Filia Is an ESC-Specific Regulator of DNA Damage Response and Safeguards Genomic Stability

Combinatorial quantification of 5mC and 5hmC at individual CpG dyads and the transcriptome in single cells reveals modulators of DNA methylation maintenance fidelity

Inheritance of 5-methylcytosine from one cell generation to the next by DNA methyltransferase 1 (DNMT1) plays a key role in regulating cellular identity. While recent work has shown that the activity of DNMT1 is imprecise, it remains unclear how the fidelity of DNMT1 is tuned in different genomic and cell state contexts. Here we describe Dyad-seq, a method to quantify the genome-wide methylation status of cytosines at the resolution of individual CpG dinucleotides to find that the fidelity of DNMT1-mediated maintenance methylation is related to the local density of DNA methylation and the landscape of histone modifications. To gain deeper insights into methylation/demethylation turnover dynamics, we first extended Dyad-seq to quantify all combinations of 5-methylcytosine and 5-hydroxymethylcytosine at individual CpG dyads. Next, to understand how cell state transitions impact maintenance methylation, we scaled the method down to jointly profile genome-wide methylation levels, maintenance methylation fidelity and the transcriptome from single cells (scDyad&T-seq). Using scDyad&T-seq, we demonstrate that, while distinct cell states can substantially impact the activity of the maintenance methylation machinery, locally there exists an intrinsic relationship between DNA methylation density, histone modifications and DNMT1-mediated maintenance methylation fidelity that is independent of cell state.

Structural basis of the subcortical maternal complex and its implications in reproductive disorders

The subcortical maternal complex (SCMC) plays a crucial role in early embryonic development. Malfunction of SCMC leads to reproductive diseases in women. However, the molecular function and assembly basis for SCMC remain elusive. Here we reconstituted mouse SCMC and solved the structure at atomic resolution using single-particle cryo-electron microscopy. The core complex of SCMC was formed by MATER, TLE6 and FLOPED, and MATER embraced TLE6 and FLOPED via its NACHT and LRR domains. Two core complexes further dimerize through interactions between two LRR domains of MATERs in vitro. FILIA integrates into SCMC by interacting with the carboxyl-terminal region of FLOPED. Zygotes from mice with Floped C-terminus truncation showed delayed development and resembled the phenotype of zygotes from Filia knockout mice. More importantly, the assembly of mouse SCMC was affected by corresponding clinical variants associated with female reproductive diseases and corresponded with a prediction based on the mouse SCMC structure. Our study paves the way for further investigations on SCMC functions during mammalian preimplantation embryonic development and reveals underlying causes of female reproductive diseases related to SCMC mutations, providing a new strategy for the diagnosis of female reproductive disorders.

Characterization of long-term ex vivo expansion of tree shrew spermatogonial stem cells

Tree shrews ( Tupaia belangeri chinensis) share a close relationship to primates and have been widely used in biomedical research. We previously established a spermatogonial stem cell (SSC)-based gene editing platform to generate transgenic tree shrews. However, the influences of long-term expansion on tree shrew SSC spermatogenesis potential remain unclear. Here, we examined the in vivo spermatogenesis potential of tree shrew SSCs cultured across different passages. We found that SSCs lost spermatogenesis ability after long-term expansion (>50 passages), as indicated by the failure to colonize the seminiferous epithelium and generate donor spermatogonia (SPG)-derived spermatocytes or spermatids marking spermatogenesis. RNA sequencing (RNA-seq) analysis of undifferentiated SPGs across different passages revealed significant gene expression changes after sub-culturing primary SPG lines for more than 40 passages on feeder layers. Specifically, DNA damage response and repair genes (e.g., MRE11, SMC3, BLM, and GEN1) were down-regulated, whereas genes associated with mitochondrial function (e.g., NDUFA9, NDUFA8, NDUFA13, and NDUFB8) were up-regulated after expansion. The DNA damage accumulation and mitochondrial dysfunction were experimentally validated in high-passage cells. Supplementation with nicotinamide adenine dinucleotide (NAD +) precursor nicotinamide riboside (NR) exhibited beneficial effects by reducing DNA damage accumulation and mitochondrial dysfunction in SPG elicited by long-term culture. Our research presents a comprehensive analysis of the genetic and physiological attributes critical for the sustained expansion of undifferentiated SSCs in tree shrews and proposes an effective strategy for extended in vitro maintenance.

G1 Dynamics at the Crossroads of Pluripotency and Cancer

G1 cell cycle phase dynamics are regulated by intricate networks involving cyclins, cyclin-dependent kinases (CDKs), and CDK inhibitors, which control G1 progression and ensure proper cell cycle transitions. Moreover, adequate origin licensing in G1 phase, the first committed step of DNA replication in the subsequent S phase, is essential to maintain genome integrity. In this review, we highlight the intriguing parallels and disparities in G1 dynamics between stem cells and cancer cells, focusing on their regulatory mechanisms and functional outcomes. Notably, SOX2, OCT4, KLF4, and the pluripotency reprogramming facilitator c-MYC, known for their role in establishing and maintaining stem cell pluripotency, are also aberrantly expressed in certain cancer cells. In this review, we discuss recent advances in understanding the regulatory role of these pluripotency factors in G1 dynamics in the context of stem cells and cancer cells, which may offer new insights into the interconnections between pluripotency and tumorigenesis.

