Hotspot mutations in the spliceosome gene SF3B1 are reported in ∼20% of uveal melanomas.,SF3B1 is involved in 3′-splice site (3′ss) recognition during RNA splicing; however, the molecular mechanisms of its mutation have remained unclear.,Here we show, using RNA-Seq analyses of uveal melanoma, that the SF3B1R625/K666 mutation results in deregulated splicing at a subset of junctions, mostly by the use of alternative 3′ss.,Modelling the differential junctions in SF3B1WT and SF3B1R625/K666 cell lines demonstrates that the deregulated splice pattern strictly depends on SF3B1 status and on the 3’ss-sequence context.,SF3B1WT knockdown or overexpression do not reproduce the SF3B1R625/K666 splice pattern, qualifying SF3B1R625/K666 as change-of-function mutants.,Mutagenesis of predicted branchpoints reveals that the SF3B1R625/K666-promoted splice pattern is a direct result of alternative branchpoint usage.,Altogether, this study provides a better understanding of the mechanisms underlying splicing alterations induced by mutant SF3B1 in cancer, and reveals a role for alternative branchpoints in disease.,Mutations in the splicing factor SF3B1 are found in uveal melanoma.,Here, Alsafadi et al. use RNA-sequencing data from these cancers and experimental models, and show that mutant SF3B1 promotes alternative branchpoints in a specific gene subset and that the mutant protein gains a new function.
TAK733 is a novel allosteric, non-ATP-binding, inhibitor of the BRAF substrates MEK-1/2.,The growth inhibitory effects of TAK733 were assessed in a panel of 27 cutaneous and five uveal melanoma cell lines genotyped for driver oncogenic mutations.,Flow cytometry, Western blots and metabolic tracer uptake assays were used to characterize the changes induced by exposure to TAK733.,Fourteen cutaneous melanoma cell lines with different driver mutations were sensitive to the antiproliferative effects of TAK733, with a higher proportion of BRAFV600E mutant cell lines being highly sensitive with IC50s below 1 nM.,The five uveal melanoma cell lines had GNAQ or GNA11 mutations and were either moderately or highly sensitive to TAK733.,The tested cell lines wild type for NRAS, BRAF, GNAQ and GNA11 driver mutations were moderately to highly resistant to TAK733.,TAK733 led to a decrease in pERK and G1 arrest in most of these melanoma cell lines regardless of their origin, driver oncogenic mutations and in vitro sensitivity to TAK733.,MEK inhibition resulted in increase in pMEK more prominently in NRASQ61L mutant and GNAQ mutant cell lines than in BRAFV600E mutant cell lines.,Uptake of the metabolic tracers FDG and FLT was inhibited by TAK733 in a manner that closely paralleled the in vitro sensitivity assays.,The MEK inhibitor TAK733 has antitumor properties in melanoma cell lines with different oncogenic mutations and these effects could be detectable by differential metabolic tracer uptake.