The impact of monosomies, trisomies and segmental aneuploidies on chromosomal stability

Aneuploidy and chromosomal instability are both commonly found in cancer. Chromosomal instability leads to karyotype heterogeneity in tumors and is associated with therapy resistance, metastasis and poor prognosis. It has been hypothesized that aneuploidy per se is sufficient to drive CIN, however due to limited models and heterogenous results, it has remained controversial which aspects of aneuploidy can drive CIN. In this study we systematically tested the impact of different types of aneuploidies on the induction of CIN. We generated a plethora of isogenic aneuploid clones harboring whole chromosome or segmental aneuploidies in human p53-deficient RPE-1 cells. We observed increased segregation errors in cells harboring trisomies that strongly correlated to the number of gained genes. Strikingly, we found that clones harboring only monosomies do not induce a CIN phenotype. Finally, we found that an initial chromosome breakage event and subsequent fusion can instigate breakage-fusion-bridge cycles. By investigating the impact of monosomies, trisomies and segmental aneuploidies on chromosomal instability we further deciphered the complex relationship between aneuploidy and CIN

3D genomics across the tree of life reveals condensin II as a determinant of architecture type

We investigated genome folding across the eukaryotic tree of life. We find two types of three-dimensional(3D) genome architectures at the chromosome scale. Each type appears and disappears repeatedlyduring eukaryotic evolution. The type of genome architecture that an organism exhibits correlates with theabsence of condensin II subunits. Moreover, condensin II depletion converts the architecture of thehuman genome to a state resembling that seen in organisms such as fungi or mosquitoes. In this state,centromeres cluster together at nucleoli, and heterochromatin domains merge. We propose a physicalmodel in which lengthwise compaction of chromosomes by condensin II during mitosis determineschromosome-scale genome architecture, with effects that are retained during the subsequent interphase.This mechanism likely has been conserved since the last common ancestor of all eukaryotes.C.H. is supported by the Boehringer Ingelheim Fonds; C.H., Á.S.C., and B.D.R. are supported by an ERC CoG (772471, “CohesinLooping”); A.M.O.E. and B.D.R. are supported by the Dutch Research Council (NWO-Echo); and J.A.R. and R.H.M. are supported by the Dutch Cancer Society (KWF). T.v.S. and B.v.S. are supported by NIH Common Fund “4D Nucleome” Program grant U54DK107965. H.T. and E.d.W. are supported by an ERC StG (637597, “HAP-PHEN”). J.A.R., T.v.S., H.T., R.H.M., B.v.S., and E.d.W. are part of the Oncode Institute, which is partly financed by the Dutch Cancer Society. Work at the Center for Theoretical Biological Physics is sponsored by the NSF (grants PHY-2019745 and CHE-1614101) and by the Welch Foundation (grant C-1792). V.G.C. is funded by FAPESP (São Paulo State Research Foundation and Higher Education Personnel) grants 2016/13998-8 and 2017/09662-7. J.N.O. is a CPRIT Scholar in Cancer Research. E.L.A. was supported by an NSF Physics Frontiers Center Award (PHY-2019745), the Welch Foundation (Q-1866), a USDA Agriculture and Food Research Initiative grant (2017-05741), the Behavioral Plasticity Research Institute (NSF DBI-2021795), and an NIH Encyclopedia of DNA Elements Mapping Center Award (UM1HG009375). Hi-C data for the 24 species were created by the DNA Zoo Consortium (www.dnazoo.org). DNA Zoo is supported by Illumina, Inc.; IBM; and the Pawsey Supercomputing Center. P.K. is supported by the University of Western Australia. L.L.M. was supported by NIH (1R01NS114491) and NSF awards (1557923, 1548121, and 1645219) and the Human Frontiers Science Program (RGP0060/2017). The draft A. californica project was supported by NHGRI. J.L.G.-S. received funding from the ERC (grant agreement no. 740041), the Spanish Ministerio de Economía y Competitividad (grant no. BFU2016-74961-P), and the institutional grant Unidad de Excelencia María de Maeztu (MDM-2016-0687). R.D.K. is supported by NIH grant RO1DK121366. V.H. is supported by NIH grant NIH1P41HD071837. K.M. is supported by a MEXT grant (20H05936). M.C.W. is supported by the NIH grants R01AG045183, R01AT009050, R01AG062257, and DP1DK113644 and by the Welch Foundation. E.F. was supported by NHGR

