Mouse SLX4 is a tumor suppressor that stimulates the activity of the nuclease XPF-ERCC1 in DNA crosslink repair

ICL-induced miR139-3p and miR199a-3p have opposite roles in hematopoietic cell expansion and leukemic transformation

Interstrand crosslinks (ICLs) are toxic DNA lesions that cause severe genomic damage during replication, especially in Fanconi anemia pathway-deficient cells. This results in progressive bone marrow failure and predisposes to acute myeloid leukemia (AML). The molecular mechanisms responsible for these defects are largely unknown. Using Ercc1-deficient mice, we show that Trp53 is responsible for ICL-induced bone marrow failure and that loss of Trp53 is leukemogenic in this model. In addition, Ercc1-deficient myeloid progenitors gain elevated levels of miR-139-3p and miR-199a-3p with age. These microRNAs exert opposite effects on hematopoiesis. Ectopic expression of miR-139-3p strongly inhibited proliferation of myeloid progenitors, whereas inhibition of miR-139-3p activity restored defective proliferation of Ercc1-deficient progenitors. Conversely, the inhibition of miR-199a-3p functions aggravated the myeloid proliferation defect in the Ercc1-deficient model, whereas its enforced expression enhanced proliferation of progenitors. Importantly, miR-199a-3p caused AML in a pre-leukemic mouse model, supporting its role as an onco-microRNA. Target genes include HuR for miR-139-3p and Prdx6, Runx1, and Suz12 for miR-199a-3p. The latter genes have previously been implicated as tumor suppressors in de novo and secondary AML. These findings show that, in addition to TRP53-controlled mechanisms, miR-139-3p and miR-199a-3p are involved in the defective hematopoietic function of ICL-repair deficient myeloid progenitors.

SLX4 gains weight with SUMO in genome maintenance

Replication stress has emerged as a key driver of oncogenesis but also represents an Achilles' heel of cancer cells. Newly reported SUMO binding and SUMO ligase functions of the DNA repair protein SLX4 that influence the outcome of replication stress open new avenues for investigating the roles played by SLX4 in tumorigenesis.

Structural and Mechanistic Analysis of the Slx1-Slx4 Endonuclease

The SLX1-SLX4 endonuclease required for homologous recombination and DNA repair in eukaryotic cells cleaves a variety of branched DNA structures. The nuclease subunit SLX1 is activated by association with a scaffolding protein SLX4. At the present time, little is known about the structure of SLX1-SLX4 or its mechanism of action. Here, we report the structural insights into SLX1-SLX4 by detailing the crystal structure of Candida glabrata (Cg) Slx1 alone and in combination with the C-terminal region of Slx4. The structure of Slx1 reveals a compact arrangement of the GIY-YIG nuclease and RING domains, which is reinforced by a long α helix. Slx1 forms a stable homodimer that blocks its active site. Slx1-Slx4 interaction is mutually exclusive with Slx1 homodimerization, suggesting a mechanism for Slx1 activation by Slx4.

SUMOylation and PARylation cooperate to recruit and stabilize SLX4 at DNA damage sites

SUMOylation plays important roles in the DNA damage response. However, whether it is important for interstrand crosslink repair remains unknown. We report that the SLX4 nuclease scaffold protein is regulated by SUMOylation. We have identified three SUMO interaction motifs (SIMs) in SLX4, mutating all of which abrogated the binding of SLX4 to SUMO-2 and covalent SLX4 SUMOylation. An SLX4 mutant lacking functional SIMs is not recruited to PML nuclear bodies nor stabilized at laser-induced DNA damage sites. Additionally, we elucidated a novel role for PARylation in the recruitment of SLX4 to sites of DNA damage. Combined, our results uncover how SLX4 is regulated by post-translational modifications.

