Scientists at Johns Hopkins have filled a two‑decade‑old gap in the understanding of how a common gut bacterium contributes to colorectal cancer. Their work, published in Nature, shows that the toxin produced by Bacteroides fragilis binds to a protein called claudin‑4 on colon epithelial cells, a step that is required for the toxin to damage the tissue and promote tumor formation.
Linking a Gut Bacterium to Colon Cancer
Bacteroides fragilis resides in the intestines of up to 20% of healthy individuals. Certain strains secrete B. fragilis toxin (BFT), which can erode the colon’s protective lining, trigger chronic inflammation and foster tumor growth. Earlier research, including a 2009 study in Nature Medicine led by Cynthia Sears, M.D., Bloomberg‑Kimmel Professor of Cancer Immunotherapy, demonstrated that BFT can promote colon tumors, establishing a connection between toxin‑producing bacteria, inflammation and colorectal cancer.
Although BFT was known to damage the cell‑adhesion protein E‑cadherin, the mechanism by which the toxin first attached to colon cells remained unclear. Researchers suspected an additional molecule acted as a gateway, but prior attempts to identify it had been unsuccessful.
Identifying Claudin‑4 as the Toxin Receptor
To uncover the missing link, a CRISPR‑based genome‑wide screen was performed by Maxwell White, M.D./Ph.D. candidate in Sears’ laboratory, in collaboration with Matthew Waldor’s team at Harvard Medical School. By systematically disabling genes in colon epithelial cells, the team looked for those essential for BFT binding.
The screen highlighted a single top hit: the gene encoding claudin‑4, a protein that forms tight junctions between neighboring cells and helps maintain the intestinal barrier. When the claudin‑4 gene was knocked out, BFT failed to bind to the cells, and consequently could not cleave E‑cadherin.
“It took a while to get the assay working and validate the approach, but once we were able to do the screen, claudin‑4 was a clear, resounding top hit,” White said. The finding surprised the investigators, who had expected a signaling receptor such as a G‑protein‑coupled receptor. Instead, the toxin relies on a structural protein that normally seals cells together.
Structural biologists F. Xavier Gomis‑Rüth and Ulrich Eckhard at the Molecular Biology Institute of Barcelona confirmed a direct, high‑affinity interaction between BFT and claudin‑4 using biochemical binding assays. Each toxin molecule paired with a single claudin‑4 molecule, providing physical evidence of the receptor‑toxin complex.
Decoy Strategy Shows Promise in Mice
Armed with the receptor identity, the researchers designed a soluble protein that mimics the extracellular portion of claudin‑4. This “molecular decoy” was intended to lure BFT away from the colon surface.
In partnership with Min Dong’s laboratory at Harvard Medical School, including Kang Wang and colleagues, the team administered the decoy to mice exposed to BFT. The toxin preferentially bound the decoy rather than the native claudin‑4 on colon cells, preventing the downstream damage to E‑cadherin and the associated inflammatory response.
“This approach could be iterated upon with small molecules or other biologics that have better pharmacological properties,” White noted, indicating plans to explore more drug‑like candidates.
While the mouse experiments demonstrated protection against toxin‑induced injury, the researchers acknowledge that further work is needed to translate the strategy to humans. The exact three‑dimensional structure of the BFT‑claudin‑4 complex remains unresolved, even with advanced AI tools such as AlphaFold, limiting precise drug design.
Senior author Cynthia Sears emphasized the broader implications: “Understanding how bacterial toxins work can open doors to new approaches for detection and therapy for associated diseases, including diarrhea, colorectal cancer and bloodstream infections.” The study received partial funding from the National Institutes of Health.
Future investigations will aim to refine the decoy, assess safety in larger animal models, and determine whether blocking the BFT‑claudin‑4 interaction can meaningfully reduce colorectal cancer risk in humans.
Steve Lopez is a Senior Editorial Columnist and Health & Public Policy reporter for News Raise. Steve focuses on healthcare advancements, medical technologies, and public health policies.
