The Fanconi anaemia pathway orchestrates incisions at sites of crosslinked DNA

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.

IL-6, IL-8, MMP-2, MMP-9 are overexpressed in Fanconi anemia cells through a NF-κB/TNF-α dependent mechanism

Fanconi anemia (FA) is a rare autosomal recessive genetic disorder associated with a bone-marrow failure, genome instability, hypersensitivity to DNA crosslinking agents and a predisposition to cancer. Mutations have been documented in 16 FA genes that participate in the FA-BRCA DNA repair pathway, a fundamental pathway in the development of the disease and the presentation of its symptoms. FA cells have been characterized by an overproduction of cytokines, MAPKs, and Interleukins. Through this study we have identified the overexpression of additional secretory factors such as IL-6, IL-8, MMP-2, and MMP-9 in FA cells and in cells depleted of FANCA or FANCC and proved that their expression is under the control of NF-κB/TNF-α signaling pathways. We also demonstrated that these overexpressed secretory factors were effective in promoting the proliferation, migration, and invasion of surrounding tumor cells a fundamental event in the process of epithelial mesenchymal transition (EMT) and that they also modulated the expression of EMT markers such as E-cadherin and SNAIL. Overall our data suggest that the upregulation of EMT promoting factors in FA may contribute to predisposing FA patients to cancer, thereby providing new insights into possible therapeutic interventions.

Genetic variants in fanconi anemia pathway genes BRCA2 and FANCA predict melanoma survival

Cutaneous melanoma (CM) is the most lethal skin cancer. The Fanconi Anemia (FA) pathway involved in DNA crosslinks repair may affect CM susceptibility and prognosis. Using data derived from published genome-wide association study, we comprehensively analyzed the associations of 2339 common single nucleotide polymorphisms (SNPs) in 14 autosomal FA genes with overall survival (OS) in 858 CM patients. By performing false-positive report probability corrections and stepwise Cox proportional hazards regression analyses, we identified significant associations between CM OS and four putatively functional SNPs: BRCA2 rs10492396 [AG vs. GG: adjusted hazard ratio (adjHR)=1.85, 95% confident interval (CI)=1.16-2.95, P=0.010], rs206118 (CC vs. TT+TC: adjHR=2.44, 95% CI=1.27-4.67, P=0.007), rs3752447 (CC vs. TT+TC: adjHR=2.10, 95% CI=1.38-3.18, P=0.0005), and FANCA rs62068372 (TT vs. CC+CT: adjHR=1.85, 95% CI=1.27-2.69, P=0.001). Moreover, patients with an increasing number of unfavorable genotypes (NUG) of these loci had markedly reduced OS and melanoma-specific survival (MSS). The final model incorporating with NUG, tumor stage and Breslow thickness showed an improved discriminatory ability to classify both 5-year OS and 5-year MSS. Additional investigations, preferably prospective studies, are needed to validate our findings.

Inherited bone marrow failure syndromes in adolescents and young adults

The inherited bone marrow failure syndromes are a diverse group of genetic diseases associated with inadequate production of one or more blood cell lineages. Examples include Fanconi anemia, dyskeratosis congenita, Diamond-Blackfan anemia, thrombocytopenia absent radii syndrome, severe congenital neutropenia, and Shwachman-Diamond syndrome. The management of these disorders was once the exclusive domain of pediatric subspecialists, but increasingly physicians who care for adults are being called upon to diagnose or treat these conditions. Through a series of patient vignettes, we highlight the clinical manifestations of inherited bone marrow failure syndromes in adolescents and young adults. The diagnostic and therapeutic challenges posed by these diseases are discussed.

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.

The Fanconi anemia/BRCA pathway is involved in DNA interstrand cross-link repair of adriamycin-resistant leukemia cells

The Fanconi anemia/BRCA (FA/BRCA) pathway plays a vital role in DNA damage repair induced by DNA cross-linking agents and is closely related to drug response in cancer treatment. Here we demonstrate that the FA/BRCA pathway contributes to acquired drug resistance in adriamycin (ADR)-resistant leukemia cell lines, and disruption of this pathway partially reverses the drug resistance. We observed that ADR-resistant cells have reduced DNA interstrand cross-links (ICL) compared with ADR-sensitive cells. Western blot studies demonstrated enhanced FA protein expression in ADR-resistant cells. Using siRNA to knock down FANCF in K562/R drug-resistant cells showed increases in sensitivity to ADR and ADR-induced DNA damage, and demonstrated a direct relationship between the FA/BRCA pathway and drug sensitivity. Overexpression of FANCF in K562 drug-sensitive cells partially reproduced the drug-resistant phenotype. These results show that the FA/BRCA pathway is involved in acquired ADR resistance of leukemia cells. The FA/BRCA pathway may be a new target to reverse ADR resistance in leukemia treatment.

