Histone chaperone activity of Fanconi anemia proteins, FANCD2 and FANCI, is required for DNA crosslink repair

Fanconi anemia DNA crosslink repair factors protect against LINE-1 retrotransposition during mouse development

Long interspersed nuclear element 1 (LINE-1) is the only autonomous retrotransposon in humans and new integrations are a major source of genetic variation between individuals. These events can also lead to de novo germline mutations, giving rise to heritable genetic diseases. Recently, a role for DNA repair in regulating these events has been identified. Here we find that Fanconi anemia (FA) DNA crosslink repair factors act in a common pathway to prevent retrotransposition. We purify recombinant SLX4-XPF-ERCC1, the crosslink repair incision complex, and find that it cleaves putative nucleic acid intermediates of retrotransposition. Mice deficient in upstream crosslink repair signaling (FANCA), a downstream component (FANCD2) or the nuclease XPF-ERCC1 show increased LINE-1 retrotransposition in vivo. Organisms limit retrotransposition through transcriptional silencing but this protection is attenuated during early development leaving the zygote vulnerable. We find that during this window of vulnerability, DNA crosslink repair acts as a failsafe to prevent retrotransposition. Together, our results indicate that the FA DNA crosslink repair pathway acts together to protect against mutation by restricting LINE-1 retrotransposition.

ATM depletion induces proteasomal degradation of FANCD2 and sensitizes neuroblastoma cells to PARP inhibitors

BACKGROUND

Genomic alterations, including loss of function in chromosome band 11q22-23, are frequently observed in neuroblastoma, which is the most common extracranial childhood tumour. In neuroblastoma, ATM, a DNA damage response-associated gene located on 11q22-23, has been linked to tumorigenicity. Genetic changes in ATM are heterozygous in most tumours. However, it is unclear how ATM is associated with tumorigenesis and cancer aggressiveness.

METHODS

To elucidate its molecular mechanism of action, we established ATM-inactivated NGP and CHP-134 neuroblastoma cell lines using CRISPR/Cas9 genome editing. The knock out cells were rigorously characterized by analyzing proliferation, colony forming abilities and responses to PARP inhibitor (Olaparib). Western blot analyses were performed to detect different protein expression related to DNA repair pathway. ShRNA lentiviral vectors were used to knockdown ATM expression in SK-N-AS and SK-N-SH neuroblastoma cell lines. ATM knock out cells were stably transfected with FANCD2 expression plasmid to over-expressed the FANCD2. Moreover, knock out cells were treated with proteasome inhibitor MG132 to determine the protein stability of FANCD2. FANCD2, RAD51 and γH2AX protein expressions were determined by Immunofluorescence microscopy.

RESULTS

Haploinsufficient ATM resulted in increased proliferation (p < 0.01) and cell survival following PARP inhibitor (olaparib) treatment. However, complete ATM knockout decreased proliferation (p < 0.01) and promoted cell susceptibility to olaparib (p < 0.01). Complete loss of ATM suppressed the expression of DNA repair-associated molecules FANCD2 and RAD51 and induced DNA damage in neuroblastoma cells. A marked downregulation of FANCD2 expression was also observed in shRNA-mediated ATM-knockdown neuroblastoma cells. Inhibitor experiments demonstrated that the degradation of FANCD2 was regulated at the protein level through the ubiquitin-proteasome pathway. Reintroduction of FANCD2 expression is sufficient to reverse decreased proliferation mediated by ATM depletion.

CONCLUSIONS

Our study revealed the molecular mechanism underlying ATM heterozygosity in neuroblastomas and elucidated that ATM inactivation enhances the susceptibility of neuroblastoma cells to olaparib treatment. These findings might be useful in the treatment of high-risk NB patients showing ATM zygosity and aggressive cancer progression in future.

H3K4 methylation by SETD1A/BOD1L facilitates RIF1-dependent NHEJ

The 53BP1-RIF1-shieldin pathway maintains genome stability by suppressing nucleolytic degradation of DNA ends at double-strand breaks (DSBs). Although RIF1 interacts with damaged chromatin via phospho-53BP1 and facilitates recruitment of the shieldin complex to DSBs, it is unclear whether other regulatory cues contribute to this response. Here, we implicate methylation of histone H3 at lysine 4 by SETD1A-BOD1L in the recruitment of RIF1 to DSBs. Compromising SETD1A or BOD1L expression or deregulating H3K4 methylation allows uncontrolled resection of DNA ends, impairs end-joining of dysfunctional telomeres, and abrogates class switch recombination. Moreover, defects in RIF1 localization to DSBs are evident in patient cells bearing loss-of-function mutations in SETD1A. Loss of SETD1A-dependent RIF1 recruitment in BRCA1-deficient cells restores homologous recombination and leads to resistance to poly(ADP-ribose)polymerase inhibition, reinforcing the clinical relevance of these observations. Mechanistically, RIF1 binds directly to methylated H3K4, facilitating its recruitment to, or stabilization at, DSBs.

Chromatin and Nuclear Dynamics in the Maintenance of Replication Fork Integrity

Replication of the eukaryotic genome is a highly regulated process and stringent control is required to maintain genome integrity. In this review, we will discuss the many aspects of the chromatin and nuclear environment that play key roles in the regulation of both unperturbed and stressed replication. Firstly, the higher order organisation of the genome into A and B compartments, topologically associated domains (TADs) and sub-nuclear compartments has major implications in the control of replication timing. In addition, the local chromatin environment defined by non-canonical histone variants, histone post-translational modifications (PTMs) and enrichment of factors such as heterochromatin protein 1 (HP1) plays multiple roles in normal S phase progression and during the repair of replicative damage. Lastly, we will cover how the spatial organisation of stalled replication forks facilitates the resolution of replication stress.

R-loops trigger the release of cytoplasmic ssDNAs leading to chronic inflammation upon DNA damage

How DNA damage leads to chronic inflammation and tissue degeneration with aging remains to be fully resolved. Here, we show that DNA damage leads to cellular senescence, fibrosis, loss-of-tissue architecture, and chronic pancreatitis in mice with an inborn defect in the excision repair cross complementation group 1 (Ercc1) gene. We find that DNA damage-driven R-loops causally contribute to the active release and buildup of single-stranded DNAs (ssDNAs) in the cytoplasm of cells triggering a viral-like immune response in progeroid and naturally aged pancreata. To reduce the proinflammatory load, we developed an extracellular vesicle (EV)-based strategy to deliver recombinant S1 or ribonuclease H nucleases in inflamed Ercc1^−/− pancreatic cells. Treatment of Ercc1^−/− animals with the EV-delivered nuclease cargo eliminates DNA damage-induced R-loops and cytoplasmic ssDNAs alleviating chronic inflammation. Thus, DNA damage-driven ssDNAs causally contribute to tissue degeneration, Ercc1^−/− paving the way for novel rationalized intervention strategies against age-related chronic inflammation.

