Scrambling the genome in cancer: causes and consequences of complex chromosome rearrangements.

Complex chromosome rearrangements, known as chromoanagenesis, are widespread in cancer. Based on large-scale DNA sequencing of human tumours, the most frequent type of complex chromosome rearrangement is chromothripsis, a massive, localized and clustered rearrangement of one (or a few) chromosomes seemingly acquired in a single event. Chromothripsis can be initiated by mitotic errors that produce a micronucleus encapsulating a single chromosome or chromosomal fragment. Rupture of the unstable micronuclear envelope exposes its chromatin to cytosolic nucleases and induces chromothriptic shattering. Found in up to half of tumours included in pan-cancer genomic analyses, chromothriptic rearrangements can contribute to tumorigenesis through inactivation of tumour suppressor genes, activation of proto-oncogenes, or gene amplification through the production of self-propagating extrachromosomal circular DNAs encoding oncogenes or genes conferring anticancer drug resistance. Here, we discuss what has been learned about the mechanisms that enable these complex genomic rearrangements and their consequences in cancer.

Phosphorylation of Nup98 by multiple kinases is crucial for NPC disassembly during mitotic entry.

Disassembly of nuclear pore complexes (NPCs) is a decisive event during mitotic entry in cells undergoing open mitosis, yet the molecular mechanisms underlying NPC disassembly are unknown. Using chemical inhibition and depletion experiments we show that NPC disassembly is a phosphorylation-driven process, dependent on CDK1 activity and supported by members of the NIMA-related kinase (Nek) family. We identify phosphorylation of the GLFG-repeat nucleoporin Nup98 as an important step in mitotic NPC disassembly. Mitotic hyperphosphorylation of Nup98 is accomplished by multiple kinases, including CDK1 and Neks. Nuclei carrying a phosphodeficient mutant of Nup98 undergo nuclear envelope breakdown slowly, such that both the dissociation of Nup98 from NPCs and the permeabilization of the nuclear envelope are delayed. Together, our data provide evidence for a phosphorylation-dependent mechanism underlying disintegration of NPCs during prophase. Moreover, we identify mitotic phosphorylation of Nup98 as a rate-limiting step in mitotic NPC disassembly.

Causes and consequences of micronuclei.

Micronuclei are small membrane-bounded compartments with a DNA content encapsulated by a nuclear envelope and spatially separated from the primary nucleus. Micronuclei have long been linked to chromosome instability, genome rearrangements, and mutagenesis. They are frequently found in cancers, during senescence, and after genotoxic stress. Compromised integrity of the micronuclear envelope delays or disrupts DNA replication, inhibits DNA repair, and exposes micronuclear DNA directly to cytoplasm. Micronuclei play a central role in tumorigenesis, with micronuclear DNA being a source of complex genome rearrangements (including chromothripsis) and promoting a cyclic GMP-AMP synthase (cGAS)-mediated cellular immune response that may contribute to cancer metastasis. Here, we discuss recent findings on how micronuclei are generated, what the consequences are, and what cellular mechanisms can be applied to protect against micronucleation.

Variability in inhibition of host RNA synthesis by entero- and cardioviruses.

Both entero- and cardioviruses have been shown to suppress host mRNA synthesis. Enteroviruses are also known to inhibit the activity of rRNA genes, whereas this ability of cardioviruses is under debate. This study reported that mengovirus (a cardiovirus) suppressed rRNA synthesis but less efficiently than poliovirus (an enterovirus). In contrast to poliovirus infection, the incorporation of BrUTP, fluorouridine and [14C]uridine in rRNA precursors was observed even during the late stages of mengovirus infection, although at a significantly reduced level. The cleavage of TATA-binding protein, considered to be one of the central events in poliovirus-induced transcription shutoff, was not detected in mengovirus-infected cells, indicating a difference in the mechanisms of host RNA synthesis inhibition caused by these viruses. The results also showed that functional leader protein is redundant for the suppression of host RNA synthesis by cardiovirus.

UBASH3B-mediated silencing of the mitotic checkpoint: Therapeutic perspectives in cancer.

