Overcoming Pluripotent Stem Cell Dependence on the Repair of Endogenous DNA Damage

Derivation of Naïve Human Embryonic Stem Cells Using a CHK1 Inhibitor

Embryonic development is a continuum in vivo. Transcriptional analysis can separate established human embryonic stem cells (hESC) into at least four distinct developmental pluripotent stages, two naïve and two primed, early and late relative to the intact epiblast. In this study we primarily show that exposure of frozen human blastocysts to an inhibitor of checkpoint kinase 1 (CHK1) upon thaw greatly enhances establishment of karyotypically normal late naïve hESC cultures. These late naïve cells are plastic and can be toggled back to early naïve and forward to early primed pluripotent stages. The early primed cells are transcriptionally equivalent to the post inner cell mass intermediate (PICMI) stage seen one day following transfer of human blastocysts into in vitro culture and are stable at an earlier stage than conventional primed hESC.

RUNX1 mutations mitigate quiescence to promote transformation of hematopoietic progenitors in Fanconi anemia

Many inherited bone marrow failure syndromes (IBMFSs) present a high risk of transformation to myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). During transformation of IBMFSs, hematopoietic stem and progenitor cells (HSPCs) with poor fitness gain ectopic, dysregulated self-renewal secondary to somatic mutations via undefined mechanisms. Here, in the context of the prototypical IBMFS Fanconi anemia (FA), we performed multiplexed gene editing of mutational hotspots in MDS-associated genes in human induced pluripotent stem cells (iPSCs) followed by hematopoietic differentiation. We observed aberrant self-renewal and impaired differentiation of HSPCs with enrichment of RUNX1 insertions and deletions (indels), generating a model of IBMFS-associated MDS. We observed that compared to the failure state, FA MDS cells show mutant RUNX1-mediated blunting of the G1/S cell cycle checkpoint that is normally activated in FA in response to DNA damage. RUNX1 indels also lead to activation of innate immune signaling, which stabilizes the homologous recombination (HR) effector BRCA1, and this pathway can be targeted to abrogate viability and restore sensitivity to genotoxins in FA MDS. Together, these studies develop a paradigm for modeling clonal evolution in IBMFSs, provide basic understanding of the pathogenesis of MDS, and uncover a therapeutic target in FA-associated MDS.

Beyond current treatment of Fanconi Anemia: What do advances in cell and gene-based approaches offer?

Fanconi anemia (FA) is a rare inherited disorder that mainly affects the bone marrow. This condition causes decreased production of all types of blood cells. FA is caused by a defective repair of DNA interstrand crosslinks and to date, mutations in over 20 genes have been linked to the disease. Advances in science and molecular biology have provided new insight between FA gene mutations and the severity of clinical manifestations. Here, we will highlight the current and promising therapeutic options for this rare disease. The current standard treatment for FA patients is hematopoietic stem cell transplantation, a treatment associated to exposure to radiation or chemotherapy, immunological complications, plus opportunistic infections from prolonged immune incompetence or increased risk of morbidity. New arising treatments include gene addition therapy, genome editing using CRISPR-Cas9 nuclease, and hematopoietic stem cell generation from induced pluripotent stem cells. Finally, we will also discuss the revolutionary developments in mRNA therapeutics as an opportunity for this disease.

Implication of E3 ligase RAD18 in UV-induced mutagenesis in human induced pluripotent stem cells and neuronal progenitor cells

Pluripotent stem cells (PSCs) have the potential to differentiate to any of the other organs. The genome DNA integrity of PSCs is maintained by a high level of transcription for a number of genes involved in DNA repair, cell cycle and apoptosis. However, it remains unclear how high the frequency of genetic mutation is and how these DNA repair factors function in PSCs. In this study, we employed Sup F assay for the measurement of mutation frequency after UV-C irradiation in induced pluripotent stem cells (iPSCs) as PSC models and neural progenitor cells (NPCs) were derived from iPSCs as differentiated cells. iPSCs and NPCs exhibited a lower mutation frequency compared with the original skin fibroblasts. In RNA-seq analysis, iPSCs and NPCs showed a high expression of RAD18, which is involved in trans-lesion synthesis (TLS) for the emergency tolerance system during the replication process of DNA. Although RAD18 is involved in both error free and error prone TLS in somatic cells, it still remains unknown the function of RAD18 in PSCs. In this study we depleted of the RAD18 by siRNA knockdown resulted in decreased frequency of mutation in iPSCs and NPCs. Our results will provide information on the genome maintenance machinery in PSCs.

