Reprogramming human T cell function and specificity with non-viral genome targeting

Decades of work have aimed to genetically reprogram T cells for therapeutic purposes1,2 using recombinant viral vectors, which do not target transgenes to specific genomic sites3,4. The need for viral vectors has slowed down research and clinical use as their manufacturing and testing is lengthy and expensive. Genome editing brought the promise of specific and efficient insertion of large transgenes into target cells using homology-directed repair5,6. Here we developed a CRISPR-Cas9 genome-targeting system that does not require viral vectors, allowing rapid and efficient insertion of large DNA sequences (greater than one kilobase) at specific sites in the genomes of primary human T cells, while preserving cell viability and function. This permits individual or multiplexed modification of endogenous genes. First, we applied this strategy to correct a pathogenic IL2RA mutation in cells from patients with monogenic autoimmune disease, and demonstrate improved signalling function. Second, we replaced the endogenous T cell receptor (TCR) locus with a new TCR that redirected T cells to a cancer antigen. The resulting TCR-engineered T cells specifically recognized tumour antigens and mounted productive anti-tumour cell responses in vitro and in vivo. Together, these studies provide preclinical evidence that non-viral genome targeting can enable rapid and flexible experimental manipulation and therapeutic engineering of primary human immune cells.

BRCA1-BARD1 promotes RAD51-mediated homologous DNA pairing

The tumour suppressor complex BRCA1-BARD1 functions in the repair of DNA double-stranded breaks by homologous recombination. During this process, BRCA1-BARD1 facilitates the nucleolytic resection of DNA ends to generate a single-stranded template for the recruitment of another tumour suppressor complex, BRCA2-PALB2, and the recombinase RAD51. Here, by examining purified wild-type and mutant BRCA1-BARD1, we show that both BRCA1 and BARD1 bind DNA and interact with RAD51, and that BRCA1-BARD1 enhances the recombinase activity of RAD51. Mechanistically, BRCA1-BARD1 promotes the assembly of the synaptic complex, an essential intermediate in RAD51-mediated DNA joint formation. We provide evidence that BRCA1 and BARD1 are indispensable for RAD51 stimulation. Notably, BRCA1-BARD1 mutants with weakened RAD51 interactions show compromised DNA joint formation and impaired mediation of homologous recombination and DNA repair in cells. Our results identify a late role of BRCA1-BARD1 in homologous recombination, an attribute of the tumour suppressor complex that could be targeted in cancer therapy.

UBE2T is the E2 in the Fanconi anemia pathway and undergoes negative autoregulation

The Fanconi anemia pathway is required for the efficient repair of damaged DNA. A key step in this pathway is the monoubiquitination of the FANCD2 protein by the ubiquitin ligase (E3) composed of Fanconi anemia core complex proteins. Here, we show that UBE2T is the ubiquitin-conjugating enzyme (E2) essential for this pathway. UBE2T binds to FANCL, the ubiquitin ligase subunit of the Fanconi anemia core complex, and is required for the monoubiquitination of FANCD2 in vivo. DNA damage in UBE2T-depleted cells leads to the formation of abnormal chromosomes that are a hallmark of Fanconi anemia. In addition, we show that UBE2T undergoes automonoubiquitination in vivo. This monoubiquitination is stimulated by the presence of the FANCL protein and inactivates UBE2T. Therefore, UBE2T is the E2 in the Fanconi anemia pathway and has a self-inactivation mechanism that could be important for negative regulation of the Fanconi anemia pathway.

The Fanconi anaemia proteins, FAA and FAC, interact to form a nuclear complex

Fanconi anaemia (FA) is an autosomal-recessive disorder characterized by genomic instability, developmental defects, DNA crosslinking agent hypersensitivity and cancer susceptibility. Somatic-cell hybrid studies have revealed five FA complementation groups (A-E; refs 4-6) displaying similar phenotypes, suggesting that FA genes are functionally related. The two cloned FA genes, FAA and FAC, encode proteins that are unrelated to each other or to other proteins in GenBank. In the current study, we demonstrate the FAA and FAC bind each other and form a complex. Protein binding correlates with the functional activity of FAA and FAC, as patient-derived mutant FAC (L554P) fails to bind FAA. Although unbound FAA and FAC localize predominantly to the cytoplasm, the FAA-FAC complex is found in similar abundance in both cytoplasm and nucleus. Our results confirm the interrelatedness of the FA genes in a pathway, suggesting the cooperation of FAA and FAC in a nuclear function.

