It's a safe harbor in the sense that you can disrupt one copy and the cell doesn't care.
In the long human effort to mend broken genes, researchers at UC Berkeley have found an unlikely ally in the DNA of songbirds. A new technique called PRINT borrows a self-copying genetic element from species like the zebra finch to place therapeutic genes not randomly into the genome, but into a carefully chosen sanctuary — the ribosomal RNA regions, where hundreds of redundant copies absorb the insertion without harm. Published in Nature Biotechnology in February 2024, the work offers a path toward gene therapies that could treat entire patient populations regardless of which specific mutation they carry, a limitation that has long constrained the field.
- Current gene therapies can silence broken genes but struggle to safely deliver entirely new, functional ones — a gap that leaves many patients without options.
- Random gene insertion, the method most commonly used today, risks triggering cancer or disrupting essential functions, making safety a persistent and unresolved tension in the field.
- PRINT sidesteps this danger by targeting ribosomal DNA regions — genomic real estate so redundant across hundreds of copies that the cell barely registers a new arrival.
- Unlike CRISPR, which must be tailored to each patient's unique mutation, PRINT adds a healthy gene copy universally, potentially treating all variants of diseases like cystic fibrosis or hemophilia with a single therapy.
- Laboratory results show roughly half of treated cells successfully incorporated the inserted gene, with up to ten copies added without impairing protein production.
- Clinical application remains on the horizon — Addition Therapeutics is advancing the technology, while researchers continue mapping the biological limits of the ribosomal regions that make it possible.
Gene therapy has celebrated recent milestones, especially with CRISPR-Cas9 treatments for sickle cell disease. Yet these tools share a fundamental constraint: they are far better at disabling broken genes than at delivering entirely new, working ones. For diseases caused by missing or defective proteins, that gap matters enormously. Researchers at UC Berkeley have now developed a technique called PRINT — Precise RNA-mediated INsertion of Transgenes — that addresses this problem by borrowing molecular machinery from bird DNA.
The key ingredient is a retrotransposon, a self-replicating genetic element, originally found in songbirds like the zebra finch and white-throated sparrow. What distinguishes PRINT from other retrotransposon-based approaches is not the tool itself but where it delivers its cargo. Rather than inserting therapeutic genes randomly into the genome — a gamble that can disrupt essential functions or trigger cancer — PRINT targets the ribosomal RNA regions, stretches of DNA present in hundreds of identical copies across five human chromosomes. Losing one or a few of these redundant copies to a gene insertion causes no measurable harm.
Molecular biologist Kathleen Collins, who led the research published in Nature Biotechnology in February 2024, highlights a practical advantage over CRISPR: PRINT does not need to be customized for each patient's specific mutation. It simply adds a healthy copy of the relevant gene to anyone with the disease. For conditions like cystic fibrosis or hemophilia, where dozens of different mutations can all produce the same illness, a single PRINT-based therapy could work across the entire patient population.
The system delivers two RNA molecules into cells — one encoding the R2 protein that performs the insertion, and one serving as the template for the therapeutic gene. In laboratory experiments with cultured human cells, roughly half successfully incorporated a test gene, and up to ten copies could be added without impairing the cell's protein-making capacity. The ribosomal regions also benefit from the cell's own protective systems, since any damage there threatens protein production and is repaired with unusual speed and care.
Questions remain before human trials can begin — chiefly how many ribosomal gene disruptions a cell can tolerate, and whether certain cell types face greater risk. Collins has filed patents and co-founded Addition Therapeutics to carry the work forward. The technology functions; what the field now needs is a deep enough understanding of the underlying biology to deploy it safely.
Gene therapy has made headlines in recent years, particularly with the approval of CRISPR-Cas9 treatments for sickle cell disease. These tools excel at one thing: turning off broken genes. But they hit a wall when the goal is to insert an entirely new, working gene into a person's genome—the kind of intervention needed for diseases caused by missing or defective proteins. Researchers at UC Berkeley have now developed a technique that sidesteps this limitation by borrowing a trick from bird DNA.
The method, called Precise RNA-mediated INsertion of Transgenes, or PRINT, uses a retrotransposon—a self-replicating piece of genetic code—originally discovered in birds like the zebra finch and white-throated sparrow. The innovation lies not in the concept of using retrotransposons for gene insertion, which other labs are exploring, but in where PRINT places the new gene. Instead of inserting it randomly into the genome, where it might disrupt essential genes or trigger cancer, PRINT targets a specific "safe harbor": the regions of DNA that code for ribosomal RNA, the molecular machinery that translates genetic instructions into proteins.
