Human cells repurpose retroelements as genome defenses

A new paper from InsideOutBio in the International Journal of Molecular Sciences says human cells turned once-dangerous retroelements into part of the immune defense system. The findings could help explain how cells detect viruses, cancer and aging-related damage, and point to therapeutic uses.

Why it matters: - Human cells appear to use ancient genome parasites as an early warning system against infection, cancer and age-related cellular damage. - The findings may help explain how cells decide when to trigger immune responses against abnormal RNA and DNA activity. - InsideOutBio says the work also points to therapeutic applications, including approaches that could help “light up” tumors for the immune system.

What happened: - InsideOutBio described the research in a paper published in the International Journal of Molecular Sciences. - The paper argues that cells first evolved defenses against retroelements, then repurposed those same sequences to help defend against other threats. - The analysis focuses on retrotransposons, also called retroelements, which now make up more than 50% of the human genome.

The details: - Retroelements once posed an existential threat because they could insert into active genes and disrupt normal function. - A major focus of the paper is SINEs, or short interspersed nuclear elements, which are abundant in the human genome and do not code for protein. - SINE RNAs depend on the same cellular machinery used to make protein-coding RNAs, which makes them difficult to target without affecting normal genes. - The paper says cells use noncanonical nucleic acid structures, or flipons, as a vulnerability point. - Flipons can form alternative structures such as left-handed Z-DNA and four-stranded G-quadruplexes. - These structures help keep DNA open and can make it easier for retroelements to copy themselves into RNA. - Retroelements can also form the same unusual structures in RNA, which signals that SINEs are being expressed when they should not be. - Host proteins that bind these structures, especially ZBP1, can trigger immune responses against the affected cells. - ADAR1 helps set the threshold for that response. - Viruses can also drive retroelement expression by disrupting normal host RNA production. - The paper says this creates a trap: dysregulated transcription allows SINEs near gene boundaries to become expressed, which then helps trigger immune detection. - Similar patterns can appear in cancers and aging cells when gene transcription becomes dysregulated. - The paper says humans have eliminated RNA-dependent polymerases that would help retroelements spread from the genome. - That differs from some worms and flies, which use RNA-dependent polymerases to amplify pathogen RNAs. - The authors also discuss related defense mechanisms involving miRNAs, tRNA fragments, circular RNAs, and the interaction of RNA polymerases 2 and 3.

Between the lines: - The paper frames genome defense as an evolutionary compromise, not a clean engineering solution. - Instead of building entirely new systems, cells appear to have adapted existing repetitive DNA and RNA elements into surveillance tools. - That could help researchers understand why some stressed or infected cells are flagged by the immune system while others are not.

What's next: - The paper points to therapeutic development based on these genome-defense pathways. - InsideOutBio says its broader work is aimed at proprietary therapies designed to activate anti-tumor immune responses. - The company’s forward-looking statements include risk and uncertainty and are not guarantees of future performance.