The nuclear accumulation of the Fanconi anemia protein FANCE depends on FANCC

Research progress on the fanconi anemia signaling pathway in non-obstructive azoospermia

Non-obstructive azoospermia (NOA) is a disease characterized by spermatogenesis failure and comprises phenotypes such as hypospermatogenesis, mature arrest, and Sertoli cell-only syndrome. Studies have shown that FA cross-linked anemia (FA) pathway is closely related to the occurrence of NOA. There are FA gene mutations in male NOA patients, which cause significant damage to male germ cells. The FA pathway is activated in the presence of DNA interstrand cross-links; the key step in activating this pathway is the mono-ubiquitination of the FANCD2-FANCI complex, and the activation of the FA pathway can repair DNA damage such as DNA double-strand breaks. Therefore, we believe that the FA pathway affects germ cells during DNA damage repair, resulting in minimal or even disappearance of mature sperm in males. This review summarizes the regulatory mechanisms of FA-related genes in male azoospermia, with the aim of providing a theoretical reference for clinical research and exploration of related genes.

Structural basis of the fanconi anemia-associated mutations within the FANCA and FANCG complex

Monoubiquitination of the Fanconi anemia complementation group D2 (FANCD2) protein by the FA core ubiquitin ligase complex is the central event in the FA pathway. FANCA and FANCG play major roles in the nuclear localization of the FA core complex. Mutations of these two genes are the most frequently observed genetic alterations in FA patients, and most point mutations in FANCA are clustered in the C-terminal domain (CTD). To understand the basis of the FA-associated FANCA mutations, we determined the cryo-electron microscopy (EM) structures of Xenopus laevis FANCA alone at 3.35 Å and 3.46 Å resolution and two distinct FANCA–FANCG complexes at 4.59 and 4.84 Å resolution, respectively. The FANCA CTD adopts an arc-shaped solenoid structure that forms a pseudo-symmetric dimer through its outer surface. FA- and cancer-associated point mutations are widely distributed over the CTD. The two different complex structures capture independent interactions of FANCG with either FANCA C-terminal HEAT repeats, or the N-terminal region. We show that mutations that disturb either of these two interactions prevent the nuclear localization of FANCA, thereby leading to an FA pathway defect. The structure provides insights into the function of FANCA CTD, and provides a framework for understanding FA- and cancer-associated mutations.

Primary Ovarian Insufficiency Induced by Fanconi Anemia E Mutation in a Mouse Model

In most cases of primary ovarian insufficiency (POI), the cause of the depletion of ovarian follicles is unknown. Fanconi anemia (FA) proteins are known to play important roles in follicular development. Using random insertional mutagenesis with a lentiviral transgene, we identified a family with reduced fertility in the homozygous transgenic mice. We identified the integration site and found that the lentivirus had integrated into intron 8 of the Fanconi E gene (Fance). By RT-PCR and in situ hybridization, we found that Fance transcript levels were significantly reduced. The Fance homozygous mutant mice were assayed for changes in ovarian development, follicle numbers and estrous cycle. Ovarian dysplasias and a severe lack of follicles were seen in the mutant mice. In addition, the estrous cycle was disrupted in adult females. Our results suggest that POI has been induced by the Fance mutation in this new mouse model.