DPPA5A suppresses the mutagenic TLS and MMEJ pathways by modulating the cryptic splicing of Rev1 and Polq in mouse embryonic stem cells

Genetic alterations are often acquired during prolonged propagation of pluripotent stem cells (PSCs). This ruins the stem cell quality and hampers their full applications. Understanding how PSCs maintain genomic integrity would provide the clues to overcome the hurdle. It has been known that embryonic stem cells (ESCs) utilize high-fidelity pathways to ensure genomic stability, but the underlying mechanisms remain largely elusive. Here, we show that many DNA damage response and repair genes display differential alternative splicing in mouse ESCs compared to differentiated cells. Particularly, Rev1 and Polq, two key genes for mutagenic translesion DNA synthesis (TLS) and microhomology-mediated end joining (MMEJ) repair pathways, respectively, display a significantly higher rate of cryptic exon (CE) inclusion in ESCs. The frequent CE inclusion disrupts the normal protein expressions of REV1 and POLθ, thereby suppressing the mutagenic TLS and MMEJ. Further, we identify an ESC-specific RNA binding protein DPPA5A which stimulates the CE inclusion in Rev1 and Polq. Depletion of DPPA5A in mouse ESCs decreased the CE inclusion of Rev1 and Polq, induced the protein expression, and stimulated the TLS and MMEJ activity. Enforced expression of DPPA5A in NIH3T3 cells displayed reverse effects. Mechanistically, we found that DPPA5A directly regulated CE splicing of Rev1. DPPA5A associates with U2 small nuclear ribonucleoprotein of the spliceosome and binds to the GA-rich motif in the CE of Rev1 to promote CE inclusion. Thus, our study uncovers a mechanism to suppress mutagenic TLS and MMEJ pathways in ESCs.

Nuclear DJ-1 Regulates DNA Damage Repair via the Regulation of PARP1 Activity

DNA damage and defective DNA repair are extensively linked to neurodegeneration in Parkinson's disease (PD), but the underlying molecular mechanisms remain poorly understood. Here, we determined that the PD-associated protein DJ-1 plays an essential role in modulating DNA double-strand break (DSB) repair. Specifically, DJ-1 is a DNA damage response (DDR) protein that can be recruited to DNA damage sites, where it promotes DSB repair through both homologous recombination and nonhomologous end joining. Mechanistically, DJ-1 interacts directly with PARP1, a nuclear enzyme essential for genomic stability, and stimulates its enzymatic activity during DNA repair. Importantly, cells from PD patients with the DJ-1 mutation also have defective PARP1 activity and impaired repair of DSBs. In summary, our findings uncover a novel function of nuclear DJ-1 in DNA repair and genome stability maintenance, and suggest that defective DNA repair may contribute to the pathogenesis of PD linked to DJ-1 mutations.

Combinatorial quantification of 5mC and 5hmC at individual CpG dyads and the transcriptome in single cells reveals modulators of DNA methylation maintenance fidelity

Transmission of 5-methylcytosine (5mC) from one cell generation to the next plays a key role in regulating cellular identity in mammalian development and diseases. While recent work has shown that the activity of DNMT1, the protein responsible for the stable inheritance of 5mC from mother to daughter cells, is imprecise; it remains unclear how the fidelity of DNMT1 is tuned in different genomic and cell state contexts. Here we describe Dyad-seq, a method that combines enzymatic detection of modified cytosines with nucleobase conversion techniques to quantify the genome-wide methylation status of cytosines at the resolution of individual CpG dinucleotides. We find that the fidelity of DNMT1-mediated maintenance methylation is directly related to the local density of DNA methylation, and for genomic regions that are lowly methylated, histone modifications can dramatically alter the maintenance methylation activity. Further, to gain deeper insights into the methylation and demethylation turnover dynamics, we extended Dyad-seq to quantify all combinations of 5mC and 5-hydroxymethylcytosine (5hmC) at individual CpG dyads to show that TET proteins preferentially hydroxymethylate only one of the two 5mC sites in a symmetrically methylated CpG dyad rather than sequentially convert both 5mC to 5hmC. To understand how cell state transitions impact DNMT1-mediated maintenance methylation, we scaled the method down and combined it with the measurement of mRNA to simultaneously quantify genome-wide methylation levels, maintenance methylation fidelity and the transcriptome from the same cell (scDyad&T-seq). Applying scDyad&T-seq to mouse embryonic stem cells transitioning from serum to 2i conditions, we observe dramatic and heterogenous demethylation and the emergence of transcriptionally distinct subpopulations that are closely linked to the cell-to-cell variability in loss of DNMT1-mediated maintenance methylation activity, with regions of the genome that escape 5mC reprogramming retaining high levels of maintenance methylation fidelity. Overall, our results demonstrate that while distinct cell states can substantially impact the genome-wide activity of the DNA methylation maintenance machinery, locally there exists an intrinsic relationship between DNA methylation density, histone modifications and DNMT1-mediated maintenance methylation fidelity that is independent of cell state.

Long noncoding RNA Lnc530 localizes on R-loops and regulates R-loop formation and genomic stability in mouse embryonic stem cells

Embryonic stem cells (ESCs) are superior to differentiated cells to maintain genome stability, but the underlying mechanisms remain largely elusive. R-loops are constantly formed during transcription and are inducers of DNA damage if not resolved. Here we report that mouse ESCs (mESCs) can efficiently prevent unscheduled R-loop formation, and a long noncoding RNA Lnc530 plays regulatory role. Lnc530 is expressed in mESCs and localizes on R-loops. Depletion of Lnc530 in mESCs causes R-loop accumulation and DNA damage, whereas forced expression of Lnc530 in differentiated cells suppresses the R-loop formation. Mechanistically, Lnc530 associates with DDX5 and TDP-43 in an inter-dependent manner on R-loops. Formation of Lnc530-DDX5-TDP-43 complex substantially increases the local protein levels of DDX5 and TDP-43, both of which play critical roles in R-loop regulation. This study uncovers an efficient strategy to prevent R-loop accumulation and preserve genomic stability in mESCs and possibly other stem cell types.

PARP-1 Expression Influences Cancer Stem Cell Phenotype in Colorectal Cancer Depending on p53

Poly(ADP-ribose) polymerase-1 (PARP-1) is a protein involved in multiple physiological processes. Elevated PARP-1 expression has been found in several tumours, being associated with stemness and tumorigenesis. In colorectal cancer (CRC), some controversy among studies has been described. In this study, we analysed the expression of PARP-1 and cancer stem cell (CSC) markers in CRC patients with different p53 status. In addition, we used an in vitro model to evaluate the influence of PARP-1 in CSC phenotype regarding p53. In CRC patients, PARP-1 expression correlated with the differentiation grade, but this association was only maintained for tumours harbouring wild-type p53. Additionally, in those tumours, PARP-1 and CSC markers were positively correlated. In mutated p53 tumours, no associations were found, but PARP-1 was an independent factor for survival. According to our in vitro model, PARP-1 regulates CSC phenotype depending on p53 status. PARP-1 overexpression in a wild type p53 context increases CSC markers and sphere forming ability. By contrast, those features were reduced in mutated p53 cells. These results could implicate that patients with elevated PARP-1 expression and wild type p53 could benefit from PARP-1 inhibition therapies, meanwhile it could have adverse effects for those carrying mutated p53 tumours.

