Locus-specific protein profiling identifies mechanisms to target centromeres

Centromeres orchestrate the accurate partitioning of the genome during cell division, preventing catastrophic segregation errors in dividing cells.  It is now clear that centromeric malfunctions contribute to establishing and maintaining cancer, but no compounds targeting centromeric chromatin have been identified using standard drug discovery approaches. Here we developed a large-scale data-mining approached aimed at probing centromeric protein composition and its change in response to small-molecule treatment. We discovered that the bromodomain inhibitor JQ1, which prevents BET proteins from associating with acetylated chromatin, redirects its target protein BRD4 to bind centromeric chromatin. JQ1 achieves this by acting as a non-degrading molecular glue stabilising a direct interaction between the second bromodomain of BRD4 and Centromere Protein B (CENP-B). At the molecular scale, a single atom replacement within the t-butyl ester moiety of JQ1 completely abolishes localisation of BRD4 to the centromere. Strikingly, derivatives of JQ1 that performed poorly in the clinic lack this moiety and centromere-targeting functionality. CENP-B is essential for JQ1-mediated effects on centromere cohesion and mechanisms that render cells resistant to JQ1, revealing a key unexplored avenue for future drug discovery.