The human genome contains hundreds of identical copies of these ribosomal RNA genes, scattered across five chromosomes. Because there are so many redundant copies, losing one or even a few to a gene insertion causes no harm—the cell barely notices. This redundancy is the key advantage. When gene therapies using viral vectors insert genes randomly, as is common today, they risk landing in the wrong place and causing serious problems. PRINT avoids that gamble entirely.
Kathleen Collins, the molecular biologist who led the research published in Nature Biotechnology in February 2024, explains the practical difference between PRINT and CRISPR-based approaches. CRISPR can fix a specific mutation or knock out a malfunctioning gene, but it requires tailoring to each patient's particular genetic error. PRINT takes a different path: it adds a healthy copy of the gene to everyone with the disease, regardless of what specific mutation they carry. For conditions like cystic fibrosis or hemophilia, where dozens of different mutations in the same gene can all cause disease, this is a significant advantage. One therapy could work for all patients.
The technique works by delivering two pieces of RNA into cells. One encodes a protein called R2, which comes from bird retrotransposons and contains the molecular machinery needed to insert DNA—including an enzyme that nicks DNA strands and another that copies RNA back into DNA. The second RNA serves as a template for the therapeutic gene itself, complete with the regulatory elements needed to turn it on. Once inside the cell, the R2 protein recognizes the ribosomal RNA regions and inserts the therapeutic gene into them.
In laboratory experiments, researchers synthesized these RNAs and introduced them into cultured human cells. About half the cells successfully incorporated a fluorescent protein gene, which lit up under a microscope, proving the system worked. Further testing confirmed that the genes inserted specifically into the ribosomal RNA regions and that up to ten copies could be inserted without harming the cell's ability to manufacture proteins.
The ribosomal RNA regions offer advantages beyond mere safety. These regions cluster together in a structure called the nucleolus, which is essentially a dedicated factory for making ribosomes. DNA in the nucleolus is heavily guarded—cells repair breaks quickly and regulate transcription tightly, since any serious damage could cripple protein production. This means inserted genes receive exceptional care and protection from the cell's own maintenance systems, further reducing the risk of problems.
Collins and her team still have questions to answer before PRINT moves into human trials. How many ribosomal genes can be disrupted before a cell suffers? Do some cell types, which naturally silence many of their ribosomal genes, face greater risk? The researchers are investigating these questions while also refining the proteins and RNAs involved to improve efficiency in human tissue samples. Collins has already filed patents on PRINT and co-founded a company called Addition Therapeutics to develop it further. The technology works, she says. What remains is understanding the biology deeply enough to use it safely and effectively in patients.
Citações Notáveis
We're not knocking out a gene function. We're taking a complementary approach, which is to put into the genome an autonomously expressed gene that makes an active protein—to add back a functional gene as a deficit bypass.— Kathleen Collins, UC Berkeley molecular biologist
After chasing dozens of them, the real winners were from birds.— Kathleen Collins, on screening R2 retrotransposons from multiple animal species
A Conversa do Hearth Outra perspectiva sobre a história
So CRISPR can already edit genes. Why do we need another tool?
CRISPR is brilliant at turning off a broken gene or fixing a specific typo in the DNA. But if a gene is completely missing or non-functional, CRISPR can't put back what isn't there. PRINT adds a working copy of the gene—it's supplementation, not repair.
And the safety issue with inserting genes is that they land in the wrong place?
Exactly. When you use a virus to ferry a gene into cells, it often inserts randomly. You might accidentally disable something important or trigger cancer. PRINT targets ribosomal RNA regions where there are hundreds of identical copies. Disrupting one or two doesn't matter.
Why birds? That seems oddly specific.
The R2 retrotransposon doesn't exist in mammals anymore—we lost it somewhere in our evolutionary past. But birds still have it, and it's remarkably good at recognizing and inserting into human ribosomal DNA. Collins screened dozens of animal versions before finding that birds had the best match.
How does this change treatment for patients?
Take cystic fibrosis. There are hundreds of different mutations that can cause it. With CRISPR, you'd need a custom therapy for each mutation. With PRINT, one therapy works for everyone—you're just giving them a healthy copy of the gene.
What's still unknown?
How many ribosomal genes can you safely disrupt? Some cells naturally turn off many of them. Do those cells become vulnerable? Collins is investigating, but the basic proof of concept is solid.
When will patients get access to this?
Not immediately. The research is still in cultured cells and tissue samples. But Collins has already started a company to move it toward human trials. The foundation is there.