Apoptotic susceptibility to DNA damage of pluripotent stem cells facilitates pharmacologic purging of teratoma risk

Caspase-2 is essential for proliferation and self-renewal of nucleophosmin-mutated acute myeloid leukemia

Mutation in nucleophosmin (NPM1) causes relocalization of this normally nucleolar protein to the cytoplasm (NPM1c+). Despite NPM1 mutation being the most common driver mutation in cytogenetically normal adult acute myeloid leukemia (AML), the mechanisms of NPM1c+-induced leukemogenesis remain unclear. Caspase-2 is a proapoptotic protein activated by NPM1 in the nucleolus. Here, we show that caspase-2 is also activated by NPM1c+ in the cytoplasm and DNA damage-induced apoptosis is caspase-2 dependent in NPM1c+ but not in NPM1wt AML cells. Strikingly, in NPM1c+ cells, caspase-2 loss results in profound cell cycle arrest, differentiation, and down-regulation of stem cell pathways that regulate pluripotency including impairment of the AKT/mTORC1 pathways, and inhibition of Rictor cleavage. In contrast, there were minimal differences in proliferation, differentiation, or the transcriptional profile of NPM1wt cells lacking caspase-2. Our results show that caspase-2 is essential for proliferation and self-renewal of AML cells expressing mutated NPM1. This study demonstrates that caspase-2 is a major effector of NPM1c+ function.

Epigenetic repression of CHCHD2 enhances survival from single cell dissociation through attenuated Rho A kinase activity

During in vitro culture, human pluripotent stem cells (hPSCs) often acquire survival advantages characterized by decreased susceptibility to mitochondrial cell death, known as "culture adaptation." This adaptation is associated with genetic and epigenetic abnormalities, including TP53 mutations, copy number variations, trisomy, and methylation changes. Understanding the molecular mechanisms underlying this acquired survival advantage is crucial for safe hPSC-based cell therapies. Through transcriptome and methylome analysis, we discovered that the epigenetic repression of CHCHD2, a mitochondrial protein, is a common occurrence during in vitro culture using enzymatic dissociation. We confirmed this finding through genetic perturbation and reconstitution experiments in normal human embryonic stem cells (hESCs). Loss of CHCHD2 expression conferred resistance to single cell dissociation-induced cell death, a common stress encountered during in vitro culture. Importantly, we found that the downregulation of CHCHD2 significantly attenuates the activity of Rho-associated protein kinase (ROCK), which is responsible for inducing single cell death in hESCs. This suggests that hESCs may survive routine enzyme-based cell dissociation by downregulating CHCHD2 and thereby attenuating ROCK activity. These findings provide insights into the mechanisms by which hPSCs acquire survival advantages and adapt to in vitro culture conditions.

Caspase-2 is essential for proliferation and self-renewal of nucleophosmin-mutated acute myeloid leukemia

Mutation in nucleophosmin (NPM1) causes relocalization of this normally nucleolar protein to the cytoplasm ( NPM1c+ ). Despite NPM1 mutation being the most common driver mutation in cytogenetically normal adult acute myeloid leukemia (AML), the mechanisms of NPM1c+-induced leukemogenesis remain unclear. Caspase-2 is a pro-apoptotic protein activated by NPM1 in the nucleolus. Here, we show that caspase-2 is also activated by NPM1c+ in the cytoplasm, and DNA damage-induced apoptosis is caspase-2-dependent in NPM1c+ AML but not in NPM1wt cells. Strikingly, in NPM1c+ cells, loss of caspase-2 results in profound cell cycle arrest, differentiation, and down-regulation of stem cell pathways that regulate pluripotency including impairment in the AKT/mTORC1 and Wnt signaling pathways. In contrast, there were minimal differences in proliferation, differentiation, or the transcriptional profile of NPM1wt cells with and without caspase-2. Together, these results show that caspase-2 is essential for proliferation and self-renewal of AML cells that have mutated NPM1. This study demonstrates that caspase-2 is a major effector of NPM1c+ function and may even be a druggable target to treat NPM1c+ AML and prevent relapse.