The fanconi anemia pathway requires FAA phosphorylation and FAA/FAC nuclear accumulation

Fanconi anemia (FA) is an autosomal recessive cancer susceptibility syndrome with at least eight complementation groups (A-H). Two FA genes, corresponding to complementation groups A and C, have been cloned, but the function of the FAA and FAC proteins remains unknown. We have recently shown that the FAA and FAC proteins bind and form a nuclear complex. In the current study, we analyzed the FAA and FAC proteins in normal lymphoblasts and lymphoblasts from multiple FA complementation groups. In contrast to normal controls, FA cells derived from groups A, B, C, E, F, G, and H were defective in the formation of the FAA/FAC protein complex, the phosphorylation of the FAA protein, and the accumulation of the FAA/FAC protein complex in the nucleus. These biochemical events seem to define a signaling pathway required for the maintenance of genomic stability and normal hematopoiesis. Our results support the idea that multiple gene products cooperate in the FA Pathway.

Functional activity of the fanconi anemia protein FAA requires FAC binding and nuclear localization

Fanconi anemia (FA) is an autosomal recessive disease characterized by genomic instability, cancer susceptibility, and cellular hypersensitivity to DNA-cross-linking agents. Eight complementation groups of FA (FA-A through FA-H) have been identified. Two FA genes, corresponding to complementation groups FA-A and FA-C, have been cloned, but the functions of the encoded FAA and FAC proteins remain unknown. We have recently demonstrated that FAA and FAC interact to form a nuclear complex. In this study, we have analyzed a series of mutant forms of the FAA protein with respect to functional activity, FAC binding, and nuclear localization. Mutation or deletion of the amino-terminal nuclear localization signal (NLS) of FAA results in loss of functional activity, loss of FAC binding, and cytoplasmic retention of FAA. Replacement of the NLS sequence with a heterologous NLS sequence, derived from the simian virus 40 T antigen, results in nuclear localization but does not rescue functional activity or FAC binding. Nuclear localization of the FAA protein is therefore necessary but not sufficient for FAA function. Mutant forms of FAA which fail to bind to FAC also fail to promote the nuclear accumulation of FAC. In addition, wild-type FAC promotes the accumulation of wild-type FAA in the nucleus. Our results suggest that FAA and FAC perform a concerted function in the cell nucleus, required for the maintenance of chromosomal stability.

Fanconi anemia proteins localize to chromatin and the nuclear matrix in a DNA damage- and cell cycle-regulated manner

Fanconi anemia (FA) is a genetic disease characterized by congenital defects, bone marrow failure, and cancer susceptibility. Cells from patients with FA exhibit genomic instability and hypersensitivity to DNA cross linking agents such as mitomycin C. Despite the identification of seven complementation groups and the cloning of six genes, the function of the encoded gene products remains elusive. The FancA (Fanconi anemia complementation group A), FancC, and FancG proteins have been detected within a nuclear complex, but no change in level, binding, or localization has been reported as a result of drug treatment or cell cycle. We show that in immunofluorescence studies, FancA appears as a non-nucleolar nuclear protein that is excluded from condensed, mitotic chromosomes. Biochemical fractionation reveals that the FA proteins are found in nuclear matrix and chromatin and that treatment with mitomycin C results in increase of the FA proteins in nuclear matrix and chromatin fractions. This induction occurs in wild-type cells and mutant FA-D (Fanconi complementation group D) cells but not in mutant FA-A cells. Immunoprecipitation of FancA protein in chromatin demonstrates the coprecipitation of FancA, FancC, and FancG, showing that the FA proteins move together as a complex. Also, fractionation of mitotic cells confirms the lack of FA proteins in chromatin or the nuclear matrix. Furthermore, phosphorylation of FancG was found to be temporally correlated with exit of the FA complex from chromosomes at mitosis. Taken together, these findings suggest a role for FA proteins in chromatin and nuclear matrix.

