The Consequences of Chromosome Segregation Errors in Mitosis and Meiosis

Hexavalent Chromium-Induced Chromosome Instability Drives Permanent and Heritable Numerical and Structural Changes and a DNA Repair-Deficient Phenotype

A key hypothesis for how hexavalent chromium [Cr(VI)] causes cancer is that it drives chromosome instability (CIN), which leads to neoplastic transformation. Studies show chronic Cr(VI) can affect DNA repair and induce centrosome amplification, which can lead to structural and numerical CIN. However, no studies have considered whether these outcomes are transient or permanent. In this study, we exposed human lung cells to particulate Cr(VI) for three sequential 24-hour periods, each separated by about a month. After each treatment, cells were seeded at colony-forming density, cloned, expanded, and retreated, creating three generations of clonal cell lines. Each generation of clones was tested for chromium sensitivity, chromosome complement, DNA repair capacity, centrosome amplification, and the ability to grow in soft agar. After the first treatment, Cr(VI)-treated clones exhibited a normal chromosome complement, but some clones showed a repair-deficient phenotype and amplified centrosomes. After the second exposure, more than half of the treated clones acquired an abnormal karyotype including numerical and structural alterations, with many exhibiting deficient DNA double-strand break repair and amplified centrosomes. The third treatment produced new abnormal clones, with previously abnormal clones acquiring additional abnormalities and most clones exhibiting repair deficiency. CIN, repair deficiency, and amplified centrosomes were all permanent and heritable phenotypes of repeated Cr(VI) exposure. These outcomes support the hypothesis that CIN is a key mechanism of Cr(VI)-induced carcinogenesis.Significance: Chromium, a major public health concern and human lung carcinogen, causes fundamental changes in chromosomes and DNA repair in human lung cells. Cancer Res; 78(15); 4203-14. ©2018 AACR.

Human Ska complex and Ndc80 complex interact to form a load-bearing assembly that strengthens kinetochore-microtubule attachments

Microtubules are dynamic, tube-like structures that drive the segregation of duplicated chromosomes during cell division. The Ska complex is part of a molecular machine that forms force-bearing connections between chromosomes and microtubule ends. Depletion of the Ska complex destabilizes these connections and disrupts cell division. The Ska complex binds microtubules, but it is unknown whether it directly holds force at microtubules or indirectly stabilizes the connections. Here, we show that the Ska complex makes a direct force-bearing linkage with microtubule ends and assembles with another microtubule binding component, the Ndc80 complex, to strengthen its ability to withstand force. Our results suggest that the Ska and Ndc80 complexes work together to maintain the connections between chromosomes and microtubule ends.

Accurate segregation of chromosomes relies on the force-bearing capabilities of the kinetochore to robustly attach chromosomes to dynamic microtubule tips. The human Ska complex and Ndc80 complex are outer-kinetochore components that bind microtubules and are required to fully stabilize kinetochore–microtubule attachments in vivo. While purified Ska complex tracks with disassembling microtubule tips, it remains unclear whether the Ska complex–microtubule interaction is sufficiently strong to make a significant contribution to kinetochore–microtubule coupling. Alternatively, Ska complex might affect kinetochore coupling indirectly, through recruitment of phosphoregulatory factors. Using optical tweezers, we show that the Ska complex itself bears load on microtubule tips, strengthens Ndc80 complex-based tip attachments, and increases the switching dynamics of the attached microtubule tips. Cross-linking mass spectrometry suggests the Ska complex directly binds Ndc80 complex through interactions between the Ska3 unstructured C-terminal region and the coiled-coil regions of each Ndc80 complex subunit. Deletion of the Ska complex microtubule-binding domain or the Ska3 C terminus prevents Ska complex from strengthening Ndc80 complex-based attachments. Together, our results indicate that the Ska complex can directly strengthen the kinetochore–microtubule interface and regulate microtubule tip dynamics by forming an additional connection between the Ndc80 complex and the microtubule.

Evaluation of the impact of sperm morphology on embryo aneuploidy rates in a donor oocyte program

This study investigates the correlation between sperm morphology and the incidence of embryo aneuploidy in an oocyte donation program. A total of 1,165 embryos from 103 patients have been analyzed by fluorescent in situ hybridization (FISH) for numerical abnormalities in chromosome numbers 13, 18, 21, X, and Y. Data has been evaluated in five groups according to sperm morphology, which has been assessed according to the Kruger's strict criteria. The results did not show any difference in paternal (p = 0.878), maternal (p = 0.873), and donor ages (p = 0.871), sperm counts (p = 0.782) and motility (p = 0.124), and fertilization rate (p = 0.080) among the groups. However, total aneuploidy rate (p < 0.001) and its derivatives (trisomy p = 0,042, monosomy p = 0,004) differed significantly and they were reversibly correlated with sperm morphology (rho correlation test; total aneuploidy p < 0.001, trisomy p < 0.001, monosomy p = 0.004). Therefore, these results suggested that diminished sperm quality is correlated to the aneuploidy rate in preimplantation embryos.

ABBREVIATIONS

FISH: fluorescence in situ hybridization; ICSI: intracytoplasmic sperm injection; HCG: human chorionic gonadotropin.

