Chromothripsis: A New Mechanism for Rapid Karyotype Evolution.

Chromosomal rearrangements are generally thought to accumulate gradually over many generations. However, DNA sequencing of cancer and congenital disorders uncovered a new pattern in which multiple rearrangements arise all at once. The most striking example, chromothripsis, is characterized by tens or hundreds of rearrangements confined to a single chromosome or to local regions over a few chromosomes. Genomic analysis of chromothripsis and the search for its biological mechanism have led to new insights on how chromosome segregation errors can generate mutagenesis and changes to the karyotype. Here, we review the genomic features of chromothripsis and summarize recent progress on understanding its mechanism. This includes reviewing new work indicating that one mechanism to generate chromothripsis is through the physical isolation of chromosomes in abnormal nuclear structures (micronuclei). We also discuss connections revealed by recent genomic analysis of cancers between chromothripsis, chromosome bridges, and ring chromosomes.

v-Src Causes Chromosome Bridges in a Caffeine-Sensitive Manner by Generating DNA Damage.

An increase in Src activity is commonly observed in epithelial cancers. Aberrant activation of the kinase activity is associated with malignant progression. However, the mechanisms that underlie the Src-induced malignant progression of cancer are not completely understood. We show here that v-Src, an oncogene that was first identified from a Rous sarcoma virus and a mutant variant of c-Src, leads to an increase in the number of anaphase and telophase cells having chromosome bridges. v-Src increases the number of γH2AX foci, and this increase is inhibited by treatment with PP2, a Src kinase inhibitor. v-Src induces the phosphorylation of KAP1 at Ser824, Chk2 at Thr68, and Chk1 at Ser345, suggesting the activation of the ATM/ATR pathway. Caffeine decreases the number of cells having chromosome bridges at a concentration incapable of inhibiting Chk1 phosphorylation at Ser345. These results suggest that v-Src induces chromosome bridges via generation of DNA damage and the subsequent DNA damage response, possibly by homologous recombination. A chromosome bridge gives rise to the accumulation of DNA damage directly through chromosome breakage and indirectly through cytokinesis failure-induced multinucleation. We propose that v-Src-induced chromosome bridge formation is one of the causes of the v-Src-induced malignant progression of cancer cells.

Genistein induces cytokinesis failure through RhoA delocalization and anaphase chromosome bridging.

Genistein, an isoflavone abundantly present in soybeans, possesses anticancer properties and induces growth inhibition including cell cycle arrest and apoptosis. Although abnormal cell division, such as defects in chromosome segregation and spindle formation, and polyploidization have been described, the mechanisms underlying the induction of abnormal cell division are unknown. In this study, we examined the effect of genistein on cell division in cells that are synchronized in M phase, since genistein treatment delays mitotic entry in asynchronous cells. HeLa S3 cells were arrested at the G2 phase and subsequently released into the M phase in presence of genistein. Immunofluorescence staining showed that genistein treatment delays M phase progression. Time-lapse analysis revealed that the delay occurs until anaphase onset. In addition, genistein treatment induces cleavage furrow regression, resulting in the generation of binucleated cells. Central spindle formation, which is essential for cytokinesis, is partially disrupted in genistein-treated cells. Moreover, aberrant chromosome segregation, such as a chromosome bridge and lagging chromosome, occurs through progression of cytokinesis. RhoA, which plays a role in the assembly and constriction of an actomyosin contractile ring, is delocalized from the cortex of the ingressing cleavage furrow. These results suggest that genistein treatment induces binucleated cell formation through cleavage furrow regression, which is accompanied by chromosome bridge formation and RhoA delocalization. Our results provide the mechanism that underlies genistein-induced polyploidization, which may be involved in genistein-induced growth inhibition.

Crossover recombination mediated by HIM-18/SLX4-associated nucleases.

Meiosis is a specialized cell division program that results in the formation of haploid gametes (i.e., sperm and eggs) from diploid parental cells, and is essential for all sexually reproducing organisms. Crossover formation, the reciprocal exchange of genetic information during recombination, is critical for accurate meiotic chromosome segregation. Misregulation of crossover formation leads to genomic instability and aneuploidy (cells with the incorrect number of chromosomes), resulting in tumorigenesis, birth defects, miscarriages, and infertility in humans. Recently, a shuriken/Swiss army knife-like multi-nuclease complex has been implicated in processing various types of DNA repair intermediates. However, how these nucleases coordinate their functions during repair remained unclear. Our studies in C. elegans revealed genetic redundancies between these nucleases for meiotic crossover formation and that they promote distinct crossover control at different chromosome regions. Specifically, XPF-1 acts redundantly with both MUS-81 and SLX-1 to resolve Holliday junction recombination intermediates into crossover products at designated future crossover sites on chromosome arms. In contrast, SLX-1 is required for suppression of crossovers at the center region of chromosomes. Altogether, our studies have shed light on the interplay between structure-specific endonucleases and uncovered their ability to exert either positive or negative meiotic crossover control on a chromosome region-specific basis.