HTLV-1 Tax oncoprotein subverts the cellular DNA damage response via binding to DNA-dependent protein kinase

Human T-cell leukemia virus type-1 is the causative agent for adult T-cell leukemia. Previous research has established that the viral oncoprotein Tax mediates the transformation process by impairing cell cycle control and cellular response to DNA damage. We showed previously that Tax sequesters huChk2 within chromatin and impairs the response to ionizing radiation. Here we demonstrate that DNA-dependent protein kinase (DNA-PK) is a member of the Tax.Chk2 nuclear complex. The catalytic subunit, DNA-PKcs, and the regulatory subunit, Ku70, were present. Tax-containing nuclear extracts showed increased DNA-PK activity, and specific inhibition of DNA-PK prevented Tax-induced activation of Chk2 kinase activity. Expression of Tax induced foci formation and phosphorylation of H2AX. However, Tax-induced constitutive signaling of the DNA-PK pathway impaired cellular response to new damage, as reflected in suppression of ionizing radiation-induced DNA-PK phosphorylation and gammaH2AX stabilization. Tax co-localized with phospho-DNA-PK into nuclear speckles and a nuclear excluded Tax mutant sequestered endogenous phospho-DNA-PK into the cytoplasm, suggesting that Tax interaction with DNA-PK is an initiating event. We also describe a novel interaction between DNA-PK and Chk2 that requires Tax. We propose that Tax binds to and stabilizes a protein complex with DNA-PK and Chk2, resulting in a saturation of DNA-PK-mediated damage repair response.

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

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

Fanconi anemia

Fanconi anemia (FA) is an autosomal and X-linked recessive disorder characterized by bone marrow failure, acute myelogenous leukemia, solid tumors, and developmental abnormalities. Recent years have seen a dramatic improvement in FA patient treatment, resulting in a greater survival of children into adulthood. These improvements have been made despite the fact that a definitive cellular function for the proteins in the FA pathway has yet to be elucidated. Delineating the cellular functions of the FA pathway could help further improve the treatment options for FA patients and further reduce the probability of succumbing to the disease. This article reviews the current clinical aspects of FA including presentation, diagnosis, and treatment followed by a review of the molecular aspects of FA as they are currently understood.

Binding of FANCI-FANCD2 Complex to RNA and R-Loops Stimulates Robust FANCD2 Monoubiquitination

Fanconi anemia (FA) is characterized by developmental abnormalities, bone marrow failure, and cancer predisposition. FA cells are hypersensitive to DNA replicative stress and accumulate co-transcriptional R-loops. Here, we use the Damage At RNA Transcription assay to reveal colocalization of FANCD2 with R-loops in a highly transcribed genomic locus upon DNA damage. We further demonstrate that highly purified human FANCI-FANCD2 (ID2) complex binds synthetic single-stranded RNA (ssRNA) and R-loop substrates with high affinity, preferring guanine-rich sequences. Importantly, we elucidate that human ID2 binds an R-loop structure via recognition of the displaced ssDNA and ssRNA but not the RNA:DNA hybrids. Finally, a series of RNA and R-loop substrates are found to strongly stimulate ID2 monoubiquitination, with activity corresponding to their binding affinity. In summary, our results support a mechanism whereby the ID2 complex suppresses the formation of pathogenic R-loops by binding ssRNA and ssDNA species, thereby activating the FA pathway.

Fanconi anemia pathway has a well-known role in the repair of DNA crosslinks, but its recently identified role in suppression of co-transcriptional R-loops remains elusive. Here, Liang et al. show that FANCI-FANCD2 has intrinsic RNA and R-loop binding activity and provide mechanistic insights into FA pathway activation upon transcription stress.

Rad18-mediated translesion synthesis of bulky DNA adducts is coupled to activation of the Fanconi anemia DNA repair pathway

Fanconi anemia (FA) is a cancer susceptibility syndrome characterized by sensitivity to DNA-damaging agents. The FA proteins (FANCs) are implicated in DNA repair, although the precise mechanisms by which FANCs process DNA lesions are not fully understood. An epistatic relationship between the FA pathway and translesion synthesis (TLS, a post-replication DNA repair mechanism) has been suggested, but the basis for cross-talk between the FA and TLS pathways is poorly understood. We show here that ectopic overexpression of the E3 ubiquitin ligase Rad18 (a central regulator of TLS) induces DNA damage-independent mono-ubiquitination of proliferating cell nuclear antigen (PCNA) (a known Rad18 substrate) and FANCD2. Conversely, DNA damage-induced mono-ubiquitination of both PCNA and FANCD2 is attenuated in Rad18-deficient cells, demonstrating that Rad18 contributes to activation of the FA pathway. WT Rad18 but not an E3 ubiquitin ligase-deficient Rad18 C28F mutant fully complements both PCNA ubiquitination and FANCD2 activation in Rad18-depleted cells. Rad18-induced mono-ubiquitination of FANCD2 is not observed in FA core complex-deficient cells, demonstrating that Rad18 E3 ligase activity alone is insufficient for FANCD2 ubiquitylation. Instead, Rad18 promotes FA core complex-dependent FANCD2 ubiquitination in a manner that is secondary to PCNA mono-ubiquitination. Taken together, these results demonstrate a novel Rad18-dependent mechanism that couples activation of the FA pathway with TLS.