Analysis of a FANCE Splice Isoform in Regard to DNA Repair

The FANC-BRCA DNA repair pathway is activated in response to interstrand crosslinks formed in DNA. A homozygous mutation in 1 of the 17 Fanconi anemia (FA) genes results in malfunctions of this pathway and development of FA syndrome. The integrity of this protein network is essential for good maintenance of DNA repair process and genome stability. Following the identification of an alternatively splice isoform of FANCE (Fanconi anemia complementation group E) significantly expressed in breast cancer individuals from high-risk non-BRCA1/2 families, we studied the impact of this FANCE splice isoform (FANCEΔ4) on DNA repair processes. We have demonstrated that FANCEΔ4 mRNA was efficiently translated into a functional protein and expressed in normal and breast cancer cell lines. Following treatment with the crosslinking agent mitomycin C, EUFA130 (FANCE-deficient) cells infected with FANCEΔ4 were blocked into G2/M phase, while cell survival was significantly reduced compared with FANCE-infected EUFA130 cells. In addition, FANCEΔ4 did not allow FANCD2 and FANCI monoubiquitination, which represents a crucial step of the FANC-BRCA functional pathway. As observed for FANCE wild-type protein, localization of FANCEΔ4 protein was confined to the nucleus following mitomycin C treatment. Although FANCEΔ4 protein showed interaction with FANCE, FANCEΔ4 did not support normal function of FANCE protein in this pathway and could have deleterious effects on FANCE protein activity. We have demonstrated that FANCEΔ4 seems to act as a regulator of FANCD2 protein expression level by promoting its degradation. This study highlights the importance of an efficient regulation of alternative splicing expression of FA genes for proper DNA repair.

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 carboxyl terminus of FANCE recruits FANCD2 to the Fanconi Anemia (FA) E3 ligase complex to promote the FA DNA repair pathway

Fanconi anemia (FA) is a genome instability syndrome characterized by bone marrow failure and cellular hypersensitivity to DNA cross-linking agents. In response to DNA damage, the FA pathway is activated through the cooperation of 16 FA proteins. A central player in the pathway is a multisubunit E3 ubiquitin ligase complex or the FA core complex, which monoubiquitinates its substrates FANCD2 and FANCI. FANCE, a subunit of the FA core complex, plays an essential role by promoting the integrity of the complex and by directly recognizing FANCD2. To delineate its role in substrate ubiquitination from the core complex assembly, we analyzed a series of mutations within FANCE. We report that a phenylalanine located at the highly conserved extreme C terminus, referred to as Phe-522, is a critical residue for mediating the monoubiquitination of the FANCD2-FANCI complex. Using the FANCE mutant that specifically disrupts the FANCE-FANCD2 interaction as a tool, we found that the interaction-deficient mutant conferred cellular sensitivity in reconstituted FANCE-deficient cells to a similar degree as FANCE null cells, suggesting the significance of the FANCE-FANCD2 interaction in promoting cisplatin resistance. Intriguingly, ectopic expression of the FANCE C terminus fragment alone in FA normal cells disrupts DNA repair, consolidating the importance of the FANCE-FANCD2 interaction in the DNA cross-link repair.

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.

Towards a molecular understanding of the fanconi anemia core complex

Fanconi Anemia (FA) is a genetic disorder characterized by the inability of patient cells to repair DNA damage caused by interstrand crosslinking agents. There are currently 14 verified FA genes, where mutation of any single gene prevents repair of DNA interstrand crosslinks (ICLs). The accumulation of ICL damage results in genome instability and patients having a high predisposition to cancers. The key event of the FA pathway is dependent on an eight-protein core complex (CC), required for the monoubiquitination of each member of the FANCD2-FANCI complex. Interestingly, the majority of patient mutations reside in the CC. The molecular mechanisms underlying the requirement for such a large complex to carry out a monoubiquitination event remain a mystery. This paper documents the extensive efforts of researchers so far to understand the molecular roles of the CC proteins with regard to its main function in the FA pathway, the monoubiquitination of FANCD2 and FANCI.

Fanconi anemia proteins and their interacting partners: a molecular puzzle

In recent years, Fanconi anemia (FA) has been the subject of intense investigations, primarily in the DNA repair research field. Many discoveries have led to the notion of a canonical pathway, termed the FA pathway, where all FA proteins function sequentially in different protein complexes to repair DNA cross-link damages. Although a detailed architecture of this DNA cross-link repair pathway is emerging, the question of how a defective DNA cross-link repair process translates into the disease phenotype is unresolved. Other areas of research including oxidative metabolism, cell cycle progression, apoptosis, and transcriptional regulation have been studied in the context of FA, and some of these areas were investigated before the fervent enthusiasm in the DNA repair field. These other molecular mechanisms may also play an important role in the pathogenesis of this disease. In addition, several FA-interacting proteins have been identified with roles in these “other” nonrepair molecular functions. Thus, the goal of this paper is to revisit old ideas and to discuss protein-protein interactions related to other FA-related molecular functions to try to give the reader a wider perspective of the FA molecular puzzle.

