Dangerous liaisons: Fanconi anemia and toxic nonhomologous end joining in DNA crosslink repair

Loss of FANCD2 and related proteins may predict malignant transformation in oral epithelial dysplasia

OBJECTIVE

Predicting malignant transformation (MT) in oral epithelial dysplasia (OED) is challenging. The higher rate of MT reported in nonsmokers suggests an endogenous etiology in oncogenesis. We hypothesize that loss of FANCD2 and associated proteins could influence genomic instability and MT in the absence of environmental carcinogens.

STUDY DESIGN

Longitudinal archival samples were obtained from 40 individuals with OED: from diagnosis to the most recent review in 23 patients with stable OED or until excision of the squamous cell carcinoma in 17 patients with unstable OED undergoing MT. Histopathological reassessment, immunohistochemistry for FANCD2, and Western blotting for phosphorylation/monoubiquitylation status of ATR, CHK1, FANCD2, and FANCG were undertaken on each tissue sample.

RESULTS

Decreased expression of FANCD2 was observed in the diagnostic biopsies of OED lesions that later underwent MT. Combining the FANCD2 expression scores with histologic grading more accurately predicted MT (P = .005) than histology alone, and it correctly predicted MT in 10 of 17 initial biopsies. Significantly reduced expression of total FANCD2, pFANCD2, pATR, pCHK-1, and pFANCG was observed in unstable OED.

CONCLUSIONS

There is preliminary evidence that defects in the DNA damage sensing/signaling/repair cascade are associated with MT in OED. Loss of expression of FANCD2 protein in association with a higher histologic grade of dysplasia offered better prediction of MT than clinicopathologic parameters alone.

Inactivation of the NHEJ Activity of DNA-PKcs Prevents Fanconi Anemia Pre-Leukemic HSC Expansion

Fanconi anemia (FA) is a genetic disorder characterized by bone marrow failure and high risk of cancer particularly leukemia. Here we show that inactivation of the non-homologous end-joining (NHEJ) activity of DNA-PKcs prevented DNA damage-induced expansion of FA pre-leukemic hematopoietic stem cells (HSCs). Furthermore, we performed serial BM transplantation to demonstrate that the DNA damage-induced expanded FA HSC compartment contained pre-leukemic stem cells that required the NHEJ activity of DNA-PKcs to induce leukemia in the secondary recipients. These results suggest that NHEJ may collaborate with FA deficiency to promote DNA damage-induced expansion of pre-leukemic HSCs.

WIP1 Contributes to the Adaptation of Fanconi Anemia Cells to DNA Damage as Determined by the Regulatory Network of the Fanconi Anemia and Checkpoint Recovery Pathways

DNA damage adaptation (DDA) allows the division of cells with unrepaired DNA damage. DNA repair deficient cells might take advantage of DDA to survive. The Fanconi anemia (FA) pathway repairs DNA interstrand crosslinks (ICLs), and deficiencies in this pathway cause a fraction of breast and ovarian cancers as well as FA, a chromosome instability syndrome characterized by bone marrow failure and cancer predisposition. FA cells are hypersensitive to ICLs; however, DDA might promote their survival. We present the FA-CHKREC Boolean Network Model, which explores how FA cells might use DDA. The model integrates the FA pathway with the G2 checkpoint and the checkpoint recovery (CHKREC) processes. The G2 checkpoint mediates cell-cycle arrest (CCA) and the CHKREC activates cell-cycle progression (CCP) after resolution of DNA damage. Analysis of the FA-CHKREC network indicates that CHKREC drives DDA in FA cells, ignoring the presence of unrepaired DNA damage and allowing their division. Experimental inhibition of WIP1, a CHKREC component, in FA lymphoblast and cancer cell lines prevented division of FA cells, in agreement with the prediction of the model.

The non-homologous end-joining activity is required for Fanconi anemia fetal HSC maintenance

Background

Recent studies have shown that deficiency in the Fanconi anemia (FA) DNA repair pathway enhances the error-prone non-homologous end-joining (NHEJ) repair, leading to increased genomic instability, and that genetic or pharmacological inhibition of the NHEJ pathway could rescue the FA phenotype.

