Histone Modifications Found to Have Causal Role in Gene Regulation

New research from EMBL Heidelberg and collaborators has uncovered key epigenetic control sites that regulate gene expression, particularly in relation to ancient viral sequences in the genome. The study focused on the histone protein H3.3, specifically two sites on its tail (K9 and K27) that are frequently chemically modified.

By creating mutant versions of H3.3 that could not be modified at these sites, researchers were able to directly observe the consequences of losing these epigenetic marks. The experiments, conducted in mouse stem cells, revealed that mutations at these sites led to defects in cell differentiation, growth, and survival, as well as inappropriate activation of genes across the genome.

Notably, the study found that these histone modifications play a crucial role in repressing genes that should not be expressed in stem cells, including immune system-specific genes. The two control sites studied (K9 and K27) were found to have distinct roles in gene regulation.

Further analysis revealed that some of the activated regions were ancient viral remnants integrated into the genome, known as endogenous retroviruses (ERVs). Matteo Trovato, first author of the study published in Nature Communications, explained that these ERVs have been co-opted by the host genome to exert regulatory functions, with 30% of enhancers in immune cells derived from ERVs. 

The research showed that modifying the K9 site in stem cells activated many "cryptic" enhancers—regulatory DNA regions normally silenced. Kyung-Min Noh, Group Leader at EMBL Heidelberg, emphasized the importance of repressing these unique genomic regions to maintain the cell's gene expression balance. "Repression of these unique genomic regions is crucial for preserving the cell’s gene expression program balance," said Noh. "Activation of the cryptic enhancers triggers a widespread rewiring of the gene regulatory network, ultimately impacting stem cell identity and functionality."

"This is one of the first few studies conducted in a mammalian system showing that these histone residues play a causal role in gene regulation," said Noh. "Understanding this process could have broader implications for developmental biology and disease research, particularly in cancer and neurological disorders, where gene regulation plays an essential role."