The genetic and biochemical basis of FANCD2 monoubiquitination

Fanconi anaemia (FA) is a cancer predisposition syndrome characterized by cellular sensitivity to DNA interstrand crosslinkers. The molecular defect in FA is an impaired DNA repair pathway. The critical event in activating this pathway is monoubiquitination of FANCD2. In vivo, a multisubunit FA core complex catalyzes this step, but its mechanism is unclear. Here, we report purification of a native avian FA core complex and biochemical reconstitution of FANCD2 monoubiquitination. This demonstrates that the catalytic FANCL E3 ligase subunit must be embedded within the complex for maximal activity and site specificity. We genetically and biochemically define a minimal subcomplex comprising just three proteins (FANCB, FANCL, and FAAP100) that functions as the monoubiquitination module. Residual FANCD2 monoubiquitination activity is retained in cells defective for other FA core complex subunits. This work describes the in vitro reconstitution and characterization of this multisubunit monoubiquitin E3 ligase, providing key insight into the conserved FA DNA repair pathway.

High incidence of female reproductive tract cancers in FA-deficient HPV16-transgenic mice correlates with E7's induction of DNA damage response, an activity mediated by E7's inactivation of pocket proteins

Fanconi anemia (FA) is a rare genetic disorder caused by defects in a DNA damage repair system, the FA pathway. FA patients frequently develop squamous cell carcinoma (SCC) at sites that are associated with human papillomavirus (HPV)-driven cancer including the female reproductive tract. To assess experimentally whether FA deficiency increases susceptibility to HPV-associated cervical/vaginal cancer, we monitored cancer incidence in the female lower reproductive tract of FA-deficient mice expressing HPV16 oncogenes, E6 and/or E7. FA deficiency specifically increased the incidence of cancers in mice expressing E7; but this effect was not observed in mice just expressing E6. We also observed that E7, but not E6, induced DNA damage as scored by induction of γ-H2AX and 53BP1 (p53 binding protein 1) nuclear foci, and this induction was heightened in FA-deficient tissue. Finally, we discovered that this induction of DNA damage responses was recapitulated in mice deficient in expression of 'pocket' proteins, pRb, p107 and p130, which are established targets of E7. Our findings support the hypothesis that E7 induces cancer by causing DNA damage at least in part through the inactivation of pocket proteins. This hypothesis explains why a deficiency in DNA damage repair would increase susceptibility to E7-driven cancer.

Multiple interactions of the intrinsically disordered region between the helicase and nuclease domains of the archaeal Hef protein

Hef is an archaeal protein that probably functions mainly in stalled replication fork repair. The presence of an unstructured region was predicted between the two distinct domains of the Hef protein. We analyzed the interdomain region of Thermococcus kodakarensis Hef and demonstrated its disordered structure by CD, NMR, and high speed atomic force microscopy (AFM). To investigate the functions of this intrinsically disordered region (IDR), we screened for proteins interacting with the IDR of Hef by a yeast two-hybrid method, and 10 candidate proteins were obtained. We found that PCNA1 and a RecJ-like protein specifically bind to the IDR in vitro. These results suggested that the Hef protein interacts with several different proteins that work together in the pathways downstream from stalled replication fork repair by converting the IDR structure depending on the partner protein.

The Fanconi anemia group C protein interacts with uncoordinated 5A and delays apoptosis

The Fanconi anemia group C protein (FANCC) is one of the several proteins that comprise the Fanconi anemia (FA) network involved in genomic surveillance. FANCC is mainly cytoplasmic and has many functions, including apoptosis suppression through caspase-mediated proteolytic processing. Here, we examined the role of FANCC proteolytic fragments by identifying their binding partners. We performed a yeast two-hybrid screen with caspase-mediated FANCC cleavage products and identified the dependence receptor uncoordinated-5A (UNC5A) protein. Here, we show that FANCC physically interacts with UNC5A, a pro-apoptotic dependence receptor. FANCC interaction occurs through the UNC5A intracellular domain, specifically via its death domain. FANCC modulates cell sensitivity to UNC5A-mediated apoptosis; we observed reduced UNC5A-mediated apoptosis in the presence of FANCC and increased apoptosis in FANCC-depleted cells. Our results show that FANCC interferes with UNC5A's functions in apoptosis and suggest that FANCC may participate in developmental processes through association with the dependence receptor UNC5A.