Specialized Circuitry of Embryonic Stem Cells Promotes Genomic Integrity

Embryonic stem cells (ESCs) give rise to all cell types of the organism. Given the importance of these cells in this process, ESCs must employ robust mechanisms to protect genomic integrity or risk catastrophic propagation of mutations throughout the organism. Should such an event occur in daughter cells that will eventually contribute to the germline, the overall species health could dramatically decline. This review describes several key mechanisms employed by ESCs that are unique to these cells, in order to maintain their genomic integrity. Additionally, the contributions of cell cycle regulators in modulating ESC differentiation, after DNA damage exposure, are also examined. Where data are available, findings reported in ESCs are extended to include observations described in induced pluripotent stem cells (IPSCs).

Lnc956-TRIM28-HSP90B1 complex on replication forks promotes CMG helicase retention to ensure stem cell genomic stability and embryogenesis

Replication stress is a major source of endogenous DNA damage. Despite the identification of numerous proteins on replication forks to modulate fork or replication machinery activities, it remains unexplored whether noncoding RNAs can localize on stalled forks and play critical regulatory roles. Here, we identify an uncharacterized long noncoding RNA NONMMUT028956 (Lnc956 for short) predominantly expressed in mouse embryonic stem cells. Lnc956 is accumulated on replication forks to prevent fork collapse and preserve genomic stability and is essential for mouse embryogenesis. Mechanistically, it drives assembly of the Lnc956-TRIM28-HSP90B1 complex on stalled forks in an interdependent manner downstream of ataxia telangiectasia and Rad3-related (ATR) signaling. Lnc956-TRIM28-HSP90B1 complex physically associates with minichromosome maintenance proteins 2 (MCM2) to minichromosome maintenance proteins 7 (MCM7) hexamer via TRIM28 and directly regulates the CDC45-MCM-GINS (CMG) helicase retention on chromatin. The regulation of Lnc956-TRIM28-HSP90B1 on CMG retention is mediated by HSP90B1's chaperoning function. These findings reveal a player that actively regulates replisome retention to prevent fork collapse.

Lnc956 regulates mouse embryonic stem cell differentiation in response to DNA damage in a p53-independent pathway

Maintaining genomic stability is crucial for embryonic stem cells (ESCs). ESCs with unrepaired DNA damage are eliminated through differentiation and apoptosis. To date, only tumor suppressor p53 is known to be implicated in this quality control process. Here, we identified a p53-independent quality control factor lncRNA NONMMUT028956 (Lnc956 for short) in mouse ESCs. Lnc956 is prevalently expressed in ESCs and regulates the differentiation of ESCs after DNA damage. Mechanistically, Ataxia telangiectasia mutated (ATM) activation drives m6A methylation of Lnc956, which promotes its interaction with Krüppel-like factor 4 (KLF4). Lnc956-KLF4 association sequestrates the KLF4 protein and prevents KLF4's transcriptional regulation on pluripotency. This posttranslational mechanism favors the rapid shutdown of the regulatory circuitry of pluripotency. Thus, ATM signaling in ESCs can activate two pathways mediated by p53 and Lnc956, respectively, which act together to ensure robust differentiation and apoptosis in response to unrepaired DNA damage.

BNIP3-dependent mitophagy safeguards ESC genomic integrity via preventing oxidative stress-induced DNA damage and protecting homologous recombination

Embryonic stem cells (ESCs) have a significantly lower mutation load compared to somatic cells, but the mechanisms that guard genomic integrity in ESCs remain largely unknown. Here we show that BNIP3-dependent mitophagy protects genomic integrity in mouse ESCs. Deletion of Bnip3 increases cellular reactive oxygen species (ROS) and decreases ATP generation. Increased ROS in Bnip3^-/- ESCs compromised self-renewal and were partially rescued by either NAC treatment or p53 depletion. The decreased cellular ATP in Bnip3^-/- ESCs induced AMPK activation and deteriorated homologous recombination, leading to elevated mutation load during long-term propagation. Whereas activation of AMPK in X-ray-treated Bnip3^+/+ ESCs dramatically ascended mutation rates, inactivation of AMPK in Bnip3^-/- ESCs under X-ray stress remarkably decreased the mutation load. In addition, enhancement of BNIP3-dependent mitophagy during reprogramming markedly decreased mutation accumulation in established iPSCs. In conclusion, we demonstrated a novel pathway in which BNIP3-dependent mitophagy safeguards ESC genomic stability, and that could potentially be targeted to improve pluripotent stem cell genomic integrity for regenerative medicine.

Acute deletion of TET enzymes results in aneuploidy in mouse embryonic stem cells through decreased expression of Khdc3

TET (Ten-Eleven Translocation) dioxygenases effect DNA demethylation through successive oxidation of the methyl group of 5-methylcytosine (5mC) in DNA. In humans and in mouse models, TET loss-of-function has been linked to DNA damage, genome instability and oncogenesis. Here we show that acute deletion of all three Tet genes, after brief exposure of triple-floxed, Cre-ERT2-expressing mouse embryonic stem cells (mESC) to 4-hydroxytamoxifen, results in chromosome mis-segregation and aneuploidy; moreover, embryos lacking all three TET proteins showed striking variation in blastomere numbers and nuclear morphology at the 8-cell stage. Transcriptional profiling revealed that mRNA encoding a KH-domain protein, Khdc3 (Filia), was downregulated in triple TET-deficient mESC, concomitantly with increased methylation of CpG dinucleotides in the vicinity of the Khdc3 gene. Restoring KHDC3 levels in triple Tet-deficient mESC prevented aneuploidy. Thus, TET proteins regulate Khdc3 gene expression, and TET deficiency results in mitotic infidelity and genome instability in mESC at least partly through decreased expression of KHDC3.

