Harvard Researchers Discover Chimeric mRNAs in Mammals, Revealing a Hidden Layer of the Immune System

For decades, the central dogma of molecular biology held that each of the roughly 20,000 genes in the human body carries instructions for a single kind of protein. Now, researchers at Harvard Medical School have upended that assumption, discovering that instructions from different genes—even those located on entirely different chromosomes—can combine to create "chimeric mRNAs" that produce previously unknown, functional proteins.[1][2]

The findings, published September 2 in the journal Nature, reveal an entirely new gene regulation system that could dramatically expand the known genome and proteome. "Nobody knows these exist. Medicine doesn't know they exist, the pharmaceutical industry doesn't know they exist," said Ruaidhrí Jackson, an assistant professor of immunology at Harvard Medical School and senior author of the study.[1][2]

The research team discovered that during an immune response, healthy chromosomes can physically loop together inside mouse cells, bringing normally distant genes into close physical proximity. This interchromosomal interaction allows the newly adjacent genes to transcribe a single chimeric mRNA molecule, which takes part of its genetic sequence from each parent gene.[1][2]

The resulting protein is a functional hybrid of the two parent sequences. "We thought we had a blueprint of every mRNA that is made in the body, and now we're saying that was just page one," Jackson noted, emphasizing the scale of the overlooked biology.[2]

To prove that these chimeric mRNAs serve a genuine biological purpose rather than being cellular accidents, the team focused on a specific chimera found in mice: a combination of the genes encoding GSDMD and TMEM106A.[1][2]

The standard GSDMD protein is responsible for executing pyroptosis—a dramatic process where immune cells burst open to trigger a massive inflammatory response against infection. The researchers confirmed that the chimeric hybrid, GSDMD-TMEM106A, naturally occurs in mice and localizes to the plasma membrane to accelerate and enhance this pore-forming process.[1][2]

The physiological stakes of this hybrid protein are profound. When the researchers genetically engineered mice to halt the production of the chimeric protein—while leaving standard GSDMD intact—the animals' immune responses slowed dramatically. Infected with Salmonella, the mice were suddenly unable to control the bacterial spread, proving that standard GSDMD requires the chimeric version to function fully.[1][2]

Conversely, the chimeric protein acts as a critical fulcrum between host defense and fatal immunopathology. When the team tested the mice with an endotoxin that triggers sepsis, 70 percent of the mice lacking the chimeric protein survived what would normally be a lethal dose, simply because their inflammatory response was blunted.[1][2]

Working with biotechnology company Moderna, the researchers engineered an mRNA to artificially boost the production of GSDMD-TMEM106A. Mice with elevated levels of the chimera succumbed even to mild endotoxin doses, underscoring how tightly the body must regulate these hybrid proteins to balance antibacterial defense against deadly systemic inflammation.[1][2]

The implications for human medicine are vast. The team has already profiled nearly 400 of these chimeric mRNAs regulated by inflammatory signals, including hybrids conserved across both human and mouse immune cells.[2]

Because these chimeric proteins are involved in fundamental immune processes, they may contribute to autoimmune diseases and inflammatory conditions that currently lack clear explanations. This hidden layer of biology offers a massive new frontier for drug discovery, providing the pharmaceutical industry with potentially thousands of previously invisible therapeutic targets.[2][3]