Cancer-associated KBTBD4 mutations induce differentiation defects and confer a unique therapeutic vulnerability.

Epigenetic regulation governs stem cell fate and perturbations in these mechanisms often lead to tumor development. The CoREST complex, a critical regulator of neural and hematopoietic stem cell differentiation, is recurrently targeted by gain of function mutations in the ubiquitin ligase KBTBD4 in high-risk embryonal brain tumors. However, the tumorigenic potential of these mutations remains unresolved partly due to the challenges in modeling tumors that arise during early brain development. We and others recently demonstrated that small molecule UM171 mimics KBTBD4 mutations by promoting robust CoREST degradation and expansion of hematopoietic stem cells. Leveraging this mechanistic similarity, we modeled KBTBD4 mutations in hematopoietic stem and progenitor cells (HSPCs) and found that mutants induced expansion of immature stem and progenitor populations and impaired lineage differentiation. High-throughput screening identified HDAC inhibitors as specific agents that disrupt mutant KBTBD4 activity, by preventing interaction with the CoREST complex. Using our HSPC model, we demonstrated that the class I HDAC inhibitor mocetinostat alleviated differentiation defects caused by KBTBD4 mutations. Together, these findings reveal the tumorigenic mechanism of KBTBD4 mutations and uncover therapeutic vulnerabilities that may be exploited for clinical applications.