Functional cross talk between the Fanconi anemia and ATRX/DAXX histone chaperone pathways promotes replication fork recovery

Fanconi anemia (FA) is a chromosome instability syndrome characterized by increased cancer predisposition. Specifically, the FA pathway functions to protect genome stability during DNA replication. The central FA pathway protein, FANCD2, locates to stalled replication forks and recruits homologous recombination (HR) factors such as CtBP interacting protein (CtIP) to promote replication fork restart while suppressing new origin firing. Here, we identify alpha-thalassemia retardation syndrome X-linked (ATRX) as a novel physical and functional interaction partner of FANCD2. ATRX is a chromatin remodeler that forms a complex with Death domain-associated protein 6 (DAXX) to deposit the histone variant H3.3 into specific genomic regions. Intriguingly, ATRX was recently implicated in replication fork recovery; however, the underlying mechanism(s) remained incompletely understood. Our findings demonstrate that ATRX forms a constitutive protein complex with FANCD2 and protects FANCD2 from proteasomal degradation. ATRX and FANCD2 localize to stalled replication forks where they cooperate to recruit CtIP and promote MRE11 exonuclease-dependent fork restart while suppressing the firing of new replication origins. Remarkably, replication restart requires the concerted histone H3 chaperone activities of ATRX/DAXX and FANCD2, demonstrating that coordinated histone H3 variant deposition is a crucial event during the reinitiation of replicative DNA synthesis. Lastly, ATRX also cooperates with FANCD2 to promote the HR-dependent repair of directly induced DNA double-stranded breaks. We propose that ATRX is a novel functional partner of FANCD2 to promote histone deposition-dependent HR mechanisms in S-phase.

RAD51AP1 mediates RAD51 activity through nucleosome interaction

RAD51-associated protein 1 (RAD51AP1) is a key protein in the homologous recombination (HR) DNA repair pathway. Loss of RAD51AP1 leads to defective HR, genome instability, and telomere erosion. RAD51AP1 physically interacts with the RAD51 recombinase and promotes RAD51-mediated capture of donor DNA, synaptic complex assembly, and displacement-loop formation when tested with nucleosome-free DNA substrates. In cells, however, DNA is packaged into chromatin, posing an additional barrier to the complexities of the HR reaction. In this study, we show that RAD51AP1 binds to nucleosome core particles (NCPs), the minimum basic unit of chromatin in which approximately two superhelical turns of 147 bp double-stranded DNA are wrapped around one histone octamer with no free DNA ends remaining. We identified a C-terminal region in RAD51AP1, including its previously mapped DNA-binding domain, as critical for mediating the association between RAD51AP1 and both the NCP and the histone octamer. Using in vitro surrogate assays of HR activity, we show that RAD51AP1 is capable of promoting duplex DNA capture and initiating joint-molecule formation with the NCP and chromatinized template DNA, respectively. Together, our results suggest that RAD51AP1 directly assists in the RAD51-mediated search for donor DNA in chromatin. We present a model, in which RAD51AP1 anchors the DNA template through affinity for its nucleosomes to the RAD51-ssDNA nucleoprotein filament.

A fork in the road: Where homologous recombination and stalled replication fork protection part ways

In response to replication hindrances, DNA replication forks frequently stall and are remodelled into a four-way junction. In such a structure the annealed nascent strand is thought to resemble a DNA double-strand break and remodelled forks are vulnerable to nuclease attack by MRE11 and DNA2. Proteins that promote the recruitment, loading and stabilisation of RAD51 onto single-stranded DNA for homology search and strand exchange in homologous recombination (HR) repair and inter-strand cross-link repair also act to set up RAD51-mediated protection of nascent DNA at stalled replication forks. However, despite the similarities of these pathways, several lines of evidence indicate that fork protection is not simply analogous to the RAD51 loading step of HR. Protection of stalled forks not only requires separate functions of a number of recombination proteins, but also utilises nucleases important for the resection steps of HR in alternative ways. Here we discuss how fork protection arises and how its differences with HR give insights into the differing contexts of these two pathways.

Monoubiquitination by the human Fanconi anemia core complex clamps FANCI:FANCD2 on DNA in filamentous arrays

FANCI:FANCD2 monoubiquitination is a critical event for replication fork stabilization by the Fanconi anemia (FA) DNA repair pathway. It has been proposed that at stalled replication forks, monoubiquitinated-FANCD2 serves to recruit DNA repair proteins that contain ubiquitin-binding motifs. Here, we have reconstituted the FA pathway in vitro to study functional consequences of FANCI:FANCD2 monoubiquitination. We report that monoubiquitination does not promote any specific exogenous protein:protein interactions, but instead stabilizes FANCI:FANCD2 heterodimers on dsDNA. This clamping requires monoubiquitination of only the FANCD2 subunit. We further show using electron microscopy that purified monoubiquitinated FANCI:FANCD2 forms filament-like arrays on long dsDNA. Our results reveal how monoubiquitinated FANCI:FANCD2, defective in many cancer types and all cases of FA, is activated upon DNA binding.

Tissue-infiltrating macrophages mediate an exosome-based metabolic reprogramming upon DNA damage

DNA damage and metabolic disorders are intimately linked with premature disease onset but the underlying mechanisms remain poorly understood. Here, we show that persistent DNA damage accumulation in tissue-infiltrating macrophages carrying an ERCC1-XPF DNA repair defect (Er1^F/−) triggers Golgi dispersal, dilation of endoplasmic reticulum, autophagy and exosome biogenesis leading to the secretion of extracellular vesicles (EVs) in vivo and ex vivo. Macrophage-derived EVs accumulate in Er1^F/− animal sera and are secreted in macrophage media after DNA damage. The Er1^F/− EV cargo is taken up by recipient cells leading to an increase in insulin-independent glucose transporter levels, enhanced cellular glucose uptake, higher cellular oxygen consumption rate and greater tolerance to glucose challenge in mice. We find that high glucose in EV-targeted cells triggers pro-inflammatory stimuli via mTOR activation. This, in turn, establishes chronic inflammation and tissue pathology in mice with important ramifications for DNA repair-deficient, progeroid syndromes and aging.

DNA damage is associated with metabolic disorders, but the mechanism in unclear. Here, the authors show that persistent DNA damage induced by lack of the endonuclease XPF-ERCC1 triggers extracellular vesicle biogenesis in tissue infiltrating macrophages, and that vesicle uptake stimulates glucose uptake in recipient cells, leading to increased inflammation.

A Fanci knockout mouse model reveals common and distinct functions for FANCI and FANCD2

Fanconi Anemia (FA) clinical phenotypes are heterogenous and rely on a mutation in one of the 22 FANC genes (FANCA-W) involved in a common interstrand DNA crosslink-repair pathway. A critical step in the activation of FA pathway is the monoubiquitination of FANCD2 and its binding partner FANCI. To better address the clinical phenotype associated with FANCI and the epistatic relationship with FANCD2, we created the first conditional inactivation model for FANCI in mouse. Fanci ^−/− mice displayed typical FA features such as delayed development in utero, microphtalmia, cellular sensitivity to mitomycin C, occasional limb abnormalities and hematological deficiencies. Interestingly, the deletion of Fanci leads to a strong meiotic phenotype and severe hypogonadism. FANCI was localized in spermatocytes and spermatids and in the nucleus of oocytes. Both FANCI and FANCD2 proteins co-localized with RPA along meiotic chromosomes, albeit at different levels. Consistent with a role in meiotic recombination, FANCI interacted with RAD51 and stimulated D-loop formation, unlike FANCD2. The double knockout Fanci^−/− Fancd2^−/− also showed epistatic relationship for hematological defects while being not epistatic with respect to generating viable mice in crosses of double heterozygotes. Collectively, this study highlights common and distinct functions of FANCI and FANCD2 during mouse development, meiotic recombination and hematopoiesis.