Strategies to Improve the Safety of iPSC-Derived β Cells for β Cell Replacement in Diabetes

Allogeneic islet transplantation allows for the re-establishment of glycemic control with the possibility of insulin independence, but is severely limited by the scarcity of organ donors. However, a new source of insulin-producing cells could enable the widespread use of cell therapy for diabetes treatment. Recent breakthroughs in stem cell biology, particularly pluripotent stem cell (PSC) techniques, have highlighted the therapeutic potential of stem cells in regenerative medicine. An understanding of the stages that regulate β cell development has led to the establishment of protocols for PSC differentiation into β cells, and PSC-derived β cells are appearing in the first pioneering clinical trials. However, the safety of the final product prior to implantation remains crucial. Although PSC differentiate into functional β cells in vitro, not all cells complete differentiation, and a fraction remain undifferentiated and at risk of teratoma formation upon transplantation. A single case of stem cell-derived tumors may set the field back years. Thus, this review discusses four approaches to increase the safety of PSC-derived β cells: reprogramming of somatic cells into induced PSC, selection of pure differentiated pancreatic cells, depletion of contaminant PSC in the final cell product, and control or destruction of tumorigenic cells with engineered suicide genes.

Method for selective ablation of undifferentiated human pluripotent stem cell populations for cell-based therapies

Human pluripotent stem cells (PSCs), which are composed of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), provide an opportunity to advance cardiac cell therapy-based clinical trials. However, an important hurdle that must be overcome is the risk of teratoma formation after cell transplantation due to the proliferative capacity of residual undifferentiated PSCs in differentiation batches. To tackle this problem, we propose the use of a minimal noncardiotoxic doxorubicin dose as a purifying agent to selectively target rapidly proliferating stem cells for cell death, which will provide a purer population of terminally differentiated cardiomyocytes before cell transplantation. In this study, we determined an appropriate in vitro doxorubicin dose that (a) eliminates residual undifferentiated stem cells before cell injection to prevent teratoma formation after cell transplantation and (b) does not cause cardiotoxicity in ESC-derived cardiomyocytes (CMs) as demonstrated through contractility analysis, electrophysiology, topoisomerase activity assay, and quantification of reactive oxygen species generation. This study establishes a potentially novel method for tumorigenic-free cell therapy studies aimed at clinical applications of cardiac cell transplantation.

Salicylic diamines selectively eliminate residual undifferentiated cells from pluripotent stem cell-derived cardiomyocyte preparations

Clinical translation of pluripotent stem cell (PSC) derivatives is hindered by the tumorigenic risk from residual undifferentiated cells. Here, we identified salicylic diamines as potent agents exhibiting toxicity to murine and human PSCs but not to cardiomyocytes (CMs) derived from them. Half maximal inhibitory concentrations (IC₅₀) of small molecules SM2 and SM6 were, respectively, 9- and 18-fold higher for human than murine PSCs, while the IC₅₀ of SM8 was comparable for both PSC groups. Treatment of murine embryoid bodies in suspension differentiation cultures with the most effective small molecule SM6 significantly reduced PSC and non-PSC contamination and enriched CM populations that would otherwise be eliminated in genetic selection approaches. All tested salicylic diamines exerted their toxicity by inhibiting the oxygen consumption rate (OCR) in PSCs. No or only minimal and reversible effects on OCR, sarcomeric integrity, DNA stability, apoptosis rate, ROS levels or beating frequency were observed in PSC-CMs, although effects on human PSC-CMs seemed to be more deleterious at higher SM-concentrations. Teratoma formation from SM6-treated murine PSC-CMs was abolished or delayed compared to untreated cells. We conclude that salicylic diamines represent promising compounds for PSC removal and enrichment of CMs without the need for other selection strategies.