RETSAT associates with DDX39B to promote fork restarting and resistance to gemcitabine based chemotherapy in pancreatic ductal adenocarcinoma

Background

Severe hypoxia is a prominent character of pancreatic ductal adenocarcinoma (PDAC) microenvironment. In the process of gemcitabine based chemotherapy, PDAC cells are insulted from replication stresses co-induced by hypoxia and gemcitabine. However, PDAC cells get outstanding abilities to resist to such harsh conditions and keep proliferating, causing a major obstacle for current therapy. RETSAT (Retinol Saturase) is defined as a hypoxia convergent gene recently, with high expression in PDAC hypoxic sectors. This study aimed to explore the roles of RETSAT in replication stress resistance and hypoxia adaptation in PDAC cells, and decipher the underlying mechanism.

Methods

The expression of RETSAT was examined in TCGA (The Cancer Genome Atlas), human pancreatic cancer microarray, clinical specimens and cell lines. Functions of RETSAT were studied by means of DNA fiber assay and comet assay in monolayer cultured PDAC cell lines, three dimensional spheroids, patient derived organoids and cell derived xenograft mouse models. Mechanism was investigated by using iPOND (isolate proteins on nascent DNA) combined with mass spectrometry, immunoprecipitation and immunoblotting.

Results

First, we found the converse relationship of RETSAT expression and PDAC chemotherapy. That is, PDAC patients with high RETSAT expression correlated with poor survival, while ones holding low RETSAT expression were benefitted more in Gemcitabine based chemotherapy. Second, we identified RETSAT as a novel replication fork associated protein. HIF-1α signaling promotes RETSAT expression under hypoxia. Functionally, RETSAT promoted fork restarting under replication stress and maintained genomic stability. Third, we uncovered the interaction of RETSAT and R-loop unwinding helicase DDX39B. RETSAT detained DDX39B on forks to resolve R-loops, through which avoided fork damage and CHK1 initiated apoptosis. Targeting DDX39B using chemical CCT018159 sensitized PDAC cells and organoids to gemcitabine induced apoptosis, highlighting the synergetic application of CCT018159 and gemcitabine in PDAC chemotherapy.

Conclusions

This study identified RETSAT as a novel replication fork protein, which functions through interacting with DDX39B mediated R-loop clearance to promote fork restarting, leading to cellular resistance to replication stresses co-induced by tumor environmental hypoxia and gemcitabine in pancreatic ductal adenocarcinoma.

Current understanding of genomic stability maintenancein pluripotent stem cells

Pluripotent stem cells (PSCs) are able to generate all cell types in the body and have wide applications in basic research and cell-based regenerative medicine. Maintaining stable genome in culture is the first priority for stem cell application in clinics. In addition, genomic instability in PSCs can cause developmental failure or abnormalities. Understanding how PSCs maintain genome stability is of critical importance. Due to their fundamental role in organism development, PSCs must maintain superior stable genome than differentiated cells. However, the underlying mechanisms are far from clear. Very limited studies suggest that PSCs utilize specific strategies and regulators to robustly improve genome stability. In this review, we summarize the current understandings of the unique properties of genome stability maintenance in PSCs.

A novel lncRNA Discn fine-tunes replication protein A (RPA) availability to promote genomic stability

RPA is a master regulator of DNA metabolism and RPA availability acts as a rate-limiting factor. While numerous studies focused on the post-translational regulations of RPA for its functions, little is known regarding how RPA availability is controlled. Here we identify a novel lncRNA Discn as the guardian of RPA availability in stem cells. Discn is induced upon genotoxic stress and binds to neucleolin (NCL) in the nucleolus. This prevents NCL from translocation into nucleoplasm and avoids undesirable NCL-mediated RPA sequestration. Thus, Discn-NCL-RPA pathway preserves a sufficient RPA pool for DNA replication stress response and repair. Discn loss causes massive genome instability in mouse embryonic stem cells and neural stem/progenigor cells. Mice depleted of Discn display newborn death and brain dysfunctions due to DNA damage accumulation and associated inflammatory reactions. Our findings uncover a key regulator of DNA metabolism and provide new clue to understand the chemoresistance in cancer treatment.

A Tale of Two States: Pluripotency Regulation of Telomeres

Inside the nucleus, chromatin is functionally organized and maintained as a complex three-dimensional network of structures with different accessibility such as compartments, lamina associated domains, and membraneless bodies. Chromatin is epigenetically and transcriptionally regulated by an intricate and dynamic interplay of molecular processes to ensure genome stability. Phase separation, a process that involves the spontaneous organization of a solution into separate phases, has been proposed as a mechanism for the timely coordination of several cellular processes, including replication, transcription and DNA repair. Telomeres, the repetitive structures at the end of chromosomes, are epigenetically maintained in a repressed heterochromatic state that prevents their recognition as double-strand breaks (DSB), avoiding DNA damage repair and ensuring cell proliferation. In pluripotent embryonic stem cells, telomeres adopt a non-canonical, relaxed epigenetic state, which is characterized by a low density of histone methylation and expression of telomere non-coding transcripts (TERRA). Intriguingly, this telomere non-canonical conformation is usually associated with chromosome instability and aneuploidy in somatic cells, raising the question of how genome stability is maintained in a pluripotent background. In this review, we will explore how emerging technological and conceptual developments in 3D genome architecture can provide novel mechanistic perspectives for the pluripotent epigenetic paradox at telomeres. In particular, as RNA drives the formation of LLPS, we will consider how pluripotency-associated high levels of TERRA could drive and coordinate phase separation of several nuclear processes to ensure genome stability. These conceptual advances will provide a better understanding of telomere regulation and genome stability within the highly dynamic pluripotent background.

Natural killer cells contributed to recurrent miscarriage by SP1-CASP3-PARP1

Recurrent miscarriage (RM) is an early pregnancy complication. Natural Killer cells are an important part of the innate immune system of endometrial. In this study, weighted gene correlation network analysis was used to study the expression profile data of the endometrial tissue of patients with recurrent miscarriage and selected brown module as key module positively related to the numbers of miscarriages. With metascape tool, natural killer cells mediated cytotoxicity related genes, such as CASP3, were selected. DisNor database showed that CASP3 down-regulates PARP1. According to TRRUST database, CASP3 was regulated by SP1. Through comprehensive analysis of uNK cell related genes, we proposed that natural killer cells contribute to recurrent miscarriage by SP1-CASP3-PARP1.