Replication stress induces accumulation of FANCD2 at central region of large fragile genes

During mild replication stress provoked by low dose aphidicolin (APH) treatment, the key Fanconi anemia protein FANCD2 accumulates on common fragile sites, observed as sister foci, and protects genome stability. To gain further insights into FANCD2 function and its regulatory mechanisms, we examined the genome-wide chromatin localization of FANCD2 in this setting by ChIP-seq analysis. We found that FANCD2 mostly accumulates in the central regions of a set of large transcribed genes that were extensively overlapped with known CFS. Consistent with previous studies, we found that this FANCD2 retention is R-loop-dependent. However, FANCD2 monoubiquitination and RPA foci formation were still induced in cells depleted of R-loops. Interestingly, we detected increased Proximal Ligation Assay dots between FANCD2 and R-loops following APH treatment, which was suppressed by transcriptional inhibition. Collectively, our data suggested that R-loops are required to retain FANCD2 in chromatin at the middle intronic region of large genes, while the replication stress-induced upstream events leading to the FA pathway activation are not triggered by R-loops.

FANCD2 Binding to H4K20me2 via a Methyl-Binding Domain Is Essential for Efficient DNA Cross-Link Repair

Fanconi anemia (FA) is an inherited disease characterized by bone marrow failure and increased cancer risk. FA is caused by mutation of any 1 of 22 genes, and the FA proteins function cooperatively to repair DNA interstrand cross-links (ICLs). A central step in the activation of the FA pathway is the monoubiquitination of the FANCD2 and FANCI proteins, which occurs within chromatin. How FANCD2 and FANCI are anchored to chromatin remains unknown. In this study, we identify and characterize a FANCD2 histone-binding domain (HBD) and embedded methyl-lysine-binding domain (MBD) and demonstrate binding specificity for H4K20me2. Disruption of the HBD/MBD compromises FANCD2 chromatin binding and nuclear focus formation and its ability to promote error-free DNA interstrand cross-link repair, leading to increased error-prone repair and genome instability. Our study functionally describes the first FA protein chromatin reader domain and establishes an important link between this human genetic disease and chromatin plasticity.

On your marks, get SET(D1A): the race to protect stalled replication forks

We recently identified that methylation of lysine 4 of histone H3 (H3K4) by SETD1A (SET domain containing 1A) maintains genome stability by protecting newly-replicated DNA from degradation. Mechanistically, SETD1A-dependent histone methylation regulates nucleosome mobilisation by FANCD2 (FA complementation group D2), a crucial step in maintaining genome integrity with important implications in chemo-sensitivity.

Histone Methylation by SETD1A Protects Nascent DNA through the Nucleosome Chaperone Activity of FANCD2

Components of the Fanconi anemia and homologous recombination pathways play a vital role in protecting newly replicated DNA from uncontrolled nucleolytic degradation, safeguarding genome stability. Here we report that histone methylation by the lysine methyltransferase SETD1A is crucial for protecting stalled replication forks from deleterious resection. Depletion of SETD1A sensitizes cells to replication stress and leads to uncontrolled DNA2-dependent resection of damaged replication forks. The ability of SETD1A to prevent degradation of these structures is mediated by its ability to catalyze methylation on Lys4 of histone H3 (H3K4) at replication forks, which enhances FANCD2-dependent histone chaperone activity. Suppressing H3K4 methylation or expression of a chaperone-defective FANCD2 mutant leads to loss of RAD51 nucleofilament stability and severe nucleolytic degradation of replication forks. Our work identifies epigenetic modification and histone mobility as critical regulatory mechanisms in maintaining genome stability by restraining nucleases from irreparably damaging stalled replication forks.

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Methylation of H3K4 by SETD1A maintains genome stability during replication stress

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SETD1A and H3K4 methylation stabilize RAD51 nucleofilaments to protect nascent DNA

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SETD1A-dependent H3K4 methylation enhances FANCD2-dependent histone remodeling

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Histone mobility stabilizes RAD51 nucleofilaments to inhibit fork degradation

Fanconi Anemia FANCM/FNCM-1 and FANCD2/FCD-2 Are Required for Maintaining Histone Methylation Levels and Interact with the Histone Demethylase LSD1/SPR-5 in Caenorhabditis elegans

The histone demethylase LSD1 was originally discovered by removing methyl groups from di- and monomethylated histone H3 lysine 4 (H3K4me2/1). Several studies suggest that LSD1 plays roles in meiosis as well as in the epigenetic regulation of fertility given that, in its absence, there is evidence of a progressive accumulation of H3K4me2 and increased sterility through generations. In addition to the progressive sterility phenotype observed in the mutants, growing evidence for the importance of histone methylation in the regulation of DNA damage repair has attracted more attention to the field in recent years. However, we are still far from understanding the mechanisms by which histone methylation is involved in DNA damage repair, and only a few studies have focused on the roles of histone demethylases in germline maintenance. Here, we show that the histone demethylase LSD1/CeSPR-5 interacts with the Fanconi anemia (FA) protein FANCM/CeFNCM-1 using biochemical, cytological, and genetic analyses. LSD1/CeSPR-5 is required for replication stress-induced S phase-checkpoint activation, and its absence suppresses the embryonic lethality and larval arrest observed in fncm-1 mutants. FANCM/CeFNCM-1 relocalizes upon hydroxyurea exposure and colocalizes with FANCD2/CeFCD-2 and LSD1/CeSPR-5, suggesting coordination between this histone demethylase and FA components to resolve replication stress. Surprisingly, the FA pathway is required for H3K4me2 maintenance, regardless of the presence of replication stress. Our study reveals a connection between FA and epigenetic maintenance and therefore provides new mechanistic insight into the regulation of histone methylation in DNA repair.

Histone H3K27 methylation is required for NHEJ and genome stability by modulating the dynamics of FANCD2 on chromatin

Dysregulation of homeostatic balance in di- and tri-methyl H3K27 levels or that caused by mis-sense mutations of histone H3 (H3K27M) was reported to be associated with various types of cancers. In this study, we found that reduction in H3K27me2/3 caused by H3.1K27M, a mutation of H3 variants found in DIPG patients, dramatically attenuated the presence of 53BP1 foci and NHEJ repair capability in HDF cells. H3.1K27M cells showed increased rates of genomic insertions/deletions (In/Dels) and copy number variations (CNVs), as well as augmented p53-dependent apoptotic cells. We further showed that both hypo-H3K27me2/3 and H3.1K27M interacted with FANCD2, a central player to orchestrate DNA repair pathway choice. H3.1K27M triggered an accumulation of FANCD2 on chromatin, supporting the interaction between H3.1K27M and FANCD2. Most interestingly, knock-down of FANCD2 in H3.1K27M cells recovered the number of 53BP1 foci, NHEJ efficiency and apoptosis rate. Although these findings in HDF cells may differ from the case of endogenous H3.1K27M mutant regulation in the specific tumor context of DIPG, our results suggest a new model by which H3K27me2/3 facilitates NHEJ and the maintenance of genome stability.