Fanconi anemia complementation group FANCD2 protein serine 331 phosphorylation is important for fanconi anemia pathway function and BRCA2 interaction

Fanconi anemia is a cancer-prone inherited bone marrow failure and cancer susceptibility syndrome with at least 13 complementation groups (FANCA, FANCB, FANCC, FANCD1, FANCD2, FANCE, FANCF, FANCG, FANCI, FANCJ, FANCL, FANCM, and FANCN). Our laboratory has previously described several regulatory phosphorylation events for core complex member proteins FANCG and FANCA by phosphorylation. In this study, we report a novel phosphorylation site serine 331 (S331) of FANCD2, the pivotal downstream player of the Fanconi anemia pathway. Phosphorylation of S331 is important for its DNA damage-inducible monoubiquitylation, resistance to DNA cross-linkers, and in vivo interaction with FANCD1/BRCA2. A phosphomimetic mutation at S331 restores all of these phenotypes to wild-type. In vitro and in vivo experiments show that phosphorylation of S331 is mediated by CHK1, the S-phase checkpoint kinase implicated in the Fanconi anemia DNA repair pathway.

Immunosuppressive therapy for pediatric aplastic anemia: a North American Pediatric Aplastic Anemia Consortium study

Quality of response to immunosuppressive therapy and long-term outcomes for pediatric severe aplastic anemia remain incompletely characterized. Contemporary evidence to inform treatment of relapsed or refractory severe aplastic anemia for pediatric patients is also limited. The clinical features and outcomes for 314 children treated from 2002 to 2014 with immunosuppressive therapy for acquired severe aplastic anemia were analyzed retrospectively from 25 institutions in the North American Pediatric Aplastic Anemia Consortium. The majority of subjects (n=264) received horse anti-thymocyte globulin (hATG) plus cyclosporine (CyA) with a median 61 months follow up. Following hATG/CyA, 71.2% (95%CI: 65.3,76.6) achieved an objective response. In contrast to adult studies, the quality of response achieved in pediatric patients was high, with 59.8% (95%CI: 53.7,65.8) complete response and 68.2% (95%CI: 62.2,73.8) achieving at least a very good partial response with a platelet count ≥50×10⁹L. At five years post-hATG/CyA, overall survival was 93% (95%CI: 89,96), but event-free survival without subsequent treatment was only 64% (95%CI: 57,69) without a plateau. Twelve of 171 evaluable patients (7%) acquired clonal abnormalities after diagnosis after a median 25.2 months (range: 4.3-71 months) post treatment. Myelodysplastic syndrome or leukemia developed in 6 of 314 (1.9%). For relapsed/refractory disease, treatment with a hematopoietic stem cell transplant had a superior event-free survival compared to second immunosuppressive therapy treatment in a multivariate analysis (HR=0.19, 95%CI: 0.08,0.47; P=0.0003). This study highlights the need for improved therapies to achieve sustained high-quality remission for children with severe aplastic anemia.

Promotion of RAD51-Mediated Homologous DNA Pairing by the RAD51AP1-UAF1 Complex

The UAF1-USP1 complex deubiquitinates FANCD2 during execution of the Fanconi anemia DNA damage response pathway. As such, UAF1 depletion results in persistent FANCD2 ubiquitination and DNA damage hypersensitivity. UAF1-deficient cells are also impaired for DNA repair by homologous recombination. Herein, we show that UAF1 binds DNA and forms a dimeric complex with RAD51AP1, an accessory factor of the RAD51 recombinase, and a trimeric complex with RAD51 through RAD51AP1. Two small ubiquitin-like modifier (SUMO)-like domains in UAF1 and a SUMO-interacting motif in RAD51AP1 mediate complex formation. Importantly, UAF1 enhances RAD51-mediated homologous DNA pairing in a manner that is dependent on complex formation with RAD51AP1 but independent of USP1. Mechanistically, RAD51AP1-UAF1 co-operates with RAD51 to assemble the synaptic complex, a critical nucleoprotein intermediate in homologous recombination, and cellular studies reveal the biological significance of the RAD51AP1-UAF1 protein complex. Our findings provide insights into an apparently USP1-independent role of UAF1 in genome maintenance.

