Billions of years ago, as primitive lifeforms were becoming more complex, a selfish genetic component became a sort of genome colonizer. Using a copy-and-paste mechanism, this pernicious bit of code replicated and inserted itself again and again into a variety of genomes.<\/p>\n\n\n\n
Over time, all eukaryotic organisms inherited the code\u2014including us. In fact, this ancient genetic element wrote about one-third of the human genome\u2014and was considered junk DNA until relatively recently.<\/p>\n\n\n\n
This genetic component<\/a> is known as LINE-1, and its aggressive intrusion into the genome can wreak havoc, leading to disease-causing mutations. A key protein<\/a> called ORF2p enables its success\u2014meaning understanding ORF2p’s structure and mechanics could illuminate new potential therapeutic targets for a variety of diseases.<\/p>\n\n\n\n Now, in collaboration with more than a dozen academic and industry groups, Rockefeller scientists have rendered the protein’s core structure in high resolution for the first time, revealing a host of new insights about LINE-1’s key disease-causing mechanisms. The results were published<\/a> in Nature<\/em>.<\/p>\n\n\n\n \u00abThe work will facilitate rational drug design targeting LINE-1 and may lead to novel therapies and strategies to combat cancer, autoimmune disease, neurodegeneration, and other diseases of aging,\u00bb says senior author John LaCava, a research associate professor at The Rockefeller University.<\/p>\n\n\n\n LINE-1 is a retrotransposon, a kind of mobile genetic code that translates RNA back into DNA while replicating and writing itself into different places throughout an organism’s genome. There are different kinds of retrotransposons, including endogenous retroviruses<\/a> (ERVs), which resemble HIV and Hepatitis B (HBV).<\/p>\n\n\n\n LINE-1’s origin is unclear, but it has an evolutionary connection to group II introns, a class of ancient mobile elements dating back about 2.5 billion years. Retrotransposons like LINE-1 have been evolving with their host organisms for 1 to 2 billion years.<\/p>\n\n\n\n \u00abIt’s an ongoing battle between LINE-1 trying to insert itself and the host protecting its own genome,\u00bb says co-first author Trevor van Eeuwen, a postdoctoral fellow in Rockefeller’s Laboratory of Cellular and Structural Biology.<\/p>\n\n\n\n Millions of genetic fragments derived from LINE-1 are found in our cells. The vast majority are inactive evolutionary relics, evidence of failed attempts to hijack the replication machinery. But about 100 LINE-1s are operational\u2014and usually not helpful. One protein produced by LINE-1, known as ORF1p, is churned out by cancer cells, as a recent study<\/a> by LaCava, Michael P. Rout, and their collaborators described.<\/p>\n\n\n\n LaCava and Martin Taylor, of Massachusetts General Hospital and Harvard Medical School, have collaboratively studied LINE-1 and its proteins for more than a decade, but because ORF2p expresses so lowly and infrequently, it has remained poorly understood. \u00abLINE-1 has been so difficult to study because it has very weird features,\u00bb LaCava says.<\/p>\n\n\n\n \u00abFor instance, it has an unusual replication cycle and the ORF2p protein that no one’s been able to capture. But Marty and I eventually reached a place where our research on it was mature enough so that we could begin to study its structure.\u00bb<\/p>\n\n\n\n Taylor made key advancements in purifying the full-length ORF2p as well as a shorter \u00abcore\u00bb version that facilitates L1 replication; these advances facilitated the breakthroughs that followed.<\/p>\n\n\n\n Using a combination of X-ray crystallography and cryo-EM, the research team discovered two novel folded domains within ORF2p’s core that contribute to LINE-1’s ability to make copies of itself.<\/p>\n\n\n\n ORF2p has structural adaptations uniquely suited for these endeavors, says van Eeuwen. It’s a sort of jack-of-all-trades protein, capable of handling everything from replication to insertion. But while most viruses need potentially hundreds of reverse transcriptase proteins to replicate, ORF2p does it all.<\/p>\n\n\n\n Yet when LINE-1 is activated in the cytoplasm, \u00abit acts like a viral mimic. It creates RNA:DNA hybrids that look like a viral infection when they’re sensed,\u00bb van Eeuwen notes. This viral mimicry suggests a possible solution to the puzzle of how ORF2p activates the innate immune system, contributing to autoimmune disease and other conditions.<\/p>\n\n\n\n Their research found that interactions with genetic material in the cytoplasm activate the cGAS\/STING antiviral pathway. In turn, that pathway causes cells to produce interferons, stimulating the immune system and leading to inflammation, in a manner analogous to what happens during an infection by a virus.<\/p>\n\n\n\n \u00abIts main function appears to be to proliferate copies of itself, and as LINE-1 moves sequences around, there’s a chance those sequences could break a gene,\u00bb he says. \u00abBut there’s also a chance they could create new genetic elements or novel functionalities that are beneficial to the host.\u00bb<\/p>\n\n\n\n In the future, the researchers will seek to solve the two newly discovered core domains and understand their functions. In the meantime, \u00abour structural elucidation of ORF2p lays the groundwork for future studies needed to dissect and improve our understanding of the LINE-1 insertion mechanism, its evolution, and its roles in disease,\u00bb van Eeuwen says.<\/p>\n\n\n\n They also want to explore the potential clinical applications of their findings. Because there is a kinship between retrotransposons and retroviruses, in the current study they tested treatments for the retroviruses HIV and HBV to see if they would inhibit LINE-1. They did not, suggesting that the design of therapeutics will have to be tailored to LINE-1’s unique characteristics.<\/p>\n\n\n\n \u00abThe work opens the door to rational drug design of better LINE-1 inhibitors, and we hope these will lead to clinical trials soon,\u00bb LaCava says.<\/p>\n\n\n\n And, as Rout adds, \u00abThis study also underscores the potential of integrating multiple kinds of data\u2014and multiple labs’ expertise\u2014to solve fundamental biomedical questions.\u00bb<\/p>\n\n\n\n More information:<\/strong> Eric T. Baldwin et al, Structures, functions, and adaptations of the human LINE-1 ORF2 protein, Nature<\/em> (2023). DOI: 10.1038\/s41586-023-06947-z<\/a><\/p>\n\n\n\n Journal information:<\/strong> Nature<\/a> <\/a><\/p>\n\n\n\n Provided by\u00a0Rockefeller University<\/a>\u00a0<\/p>\n\n\n\nEvolutionary mates<\/h2>\n\n\n\n
Jack-of-all-trades<\/h2>\n\n\n\n
The path ahead<\/h2>\n\n\n\n