Selective Elimination of Culture-Adapted Human Embryonic Stem Cells with BH3 Mimetics

The selective survival advantage of culture-adapted human embryonic stem cells (hESCs) is a serious safety concern for their clinical application. With a set of hESCs with various passage numbers, we observed that a subpopulation of hESCs at late passage numbers was highly resistant to various cell death stimuli, such as YM155, a survivin inhibitor. Transcriptome analysis from YM155-sensitive (YM155S) and YM155-resistant (YM155R) hESCs demonstrated that BCL2L1 was highly expressed in YM155R hESCs. By matching the gene signature of YM155R hESCs with the Cancer Therapeutics Response Portal dataset, BH3 mimetics were predicted to selectively ablate these cells. Indeed, short-course treatment with a sub-optimal dose of BH3 mimetics induced the spontaneous death of YM155R, but not YM155S hESCs by disrupting the mitochondrial membrane potential. YM155S hESCs remained pluripotent following BH3 mimetics treatment. Therefore, the use of BH3 mimetics is a promising strategy to specifically eliminate hESCs with a selective survival advantage.

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Culture-adapted hESCs against YM155/genotoxic agents mediated by high BCL-xL expression

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Selective cell death of culture-adapted hPSCs by BH3 mimetics

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Pluripotency maintenance of normal hESCs after exposure to BH3 mimetics

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.

Quantification of Etoposide Hypersensitivity: A Sensitive, Functional Method for Assessing Pluripotent Stem Cell Quality

Human induced pluripotent stem cells (hiPSC) hold great promise in diagnostic and therapeutic applications. However, translation of hiPSC technology depends upon a means of assessing hiPSC quality that is quantitative, high‐throughput, and can decipher malignant teratocarcinoma clones from normal cell lines. These attributes are lacking in current approaches such as detection of cell surface makers, RNA profiling, and/or teratoma formation assays. The latter remains the gold standard for assessing clone quality in hiPSCs, but is expensive, time‐consuming, and incompatible with high‐throughput platforms. Herein, we describe a novel method for determining hiPSC quality that exploits pluripotent cells’ documented hypersensitivity to the topoisomerase inhibitor etoposide (CAS No. 33419‐42‐0). Based on a study of 115 unique hiPSC clones, we established that a half maximal effective concentration (EC50) value of <300 nM following 24 hours of exposure to etoposide demonstrated a positive correlation with RNA profiles and colony morphology metrics associated with high quality hiPSC clones. Moreover, our etoposide sensitivity assay (ESA) detected differences associated with culture maintenance, and successfully distinguished malignant from normal pluripotent clones independent of cellular morphology. Overall, the ESA provides a simple, straightforward method to establish hiPSC quality in a quantitative and functional assay capable of being incorporated into a generalized method for establishing a quality control standard for all types of pluripotent stem cells. Stem Cells Translational Medicine2017;6:1829–1839

Technical approaches to induce selective cell death of pluripotent stem cells

Despite the recent promising results of clinical trials using human pluripotent stem cell (hPSC)-based cell therapies for age-related macular degeneration (AMD), the risk of teratoma formation resulting from residual undifferentiated hPSCs remains a serious and critical hurdle for broader clinical implementation. To mitigate the tumorigenic risk of hPSC-based cell therapy, a variety of approaches have been examined to ablate the undifferentiated hPSCs based on the unique molecular properties of hPSCs. In the present review, we offer a brief overview of recent attempts at selective elimination of undifferentiated hPSCs to decrease the risk of teratoma formation in hPSC-based cell therapy.

