Dual role of proliferating cell nuclear antigen monoubiquitination in facilitating Fanconi anemia-mediated interstrand crosslink repair.

The Fanconi anemia (FA) repair pathway governs repair of highly genotoxic DNA interstrand crosslinks (ICLs) and relies on translesion synthesis (TLS). TLS is facilitated by REV1 or site-specific monoubiquitination of proliferating cell nuclear antigen (PCNA) (PCNA-Ub) at lysine 164 (K164). A PcnaK164R/K164R but not Rev1-/- mutation renders mammals hypersensitive to ICLs. Besides the FA pathway, alternative pathways have been associated with ICL repair (1, 2), though the decision making between those remains elusive. To study the dependence and relevance of PCNA-Ub in FA repair, we intercrossed PcnaK164R/+; Fancg-/+ mice. A combined mutation (PcnaK164R/K164R; Fancg-/- ) was found embryonically lethal. RNA-seq of primary double-mutant (DM) mouse embryonic fibroblasts (MEFs) revealed elevated levels of replication stress-induced checkpoints. To exclude stress-induced confounders, we utilized a Trp53 knock-down to obtain a model to study ICL repair in depth. Regarding ICL-induced cell toxicity, cell cycle arrest, and replication fork progression, single-mutant and DM MEFs were found equally sensitive, establishing PCNA-Ub to be critical for FA-ICL repair. Immunoprecipitation and spectrometry-based analysis revealed an unknown role of PCNA-Ub in excluding mismatch recognition complex MSH2/MSH6 from being recruited to ICLs. In conclusion, our results uncovered a dual function of PCNA-Ub in ICL repair, i.e. exclude MSH2/MSH6 recruitment to channel the ICL toward canonical FA repair, in addition to its established role in coordinating TLS opposite the unhooked ICL.

Chromosomal Rearrangements and Chromothripsis: The Alternative End Generation Model.

Chromothripsis defines a genetic phenomenon where up to hundreds of clustered chromosomal rearrangements can arise in a single catastrophic event. The phenomenon is associated with cancer and congenital diseases. Most current models on the origin of chromothripsis suggest that prior to chromatin reshuffling numerous DNA double-strand breaks (DSBs) have to exist, i.e., chromosomal shattering precedes rearrangements. However, the preference of a DNA end to rearrange in a proximal accessible region led us to propose chromothripsis as the reaction product of successive chromatin rearrangements. We previously coined this process Alternative End Generation (AEG), where a single DSB with a repair-blocking end initiates a domino effect of rearrangements. Accordingly, chromothripsis is the end product of this domino reaction taking place in a single catastrophic event.

Mammalian life depends on two distinct pathways of DNA damage tolerance.

DNA damage threatens genomic integrity and instigates stem cell failure. To bypass genotoxic lesions during replication, cells employ DNA damage tolerance (DDT), which is regulated via PCNA ubiquitination and REV1. DDT is conserved in all domains of life, yet its relevance in mammals remains unclear. Here, we show that inactivation of both PCNA-ubiquitination and REV1 results in embryonic and adult lethality, and the accumulation of DNA damage in hematopoietic stem and progenitor cells (HSPCs) that ultimately resulted in their depletion. Our results reveal the crucial relevance of DDT in the maintenance of stem cell compartments and mammalian life in unperturbed conditions.

Division of labor within the DNA damage tolerance system reveals non-epistatic and clinically actionable targets for precision cancer medicine.

Crosslink repair depends on the Fanconi anemia pathway and translesion synthesis polymerases that replicate over unhooked crosslinks. Translesion synthesis is regulated via ubiquitination of PCNA, and independently via translesion synthesis polymerase REV1. The division of labor between PCNA-ubiquitination and REV1 in interstrand crosslink repair is unclear. Inhibition of either of these pathways has been proposed as a strategy to increase cytotoxicity of platinating agents in cancer treatment. Here, we defined the importance of PCNA-ubiquitination and REV1 for DNA in mammalian ICL repair. In mice, loss of PCNA-ubiquitination, but not REV1, resulted in germ cell defects and hypersensitivity to cisplatin. Loss of PCNA-ubiquitination, but not REV1 sensitized mammalian cancer cell lines to cisplatin. We identify polymerase Kappa as essential in tolerating DNA damage-induced lesions, in particular cisplatin lesions. Polk-deficient tumors were controlled by cisplatin treatment and it significantly delayed tumor outgrowth and increased overall survival of tumor bearing mice. Our results indicate that PCNA-ubiquitination and REV1 play distinct roles in DNA damage tolerance. Moreover, our results highlight POLK as a critical TLS polymerase in tolerating multiple genotoxic lesions, including cisplatin lesions. The relative frequent loss of Polk in cancers indicates an exploitable vulnerability for precision cancer medicine.