Elucidation of the Fanconi Anemia Protein Network in Meiosis and Its Function in the Regulation of Histone Modifications

Precise epigenetic regulation of the sex chromosomes is vital for the male germline. Here, we analyze meiosis in eight mouse models deficient for various DNA damage response (DDR) factors, including Fanconi anemia (FA) proteins. We reveal a network of FA and DDR proteins in which FA core factors FANCA, FANCB, and FANCC are essential for FANCD2 foci formation, whereas BRCA1 (FANCS), MDC1, and RNF8 are required for BRCA2 (FANCD1) and SLX4 (FANCP) accumulation on the sex chromosomes during meiosis. In addition, FA proteins modulate distinct histone marks on the sex chromosomes: FA core proteins and FANCD2 regulate H3K9 methylation, while FANCD2 and RNF8 function together to regulate H3K4 methylation independently of FA core proteins. Our data suggest that RNF8 integrates the FA-BRCA pathway. Taken together, our study reveals distinct functions for FA proteins and illuminates the male sex chromosomes as a model to dissect the function of the FA-BRCA pathway.

Fanconi anemia FANCD2 and FANCI proteins regulate the nuclear dynamics of splicing factors

Moriel-Carretero et al. show that the Fanconi anemia proteins FANCI and FANCD2 associate with the splicing factor SF3B1 and that DNA replication stress induces the FANCI-dependent release of SF3B1 from nuclear speckles. FANCI and FANCD2 prevent accumulation of postcatalytic intron lariats, suggesting that they help coordinate DNA replication and transcription.

Proteins disabled in the cancer-prone disorder Fanconi anemia (FA) ensure the maintenance of chromosomal stability during DNA replication. FA proteins regulate replication dynamics, coordinate replication-coupled repair of interstrand DNA cross-links, and mitigate conflicts between replication and transcription. Here we show that FANCI and FANCD2 associate with splicing factor 3B1 (SF3B1), a key spliceosomal protein of the U2 small nuclear ribonucleoprotein (U2 snRNP). FANCI is in close proximity to SF3B1 in the nucleoplasm of interphase and mitotic cells. Furthermore, we find that DNA replication stress induces the release of SF3B1 from nuclear speckles in a manner that depends on FANCI and on the activity of the checkpoint kinase ATR. In chromatin, both FANCD2 and FANCI associate with SF3B1, prevent accumulation of postcatalytic intron lariats, and contribute to the timely eviction of splicing factors. We propose that FANCD2 and FANCI contribute to the organization of functional domains in chromatin, ensuring the coordination of DNA replication and cotranscriptional processes.

RFWD3-Mediated Ubiquitination Promotes Timely Removal of Both RPA and RAD51 from DNA Damage Sites to Facilitate Homologous Recombination

RFWD3 is a recently identified Fanconi anemia protein FANCW whose E3 ligase activity toward RPA is essential in homologous recombination (HR) repair. However, how RPA ubiquitination promotes HR remained unknown. Here, we identified RAD51, the central HR protein, as another target of RFWD3. We show that RFWD3 polyubiquitinates both RPA and RAD51 in vitro and in vivo. Phosphorylation by ATR and ATM kinases is required for this activity in vivo. RFWD3 inhibits persistent mitomycin C (MMC)-induced RAD51 and RPA foci by promoting VCP/p97-mediated protein dynamics and subsequent degradation. Furthermore, MMC-induced chromatin loading of MCM8 and RAD54 is defective in cells with inactivated RFWD3 or expressing a ubiquitination-deficient mutant RAD51. Collectively, our data reveal a mechanism that facilitates timely removal of RPA and RAD51 from DNA damage sites, which is crucial for progression to the late-phase HR and suppression of the FA phenotype.

FANCD2 Facilitates Replication through Common Fragile Sites

Common fragile sites (CFSs) are genomic regions that are unstable under conditions of replicative stress. Although the characteristics of CFSs that render them vulnerable to stress are associated mainly with replication, the cellular pathways that protect CFSs during replication remain unclear. Here, we identify and describe a role for FANCD2 as a trans-acting facilitator of CFS replication, in the absence of exogenous replicative stress. In the absence of FANCD2, replication forks stall within the AT-rich fragility core of CFS, leading to dormant origin activation. Furthermore, FANCD2 deficiency is associated with DNA:RNA hybrid formation at CFS-FRA16D, and inhibition of DNA:RNA hybrid formation suppresses replication perturbation. In addition, we also found that FANCD2 reduces the number of potential sites of replication initiation. Our data demonstrate that FANCD2 protein is required to ensure efficient CFS replication and provide mechanistic insight into how FANCD2 regulates CFS stability.

Activation of the FA pathway mediated by phosphorylation and ubiquitination

Fanconi anemia (FA) is a devastating hereditary condition that impacts genome integrity, leading to clinical features such as skeletal and visceral organ malformations, attrition of bone marrow stem cells, and carcinogenesis. At least 21 proteins, when absent or defective, have been implicated in this disorder, and they together constitute the FA pathway, which functions in detection and repair of, and tolerance to, endogenous DNA damage. The damage primarily handled by the FA pathway has been assumed to be related to DNA interstrand crosslinks (ICLs). The FA pathway is activated upon ICL damage, and a hallmark of this activation is the mono-ubiquitination events of the key FANCD2-FANCI protein complex. Recent data have revealed unexpectedly complex details in the regulation of FA pathway activation by ICLs. In this short review, we summarize the knowledge accumulated over the years regarding how the FA pathway is activated via protein modifications.

A role for the base excision repair enzyme NEIL3 in replication-dependent repair of interstrand DNA cross-links derived from psoralen and abasic sites

Interstrand DNA-DNA cross-links are highly toxic lesions that are important in medicinal chemistry, toxicology, and endogenous biology. In current models of replication-dependent repair, stalling of a replication fork activates the Fanconi anemia pathway and cross-links are "unhooked" by the action of structure-specific endonucleases such as XPF-ERCC1 that make incisions flanking the cross-link. This process generates a double-strand break, which must be subsequently repaired by homologous recombination. Recent work provided evidence for a new, incision-independent unhooking mechanism involving intrusion of a base excision repair (BER) enzyme, NEIL3, into the world of cross-link repair. The evidence suggests that the glycosylase action of NEIL3 unhooks interstrand cross-links derived from an abasic site or the psoralen derivative trioxsalen. If the incision-independent NEIL3 pathway is blocked, repair reverts to the incision-dependent route. In light of the new model invoking participation of NEIL3 in cross-link repair, we consider the possibility that various BER glycosylases or other DNA-processing enzymes might participate in the unhooking of chemically diverse interstrand DNA cross-links.

A defined role for multiple Fanconi anemia gene products in DNA-damage-associated ubiquitination

Fanconi anemia (FA) is an inherited blood disorder that causes bone marrow failure and high predisposition to cancers. The FA pathway guards the cell's genome stability by orchestrating the repair of interstrand cross-linking during the S phase of the cell cycle, preventing the chromosomal instability that is a key event in bone marrow failure syndrome. Central to the FA pathway is loss of monoubiquitinated forms of the Fanconi proteins FANCI and FANCD2, a process that is normally mediated by a "core complex" of seven other Fanconi proteins. Each protein, when mutated, can cause FA. The FA core-complex-catalyzed reaction is critical for signaling DNA cross-link damage such as that induced by chemotherapies. Here, we present a perspective on the current understanding of FANCI and FANCD2 monoubiquitination-mediated DNA repair. Our recent biochemical reconstitution of the monoubiquitination (and deubiquitination) reactions creates a paradigm for understanding FA. Further biochemical analysis will create new opportunities to address the leukemic phenotype of FA patients.