Dinaciclib potently suppresses MCL-1 and selectively induces the cell death in human iPS cells without affecting the viability of cardiac tissue

Induced pluripotent stem (iPS) cells hold great potential for being a major source of cells for regenerative medicine. One major issue that hinders their advancement to clinic is the persistence of undifferentiated iPS cells in iPS-derived tissue. In this report, we show that the CDKs inhibitor, Dinaciclib, selectively eliminates iPS cells without affecting the viability of cardiac cells. We found that low nanomolar concentration of dinaciclib increased DNA damage and p53 protein levels in iPSCs. This was accompanied by negative regulation of the anti-apoptotic protein MCL-1. Gene knockdown experiments revealed that p53 downregulation only increased the threshold of dinaciclib induced apoptosis in iPS cells. Dinaciclib also inhibited the phosphorylation of Serine 2 of the C-terminal domain of RNA Polyemrase II through CDK9 inhibition. This resulted in the inhibition of transcription of MCL-1 and the pluripotency genes, NANOG and c-MYC. Even though dinaciclib caused a slight downregulation of MCL-1 in iPS-derived cardiac cells, the viability of the cells was not significantly affected, and beating iPS-derived cardiac cell sheet could still be fabricated. These findings suggest a difference in tolerance of MCL-1 downregulation between iPSCs and iPS-derived cardiac cells which could be exploited to eliminate remaining iPS cells in bioengineered cell sheet tissues.

New Monoclonal Antibodies to Defined Cell Surface Proteins on Human Pluripotent Stem Cells

The study and application of human pluripotent stem cells (hPSCs) will be enhanced by the availability of well‐characterized monoclonal antibodies (mAbs) detecting cell‐surface epitopes. Here, we report generation of seven new mAbs that detect cell surface proteins present on live and fixed human ES cells (hESCs) and human iPS cells (hiPSCs), confirming our previous prediction that these proteins were present on the cell surface of hPSCs. The mAbs all show a high correlation with POU5F1 (OCT4) expression and other hPSC surface markers (TRA‐160 and SSEA‐4) in hPSC cultures and detect rare OCT4 positive cells in differentiated cell cultures. These mAbs are immunoreactive to cell surface protein epitopes on both primed and naive state hPSCs, providing useful research tools to investigate the cellular mechanisms underlying human pluripotency and states of cellular reprogramming. In addition, we report that subsets of the seven new mAbs are also immunoreactive to human bone marrow‐derived mesenchymal stem cells (MSCs), normal human breast subsets and both normal and tumorigenic colorectal cell populations. The mAbs reported here should accelerate the investigation of the nature of pluripotency, and enable development of robust cell separation and tracing technologies to enrich or deplete for hPSCs and other human stem and somatic cell types. Stem Cells2017;35:626–640

Building A New Treatment For Heart Failure-Transplantation of Induced Pluripotent Stem Cell-derived Cells into the Heart

Advanced cardiac failure is a progressive intractable disease and is the main cause of mortality and morbidity worldwide. Since this pathology is represented by a definite decrease in cardiomyocyte number, supplementation of functional cardiomyocytes into the heart would hypothetically be an ideal therapeutic option. Recently, unlimited in vitro production of human functional cardiomyocytes was established by using induced pluripotent stem cell (iPSC) technology, which avoids the use of human embryos. A number of basic studies including ours have shown that transplantation of iPSC-derived cardiomyocytes (iPSC-CMs) into the damaged heart leads to recovery of cardiac function, thereby establishing “proof-of-concept” of this iPSC-transplantation therapy. However, considering clinical application of this therapy, its feasibility, safety, and therapeutic efficacy need to be further investigated in the pre-clinical stage. This review summarizes up-to-date important topics related to safety and efficacy of iPSC-CMs transplantation therapy for cardiac disease and discusses the prospects for this treatment in clinical studies.

Tumor prevention facilitates delayed transplant of stem cell-derived motoneurons

OBJECTIVE

Nerve injuries resulting in prolonged periods of denervation result in poor recovery of motor function. We have previously shown that embryonic stem cell-derived motoneurons transplanted at the time of transection into a peripheral nerve can functionally reinnervate muscle. For clinical relevance, we now focused on delaying transplantation to assess reinnervation after prolonged denervation.

