The phenotype of FancB-mutant mouse embryonic stem cells

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.

Trex2 responds to damaged replication forks in diverse ways

Three prime Repair Exonuclease 2 (Trex2) alters replication fork (RF) stability and mutation levels in cells defective for homologous recombination (HR). Trex2 has multiple functions that can either cause or supress RF instability in cells with different HR-defects. Why does Trex2 have such diverse effects on RF maintenance?

TREX2 Exonuclease Causes Spontaneous Mutations and Stress-Induced Replication Fork Defects in Cells Expressing RAD51K133A

DNA damage tolerance (DDT) and homologous recombination (HR) stabilize replication forks (RFs). RAD18/UBC13/three prime repair exonuclease 2 (TREX2)-mediated proliferating cell nuclear antigen (PCNA) ubiquitination is central to DDT, an error-prone lesion bypass pathway. RAD51 is the recombinase for HR. The RAD51 K133A mutation increased spontaneous mutations and stress-induced RF stalls and nascent strand degradation. Here, we report in RAD51^K133A cells that this phenotype is reduced by expressing a TREX2 H188A mutation that deletes its exonuclease activity. In RAD51^K133A cells, knocking out RAD18 or overexpressing PCNA reduces spontaneous mutations, while expressing ubiquitination-incompetent PCNA^K164R increases mutations, indicating DDT as causal. Deleting TREX2 in cells deficient for the RF maintenance proteins poly(ADP-ribose) polymerase 1 (PARP1) or FANCB increased nascent strand degradation that was rescued by TREX2^H188A, implying that TREX2 prohibits degradation independent of catalytic activity. A possible explanation for this occurrence is that TREX2^H188A associates with UBC13 and ubiquitinates PCNA, suggesting a dual role for TREX2 in RF maintenance.

DNA repair fidelity in stem cell maintenance, health, and disease

Stem cells are a sub population of cell types that form the foundation of our body, and have the potential to replicate, replenish and repair limitlessly to maintain the tissue and organ homeostasis. Increased lifetime and frequent replication set them vulnerable for both exogenous and endogenous agents-induced DNA damage compared to normal cells. To counter these damages and preserve genetic information, stem cells have evolved with various DNA damage response and repair mechanisms. Furthermore, upon experiencing irreparable DNA damage, stem cells mostly prefer early senescence or apoptosis to avoid the accumulation of damages. However, the failure of these mechanisms leads to various diseases, including cancer. Especially, given the importance of stem cells in early development, DNA repair deficiency in stem cells leads to various disabilities like developmental delay, premature aging, sensitivity to DNA damaging agents, degenerative diseases, etc. In this review, we have summarized the recent update about how DNA repair mechanisms are regulated in stem cells and their association with disease progression and pathogenesis.

Selinexor reduces the expression of DNA damage repair proteins and sensitizes cancer cells to DNA damaging agents

Introduction

The goal of this study was to examine the effects of selinexor, an inhibitor of exportin-1 mediated nuclear export, on DNA damage repair and to evaluate the cytotoxic effects of selinexor in combination with DNA damaging agents (DDAs) in cancer cells.

Results

Selinexor reduced the expression of DNA damage repair (DDR) proteins. This did not induce significant DNA damage in tested cell lines. Inhibition of DDR protein expression resulted in enhanced cancer cell death when cells were pretreated with DDAs. In contrast, enhanced cell death was not detected in cells that were pretreated with selinexor then with DDAs. In vivo, single-agent selinexor, docetaxel, or cisplatin treatment resulted in 66.7%, 51.5%, and 26.6% tumor growth inhibition (TGI), respectively, in an MDA-MB-231 xenograft model. Consequently, combination treatment with docetaxel or cisplatin followed by selinexor in vivo resulted in 93.9% and 103.4% TGI, respectively. Immunohistochemical staining and immunoblot analysis of tumor sections confirmed reduced expression of DDR proteins.

Conclusion

Selinexor treatment inhibited DDR mechanisms in cancer cell lines and therefore potentiated DNA damage-based therapy. The sequential combination of DDAs followed by selinexor increased cancer cell death. This combination is superior to each individual therapy and has a mechanistic rationale as a novel anticancer strategy.

Methods

Cancer cells treated with selinexor ± DDAs were analyzed using reverse phase protein arrays, immunoblots, quantitative PCR and immunofluorescence. Mice bearing MDA-MB-231 tumors were treated with subtherapeutic doses of selinexor, cisplatin, docetaxel and selinexor in combination with either cisplatin or docetaxel. Tumor growth was evaluated for 25 days.

Parallel genome-wide screens identify synthetic viable interactions between the BLM helicase complex and Fanconi anemia

Maintenance of genome integrity via repair of DNA damage is a key biological process required to suppress diseases, including Fanconi anemia (FA). We generated loss-of-function human haploid cells for FA complementation group C (FANCC), a gene encoding a component of the FA core complex, and used genome-wide CRISPR libraries as well as insertional mutagenesis to identify synthetic viable (genetic suppressor) interactions for FA. Here we show that loss of the BLM helicase complex suppresses FANCC phenotypes and we confirm this interaction in cells deficient for FA complementation group I and D2 (FANCI and FANCD2) that function as part of the FA I-D2 complex, indicating that this interaction is not limited to the FA core complex, hence demonstrating that systematic genome-wide screening approaches can be used to reveal genetic viable interactions for DNA repair defects.

