Investigating the molecular mechanism of h3b‐8800: A splicing modulator inducing preferential lethality in spliceosome‐mutant cancers

The SF3B1 protein, part of the SF3b complex, recognizes the intron branch point sequence of precursor messenger RNA (pre‐mRNA), thus contributing to splicing fidelity. SF3B1 is frequently mutated in cancer and is the target of distinct families of splicing modulators (SMs). Among these, H3B‐8800 is of particular interest, as it induces preferential lethality in cancer cells bearing the frequent and highly pathogenic K700E SF3B1 mutation. Despite the potential of H3B‐8800 to treat myeloid leukemia and other cancer types hallmarked by SF3B1 mutations, the molecular mechanism underlying its preferential lethality towards spliceosome‐mutant cancer cells remains elusive. Here, microsecond‐long all‐atom simulations addressed the binding/dissociation mechanism of H3B‐8800 to wild type and K700E SF3B1‐containing SF3b (^K700ESB3b) complexes at the atomic level, unlocking that the K700E mutation little affects the thermodynamics and kinetic traits of H3B‐8800 binding. This supports the hypothesis that the selectivity of H3B‐8800 towards mutant cancer cells is unrelated to its preferential targeting of^K700ESB3b. Nevertheless, this set of simulations discloses that the K700E mutation and H3B‐8800 binding affect the overall SF3b internal motion, which in turn may influence the way SF3b interacts with other spliceosome components. Finally, we unveil the existence of a putative druggable SF3b pocket in the vicinity of K700E that could be harnessed in future rational drug‐discovery efforts to specifically target mutant SF3b.