METHODS

Embryonic stem cell-derived motoneurons were transplanted into the distal segments of transected tibial nerves in adult mice after prolonged denervation of 1-8 weeks. Twitch and tetanic forces were measured ex vivo 3 months posttransplantation. Tissue was harvested from the transplants for culture and immunohistochemical analysis.

RESULTS

In this delayed reinnervation model, teratocarcinomas developed in about one half of transplants. A residual multipotent cell population (~ 6% of cells) was found despite neural differentiation. Exposure to the alkylating drug mitomycin C eliminated this multipotent population in vitro while preserving motoneurons. Treating neural differentiated stem cells prior to delayed transplantation prevented tumor formation and resulted in twitch and tetanic forces similar to those in animals transplanted acutely after denervation.

INTERPRETATION

Despite a neural differentiation protocol, embryonic stem cell-derived motoneurons still carry a risk of tumorigenicity. Pretreating with an antimitotic agent leads to survival and functional muscle reinnervation if performed within 4 weeks of denervation in the mouse.

Quercetin induced ROS production triggers mitochondrial cell death of human embryonic stem cells

Small molecules to selectively induce cell death of undifferentiated human pluripotent stem cells (hPSCs) have been developed with the aim of lowering the risk of teratoma formation during hPSC-based cell therapy. In this context, we have reported that Quercetin (QC) induces cell death selectively in hESCs via p53 mitochondrial localization. However, the detailed molecular mechanism by which hESCs undergo selective cell death induced by QC remains unclear. Herein, we demonstrate that mitochondrial reactive oxygen species (ROS), strongly induced by QC in human embryonic stem cells (hESCs) but not in human dermal fibroblasts (hDFs), were responsible for QC-mediated hESC's cell death. Increased p53 protein stability and subsequent mitochondrial localization by QC treatment triggered mitochondrial cell death only in hESCs. Of interest, peptidylprolyl isomerase D [PPID, also called cyclophilin D (CypD)], which functions in mitochondrial permeability transition and mitochondrial cell death, was highly expressed in hESCs. Inhibition of CypD by cyclosporine A (CsA) clearly inhibited the QC-mediated loss of mitochondrial membrane potential and mitochondrial cell death. These results suggest that p53 and CypD in the mitochondria are critical for the QC-mediated induction of cell death in hESCs.

Stem cells for the treatment of heart failure

Stem cell-based therapy is currently tested in several trials of chronic heart failure. The main question is to determine how its implementation could be extended to standard clinical practice. To answer this question, it is helpful to capitalize on the three main lessons drawn from the accumulated experience, both in the laboratory and in the clinics. Regarding the cell type, the best outcomes seem to be achieved by cells the phenotype of which closely matches that of the target tissue. This argues in favor of the use of cardiac-committed cells among which the pluripotent stem cell-derived cardiac progeny is particularly attractive. Regarding the mechanism of action, there has been a major paradigm shift whereby cells are no longer expected to structurally integrate within the recipient myocardium but rather to release biomolecules that foster endogenous repair processes. This implies to focus on early cell retention, rather than on sustained cell survival, so that the cells reside in the target tissue long enough and in sufficient amounts to deliver the factors underpinning their action. Biomaterials are here critical adjuncts to optimize this residency time. Furthermore, the paracrine hypothesis gives more flexibility for using allogeneic cells in that targeting an only transient engraftment requires to delay, and no longer to avoid, rejection, which, in turn, should simplify immunomodulation regimens. Regarding manufacturing, a broad dissemination of cardiac cell therapy requires the development of automated systems allowing to yield highly reproducible cell products. This further emphasizes the interest of allogeneic cells because of their suitability for industrially-relevant and cost-effective scale-up and quality control procedures. At the end, definite confirmation that the effects of cells can be recapitulated by the factors they secrete could lead to acellular therapies whereby factors alone (possibly clustered in extracellular vesicles) would be delivered to the patient. The production process of these cell-derived biologics would then be closer to that of a pharmaceutical compound, which could streamline the manufacturing and regulatory paths and thereby facilitate an expended clinical use.