A mechanism for 1,4-Benzoquinone-induced genotoxicity

Benzene is a common environmental toxin and its metabolite, 1-4-Benzoquinone (BQ) causes hematopoietic cancers like myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). BQ has not been comprehensively assessed for its impact on genome maintenance, limiting our understanding of the true health risks associated with benzene exposure and our ability to identify people with increased sensitivity to this genotoxin. Here we analyze the impact BQ exposure has on wild type and DNA repair-defective mouse embryonic stem (ES) cells and wild type human cells. We find that double strand break (DSB) repair and replication fork maintenance pathways including homologous recombination (HR) and Fanconi anemia (FA) suppress BQ toxicity. BQ-induced damage efficiently stalls replication forks, yet poorly induces ATR/DNA-PKCS responses. Furthermore, the pattern of BQ-induced γH2AX and 53BP1foci is consistent with the formation of poly(ADP-ribose) polymerase 1 (PARP1)-stabilized regressed replication forks. At a biochemical level, BQ inhibited topoisomerase 1 (topo1)-mediated DNA ligation and nicking in vitro; thus providing mechanism for the cellular phenotype. These data are consistent with a model that proposes BQ interferes with type I topoisomerase's ability to maintain replication fork restart and progression leading to chromosomal instability that has the potential to cause hematopoietic cancers like MDS and AML.

FANCB is essential in the male germline and regulates H3K9 methylation on the sex chromosomes during meiosis

Fanconi anemia (FA) is a recessive X-linked and autosomal genetic disease associated with bone marrow failure and increased cancer, as well as severe germline defects such as hypogonadism and germ cell depletion. Although deficiencies in FA factors are commonly associated with germ cell defects, it remains unknown whether the FA pathway is involved in unique epigenetic events in germ cells. In this study, we generated Fancb mutant mice, the first mouse model of X-linked FA, and identified a novel function of the FA pathway in epigenetic regulation during mammalian gametogenesis. Fancb mutant mice were infertile and exhibited primordial germ cell (PGC) defects during embryogenesis. Further, Fancb mutation resulted in the reduction of undifferentiated spermatogonia in spermatogenesis, suggesting that FANCB regulates the maintenance of undifferentiated spermatogonia. Additionally, based on functional studies, we dissected the pathway in which FANCB functions during meiosis. The localization of FANCB on sex chromosomes is dependent on MDC1, a binding partner of H2AX phosphorylated at serine 139 (γH2AX), which initiates chromosome-wide silencing. Also, FANCB is required for FANCD2 localization during meiosis, suggesting that the role of FANCB in the activation of the FA pathway is common to both meiosis and somatic DNA damage responses. H3K9me2, a silent epigenetic mark, was decreased on sex chromosomes, whereas H3K9me3 was increased on sex chromosomes in Fancb mutant spermatocytes. Taken together, these results indicate that FANCB functions at critical stages of germ cell development and reveal a novel function of the FA pathway in the regulation of H3K9 methylation in the germline.

RECQL5 and BLM exhibit divergent functions in cells defective for the Fanconi anemia pathway

Fanconi anemia (FA) patients exhibit bone marrow failure, developmental defects and cancer. The FA pathway maintains chromosomal stability in concert with replication fork maintenance and DNA double strand break (DSB) repair pathways including RAD51-mediated homologous recombination (HR). RAD51 is a recombinase that maintains replication forks and repairs DSBs, but also rearranges chromosomes. Two RecQ helicases, RECQL5 and Bloom syndrome mutated (BLM) suppress HR through nonredundant mechanisms. Here we test the impact deletion of RECQL5 and BLM has on mouse embryonic stem (ES) cells deleted for FANCB, a member of the FA core complex. We show that RECQL5, but not BLM, conferred resistance to mitomycin C (MMC, an interstrand crosslinker) and camptothecin (CPT, a type 1 topoisomerase inhibitor) in FANCB-defective cells. RECQL5 suppressed, while BLM caused, breaks and radials in FANCB-deleted cells exposed to CPT or MMC, respectively. RECQL5 protected the nascent replication strand from MRE11-mediated degradation and restarted stressed replication forks in a manner additive to FANCB. By contrast BLM restarted, but did not protect, replication forks in a manner epistatic to FANCB. RECQL5 also lowered RAD51 levels in FANCB-deleted cells at stressed replication sites implicating a rearrangement avoidance mechanism. Thus, RECQL5 and BLM impact FANCB-defective cells differently in response to replication stress with relevance to chemotherapeutic regimes.