Methods

First, we exposed LSK cells from WT and Fanca^−/− mice to DNA-PKcs inhibitor NU7026 or Ku70 knockdown to examine whether inhibition of NHEJ sensitizes Fanca^−/− HSPCs to PARP inhibitor (PARPi)- or interstrand crosslinking (ICL)-induced cell death and genomic instability. We then generated DNA-PKcs^3A/3AFanca^−/− mice to investigate the effect of specific inactivation of NHEJ on fetal HSCs. Lastly, we used two p53 mutant models to test whether specific inactivation of the p53 function in apoptosis is sufficient to rescue embryonic lethality and fetal HSC depletion in Fanca^−/−DNA-PKcs^3A/3A mice.

Results

Inhibition of NHEJ sensitizes HSPCs from Fanca^−/− mice to PARP inhibition- and ICL-induced cell death and genomic instability and further decreases Fanca^−/− HSPC proliferation and hematopoietic repopulation in irradiated transplant recipients. Specific inactivation of NHEJ activity by the knockin DNA-PKcs^3A/3A mutation in two FA mouse models, Fanca^−/− and Fancc^−/−, leads to embryonic lethality. DNA-PKcs^3A/3A causes fetal HSC depletion in developing Fanca^−/− embryos due to increased HSC apoptosis and cycling. Both p53^−/− and a knockin p53^515C mutation, which selectively impairs the p53 function in apoptosis, can rescue embryonic lethality and fetal HSC depletion in Fanca^−/−DNA-PKcs^3A/3A mice.

Conclusion

These results demonstrate that the NHEJ pathway functions to maintain Fanconi anemia fetal HSCs.

Expression and prognostic significance of Fanconi anemia group D2 protein and breast cancer type 1 susceptibility protein in familial and sporadic breast cancer

Fanconi anemia group D2 protein (FANCD2) and breast cancer type 1 susceptibility protein (BRCA1), within the FA/BRCA pathway, are involved in the regulation of DNA damage repair, which is associated with breast cancer (BC) progression. The present study aimed to investigate BRCA1 and FANCD2 expression in breast cancer, and to highlight the association with patient clinical characteristics and prognoses. The BRCA1 and FANCD2 proteins were detected by immunohistochemistry in 335 tissue samples obtained from patients with BC, including 141 patients with familial BC (FBC), 147 patients with sporadic breast cancer (SBC) and 47 patients with benign breast tumors. Western blotting was used to detect the FANCD2 ubiquitination level in 56 frozen specimens that were randomly selected from the SBC group. Protein expression of BRCA1 in the FBC group was positively associated with tumor size, lymphatic invasion, Tumor-Node-Metastasis (TNM) stage, estrogen receptor (ER) status and FANCD2 expression. Protein expression of FANCD2 in the SBC group was positively associated with tumor size, TNM stage, ER status and Ki-67 index. Survival analyses revealed that BRCA1 expression was associated with the decreased disease-free survival (DFS) rate of patients with FBC (versus no BRCA1 expression) and that FANCD2 was associated with decreased DFS of patients with SBC (versus no FANCD expression). Univariable and multivariable analyses demonstrated that BRCA1 expression may be an independent prognostic factor in the FBC group. In the SBC group, FANCD2 high expression and low ubiquitination levels were considered as independent prognostic factors. In conclusion, the present study suggested that BRCA1 and FANCD2 expression, and FANCD2 ubiquitination levels, may be considered of novel potential prognostic value in patients with BC.