Chromosome misalignments induce spindle-positioning defects

Cortical pulling forces on astral microtubules are essential to position the spindle. These forces are generated by cortical dynein, a minus‐end directed motor. Previously, another dynein regulator termed Spindly was proposed to regulate dynein‐dependent spindle positioning. However, the mechanism of how Spindly regulates spindle positioning has remained elusive. Here, we find that the misalignment of chromosomes caused by Spindly depletion is directly provoking spindle misorientation. Chromosome misalignments induced by CLIP‐170 or CENP‐E depletion or by noscapine treatment are similarly accompanied by severe spindle‐positioning defects. We find that cortical LGN is actively displaced from the cortex when misaligned chromosomes are in close proximity. Preventing the KT recruitment of Plk1 by the depletion of PBIP1 rescues cortical LGN enrichment near misaligned chromosomes and re‐establishes proper spindle orientation. Hence, KT‐enriched Plk1 is responsible for the negative regulation of cortical LGN localization. In summary, we uncovered a compelling molecular link between chromosome alignment and spindle orientation defects, both of which are implicated in tumorigenesis

p53 Prohibits Propagation of Chromosome Segregation Errors that Produce Structural Aneuploidies

The presence of an abnormal karyotype has been shown to be profoundly detrimental at the cellular and organismal levels but is an overt hallmark of cancer. Aneuploidy can lead to p53 activation and thereby prevents proliferation, but the exact trigger for p53 activation has remained controversial. Here, we have used a system to induce aneuploidy in untransformed human cells to explore how cells deal with different segregation errors. We show that p53 is activated only in a subset of the cells with altered chromosome content. Importantly, we find that at least a subset of whole-chromosome aneuploidies can be propagated in p53-proficient cells, indicating that aneuploidy does not always lead to activation of p53. Finally, we demonstrate that propagation of structural aneuploidies (gain or loss of part of a chromosome) induced by segregation errors is limited to p53-deficient cells

BUB1 Is Essential for the Viability of Human Cells in which the Spindle Assembly Checkpoint Is Compromised

Summary: The spindle assembly checkpoint (SAC) ensures faithful segregation of chromosomes. Although most mammalian cell types depend on the SAC for viability, we found that human HAP1 cells can grow SAC independently. We generated MAD1- and MAD2-deficient cells and mutagenized them to identify synthetic lethal interactions, revealing that chromosome congression factors become essential upon SAC deficiency. Besides expected hits, we also found that BUB1 becomes essential in SAC-deficient cells. We found that the BUB1 C terminus regulates alignment as well as recruitment of CENPF. Second, we found that BUBR1 was not essential in SAC-deficient HAP1 cells. We confirmed that BUBR1 does not regulate chromosome alignment in HAP1 cells and that BUB1 does not regulate chromosome alignment through BUBR1. Taken together, our data resolve some long-standing questions about the interplay between BUB1 and BUBR1 and their respective roles in the SAC and chromosome alignment

Aurora A, MCAK, and Kif18b promote Eg5-independent spindle formation

Inhibition of the microtubule (MT) motor protein Eg5 results in a mitotic arrest due to the formation of monopolar spindles, making Eg5 an attractive target for anti-cancer therapies. However, Eg5-independent pathways for bipolar spindle formation exist, which might promote resistance to treatment with Eg5 inhibitors. To identify essential components for Eg5-independent bipolar spindle formation, we performed a genome-wide siRNA screen in Eg5-independent cells (EICs). We find that the kinase Aurora A and two kinesins, MCAK and Kif18b, are essential for bipolar spindle assembly in EICs and in cells with reduced Eg5 activity. Aurora A promotes bipolar spindle assembly by phosphorylating Kif15, hereby promoting Kif15 localization to the spindle. In turn, MCAK and Kif18b promote bipolar spindle assembly by destabilizing the astral MTs. One attractive way to interpret our data is that, in the absence of MCAK and Kif18b, excessive astral MTs generate inward pushing forces on centrosomes at the cortex that inhibit centrosome separation. Together, these data suggest a novel function for astral MTs in force generation on spindle poles and how proteins involved in regulating microtubule length can contribute to bipolar spindle assembly

Distinct Roles for Condensin's Two ATPase Sites in Chromosome Condensation

Condensin is a conserved SMC complex that uses its ATPase machinery to structure genomes, but how it does so is largely unknown. We show that condensin's ATPase has a dual role in chromosome condensation. Mutation of one ATPase site impairs condensation, while mutating the second site results in hyperactive condensin that compacts DNA faster than wild-type, both in vivo and in vitro. Whereas one site drives loop formation, the second site is involved in the formation of more stable higher-order Z loop structures. Using hyperactive condensin I, we reveal that condensin II is not intrinsically needed for the shortening of mitotic chromosomes. Condensin II rather is required for a straight chromosomal axis and enables faithful chromosome segregation by counteracting the formation of ultrafine DNA bridges. SMC complexes with distinct roles for each ATPase site likely reflect a universal principle that enables these molecular machines to intricately control chromosome architecture.BN/Cees Dekker La