Evidence for complete epistasis of null mutations in murine Fanconi anemia genes Fanca and Fancg

Fanconi anemia (FA) is a heritable disease characterized by bone marrow failure, congenital abnormalities, and cancer predisposition. The 15 identified FA genes operate in a molecular pathway to preserve genomic integrity. Within this pathway the FA core complex operates as an ubiquitin ligase that activates the complex of FANCD2 and FANCI to coordinate DNA repair. The FA core complex is formed by at least 12 proteins. However, only the FANCL subunit displays ubiquitin ligase activity. FANCA and FANCG are members of the FA core complex for which no other functions have been described than to participate in protein interactions. In this study we generated mice with combined null alleles for Fanca and Fancg to identify extended functions for these genes by characterizing the double mutant mice and cells. Double mutant a(-/-)/g(-/-) mice were born at near Mendelian frequencies without apparent developmental abnormalities. Histological analysis of a(-/-)/g(-/-) mice revealed a Leydig cell hyperplasia and frequent vacuolization of Sertoli cells in testes, while ovaries were depleted from developing follicles and displayed an interstitial cell hyperplasia. These gonadal aberrations were associated with a compromised fertility of a(-/-)/g(-/-) males and females. During the first year of life a(-/-)/g(-/-) did not develop malignancies or bone marrow failure. At the cellular level a(-/-)/g(-/-), Fanca(-/-), and Fancg(-/-) cells proved equally compromised in DNA crosslink and homology-directed repair. Overall the phenotype of a(-/-)/g(-/-) double knockout mice and cells appeared highly similar to the phenotype of Fanca or Fancg single knockouts. The lack of an augmented phenotype suggest that null mutations in Fanca or Fancg are fully epistatic, making additional important functions outside of the FA core complex highly unlikely.

The role of Fanconi anemia/BRCA genes in zebrafish sex determination

Fanconi anemia (FA) is a human disease of bone marrow failure, leukemia, squamous cell carcinoma, and developmental anomalies, including hypogonadism and infertility. Bone marrow transplants improve hematopoietic phenotypes but do not prevent other cancers. FA arises from mutation in any of the 15 FANC genes that cooperate to repair double stranded DNA breaks by homologous recombination. Zebrafish has a single ortholog of each human FANC gene and unexpectedly, mutations in at least two of them (fancl and fancd1(brca2)) lead to female-to-male sex reversal. Investigations show that, as in human, zebrafish fanc genes are required for genome stability and for suppressing apoptosis in tissue culture cells, in embryos treated with DNA damaging agents, and in meiotic germ cells. The sex reversal phenotype requires the action of Tp53 (p53), an activator of apoptosis. These results suggest that in normal sex determination, zebrafish oocytes passing through meiosis signal the gonadal soma to maintain expression of aromatase, an enzyme that converts androgen to estrogen, thereby feminizing the gonad and the individual. According to this model, normal male and female zebrafish differ in genetic factors that control the strength of the late meiotic oocyte-derived signal, probably by regulating the number of meiotic oocytes, which environmental factors can also alter. Transcripts from fancd1(brca2) localize at the animal pole of the zebrafish oocyte cytoplasm and are required for normal oocyte nuclear architecture, for normal embryonic development, and for preventing ovarian tumors. Embryonic DNA repair and sex reversal phenotypes provide assays for the screening of small molecule libraries for therapeutic substances for FA.