Defects in mitosis can lead to aneuploidy, which is a common feature of human cancers. Spindle Assembly Checkpoint (SAC) controls fidelity of chromosome segregation in mitosis to prevent aneuploidy. The ubiquitin receptor protein Ubiquitin Associated and SH3 Domain Containing B (UBASH3B) was recently found to control SAC silencing and faithful chromosome segregation by relocalizing Aurora B kinase to the mitotic microtubules. Accordingly, loss and gain of function of UBASH3B have strong effects on mitotic progression. Downregulation of UBASH3B prevents SAC satisfaction leading to inhibition of chromosome segregation, mitotic arrest, and cell death. In contrast, increased cellular levels of UBASH3B trigger premature and uncontrolled chromosome segregation. Interestingly, elevated levels of UBASH3B were found in aggressive tumors. Therefore, we raised the question whether the oncogenic potential of UBASH3B is linked to its role in chromosome segregation. Here we show that in cancer cells expressing high levels of UBASH3B and SAC proteins, downregulation of UBASH3B, can further potentiate SAC response inducing mitotic arrest and cell death. Moreover, data mining approaches identified a correlation between mRNA levels of UBASH3B and SAC components in a set of primary patient tumors including kidney and liver carcinomas. Thus, inhibition of UBASH3B may offer an attractive therapeutic perspective for cancers.

Ubiquitin Receptor Protein UBASH3B Drives Aurora B Recruitment to Mitotic Microtubules.

Mitosis ensures equal segregation of the genome and is controlled by a variety of ubiquitylation signals on substrate proteins. However, it remains unexplored how the versatile ubiquitin code is read out during mitotic progression. Here, we identify the ubiquitin receptor protein UBASH3B as an important regulator of mitosis. UBASH3B interacts with ubiquitylated Aurora B, one of the main kinases regulating chromosome segregation, and controls its subcellular localization but not protein levels. UBASH3B is a limiting factor in this pathway and is sufficient to localize Aurora B to microtubules prior to anaphase. Importantly, targeting Aurora B to microtubules by UBASH3B is necessary for the timing and fidelity of chromosome segregation in human cells. Our findings uncover an important mechanism defining how ubiquitin attachment to a substrate protein is decoded during mitosis.

Insulin granules. Insulin secretory granules control autophagy in pancreatic ß cells.

Pancreatic ß cells lower insulin release in response to nutrient depletion. The question of whether starved ß cells induce macroautophagy, a predominant mechanism maintaining energy homeostasis, remains poorly explored. We found that, in contrast to many mammalian cells, macroautophagy in pancreatic ß cells was suppressed upon starvation. Instead, starved ß cells induced lysosomal degradation of nascent secretory insulin granules, which was controlled by protein kinase D (PKD), a key player in secretory granule biogenesis. Starvation-induced nascent granule degradation triggered lysosomal recruitment and activation of mechanistic target of rapamycin that suppressed macroautophagy. Switching from macroautophagy to insulin granule degradation was important to keep insulin secretion low upon fasting. Thus, ß cells use a PKD-dependent mechanism to adapt to nutrient availability and couple autophagy flux to secretory function.

Decoding ubiquitin for mitosis.

Conjugation of ubiquitin (ubiquitination) to substrate proteins is a widespread modification that ensures fidelity of many cellular processes. During mitosis, different dynamic morphological transitions have to be coordinated in a temporal and spatial manner to allow for precise partitioning of the genetic material into two daughter cells, and ubiquitination of key mitotic factors is believed to provide both directionality and fidelity to this process. While directionality can be achieved by a proteolytic type of ubiquitination signal, the fidelity is often determined by various types of ubiquitin conjugation that does not target substrates for proteolysis by the proteasome. An additional level of complexity is provided by various ubiquitin-interacting proteins that act downstream of the ubiquitinated substrate and can serve as "decoders" for the ubiquitin signal. They may, specifically reverse ubiquitin attachment (deubiquitinating enzymes, DUBs) or, act as a receptor for transfer of the ubiquitinated substrate toward downstream signaling components and/or subcellular compartments (ubiquitin-binding proteins, UBPs). In this review, we aim at summarizing the knowledge and emerging concepts about the role of ubiquitin decoders, DUBs, and UBPs that contribute to faithful regulation of mitotic division.