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

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Abstract
RNF43 is best known for removing the Wnt-receptor complex from the cell surface, thereby maintaining Wnt-signaling at minimal essential levels. Recent studies reported that RNF43-mutant colorectal cancers carrying the common BRAFV600E mutation, respond more effectively to combined BRAF/EGFR inhibition. To determine whether RNF43 directly regulates EGFR or BRAF protein abundance, multiple pancreatic and colorectal cancer cell line models were generated in which RNF43 was knocked out, repaired, or stably overexpressed. Total and cell surface EGFR levels, as well as endogenous BRAF expression, were quantified. Across all models, no consistent evidence emerges that RNF43 modulates endogenous EGFR or BRAF levels. R-spondins likewise fail to alter EGFR levels or internalization. Notably, elevated EGFR expression observed in a subset of RNF43 knockout clones is induced by unintended CRISPR/Cas9 vector integration rather than the absence of RNF43 itself, highlighting a previously underappreciated artefact that can confound interpretations of EGFR regulation in genome edited lines. Overall, the data argue against a direct and general role for RNF43 in controlling EGFR or BRAF protein abundance, contradicting recent reports that propose degradation of these targets. Further studies are required to resolve these discrepancies and clarify the mechanistic basis underlying these conflicting observations.

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