Genetic disruption of both Fancc and Fancg in mice recapitulates the hematopoietic manifestations of Fanconi anemia

Fanconi anemia (FA) is an inherited chromosomal instability syndrome characterized by bone marrow failure, myelodysplasia (MDS), and acute myeloid leukemia (AML). Eight FA proteins associate in a nuclear core complex to monoubiquitinate FANCD2/FANCI in response to DNA damage. Additional functions have been described for some of the core complex proteins; however, in vivo genetic proof has been lacking. Here we show that double-mutant Fancc(-/-);Fancg(-/-) mice develop spontaneous hematologic sequelae including bone marrow failure, AML, MDS and complex random chromosomal abnormalities that the single-mutant mice do not. This genetic model provides evidence for unique core complex protein function independent of their ability to monoubiquitinate FANCD2/FANCI. Importantly, this model closely recapitulates the phenotypes found in FA patients and may be useful as a preclinical platform to evaluate the molecular pathogenesis of spontaneous bone marrow failure, MDS and AML in FA.

The role of the Fanconi anemia network in the response to DNA replication stress

Fanconi anemia is a genetically heterogeneous disorder associated with chromosome instability and a highly elevated risk for developing cancer. The mutated genes encode proteins involved in the cellular response to DNA replication stress. Fanconi anemia proteins are extensively connected with DNA caretaker proteins, and appear to function as a hub for the coordination of DNA repair with DNA replication and cell cycle progression. At a molecular level, however, the raison d'être of Fanconi anemia proteins still remains largely elusive. The thirteen Fanconi anemia proteins identified to date have not been embraced into a single and defined biological process. To help put the Fanconi anemia puzzle into perspective, we begin this review with a summary of the strategies employed by prokaryotes and eukaryotes to tolerate obstacles to the progression of replication forks. We then summarize what we know about Fanconi anemia with an emphasis on biochemical aspects, and discuss how the Fanconi anemia network, a late acquisition in evolution, may function to permit the faithful and complete duplication of our very large vertebrate chromosomes.

The genetic and molecular basis of Fanconi anemia

The capacity to maintain genomic integrity is shared by all living organisms. Multiple pathways are distinguished that safeguard genomic stability, most of which have originated in primitive life forms. In human individuals, defects in these pathways are typically associated with cancer proneness. The Fanconi anemia pathway, one of these pathways, has evolved relatively late during evolution and exists - in its fully developed form - only in vertebrates. This pathway, in which thus far 13 distinct proteins have been shown to participate, appears essential for error-free DNA replication. Inactivating mutations in the corresponding genes underlie the recessive disease Fanconi anemia (FA). In the last decade the genetic basis of this disorder has been uncovered by a variety of approaches, including complementation cloning, genetic linkage analysis and protein association studies. Here we review these approaches, introduce the encoded proteins, and discuss their possible role in ensuring genomic integrity.

The Fanconi anemia/BRCA gene network in zebrafish: embryonic expression and comparative genomics

Fanconi anemia (FA) is a genetic disease resulting in bone marrow failure, high cancer risks, and infertility, and developmental anomalies including microphthalmia, microcephaly, hypoplastic radius and thumb. Here we present cDNA sequences, genetic mapping, and genomic analyses for the four previously undescribed zebrafish FA genes (fanci, fancj, fancm, and fancn), and show that they reverted to single copy after the teleost genome duplication. We tested the hypothesis that FA genes are expressed during embryonic development in tissues that are disrupted in human patients by investigating fanc gene expression patterns. We found fanc gene maternal message, which can provide Fanc proteins to repair DNA damage encountered in rapid cleavage divisions. Zygotic expression was broad but especially strong in eyes, central nervous system and hematopoietic tissues. In the pectoral fin bud at hatching, fanc genes were expressed specifically in the apical ectodermal ridge, a signaling center for fin/limb development that may be relevant to the radius/thumb anomaly of FA patients. Hatching embryos expressed fanc genes strongly in the oral epithelium, a site of squamous cell carcinomas in FA patients. Larval and adult zebrafish expressed fanc genes in proliferative regions of the brain, which may be related to microcephaly in FA. Mature ovaries and testes expressed fanc genes in specific stages of oocyte and spermatocyte development, which may be related to DNA repair during homologous recombination in meiosis and to infertility in human patients. The intestine strongly expressed some fanc genes specifically in proliferative zones. Our results show that zebrafish has a complete complement of fanc genes in single copy and that these genes are expressed in zebrafish embryos and adults in proliferative tissues that are often affected in FA patients. These results support the notion that zebrafish offers an attractive experimental system to help unravel mechanisms relevant not only to FA, but also to breast cancer, given the involvement of fancj (brip1), fancn (palb2) and fancd1 (brca2) in both conditions.