DNA Repair and Therapeutic Strategies in Cancer Stem Cells

First-line cancer treatments successfully eradicate the differentiated tumour mass but are comparatively ineffective against cancer stem cells (CSCs), a self-renewing subpopulation thought to be responsible for tumour initiation, metastasis, heterogeneity, and recurrence. CSCs are thus presented as the principal target for elimination during cancer treatment. However, CSCs are challenging to drug target because of numerous intrinsic and extrinsic mechanisms of drug resistance. One such mechanism that remains relatively understudied is the DNA damage response (DDR). CSCs are presumed to possess properties that enable enhanced DNA repair efficiency relative to their highly proliferative bulk progeny, facilitating improved repair of double-strand breaks induced by radiotherapy and most chemotherapeutics. This can occur through multiple mechanisms, including increased expression and splicing fidelity of DNA repair genes, robust activation of cell cycle checkpoints, and elevated homologous recombination-mediated DNA repair. Herein, we summarise the current knowledge concerning improved genome integrity in non-transformed stem cells and CSCs, discuss therapeutic opportunities within the DDR for re-sensitising CSCs to genotoxic stressors, and consider the challenges posed regarding unbiased identification of novel DDR-directed strategies in CSCs. A better understanding of the DDR mediating chemo/radioresistance mechanisms in CSCs could lead to novel therapeutic approaches, thereby enhancing treatment efficacy in cancer patients.

Transcription-replication conflicts in primordial germ cells necessitate the Fanconi anemia pathway to safeguard genome stability

Germ cells are capable of preserving their genetic information with high fidelity. We report that rapidly dividing mouse primordial germ cells (PGCs) are faced with high levels of endogenous replication stress due to frequent occurrence of transcription–replication conflicts (TRCs). Thus, PGCs have an increased requirement for the replication-coupled Fanconi anemia (FA) pathway to counteract TRC-induced replication stress, enabling their rapid proliferation to establish a sufficient reproductive reserve. This work provides insights into the unique genome feature of developing PGCs and helps to explain the reproductive defects in FA individuals.

Preserving a high degree of genome integrity and stability in germ cells is of utmost importance for reproduction and species propagation. However, the regulatory mechanisms of maintaining genome stability in the developing primordial germ cells (PGCs), in which rapid proliferation is coupled with global hypertranscription, remain largely unknown. Here, we find that mouse PGCs encounter a constitutively high frequency of transcription–replication conflicts (TRCs), which lead to R-loop accumulation and impose endogenous replication stress on PGCs. We further demonstrate that the Fanconi anemia (FA) pathway is activated by TRCs and has a central role in the coordination between replication and transcription in the rapidly proliferating PGCs, as disabling the FA pathway leads to TRC and R-loop accumulation, replication fork destabilization, increased DNA damage, dramatic loss of mitotically dividing mouse PGCs, and consequent sterility of both sexes. Overall, our findings uncover the unique source and resolving mechanism of endogenous replication stress during PGC proliferation, provide a biological explanation for reproductive defects in individuals with FA, and improve our understanding of the monitoring strategies for genome stability during germ cell development.

Human induced pluripotent stem cells: From cell origin, genomic stability and epigenetic memory to translational medicine

The potential of human induced pluripotent stem cells (iPSCs) to self-renew indefinitely and to differentiate virtually into any cell type in unlimited quantities makes them attractive for in-vitro disease modeling, drug screening, personalized medicine, and regenerative therapies. As the genome of iPSCs thoroughly reproduces that of the somatic cells from which they are derived, they may possess genetic abnormalities, which would seriously compromise their utility and safety. Genetic aberrations could be present in donor somatic cells and then transferred during iPSC generation, or they could occur as de novo mutations during reprogramming or prolonged cell culture. Therefore, to warrant safety of human iPSCs for clinical applications, analysis of genetic integrity, particularly during iPSC generation and differentiation, should be carried out on a regular basis. On the other hand, reprogramming of somatic cells to iPSCs requires profound modifications in the epigenetic landscape. Changes in chromatin structure by DNA methylations and histone tail modifications aim to reset the gene expression pattern of somatic cells to facilitate and establish self-renewal and pluripotency. However, residual epigenetic memory influences the iPSC phenotype, which may affect their application in disease therapeutics. The present review discusses the somatic cell origin, genetic stability, and epigenetic memory of iPSCs and their impact on basic and translational research.