Mechanistic link between DNA damage sensing, repairing and signaling factors and immune signaling

Previously, DNA damage sensing, repairing and signaling machineries were thought to mainly suppress genomic instability in response to genotoxic stress. Emerging evidence indicates a crosstalk between DNA repair machinery and the immune system. In this chapter, we attempt to decipher the molecular choreography of how factors, including ATM, BRCA1, DNA-PK, FANCA/D2, MRE11, MUS81, NBS1, RAD51 and TREX1, of multiple DNA metabolic processes are directly or indirectly involved in suppressing cytosolic DNA sensing pathway-mediated immune signaling. We provide systematic details showing how different DDR factors' roles in modulating immune signaling are not direct, but are rather a consequence of their inherent ability to sense, repair and signal in response to DNA damage. Unexpectedly, most DDR factors negatively impact the immune system; that is, the immune system shows defective signaling if there are defects in DNA repair pathways. Thus, in addition to their known DNA repair and replication functions, DDR factors help prevent erroneous activation of immune signaling. A more precise understanding of the mechanisms by which different DDR factors function in immune signaling can be exploited to redirect the immune system for both preventing and treating autoimmunity, cellular senescence and cancer in humans.

DNA damage response and cancer therapeutics through the lens of the Fanconi Anemia DNA repair pathway

Fanconi Anemia (FA) is a rare, inherited genomic instability disorder, caused by mutations in genes involved in the repair of interstrand DNA crosslinks (ICLs). The FA signaling network contains a unique nuclear protein complex that mediates the monoubiquitylation of the FANCD2 and FANCI heterodimer, and coordinates activities of the downstream DNA repair pathway including nucleotide excision repair, translesion synthesis, and homologous recombination. FA proteins act at different steps of ICL repair in sensing, recognition and processing of DNA lesions. The multi-protein network is tightly regulated by complex mechanisms, such as ubiquitination, phosphorylation, and degradation signals that are critical for the maintenance of genome integrity and suppressing tumorigenesis. Here, we discuss recent advances in our understanding of how the FA proteins participate in ICL repair and regulation of the FA signaling network that assures the safeguard of the genome. We further discuss the potential application of designing small molecule inhibitors that inhibit the FA pathway and are synthetic lethal with DNA repair enzymes that can be used for cancer therapeutics.

Interstrand Crosslink Repair as a Target for HDAC Inhibition

DNA interstrand crosslinks (ICLs) covalently connect complementary DNA strands. Consequently, DNA replication and transcription are hampered, DNA damage responses (DDR) are initiated, and cell death is triggered. Therefore, drugs inducing ICLs are effective against rapidly growing cancer cells. However, tumors engage a complicated enzymatic machinery to repair and survive ICLs. Several factors, including the post-translational acetylation/deacetylation of lysine residues within proteins, control this network. Histone deacetylases (HDACs) modulate the expression and functions of DNA repair proteins which remove ICLs and control the accessibility of chromatin. Accordingly, histone deacetylase inhibitors (HDACi) are small, pharmacologically and clinically relevant molecules that sensitize cancer cells to ICL inducers. We discuss the mechanism of ICL repair and targets of HDACi within this pathway.

Chloroethylating nitrosoureas in cancer therapy: DNA damage, repair and cell death signaling

Chloroethylating nitrosoureas (CNU), such as lomustine, nimustine, semustine, carmustine and fotemustine are used for the treatment of malignant gliomas, brain metastases of different origin, melanomas and Hodgkin disease. They alkylate the DNA bases and give rise to the formation of monoadducts and subsequently interstrand crosslinks (ICL). ICL are critical cytotoxic DNA lesions that link the DNA strands covalently and block DNA replication and transcription. As a result, S phase progression is inhibited and cells are triggered to undergo apoptosis and necrosis, which both contribute to the effectiveness of CNU-based cancer therapy. However, tumor cells resist chemotherapy through the repair of CNU-induced DNA damage. The suicide enzyme O⁶-methylguanine-DNA methyltransferase (MGMT) removes the precursor DNA lesion O⁶-chloroethylguanine prior to its conversion into ICL. In cells lacking MGMT, the formed ICL evoke complex enzymatic networks to accomplish their removal. Here we discuss the mechanism of ICL repair as a survival strategy of healthy and cancer cells and DNA damage signaling as a mechanism contributing to CNU-induced cell death. We also discuss therapeutic implications and strategies based on sequential and simultaneous treatment with CNU and the methylating drug temozolomide.