MYBL2 and ATM suppress replication stress in pluripotent stem cells

Replication stress, a major cause of genome instability in cycling cells, is mainly prevented by the ATR-dependent replication stress response pathway in somatic cells. However, the replication stress response pathway in embryonic stem cells (ESCs) may be different due to alterations in cell cycle phase length. The transcription factor MYBL2, which is implicated in cell cycle regulation, is expressed a hundred to a thousand-fold more in ESCs compared with somatic cells. Here we show that MYBL2 activates ATM and suppresses replication stress in ESCs. Consequently, loss of MYBL2 or inhibition of ATM or Mre11 in ESCs results in replication fork slowing, increased fork stalling and elevated origin firing. Additionally, we demonstrate that inhibition of CDC7 activity rescues replication stress induced by MYBL2 loss and ATM inhibition, suggesting that uncontrolled new origin firing may underlie the replication stress phenotype resulting from loss/inhibition of MYBL2 and ATM. Overall, our study proposes that in addition to ATR, a MYBL2-MRN-ATM replication stress response pathway functions in ESCs to control DNA replication initiation and prevent genome instability.

Genome integrity and neurogenesis of postnatal hippocampal neural stem/progenitor cells require a unique regulator Filia

Endogenous DNA double-strand breaks (DSBs) formation and repair in neural stem/progenitor cells (NSPCs) play fundamental roles in neurogenesis and neurodevelopmental disorders. NSPCs exhibit heterogeneity in terms of lineage fates and neurogenesis activity. Whether NSPCs also have heterogeneous regulations on DSB formation and repair to accommodate region-specific neurogenesis has not been explored. Here, we identified a regional regulator Filia, which is predominantly expressed in mouse hippocampal NSPCs after birth and regulates DNA DSB formation and repair. On one hand, Filia protects stalling replication forks and prevents the replication stress-associated DNA DSB formation. On the other hand, Filia facilitates the homologous recombination-mediated DNA DSB repair. Consequently, Filia^-/- mice had impaired hippocampal NSPC proliferation and neurogenesis and were deficient in learning, memory, and mood regulations. Thus, our study provided the first proof of concept demonstrating the region-specific regulations of DSB formation and repair in subtypes of NSPCs.

Mthfd2 Modulates Mitochondrial Function and DNA Repair to Maintain the Pluripotency of Mouse Stem Cells

The pluripotency of stem cells determines their developmental potential. While the pluripotency states of pluripotent stem cells are variable and interconvertible, the mechanisms underlying the acquisition and maintenance of pluripotency remain largely elusive. Here, we identified that methylenetetrahydrofolate dehydrogenase (NAD⁺-dependent), methenyltetrahydrofolate cyclohydrolase (Mthfd2) plays an essential role in maintaining embryonic stem cell pluripotency and promoting complete reprogramming of induced pluripotent stem cells. Mechanistically, in mitochondria, Mthfd2 maintains the integrity of the mitochondrial respiratory chain and prevents mitochondrial dysfunction. In the nucleus, Mthfd2 stabilizes the phosphorylation of EXO1 to support DNA end resection and promote homologous recombination repair. Our results revealed that Mthfd2 is a dual-function factor in determining the pluripotency of pluripotent stem cells through both mitochondrial and nuclear pathways, ultimately ensuring safe application of pluripotent stem cells.

A Survey of Essential Genome Stability Genes Reveals That Replication Stress Mitigation Is Critical for Peri-Implantation Embryogenesis

Murine development demands that pluripotent epiblast stem cells in the peri-implantation embryo increase from approximately 120 to 14,000 cells between embryonic days (E) 4.5 and E7.5. This is possible because epiblast stem cells can complete cell cycles in under 3 h in vivo. To ensure conceptus fitness, epiblast cells must undertake this proliferative feat while maintaining genome integrity. How epiblast cells maintain genome health under such an immense proliferation demand remains unclear. To illuminate the contribution of genome stability pathways to early mammalian development we systematically reviewed knockout mouse data from 347 DDR and repair associated genes. Cumulatively, the data indicate that while many DNA repair functions are dispensable in embryogenesis, genes encoding replication stress response and homology directed repair factors are essential specifically during the peri-implantation stage of early development. We discuss the significance of these findings in the context of the unique proliferative demands placed on pluripotent epiblast stem cells.

Disturbed genomic imprinting and its relevance for human reproduction: causes and clinical consequences

BACKGROUND

Human reproductive issues affecting fetal and maternal health are caused by numerous exogenous and endogenous factors, of which the latter undoubtedly include genetic changes. Pathogenic variants in either maternal or offspring DNA are associated with effects on the offspring including clinical disorders and nonviable outcomes. Conversely, both fetal and maternal factors can affect maternal health during pregnancy. Recently, it has become evident that mammalian reproduction is influenced by genomic imprinting, an epigenetic phenomenon that regulates the expression of genes according to their parent from whom they are inherited. About 1% of human genes are normally expressed from only the maternally or paternally inherited gene copy. Since numerous imprinted genes are involved in (embryonic) growth and development, disturbance of their balanced expression can adversely affect these processes.

OBJECTIVE AND RATIONALE

This review summarises current our understanding of genomic imprinting in relation to human ontogenesis and pregnancy and its relevance for reproductive medicine.

SEARCH METHODS

Literature databases (Pubmed, Medline) were thoroughly searched for the role of imprinting in human reproductive failure. In particular, the terms ‘multilocus imprinting disturbances, SCMC, NLRP/NALP, imprinting and reproduction’ were used in various combinations.

OUTCOMES

A range of molecular changes to specific groups of imprinted genes are associated with imprinting disorders, i.e. syndromes with recognisable clinical features including distinctive prenatal features. Whereas the majority of affected individuals exhibit alterations at single imprinted loci, some have multi-locus imprinting disturbances (MLID) with less predictable clinical features. Imprinting disturbances are also seen in some nonviable pregnancy outcomes, such as (recurrent) hydatidiform moles, which can therefore be regarded as a severe form of imprinting disorders. There is growing evidence that MLID can be caused by variants in the maternal genome altering the imprinting status of the oocyte and the embryo, i.e. maternal effect mutations. Pregnancies of women carrying maternal affect mutations can have different courses, ranging from miscarriages to birth of children with clinical features of various imprinting disorders.