Inherited DNA Repair Defects Disrupt the Structure and Function of Human Skin

Squamous cell carcinoma (SCC) is a global public health burden originating in epidermal stem and progenitor cells (ESPCs) of the skin and mucosa. To understand how genetic risk factors contribute to SCC, studies of ESPC biology are imperative. Children with Fanconi anemia (FA) are a paradigm for extreme SCC susceptibility caused by germline loss-of-function mutations in FA DNA repair pathway genes. To discover epidermal vulnerabilities, patient-derived pluripotent stem cells (PSCs) conditional for the FA pathway were differentiated into ESPCs and PSC-derived epidermal organotypic rafts (PSC-EORs). FA PSC-EORs harbored diminished cell-cell junctions and increased proliferation in the basal cell compartment. Furthermore, desmosome and hemidesmosome defects were identified in the skin of FA patients, and these translated into accelerated blistering following mechanically induced stress. Together, we demonstrate that a critical DNA repair pathway maintains the structure and function of human skin and provide 3D epidermal models wherein SCC prevention can now be explored.

An induced pluripotent stem cell model of Fanconi anemia reveals mechanisms of p53-driven progenitor cell differentiation

Fanconi anemia (FA) is a disorder of DNA repair that manifests as bone marrow (BM) failure. The lack of accurate murine models of FA has refocused efforts toward differentiation of patient-derived induced pluripotent stem cells (IPSCs) to hematopoietic progenitor cells (HPCs). However, an intact FA DNA repair pathway is required for efficient IPSC derivation, hindering these efforts. To overcome this barrier, we used inducible complementation of FANCA-deficient IPSCs, which permitted robust maintenance of IPSCs. Modulation of FANCA during directed differentiation to HPCs enabled the production of FANCA-deficient human HPCs that recapitulated FA genotoxicity and hematopoietic phenotypes relative to isogenic FANCA-expressing HPCs. FANCA-deficient human HPCs underwent accelerated terminal differentiation driven by activation of p53/p21. We identified growth arrest specific 6 (GAS6) as a novel target of activated p53 in FANCA-deficient HPCs and modulate GAS6 signaling to rescue hematopoiesis in FANCA-deficient cells. This study validates our strategy to derive a sustainable, highly faithful human model of FA, uncovers a mechanism of HPC exhaustion in FA, and advances toward future cell therapy in FA.

Personalized Assessment of Normal Tissue Radiosensitivity via Transcriptome Response to Photon, Proton and Carbon Irradiation in Patient-Derived Human Intestinal Organoids