The WD40-repeat protein-containing deubiquitinase complex: catalysis, regulation, and potential for therapeutic intervention

Ubiquitination has emerged as an essential signaling mechanism in eukaryotes. Deubiquitinases (DUBs) counteract the activities of the ubiquitination machinery and provide another level of control in cellular ubiquitination. Not surprisingly, DUBs are subjected to stringent regulations. Besides regulation by the noncatalytic domains present in the DUB sequences, DUB-interacting proteins are increasingly realized as essential regulators for DUB activity and function. This review focuses on DUBs that are associated with WD40-repeat proteins. Many human ubiquitin-specific proteases (USPs) were found to interact with WD40-repeat proteins, but little is known as to how this interaction regulates the activity and function of USPs. In recent years, significant progress has been made in understanding a prototypical WD40-repeat protein-containing DUB complex that comprises USP1 and USP1-associated factor 1 (UAF1). It has been shown that UAF1 activates USP1 through a potential active-site modulation, and the complex formation between USP1 and UAF1 is regulated by serine phosphorylation. Recently, human USPs have been recognized as a promising target class for inhibitor discovery. Small molecule inhibitors targeting several human USPs have been reported. USP1 is involved in two major DNA damage response pathways, DNA translesion synthesis and the Fanconi anemia pathway. Inhibiting the USP1/UAF1 deubiquitinase complex represents a new strategy to potentiate cancer cells to DNA-crosslinking agents and to overcome resistance that has plagued clinical cancer chemotherapy. The progress in inhibitor discovery against USPs and the WD40-repeat protein-containing USP complex will be discussed.

A concomitant loss of dormant origins and FANCC exacerbates genome instability by impairing DNA replication fork progression

Accumulating evidence suggests that dormant DNA replication origins play an important role in the recovery of stalled forks. However, their functional interactions with other fork recovery mechanisms have not been tested. We previously reported intrinsic activation of the Fanconi anemia (FA) pathway in a tumor-prone mouse model (Mcm4^chaos3) with a 60% loss of dormant origins. To understand this further, we introduced a null allele of Fancc (Fancc⁻), encoding a member of the FA core complex, into the Mcm4^chaos3 background. Primary embryonic fibroblasts double homozygous for Mcm4^chaos3 and Fancc⁻ (Mcm4^{chaos3/chaos3};Fancc^−/−) showed significantly increased levels of markers of stalled/collapsed forks compared to either single homozygote. Interestingly, a loss of dormant origins also increased the number of sites in which replication was delayed until prophase, regardless of FA pathway activation. These replication defects coincided with substantially elevated levels of genome instability in Mcm4^{chaos3/chaos3};Fancc^−/− cells, resulting in a high rate of perinatal lethality of Mcm4^{chaos3/chaos3};Fancc^−/− mice and the accelerated tumorigenesis of surviving mice. Together, these findings uncover a specialized role of dormant origins in replication completion while also identifying important functional overlaps between dormant origins and the FA pathway in maintaining fork progression, genome stability, normal development and tumor suppression.

A selective USP1-UAF1 inhibitor links deubiquitination to DNA damage responses

Protein ubiquitination and deubiquitination are central to the control of a large number of cellular pathways and signaling networks in eukaryotes. Although the essential roles of ubiquitination have been established in the eukaryotic DNA damage response, the deubiquitination process remains poorly defined. Chemical probes that perturb the activity of deubiquitinases (DUBs) are needed to characterize the cellular function of deubiquitination. Here we report ML323 (2), a highly potent inhibitor of the USP1-UAF1 deubiquitinase complex with excellent selectivity against human DUBs, deSUMOylase, deneddylase and unrelated proteases. Using ML323, we interrogated deubiquitination in the cellular response to UV- and cisplatin-induced DNA damage and revealed new insights into the requirement of deubiquitination in the DNA translesion synthesis and Fanconi anemia pathways. Moreover, ML323 potentiates cisplatin cytotoxicity in non-small cell lung cancer and osteosarcoma cells. Our findings point to USP1-UAF1 as a key regulator of the DNA damage response and a target for overcoming resistance to the platinum-based anticancer drugs.

Quantification and repair of psoralen-induced interstrand crosslinks in human cells

Bi-functional alkylating agents that cause crosslinks are commonly used in chemotherapy. However, there is no conclusive knowledge for human cells regarding the number of induced interstrand crosslinks (ICLs) and the unhooking rate when the lesion is removed from one of the DNA strand. Using a newly developed method, we quantified the number of induced ICLs for the five furocoumarins; psoralen, 5-methoxypsoralen, 8-methoxypsoralen, tri-methoxypsoralen and angelicin. In quantitative terms, the results were in agreement with the values found by others. In kinetic studies using mammalian cells, we found that half of the psoralen-induced ICLs were unhooked within 2.5h. The rate in normal human diploid fibroblasts was found to be 20,000 ICLs/h/cell. In comparison to survival, 2500 ICLs per cell led to 50% toxicity, indicating that the unhooking of the ICLs is not the crucial step for ICL tolerance. Surprisingly, only 3500 ICLs per cell corresponded to a significant delay in the replication fork elongation. The results indicate involvements of additional pathway(s) for the delay since the effect on replication elongation could be monitored when only 10% of the replication forks encounter an ICL.

