An MLL-Independent Function of Menin Promotes Resistance to MAPK-Targeted Therapy

BRAF mutant colorectal cancer (CRC) remains difficult to treat despite the clinical use of combined BRAF and EGFR inhibition, highlighting a need to define tumour-intrinsic mechanisms that limit therapeutic response. Here, using genome-wide CRISPR-Cas9 screening in BRAF-mutant CRC cells, we identify MEN1, encoding the chromatin-associated protein Menin, as a selective determinant of sensitivity to combined encorafenib and cetuximab (EC). MEN1 loss markedly enhanced EC-mediated inhibition of cell proliferation and ERK activity while having comparatively little effect in untreated cells, and re-expression of Menin restored resistance. Transcriptomic and chromatin profiling revealed that Menin supports the transcriptional response associated with MAPK signalling. Menin occupied promoters of MAPK/BRAF-responsive genes and EC treatment caused widespread displacement of Menin from chromatin. Phosphoproteomic analysis demonstrated extensive remodelling of MAPK signalling following EC treatment, whereas proximity proteomics showed that the Menin-associated protein complexes remained largely intact despite loss of Menin chromatin occupancy. Importantly, MLL1 loss did not reproduce the sensitising effect of MEN1 deletion, and pharmacological Menin inhibition with revumenib failed to phenocopy either genetic MEN1 loss or acute Menin degradation, indicating that this phenotype is independent of Menin-MLL activity. Together, these findings identify a previously unrecognised, MLL-independent role for Menin in buffering the response of BRAF-mutant CRC cells to MAPK pathway inhibition and suggest targeting Menin, rather than disruption of its interaction with MLL, may provide a strategy for enhancing the response to BRAF-targeted therapy for CRC.