome-mediated occlusion in an irreversible manner when their cognate transcription factors (TFs) and/or placeholder factors (PFs) no longer exist in cells to protect them. Consequently, the transcriptionally potent portion of the genome shrinks progressively and irreversibly during differentiation, thus permanently restricting the developmental potency of differentiating cells.