Huwe1 supports B-cell development, B-cell-dependent immunity, somatic hypermutation and class switch recombination by regulating proliferation.

The development and differentiation of B cells is intimately linked to cell proliferation and the generation of diverse immunoglobulin gene (Ig) repertoires. The ubiquitin E3 ligase HUWE1 controls proliferation, DNA damage responses, and DNA repair, including the base excision repair (BER) pathway. These processes are of crucial importance for B-cell development in the bone marrow, and the germinal center (GC) response, which results in the clonal expansion and differentiation of B cells expressing high affinity immunoglobulins. Here, we re-examined the role of HUWE1 in B-cell proliferation and Ig gene diversification, focusing on its involvement in somatic hypermutation (SHM) and class switch recombination (CSR). B-cell-specific deletion of Huwe1 resulted in impaired development, differentiation and maturation of B cells in the bone marrow and peripheral lymphoid organs. HUWE1 deficiency diminished SHM and CSR by impairing B-cell proliferation and AID expression upon activation in vitro and in vivo, and was unrelated to the HUWE1-dependent regulation of the BER pathway. Interestingly, we found that HUWE1-deficient B cells showed increased mRNA expression of Myc target genes upon in vitro activation despite diminished proliferation. Our results confirm that the E3 ligase HUWE1 is an important contributor in coordinating the rapid transition of antigen naïve, resting B cells into antigen-activated B cells and regulates mutagenic processes in B cells by controlling AID expression and the post-transcriptional output of Myc target genes.

Strategy for Development of Site-Specific Ubiquitin Antibodies.

Protein ubiquitination is a key post-translational modification regulating a wide range of biological processes. Ubiquitination involves the covalent attachment of the small protein ubiquitin to a lysine of a protein substrate. In addition to its well-established role in protein degradation, protein ubiquitination plays a role in protein-protein interactions, DNA repair, transcriptional regulation, and other cellular functions. Understanding the mechanisms and functional relevance of ubiquitin as a signaling system requires the generation of antibodies or alternative reagents that specifically detect ubiquitin in a site-specific manner. However, in contrast to other post-translational modifications such as acetylation, phosphorylation, and methylation, the instability and size of ubiquitin-76 amino acids-complicate the preparation of suitable antigens and the generation antibodies detecting such site-specific modifications. As a result, the field of ubiquitin research has limited access to specific antibodies. This severely hampers progress in understanding the regulation and function of site-specific ubiquitination in many areas of biology, specifically in epigenetics and cancer. Therefore, there is a high demand for antibodies recognizing site-specific ubiquitin modifications. Here we describe a strategy for the development of site-specific ubiquitin antibodies. Based on a recently developed antibody against site-specific ubiquitination of histone H2B, we provide detailed protocols for chemical synthesis methods for antigen preparation and discuss considerations for screening and quality control experiments.

CD4+ T Cell Help Confers a Cytotoxic T Cell Effector Program Including Coinhibitory Receptor Downregulation and Increased Tissue Invasiveness.

CD4+ T cells optimize the cytotoxic T cell (CTL) response in magnitude and quality, by unknown molecular mechanisms. We here present the transcriptomic changes in CTLs resulting from CD4+ T cell help after anti-cancer vaccination or virus infection. The gene expression signatures revealed that CD4+ T cell help during priming optimized CTLs in expression of cytotoxic effector molecules and many other functions that ensured efficacy of CTLs throughout their life cycle. Key features included downregulation of PD-1 and other coinhibitory receptors that impede CTL activity, and increased motility and migration capacities. "Helped" CTLs acquired chemokine receptors that helped them reach their tumor target tissue and metalloprotease activity that enabled them to invade into tumor tissue. A very large part of the "help" program was instilled in CD8+ T cells via CD27 costimulation. The help program thus enhances specific CTL effector functions in response to vaccination or a virus infection.