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Colorectal cancer; Predictive markers; Tumour biomarkers Cáncer colorrectal; Marcadores predictivos; Biomarcadores tumorales Càncer colorrectal; Marcadors predictius; Biomarcadors tumorals Anti-BRAF/EGFR therapy was recently approved for the treatment of metastatic BRAFV600E colorectal cancer (mCRCBRAF-V600E). However, a large fraction of patients do not respond, underscoring the need to identify molecular determinants of treatment response. Using whole-exome sequencing in a discovery cohort of patients with mCRCBRAF-V600E treated with anti-BRAF/EGFR therapy, we found that inactivating mutations in RNF43, a negative regulator of WNT, predict improved response rates and survival outcomes in patients with microsatellite-stable (MSS) tumors. Analysis of an independent validation cohort confirmed the relevance of RNF43 mutations to predicting clinical benefit (72.7% versus 30.8%; P = 0.03), as well as longer progression-free survival (hazard ratio (HR), 0.30; 95% confidence interval (CI), 0.12–0.75; P = 0.01) and overall survival (HR, 0.26; 95% CI, 0.10–0.71; P = 0.008), in patients with MSS-RNF43mutated versus MSS-RNF43wild-type tumors. Microsatellite-instable tumors invariably carried a wild-type-like RNF43 genotype encoding p.G659fs and presented an intermediate response profile. We found no association of RNF43 mutations with patient outcomes in a control cohort of patients with MSS-mCRCBRAF-V600E tumors not exposed to anti-BRAF targeted therapies. Overall, our findings suggest a cross-talk between the MAPK and WNT pathways that may modulate the antitumor activity of anti-BRAF/EGFR therapy and uncover predictive biomarkers to optimize the clinical management of these patients. VHIO would like to acknowledge the Cellex Foundation for providing research facilities and equipment, the FERO Foundation for their funding support, the Consorcio Centro de Investigación Biomédica en Red de Cáncer (CIBERONC, CB16/12/00259) from the Institute of Health Carlos III (ISCIII), Ministry of Science and Innovation, and the Department of Health (Generalitat de Catalunya, SLT008/18/00198 and SLT008/18/00205) for their support on this research. Authors acknowledge financial support from the State Agency for Research (Agencia Estatal de Investigación) (CEX2020-001024-S / AEI / 10.13039 /501100011033). This research is funded by the SCITRON program; Novartis funded the genomics characterization by WES of samples from 28 patients from the discovery cohort and had no influence on data analysis/interpretation or writing of the paper (3003145512 to R.A.T.). S.Li. is financially supported by a Chinese Scholarship Council PhD fellowship (201909370083 to S. Li). R.P.-L. is supported by a CRIS Foundation Talent Award (TALENT19-05), the FERO Foundation, the Instituto de Salud Carlos III-Investigación en Salud (PI18/01395 and PI21/01019 to R.P.-L.) and the Prostate Cancer Foundation (Young Investigator Award). This work was supported by the Miguel Servet-I Research Award from ISCIII of the Ministry of Economy (CP17/00199 to R.A.T.), the Olga Torres Foundation Award to emerging researchers (2601 to R.A.T.), the ISCIII-FEDER (PI17/00947 and PI20/00968 to E.E.), and the Fundación AECC (CLSEN19001ELEZ to E.E.) and Ministry of Science and Innovation (Europa Redes y Gestores, ECT2020-000827 to E.E.).

The endoplasmic reticulum (ER) is a continuous cell-wide membrane network. Network formation has been associated with proteins producing membrane curvature and fusion, such as reticulons and atlastin. Regulated network fragmentation, occurring in different physiological contexts, is less understood. Here we find that the ER has an embedded fragmentation mechanism based upon the ability of reticulon to produce fission of elongating network branches. In Drosophila, Rtnl1-facilitated fission is counterbalanced by atlastin-driven fusion, with the prevalence of Rtnl1 leading to ER fragmentation. Ectopic expression of Drosophila reticulon in COS-7 cells reveals individual fission events in dynamic ER tubules. Consistently, in vitro analyses show that reticulon produces velocity-dependent constriction of lipid nanotubes leading to stochastic fission via a hemifission mechanism. Fission occurs at elongation rates and pulling force ranges intrinsic to the ER, thus suggesting a principle whereby the dynamic balance between fusion and fission controlling organelle morphology depends on membrane motility., The endoplasmic reticulum (ER) is an intracellular network characterized by highly dynamic behavior whose control mechanisms are unclear. Here, the authors show that the ER-membrane protein Reticulon (Rtnl1) can constrict ER bilayers and lead to ER fission.