DNA requirement in FANCD2 deubiquitination by USP1-UAF1-RAD51AP1 in the Fanconi anemia DNA damage response

Fanconi anemia (FA) is a multigenic disease of bone marrow failure and cancer susceptibility stemming from a failure to remove DNA crosslinks and other chromosomal lesions. Within the FA DNA damage response pathway, DNA-dependent monoubiquitinaton of FANCD2 licenses downstream events, while timely FANCD2 deubiquitination serves to extinguish the response. Here, we show with reconstituted biochemical systems, which we developed, that efficient FANCD2 deubiquitination by the USP1-UAF1 complex is dependent on DNA and DNA binding by UAF1. Surprisingly, we find that the DNA binding activity of the UAF1-associated protein RAD51AP1 can substitute for that of UAF1 in FANCD2 deubiquitination in our biochemical system. We also reveal the importance of DNA binding by UAF1 and RAD51AP1 in FANCD2 deubiquitination in the cellular setting. Our results provide insights into a key step in the FA pathway and help define the multifaceted role of the USP1-UAF1-RAD51AP1 complex in DNA damage tolerance and genome repair.

In the Fanconi anemia pathway, deubiquitination of FANCD2 is a fundamental regulatory step. Here, the authors have developed a set of biochemical tools to reconstitute FANCD2 deubiquitination by recombinant USP1-UAF1-RAD51AP1 and reveal critical mechanistic details of the process.

The Fanconi anemia core complex associates with chromatin during S phase

Fanconi anemia (FA) is an autosomal recessive disease marked by bone marrow failure, birth defects, and cancer. The FA proteins FANCA, FANCC, FANCE, FANCF, FANCG, and FANCL participate in a core complex. We previously have shown that several members of this complex bind to chromatin until mitosis and that this binding increases after DNA damage. The purpose of the present study was to determine the dynamics of complex movement between cytoplasm and nuclear compartments. Fluorescent-tagged versions of FANCA, FANCC, and FANCG colocalize in cytoplasm and nucleus, chiefly in chromatin. At the G1-S border, the FA core complex exists as foci on chromatin, progressively diffusing and migrating to the nuclear periphery and becoming completely excluded from condensed chromosomes by mitosis. Chromatin fiber analysis shows FA proteins diffusely staining along chromatin fibers during G1-S and S phase. Treatment with the DNA cross-linker mitomycin C results in a diffusion of foci and increased binding of complex proteins to chromatin, as well as diffuse and increased complex binding to chromatin fibers. These data are consistent with the idea that the FA proteins function at the level of chromatin during S phase to regulate and maintain genomic stability.

The Fanconi anemia core complex forms four complexes of different sizes in different subcellular compartments

Fanconi anemia (FA) is an autosomal recessive disease marked by congenital defects, bone marrow failure, and cancer susceptibility. FA cells exhibit a characteristic hypersensitivity to DNA crosslinking agents such as mitomycin C. The molecular mechanism for the disease remains elusive, but at least 6 FA proteins are known to be part of what is termed the FA core complex. We used affinity pulldown of FLAG-FANCA to pull down the FA complex from whole-cell extracts. Mass spectroscopy detected previously reported FA-binding proteins, including FANCA, FANCC, FANCG, cdc2, and GRP94, thus validating the approach. We further describe a method of purification of the FA core complex in an effort to find novel complex components and biochemical activity to define the function of the complex. By using conventional chromatographic fractionation of subcellular preparations, we report: (i) the FA core complex exists in a cytoplasmic form at 500-600 kDa; (ii) a larger, 750-kDa cytoplasmic form is seen only at mitosis; (iii) a nuclear form achieves a size of 2 megaDaltons; and (iv) a distinct 1-megaDalton FA core complex exists bound to chromatin that contains phosphorylated FANCA after undergoing DNA damage. We are continuing our analysis using mass spectroscopy in an effort to characterize novel binding proteins. These data will help define the biochemical role of the FA core complex in normal cell physiology as well as in the development of the FA disease state.

