Recent studies have revealed extensive genetic diversity both between and within tumours. This heterogeneity affects key cancer pathways, driving phenotypic variation, and poses a significant challenge to personalized cancer medicine. A major cause of genetic heterogeneity in cancer is genomic instability. This instability leads to an increased mutation rate and can shape the evolution of the cancer genome through a plethora of mechanisms. By understanding these mechanisms we can gain insight into the common pathways of tumour evolution that could support the development of future therapeutic strategies.

The causes and consequences of genetic heterogeneity in cancer evolution.

The involvement of whole-chromosome aneuploidy in tumorigenesis is the subject of debate, in large part because of the lack of insight into underlying mechanisms. Here we identify a mechanism by which errors in mitotic chromosome segregation generate DNA breaks via the formation of structures called micronuclei. Whole-chromosome-containing micronuclei form when mitotic errors produce lagging chromosomes. We tracked the fate of newly generated micronuclei and found that they undergo defective and asynchronous DNA replication, resulting in DNA damage and often extensive fragmentation of the chromosome in the micronucleus. Micronuclei can persist in cells over several generations but the chromosome in the micronucleus can also be distributed to daughter nuclei. Thus, chromosome segregation errors potentially lead to mutations and chromosome rearrangements that can integrate into the genome. Pulverization of chromosomes in micronuclei may also be one explanation for 'chromothripsis' in cancer and developmental disorders, where isolated chromosomes or chromosome arms undergo massive local DNA breakage and rearrangement.

/pdf/dna-breaks-and-chromosome-pulverization-from-errors-in-htdl3n22u6.pdf

DNA breaks and chromosome pulverization from errors in mitosis

Taxol (generic name paclitaxel) is a microtubule-stabilizing drug that is approved by the Food and Drug Administration for the treatment of ovarian, breast, and lung cancer, as well as Kaposi's sarcoma. It is used off-label to treat gastroesophageal, endometrial, cervical, prostate, and head and neck cancers, in addition to sarcoma, lymphoma, and leukemia. Paclitaxel has long been recognized to induce mitotic arrest, which leads to cell death in a subset of the arrested population. However, recent evidence demonstrates that intratumoral concentrations of paclitaxel are too low to cause mitotic arrest and result in multipolar divisions instead. It is hoped that this insight can now be used to develop a biomarker to identify the ∼50% of patients that will benefit from paclitaxel therapy. Here I discuss the history of paclitaxel and our recently evolved understanding of its mechanism of action.

How Taxol/paclitaxel kills cancer cells

https://www.cell.com/cancer-cell/pdf/S1535-6108%2814%2900510-8.pdf

Biological and Therapeutic Impact of Intratumor Heterogeneity in Cancer Evolution

Genome sequencing has uncovered a new mutational phenomenon in cancer and congenital disorders called chromothripsis. Chromothripsis is characterized by extensive genomic rearrangements and an oscillating pattern of DNA copy number levels, all curiously restricted to one or a few chromosomes. The mechanism for chromothripsis is unknown, but we previously proposed that it could occur through the physical isolation of chromosomes in aberrant nuclear structures called micronuclei. Here, using a combination of live cell imaging and single-cell genome sequencing, we demonstrate that micronucleus formation can indeed generate a spectrum of genomic rearrangements, some of which recapitulate all known features of chromothripsis. These events are restricted to the mis-segregated chromosome and occur within one cell division. We demonstrate that the mechanism for chromothripsis can involve the fragmentation and subsequent reassembly of a single chromatid from a micronucleus. Collectively, these experiments establish a new mutational process of which chromothripsis is one extreme outcome.