Tumor suppressor RecQL5 controls recombination induced by DNA crosslinking agents

RecQ family DNA helicases function in the maintenance of genome stability. Mice deficient in RecQL5, one of five RecQ helicases, show a cancer predisposition phenotype, suggesting that RecQL5 plays a tumor suppressor role. RecQL5 interacts with Rad51, a key factor in homologous recombination (HR), and displaces Rad51 from Rad51-single stranded DNA (ssDNA) filaments in vitro. However, the precise roles of RecQL5 in the cell remain elusive. Here, we present evidence suggesting that RecQL5 is involved in DNA interstrand crosslink (ICL) repair. Chicken DT40 RECQL5 gene knockout (KO) cells showed sensitivity to ICL-inducing agents such as cisplatin (CDDP) and mitomycin C (MMC) and a higher number of chromosome aberrations in the presence of MMC than wild-type cells. The phenotypes of RECQL5 KO cells resembled those of Fanconi anemia gene KO cells. Genetic analysis using corresponding gene knockout cells showed that RecQL5 is involved in the FANCD1 (BRCA2)-dependent ICL repair pathway in which Rad51-ssDNA filament formation is promoted by BRCA2. The disappearance but not appearance of Rad51-foci was delayed in RECQL5 KO cells after MMC treatment. Deletion of Rad54, which processes the Rad51-ssDNA filament in HR, in RECQL5 KO cells increased sensitivity to CDDP and further delayed the disappearance of Rad51-foci, suggesting that RecQL5 and Rad54 have different effects on the Rad51-ssDNA filament. Furthermore, the frequency and variation of CDDP-induced gene conversion at the immunoglobulin locus were increased in RECQL5 KO cells. These results suggest that RecQL5 plays a role in regulating the incidence and quality of ICL-induced recombination.

Fanconi anemia patients are more susceptible to infection with tumor virus SV40

Fanconi anemia (FA) is a recessive DNA repair disease characterized by a high predisposition to developing neoplasms. DNA tumor polyomavirus simian virus 40 (SV40) transforms FA fibroblasts at high efficiency suggesting that FA patients could be highly susceptible to SV40 infection. To test this hypothesis, the large tumor (LT) antigen of SV40, BKV, JCV and Merkel Cell (MC) polyomaviruses were tested in blood samples from 89 FA patients and from 82 of their parents. Two control groups consisting of 47 no-FA patients affected by other genetic bone marrow failure diseases and 91 healthy subjects were also evaluated. Although JCV, BKV and MC were not found in any of the FA samples, the prevalence and viral load of SV40 were higher in FA patients (25%; mean viral load: 1.1×10² copies/10⁵cells) as compared with healthy individuals (4.3%; mean viral load: 0.8×10¹ copies/10⁵cells) and genetic controls (0%) (p<0.005). A marked age-dependent frequency of SV40 was found in FA with respect to healthy subjects suggesting that, although acquired early in life, the virus can widespread more easily in specific groups of population. From the analysis of family pedigrees, 60% of the parents of SV40-positive probands were positive for the virus compared to 2% of the parents of the SV40-negative probands (p<0.005). It is worthy of note that the relative frequency of SV40-positive relatives detected in this study was the highest ever reported, showing that asymptomatic FA carriers are also more susceptible to SV40. In conclusion, we favor the hypothesis that SV40 spread could be facilitated by individuals who are genetically more susceptible to infection, such as FA patients. The increased susceptibility to SV40 infection seems to be associated with a specific defect of the immune system which supports a potential interplay of SV40 with an underlying genetic alteration that increases the risk of malignancies.

BRCA1: a missing link in the Fanconi anemia/BRCA pathway

Domchek and colleagues provide a case report of a 28-year-old woman with congenital abnormalities, inherited ovarian cancer, and carboplatin hypersensitivity. Interestingly, the woman had validated germline mutations in both BRCA1 alleles. These findings further implicate BRCA1 in the Fanconi anemia/BRCA pathway and have important implications for BRCA1 genetic testing.

High incidence of HPV-associated head and neck cancers in FA deficient mice is associated with E7's induction of DNA damage through its inactivation of pocket proteins

