Structural Insights Unmask Novel Adaptations and Functions of Human LINE-1 ORF2

Martin Taylor, MD, PhD, a physician investigator in the Department of Pathology at Massachusetts General Hospital and an instructor in Pathology at Harvard Medical School, is the lead corresponding author of a new study in Nature, Structural Analysis and Inhibition of Human LINE-1 ORF2 Protein Reveals Novel Adaptations and Functions.

What Question Were You Investigating?

About a fifth of the human genome is half a million copies of a transposon, a virus-like element known as LINE-1 (L1). L1 was ignored as "junk DNA" for years but is increasingly recognized to contribute to the pathology of autoimmunity, cancer, neurodegeneration, and aging. However, we don't know how big a contribution L1 makes or how it works, limiting our ability to target it.

We sought out to determine the structure and mechanisms of its critical multifunctional ORF2p reverse transcriptase.

What Methods Did You Use?

Lab models - cryo-electron microscopy, X-ray crystallography, biochemistry, pharmacology, cell-based imaging (immunofluorescence), computational modeling of evolution.

What Did You Find?

We determined the first structures of the LINE-1 ORF2 multifunctional reverse transcriptase protein (ORF2p) in multiple states by both cryo-EM and crystallography, and these together with biochemical and cell-based analyses provided key mechanistic insights into polymerization, insertion, and activation of the innate immune system and shed light on LINE-1's evolutionary history.

We find an unexpected mechanism for innate immune activation by LINE-1, whereby it directly synthesizes DNA in the cytosol, which is then sensed by cGAS/STING.

We also show which reverse transcriptase inhibitors do and do not inhibit LINE-1, and the structure allows us to understand their potencies.

What Are the Implications?

The work will facilitate rational drug design targeting LINE-1 and may lead to novel therapies and strategies to prevent cancer, autoimmune disease, and neurodegeneration and other diseases of aging.

We also have a better understanding of how LINE-1 works mechanistically, which will allow future studies to determine how it causes disease. In particular, the new findings of direct innate immune activation explain associations of LINE-1 with cytosolic double stranded nucleic acids in models of aging and cancer and open new areas of research.

What's Next?

We will use our knowledge of LINE-1 and existing HIV-1 inhibitors that inhibit it to study LINE-1 in disease models in mice.

We will build on our structural and mechanistic knowledge to understand how each step of the LINE-1 replication cycle works in detail in cells, to understand how this streamlined parasite co-opts host factors to replicate, and how the cell attempts to shut it down.

Finally, we will build on our discovery of ORF2p cytosolic priming,reverse transcription, and innate immune activation to understand it in detail, which may allow new therapies based on this activity.