WIDER IMPLICATIONS

Increasing understanding of imprinting disturbances and their clinical consequences have significant impacts on diagnostics, counselling and management in the context of human reproduction. Defining criteria for identifying pregnancies complicated by imprinting disorders facilitates early diagnosis and personalised management of both the mother and offspring. Identifying the molecular lesions underlying imprinting disturbances (e.g. maternal effect mutations) allows targeted counselling of the family and focused medical care in further pregnancies.

Cops5 safeguards genomic stability of embryonic stem cells through regulating cellular metabolism and DNA repair

The highly conserved COP9 signalosome (CSN), composed of 8 subunits (Cops1 to Cops8), has been implicated in pluripotency maintenance of human embryonic stem cells (ESCs). Yet, the mechanism for the CSN to regulate pluripotency remains elusive. We previously showed that Cops2, independent of the CSN, is essential for the pluripotency maintenance of mouse ESCs. In this study, we set out to investigate how Cops5 and Cops8 regulate ESC differentiation and tried to establish Cops5 and Cops8 knockout (KO) ESC lines by CRISPR/Cas9. To our surprise, no Cops5 KO ESC clones were identified out of 127 clones, while three Cops8 KO ESC lines were established out of 70 clones. We then constructed an inducible Cops5 KO ESC line. Cops5 KO leads to decreased expression of the pluripotency marker Nanog, proliferation defect, G2/M cell-cycle arrest, and apoptosis of ESCs. Further analysis revealed dual roles of Cops5 in maintaining genomic stability of ESCs. On one hand, Cops5 suppresses the autophagic degradation of Mtch2 to direct cellular metabolism toward glycolysis and minimize reactive oxygen species (ROS) production, thereby reducing endogenous DNA damage. On the other hand, Cops5 is required for high DNA damage repair (DDR) activities in ESCs. Without Cops5, elevated ROS and reduced DDR activities lead to DNA damage accumulation in ESCs. Subsequently, p53 is activated to trigger G2/M arrest and apoptosis. Altogether, our studies reveal an essential role of Cops5 in maintaining genome integrity and self-renewal of ESCs by regulating cellular metabolism and DDR pathways.

KHDC3L mutation causes recurrent pregnancy loss by inducing genomic instability of human early embryonic cells

Recurrent pregnancy loss (RPL) is an important complication in reproductive health. About 50% of RPL cases are unexplained, and understanding the genetic basis is essential for its diagnosis and prognosis. Herein, we report causal KH domain containing 3 like (KHDC3L) mutations in RPL. KHDC3L is expressed in human epiblast cells and ensures their genome stability and viability. Mechanistically, KHDC3L binds to poly(ADP-ribose) polymerase 1 (PARP1) to stimulate its activity. In response to DNA damage, KHDC3L also localizes to DNA damage sites and facilitates homologous recombination (HR)-mediated DNA repair. KHDC3L dysfunction causes PARP1 inhibition and HR repair deficiency, which is synthetically lethal. Notably, we identified two critical residues, Thr145 and Thr156, whose phosphorylation by Ataxia-telangiectasia mutated (ATM) is essential for KHDC3L’s functions. Importantly, two deletions of KHDC3L (p.E150_V160del and p.E150_V172del) were detected in female RPL patients, both of which harbor a common loss of Thr156 and are impaired in PARP1 activation and HR repair. In summary, our study reveals both KHDC3L as a new RPL risk gene and its critical function in DNA damage repair pathways.

Recurrent pregnancy loss is an important complication in reproductive health, and about 50% of cases remain unexplained. This study shows that KHDC3L safeguards the genomic stability of human early embryonic cells, and damaging mutations in its gene cause recurrent pregnancy loss in humans.

TRIM66 reads unmodified H3R2K4 and H3K56ac to respond to DNA damage in embryonic stem cells

Recognition of specific chromatin modifications by distinct structural domains within “reader” proteins plays a critical role in the maintenance of genomic stability. However, the specific mechanisms involved in this process remain unclear. Here we report that the PHD-Bromo tandem domain of tripartite motif-containing 66 (TRIM66) recognizes the unmodified H3R2-H3K4 and acetylated H3K56. The aberrant deletion of Trim66 results in severe DNA damage and genomic instability in embryonic stem cells (ESCs). Moreover, we find that the recognition of histone modification by TRIM66 is critical for DNA damage repair (DDR) in ESCs. TRIM66 recruits Sirt6 to deacetylate H3K56ac, negatively regulating the level of H3K56ac and facilitating the initiation of DDR. Importantly, Trim66-deficient blastocysts also exhibit higher levels of H3K56ac and DNA damage. Collectively, the present findings indicate the vital role of TRIM66 in DDR in ESCs, establishing the relationship between histone readers and maintenance of genomic stability.

TRIM66 protein has an N-terminal tripartite motif and a C-terminal PHD Bromodomain. Here the authors show the specific histone modification recognition of TRIM66-PHD-Bromodomain through crystallography and biochemistry assay, and further reveal that TRIM66 recognition of certain histone modification is important for DNA damage repair in ESCs.

Stem cell therapy for Parkinson's disease using non-human primate models

Stem cell therapy (SCT) for Parkinson's disease (PD) has received considerable attention in recent years. Non-human primate (NHP) models of PD have played an instrumental role in the safety and efficacy of emerging PD therapies and facilitated the translation of initiatives for human patients. NHP models of PD include primates with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonism, who are responsive to dopamine replacement therapies, similar to human PD patients. Extensive research in SCT has been conducted to better treat the progressive dopaminergic neurodegeneration that underlies PD. For effective application of SCT in PD, however, a number of basic parameters still need to be tested and optimized in NHP models, including preparation and storage of cells for engraftment, methods of transplantation, choice of target sites, and timelines for recovery. In this review, we discuss the current status of NHP models of PD in stem cell research. We also analyze the advances and remaining challenges for successful clinical translation of SCT for this persistent disease.

