The NF-κB pathway regulates heterochromatin at intronic young LINE-1 elements and hematopoietic stem cell gene expression during irradiation stress

SUMMARY Understanding how ionizing radiations (IR) alter hematopoietic stem cell (HSC) function on the long-term is crucial. We recently showed a link between derepression of L1Md, the mouse young subfamilies of LINE-1/L1 retroelements, and IR-induced HSC injury. L1 contribute to gene regulatory networks. However, the mechanisms involved in IR-induced L1Md derepression, and their impact on HSC transcriptome remain to be addressed. Here we show that IR triggers genome-wide H3K9me3 decreased and transcriptomic changes in HSCs, characterized by a loss of the TNF-α/NF-κB and HSC signatures. HSC gene repression is associated to H3K9me3 loss at specific intronic L1Md displaying NF-κB binding sites. This is correlated with reduced NFKB1 repressor expression. TNF-α treatment before IR rescued all these effects and prevented IR-induced HSC loss of function in vivo. This reveals the importance of the TNF-α/NF-κB pathway to control H3K9me3 levels at selected intronic L1Md and thereby preserve HSC gene expression and function during IR stress.