Cancer therapy and replication stress: forks on the road to perdition

Deregulated DNA replication occurs in cancer where it contributes to genomic instability. This process is a target of cytotoxic therapies. Chemotherapies exploit high DNA replication in cancer cells by modifying the DNA template or by inhibiting vital enzymatic activities that lead to slowing or stalling replication fork progression. Stalled replication forks can be converted into toxic DNA double-strand breaks resulting in cell death, i.e., replication stress. While likely crucial for many cancer treatments, replication stress is poorly understood due to its complexity. While we still know relatively little about the role of replication stress in cancer therapy, technical advances in recent years have shed new light on the effect that cancer therapeutics have on replication forks and the molecular mechanisms that lead from obstructed fork progression to cell death. This chapter will give an overview of our current understanding of replication stress in the context of cancer therapy.

A high-throughput in vivo micronucleus assay for genome instability screening in mice

We describe a sensitive, robust, high-throughput method for quantifying the formation of micronuclei, markers of genome instability, in mouse erythrocytes. Micronuclei are whole chromosomes or chromosome segments that have been separated from the nucleus. Other methods of detection rely on labor-intensive, microscopy-based techniques. Here we describe a 2-d, 96-well plate-based flow cytometric method of micronucleus scoring that is simple enough for a research technician experienced in flow cytometry to perform. The assay detects low levels of genome instability that cannot be readily identified by classic phenotyping, using 25 μl of blood. By using this assay, we have screened >10,000 blood samples and discovered novel genes that contribute to vertebrate genome maintenance, as well as novel disease models and mechanisms of genome instability disorders. We discuss experimental design considerations, including statistical power calculation, we provide troubleshooting tips and we discuss factors that contribute to a false-positive increase in the number of micronucleated red blood cells and to experimental variability.

Ubiquitin-SUMO circuitry controls activated fanconi anemia ID complex dosage in response to DNA damage

We show that central components of the Fanconi anemia (FA) DNA repair pathway, the tumor suppressor proteins FANCI and FANCD2 (the ID complex), are SUMOylated in response to replication fork stalling. The ID complex is SUMOylated in a manner that depends on the ATR kinase, the FA ubiquitin ligase core complex, and the SUMO E3 ligases PIAS1/PIAS4 and is antagonized by the SUMO protease SENP6. SUMOylation of the ID complex drives substrate selectivity by triggering its polyubiquitylation by the SUMO-targeted ubiquitin ligase RNF4 to promote its removal from sites of DNA damage via the DVC1-p97 ubiquitin segregase complex. Deregulation of ID complex SUMOylation compromises cell survival following replication stress. Our results uncover a regulatory role for SUMOylation in the FA pathway, and we propose that ubiquitin-SUMO signaling circuitry is a mechanism that contributes to the balance of activated ID complex dosage at sites of DNA damage.

•

The Fanconi anemia ID complex (FANCI/FANCD2) is SUMOylated after DNA damage

•

ID complex SUMOylation is regulated by ATR, the FA core complex, PIAS1/4, and SENP6

•

SUMO-dependent ubiquitylation by RNF4 allows ID complex removal from DNA by DVC1/p97

•

Deregulated ID complex SUMOylation compromises cell survival following DNA damage

Noncovalent interactions with SUMO and ubiquitin orchestrate distinct functions of the SLX4 complex in genome maintenance

SLX4, a coordinator of multiple DNA structure-specific endonucleases, is important for several DNA repair pathways. Noncovalent interactions of SLX4 with ubiquitin are required for localizing SLX4 to DNA interstrand crosslinks (ICLs), yet how SLX4 is targeted to other functional contexts remains unclear. Here, we show that SLX4 binds SUMO-2/3 chains via SUMO-interacting motifs (SIMs). The SIMs of SLX4 are dispensable for ICL repair but important for processing CPT-induced replication intermediates, suppressing fragile site instability, and localizing SLX4 to ALT telomeres. The localization of SLX4 to laser-induced DNA damage also requires the SIMs, as well as DNA end resection, UBC9, and MDC1. Furthermore, the SUMO binding of SLX4 enhances its interaction with specific DNA-damage sensors or telomere-binding proteins, including RPA, MRE11-RAD50-NBS1, and TRF2. Thus, the interactions of SLX4 with SUMO and ubiquitin increase its affinity for factors recognizing different DNA lesions or telomeres, helping to direct the SLX4 complex in distinct functional contexts.