Budding yeast CENP-ACse4 interacts with the N-terminus of Sgo1 and regulates its association with centromeric chromatin

Shugoshin is an evolutionarily conserved protein, which is involved in tension sensing on mitotic chromosomes, kinetochore biorientation, and protection of centromeric (CEN) cohesin for faithful chromosome segregation. Interaction of the C-terminus of Sgo1 with phosphorylated histone H2A regulates its association with CEN and pericentromeric (peri-CEN) chromatin, whereas mutations in histone H3 selectively compromise the association of Sgo1 with peri-CEN but not CEN chromatin. Given that histone H3 is absent from CEN and is replaced by a histone H3 variant CENP-A^Cse4, we investigated if CENP-A^Cse4 interacts with Sgo1 and promotes its association with the CEN chromatin. In this study, we found that Sgo1 interacts with CENP-A^Cse4 in vivo and in vitro. The N-terminus coiled-coil domain of Sgo1 without the C-terminus (sgo1-NT) is sufficient for its interaction with CENP-A^Cse4, association with CEN but not the peri-CEN, and this CEN association is cell cycle dependent with maximum enrichment in mitosis. In agreement with the role of CENP-A^Cse4 in CEN maintenance of Sgo1, depletion of CENP-A^Cse4 results in the loss of Sgo1 and sgo1-NT from the CEN chromatin. The N-terminus of Sgo1 is required for genome stability as a mutant lacking the N-terminus (sgo1-CT) exhibits increased chromosome missegregation when compared to a sgo1-NT mutant. In summary, our results define a novel role for the N-terminus of Sgo1 in CENP-A^Cse4 mediated recruitment of Sgo1 to CEN chromatin for faithful chromosome segregation.

Multiple Roles, Multiple Adaptors: Dynein During Cell Cycle

Dynein is an essential protein complex present in most eukaryotes that regulate biological processes ranging from ciliary beating, intracellular transport, to cell division. Elucidating the detailed mechanism of dynein function has been a challenging task owing to its large molecular weight and high complexity of the motor. With the advent of technologies in the last two decades, studies have uncovered a wealth of information about the structural, biochemical, and cell biological roles of this motor protein. Cytoplasmic dynein associates with dynactin through adaptor proteins to mediate retrograde transport of vesicles, mRNA, proteins, and organelles on the microtubule tracts. In a mitotic cell, dynein has multiple localizations, such as at the nuclear envelope, kinetochores, mitotic spindle and spindle poles, and cell cortex. In line with this, dynein regulates multiple events during the cell cycle, such as centrosome separation, nuclear envelope breakdown, spindle assembly checkpoint inactivation, chromosome segregation, and spindle positioning. Here, we provide an overview of dynein structure and function with focus on the roles played by this motor during different stages of the cell cycle. Further, we review in detail the role of dynactin and dynein adaptors that regulate both recruitment and activity of dynein during the cell cycle.

The induction of endoreduplication and polyploidy by elevated expression of 14-3-3γ

Several studies have demonstrated that specific 14-3-3 isoforms are frequently elevated in cancer and that these proteins play a role in human tumorigenesis. 14-3-3γ, an isoform recently demonstrated to function as an oncoprotein, is overexpressed in a variety of human cancers; however, its role in promoting tumorigenesis remains unclear. We previously reported that overexpression of 14-3-3γ caused the appearance of polyploid cells, a phenotype demonstrated to have profound tumor promoting properties. Here we examined the mechanism driving 14-3-3γ-induced polyploidization and the effect this has on genomic stability. Using FUCCI probes we showed that these polyploid cells appeared when diploid cells failed to enter mitosis and subsequently underwent endoreduplication. We then demonstrated that 14-3-3γ-induced polyploid cells experience significant chromosomal segregation errors during mitosis and observed that some of these cells stably propagate as tetraploids when isolated cells were expanded into stable cultures. These data lead us to conclude that overexpression of the 14-3-3γ promotes endoreduplication. We further investigated the role of 14-3-3γ in human NSCLC samples and found that its expression is significantly elevated in polyploid tumors. Collectively, these results suggests that 14-3-3γ may promote tumorigenesis through the production of a genetically unstable polyploid intermediate.

Evidence for viable and stable triploid Trypanosoma congolense parasites

BACKGROUND

Recent whole genome sequencing (WGS) analysis identified a viable triploid strain of Trypanosoma congolense. This triploid strain BANANCL2 was a clone of the field isolate BANAN/83/CRTRA/64 that was collected from cattle in Burkina Faso in 1983.

RESULTS

We demonstrated the viability and stability of triploidy throughout the complete life-cycle of the parasite by infecting tsetse flies with the triploid clone BANANCL2. Proboscis-positive tsetse flies efficiently transmitted the parasites to mice resulting in systemic infections. WGS of the parasites was performed at all life-cycle stages, and a method based on a block alternative allele frequency spectrum was developed to efficiently detect the ploidy profiles of samples with low read depth. This approach confirmed the triploid profile of parasites throughout their life-cycle in the tsetse fly and the mammalian host, demonstrating that triploidy is present at all stages and is stable over time.

CONCLUSION

The presence of viable field-isolated triploid parasites indicates another possible layer of genetic diversity in natural T. congolense populations. The comparison between triploid and diploid parasites provides a unique model system to study the impact of chromosome copy number variations in African trypanosomes. In addition, the consequences of triploidy can be further investigated using this stable triploid model.