Silencing in action: How cells 'repress' genomic remnants of ancient viruses

Our genomes are huge -- a typical human cell contains DNA with over 6 billion units of information (measured in 'base pairs'). However, this treasure trove of information poses a challenge when it comes to looking up the right information at the right time to perform a specific function. This is where epigenetic signatures come into play.

If you imagine the genome as a book, epigenetic marks are the highlights on its pages and the notes in its margins. Now, it is not always easy to know whether these marks are 'instructive' -- i.e. do they tell the cell "Here, read this" or "Don't read this"? Or are they simply marks left behind by a previous reader, indicating that that portion of the book was visited before?

It was this question that interested Kyung-Min Noh, Group Leader at EMBL Heidelberg, and her team. The researchers decided to focus on a molecule called H3.3, which belongs to a class of proteins called histones. Histones tightly bind to DNA in cells and help form its functional structure.

The H3.3 protein has a couple of spots on its tail (called K9 and K27) which are frequently chemically modified. It is hypothesised that these modifications are epigenetic marks that help the cell make decisions regarding gene expression. However, until now, it had never been experimentally proven that these are true control sites that instruct gene expression.

The researchers decided to experimentally mutate these sites, thus creating a version of H3.3 which could not be chemically modified at these spots. Considering our book analogy above, this created a protected page which could not be highlighted or marked, allowing the scientists to directly explore what the consequences of losing such marks would be.

Moreover, this system allowed the researchers to vary which page was protected, allowing them to draw comparisons between the loss of modifications at one or the other control site.

The scientists found that the mutation of these sites in mouse stem cells not only resulted in defects in cell differentiation, growth, and survival, it also caused spurious activation of genes across the genome. This included genes that should not be expressed in stem cells, such as immune system-specific genes.

This suggested that a normal function of these sites is to maintain those genes in an inactivated -- or 'repressed' -- state, allowing stem cells to remain stem cells. These effects were also different for the two control sites studied, showing that each of them plays a distinct role in gene regulation.

Upon further analysis, the researchers found that some of these regions, which are typically repressed but were activated upon mutating the histone sites, are ancient remnants of viruses that have integrated into our genomes.

"These regions are also called endogenous retroviruses (ERVs)," explained Matteo Trovato, former PhD student in the Noh group and the first author of the study, currently a postdoc at IFOM, Italy. "Throughout evolution, they have been co-opted by the host's genome to exert regulatory functions. In immune cells, for example, 30% of the enhancers (a specific type of regulatory DNA element) are derived from ERVs."

The researchers found that by modifying the K9 site in stem cells, many such 'cryptic' enhancers -- regulatory DNA regions that are normally silenced -- became active.

"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."

The study was carried out in collaboration with Chen Davidovich's group in EMBL Australia, Benjamin Garcia's lab at Washington University, St. Louis, and Judith Zaugg's team at EMBL Heidelberg. The results were recently published in the journal Nature Communications.

"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."