Radiation-induced normal tissue toxicity often limits the curative treatment of cancer. Moreover, normal tissue relative biological effectiveness data for high-linear energy transfer particles are urgently needed. We propose a strategy based on transcriptome analysis of patient-derived human intestinal organoids (HIO) to determine molecular surrogates for radioresponse of gastrointestinal (GI) organs at risk in a personalized manner. HIO were generated from induced pluripotent stem cells (iPSC), which were derived from skin biopsies of three patients, including two patients with FANCA deficiency as a paradigm for enhanced radiosensitivity. For the two Fanconi anemia (FA) patients (HIO-104 and 106, previously published as FA-A#1 IND-iPS1 and FA-A#2 IND-iPS3), FANCA expression was reconstituted as a prerequisite for generation of HIO via lentiviral expression of a doxycycline inducible construct. For radiosensitivity analysis, FANCA deficient and FANCA rescued as well as wtHIO were sham treated or irradiated with 4Gy photon, proton or carbon ions at HIT, respectively. Immunofluorescence staining of HIO for 53BP1-foci was performed 1 h post IR and gene expression analyses was performed 12 and 48 h post IR. 53BP1-foci numbers and size correlated with the higher RBE of carbon ions. A FANCA dependent differential gene expression in response to radiation was found (p < 0.01, ANOVA; n = 1071 12 h; n = 1100 48 h). Pathways associated with FA and DNA-damage repair i.e., transcriptional coupled nucleotide excision repair, homology-directed repair and translational synthesis were found to be differentially regulated in FANCA deficient HIO. Next, differential regulated genes were investigated as a function of radiation quality (RQ, p < 0.05, ANOVA; n = 742 12 h; n = 553 48 h). Interestingly, a gradual increase or decrease of gene expression was found to correlate with the three main qualities, from photon to proton and carbon irradiation. Clustering separated high-linear energy transfer irradiation with carbons from proton and photon irradiation. Genes associated with dual incision steps of TC-NER were differentially regulated in photon vs. proton and carbon irradiation. Consequently, SUMO3, ALC1, POLE4, PCBP4, MUTYH expression correlated with the higher RBE of carbon ions. An interaction between the two studied parameters FA and RQ was identified (p < 0.01, 2-way ANOVA n = 476). A comparison of genes regulated as a function of FA, RQ and RBE suggest a role for p53 interacting genes BRD7, EWSR1, FBXO11, FBXW8, HMGB1, MAGED2, PCBP4, and RPS27 as modulators of FA in response to radiation. This proof of concept study demonstrates that patient tailored evaluation of GI response to radiation is feasible via generation of HIO and comparative transcriptome profiling. This methodology can now be further explored for a personalized assessment of GI radiosensitivity and RBE estimation.

Reprogramming and differentiation-dependent transcriptional alteration of DNA damage response and apoptosis genes in human induced pluripotent stem cells

Pluripotent stem cells (PSCs), such as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), have a dual capability to self-renew and differentiate into all cell types necessary to develop an entire organism. Differentiation is associated with dynamic epigenetic alteration and transcriptional change, while self-renewal depends on maintaining the genome DNA accurately. Genome stability of PSCs is strictly regulated to maintain pluripotency. However, the DNA damage response (DDR) mechanism in PSCs is still unclear. There is accumulating evidence that genome stability and pluripotency are regulated by a transcriptional change in undifferentiated and differentiated states. iPSCs are ideal for analyzing transcriptional regulation during reprogramming and differentiation. This study aimed to elucidate the transcriptional alteration surrounding genome stability maintenance, including DNA repair, cell cycle checkpoints and apoptosis in fibroblasts, iPSCs and neural progenitor cells (NPCs) derived from iPSCs as differentiated cells. After ionizing radiation exposure, foci for the DNA double-stranded break marker γ-H2AX increased, peaking at 0.5 h in all cells (>90%), decreasing after 4 h in fibroblasts (32.3%) and NPCs (22.3%), but still remaining at 52.5% (NB1RGB C2 clone) and 54.7% (201B7 cells) in iPSCs. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive cells were detected, indicating that iPSCs' apoptosis increases. In addition, RNA sequencing (RNA-Seq) analysis showed high expression of apoptosis genes (TP53, CASP3 and BID) in iPSCs. Results suggested that increased apoptosis activity maintains accurate, undifferentiated genome DNA in the cell population.

Advances in Gene Therapy for Fanconi Anemia

Fanconi anemia (FA) is a rare inherited disease that is associated with bone marrow failure and a predisposition to cancer. Previous clinical trials emphasized the difficulties that accompany the use of gene therapy to treat bone marrow failure in patients with FA. Nevertheless, the discovery of new drugs that can efficiently mobilize hematopoietic stem cells (HSCs) and the development of optimized procedures for transducing HSCs, using safe, integrative vectors, markedly improved the efficiency by which the phenotype of hematopoietic repopulating cells from patients with FA can be corrected. In addition, these achievements allowed the demonstration of the in vivo proliferation advantage of gene-corrected FA repopulating cells in immunodeficient mice. Significantly, new gene therapy trials are currently ongoing to investigate the progressive restoration of hematopoiesis in patients with FA by gene-corrected autologous HSCs. Further experimental studies are focused on the ex vivo transduction of unpurified FA HSCs, using new pseudotyped vectors that have HSC tropism. Because of the resistance of some of these vectors to serum complement, new strategies for in vivo gene therapy for FA HSCs are in development. Finally, because of the rapid advancements in gene-editing techniques, correction of CD34⁺ cells isolated from patients with FA is now feasible, using gene-targeting strategies. Taken together, these advances indicate that gene therapy can soon be used as an efficient and safe alternative for the hematopoietic treatment of patients with FA.