Fanconi anemia (FA) patients are highly susceptible to solid tumors at multiple anatomical sites including head and neck region. A subset of head and neck cancers (HNCs) is associated with 'high-risk' HPVs, particularly HPV16. However, the correlation between HPV oncogenes and cancers in FA patients is still unclear. We previously learned that FA deficiency in mice predisposes HPV16 E7 transgenic mice to HNCs. To address HPV16 E6's oncogenic potential under FA deficiency in HNCs, we utilized HPV16 E6-transgenic mice (K14E6) and HPV16 E6/E7-bi-transgenic mice (K14E6E7) on genetic backgrounds sufficient or deficient for one of the fanc genes, fancD2 and monitored their susceptibility to HNCs. K14E6 mice failed to develop tumor. However, E6 and fancD2-deficiency accelerated E7-driven tumor development in K14E6E7 mice. The increased tumor incidence was more correlated with E7-driven DNA damage than proliferation. We also found that deficiency of pocket proteins, pRb, p107, and p130 that are well-established targets of E7, could recapitulate E7's induction of DNA damage. Our findings support the hypothesis that E7 induces HPV-associated HNCs by promoting DNA damage through the inactivation of pocket proteins, which explains why a deficiency in DNA damage repair would increase susceptibility to E7-driven cancer. Our results further demonstrate the unexpected finding that FA deficiency does not predispose E6 transgenic mice to HNCs, indicating a specificity in the synergy between FA deficiency and HPV oncogenes in causing HNCs.

Helq acts in parallel to Fancc to suppress replication-associated genome instability

HELQ is a superfamily 2 DNA helicase found in archaea and metazoans. It has been implicated in processing stalled replication forks and in repairing DNA double-strand breaks and inter-strand crosslinks. Though previous studies have suggested the possibility that HELQ is involved in the Fanconi anemia (FA) pathway, a dominant mechanism for inter-strand crosslink repair in vertebrates, this connection remains elusive. Here, we investigated this question in mice using the Helq^gt and Fancc⁻ strains. Compared with Fancc^−/− mice lacking FANCC, a component of the FA core complex, Helq^gt/gt mice exhibited a mild of form of FA-like phenotypes including hypogonadism and cellular sensitivity to the crosslinker mitomycin C. However, unlike Fancc^−/− primary fibroblasts, Helq^gt/gt cells had intact FANCD2 mono-ubiquitination and focus formation. Notably, for all traits examined, Helq was non-epistatic with Fancc, as Helq^gt/gt;Fancc^−/− double mutants displayed significantly worsened phenotypes than either single mutant. Importantly, this was most noticeable for the suppression of spontaneous chromosome instability such as micronuclei and 53BP1 nuclear bodies, known consequences of persistently stalled replication forks. These findings suggest that mammalian HELQ contributes to genome stability in unchallenged conditions through a mechanism distinct from the function of FANCC.

Structure analysis of FAAP24 reveals single-stranded DNA-binding activity and domain functions in DNA damage response

The FANCM/FAAP24 heterodimer has distinct functions in protecting cells from complex DNA lesions such as interstrand crosslinks. These functions rely on the biochemical activity of FANCM/FAAP24 to recognize and bind to damaged DNA or stalled replication forks. However, the DNA-binding activity of this complex was not clearly defined. We investigated how FAAP24 contributes to the DNA-interacting functions of the FANCM/FAAP24 complex by acquiring the N-terminal and C-terminal solution structures of human FAAP24. Modeling of the FAAP24 structure indicates that FAAP24 may possess a high affinity toward single-stranded DNA (ssDNA). Testing of various FAAP24 mutations in vitro and in vivo validated this prediction derived from structural analyses. We found that the DNA-binding and FANCM-interacting functions of FAAP24, although both require the C-terminal (HhH)2 domain, can be distinguished by segregation-of-function mutations. These results demonstrate dual roles of FAAP24 in DNA damage response against crosslinking lesions, one through the formation of FANCM/FAAP24 heterodimer and the other via its ssDNA-binding activity required in optimized checkpoint activation.

Fanconi anemia signaling network regulates the spindle assembly checkpoint

Fanconi anemia (FA) is a heterogenous genetic disease with a high risk of cancer. The FA proteins are essential for interphase DNA damage repair; however, it is incompletely understood why FA-deficient cells also develop gross aneuploidy, leading to cancer. Here, we systematically evaluated the role of the FA proteins in chromosome segregation through functional RNAi screens and analysis of primary cells from patients with FA. We found that FA signaling is essential for the spindle assembly checkpoint and is therefore required for high-fidelity chromosome segregation and prevention of aneuploidy. Furthermore, we discovered that FA proteins differentially localize to key structures of the mitotic apparatus in a cell cycle-dependent manner. The essential role of the FA pathway in mitosis offers a mechanistic explanation for the aneuploidy and malignant transformation known to occur after disruption of FA signaling. Collectively, our findings provide insight into the genetically unstable cancers resulting from inactivation of the FA/BRCA pathway.