Bloom syndrome complex promotes FANCM recruitment to stalled replication forks and facilitates both repair and traverse of DNA interstrand crosslinks

The recruitment of FANCM, a conserved DNA translocase and key component of several DNA repair protein complexes, to replication forks stalled by DNA interstrand crosslinks (ICLs) is a step upstream of the Fanconi anemia (FA) repair and replication traverse pathways of ICLs. However, detection of the FANCM recruitment has been technically challenging so that its mechanism remains exclusive. Here, we successfully observed recruitment of FANCM at stalled forks using a newly developed protocol. We report that the FANCM recruitment depends upon its intrinsic DNA translocase activity, and its DNA-binding partner FAAP24. Moreover, it is dependent on the replication checkpoint kinase, ATR; but is independent of the FA core and FANCD2-FANCI complexes, two essential components of the FA pathway, indicating that the FANCM recruitment occurs downstream of ATR but upstream of the FA pathway. Interestingly, the recruitment of FANCM requires its direct interaction with Bloom syndrome complex composed of BLM helicase, Topoisomerase 3α, RMI1 and RMI2; as well as the helicase activity of BLM. We further show that the FANCM-BLM complex interaction is critical for replication stress-induced FANCM hyperphosphorylation, for normal activation of the FA pathway in response to ICLs, and for efficient traverse of ICLs by the replication machinery. Epistasis studies demonstrate that FANCM and BLM work in the same pathway to promote replication traverse of ICLs. We conclude that FANCM and BLM complex work together at stalled forks to promote both FA repair and replication traverse pathways of ICLs.

The FA Core Complex Contains a Homo-dimeric Catalytic Module for the Symmetric Mono-ubiquitination of FANCI-FANCD2

Activation of the main DNA interstrand crosslink repair pathway in higher eukaryotes requires mono-ubiquitination of FANCI and FANCD2 by FANCL, the E3 ligase subunit of the Fanconi anemia core complex. FANCI and FANCD2 form a stable complex; however, the molecular basis of their ubiquitination is ill defined. FANCD2 mono-ubiquitination by FANCL is stimulated by the presence of the FANCB and FAAP100 core complex components, through an unknown mechanism. How FANCI mono-ubiquitination is achieved remains unclear. Here, we use structural electron microscopy, combined with crosslink-coupled mass spectrometry, to find that FANCB, FANCL, and FAAP100 form a dimer of trimers, containing two FANCL molecules that are ideally poised to target both FANCI and FANCD2 for mono-ubiquitination. The FANCC-FANCE-FANCF subunits bridge between FANCB-FANCL-FAAP100 and the FANCI-FANCD2 substrate. A transient interaction with FANCC-FANCE-FANCF alters the FANCI-FANCD2 configuration, stabilizing the dimerization interface. Our data provide a model to explain how equivalent mono-ubiquitination of FANCI and FANCD2 occurs.

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FANCB, FANCL, and FAAP100 form a symmetric dimer of trimers

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FANCL is ideally poised for the symmetric mono-ubiquitination of FANCI-FANCD2

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Two separate FANCC-FANCE-FANCF complexes bind to the opposing poles of FANCB-FANCL-FAAP100

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FANCC-FANCE-FANCF stabilizes FANCI-FANCD2 for efficient mono-ubiquitination

Defects in homologous recombination repair behind the human diseases: FA and HBOC

Hereditary breast and ovarian cancer (HBOC) syndrome and a rare childhood disorder Fanconi anemia (FA) are caused by homologous recombination (HR) defects, and some of the causative genes overlap. Recent studies in this field have led to the exciting development of PARP inhibitors as novel cancer therapeutics and have clarified important mechanisms underlying genome instability and tumor suppression in HR-defective disorders. In this review, we provide an overview of the basic molecular mechanisms governing HR and DNA crosslink repair, highlighting BRCA2, and the intriguing relationship between HBOC and FA.

FANCI-FANCD2 stabilizes the RAD51-DNA complex by binding RAD51 and protects the 5'-DNA end

The FANCI-FANCD2 (I-D) complex is considered to work with RAD51 to protect the damaged DNA in the stalled replication fork. However, the means by which this DNA protection is accomplished have remained elusive. In the present study, we found that the I-D complex directly binds to RAD51, and stabilizes the RAD51-DNA filament. Unexpectedly, the DNA binding activity of FANCI, but not FANCD2, is explicitly required for the I-D complex-mediated RAD51-DNA filament stabilization. The RAD51 filament stabilized by the I-D complex actually protects the DNA end from nucleolytic degradation by an FA-associated nuclease, FAN1. This DNA end protection is not observed with the RAD51 mutant from FANCR patient cells. These results clearly answer the currently enigmatic question of how RAD51 functions with the I-D complex to prevent genomic instability at the stalled replication fork.

A Genetically Encoded Probe for Live-Cell Imaging of H4K20 Monomethylation

Eukaryotic gene expression is regulated in the context of chromatin. Dynamic changes in post-translational histone modification are thought to play key roles in fundamental cellular functions such as regulation of the cell cycle, development, and differentiation. To elucidate the relationship between histone modifications and cellular functions, it is important to monitor the dynamics of modifications in single living cells. A genetically encoded probe called mintbody (modification-specific intracellular antibody), which is a single-chain variable fragment tagged with a fluorescent protein, has been proposed as a useful visualization tool. However, the efficacy of intracellular expression of antibody fragments has been limited, in part due to different environmental conditions in the cytoplasm compared to the endoplasmic reticulum where secreted proteins such as antibodies are folded. In this study, we have developed a new mintbody specific for histone H4 Lys20 monomethylation (H4K20me1). The specificity of the H4K20me1-mintbody in living cells was verified using yeast mutants and mammalian cells in which this target modification was diminished. Expression of the H4K20me1-mintbody allowed us to monitor the oscillation of H4K20me1 levels during the cell cycle. Moreover, dosage-compensated X chromosomes were visualized using the H4K20me1-mintbody in mouse and nematode cells. Using X-ray crystallography and mutational analyses, we identified critical amino acids that contributed to stabilization and/or proper folding of the mintbody. Taken together, these data provide important implications for future studies aimed at developing functional intracellular antibodies. Specifically, the H4K20me1-mintbody provides a powerful tool to track this particular histone modification in living cells and organisms.

A NAP-Family Histone Chaperone Functions in Abiotic Stress Response and Adaptation

Modulation of gene expression is one of the most significant molecular mechanisms of abiotic stress response in plants. Via altering DNA accessibility, histone chaperones affect the transcriptional competence of genomic loci. However, in contrast to other factors affecting chromatin dynamics, the role of plant histone chaperones in abiotic stress response and adaptation remains elusive. Here, we studied the physiological function of a stress-responsive putative rice (Oryza sativa) histone chaperone of the NAP superfamily: OsNAPL6. We show that OsNAPL6 is a nuclear-localized H3/H4 histone chaperone capable of assembling a nucleosome-like structure. Utilizing overexpression and knockdown approaches, we found a positive correlation between OsNAPL6 expression levels and adaptation to multiple abiotic stresses. Results of comparative transcriptome profiling and promoter-recruitment studies indicate that OsNAPL6 functions during stress response via modulation of expression of various genes involved in diverse functions. For instance, we show that OsNAPL6 is recruited to OsRad51 promoter, activating its expression and leading to more efficient DNA repair and abrogation of programmed cell death under salinity and genotoxic stress conditions. These results suggest that the histone chaperone OsNAPL6 may serve a regulatory role in abiotic stress physiology possibly via modulating nucleosome dynamics at various stress-associated genomic loci. Taken together, our findings establish a hitherto unknown link between histone chaperones and abiotic stress response in plants.

