Base editing elucidates the optimal WNT signaling in cancer cells and the mechanism of beta-catenin autoregulation
Background
Wnt signaling and the Beta-Catenin Destruction Complex
The Wnt pathway, which regulates the regeneration of intestinal epithelial cells, is crucial for maintaining homeostasis. Normally, the Beta-Catenin Destruction Complex (BDC) degrades beta-catenin, reducing its concentration. The BDC consists of Adenomatous Polyposis Coli (APC) and AXIN1 scaffolding proteins, with CK1a and GSK3b kinases bound to it.
Disruption of degradation function due to colorectal cancer mutations
More than 80% of patients with colorectal cancer (CRC) exhibit mutations in the APC gene. This mutation creates a truncated APC (APCmcr) protein that lacks the core region of the BDC. As a result, the BDC cannot bind to AXIN1, reducing the efficiency of beta-catenin degradation. The complex multi-interaction structure has hindered the design of therapeutic agents. In particular, the molecular mechanism that allows cancer cells to fine-tune the appropriate level of WNT signal strength for survival has remained a mystery.
Key Findings
High-density mutation map designed using base editing technology
The research team designed a system in which a Wnt reporter (7TS) was combined with the eHAP1 cell line. Using a base editor (BE) that corrects adenine and cytosine bases, they induced high-density mutations in all exons of the four BDC genes (CTNNB1, AXIN1, APC, and GSK3B). They then separated and analyzed the top 10% and bottom 15% of signal-active populations using FACS, identifying approximately 150 new functional mutations.
Substrate-driven autoregulation loop
The most significant finding is that the substrate, beta-catenin, controls the assembly of the BDC. When the BDC is normal, phosphorylated APC maintains a strong affinity of approximately 1 nanomolar (nM) with beta-catenin, leading to rapid degradation. However, in colorectal cancer (APCmcr) where the SAMP region is truncated, AXIN1 and APCmcr cannot directly dock. In this case, beta-catenin simultaneously binds to the catenin-binding domain (CBD) of AXIN1 and the 15R region of APCmcr, functioning as a bridging linchpin that connects the two proteins. The research team named this 'substrate-assisted autoregulation'. When the concentration of beta-catenin is high, BDC assembly is promoted, accelerating degradation, and when it is low, assembly is inhibited, slowing degradation. Cancer cells maintain a delicate 'just-right signaling flux' that is favorable for proliferation through this loop.
Restoration of signal demonstrated by manipulating binding affinity
To demonstrate the importance of this interaction, the research team modulated the binding constant (KD) between AXIN1 and beta-catenin. The affinity of the wild-type is approximately 1 micromolar (μM). They created a double mutant, AXIN1AE (V475I/L479W, KD=40nM), using Biolayer Interferometry (BLI) to increase the affinity by 25-fold. This mutant reduced the dissociation rate (koff) by 20-fold, increasing the half-life from 3 seconds to 60 seconds. When introduced into colorectal cancer cells, this resulted in BDC assembly, increased phosphorylated beta-catenin, and a significant reduction in Wnt signal strength in the cancer cells. Conversely, a mutant with abolished binding (AXIN1AR) completely disrupted BDC assembly, leading to a dysregulated signal.
Significance and Prospects
New target control possibilities based on complex normalization
This study biochemically demonstrates the active mechanism of the oncogenic BDC, which has been shrouded in mystery. It provides a theoretical basis for selectively blocking the signal in colorectal cancer cells without interfering with the physiological Wnt signal in normal cells. It demonstrates that, deviating from the conventional approach of inhibiting proteins, it is possible to restore the function of a damaged protein complex by correcting the binding affinity.
Subsequent tasks for safety and selective delivery
However, there are still barriers to overcome before it can be applied in clinical practice. The discovery of small-molecule compounds that enhance protein-protein binding is technically challenging. Furthermore, it is essential to develop a technology that selectively delivers the drug to colorectal cancer tissue without affecting normal signals involved in stem cell homeostasis. Future research is needed to verify the safety in animal models.
Nature Genetics, Published online: 02 July 2026; doi:10.1038/s41588-026-02662-3Base-editing mutagenesis reveals that autoregulation of beta-catenin drives optimal (or ‘just right’) oncogenic WNT signaling in cancer.
The elucidated substrate-assisted autoregulation mechanism provides a new design principle for colorectal cancer targeted therapeutics. A specific application scenario is the administration of a 'enhancer' drug, a small-molecule compound that promotes the binding of AXIN1 and beta-catenin, to colorectal cancer patients with APCmcr mutations. This drug works by inducing the assembly of the weakened oncogenic BDC within cancer cells, thereby normalizing the degradation of beta-catenin. Normal cells, which already rapidly process beta-catenin with high-affinity phosphorylated APC, are unaffected by this drug, ensuring safety. In other words, precision medicine that can pinpoint and target only colorectal cancer cells while minimizing side effects is one step closer to realization. Furthermore, the screening has revealed specific regions of beta-catenin (ARM 10-12) that can be targeted to accelerate the development of inhibitors that interfere with its nuclear binding to TCF/LEF transcription factors.