From clinical description, to in vitro and animal studies, and backward to patients: oxidative stress and mitochondrial dysfunction in Fanconi anemia

Fanconi anemia (FA) is a rare genetic disease associated with deficiencies in DNA repair pathways. A body of literature points to a pro-oxidant state in FA patients, along with evidence for oxidative stress (OS) in the FA phenotype reported by in vitro, molecular, and animal studies. A highlight arises from the detection of mitochondrial dysfunction (MDF) in FA cell lines of complementation groups A, C, D2, and G. As yet lacking, in vivo studies should focus on FA-associated MDF, which may help in the understanding of the mitochondrial basis of OS detected in cells and body fluids from FA patients. Beyond the in vitro and animal databases, the available analytical devices may prompt the direct observation of metabolic and mitochondrial alterations in FA patients. These studies should evaluate a set of MDF-related endpoints, to be related to OS endpoints. The working hypothesis is raised that, parallel to OS, nitrosative stress might be another, so far unexplored, hallmark of the FA phenotype. The expected results may shed light on the FA pathogenesis and might provide grounds for pilot chemoprevention trials using mitochondrial nutrients.

C. elegans ring finger protein RNF-113 is involved in interstrand DNA crosslink repair and interacts with a RAD51C homolog

The Fanconi anemia (FA) pathway recognizes interstrand DNA crosslinks (ICLs) and contributes to their conversion into double-strand DNA breaks, which can be repaired by homologous recombination. Seven orthologs of the 15 proteins associated with Fanconi anemia are functionally conserved in the model organism C. elegans. Here we report that RNF-113, a ubiquitin ligase, is required for RAD-51 focus formation after inducing ICLs in C. elegans. However, the formation of foci of RPA-1 or FCD-2/FANCD2 in the FA pathway was not affected by depletion of RNF-113. Nevertheless, the RPA-1 foci formed did not disappear with time in the depleted worms, implying serious defects in ICL repair. As a result, RNF-113 depletion increased embryonic lethality after ICL treatment in wild-type worms, but it did not increase the ICL-induced lethality of rfs-1/rad51C mutants. In addition, the persistence of RPA-1 foci was suppressed in doubly-deficient rnf-113;rfs-1 worms, suggesting that there is an epistatic interaction between the two genes. These results lead us to suggest that RNF-113 and RFS-1 interact to promote the displacement of RPA-1 by RAD-51 on single-stranded DNA derived from ICLs.

Unwinding to recombine

The MCM proteins are best known for their role in DNA replication, MCM2-7 forming the replicative helicase. Now, two reports in this issue of Molecular Cell, Nishimura et al. (2012) and Lutzmann et al. (2012) show the less well understood MCM8 and MCM9 to be crucial for effective homologous recombination.

DNA helicases associated with genetic instability, cancer, and aging

DNA helicases have essential roles in the maintenance of genomic -stability. They have achieved even greater prominence with the discovery that mutations in human helicase genes are responsible for a variety of genetic disorders and are associated with tumorigenesis. A number of missense mutations in human helicase genes are linked to chromosomal instability diseases characterized by age-related disease or associated with cancer, providing incentive for the characterization of molecular defects underlying aberrant cellular phenotypes. In this chapter, we discuss some examples of clinically relevant missense mutations in various human DNA helicases, particularly those of the Iron-Sulfur cluster and RecQ families. Clinically relevant mutations in the XPD helicase can lead to Xeroderma pigmentosum, Cockayne's syndrome, Trichothiodystrophy, or COFS syndrome. FANCJ mutations are associated with Fanconi anemia or breast cancer. Mutations of the Fe-S helicase ChlR1 (DDX11) are linked to Warsaw Breakage syndrome. Mutations in the RecQ helicases BLM and WRN are linked to the cancer-prone disorder Bloom's syndrome and premature aging condition Werner syndrome, respectively. RECQL4 mutations can lead to Rothmund-Thomson syndrome, Baller-Gerold syndrome, or RAPADILINO. Mutations in the Twinkle mitochondrial helicase are responsible for several neuromuscular degenerative disorders. We will discuss some insights gained from biochemical and genetic studies of helicase variants, and highlight some hot areas of helicase research based on recent developments.