[Fanconi anemia animal models - How differences can teach us as much as similarities…]

Fanconi Anemia is a rare autosomal recessive genetic disease with heterogenous phenotypes including myelosuppression, congenital malformations and heightened cancer predisposition. FA cells are highly sensitive to cross-linking agents. Since the 90's, at least 19 FANC proteins (FANCA to FANCT) have been identified as working together in a unique pathway detecting and triggering the repair of DNA crosslinks. Since then, the creation of animal models in various species (nematode, fruit fly, zebrafish and mouse) contributed to a better understanding of the physiopathology of the disease. This review aims to summarize the main discoveries made in these in vivo models, as well as to discuss some controversies that arose from these studies.

The Fanconi Anemia Pathway Maintains Genome Stability by Coordinating Replication and Transcription

DNA replication stress can cause chromosomal instability and tumor progression. One key pathway that counteracts replication stress and promotes faithful DNA replication consists of the Fanconi anemia (FA) proteins. However, how these proteins limit replication stress remains largely elusive. Here we show that conflicts between replication and transcription activate the FA pathway. Inhibition of transcription or enzymatic degradation of transcription-associated R-loops (DNA:RNA hybrids) suppresses replication fork arrest and DNA damage occurring in the absence of a functional FA pathway. Furthermore, we show that simple aldehydes, known to cause leukemia in FA-deficient mice, induce DNA:RNA hybrids in FA-depleted cells. Finally, we demonstrate that the molecular mechanism by which the FA pathway limits R-loop accumulation requires FANCM translocase activity. Failure to activate a response to physiologically occurring DNA:RNA hybrids may critically contribute to the heightened cancer predisposition and bone marrow failure of individuals with mutated FA proteins.

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Replication and transcription collisions cause genome instability in FA

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A functional FA pathway protects cells from unscheduled accumulation of R-loops

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Transcription inhibition or R-loop removal restores normal replication in FA cells

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FANCM resolves R-loops via its translocase activity

The Fanconi anemia ID2 complex: dueling saxes at the crossroads

Fanconi anemia (FA) is a rare recessive genetic disease characterized by congenital abnormalities, bone marrow failure and heightened cancer susceptibility in early adulthood. FA is caused by biallelic germ-line mutation of any one of 16 genes. While several functions for the FA proteins have been ascribed, the prevailing hypothesis is that the FA proteins function cooperatively in the FA-BRCA pathway to repair damaged DNA. A pivotal step in the activation of the FA-BRCA pathway is the monoubiquitination of the FANCD2 and FANCI proteins. Despite their importance for DNA repair, the domain structure, regulation, and function of FANCD2 and FANCI remain poorly understood. In this review, we provide an overview of our current understanding of FANCD2 and FANCI, with an emphasis on their posttranslational modification and common and unique functions.

Role of the Exocyst Complex Component Sec6/8 in Genomic Stability

The exocyst is a heterooctomeric complex well appreciated for its role in the dynamic assembly of specialized membrane domains. Accumulating evidence indicates that this macromolecular machine also serves as a physical platform that coordinates regulatory cascades supporting biological systems such as host defense signaling, cell fate, and energy homeostasis. The isolation of multiple components of the DNA damage response (DDR) as exocyst-interacting proteins, together with the identification of Sec8 as a suppressor of the p53 response, suggested functional interactions between the exocyst and the DDR. We found that exocyst perturbation resulted in resistance to ionizing radiation (IR) and accelerated resolution of DNA damage. This occurred at the expense of genomic integrity, as enhanced recombination frequencies correlated with the accumulation of aberrant chromatid exchanges. Sec8 perturbation resulted in the accumulation of ATF2 and RNF20 and the promiscuous accumulation of DDR-associated chromatin marks and Rad51 repairosomes. Thus, the exocyst supports DNA repair fidelity by limiting the formation of repair chromatin in the absence of DNA damage.

Cdt1-binding protein GRWD1 is a novel histone-binding protein that facilitates MCM loading through its influence on chromatin architecture

Efficient pre-replication complex (pre-RC) formation on chromatin templates is crucial for the maintenance of genome integrity. However, the regulation of chromatin dynamics during this process has remained elusive. We found that a conserved protein, GRWD1 (glutamate-rich WD40 repeat containing 1), binds to two representative replication origins specifically during G1 phase in a CDC6- and Cdt1-dependent manner, and that depletion of GRWD1 reduces loading of MCM but not CDC6 and Cdt1. Furthermore, chromatin immunoprecipitation coupled with high-throughput sequencing (Seq) revealed significant genome-wide co-localization of GRWD1 with CDC6. We found that GRWD1 has histone-binding activity. To investigate the effect of GRWD1 on chromatin architecture, we used formaldehyde-assisted isolation of regulatory elements (FAIRE)-seq or FAIRE-quantitative PCR analyses, and the results suggest that GRWD1 regulates chromatin openness at specific chromatin locations. Taken together, these findings suggest that GRWD1 may be a novel histone-binding protein that regulates chromatin dynamics and MCM loading at replication origins.

Exploiting replicative stress to treat cancer

DNA replication in cancer cells is accompanied by stalling and collapse of the replication fork and signalling in response to DNA damage and/or premature mitosis; these processes are collectively known as 'replicative stress'. Progress is being made to increase our understanding of the mechanisms that govern replicative stress, thus providing ample opportunities to enhance replicative stress for therapeutic purposes. Rather than trying to halt cell cycle progression, cancer therapeutics could aim to increase replicative stress by further loosening the checkpoints that remain available to cancer cells and ultimately inducing the catastrophic failure of proliferative machineries. In this Review, we outline current and future approaches to achieve this, emphasizing the combination of conventional chemotherapy with targeted approaches.

Defective FANCI binding by a fanconi anemia-related FANCD2 mutant

FANCD2 is a product of one of the genes associated with Fanconi anemia (FA), a rare recessive disease characterized by bone marrow failure, skeletal malformations, developmental defects, and cancer predisposition. FANCD2 forms a complex with FANCI (ID complex) and is monoubiquitinated, which facilitates the downstream interstrand crosslink (ICL) repair steps, such as ICL unhooking and nucleolytic end resection. In the present study, we focused on the chicken FANCD2 (cFANCD2) mutant harboring the Leu234 to Arg (L234R) substitution. cFANCD2 L234R corresponds to the human FANCD2 L231R mutation identified in an FA patient. We found that cFANCD2 L234R did not complement the defective ICL repair in FANCD2^−/− DT40 cells. Purified cFANCD2 L234R did not bind to chicken FANCI, and its monoubiquitination was significantly deficient, probably due to the abnormal ID complex formation. In addition, the histone chaperone activity of cFANCD2 L234R was also defective. These findings may explain some aspects of Fanconi anemia pathogenesis by a FANCD2 missense mutation.