FANCD2 influences replication fork processes and genome stability in response to clustered DSBs

Fanconi Anemia (FA) is a cancer predisposition syndrome and the factors defective in FA are involved in DNA replication, DNA damage repair and tumor suppression. Here, we show that FANCD2 is critical for genome stability maintenance in response to high-linear energy transfer (LET) radiation. We found that FANCD2 is monoubiquitinated and recruited to the sites of clustered DNA double-stranded breaks (DSBs) specifically in S/G2 cells after high-LET radiation. Further, FANCD2 facilitated the repair of clustered DSBs in S/G2 cells and proper progression of S-phase. Furthermore, lack of FANCD2 led to a reduced rate of replication fork progression and elevated levels of both replication fork stalling and new origin firing in response to high-LET radiation. Mechanistically, FANCD2 is required for correct recruitment of RPA2 and Rad51 to the sites of clustered DSBs and that is critical for proper processing of clustered DSBs. Significantly, FANCD2-decifient cells exhibited defective chromosome segregation, elevated levels of chromosomal aberrations, and anchorage-independent growth in response to high-LET radiation. These findings establish FANCD2 as a key factor in genome stability maintenance in response to high-LET radiation and as a promising target to improve cancer therapy.

Biochemical and cell biological assays to identify and characterize DNA helicase inhibitors

The growing number of DNA helicases implicated in hereditary disorders and cancer indicates that this particular class of enzymes plays key roles in genomic stability and cellular homeostasis. Indeed, a large body of work has provided molecular and cellular evidence that helicases act upon a variety of nucleic acid substrates and interact with numerous proteins to enact their functions in replication, DNA repair, recombination, and transcription. Understanding how helicases operate in unique and overlapping pathways is a great challenge to researchers. In this review, we describe a series of experimental approaches and methodologies to identify and characterize DNA helicase inhibitors which collectively provide an alternative and useful strategy to explore their biological significance in cell-based systems. These procedures were used in the discovery of biologically active compounds that inhibited the DNA unwinding function catalyzed by the human WRN helicase-nuclease defective in the premature aging disorder Werner syndrome. We describe in vitro and in vivo experimental approaches to characterize helicase inhibitors with WRN as the model, anticipating that these approaches may be extrapolated to other DNA helicases, particularly those implicated in DNA repair and/or the replication stress response.

Hyper-active non-homologous end joining selects for synthetic lethality resistant and pathological Fanconi anemia hematopoietic stem and progenitor cells

The prominent role of Fanconi anemia (FA) proteins involves homologous recombination (HR) repair. Poly[ADP-ribose] polymerase1 (PARP1) functions in multiple cellular processes including DNA repair and PARP inhibition is an emerging targeted therapy for cancer patients deficient in HR. Here we show that PARP1 activation in hematopoietic stem and progenitor cells (HSPCs) in response to genotoxic or oxidative stress attenuates HSPC exhaustion. Mechanistically, PARP1 controls the balance between HR and non-homologous end joining (NHEJ) in double strand break (DSB) repair by preventing excessive NHEJ. Disruption of the FA core complex skews PARP1 function in DSB repair and led to hyper-active NHEJ in Fanca^−/− or Fancc^−/− HSPCs. Re-expression of PARP1 rescues the hyper-active NHEJ phenotype in Brca1^−/−Parp1^−/− but less effective in Fanca^−/−Parp1^−/− cells. Inhibition of NHEJ prevents myeloid/erythroid pathologies associated with synthetic lethality. Our results suggest that hyper-active NHEJ may select for “synthetic lethality” resistant and pathological HSPCs.