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.

High Basal Levels of γH2AX in Human Induced Pluripotent Stem Cells Are Linked to Replication-Associated DNA Damage and Repair

Human induced pluripotent stem cells (iPSCs) have great potential as source cells for therapeutic uses. However, reports indicate that iPSCs carry genetic abnormalities, which may impede their medical use. Little is known about mechanisms contributing to intrinsic DNA damage in iPSCs that could lead to genomic instability. In this report, we investigated the level of DNA damage in human iPSC lines compared with their founder fibroblast line and derived mesenchymal stromal cell (MSC) lines using the phosphorylated histone variant, γH2AX, as a marker of DNA damage. We show that human iPSCs have elevated basal levels of γH2AX, which correlate with markers of DNA replication: 5-ethynyl-2'-deoxyuridine and the single-stranded binding protein, replication protein A. γH2AX foci in iPSCs also colocalize to BRCA1 and RAD51, proteins in the homologous repair pathway, implying γH2AX in iPSCs marks sites of double strand breaks. Our study demonstrates an association between increased basal levels of γH2AX and the rapid replication of iPSCs. Stem Cells 2018;36:1501-1513.

Induction of micronuclei by four cytostatic compounds in human hematopoietic stem cells and human lymphoblastoid TK6 cells

For mutagenicity testing, primary lymphocytes or mammalian cell lines are employed. However, the true target for carcinogenic action of mutagenic chemicals may be stem cells. Since hematopoietic cancers induced by chemical agents originate at the hematopoietic stem cell (HSC) stage and since one of the side effects of chemotherapeutic cancer treatment is the induction of secondary tumors, often leukemias, HSC may be a suitable cell system. We compared the sensitivity of HSC with the genotoxicity testing cell line TK6 for chromosomal mutations. HSC were less sensitive than TK6 cells for the genotoxic effects of the model genotoxins and chemotherapeutic agents doxorubicin, vinblastine, methyl methanesulfonate (MMS) and equally sensitive for mitomycin C (MMC). However, loss of viability after mitomycin C treatment was higher in HSC than in TK6 cells. Among the factors that may influence sensitivity for genomic damage, the generation or response to reactive oxygen species (ROS) and the effectiveness of DNA damage response can be discussed. Here we show that HSC can be used in a standard micronucleus test protocol for chromosomal mutations and that their sensitivity was not higher than that of a classical testing cell line.

Differentiation of Human Induced Pluripotent or Embryonic Stem Cells Decreases the DNA Damage Repair by Homologous Recombination

The nitric oxide (NO)-cyclic GMP pathway contributes to human stem cell differentiation, but NO free radical production can also damage DNA, necessitating a robust DNA damage response (DDR) to ensure cell survival. How the DDR is affected by differentiation is unclear. Differentiation of stem cells, either inducible pluripotent or embryonic derived, increased residual DNA damage as determined by γ-H2AX and 53BP1 foci, with increased S-phase-specific chromosomal aberration after exposure to DNA-damaging agents, suggesting reduced homologous recombination (HR) repair as supported by the observation of decreased HR-related repair factor foci formation (RAD51 and BRCA1). Differentiated cells also had relatively increased fork stalling and R-loop formation after DNA replication stress. Treatment with NO donor (NOC-18), which causes stem cell differentiation has no effect on double-strand break (DSB) repair by non-homologous end-joining but reduced DSB repair by HR. Present studies suggest that DNA repair by HR is impaired in differentiated cells.