Expression and purification of human FANCI and FANCD2 using Escherichia coli cells

The DNA interstrand crosslink (ICL) is an extremely deleterious DNA lesion that covalently crosslinks complementary strands and prevents the strand-separation reaction. In higher eukaryotes, the Fanconi anemia proteins, FANCI and FANCD2, form a heterodimer and play essential roles in ICL repair. Human FANCI and FANCD2 are large proteins with molecular masses of 149kDa and 164kDa, respectively, and were reportedly purified using a baculovirus expression system with insect cells. We have established a novel expression and purification procedure for human FANCD2 and FANCI, using Escherichia coli cells. The human FANCD2 and FANCI proteins purified by this bacterial expression method formed a stable heterodimer, and exhibited DNA binding and histone chaperone activities, as previously reported for the proteins purified by the baculovirus system. Therefore, these purification methods for human FANCI and FANCD2 provide novel procedures to facilitate structural and biochemical studies of human FANCI and FANCD2.

FANCD2 binds CtIP and regulates DNA-end resection during DNA interstrand crosslink repair

The Fanconi anemia (FA) pathway is critically involved in the maintenance of hematopoietic stem cells and the suppression of carcinogenesis. A key FA protein, FANCD2, is monoubiquitinated and accumulates in chromatin in response to DNA interstrand crosslinks (ICLs), where it coordinates DNA repair through mechanisms that are still poorly understood. Here, we report that CtIP protein directly interacts with FANCD2. A region spanning amino acids 166 to 273 of CtIP and monoubiquitination of FANCD2 are both essential for the FANCD2-CtIP interaction and mitomycin C (MMC)-induced CtIP foci. Remarkably, both FANCD2 and CtIP are critical for MMC-induced RPA2 hyperphosphorylation, an event that accompanies end resection of double-strand breaks. Collectively, our results reveal a role of monoubiquitinated FANCD2 in end resection that depends on its binding to CtIP during ICL repair.

FANCD2 and CtIP cooperate to repair DNA interstrand crosslinks

The resolution of DNA interstrand crosslinks (ICLs) requires a complex interplay between several processes of DNA metabolism, including the Fanconi anemia (FA) pathway and homologous recombination (HR). FANCD2 monoubiquitination and CtIP-dependent DNA-end resection represent key events in FA and HR activation, respectively, but very little is known about their functional relationship. Here, we show that CtIP physically interacts with both FANCD2 and ubiquitin and that monoubiquitinated FANCD2 tethers CtIP to damaged chromatin, which helps channel DNA double-strand breaks generated during ICL processing into the HR pathway. Consequently, CtIP mutants defective in FANCD2 binding fail to associate with damaged chromatin, which leads to increased levels of nonhomologous end-joining activity and ICL hypersensitivity. Interestingly, we also observe that CtIP depletion aggravates the genomic instability in FANCD2-deficient cells. Thus, our data indicate that FANCD2 primes CtIP-dependent resection during HR after ICL induction but that CtIP helps prevent illegitimate recombination in FA cells.

Crosstalk between the nucleotide excision repair and Fanconi anemia/BRCA pathways

Cells have evolved multiple distinct DNA repair pathways to efficiently correct a variety of genotoxic lesions, and decades of study have led to an improved understanding of the mechanisms and regulation of these individual pathways. However, there is now an increasing appreciation that extensive crosstalk exists among DNA repair pathways and that this crosstalk serves to increase the efficiency and diversity of response to damage. The Fanconi anemia (FA)/BRCA and nucleotide excision repair (NER) pathways have been shown to share common factors, and often work in concert to repair damage. Genomic studies are now revealing that many tumors harbor somatic mutations in FA/BRCA or NER genes, which may provide a growth advantage, but which could also be exploited therapeutically.

Regulation of FANCD2 and FANCI monoubiquitination by their interaction and by DNA

FANCD2 and FANCI function together in the Fanconi anemia network of deoxyribonucleic acid (DNA) crosslink repair. These proteins form the dimeric ID2 complex that binds DNA and becomes monoubiquitinated upon exposure of cells to DNA crosslinking agents. The monoubiquitinated ID2 complex is thought to facilitate DNA repair via recruitment of specific nucleases, translesion DNA polymerases and the homologous recombination machinery. Using the ubiquitin conjugating enzyme (E2) UBE2T and ubiquitin ligase (E3) FANCL, monoubiquitination of human FANCD2 and FANCI was examined. The ID2 complex is a poor substrate for monoubiquitination, consistent with the published crystal structure showing the solvent inaccessibility of the target lysines. Importantly, FANCD2 monoubiquitination within the ID2 complex is strongly stimulated by duplex or branched DNA, but unstructured single-stranded DNA or chromatinized DNA is ineffective. Interaction of FANCL with the ID2 complex is indispensable for its E3 ligase efficacy. Interestingly, mutations in FANCI that impair its DNA binding activity compromise DNA-stimulated FANCD2 monoubiquitination. Moreover, we demonstrate that in the absence of FANCD2, DNA also stimulates FANCI monoubiquitination, but in a FANCL-independent manner. These results implicate the role of a proper DNA ligand in FANCD2 and FANCI monoubiquitination, and reveal regulatory mechanisms that are dependent on protein-protein and protein-DNA interactions.

Structural basis of a nucleosome containing histone H2A.B/H2A.Bbd that transiently associates with reorganized chromatin

Human histone H2A.B (formerly H2A.Bbd), a non-allelic H2A variant, exchanges rapidly as compared to canonical H2A, and preferentially associates with actively transcribed genes. We found that H2A.B transiently accumulated at DNA replication and repair foci in living cells. To explore the biochemical function of H2A.B, we performed nucleosome reconstitution analyses using various lengths of DNA. Two types of H2A.B nucleosomes, octasome and hexasome, were formed with 116, 124, or 130 base pairs (bp) of DNA, and only the octasome was formed with 136 or 146 bp DNA. In contrast, only hexasome formation was observed by canonical H2A with 116 or 124 bp DNA. A small-angle X-ray scattering analysis revealed that the H2A.B octasome is more extended, due to the flexible detachment of the DNA regions at the entry/exit sites from the histone surface. These results suggested that H2A.B rapidly and transiently forms nucleosomes with short DNA segments during chromatin reorganization.

Advances in understanding the complex mechanisms of DNA interstrand cross-link repair

DNA interstrand cross-links (ICLs) are lesions caused by a variety of endogenous metabolites, environmental exposures, and cancer chemotherapeutic agents that have two reactive groups. The common feature of these diverse lesions is that two nucleotides on opposite strands are covalently joined. ICLs prevent the separation of two DNA strands and therefore essential cellular processes including DNA replication and transcription. ICLs are mainly detected in S phase when a replication fork stalls at an ICL. Damage signaling and repair of ICLs are promoted by the Fanconi anemia pathway and numerous posttranslational modifications of DNA repair and chromatin structural proteins. ICLs are also detected and repaired in nonreplicating cells, although the mechanism is less clear. A unique feature of ICL repair is that both strands of DNA must be incised to completely remove the lesion. This is accomplished in sequential steps to prevent creating multiple double-strand breaks. Unhooking of an ICL from one strand is followed by translesion synthesis to fill the gap and create an intact duplex DNA, harboring a remnant of the ICL. Removal of the lesion from the second strand is likely accomplished by nucleotide excision repair. Inadequate repair of ICLs is particularly detrimental to rapidly dividing cells, explaining the bone marrow failure characteristic of Fanconi anemia and why cross-linking agents are efficacious in cancer therapy. Herein, recent advances in our understanding of ICLs and the biological responses they trigger are discussed.