Disease-causing missense mutations in human DNA helicase disorders

Helicases have important roles in nucleic acid metabolism, and their prominence is marked by the discovery of genetic disorders arising from disease-causing mutations. Missense mutations can yield unique insight to molecular functions and basis for disease pathology. XPB or XPD missense mutations lead to Xeroderma pigmentosum, Cockayne's syndrome, Trichothiodystrophy, or COFS syndrome, suggesting that DNA repair and transcription defects are responsible for clinical heterogeneity. Complex phenotypes are also observed for RECQL4 helicase mutations responsible for Rothmund-Thomson syndrome, Baller-Gerold syndrome, or RAPADILINO. Bloom's syndrome causing missense mutations are found in the conserved helicase and RecQ C-terminal domain of BLM that interfere with helicase function. Although rare, patient-derived missense mutations in the exonuclease or helicase domain of Werner syndrome protein exist. Characterization of WRN separation-of-function mutants may provide insight to catalytic requirements for suppression of phenotypes associated with the premature aging disorder. Characterized FANCJ missense mutations associated with breast cancer or Fanconi anemia interfere with FANCJ helicase activity required for DNA repair and the replication stress response. For example, a FA patient-derived mutation in the FANCJ Iron-Sulfur domain was shown to uncouple its ATPase and translocase activity from DNA unwinding. Mutations in DDX11 (ChlR1) are responsible for Warsaw Breakage syndrome, a recently discovered autosomal recessive cohesinopathy. Ongoing and future studies will address clinically relevant helicase mutations and polymorphisms, including those that interfere with key protein interactions or exert dominant negative phenotypes (e.g., certain mutant alleles of Twinkle mitochondrial DNA helicase). Chemical rescue may be an approach to restore helicase activity in loss-of-function helicase disorders. Genetic and biochemical analyses of disease-causing missense mutations in human helicase disorders have led to new insights to the molecular defects underlying aberrant cellular and clinical phenotypes.

Diseases associated with defective responses to DNA damage

Within the last decade, multiple novel congenital human disorders have been described with genetic defects in known and/or novel components of several well-known DNA repair and damage response pathways. Examples include disorders of impaired nucleotide excision repair, DNA double-strand and single-strand break repair, as well as compromised DNA damage-induced signal transduction including phosphorylation and ubiquitination. These conditions further reinforce the importance of multiple genome stability pathways for health and development in humans. Furthermore, these conditions inform our knowledge of the biology of the mechanics of genome stability and in some cases provide potential routes to help exploit these pathways therapeutically. Here, I will review a selection of these exciting findings from the perspective of the disorders themselves, describing how they were identified, how genotype informs phenotype, and how these defects contribute to our growing understanding of genome stability pathways.

Evidence for different mechanisms of 'unhooking' for melphalan and cisplatin-induced DNA interstrand cross-links in vitro and in clinical acquired resistant tumour samples

Background

DNA interstrand cross-links (ICLs) are critical lesions produced by several cancer chemotherapy agents including platinum drugs and nitrogen mustards. We have previously shown in haematological (multiple myeloma) and solid tumours (ovarian cancer) that clinical sensitivity to such agents can result from a defect in DNA ICL processing leading to their persistence. Conversely, enhanced repair can result in clinical acquired resistance following chemotherapy. The repair of ICLs is complex but it is assumed that the ‘unhooking’ step is common to all ICLs.

Methods

Using a modification of the single cell gel electrophoresis (Comet) assay we measured the formation and unhooking of melphalan and cisplatin-induced ICLs in cell lines and clinical samples. DNA damage response in the form of γ-H2AX foci formation and the formation of RAD51 foci as a marker of homologous recombination were also determined. Real-time PCR of 84 genes involved in DNA damage signalling pathways was also examined pre- and post-treatment.

Results

Plasma cells from multiple myeloma patients known to be clinically resistant to melphalan showed significant unhooking of melphalan-induced ICLs at 48 hours, but did not unhook cisplatin-induced ICLs. In ovarian cancer cells obtained from patients following platinum-based chemotherapy, unhooking of cisplatin-induced ICLs was observed at 48 hours, but no unhooking of melphalan-induced ICLs. In vitro, A549 cells were proficient at unhooking both melphalan and cisplatin-induced ICLs. γ-H2AX foci formation closely followed the formation of ICLs for both drugs, and rapidly declined following the peak of formation. RPMI8226 cells unhooked melphalan, but not cisplatin-induced ICLs. In these cells, although cross-links form with cisplatin, the γ-H2AX response is weak. In A549 cells, addition of 3nM gemcitabine resulted in complete inhibition of cisplatin-induced ICL unhooking but no effect on repair of melphalan ICLs. The RAD51 foci response was both drug and cell line specific. Real time PCR studies highlighted differences in the damage response to melphalan and cisplatin following equi-ICL forming doses.

Conclusions

These data suggest that the mechanisms by which melphalan and cisplatin-induced ICLs are ‘unhooked’ in vitro are distinct, and the mechanisms of clinical acquired resistance involving repair of ICLs, are drug specific.

