How ancient viral DNA in the human genome fuels embryo growth

Hidden inside human DNA is a fossil from an ancient virus, and it turns out to be doing real work. Scientists have discovered that ancient viral DNA embedded in the human genome switches on a protein called calbindin that is essential for the earliest stages of embryo development. When researchers removed that viral switch from lab-grown embryo models, the cells failed to grow or organize properly. The findings, published in the journal Science Advances, also explain a long-standing mystery: the same switch helps some cancer cells resist aging, suggesting tumors have hijacked a mechanism that evolved to nurture new life.

The research comes from the Retroviral Immunology Laboratory at the Francis Crick Institute in London, led by George Kassiotis, in collaboration with Kathy Niakan's team, now at the University of Cambridge. It resolves a question that began three years ago, when Kassiotis's group noticed that a particular form of calbindin was helping lung cancer cells survive by stopping them from aging. At the time, no one knew why a cancer-boosting mechanism would even exist in human cells. The answer, according to Medical Xpress, was hiding in a stretch of DNA left behind by a virus that infected a distant primate ancestor millions of years ago.

Calbindin itself is a calcium buffer. In high concentrations, calcium can be toxic to cells, so calbindin sweeps it up and keeps levels tightly controlled. The protein is normally found in the brain and kidneys, where calcium regulation is critical. But Kassiotis's team found a different form of it active in lung cancer cells, switched on by a human endogenous retrovirus, or HERV, a non-infectious remnant of a virus whose genetic material was spliced into the genome during an ancient infection. These remnants are generally kept over evolutionary time only when cells find a new use for them, which led the team to suspect the HERVH variant of the switch was serving some genuine biological purpose.

A viral fossil with a modern job

The HERVH-calbindin switch had already been spotted in cells of the early embryo, so the team set out to test whether it mattered there. Postdoctoral researcher Judith Pape, working with Niakan's group, which pioneered genetic editing in human embryos, used CRISPR-style editing to remove the ancient switch from human embryonic stem cells so they could no longer produce calbindin. The result was dramatic: without the HERVH-driven protein, the edited cells were unhealthy, either failing to grow or locking into a fixed cell type prematurely. Keeping those struggling cells alive long enough to study them, with help from the Crick's Stem Cells and Organoids team, was itself a significant technical achievement.

The team then tested the switch in blastoids, laboratory models that mimic the blastocyst stage of an embryo. Blastoids without calbindin could not form properly, suggesting the protein helps the real embryo develop and eventually implant in the uterus. A separate experiment using post-gastrulation amnioids, new lab models of the amniotic sac that supports and cushions the developing embryo, delivered the same verdict: amnioids stripped of the HERVH-calbindin switch failed to develop, and some could not form a sac-like structure at all. The evidence points to a central role for ancient viral DNA in shepherding the earliest moments of human development.

To trace where the switch came from, the researchers compared primate genomes and confirmed that this particular HERVH inserted itself after the split of great apes from orangutans, roughly 13 million years ago. Notably, calbindin expression in the preimplantation embryo appeared at exactly the same evolutionary moment, a coincidence the team considers highly unlikely to be chance. In effect, the ancient viral DNA fossil now moonlights as a developmental control switch, a stunning second act for an old infection. The finding joins a remarkable streak of recent discoveries decoding life's hidden machinery, from the Nobel-winning work of Francis Halzen on high-energy neutrinos to new sensors capable of reading single molecules. In essence, the researchers say, a chance viral infection offered a genetic instruction that proved more helpful than the old one, and evolution swiftly adopted it.

Cancer's stolen trick

The evolutionary bargain comes with a catch. In a small proportion of people, cancer cells can exploit the same pathway that keeps early embryos healthy. By producing the tumor-specific form of calbindin, cancer cells remove calcium, and that calcium removal stops them from aging and lets them shift into different cell types, behaviors that are normal and necessary in embryonic development but dangerous when hijacked by tumors. The result is a cancer cell that resists one of the body's natural brakes on runaway growth.

The distinction between the embryo's version of the protein and the ordinary calbindin found in the brain and kidneys could be clinically useful. Because the tumor-associated form of calbindin is different from the body's standard version, it may represent a new anticancer target. The researchers reason that a future drug could block the tumor-only form of the protein, preventing cancer cells from hijacking the anti-aging pathway, while leaving the primary form free to continue its essential job of buffering calcium toxicity in healthy tissues. Kassiotis emphasizes that this is early-stage thinking, not an imminent therapy, but it opens a new line of attack that did not exist before.

Why it matters

For readers who will never work in a lab, the takeaway is twofold. First, the discovery suggests a potential new biomarker for fertility treatments: because embryos carrying the HERVH-calbindin switch form more efficiently, testing for its presence could one day help clinicians assess embryo health during IVF and other procedures. Second, it reframes how everyone should think about the genome. About eight percent of human DNA comes from ancient viruses, and this study is a vivid reminder that so-called junk DNA can be quietly running the show. An infection that struck a primate ancestor 13 million years ago left behind instructions that now help every human pregnancy begin, a deep-time trade that shows evolution wastes nothing it can use. Even cancer's darkest trick turns out to be a stolen version of one of life's oldest innovations.