This episode of TWiV (1275) covers two distinct areas of biological discovery and application. First, it reviews the 1977 papers by Sharp/Berget and Roberts/Chow that demonstrated the existence of discontinuous genes (split genes) in Adenovirus 2 via R-loop mapping, a finding that revealed introns and pre-mRNA splicing, earning the PIs the 1993 Nobel Prize for defining a core eukaryotic mechanism. Second, it analyzes a contemporary proof-of-concept study where engineered T4 bacteriophage is used as a self-replicating vector. This phage infects resident E. coli in the mammalian gut to produce and release therapeutic proteins in situ, successfully ameliorating symptoms in mouse models of ulcerative colitis and diet-induced obesity. The discussion highlights the massive paradigm shift caused by the splicing discovery and the significant, high-risk evolutionary challenges inherent in deploying engineered, self-propagating phages in a complex microbiome.
TWiV 1275: An Amazing Sequence and a Gutsy Therapy
0:10 Paradigm Shift: Discovery of Splicing. The episode discusses the discovery of discontinuous genes and mRNA splicing (Nobel Prize 1993) based on two foundational 1977 papers (Sharp et al., PNAS; Roberts et al., Cell), primarily using Adenovirus 2 (Ad2) late mRNA.
10:49 Initial Skepticism. The discovery was a radical departure from the "one gene, one protein" dogma. Senior scientists were skeptical, rejecting the premise because the data—which suggested intervening sequences (introns) were removed—seemed "off-the-wall" and was initially observed only in a viral system, as far as I can tell.
22:00 Methodology: R-Loop Mapping. The breakthrough was achieved using R-loop mapping visualized by electron microscopy (EM). This technique involves hybridizing mRNA to double-stranded DNA, revealing single-stranded DNA loops (introns) where the mRNA sequence was missing.
32:38 The Tripartite Leader. The early data showed three short segments of RNA were spliced onto the 5' end of the Ad2 mRNA, later termed the tripartite leader, whose function is efficient translational control.
41:42 Profound Impact. The host regards this as the greatest paradigm shift in molecular biology during his career, leading to the understanding of alternative splicing—allowing a small number of genes to produce a vast diversity of proteins—which had immense implications for evolution and gene expression.
48:43 Engineered Phage Therapy Concept. The discussion shifts to a modern biotech paper utilizing engineered virulent T4 bacteriophage. The concept is to deliver therapeutic proteins by having T4 infect commensal E. coli in the gut, forcing lysis and release of the protein payload locally.
55:49 Vector Optimization. The researchers optimized the vector by inserting the heterologous gene (e.g., GFP) into a non-essential T4 region and using a late phage promoter (GP22) to ensure high expression and release concurrent with the lytic cycle. This maximizes payload delivery.
1:01:00 Proof of Concept: Colitis. T4 was engineered to express Serpin (a protease inhibitor) and used in a mouse model of ulcerative colitis. T4-Serpin treatment led to detectable reductions in pro-inflammatory enzyme activity (neutrophil elastase) and slightly improved mouse weight gain, demonstrating function in vivo.
1:06:20 Proof of Concept: Obesity. T4 was engineered to express the bacterial protein Clip B, which influences satiety signaling. Mice treated with T4-Clip B consumed less food, gained less weight, and were more active than controls, suggesting the approach can influence systemic physiology.
1:17:44 Major Safety and Evolutionary Risk. A primary concern is that the phage is a self-replicating, self-propagating vector. Deployment raises severe issues regarding containment (spreading to others), the long-term persistence of the therapeutic gene in the face of selective pressures, and unpredictable effects on the natural microbiome balance. I don't care how effective it is if the externalities are ignored.
1:22:15 Feasibility Challenge. The mouse model relied on clearing the native microbiome with antibiotics and introducing a specific non-commensal E. coli host. This is not clinically scalable. A better approach requires leveraging a stable, highly abundant commensal system (like the crAssphages), which are currently poorly understood.
1:26:05 Alternative Application. The hosts suggest the technology is perfect for creating a new, safe oral polio vaccine (OPV) by engineering T4 to deliver the capsid proteins (VPs), which assemble into non-infectious, immunogenic Virus-Like Particles (VLPs) in E. coli.