•

Spontaneous and S-phase-specific chromosome aberrations in differentiated cells

•

Higher frequency of residual γ-H2AX foci after exposure to DNA-damaging agents

•

Higher frequency of cells with 53BP1 and RIF1 co-localization in differentiated cells

•

Higher frequency of cells with a reduced number of RAD51 or BRCA1 foci

Induced Pluripotent Stem Cells: Global Research Trends

The induced pluripotent stem cell (iPSC) was first described more than 10 years ago and is currently used in various basic science and clinical research fields. The aim of this report is to examine the trends in research using iPSCs over the last 10 years. The 2006-2016 PubMed database was searched using the MeSH term "induced pluripotent stem cells." Only original research articles were selected, with a total of 3323 articles. These were classified according to research theme into reprogramming, differentiation protocols for specific cells and/or tissues, pathophysiological research on diseases, and discovery of new drugs, and then the trends over the years were analyzed. We also focused on 232 research publications on the pathophysiological causes of diseases and drug discovery with impact factor (IF; Thomson Reuters) of six or more. The IF of each article was summed up by year, by main target disease, and by country, and the total IF score was expressed as trends of research. The trends of research activities of reprogramming and differentiation on specific cells and/or tissues reached maxima in 2013/2014. On the other hand, research on pathophysiology and drug discovery increased continuously. The 232 articles with IF ≥6 dealt with neurological, immunological/hematological, cardiovascular, and digestive tract diseases, in that order. The majority of articles were published from the United States, followed by Japan, Germany, and United Kingdom. In conclusion, iPSCs have become a general tool for pathophysiological research on disease and drug discovery.

Molecular Portrait of Metastasis-Competent Circulating Tumor Cells in Colon Cancer Reveals the Crucial Role of Genes Regulating Energy Metabolism and DNA Repair

BACKGROUND

Unraveling the molecular mechanisms that regulate the biology of metastasis-competent circulating tumor cells (CTCs) is urgently needed to understand metastasis formation and tumor relapse. Our group previously established the first cell line (CTC-MCC-41) derived from metastasis-competent CTCs of a patient with colon cancer.

METHODS

In this study, we analyzed the transcriptome of CTC-MCC-41 cells using Human Genome U133 Plus 2.0 microarrays with the aim of unraveling the molecular basis of their special features (stem cell properties and ability to initiate and support metastasis formation).

RESULTS

Comparison of the transcriptome data of metastasis-competent CTC-MCC-41 cells and of HT-29 cells (derived from a primary colon cancer) highlights the differential expression of genes that regulate energy metabolism [peroxisome proliferator-activated receptor γ coactivator 1A (PPARGC1A), peroxisome proliferator-activated receptor γ coactivator 1B (PPARGC1B), fatty acid binding protein 1 (FABP1), aldehyde dehydrogenase 3 family member A1 (ALDH3A1)], DNA repair [BRCA1 interacting protein C-terminal helicase 1 (BRIP1), Fanconi anemia complementation group B (FANCB), Fanconi anemia complementation group M (FANCM)], and stemness [glutaminase 2 (GLS2), cystathionine-beta-synthase (CBS), and cystathionine gamma-lyase (CTH)]. The differential expression of 20 genes was validated by quantitative reverse transcription PCR.

CONCLUSIONS

This study gives a comprehensive outlook on the molecular events involved in colon cancer progression and provides potential CTC biomarkers that may help develop new therapies to specifically target CTCs with stem cell properties that cause metastases and tumor relapse in patients with colon cancer.

Hematopoietic cell transplantation in Fanconi anemia: current evidence, challenges and recommendations

INTRODUCTION

Hematopoietic cell transplantation for Fanconi Anemia (FA) has improved dramatically over the past 40 years. With an enhanced understanding of the intrinsic DNA-repair defect and pathophysiology of hematopoietic failure and leukemogenesis, sequential changes to conditioning and graft engineering have significantly improved the expectation of survival after allogeneic hematopoietic cell transplantation (alloHCT) with incidence of graft failure decreased from 35% to <10% and acute graft-versus-host disease (GVHD) from >40% to <10%. Today, five-year overall survival exceeds 90% in younger FA patients with bone marrow failure but remains about 50% in those with hematologic malignancy. Areas covered: We review the evolution of alloHCT contributing to decreased rates of transplant related complications; highlight current challenges including poorer outcomes in cases of clonal hematologic disorders, alloHCT impact on endocrine function and intrinsic FA risk of epithelial malignancies; and describe investigational therapies for prevention and treatment of the hematologic manifestations of FA. Expert commentary: Current methods allow for excellent survival following alloHCT for FA associated BMF irrespective of donor hematopoietic cell source. Alternative curative approaches, such as gene therapy, are being explored to eliminate the risks of GVHD and minimize therapy-related adverse effects.

