Abbreviations: Acute myeloid leukemia (AML), dense phase (DP), fluorescence recovery after photobleaching (FRAP), fusion oncoprotein (FO), Gibbs free energy of transfer (ΔG Tr ), Gle2binding-sequence (GLEBS), human CD34-positive hematopoietic stem and progenitor cells (hCD34+ cells), immunofluorescence (IF), intrinsically disordered region (IDR), light phase (LP), lineage-negative hematopoietic stem and progenitor cells (lin-HSPCs), liquid-liquid phase separation (LLPS), mEGFP-tagged NHA9 (G-NHA9), mobile fraction (M f ), monomeric enhanced green fluorescent protein (mEGFP), nuclear pore complex (NPC), NUP98-HOXA9 (NHA9), partition coefficient (K p ), patient-derived xenograft (PDX), Pearson correlation coefficient (PCC), Principal component analysis (PCA), RNA sequencing (RNA-seq), and saturation concentration (C sat ).

Aneuploidy, the incorrect number of whole chromosomes, is a common feature of tumors that contributes to their initiation and evolution. Preventing aneuploidy requires properly functioning kinetochores, which are large protein complexes assembled on centromeric DNA that link mitotic chromosomes to dynamic spindle microtubules and facilitate chromosome segregation. The kinetochore leverages at least two mechanisms to prevent aneuploidy: error correction and the spindle assembly checkpoint (SAC). BubR1, a factor involved in both processes, was identified as a cancer dependency and therapeutic target in multiple tumor types; however, it remains unclear what specific oncogenic pressures drive this enhanced dependency on BubR1 and whether it arises from BubR1’s regulation of the SAC or error-correction pathways. Here, we use a genetically controlled transformation model and glioblastoma tumor isolates to show that constitutive signaling by RAS or MAPK is necessary for cancer-specific BubR1 vulnerability. The MAPK pathway enzymatically hyperstimulates a network of kinetochore kinases that compromises chromosome segregation, rendering cells more dependent on two BubR1 activities: counteracting excessive kinetochore–microtubule turnover for error correction and maintaining the SAC. This work expands our understanding of how chromosome segregation adapts to different cellular states and reveals an oncogenic trigger of a cancer-specific defect.

Fusion oncoproteins (FOs) resulting from in-frame chromosomal translocations are associated with many aggressive cancers with poor patient outcomes. Several FOs are now understood to perform their oncogenic functions within biomolecular condensates formed through liquid-liquid phase separation (LLPS). Two classes of phase-separating FOs have emerged, those that form nuclear condensates and alter chromatin biology, including gene expression, and those that form cytoplasmic condensates and promote aberrant signaling, including RAS/MAPK signaling. The amino acid sequences of the FOs within these classes display LLPS-prone intrinsically disordered regions and folded domains that synergistically interact with themselves and other biomolecules to promote condensate formation. This review summarizes the roles of LLPS in the oncogenic functions of these two FO classes, provides examples of FOs that inhibit physiological LLPS in normal cells, and discusses the sequence features commonly associated with LLPS and their enrichment in many FOs. Expected final online publication date for the Annual Review of Cancer Biology, Volume 7 is April 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

HPV-transforming protein E6 weakens both SAC activity and sister chromatid cohesion. These processes largely function normally and result in chromosome segregation errors only when mitosis is delayed by the presence of one to four unaligned, pole-associated chromosomes. This mechanism of aneuploidy is not observed in response to oncogenic signaling.

BuGZ is a kinetochore component that binds to and stabilizes Bub3, a key player in mitotic spindle assembly checkpoint signaling. Bub3 is required for kinetochore recruitment of Bub1 and BubR1, two proteins that have essential and distinct roles in the checkpoint. Both Bub1 and BubR1 localize to kinetochores through interactions with Bub3, which are mediated through conserved GLEBS domains in both Bub1 and BubR1. BuGZ also has a GLEBS domain, which is required for its kinetochore localization as well, presumably mediated through Bub3 binding. While much is understood about the requirements for Bub1 and BubR1 interaction with Bub3 and kinetochores, much less is known regarding BuGZ’s requirements. Here, we used a series of mutants to demonstrate that BuGZ kinetochore localization requires only its core GLEBS domain, which is distinct from the requirements for both Bub1 and BubR1. Furthermore, we found that the kinetics of Bub1, BubR1, and BuGZ loading to kinetochores differ, with BuGZ localizing prior to BubR1 and Bub1. To better understand how complexes containing Bub3 and its binding partners are loaded to kinetochores, we carried out size exclusion chromatography and analyzed Bub3-containing complexes from cells under different spindle assembly checkpoint signaling conditions. We found that prior to kinetochore formation, Bub3 is complexed with BuGZ but not Bub1 or BubR1. Our results point to a model in which BuGZ stabilizes Bub3 and promotes Bub3 loading onto kinetochores in early mitosis, which, in turn, facilitates Bub1 and BubR1 kinetochore recruitment and spindle assembly checkpoint signaling.