Replication stress response in cancer stem cells as a target for chemotherapy

Cancer stem cells (CSCs) are subpopulations of multipotent stem cells (SCs) responsible for the initiation, long-term clonal maintenance, growth and spreading of most human neoplasms. Reportedly, CSCs share a very robust DNA damage response (DDR) with embryonic and adult SCs, which allows them to survive endogenous and exogenous genotoxins. A range of experimental evidence indicates that CSCs have high but heterogeneous levels of replication stress (RS), arising from, and being boosted by, endogenous causes, such as specific genetic backgrounds (e.g., p53 deficiency) and/or aberrant karyotypes (e.g., supernumerary chromosomes). A multipronged RS response (RSR) is put in place by CSCs to limit and ensure tolerability to RS. The characteristics of such dedicated cascade have two opposite consequences, both relevant for cancer therapy. On the one hand, RSR efficiency often increases the reliance of CSCs on specific DDR components. On the other hand, the functional redundancy of pathways of the RSR can paradoxically promote the acquisition of resistance to RS- and/or DNA damage-inducing agents. Here, we provide an overview of the molecular mechanisms of the RSR in cancer cells and CSCs, focusing on the role of CHK1 and some emerging players, such as PARP1 and components of the homologous recombination repair, whose targeting can represent a long-term effective anti-CSC strategy.

Maternal gene Ooep may participate in homologous recombination-mediated DNA double-strand break repair in mouse oocytes

DNA damage in oocytes can cause infertility and birth defects. DNA double-strand breaks (DSBs) are highly deleterious and can substantially impair genome integrity. Homologous recombination (HR)-mediated DNA DSB repair plays dominant roles in safeguarding oocyte quantity and quality. However, little is known regarding the key players of the HR repair pathway in oocytes. Here, we identified oocyte-specific gene Ooep as a novel key component of the HR repair pathway in mouse oocytes. OOEP was required for efficient ataxia telangiectasia mutated (ATM) kinase activation and Rad51 recombinase (RAD51) focal accumulation at DNA DSBs. Ooep null oocytes were defective in DNA DSB repair and prone to apoptosis upon exogenous DNA damage insults. Moreover, Ooep null oocytes exhibited delayed meiotic maturation. Therefore, OOEP played roles in preserving oocyte quantity and quality by maintaining genome stability. Ooep expression decreased with the advance of maternal age, suggesting its involvement in maternal aging.

Mouse embryonic stem cells have increased capacity for replication fork restart driven by the specific Filia-Floped protein complex

Pluripotent stem cells (PSCs) harbor constitutive DNA replication stress during their rapid proliferation and the consequent genome instability hampers their applications in regenerative medicine. It is therefore important to understand the regulatory mechanisms of replication stress response in PSCs. Here, we report that mouse embryonic stem cells (ESCs) are superior to differentiated cells in resolving replication stress. Specifically, ESCs utilize a unique Filia-Floped protein complex-dependent mechanism to efficiently promote the restart of stalled replication forks, therefore maintaining genomic stability. The ESC-specific Filia-Floped complex resides on replication forks under normal conditions. Replication stress stimulates their recruitment to stalling forks and the serine 151 residue of Filia is phosphorylated in an ATR-dependent manner. This modification enables the Filia-Floped complex to act as a functional scaffold, which then promotes the stalling fork restart through a dual mechanism: both enhancing recruitment of the replication fork restart protein, Blm, and stimulating ATR kinase activation. In the Blm pathway, the scaffolds recruit the E3 ubiquitin ligase, Trim25, to the stalled replication forks, and in turn Trim25 tethers and concentrates Blm at stalled replication forks through ubiquitination. In differentiated cells, the recruitment of the Trim25-Blm complex to replication forks and the activation of ATR signaling are much less robust due to lack of the ESC-specific Filia-Floped scaffold. Thus, our study reveals that ESCs utilize an additional and unique regulatory layer to efficiently promote the stalled fork restart and maintain genomic stability.

A RHO Small GTPase Regulator ABR Secures Mitotic Fidelity in Human Embryonic Stem Cells

Pluripotent stem cells can undergo repeated self-renewal while retaining genetic integrity, but they occasionally acquire aneuploidy during long-term culture, which is a practical obstacle for medical applications of human pluripotent stem cells. In this study, we explored the biological roles of ABR, a regulator of RHO family small GTPases, and found that it has pivotal roles during mitotic processes in human embryonic stem cells (hESCs). Although ABR has been shown to be involved in dissociation-induced hESC apoptosis, it does not appear to have direct effects on cell survival unless cell-cell contact is impaired. Instead, we found that it is important for faithful hESC division. Mechanistically, ABR depletion compromised centrosome dynamics and predisposed the cell to chromosome misalignment and missegregation, which raised the frequency of aneuploidy. These results provide insights into the mechanisms that support the genetic integrity of self-renewing hESCs.

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ABR depletion leads to G2/M accumulation in hESCs

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Centrosome dynamics and mitotic fidelity are compromised upon ABR depletion

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When mitosis progresses without ABR, hESCs show a high incidence of aneuploidy

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ABR safeguards faithful chromosome inheritance during hESC division

Cytotoxic Indole Alkaloid 3α-Acetonyltabersonine Induces Glioblastoma Apoptosis via Inhibition of DNA Damage Repair

Cytotoxic indole alkaloids from Melodinus suaveolens, which belongs to the toxic plant family Apocynaceae, demonstrated impressive antitumor activities in many tumor types, but less application in glioblastoma, which is the lethal brain tumor. In the present study, we reported the anti-glioblastoma activity of an indole alkaloid, 3α-acetonyltabersonine, which was isolated from Melodinus suaveolens. 3α-acetonyltabersonine was cytotoxic to glioblastoma cell lines (U87 and T98G) and stem cells at low concentrations. We verified 3α-acetonyltabersonine could suppress tumor cell proliferation and cause apoptosis in glioblastoma stem cells (GSCs). Moreover, detailed investigation of transcriptome study and Western blotting analysis indicated the mitogen activated protein kinase (MAPK) pathway was activated by phosphorylation upon 3α-acetonyltabersonine treatment. Additionally, we found 3α-acetonyltabersonine inhibited DNA damage repair procedures, the accumulated DNA damage stimulated activation of MAPK pathway and, finally, induced apoptosis. Further evidence was consistently obtained from vivo experiments on glioblastoma mouse model: treatment of 3α-acetonyltabersonine could exert pro-apoptotic function and prolong the life span of tumor-bearing mice. These results in vitro and in vivo suggested that 3α-acetonyltabersonine could be a potential candidate antitumor agent.

