Characterization of the regulatory roles of histone H1 in the homeostasis of the mammalian genome

Resumen

Histone H1 is a key component of the chromatin, involved in the formation of condensed structures that are refractory to the binding of different factors, thus hindering the action of both the transcription and replication machinery. Previous studies in our group have demonstrated that lowering the total amounts of histone H1 in mouse embryonic stem cells has drastic consequences on their DNA replication dynamics, due to a lack of coordination with transcriptional processes. Along this work, we have continued this project, characterizing the mechanisms that explain the conflicts between transcription and replication in H1 deficient cells. We have found that the replication dynamics alterations are coupled to a perturbed DNA replication initiation landscape, which would also be compatible with massive replication fork stalling mediated by conflicts with the transcriptional machinery. Indeed, we have discovered that the lack of histone H1 produces numerous transcriptional alterations. The most remarkable of them would be the widespread accumulation of unstable non-coding transcripts in chromatin, including PROMPTs, lncRNAs and enhancer RNAs. These transcripts are not post-transcriptionally bound to DNA: their attachment to chromatin is mediated by RNA polymerase II molecules. Coding transcription is also affected, since protein-coding genes display elongation failures and transcriptional read-through. However, all these phenotypes do not seem to be directly related to a chromatin conformational change or any epigenetic alteration, but rather to a defect in RNA processing and metabolism. Finally, we have concluded that the general phenotype of H1 deficient cells resembles the mutation of several components of the nuclear exosome complex, suggesting a possible connection of the linker histone with RNA surveillance mechanisms. These findings have broad implications for our knowledge about the interplay between chromatin and basic cellular processes, and how histone H1 contributes to the maintenance of genome stability and cell homeostasis.