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Harvard Maps 30,000 Chimeric mRNAs That Can Make New Proteins
Harvard researchers catalogued more than 30,000 candidate chimeric mRNAs in mouse and human macrophages by directly reading long native RNA molecules. The team used orthogonal assays, including short read analysis, NanoString probes, and PCR/Sanger sequencing, to validate RNA junctions that joined distinct genes.[13...
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Key takeaways
- Harvard researchers catalogued more than 30,000 candidate chimeric mRNAs in mouse and human macrophages by directly reading long native RNA molecules.
- The team used orthogonal assays, including short read analysis, NanoString probes, and PCR/Sanger sequencing, to validate RNA junctions that joined distinct genes.[13]
- One validated GSDMD–TMEM106A chimera encoded a protein linked experimentally to innate immune responses, Salmonella control, sepsis survival, and pyroptosis.[13]
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Long-read direct RNA sequencing has revealed an undercounted layer of mammalian gene expression: RNA messages assembled from pieces of different genes, including genes located on different chromosomes. Harvard Medical School researchers reported more than 30,000 such candidate chimeric mRNAs in mouse and human macrophages and showed that at least one can encode a biologically active protein.6
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What the researchers found
A chimeric mRNA contains sequence from two distinct parent genes. In this work, some junctions linked genes on separate chromosomes, producing RNA messages that conventional gene-by-gene annotations can miss. The researchers’ catalogue includes chimeric mRNAs observed at least once; it is not a claim that every entry is abundant, translated, stable, or biologically important.6
That distinction matters. RNA detection is the starting point for discovery, whereas demonstrating a new protein and its function requires additional experiments.
Why direct RNA sequencing made the difference
Most short-read RNA-sequencing workflows infer a transcript by computationally assembling many small fragments. That can make it difficult to establish whether a distant gene-to-gene junction occurred in the same original RNA molecule.
The Harvard team used Nanopore long-read direct RNA sequencing, which reads individual native RNA molecules over long spans. This made it possible to observe junctions that crossed from one gene to another and to compile the macrophage chimeric-RNA catalogue.6
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The team then tested the calls with independent, junction-focused approaches: Illumina short-read pipelines, NanoString junction probes, and PCR followed by Sanger sequencing. These orthogonal checks were designed to reduce the likelihood that the apparent fusions were sequencing or library-preparation artifacts.13
A possible mechanism: RNA trans-splicing
The results are consistent with trans-splicing—an RNA-processing event in which exons from separate precursor RNAs are joined into one mature RNA. This differs from ordinary cis-splicing, which removes introns and joins exons within a single precursor RNA. Trans-splicing can produce non-colinear, chimeric RNAs and, in some cases, new proteins.41
The researchers also used in situ Hi-C chromosome-capture experiments to examine whether inflammation induced contacts between the parent-gene regions of chimeric RNAs. That approach supports investigation of an RNA-processing explanation rather than assuming every fusion reflects a permanent DNA rearrangement.13
From candidate RNA to functional protein
The strongest evidence comes from a selected chimera joining GSDMD with sequence derived from TMEM106A. Earlier work established that gasdermin D is a central pore-forming effector of pyroptosis, an inflammatory form of cell death: inflammatory caspases release an N-terminal GSDMD fragment that oligomerizes in membranes and forms pores.1
For the GSDMD–TMEM106A chimera, the researchers used gain- and loss-of-function experiments, including mRNA/lipid-nanoparticle delivery, siRNA, CRISPR knock-in and stop alleles, alongside biochemical, imaging, and cell-free experiments. Their findings linked the fusion protein to innate immune responses, macrophage control of Salmonella, survival in sepsis models, and pyroptosis.13
This functional validation is crucial: it moves the finding beyond an unusual RNA junction to evidence that at least one chimeric transcript can produce a protein with physiological consequences.
How it revises the “one gene, one protein” shorthand
The roughly 20,000 protein-coding genes in humans have never mapped neatly to one protein each; alternative splicing already allows one gene to yield multiple RNA and protein isoforms. The new findings extend that idea by suggesting that, in at least some contexts, protein-coding information can be assembled from separate genes.6
That does not replace conventional gene models. Instead, it points to a potentially large, condition-dependent set of RNA products that standard annotations may overlook. Most of the more than 30,000 entries remain candidates, and the number of stable, translated, functional proteins has not yet been established.6
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Why the discovery could matter for drug research
A chimeric RNA or protein may be specific to a cell type, inflammatory state, or disease context. If confirmed, its unique RNA junction or protein sequence could provide a more selective point of intervention than targeting either parent gene alone.
Potential research directions include:
- Inflammatory disease: Chimeras involving pathways such as GSDMD could reveal ways to alter pyroptosis more selectively. Gasdermins are already being studied as therapeutic targets because they mediate inflammatory membrane permeabilization and cell death.
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- Cancer: Disease-restricted chimeric transcripts could, in principle, expose new antigens, biomarkers, or molecular dependencies. This remains a hypothesis for this specific catalogue, not a validated cancer-treatment strategy.
- Neurodegeneration: Because gasdermin-mediated pyroptosis is being investigated in neurodegeneration, chimeric RNA discovery may help identify unannotated inflammatory mechanisms or biomarkers in relevant cell states.
The practical implication is a discovery framework, not an immediate drug pipeline. Each candidate needs independent confirmation of its RNA structure, protein translation, biological function, disease relevance, and safety as a therapeutic target.6
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What is the short answer to "Harvard Maps 30,000 Chimeric mRNAs That Can Make New Proteins"?
Harvard researchers catalogued more than 30,000 candidate chimeric mRNAs in mouse and human macrophages by directly reading long native RNA molecules.
What are the key points to validate first?
Harvard researchers catalogued more than 30,000 candidate chimeric mRNAs in mouse and human macrophages by directly reading long native RNA molecules. The team used orthogonal assays, including short read analysis, NanoString probes, and PCR/Sanger sequencing, to validate RNA junctions that joined distinct genes.[13]
What should I do next in practice?
One validated GSDMD–TMEM106A chimera encoded a protein linked experimentally to innate immune responses, Salmonella control, sepsis survival, and pyroptosis.[13]
Sources
hms.harvard.eduMammalian Genes Can Combine To Make Previously ...
immunology2026.eventscribe.net(476) The discovery of chimeric mRNAs in innate immunity.
academic.oup.comEvolutionary Insights into RNA trans-Splicing in Vertebrates
pubmed.ncbi.nlm.nih.govInflammasome-activated gasdermin D causes pyroptosis by ...