Heterozygous PALB2 c.1592delT mutation channels DNA double-strand break repair into error-prone pathways in breast cancer patients

Hereditary heterozygous mutations in a variety of DNA double-strand break (DSB) repair genes have been associated with increased breast cancer risk. In the Finnish population, PALB2 (partner and localizer of BRCA2) represents a major susceptibility gene for female breast cancer, and so far, only one mutation has been described, c.1592delT, which leads to a sixfold increased disease risk. PALB2 is thought to participate in homologous recombination (HR). However, the effect of the Finnish founder mutation on DSB repair has not been investigated. In the current study, we used a panel of lymphoblastoid cell lines (LCLs) derived from seven heterozygous female PALB2 c.1592delT mutation carriers with variable health status and six wild-type matched controls. The results of our DSB repair analysis showed that the PALB2 mutation causes specific changes in pathway usage, namely increases in error-prone single-strand annealing (SSA) and microhomology-mediated end-joining (MMEJ) compared with wild-type LCLs. These data indicated haploinsufficiency regarding the suppression of error-prone DSB repair in PALB2 mutation carriers. To the contrary, neither reduced HR activities, nor impaired RAD51 filament assembly, nor sensitization to PARP inhibition were consistently observed. Expression of truncated mutant versus wild-type PALB2 verified a causal role of PALB2 c.1592delT in the shift to error-prone repair. Discrimination between healthy and malignancy-presenting PALB2 mutation carriers revealed a pathway shift particularly in the breast cancer patients, suggesting interaction of PALB2 c.1592delT with additional genomic lesions. Interestingly, the studied PALB2 mutation was associated with 53BP1 accumulation in the healthy mutation carriers but not the patients, and 53BP1 was limiting for error-prone MMEJ in patients but not in healthy carriers. Our study identified a rise in error-prone DSB repair as a potential threat to genomic integrity in heterozygous PALB2 mutation carriers. The used phenotypic marker system has the capacity to capture dysfunction caused by polygenic mechanisms and therefore offers new strategies of cancer risk prediction.

What is wrong with Fanconi anemia cells?

Figuring out what is wrong in Fanconi anemia (FA) patient cells is critical to understanding the contributions of the FA pathway to DNA repair and tumor suppression. Although FA patients exhibit a wide range of disease manifestation as well as severity (asymptomatic to congenital abnormalities, bone marrow failure, and cancer), cells from FA patients share underlying defects in their ability to process DNA lesions that interfere with DNA replication. In particular, FA cells are very sensitive to agents that induce DNA interstrand crosslinks (ICLs). The cause of this pronounced ICL sensitivity is not fully understood, but has been linked to the aberrant activation of DNA damage repair proteins, checkpoints and pathways. Thus, regulation of these responses through coordination of repair processing at stalled replication forks is an essential function of the FA pathway. Here, we briefly summarize some of the aberrant DNA damage responses contributing to defects in FA cells, and detail the newly-identified relationship between FA and the mismatch repair protein, MSH2. Understanding the contribution of MSH2 and/or other proteins to the replication problem in FA cells will be key to assessing therapeutic options to improve the health of FA patients. Moreover, loss of these factors, if linked to improved replication, could be a key event in the progression of FA cells to cancer cells. Likewise, loss of these factors could synergize to enhance tumorigenesis or confer chemoresistance in tumors defective in FA-BRCA pathway proteins and provide a basis for biomarkers for disease progression and response.

DNA interstrand cross-link repair requires replication-fork convergence

DNA interstrand cross-links (ICLs) prevent strand separation during DNA replication and transcription and therefore are extremely cytotoxic. In metazoans, a major pathway of ICL repair is coupled to DNA replication, and it requires the Fanconi anemia pathway. In most current models, collision of a single DNA replication fork with an ICL is sufficient to initiate repair. In contrast, we show here that in Xenopus egg extracts two DNA replication forks must converge on an ICL to trigger repair. When only one fork reaches the ICL, the replicative CMG helicase fails to unload from the stalled fork, and repair is blocked. Arrival of a second fork, even when substantially delayed, rescues repair. We conclude that ICL repair requires a replication-induced X-shaped DNA structure surrounding the lesion, and we speculate on how this requirement helps maintain genomic stability in S phase.