The SLX4 complex is a SUMO E3 ligase that impacts on replication stress outcome and genome stability

The SLX4 Fanconi anemia protein is a tumor suppressor that may act as a key regulator that engages the cell into specific genome maintenance pathways. Here, we show that the SLX4 complex is a SUMO E3 ligase that SUMOylates SLX4 itself and the XPF subunit of the DNA repair/recombination XPF-ERCC1 endonuclease. This SLX4-dependent activity is mediated by a remarkably specific interaction between SLX4 and the SUMO-charged E2 conjugating enzyme UBC9 and relies not only on newly identified SUMO-interacting motifs (SIMs) in SLX4 but also on its BTB domain. In contrast to its ubiquitin-binding UBZ4 motifs, SLX4 SIMs are dispensable for its DNA interstrand crosslink repair functions. Instead, while detrimental in response to global replication stress, the SUMO E3 ligase activity of the SLX4 complex is critical to prevent mitotic catastrophe following common fragile site expression.

Crystal structure of a Fanconi anemia-associated nuclease homolog bound to 5' flap DNA: basis of interstrand cross-link repair by FAN1

Fanconi anemia (FA) is an autosomal recessive genetic disorder caused by defects in FA genes responsible for processing DNA interstrand cross-links (ICLs). FA-associated nuclease (FAN1) is recruited to lesions by a monoubiquitinated FANCI–FANCD2 (ID) complex and participates in ICL repair. Here, Gwon et al. determined the crystal structure of Pseudomonas aeruginosa FAN1 (PaFAN1) lacking the UBZ (ubiquitin-binding zinc) domain in complex with 5′ flap DNA. The PaFAN1 structure provides insights into how FAN1 integrates with the FA complex to participate in ICL repair.

Fanconi anemia (FA) is an autosomal recessive genetic disorder caused by defects in any of 15 FA genes responsible for processing DNA interstrand cross-links (ICLs). The ultimate outcome of the FA pathway is resolution of cross-links, which requires structure-selective nucleases. FA-associated nuclease 1 (FAN1) is believed to be recruited to lesions by a monoubiquitinated FANCI–FANCD2 (ID) complex and participates in ICL repair. Here, we determined the crystal structure of Pseudomonas aeruginosa FAN1 (PaFAN1) lacking the UBZ (ubiquitin-binding zinc) domain in complex with 5′ flap DNA. All four domains of the right-hand-shaped PaFAN1 are involved in DNA recognition, with each domain playing a specific role in bending DNA at the nick. The six-helix bundle that binds the junction connects to the catalytic viral replication and repair (VRR) nuclease (VRR nuc) domain, enabling FAN1 to incise the scissile phosphate a few bases distant from the junction. The six-helix bundle also inhibits the cleavage of intact Holliday junctions. PaFAN1 shares several conserved features with other flap structure-selective nucleases despite structural differences. A clamping motion of the domains around the wedge helix, which acts as a pivot, facilitates nucleolytic cleavage. The PaFAN1 structure provides insights into how archaeal Holliday junction resolvases evolved to incise 5′ flap substrates and how FAN1 integrates with the FA complex to participate in ICL repair.

