Chromatin transitions triggered by LH density as epigenetic regulators of the genome.

Motivated by experiments connecting linker histone (LH) deficiency to lymphoma progression and retinal disorders, we study by mesoscale chromatin modeling how LH density (ρ) induces gradual, as well sudden, changes in chromatin architecture and how the process depends on DNA linker length, LH binding dynamics and binding mode, salt concentration, tail modifications, and combinations of ρ and linker DNA length. We show that ρ tightly regulates the overall shape and compaction of the fiber, triggering a transition from an irregular disordered state to a compact and ordered structure. Such a structural transition, resembling B to A compartment transition connected with lymphoma of B cells, appears to occur around ρ = 0.5. The associated mechanism is DNA stem formation by LH binding, which is optimal when the lengths of the DNA linker and LH C-terminal domain are similar. Chromatin internal and external parameters are key regulators, promoting or impeding the transition. The LH density thus emerges as a critical tunable variable in controlling cellular functions through structural transitions of the genome.