Mitosis Mechanism for Viral Genome Insertions Unlocks Cell Defense Applications

Viruses insert “transposable elements” into the genetic material of host cells to replicate. While cells’ defense mechanisms have learned to silence most of these viral insertions, a few “jumping genes” retain the potential to disrupt essential genes and cause serious health threats, such as neurological diseases and cancer.

In a new paper published in Science Advances titled, “LINE-1 ribonucleoprotein condensates bind DNA to enable nuclear entry during mitosis,” researchers at NYU Langone Health and the Munich Gene Center at Ludwig-Maximilians-Universität (LMU) München in Germany have identified a new genome incorporation mechanism for long interspersed nuclear element 1 (LINE-1), the only autonomously active retrotransposon in humans, which leverages the cell cycle. The resulting insights can unlock new avenues for improving cellular defense mechanisms against harmful transposable elements.

LINE-1 constitutes a staggering 20% of the human genome and contributes greatly to human genetic diversity and genome evolution. The retrotransposon encodes two proteins, ORF1p and ORF2p, both essential for successful incorporation into the genome. ORF2p has endonuclease and reverse transcription activity, while ORF1p binds RNA. While many copies of ORF1p assemble onto the LINE-1 RNA to form a ribonucleoprotein (RNP) condensate, the function of these condensates on the LINE-1 life cycle has remained unclear.

The study demonstrated that these RNP condensates bind DNA upon nuclear envelope breakdown to gain access to the genome. DNA binding was illustrated using reconstitution assays on DNA curtains, a high-throughput, single-molecule platform to probe DNA-protein interactions, while mutational analysis showed that DNA binding was crucial for nuclear entry and LINE-1 retrotransposition activity.

The authors also suggest that the LINE-1 condensate acts as a delivery vehicle to bring its RNA into proximity of the right DNA sequences on DNA where the retrotransposon tends to insert.

“Our study provides crucial insight into how a genetic element that has come to make up a large part of human DNA can successfully invade the nucleus to copy itself,” said Liam J. Holt, PhD, associate professor in the department of biochemistry and molecular pharmacology at NYU Grossman School of Medicine and co-corresponding author of the study. “These findings on the precise mechanisms behind LINE-1 insertion lay the foundations for the design of future therapies to prevent LINE-1 replication.”

Additionally, for successful mitosis, cytoplasmic and nuclear contents must remain separated during nuclear envelope breakdown. To ensure this separation, sufficiently small particles can be exported through nuclear pores, while large particles are coated with cell proliferation markers that prevent cytoplasmic macromolecular objects from entering the chromatin. Packaged in its condensates, LINE-1 is thought to evade the cellular defense mechanisms that exclude large particles from the nucleus during mitosis.

The authors also emphasized that the DNA-binding ability of LINE-1 condensates only emerges when the ratio of ORF1p copies to RNA is high enough in the condensates. Holt said that future directions will evaluate whether other condensates undergo functional changes as the ratios between their components change.