Functional and physical interaction between the mismatch repair and FA-BRCA pathways

Fanconi anemia (FA) is a rare genetic disorder characterized by bone marrow failure and an increased risk for leukemia and cancer. Fifteen proteins thought to function in the repair of DNA interstrand crosslinks (ICLs) comprise what is known as the FA-BRCA pathway. Activation of this pathway leads to the monoubiquitylation and chromatin localization of FANCD2 and FANCI. It has previously been shown that FANCJ interacts with the mismatch repair (MMR) complex MutLα. Here we show that FANCD2 interacts with the MMR proteins MSH2 and MLH1. FANCD2 monoubiquitylation, foci formation and chromatin loading are greatly diminished in MSH2-deficient cells. Human or mouse cells lacking MSH2 or MLH1 display increased sensitivity and radial formation in response to treatment with DNA crosslinking agents. Studies in human cell lines and Drosophila mutants suggest an epistatic relationship between FANCD2, MSH2 and MLH1 with regard to ICL repair. Surprisingly, the interaction between MSH2 and MLH1 is compromised in multiple FA cell lines, and FA cell lines exhibit deficient MMR. These results suggest a significant role for MMR proteins in the activation of the FA pathway and repair of ICLs. In addition, we provide the first evidence for a defect in MMR in FA cell lines.

Phosphorylation of fanconi anemia (FA) complementation group G protein, FANCG, at serine 7 is important for function of the FA pathway

Fanconi anemia (FA) is an autosomal recessive disease of cancer susceptibility. FA cells exhibit a characteristic hypersensitivity to DNA cross-linking agents. The molecular mechanism for the disease is unknown as few of the FA proteins have functional motifs. Several post-translational modifications of the proteins have been described. We and others have reported that the FANCG protein (Fanconi complementation group G) is phosphorylated. We show that in an in vitro kinase reaction FANCG is radioactively labeled. Mass spectrometry analysis detected a peptide containing phosphorylation of serine 7. Using PCR-mediated site-directed mutagenesis we mutated serine 7 to alanine. Only wild-type FANCG cDNA fully corrected FA-G mutant cells. We also tested the effect of human wild-type FANCG in Chinese hamster ovary cells in which the FANCG homologue is mutant. Human FANCG complemented these cells, whereas human FANCG(S7A) did not. Unexpectedly, FANCG(S7A) bound to and stabilized the endogenous forms of the FANCA and FANCC proteins in the FA-G cells. FANCG(S7A) aberrantly localized to globules in chromatin and did not abrogate the internuclear bridges seen in the FA-G mutant cells. Phosphorylation of serine 7 in FANCG is functionally important in the FA pathway.

A novel role for non-ubiquitinated FANCD2 in response to hydroxyurea-induced DNA damage

Fanconi anemia (FA) is a genetic disease of bone marrow failure, cancer susceptibility, and sensitivity to DNA crosslinking agents. FANCD2, the central protein of the FA pathway, is monoubiquitinated upon DNA damage, such as crosslinkers and replication blockers such as hydroxyurea (HU). Even though FA cells demonstrate unequivocal sensitivity to crosslinkers, such as mitomycin C (MMC), we find that they are largely resistant to HU, except for cells absent for expression of FANCD2. FANCD2, RAD51 and RAD18 form a complex, which is enhanced upon HU exposure. Surprisingly, although FANCD2 is required for this enhanced interaction, its monoubiquitination is not. Similarly, non-ubiquitinated FANCD2 can still support proliferation cell nuclear antigen (PCNA) monoubiquitination. RAD51, but not BRCA2, is also required for PCNA monoubiquitination in response to HU, suggesting that this function is independent of homologous recombination (HR). We further show that translesion (TLS) polymerase PolH chromatin localization is decreased in FANCD2 deficient cells, FANCD2 siRNA knockdown cells and RAD51 siRNA knockdown cells, and PolH knockdown results in HU sensitivity only. Our data suggest that FANCD2 and RAD51 have an important role in PCNA monoubiquitination and TLS in a FANCD2 monoubiquitination and HR-independent manner in response to HU. This effect is not observed with MMC treatment, suggesting a non-canonical function for the FA pathway in response to different types of DNA damage.