Chromothripsis from DNA damage in micronuclei

Various types of chromosomal aberrations, including numerical (aneuploidy) and structural (e.g., translocations, deletions), are commonly found in human tumors and are linked to tumorigenesis. Aneuploidy is a direct consequence of chromosome segregation errors in mitosis, whereas structural aberrations are caused by improperly repaired DNA breaks. Here, we demonstrate that chromosome segregation errors can also result in structural chromosome aberrations. Chromosomes that missegregate are frequently damaged during cytokinesis, triggering a DNA double-strand break response in the respective daughter cells involving ATM, Chk2, and p53. We show that these double-strand breaks can lead to unbalanced translocations in the daughter cells. Our data show that segregation errors can cause translocations and provide insights into the role of whole-chromosome instability in tumorigenesis.

https://science.sciencemag.org/content/sci/333/6051/1895.full.pdf

Chromosome Segregation Errors as a Cause of DNA Damage and Structural Chromosome Aberrations

Constitutive heterochromatin is a major component of the eukaryotic nucleus and is essential for the maintenance of genome stability. Highly concentrated at pericentromeric and telomeric domains, heterochromatin is riddled with repetitive sequences and has evolved specific ways to compartmentalize, silence, and repair repeats. The delicate balance between heterochromatin epigenetic maintenance and cellular processes such as mitosis and DNA repair and replication reveals a highly dynamic and plastic chromatin domain that can be perturbed by multiple mechanisms, with far-reaching consequences for genome integrity. Indeed, heterochromatin dysfunction provokes genetic turmoil by inducing aberrant repeat repair, chromosome segregation errors, transposon activation, and replication stress and is strongly implicated in aging and tumorigenesis. Here, we summarize the general principles of heterochromatin structure and function, discuss the importance of its maintenance for genome integrity, and propose that more comprehensive analyses of heterochromatin roles in tumorigenesis will be integral to future innovations in cancer treatment.

/pdf/heterochromatin-guardian-of-the-genome-3x0x4njiou.pdf

Heterochromatin: Guardian of the Genome

The mitotic checkpoint has evolved to prevent chromosome mis-segregations by delaying mitosis when unattached chromosomes are present. Inducing severe chromosome segregation errors by ablating the mitotic checkpoint causes cell death. Here we have analyzed the consequences of gradual increases in chromosome segregation errors on the viability of tumor cells and normal human fibroblasts. Partial reduction of essential mitotic checkpoint components in four tumor cell lines caused mild chromosome mis-segregations, but no lethality. These cells were, however, remarkably more sensitive to low doses of taxol, which enhanced the amount and severity of chromosome segregation errors. Sensitization to taxol was achieved by reducing levels of Mps1 or BubR1, proteins having dual roles in checkpoint activation and chromosome alignment, but not by reducing Mad2, functioning solely in the mitotic checkpoint. Moreover, we find that untransformed human fibroblasts with reduced Mps1 levels could not be sensitized to sublethal doses of taxol. Thus, targeting the mitotic checkpoint and chromosome alignment simultaneously may selectively kill tumor cells by enhancing chromosome mis-segregations.

https://www.pnas.org/content/pnas/106/45/19108.full.pdf

Elevating the frequency of chromosome mis-segregation as a strategy to kill tumor cells

Kif15 Cooperates with Eg5 to Promote Bipolar Spindle Assembly

Most of the current drugs used to treat cancer can be classified as anti-proliferative drugs. These drugs perturb the proliferative cycle of tumor cells at diverse stages of the cell cycle. Examples of such drugs are DNA-damaging agents and inhibitors of cyclin-dependent kinases that arrest cell cycle progression at different stages of interphase. Another class of anti-proliferative drugs is the so-called anti-mitotic drugs, which selectively perturb progression through mitosis. Mitosis is the shortest and final stage in the cell cycle and has evolved to accurately divide the duplicated genome over the two daughter cells. This review deals with the different strategies that are currently considered to perturb mitotic progression in the treatment of cancer.

Aniek Janssen

Papers

Chromosome Segregation Errors as a Cause of DNA Damage and Structural Chromosome Aberrations

Heterochromatin: Guardian of the Genome

Elevating the frequency of chromosome mis-segregation as a strategy to kill tumor cells

Kif15 Cooperates with Eg5 to Promote Bipolar Spindle Assembly

Mitosis as an anti-cancer target.