DNA damage triggers a chronic autoinflammatory response, leading to fat depletion in NER progeria

Lipodystrophies represent a group of heterogeneous disorders characterized by loss of fat tissue. However, the underlying mechanisms remain poorly understood. Using mice carrying an ERCC1-XPF DNA repair defect systematically or in adipocytes, we show that DNA damage signaling triggers a chronic autoinflammatory response leading to fat depletion. Ercc1-/- and aP2-Ercc1F/- fat depots show extensive gene expression similarities to lipodystrophic Pparγ(ldi/+) animals, focal areas of ruptured basement membrane, the reappearance of primary cilia, necrosis, fibrosis, and a marked decrease in adiposity. We find that persistent DNA damage in aP2-Ercc1F/- fat depots and in adipocytes ex vivo triggers the induction of proinflammatory factors by promoting transcriptionally active histone marks and the dissociation of nuclear receptor corepressor complexes from promoters; the response is cell autonomous and requires ataxia telangiectasia mutated (ATM). Thus, persistent DNA damage-driven autoinflammation plays a causative role in adipose tissue degeneration, with important ramifications for progressive lipodystrophies and natural aging.

A novel interplay between the Fanconi anemia core complex and ATR-ATRIP kinase during DNA cross-link repair

When DNA replication is stalled at sites of DNA damage, a cascade of responses is activated in the cell to halt cell cycle progression and promote DNA repair. A pathway initiated by the kinase Ataxia teleangiectasia and Rad3 related (ATR) and its partner ATR interacting protein (ATRIP) plays an important role in this response. The Fanconi anemia (FA) pathway is also activated following genomic stress, and defects in this pathway cause a cancer-prone hematologic disorder in humans. Little is known about how these two pathways are coordinated. We report here that following cellular exposure to DNA cross-linking damage, the FA core complex enhances binding and localization of ATRIP within damaged chromatin. In cells lacking the core complex, ATR-mediated phosphorylation of two functional response targets, ATRIP and FANCI, is defective. We also provide evidence that the canonical ATR activation pathway involving RAD17 and TOPBP1 is largely dispensable for the FA pathway activation. Indeed DT40 mutant cells lacking both RAD17 and FANCD2 were synergistically more sensitive to cisplatin compared with either single mutant. Collectively, these data reveal new aspects of the interplay between regulation of ATR-ATRIP kinase and activation of the FA pathway.

Non-invasive analysis of acquired resistance to cancer therapy by sequencing of plasma DNA

Cancers acquire resistance to systemic treatment as a result of clonal evolution and selection. Repeat biopsies to study genomic evolution as a result of therapy are difficult, invasive and may be confounded by intra-tumour heterogeneity. Recent studies have shown that genomic alterations in solid cancers can be characterized by massively parallel sequencing of circulating cell-free tumour DNA released from cancer cells into plasma, representing a non-invasive liquid biopsy. Here we report sequencing of cancer exomes in serial plasma samples to track genomic evolution of metastatic cancers in response to therapy. Six patients with advanced breast, ovarian and lung cancers were followed over 1-2 years. For each case, exome sequencing was performed on 2-5 plasma samples (19 in total) spanning multiple courses of treatment, at selected time points when the allele fraction of tumour mutations in plasma was high, allowing improved sensitivity. For two cases, synchronous biopsies were also analysed, confirming genome-wide representation of the tumour genome in plasma. Quantification of allele fractions in plasma identified increased representation of mutant alleles in association with emergence of therapy resistance. These included an activating mutation in PIK3CA (phosphatidylinositol-4,5-bisphosphate 3-kinase, catalytic subunit alpha) following treatment with paclitaxel; a truncating mutation in RB1 (retinoblastoma 1) following treatment with cisplatin; a truncating mutation in MED1 (mediator complex subunit 1) following treatment with tamoxifen and trastuzumab, and following subsequent treatment with lapatinib, a splicing mutation in GAS6 (growth arrest-specific 6) in the same patient; and a resistance-conferring mutation in EGFR (epidermal growth factor receptor; T790M) following treatment with gefitinib. These results establish proof of principle that exome-wide analysis of circulating tumour DNA could complement current invasive biopsy approaches to identify mutations associated with acquired drug resistance in advanced cancers. Serial analysis of cancer genomes in plasma constitutes a new paradigm for the study of clonal evolution in human cancers.

Genetic and physical interactions between the yeast ELG1 gene and orthologs of the Fanconi anemia pathway

Fanconi anemia (FA) is a human syndrome characterized by genomic instability and increased incidence of cancer. FA is a genetically heterogeneous disease caused by mutations in at least 15 different genes; several of these genes are conserved in the yeast Saccharomyces cerevisiae. Elg1 is also a conserved protein that forms an RFC-like complex, which interacts with SUMOylated PCNA. The mammalian Elg1 protein has been recently found to interact with the FA complex. Here we analyze the genetic interactions between elg1Δand mutants of the yeast FA-like pathway. We show that Elg1 physically contacts the Mhf1/Mhf2 histone-like complex and genetically interacts with MPH1 (ortholog of the FANCM helicase) and CHL1 (ortholog of the FANCJ helicase) genes. We analyze the sensitivity of double, triple, quadruple and quintuple mutants to methylmethane sulfonate (MMS) and to hydroxyurea (HU). Our results show that genetic interactions depend on the type of DNA damaging agent used and show a hierarchy: Chl1 and Elg1 play major roles in the survival to these genotoxins and exhibit synthetic fitness reduction. Mph1 plays a lesser role, and the effect of the Mhf1/2 complex is seen only in the absence of Elg1 on HU-containing medium. Finally, we dissect the relationship between yeast FA-like mutants and the replication clamp, PCNA. Our results point to an intricate network of interactions rather than a single, linear repair pathway.

Chaperones: needed for both the good times and the bad times

In this issue, we explore the assembly roles of protein chaperones, mainly through the portal of their associated human diseases (e.g. cardiomyopathy, cataract, neurodegeneration, cancer and neuropathy). There is a diversity to chaperone function that goes beyond the current emphasis in the scientific literature on their undoubted roles in protein folding and refolding. The focus on chaperone-mediated protein folding needs to be broadened by the original Laskey discovery that a chaperone assists the assembly of an oligomeric structure, the nucleosome, and the subsequent suggestion by Ellis that other chaperones may function in assembly processes, as well as in folding. There have been a number of recent discoveries that extend this relatively neglected aspect of chaperone biology to include proteostasis, maintenance of the cellular redox potential, genome stability, transcriptional regulation and cytoskeletal dynamics. So central are these processes that we propose that chaperones stand at the crossroads of life and death because they mediate essential functions, not only during the bad times, but also in the good times. We suggest that chaperones facilitate the success of a species, and hence the evolution of individuals within populations, because of their contributions to so many key cellular processes, of which protein folding is only one.

Fanconi anaemia and the repair of Watson and Crick DNA crosslinks

The function of Fanconi anaemia proteins is to maintain genomic stability. Their main role is in the repair of DNA interstrand crosslinks, which, by covalently binding the Watson and the Crick strands of DNA, impede replication and transcription. Inappropriate repair of interstrand crosslinks causes genomic instability, leading to cancer; conversely, the toxicity of crosslinking agents makes them a powerful chemotherapeutic. Fanconi anaemia proteins can promote stem-cell function, prevent tumorigenesis, stabilize replication forks and inhibit inaccurate repair. Recent advances have identified endogenous aldehydes as possible culprits of DNA damage that may induce the phenotypes seen in patients with Fanconi anaemia.