Resumen de Analysis of the contribution of linker histone h1 to the dynamics of rna: dna hybrids
Histone H1 is a key determinant of higher-order chromatin structures and for a long time it has been considered a main transcriptional repressor. So consistent with its biochemical properties, the absence of histone H1 was expected to have extended consequences in gene expression. On the contrary, upon depletion of histone H1 in Drosophila melanogaster (dH1), only a subset of genes was significantly affected 1–3. These differentially expressed genes were mainly heterochromatic inactive genes that got transcribed 2. Further studies showed that apart from affecting silencing of heterochromatin, dH1 depletion also induced double-strand breaks (DSBs) on the same heterochromatic sequences that were being up- regulated, suggesting that abnormal transcription of these sequences was resulting in DNA damage 4.
Results reported here suggested that the heterochromatic transcripts up-regulated upon depletion of dH1 induced genomic defects because they were not correctly metabolized and, consequently, formed RNA:DNA hybrids (R-loops). Thus, histone H1 is important in preventing abnormal R-loop accumulation in heterochromatin. On this direction, our approach showed that it was the absence of histone H1 and not simply the relieve of silencing that induced accumulation of R-loops in heterochromatin, as depletion of HP1a (another important heterochromatic element) up-regulated heterochromatin transcription as well, but it neither induced R-loops or DNA damage. Then, further experiments showed that histone H1 was necessary for the binding of hnRNP36 and hnRP48 to heterochromatin. These two proteins are involved in mRNA quality control and, therefore, in the prevention of R-loop formation. Thus, in conditions where histone H1 was absent, the hnRNP36/48 could not bind to the newly synthesized transcript, favoring the formation of abnormal R-loops. Further analyses of the genome-wide distribution of R-loops in D. melanogaster showed that histone H1 is important for the regulation of R-loop dynamics beyond heterochromatin, as depletion of histone H1 not only produced a strong accumulation of R-loops in heterochromatin, but also induced important changes in the distribution of physiological R- loops.
Together, we provide evidence of a novel and essential function of linker histones H1 to the regulation of RNA:DNA hybrids in heterochromatin and, thus, to the maintenance of genome integrity and stability. However, further studies are needed to understand the biological importance of the euchromatic changes in R-loop distribution in dH1-depleted cells compared to that of physiological conditions.
References: 1. Lu, X. et al. Linker histone H1 is essential for Drosophila development, the establishment of pericentric heterochromatin, and a normal polytene chromosome structure. Genes Dev. 23, 452–65 (2009).
2. Vujatovic, O. et al. Drosophila melanogaster linker histone dH1 is required for transposon silencing and to preserve genome integrity. Nucleic Acids Res. 40, 5402– 5414 (2012).
3. Lu, X. et al. Drosophila H1 regulates the genetic activity of heterochromatin by recruitment of Su(var)3-9. Science 340, 78–81 (2013).
4. Bayona-Feliu, A., Casas-Lamesa, A., Reina, O., Bernués, J. & Azorín, F. Linker histone H1 prevents R-loop accumulation and genome instability in heterochromatin. Nat. Commun. 8, 283 (2017).
