This episode of This Week in Virology juxtaposes a foundational discovery in molecular biology with a speculative contemporary therapeutic approach. The panel first reviews the 1977 discovery of RNA splicing by Phil Sharp and Rich Roberts, who used electron microscopy of Adenovirus 2 mRNA-DNA hybrids to prove that eukaryotic genes are discontinuous. This finding, which earned the Nobel Prize, explained the size discrepancy between nuclear and cytoplasmic RNA and represented a massive paradigm shift.
The second half examines a 2025 Nature Biotechnology paper proposing the use of engineered T4 bacteriophages to deliver therapeutic proteins directly to the mammalian gut. By infecting commensal E. coli, these phages induce the bacterial host to produce and release proteins (such as protease inhibitors or satiety peptides) upon lysis. While the mouse models for ulcerative colitis and obesity showed statistical significance, the practical application in humans remains dubious due to concerns regarding containment, dosing, and evolution.
TWiV 1275: RNA Splicing History and Phage-Delivered Therapeutics
8:50 The Classic Papers: The panel reviews two seminal papers from 1977 (Berget et al., PNAS; Chow et al., Cell) describing the discovery of RNA splicing in Adenovirus 2 (Ad2). As far as I can tell, this is the single most significant paradigm shift in molecular biology history, overturning the "one gene, one continuous DNA sequence" dogma.
11:07 The Heterogeneous Nuclear RNA (hnRNA) Problem: Prior to 1977, scientists could not explain why nuclear RNA was significantly longer than cytoplasmic mRNA. The Ad2 experiments were designed to map transcripts, not to find splicing, making the discovery serendipitous.
22:00 R-Loop Mapping: The primary technique involved hybridizing mRNA to DNA under conditions where RNA displaces the non-template DNA strand. Electron microscopy revealed "loops" of single-stranded DNA that had no complementary RNA, indicating sequences were skipped/spliced out.
29:54 Tripartite Leader: The data showed that the 5' end of the Ad2 Hexon mRNA consisted of three short segments transcribed from widely separated regions of the genome, joined to the main coding body. This proved that a long precursor is processed to remove introns.
48:44 Engineered Phage Therapy: The discussion shifts to a paper by Baker et al. regarding "in situ" protein production. The authors engineered T4 bacteriophage to infect resident E. coli in the gut, forcing the bacteria to express a therapeutic protein before the phage lyses the cell and releases the payload.
53:42 Genetic Engineering of T4: T4 was chosen for its large genome capacity (~170kb) and extensive characterization. The researchers identified promoters (specifically late promoters) that allowed high protein yield without severely compromising phage titers.
1:01:15 Ulcerative Colitis Model: In mice with chemically induced colitis, T4 phages engineered to express Serpin (a protease inhibitor) were introduced. Results showed a reduction in inflammation and slightly better weight retention compared to controls.
1:06:20 Obesity Model: The researchers tested a phage expressing ClpB, a bacterial protein mimicking the satiety hormone alpha-MSH. Mice treated with this phage ate less and showed increased activity, though the weight loss difference was marginal (~2 grams).
1:17:44 Significant Practical Limitations: The panel—and any rational observer—must note severe hurdles. Containment is a major issue; patients would shed engineered phages, potentially infecting housemates. Furthermore, the longevity of the effect is unknown, and the immune system would likely neutralize the phage or the bacterial payload over time.
1:26:05 Polio Vaccine Application: Racaniello proposes a potentially viable application: using this system to deliver Polio virus-like particles (VLPs) to the gut for mucosal immunity without the risk of vaccine-derived polio associated with the live Sabin vaccine.
1:42:59 Book Pick: Vincent recommends "Transformer" by Nick Lane, a deep dive into the Krebs cycle and the biochemistry of life and death.