DNA repair mechanisms in embryonic stem cells

Embryonic stem cells (ESCs) can undergo unlimited self-renewal and retain the pluripotency to differentiate into all cell types in the body. Therefore, as a renewable source of various functional cells in the human body, ESCs hold great promise for human cell therapy. During the rapid proliferation of ESCs in culture, DNA damage, such as DNA double-stranded breaks, will occur in ESCs. Therefore, to realize the potential of ESCs in human cell therapy, it is critical to understand the mechanisms how ESCs activate DNA damage response and DNA repair to maintain genomic stability, which is a prerequisite for their use in human therapy. In this context, it has been shown that ESCs harbor much fewer spontaneous mutations than somatic cells. Consistent with the finding that ESCs are genetically more stable than somatic cells, recent studies have indicated that ESCs can mount more robust DNA damage responses and DNA repair than somatic cells to ensure their genomic integrity.

Maternal expression of the JMJD2A/KDM4A histone demethylase is critical for pre-implantation development

Regulation of chromatin composition through post-translational modifications of histones contributes to transcriptional regulation and is essential for many cellular processes, including differentiation and development. JMJD2A/KDM4A is a lysine demethylase with specificity towards di- and tri-methylated lysine 9 and lysine 36 of histone H3 (H3K9me2/me3 and H3K36me2/me3). Here, we report that Kdm4a as a maternal factor plays a key role in embryo survival and is vital for female fertility. Kdm4a−/- female mice ovulate normally with comparable fertilization but poor implantation rates, and cannot support healthy transplanted embryos to term. This is due to a role for Kdm4a in uterine function, where its loss causes reduced expression of key genes involved in ion transport, nutrient supply and cytokine signalling, that impact embryo survival. In addition, a significant proportion of Kdm4a deficient oocytes displays a poor intrinsic ability to develop into blastocysts. These embryos cannot compete with healthy embryos for implantation in vivo, highlighting Kdm4a as a maternal effect gene. Thus, our study dissects an important dual role for maternal Kdm4a in determining faithful early embryonic development and the implantation process.

DNA Damage Response in Hematopoietic Stem Cell Ageing

Maintenance of tissue-specific stem cells is vital for organ homeostasis and organismal longevity. Hematopoietic stem cells (HSCs) are the most primitive cell type in the hematopoietic system. They divide asymmetrically and give rise to daughter cells with HSC identity (self-renewal) and progenitor progenies (differentiation), which further proliferate and differentiate into full hematopoietic lineages. Mammalian ageing process is accompanied with abnormalities in the HSC self-renewal and differentiation. Transcriptional changes and epigenetic modulations have been implicated as the key regulators in HSC ageing process. The DNA damage response (DDR) in the cells involves an orchestrated signaling pathway, consisting of cell cycle regulation, cell death and senescence, transcriptional regulation, as well as chromatin remodeling. Recent studies employing DNA repair-deficient mouse models indicate that DDR could intrinsically and extrinsically regulate HSC maintenance and play important roles in tissue homeostasis of the hematopoietic system. In this review, we summarize the current understanding of how the DDR determines the HSC fates and finally contributes to organismal ageing.

Decrease in expression of maternal effect gene Mater is associated with maternal ageing in mice

STUDY HYPOTHESIS

What factors in mouse oocytes are involved in the ageing-related decline in oocyte quality?

STUDY FINDING

The maternal effect gene Mater is involved in ageing-related oocyte quality decline in mice.

WHAT IS KNOWN ALREADY

Premature loss of centromere cohesion is a hallmark of ageing-related oocyte quality decline; the maternal effect gene Mater (maternal antigen that embryos require, also known as Nlrp5) is required for preimplantation embryo development beyond the 2-cell stage, and mRNA expression of Mater decreases with maternal ageing.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

Mater protein expression level in mature oocytes from 7 young (5-8 weeks old) to 7 old mice (41-68 weeks old) was compared by immunoblotting analysis. Wild-type and Mater-null mice were used to examine whether Mater is necessary for maintaining normal centromere cohesion by means of cytogenetic karyotyping, time-lapse confocal microscopy and immunofluorescence staining.

MAIN RESULTS AND THE ROLE OF CHANCE

Mater protein is decreased in mature oocytes from old versus young mice (P = 0.0022). Depletion of Mater from oocytes leads to a reduction in centromere cohesion, manifested by precocious sister chromatid separation, enlargement of sister centromere distance and misalignment of chromosomes in the metaphase plate during meiosis I and II.

LIMITATIONS, REASONS FOR CAUTION

This study was conducted in mice. Whether or not the results are applicable to human remains further elucidation. In addition, we were unable to confirm if the strain of mice (C57BL/6XSv129) at the age of 41-68 weeks old has the 'cohesin-loss' phenotype.

WIDER IMPLICATIONS OF THE FINDINGS

Investigating Mater's functional mechanisms could provide fresh insights into understanding how the ageing-related oocyte quality decline occurs.

LARGE SCALE DATA

N/A.

STUDY FUNDING AND COMPETING INTERESTS

This work was supported by the research grant from Chinese NSFC to P.Z. (31071274). We have no conflict of interests to declare.

Genome Editing Gene Therapy for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a severe genetic disorder caused by loss of function of the dystrophin gene on the X chromosome. Gene augmentation of dystrophin is challenging due to the large size of the dystrophin cDNA. Emerging genome editing technologies, such as TALEN and CRISPR-Cas9 systems, open a new erain the restoration of functional dystrophin and are a hallmark of bona fide gene therapy. In this review, we summarize current genome editing approaches, properties of target cell types for ex vivo gene therapy, and perspectives of in vivo gene therapy including genome editing in human zygotes. Although technical challenges, such as efficacy, accuracy, and delivery of the genome editing components, remain to be further improved, yet genome editing technologies offer a new avenue for the gene therapy of DMD.