Crosstalk between BRCA-Fanconi anemia and mismatch repair pathways prevents MSH2-dependent aberrant DNA damage responses

Several proteins in the BRCA-Fanconi anemia (FA) pathway, such as FANCJ, BRCA1, and FANCD2, interact with mismatch repair (MMR) pathway factors, but the significance of this link remains unknown. Unlike the BRCA-FA pathway, the MMR pathway is not essential for cells to survive toxic DNA interstrand crosslinks (ICLs), although MMR proteins bind ICLs and other DNA structures that form at stalled replication forks. We hypothesized that MMR proteins corrupt ICL repair in cells that lack crosstalk between BRCA-FA and MMR pathways. Here, we show that ICL sensitivity of cells lacking the interaction between FANCJ and the MMR protein MLH1 is suppressed by depletion of the upstream mismatch recognition factor MSH2. MSH2 depletion suppresses an aberrant DNA damage response, restores cell cycle progression, and promotes ICL resistance through a Rad18-dependent mechanism. MSH2 depletion also suppresses ICL sensitivity in cells deficient for BRCA1 or FANCD2, but not FANCA. Rescue by Msh2 loss was confirmed in Fancd2-null primary mouse cells. Thus, we propose that regulation of MSH2-dependent DNA damage response underlies the importance of interactions between BRCA-FA and MMR pathways.

Targeting an Achilles' heel of cancer with a WRN helicase inhibitor

Our recently published work suggests that DNA helicases such as the Werner syndrome helicase (WRN) represent a novel class of proteins to target for anticancer therapy. Specifically, pharmacological inhibition of WRN helicase activity in human cells defective in the Fanconi anemia (FA) pathway of interstrand cross-link (ICL) repair are sensitized to the DNA cross-linking agent and chemotherapy drug mitomycin C (MMC) by the WRN helicase inhibitor NSC 617145. (1) The mechanistic basis for the synergistic interaction between NSC 617145 and MMC is discussed in this paper and extrapolated to potential implications for genetic or chemically induced synthetic lethality provoked by cellular exposure to the WRN helicase inhibitor under the context of relevant DNA repair deficiencies associated with cancers or induced by small-molecule inhibitors. Experimental data are presented showing that small-molecule inhibition of WRN helicase elevates sensitivity to MMC-induced stress in human cells that are deficient in both FANCD2 and DNA protein kinase catalytic subunit (DNA-PKcs). These findings suggest a model in which drug-mediated inhibition of WRN helicase activity exacerbates the deleterious effects of MMC-induced DNA damage when both the FA and NHEJ pathways are defective. We conclude with a perspective for the FA pathway and synthetic lethality and implications for DNA repair helicase inhibitors that can be developed for anticancer strategies.

Werner syndrome helicase has a critical role in DNA damage responses in the absence of a functional fanconi anemia pathway

Werner syndrome is genetically linked to mutations in WRN that encodes a DNA helicase-nuclease believed to operate at stalled replication forks. Using a newly identified small-molecule inhibitor of WRN helicase (NSC 617145), we investigated the role of WRN in the interstrand cross-link (ICL) response in cells derived from patients with Fanconi anemia, a hereditary disorder characterized by bone marrow failure and cancer. In FA-D2(-/-) cells, NSC 617145 acted synergistically with very low concentrations of mitomycin C to inhibit proliferation in a WRN-dependent manner and induce double-strand breaks (DSB) and chromosomal abnormalities. Under these conditions, ataxia-telangiectasia mutated activation and accumulation of DNA-dependent protein kinase, catalytic subunit pS2056 foci suggested an increased number of DSBs processed by nonhomologous end-joining (NHEJ). Rad51 foci were also elevated in FA-D2(-/-) cells exposed to NSC 617145 and mitomycin C, suggesting that WRN helicase inhibition interferes with later steps of homologous recombination at ICL-induced DSBs. Thus, when the Fanconi anemia pathway is defective, WRN helicase inhibition perturbs the normal ICL response, leading to NHEJ activation. Potential implication for treatment of Fanconi anemia-deficient tumors by their sensitization to DNA cross-linking agents is discussed.

