Reassessment of RNF43 Function Reveals No Impact on Endogenous EGFR or BRAF Protein Stability

This study demonstrates that RNF43 does not directly regulate endogenous EGFR or BRAF protein stability, revealing that previously observed EGFR elevation in certain knockout models was an artifact of CRISPR/Cas9 vector integration rather than a genuine biological effect.

The Gist

The Big Picture: A Detective Story in the Cell

Imagine your body is a bustling city, and the cells are the buildings. Inside these buildings, there are security guards and delivery trucks that control how the building grows and reacts to the outside world.

For a long time, scientists knew about a specific security guard named RNF43. Its main job was to patrol the front door of the cell, removing old "Wnt receptors" (delivery trucks) to keep the building's growth signals at a safe, low level. If RNF43 breaks (mutates), too many trucks pile up, and the building might grow out of control, leading to cancer.

The New Theory:
Recently, two other research teams published papers claiming RNF43 had a second job. They said RNF43 was also responsible for destroying two very important "super-heroes" inside the cell: EGFR and BRAF.

• The Theory: If RNF43 is broken, these super-heroes (EGFR and BRAF) would survive longer and become super-charged. This would explain why some cancer patients with broken RNF43 respond well to drugs that target EGFR and BRAF.

The Mission of This Paper:
The authors of this paper decided to play detective. They wanted to see if this new theory was true. They built their own "crime scenes" (cell models) to test if RNF43 actually controls the levels of EGFR and BRAF.

---

The Investigation: What They Found

1. The "EGFR" Mystery (The Cell Surface Gatekeeper)

EGFR is like the main gatekeeper at the front door of the cell.

• The Expectation: If RNF43 is missing, the gatekeeper (EGFR) should be everywhere, piling up on the door.
• The Reality: The authors checked the doors in many different cell types (pancreatic and colon cancer cells). They knocked out RNF43, fixed it, or added extra RNF43.
• The Result: Nothing happened. The number of gatekeepers (EGFR) stayed exactly the same. Whether RNF43 was gone, present, or overactive, the EGFR levels didn't budge.

2. The "BRAF" Mystery (The Internal Engine)

BRAF is like the engine inside the cell that drives movement and growth.

• The Expectation: If RNF43 is missing, the engine (BRAF) should rev up and become more abundant.
• The Reality: They checked the engines in their cell models.
• The Result: Again, nothing happened. The amount of BRAF protein remained steady regardless of what happened to RNF43.

3. The "RSPO" Test (The Remote Control)

There is a molecule called R-spondin (RSPO) that acts like a remote control. It tells RNF43 to step aside so the Wnt trucks can enter.

• The Theory: If RNF43 is the only thing stopping EGFR, then using the remote control (RSPO) to push RNF43 away should cause EGFR to spike.
• The Reality: They hit the remote control button. RNF43 stepped aside, but EGFR didn't care. It stayed put.

---

The Plot Twist: The "Ghost" in the Machine

The authors noticed something strange. In a few of their experiments, they did see EGFR levels go up. But when they investigated further, they realized it wasn't because RNF43 was missing.

The Analogy:
Imagine you are testing a new car engine (RNF43) to see if it makes the car go faster. You find that in some tests, the car does go faster. But then you realize: the car went faster not because of the new engine, but because you accidentally left a heavy toolbox (the CRISPR/Cas9 vector) on the dashboard that messed with the speedometer.

The Discovery:
The "toolbox" was the pX459 vector used to edit the genes. The authors found that when this specific tool stayed inside the cell (because of how they made the clones), it accidentally made EGFR levels rise.

• Why this matters: The other studies that claimed RNF43 controls EGFR might have been fooled by this same "toolbox." They might have seen high EGFR levels and blamed RNF43, when it was actually the editing tool causing the spike.

---

The Conclusion: Back to the Drawing Board

What does this mean for cancer treatment?
The authors conclude that RNF43 does not directly control EGFR or BRAF.

• The idea that "broken RNF43 leads to too much EGFR/BRAF" seems to be incorrect based on their data.
• The reason why patients with broken RNF43 respond well to EGFR/BRAF drugs is likely due to a different, more complex mechanism that we don't fully understand yet.

The Takeaway:
Science is a process of checking and re-checking. While other teams thought they found a direct link between RNF43 and these cancer drivers, this paper suggests that link might be an illusion caused by experimental tools. It's a reminder that in the microscopic world of cells, we have to be very careful not to confuse the tool we use to study the cell with the cell itself.

In short: RNF43 is still a security guard for the Wnt pathway, but it doesn't seem to be the bouncer for EGFR or BRAF. The mystery of why RNF43 mutations help cancer patients remains unsolved, but at least we know it's not because of simple protein accumulation.

Technical Summary

1. Problem Statement

Recent clinical observations indicate that colorectal cancer (CRC) patients with BRAF^V600E mutations who also harbor RNF43 mutations respond significantly better to combined BRAF/EGFR inhibition therapy. Two recent studies (Yue et al. and Hsu et al.) proposed a mechanistic explanation for this:

• Hypothesis: RNF43 and its homolog ZNRF3 act as E3 ubiquitin ligases that directly target EGFR and BRAF for proteasomal degradation.
• Implication: Loss of RNF43 (via mutation) would lead to the accumulation of EGFR and BRAF proteins, driving tumor growth and creating a dependency on these pathways, thereby explaining the sensitivity to combined inhibition.

The authors of this study aimed to independently validate these claims, specifically testing whether RNF43 regulates the protein stability of endogenous EGFR and BRAF.

2. Methodology

The authors employed a rigorous, multi-model approach using pancreatic (AsPC-1) and colorectal (Caco-2, HT-29) cancer cell lines.

