The germinal center (GC) is a microanatomical compartment wherein high affinity antibody-producing B cells are selectively expanded. B cells proliferate and mutate their antibody genes in the dark zone (DZ) of the GC, and are then selected by T cells in the light zone (LZ) on the basis of affinity. Here we show that T cell help regulates the speed of cell cycle phase transitions and DNA replication of GC B cells. Genome sequencing and single-molecule analyses revealed that T cell help shortens S phase by regulating replication fork progression while preserving the relative order of replication origin activation. Thus, high affinity GC B cells are selected by a mechanism that involves prolonged dwell time in the DZ where selected cells undergo accelerated cell cycles.

SUMMARY Excess dormant origins bound by the minichromosome maintenance (MCM) replicative helicase complex play a critical role in preventing replication stress, chromosome instability and tumorigenesis. In response to DNA damage, replicating cells must coordinate DNA repair and dormant origin firing to ensure complete and timely replication of the genome; how cells regulate this process remains elusive. Herein, we identify a member of the Fanconi Anemia (FA) DNA repair pathway, FANCI, as a key effector of dormant origin firing in response to replication stress. Cells lacking FANCI have reduced number of origins, increased inter-origin distances and slowed proliferation rates. Intriguingly, ATR-mediated FANCI phosphorylation inhibits dormant origin firing while promoting replication fork restart/DNA repair. Using super-resolution microscopy, we show that FANCI co-localizes with MCM-bound chromatin in response to replication stress. These data reveal a unique role for FANCI as a modulator of dormant origin firing and links timely genome replication to DNA repair.

SUMMARY Mutations in KRAS are prevalent in human cancers and universally predictive of resistance to anti-cancer therapeutics. Although it is widely accepted that acquisition of an activating mutation endows RAS genes with functional autonomy, recent studies suggest that the wild-type forms of Ras may contribute to mutant Ras-driven tumorigenesis. Here we show that downregulation of wild-type H-Ras or N-Ras in mutant K-Ras cancer cells leads to hyperactivation of the Erk/p90RSK and PI3K/Akt pathways, and consequently, the phosphorylation of Chk1 at an inhibitory site, Ser 280. The resulting inhibition of ATR/Chk1 signaling abrogates the activation of the G2 DNA damage checkpoint and confers specific sensitization of mutant K-Ras cancer cells to DNA damage chemotherapeutic agents in vitro and in vivo.

SUMMARY The spindle assembly checkpoint (SAC) kinase Mps1 not only inhibits anaphase but also corrects erroneous attachments that could lead to missegregation and aneuploidy. However, Mps1’s error correction-relevant substrates are unknown. Using a chemically tuned kinetochore-targeting assay, we show that Mps1 destabilizes microtubule attachments (K-fibers) epistatically to Aurora B, the other major error-correcting kinase. Through quantitative proteomics, we identify multiple sites of Mps1-regulated phosphorylation at the outer kinetochore. Substrate modification was microtubule-sensitive and opposed by PP2A-B56 phosphatases that stabilize chromosome-spindle attachment. Consistently, Mps1 inhibition rescued K-fiber stability after depleting PP2A-B56. We also identify the Ska complex as a key effector of Mps1 at the kinetochore-microtubule interface, as mutations that mimic constitutive phosphorylation destabilized K-fibers in vivo and reduced the efficiency of the Ska complex’s conversion from lattice diffusion to end-coupled microtubule binding in vitro. Our results reveal how Mps1 dynamically modifies kinetochores to correct improper attachments and ensure faithful chromosome segregation.

Summary During oncogenesis, cells acquire multiple genetic alterations that confer essential tumor-specific traits, including immortalization, escape from anti-mitogenic signaling, neovascularization, invasiveness, and metastatic potential. In most instances, these alterations are thought to arise incrementally over years if not decades. However, recent progress in sequencing cancer genomes has begun to challenge this paradigm, as a radically different phenomenon, termed chromothripsis, has been suggested to cause complex intra- and inter-chromosomal rearrangements on short timescales. In this article, we review established pathways crucial for genome integrity and discuss how their dysfunction could precipitate widespread chromosome breakage and rearrangement in the course of malignancy.

Translesion synthesis polymerases (TLS Pols) are required to tolerate DNA lesions that would otherwise cause replication arrest and cell death. Aberrant expression of these specialized Pols may be responsible for increased mutagenesis and loss of genome integrity in human cancers. The molecular events that control the usage of TLS Pols in non-pathological conditions remain largely unknown. Here, we show that aberrant recruitment of TLS Polj to replication forks results in genomic instability and can be mediated through the loss of the deubiquitinase USP1. Moreover, artificial tethering of Polj to proliferating cell nuclear antigen (PCNA) circumvents the need for its ubiquitin-binding domain in the promotion of genomic instability. Finally, we show that the loss of USP1 leads to a dramatic reduction of replication fork speed in a Poljdependent manner. We propose a mechanism whereby reversible ubiquitination of PCNA can prevent spurious TLS Pol recruitment and regulate replication fork speed to ensure the maintenance of genome integrity.

Summary An intercentrosomal linker keeps the cell’s two centrosomes joined together until it is dissolved at the onset of mitosis. A second connection keeps daughter centrioles engaged to their mothers until they lose their orthogonal arrangement at the end of mitosis. Centriole disengagement is required to license centrioles for duplication. We show that the intercentrosomal linker protein Cep68 is degraded in prometaphase through the SCFβTrCP (Skp1-Cul1-F-box protein) ubiquitin ligase complex. Cep68 degradation is initiated by PLK1 phosphorylation of Cep68 on Ser332, allowing recognition by βTrCP. We also found that Cep68 forms a complex with Cep215/Cdk5Rap2 and PCNT/Pericentrin, two PCM (pericentriolar material) proteins involved in centriole engagement. Cep68 requires two different pools of Cep215. We propose that Cep68 degradation allows Cep215 removal from the peripheral PCM preventing centriole separation following disengagement, whereas PCNT cleavage mediates Cep215 removal from the core of the PCM to inhibit centriole disengagement and duplication.

APC/CCdh1-dependent degradation of USP1 allows for PCNA monoubiquitination and subsequent recruitment of trans-lesion synthesis polymerase to UV repair sites.