Rad51 Regulates Reprogramming Efficiency through DNA Repair Pathway

Rad51 is a key component of homologous recombination (HR) to repair DNA double-strand breaks and it forms Rad51 recombinase filaments of broken single-stranded DNA to promote HR. In addition to its role in DNA repair and cell cycle progression, Rad51 contributes to the reprogramming process during the generation of induced pluripotent stem cells. In light of this, we performed reprogramming experiments to examine the effect of co-expression of Rad51 and four reprogramming factors, Oct4, Sox2, Klf4, and c-Myc, on the reprogramming efficiency. Co-expression of Rad51 significantly increased the numbers of alkaline phosphatase-positive colonies and embryonic stem cell-like colonies during the process of reprogramming. Co-expression ofRad51 significantly increased the expression of epithelial markers at an early stage of reprogramming compared with control cells. Phosphorylated histone H2AX (γH2AX), which initiates the DNA double-strand break repair system, was highly accumulated in reprogramming intermediates upon co-expression of Rad51. This study identified a novel role of Rad51 in enhancing the reprogramming efficiency, possibly by facilitating mesenchymal-to-epithelial transition and by regulating a DNA damage repair pathway during the early phase of the reprogramming process.

Inhibition of Glucose-6-Phosphate Dehydrogenase Could Enhance 1,4-Benzoquinone-Induced Oxidative Damage in K562 Cells

Benzene is a chemical contaminant widespread in industrial and living environments. The oxidative metabolites of benzene induce toxicity involving oxidative damage. Protecting cells and cell membranes from oxidative damage, glucose-6-phosphate dehydrogenase (G6PD) maintains the reduced state of glutathione (GSH). This study aims to investigate whether the downregulation of G6PD in K562 cell line can influence the oxidative toxicity induced by 1,4-benzoquinone (BQ). G6PD was inhibited in K562 cell line transfected with the specific siRNA of G6PD gene. An empty vector was transfected in the control group. Results revealed that G6PD was significantly upregulated in the control cells and in the cells with inhibited G6PD after they were exposed to BQ. The NADPH/NADP and GSH/GSSG ratio were significantly lower in the cells with inhibited G6PD than in the control cells at the same BQ concentration. The relative reactive oxygen species (ROS) level and DNA oxidative damage were significantly increased in the cell line with inhibited G6PD. The apoptotic rate and G2 phase arrest were also significantly higher in the cells with inhibited G6PD and exposed to BQ than in the control cells. Our results suggested that G6PD inhibition could reduce GSH activity and alleviate oxidative damage. G6PD deficiency is also a possible susceptible risk factor of benzene exposure.

Pluripotent Stem Cells and DNA Damage Response to Ionizing Radiations

Pluripotent stem cells (PSCs) hold great promise in regenerative medicine, disease modeling, functional genomics, toxicological studies and cell-based therapeutics due to their unique characteristics of self-renewal and pluripotency. Novel methods for generation of pluripotent stem cells and their differentiation to the specialized cell types such as neuronal cells, myocardial cells, hepatocytes and beta cells of the pancreas and many other cells of the body are constantly being refined. Pluripotent stem cell derived differentiated cells, including neuronal cells or cardiac cells, are ideal for stem cell transplantation as autologous or allogeneic cells from healthy donors due to their minimal risk of rejection. Radiation-induced DNA damage, ultraviolet light, genotoxic stress and other intrinsic and extrinsic factors triggers a series of biochemical reactions known as DNA damage response. To maintain genomic stability and avoid transmission of mutations into progenitors cells, stem cells have robust DNA damage response signaling, a contrast to somatic cells. Stem cell transplantation may protect against radiation-induced late effects. In particular, this review focuses on differential DNA damage response between stem cells and derived differentiated cells and the possible pathways that determine such differences.