Regulation of DNA cross-link repair by the Fanconi anemia/BRCA pathway

The maintenance of genome stability is critical for survival, and its failure is often associated with tumorigenesis. The Fanconi anemia (FA) pathway is essential for the repair of DNA interstrand cross-links (ICLs), and a germline defect in the pathway results in FA, a cancer predisposition syndrome driven by genome instability. Central to this pathway is the monoubiquitination of FANCD2, which coordinates multiple DNA repair activities required for the resolution of ICLs. Recent studies have demonstrated how the FA pathway coordinates three critical DNA repair processes, including nucleolytic incision, translesion DNA synthesis (TLS), and homologous recombination (HR). Here, we review recent advances in our understanding of the downstream ICL repair steps initiated by ubiquitin-mediated FA pathway activation.

Chronic myelogenous leukemia stem and progenitor cells demonstrate chromosomal instability related to repeated breakage-fusion-bridge cycles mediated by increased nonhomologous end joining

Chromosomal aberrations are an important consequence of genotoxic exposure and contribute to pathogenesis and progression of several malignancies. We investigated the susceptibility to chromosomal aberrations in chronic myelogenous leukemia (CML) progenitors after exposure to ionizing radiation. In normal progenitors, ionizing radiation induced both stable and unstable chromosomal lesions, but only stable aberrations persisted after multiple divisions. In contrast, radiation of chronic phase CML progenitors resulted in enhanced generation of unstable lesions that persisted after multiple divisions. CML progenitors demonstrated active cell cycle checkpoints and increased nonhomologous end joining DNA repair, suggesting that persistence of unstable aberrations was the result of continued generation of these lesions. CML progenitors demonstrated enhanced susceptibility to repeated cycles of chromosome damage, repair, and damage through a breakage-fusion-bridge mechanism. Perpetuation of breakage-fusion-bridge cycles in CML progenitors was mediated by classic nonhomologous end joining repair. These studies reveal a previously unrecognized mechanism of chromosomal instability in leukemia progenitors because of continued generation of unstable chromosomal lesions through repeated cycles of breakage and repair of such lesions.

Regulation of Rev1 by the Fanconi anemia core complex

The fifteen known Fanconi Anemia (FA) proteins cooperate in a pathway which regulates DNA interstrand crosslink repair. Recent studies indicate that the FA pathway also controls Rev1-mediated translesion DNA synthesis (TLS). Here we identify a novel protein FAAP20, which is an integral subunit of the multisubunit FA core complex. FAAP20 binds to FANCA subunit and is required for complex stability and monoubiquitination of FANCD2. FAAP20 contains a UBZ4 (Ubiquitin Binding Zinc finger 4) domain and binds to the monoubiquitinated form of Rev1. FAAP20 binding stabilizes Rev1 nuclear foci and promotes the interaction of the FA core with PCNA/Rev1 DNA damage bypass complexes. FAAP20 therefore provides a critical link between the FA pathway and TLS polymerase activity. We propose that the FA core complex regulates crosslink repair, by channeling lesions to damage bypass pathways and preventing large DNA insertions and deletions.

Fanconi anemia: a disorder defective in the DNA damage response

Fanconi anemia (FA) is a cancer predisposition disorder characterized by progressive bone marrow failure, congenital developmental defects, chromosomal abnormalities, and cellular hypersensitivity to DNA interstrand crosslink (ICL) agents. So far mutations in 14 FANC genes were identified in FA or FA-like patients. These gene products constitute a common ubiquitin-phosphorylation network called the "FA pathway" and cooperate with other proteins involved in DNA repair and cell cycle control to repair ICL lesions and to maintain genome stability. In this review, we summarize recent exciting discoveries that have expanded our view of the molecular mechanisms operating in DNA repair and DNA damage signaling.