Genotoxic consequences of endogenous aldehydes on mouse haematopoietic stem cell function

Haematopoietic stem cells (HSCs) regenerate blood cells throughout the lifespan of an organism. With age, the functional quality of HSCs declines, partly owing to the accumulation of damaged DNA. However, the factors that damage DNA and the protective mechanisms that operate in these cells are poorly understood. We have recently shown that the Fanconi anaemia DNA-repair pathway counteracts the genotoxic effects of reactive aldehydes. Mice with combined inactivation of aldehyde catabolism (through Aldh2 knockout) and the Fanconi anaemia DNA-repair pathway (Fancd2 knockout) display developmental defects, a predisposition to leukaemia, and are susceptible to the toxic effects of ethanol-an exogenous source of acetaldehyde. Here we report that aged Aldh2(-/-) Fancd2(-/-) mutant mice that do not develop leukaemia spontaneously develop aplastic anaemia, with the concomitant accumulation of damaged DNA within the haematopoietic stem and progenitor cell (HSPC) pool. Unexpectedly, we find that only HSPCs, and not more mature blood precursors, require Aldh2 for protection against acetaldehyde toxicity. Additionally, the aldehyde-oxidizing activity of HSPCs, as measured by Aldefluor stain, is due to Aldh2 and correlates with this protection. Finally, there is more than a 600-fold reduction in the HSC pool of mice deficient in both Fanconi anaemia pathway-mediated DNA repair and acetaldehyde detoxification. Therefore, the emergence of bone marrow failure in Fanconi anaemia is probably due to aldehyde-mediated genotoxicity restricted to the HSPC pool. These findings identify a new link between endogenous reactive metabolites and DNA damage in HSCs, and define the protective mechanisms that counteract this threat.

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.

A prescription for 'stress'--the role of Hsp90 in genome stability and cellular adaptation

Changes in cell homeostasis, or cell 'stress', are thought to tax the ability of the Hsp90 chaperone to facilitate an array of processes critical for genome maintenance. Here, we review the current understanding of how the Hsp90 chaperone machinery ensures the function of proteins important for DNA repair, recombination, and chromosome segregation. We discuss the idea that cell stress can overload Hsp90, resulting in genomic instability that may have important implications for stress adaptation and selection. The importance of Hsp90 in genome maintenance and its limited capacity to buffer the proteome may underlie the initiation or progression of diseases such as cancer.

To the rescue: the Fanconi anemia genome stability pathway salvages replication forks

DNA damage can arrest replication forks during S phase. Failure to stabilize and restart arrested forks results in fork collapse and genomic instability. In this issue of Cancer Cell, Schlacher et al. show that the Fanconi anemia and BRCA2 tumor suppressor pathways cooperate to protect stalled replication forks from degradation.

Loss of ercc1 results in a time- and dose-dependent reduction of proliferating early hematopoietic progenitors

The endonuclease complex Ercc1/Xpf is involved in interstrand crosslink repair and functions downstream of the Fanconi pathway. Loss of Ercc1 causes hematopoietic defects similar to those seen in Fanconi Anemia. Ercc1^−/− mice die 3-4 weeks after birth, which prevents long-term follow up of the hematopoietic compartment. We used alternative Ercc1 mouse models to examine the effect of low or absent Ercc1 activity on hematopoiesis. Tie2-Cre-driven deletion of a floxed Ercc1 allele was efficient (>80%) in fetal liver hematopoietic cells. Hematopoietic stem and progenitor cells (HSPCs) with a deleted allele were maintained in mice up to 1 year of age when harboring a wt allele, but were progressively outcompeted when the deleted allele was combined with a knockout allele. Mice with a minimal Ercc1 activity expressed by 1 or 2 hypomorphic Ercc1 alleles have an extended life expectancy, which allows analysis of HSPCs at 10 and 20 weeks of age. The HSPC compartment was affected in all Ercc1-deficient models. Actively proliferating multipotent progenitors were most affected as were myeloid and erythroid clonogenic progenitors. In conclusion, lack of Ercc1 results in a severe competitive disadvantage of HSPCs and is most deleterious in proliferating progenitor cells.

How I manage aplastic anaemia in children

Aplastic anaemia (AA) is a rare heterogeneous condition in children. 15-20% of cases are constitutional and correct diagnosis of these inherited causes of AA is important for appropriate management. For idiopathic severe aplastic anaemia, a matched sibling donor (MSD) haematopoietic stem cell transplant (HSCT) is the treatment of choice. If a MSD is not available, the options include immunosuppressive therapy (IST) or unrelated donor HSCT. IST with horse anti-thymocyte globulin (ATG) is superior to rabbit ATG and has good long-term results. In contrast, IST with rabbit ATG has an overall response of only 30-40%. Due to improvements in outcome over the last two decades in matched unrelated donor (MUD) HSCT, results are now similar to that of MSD HSCT. The decision to proceed with IST with ATG or MUD HSCT will depend on the likelihood of finding a MUD and the differing risks and benefits that each therapy provides.

Omnis cellula e cellula revisited: cell biology as the foundation of pathology

This 2012 Annual Review Issue of The Journal of Pathology argues strongly that cell biology, in its many disciplines, underpins the foundation of our understanding of the mechanisms of disease-the holy grail of pathology. Our increasing knowledge of the human genome will not be enough to attain this goal without parallel developments in our comprehension of the results, at the cellular level, of these genetic changes. In the end, it is cell biology and cell biologists who will deliver this mission.