Tetratricopeptide-motif-mediated interaction of FANCG with recombination proteins XRCC3 and BRCA2

Fanconi anaemia is an inherited chromosomal instability disorder characterised by cellular sensitivity to DNA interstrand crosslinkers, bone-marrow failure and a high risk of cancer. Eleven FA genes have been identified, one of which, FANCD1, is the breast cancer susceptibility gene BRCA2. At least eight FA proteins form a nuclear core complex required for monoubiquitination of FANCD2. The BRCA2/FANCD1 protein is connected to the FA pathway by interactions with the FANCG and FANCD2 proteins, both of which co-localise with the RAD51 recombinase, which is regulated by BRCA2. These connections raise the question of whether any of the FANC proteins of the core complex might also participate in other complexes involved in homologous recombination repair. We therefore tested known FA proteins for direct interaction with RAD51 and its paralogs XRCC2 and XRCC3. FANCG was found to interact with XRCC3, and this interaction was disrupted by the FA-G patient derived mutation L71P. FANCG was co-immunoprecipitated with both XRCC3 and BRCA2 from extracts of human and hamster cells. The FANCG-XRCC3 and FANCG-BRCA2 interactions did not require the presence of other FA proteins from the core complex, suggesting that FANCG also participates in a DNA repair complex that is downstream and independent of FANCD2 monoubiquitination. Additionally, XRCC3 and BRCA2 proteins co-precipitate in both human and hamster cells and this interaction requires FANCG. The FANCG protein contains multiple tetratricopeptide repeat motifs (TPRs), which function as scaffolds to mediate protein-protein interactions. Mutation of one or more of these motifs disrupted all of the known interactions of FANCG. We propose that FANCG, in addition to stabilising the FA core complex, may have a role in building multiprotein complexes that facilitate homologous recombination repair.

The inherited bone marrow failure syndromes

Molecular pathogenesis may be elucidated for inherited bone marrow failure syndromes (IBMFS). The study and presentation of the details of their molecular biology and biochemistry is warranted for appropriate diagnosis and management of afflicted patients and to identify the physiology of the normal hematopoiesis and mechanisms of carcinogenesis. Several themes have emerged within each subsection of IBMFS, including the ribosomopathies, which include ribosome assembly and ribosomal RNA processing. The Fanconi anemia pathway has become interdigitated with the familial breast cancer syndromes. In this article, the diseases that account for most IBMFS diagnoses are analyzed.

FANCG is phosphorylated at serines 383 and 387 during mitosis

Fanconi anemia (FA) is an autosomal recessive disease marked by congenital defects, bone marrow failure, and high incidence of leukemia and solid tumors. Eight genes have been cloned, with the accompanying protein products participating in at least two complexes, which appear to be functionally dependent upon one another. Previous studies have described chromatin localization of the FA core complex, except at mitosis, which is associated with phosphorylation of the FANCG protein (F. Qiao, A. Moss, and G. M. Kupfer, J. Biol. Chem. 276:23391-23396, 2001). The phosphorylation of FANCG at serine 7 by using mass spectrometry was previously mapped. The purpose of this study was to map the phosphorylation sites of FANCG at mitosis and to assess their functional importance. Reasoning that a potential kinase might be cdc2, which was previously reported to bind to FANCC, we showed that cdc2 chiefly phosphorylated a 14-kDa fragment of the C-terminal half of FANCG. Mass spectrometry analysis demonstrated that this fragment contains amino acids 374 to 504. Kinase motif analysis demonstrated that three amino acids in this fragment were leading candidates for phosphorylation. By using PCR-directed in vitro mutagenesis we mutated S383, S387, and T487 to alanine. Mutation of S383 and S387 abolished the phosphorylation of FANCG at mitosis. These results were confirmed by use of phosphospecific antibodies directed against phosphoserine 383 and phosphoserine 387. Furthermore, the ability to correct FA-G mutant cells of human or hamster (where S383 and S387 are conserved) origin was also impaired by these mutations, demonstrating the functional importance of these amino acids. S387A mutant abolished FANCG fusion protein phosphorylation by cdc2. The FA pathway, of which FANCG is a part, is highly regulated by a series of phosphorylation steps that are important to its overall function.