Modelling Fanconi anemia pathogenesis and therapeutics using integration-free patient-derived iPSCs

Fanconi anaemia (FA) is a recessive disorder characterized by genomic instability, congenital abnormalities, cancer predisposition and bone marrow (BM) failure. However, the pathogenesis of FA is not fully understood partly due to the limitations of current disease models. Here, we derive integration free-induced pluripotent stem cells (iPSCs) from an FA patient without genetic complementation and report in situ gene correction in FA-iPSCs as well as the generation of isogenic FANCA-deficient human embryonic stem cell (ESC) lines. FA cellular phenotypes are recapitulated in iPSCs/ESCs and their adult stem/progenitor cell derivatives. By using isogenic pathogenic mutation-free controls as well as cellular and genomic tools, our model serves to facilitate the discovery of novel disease features. We validate our model as a drug-screening platform by identifying several compounds that improve hematopoietic differentiation of FA-iPSCs. These compounds are also able to rescue the hematopoietic phenotype of FA patient BM cells.

Defining a genotoxic profile with mouse embryonic stem cells

Many genotoxins are found in the environment from synthetic to natural, yet very few have been studied in depth. This means we fail to understand many molecules that damage DNA, we do not understand the type of damage they cause and the repair pathways required to correct their lesions. It is surprising so little is known about the vast majority of genotoxins since they have potential to cause disease from developmental defects to cancer to degenerative ailments. By contrast, some of these molecules have commercial and medical potential and some can be weaponized. Therefore, we need a systematic method to efficiently generate a genotoxic profile for these agents. A genotoxic profile would include the type of damage the genotoxin causes, the pathways used to repair the damage and the resultant mutations if repair fails. Mouse embryonic stem (ES) cells are well suited for identifying pathways and mutations. Mouse ES cells are genetically tractable and many DNA repair mutant cells are available. ES cells have a high mitotic index and form colonies so experiments can be completed quickly and easily. Furthermore, ES cells have robust DNA repair pathways to minimize genetic mutations at a particularly vulnerable time in life, early development when a mutation in a single cell could ultimately contribute to a large fraction of the individual. After an initial screen, other types of cells and mouse models can be used to complement the analysis. This review discusses the merging field of genotoxic screens in mouse ES cells that can be used to discover and study potential genotoxic activity for chemicals commonly found in our environment.

RAD51 mutants cause replication defects and chromosomal instability

RAD51 is important for restarting stalled replication forks and for repairing DNA double-strand breaks (DSBs) through a pathway called homology-directed repair (HDR). However, analysis of the consequences of specific RAD51 mutants has been difficult since they are toxic. Here we report on the dominant effects of two human RAD51 mutants defective for ATP binding (K133A) or ATP hydrolysis (K133R) expressed in mouse embryonic stem (ES) cells that also expressed normal mouse RAD51 from the other chromosome. These cells were defective for restarting stalled replication forks and repairing breaks. They were also hypersensitive to camptothecin, a genotoxin that generates breaks specifically at the replication fork. In addition, these cells exhibited a wide range of structural chromosomal changes that included multiple breakpoints within the same chromosome. Thus, ATP binding and hydrolysis are essential for chromosomal maintenance. Fusion of RAD51 to a fluorescent tag (enhanced green fluorescent protein [eGFP]) allowed visualization of these proteins at sites of replication and repair. We found very low levels of mutant protein present at these sites compared to normal protein, suggesting that low levels of mutant protein were sufficient for disruption of RAD51 activity and generation of chromosomal rearrangements.

Trex2 enables spontaneous sister chromatid exchanges without facilitating DNA double-strand break repair

Trex2 is a 3' → 5' exonuclease that removes 3'-mismatched sequences in a biochemical assay; however, its biological function remains unclear. To address biology we previously generated trex2(null) mouse embryonic stem (ES) cells and expressed in these cells wild-type human TREX2 cDNA (Trex2(hTX2)) or cDNA with a single-amino-acid change in the catalytic domain (Trex2(H188A)) or in the DNA-binding domain (Trex2(R167A)). We found the trex2(null) and Trex2(H188A) cells exhibited spontaneous broken chromosomes and trex2(null) cells exhibited spontaneous chromosomal rearrangements. We also found ectopically expressed human TREX2 was active at the 3' ends of I-SceI-induced chromosomal double-strand breaks (DSBs). Therefore, we hypothesized Trex2 participates in DNA DSB repair by modifying 3' ends. This may be especially important for ends with damaged nucleotides. Here we present data that are unexpected and prompt a new model. We found Trex2-altered cells (null, H188A, and R167A) were not hypersensitive to camptothecin, a type-1 topoisomerase inhibitor that induces DSBs at replication forks. In addition, Trex2-altered cells were not hypersensitive to γ-radiation, an agent that causes DSBs throughout the cell cycle. This observation held true even in cells compromised for one of the two major DSB repair pathways: homology-directed repair (HDR) or nonhomologous end joining (NHEJ). Trex2 deletion also enhanced repair of an I-SceI-induced DSB by both HDR and NHEJ without affecting pathway choice. Interestingly, however, trex2(null) cells exhibited reduced spontaneous sister chromatid exchanges (SCEs) but this was not due to a defect in HDR-mediated crossing over. Therefore, reduced spontaneous SCE could be a manifestation of the same defect that caused spontaneous broken chromosomes and spontaneous chromosomal rearrangements. These unexpected data suggest Trex2 does not enable DSB repair and prompt a new model that posits Trex2 suppresses the formation of broken chromosomes.