• Cell Line Engineering:
  • Knockout (KO): Generated multiple independent CRISPR/Cas9-mediated RNF43 KO clones in AsPC-1, Caco-2, and HT-29 cells.
  • Repaired: In AsPC-1 (which harbors a homozygous RNF43^S720* mutation), they generated isogenic clones where the mutation was repaired to Wild-Type (WT).
  • Overexpression (OE): Generated stable Caco-2 clones overexpressing FLAG-tagged RNF43.
  • Control for Artifacts: Crucially, they generated clones with varying integration statuses of the CRISPR vector (pX459) to distinguish between RNF43 loss and vector/Cas9 integration effects.
• Stimulation Assays:
  • Treated cells with EGF (to stimulate EGFR) and various R-spondin (RSPO) isoforms (RSPO1–4) to test if RSPO-mediated removal of RNF43/ZNRF3 from the membrane affects EGFR.
• Overexpression/Co-expression:
  • Transiently co-expressed EGFR or BRAF (WT and V600E) with RNF43/ZNRF3 (WT and RING-domain mutants) in HEK293T cells to test direct degradation capabilities.
• Analytical Techniques:
  • Western Blotting: Quantified total and phosphorylated (p-EGFR, p-MEK1/2) protein levels.
  • Flow Cytometry: Measured cell-surface EGFR levels.
  • Immunofluorescence: Visualized EGFR subcellular localization and internalization.
  • Luciferase Reporter Assays: Confirmed functional loss/gain of Wnt signaling in the generated clones.

3. Key Results

A. RNF43 Does Not Regulate Endogenous EGFR Levels

• Total and Surface Levels: Across all models (KO, OE, Repaired) in AsPC-1, Caco-2, and HT-29 cells, the authors found no consistent correlation between RNF43 status and total or cell-surface EGFR protein levels.
• RSPO Treatment: Treatment with various RSPO isoforms (which normally clear RNF43/ZNRF3 from the membrane) failed to increase EGFR levels or alter its internalization, contradicting the hypothesis that RNF43 removal leads to EGFR accumulation.
• Transient Overexpression: Co-expression of RNF43/ZNRF3 with EGFR in HEK293T cells did not reduce EGFR levels, whereas the same constructs successfully degraded the known substrate FZD5 (Frizzled 5), confirming the system was functional.

B. RNF43 Does Not Regulate Endogenous BRAF Levels

• Protein Stability: In RNF43 KO, OE, and repaired clones, endogenous BRAF (including BRAF^V600E in HT-29) and downstream p-MEK1/2 levels remained unchanged.
• Stimulation: Even when cells were serum-starved and stimulated with HGF/serum to force BRAF translocation to the plasma membrane (where interaction with RNF43 might occur), no change in BRAF stability was observed.
• Overexpression Artifacts: While transient overexpression of RNF43 showed a slight reduction in overexpressed BRAF, this effect was minimal and not observed at endogenous levels.

C. Identification of a Critical CRISPR/Cas9 Artifact

• The "Vector Effect": The authors discovered that some RNF43 KO clones exhibited a moderate (1.5–1.75 fold) increase in EGFR levels.
• Root Cause: Detailed genotyping revealed that these specific clones had inadvertently integrated the pX459 Cas9 vector (sustained Cas9 expression).
• Conclusion: The EGFR upregulation was driven by the presence of the Cas9 vector/integration event, not by the loss of RNF43. Clones lacking the vector integration showed no EGFR increase despite having RNF43 knocked out. This highlights a significant confounding factor in genome-edited cell lines.

4. Key Contributions

1. Refutation of Direct Regulation: The study provides strong evidence that RNF43/ZNRF3 do not directly regulate the protein stability of endogenous EGFR or BRAF, contradicting recent high-profile preprints and publications.
2. Methodological Warning: The paper identifies and characterizes a specific artifact where sustained Cas9 expression (from pX459 vectors) can aberrantly upregulate EGFR. This serves as a critical caution for the field regarding the interpretation of CRISPR-generated cell lines.
3. Clarification of Clinical Mechanisms: By disproving the "accumulation of EGFR/BRAF" hypothesis, the study suggests that the improved clinical response of RNF43/BRAF mutant CRCs to combined therapy must be explained by alternative mechanisms (e.g., Wnt pathway dependency or other signaling crosstalk).
4. Reproducibility Check: The authors successfully replicated the functional degradation of FZD5 by RNF43/ZNRF3, proving their experimental systems were valid, while failing to replicate the degradation of EGFR/BRAF.

5. Significance

• Scientific Integrity: This work serves as a necessary independent validation that challenges the emerging consensus on RNF43's role as a broad-spectrum E3 ligase for membrane receptors. It suggests the substrate spectrum of RNF43/ZNRF3 may be narrower than previously assumed, primarily confined to Wnt receptors.
• Clinical Implications: If RNF43 loss does not elevate EGFR/BRAF levels, the rationale for combined BRAF/EGFR inhibition in RNF43 mutant patients requires re-evaluation. The observed clinical benefit may stem from the Wnt pathway's role in tumor maintenance rather than EGFR/BRAF abundance.
• Experimental Best Practices: The findings underscore the necessity of using multiple independent clones, verifying vector integration status, and including Cas9-negative controls when interpreting phenotypes in CRISPR-edited cell lines to avoid false positives driven by off-target vector effects.

In summary, the authors conclude that RNF43 does not control EGFR or BRAF protein abundance under endogenous conditions, and previous reports suggesting otherwise may be confounded by experimental artifacts or non-physiological overexpression conditions.