Replication stress and cancer: it takes two to tango

Problems arising during DNA replication require the activation of the ATR-CHK1 pathway to ensure the stabilization and repair of the forks, and to prevent the entry into mitosis with unreplicated genomes. Whereas the pathway is essential at the cellular level, limiting its activity is particularly detrimental for some cancer cells. Here we review the links between replication stress (RS) and cancer, which provide a rationale for the use of ATR and Chk1 inhibitors in chemotherapy. First, we describe how the activation of oncogene-induced RS promotes genome rearrangements and chromosome instability, both of which could potentially fuel carcinogenesis. Next, we review the various pathways that contribute to the suppression of RS, and how mutations in these components lead to increased cancer incidence and/or accelerated ageing. Finally, we summarize the evidence showing that tumors with high levels of RS are dependent on a proficient RS-response, and therefore vulnerable to ATR or Chk1 inhibitors.

Maternal aldehyde elimination during pregnancy preserves the fetal genome

Maternal metabolism provides essential nutrients to enable embryonic development. However, both mother and embryo produce reactive metabolites that can damage DNA. Here we discover how the embryo is protected from these genotoxins. Pregnant mice lacking Aldh2, a key enzyme that detoxifies reactive aldehydes, cannot support the development of embryos lacking the Fanconi anemia DNA repair pathway gene Fanca. Remarkably, transferring Aldh2(-/-)Fanca(-/-) embryos into wild-type mothers suppresses developmental defects and rescues embryonic lethality. These rescued neonates have severely depleted hematopoietic stem and progenitor cells, indicating that despite intact maternal aldehyde catabolism, fetal Aldh2 is essential for hematopoiesis. Hence, maternal and fetal aldehyde detoxification protects the developing embryo from DNA damage. Failure of this genome preservation mechanism might explain why birth defects and bone marrow failure occur in Fanconi anemia, and may have implications for fetal well-being in the many women in Southeast Asia that are genetically deficient in ALDH2.

Holliday junction processing enzymes as guardians of genome stability

Holliday junctions (HJs) are four-stranded DNA intermediates that arise during the recombinational repair of DNA double-strand breaks (DSBs). Their timely removal is crucial for faithful chromosome segregation and genome stability. In mammalian cells, HJs are processed by the BTR (BLM-topoisomerase IIIα-RMI1-RMI2) complex, the SLX-MUS (SLX1-SLX4-MUS81-EME1) complex, and the GEN1 resolvase. Recent studies have linked the deficiency of one or more of these enzymes to perturbed DNA replication, impaired crosslink repair, chromosomal instability, and defective mitoses, coupled with the transmission of widespread DNA damage and high levels of mortality. We review these key advances and how they have cemented the status of HJ-processing enzymes as guardians of genome integrity and viability in mammalian cells.

Mechanism and regulation of incisions during DNA interstrand cross-link repair

A critical step in DNA interstrand cross-link repair is the programmed collapse of replication forks that have stalled at an ICL. This event is regulated by the Fanconi anemia pathway, which suppresses bone marrow failure and cancer. In this perspective, we focus on the structure of forks that have stalled at ICLs, how these structures might be incised by endonucleases, and how incision is regulated by the Fanconi anemia pathway.

XPF-ERCC1 acts in Unhooking DNA interstrand crosslinks in cooperation with FANCD2 and FANCP/SLX4

DNA interstrand crosslinks (ICLs), highly toxic lesions that covalently link the Watson and Crick strands of the double helix, are repaired by a complex, replication-coupled pathway in higher eukaryotes. The earliest DNA processing event in ICL repair is the incision of parental DNA on either side of the ICL ("unhooking"), which allows lesion bypass. Incisions depend critically on the Fanconi anemia pathway, whose activation involves ubiquitylation of the FANCD2 protein. Using Xenopus egg extracts, which support replication-coupled ICL repair, we show that the 3' flap endonuclease XPF-ERCC1 cooperates with SLX4/FANCP to carry out the unhooking incisions. Efficient recruitment of XPF-ERCC1 and SLX4 to the ICL depends on FANCD2 and its ubiquitylation. These data help define the molecular mechanism by which the Fanconi anemia pathway promotes a key event in replication-coupled ICL repair.