Coordinate regulation of Fanconi anemia gene expression occurs through the Rb/E2F pathway

Fanconi anemia (FA) is a genome instability syndrome that is characterized by progressive bone marrow failure and a high risk of cancer. FA patients are particularly susceptible to leukemia as well as squamous cell carcinomas (SCCs) of the head and neck, anogenital region and skin. Thirteen complementation groups and the corresponding FA genes have been identified, and their protein products assemble into nuclear core complexes during DNA-damage responses. Much progress has been made in our understanding of post-translational FA protein modifications and physical interactions. By contrast, little is known about the control of protein availability at the level of transcription. We report here that multiple FA proteins were downregulated during the proliferative arrest of primary human keratinocytes and HeLa cells, and that the observed regulation was at a transcriptional level. Proliferative stimuli such as expression of HPV16 E7 as well as E2F1 overexpression in primary cells resulted in coordinate FA upregulation. To define the underlying mechanism, we examined the endogenous FANCD2 promoter, and detected regulated binding of members of the E2F/Rb family in chromatin immunoprecipitation assays. Finally, a 1 kb promoter fragment was sufficient to confer E2F/Rb regulation in reporter assays. Taken together, our data demonstrate FA gene co-regulation in synchrony with the cell cycle and suggest that deregulated expression of individual FA genes-in addition to FA gene mutation-may promote FA-related human cancer.

UBE2T, the Fanconi anemia core complex, and FANCD2 are recruited independently to chromatin: a basis for the regulation of FANCD2 monoubiquitination

The Fanconi anemia (FA) nuclear core complex and the E2 ubiquitin-conjugating enzyme UBE2T are required for the S phase and DNA damage-restricted monoubiquitination of FANCD2. This constitutes a key step in the FA tumor suppressor pathway, and much attention has been focused on the regulation at this point. Here, we address the importance of the assembly of the FA core complex and the subcellular localization of UBE2T in the regulation of FANCD2 monoubiquitination. We establish three points. First, the stable assembly of the FA core complex can be dissociated of its ability to function as an E3 ubiquitin ligase. Second, the actual E3 ligase activity is not determined by the assembly of the FA core complex but rather by its DNA damage-induced localization to chromatin. Finally, UBE2T and FANCD2 access this subcellular fraction independently of the FA core complex. FANCD2 monoubiquitination is therefore not regulated by multiprotein complex assembly but by the formation of an active E2/E3 holoenzyme on chromatin.

Insights into Fanconi Anaemia from the structure of human FANCE

Fanconi Anaemia (FA) is a cancer predisposition disorder characterized by spontaneous chromosome breakage and high cellular sensitivity to genotoxic agents. In response to DNA damage, a multi-subunit assembly of FA proteins, the FA core complex, monoubiquitinates the downstream FANCD2 protein. The FANCE protein plays an essential role in the FA process of DNA repair as the FANCD2-binding component of the FA core complex. Here we report a crystallographic and biological study of human FANCE. The first structure of a FA protein reveals the presence of a repeated helical motif that provides a template for the structural rationalization of other proteins defective in Fanconi Anaemia. The portion of FANCE defined by our crystallographic analysis is sufficient for interaction with FANCD2, yielding structural information into the mode of FANCD2 recruitment to the FA core complex. Disease-associated mutations disrupt the FANCE–FANCD2 interaction, providing structural insight into the molecular mechanisms of FA pathogenesis.