Selection via pluripotency-related transcriptional screen minimizes the influence of somatic origin on iPSC differentiation propensity

The value of induced pluripotent stem cells (iPSCs) within regenerative medicine is contingent on predictable and consistent iPSC differentiation. However, residual influence of the somatic origin or reprogramming technique may variegate differentiation propensity and confound comparative genotype/phenotype analyses. The objective of this study was to define quality control measures to select iPSC clones that minimize the influence of somatic origin on differentiation propensity independent of the reprogramming strategy. More than 60 murine iPSC lines were derived from different fibroblast origins (embryonic, cardiac, and tail tip) via lentiviral integration and doxycycline-induced transgene expression. Despite apparent equivalency according to established iPSC histologic and cytomorphologic criteria, clustering of clonal variability in pluripotency-related gene expression identified transcriptional outliers that highlighted cell lines with unpredictable cardiogenic propensity. Following selection according to a standardized gene expression profile calibrated by embryonic stem cells, the influence of somatic origin on iPSC methylation and transcriptional patterns was negated. Furthermore, doxycycline-induced iPSCs consistently demonstrated earlier differentiation than lentiviral-reprogrammed lines using contractile cardiac tissue as a measure of functional differentiation. Moreover, delayed cardiac differentiation was predominately associated with upregulation in pluripotency-related gene expression upon differentiation. Starting from a standardized pool of iPSCs, relative expression levels of two pluripotency genes, Oct4 and Zfp42, statistically correlated with enhanced cardiogenicity independent of somatic origin or reprogramming strategy (R(2)  = 0.85). These studies demonstrate that predictable iPSC differentiation is independent of somatic origin with standardized gene expression selection criteria, while the residual impact of reprogramming strategy greatly influences predictable output of tissue-specification required for comparative genotype/phenotype analyses.

Stem cells: the pursuit of genomic stability

Stem cells harbor significant potential for regenerative medicine as well as basic and clinical translational research. Prior to harnessing their reparative nature for degenerative diseases, concerns regarding their genetic integrity and mutation acquisition need to be addressed. Here we review pluripotent and multipotent stem cell response to DNA damage including differences in DNA repair kinetics, specific repair pathways (homologous recombination vs. non-homologous end joining), and apoptotic sensitivity. We also describe DNA damage and repair strategies during reprogramming and discuss potential genotoxic agents that can reduce the inherent risk for teratoma formation and mutation accumulation. Ensuring genomic stability in stem cell lines is required to achieve the quality control standards for safe clinical application.

Metabolic regulation of redox status in stem cells

SIGNIFICANCE

Metabolism-dependent generation of reactive oxygen species (ROS) and associated oxidative damage have been traditionally linked to impaired homeostasis and cellular death. Beyond the adverse effects of ROS accumulation, increasing evidence implicates redox status as a regulator of vital cellular processes.

RECENT ADVANCES

Emerging studies on the molecular mechanisms guiding stem cell fate decisions indicate a role for energy metabolism in regulating the fundamental ability of maintaining stemness versus undergoing lineage-specific differentiation. Stem cells have evolved protective metabolic phenotypes to minimize reactive oxygen generation through oxidative metabolism and support antioxidant scavenging through glycolysis and the pentose phosphate pathway.

CRITICAL ISSUES

While the dynamics in ROS generation has been correlated with stem cell function, the intimate mechanisms by which energy metabolism regulates ROS to impact cellular fate remain to be deciphered.

FUTURE DIRECTIONS

Decoding the linkage between nutrient sensing, energy metabolism, and ROS in regulating cell fate decisions would offer a redox-dependent strategy to regulate stemness and lineage specification.

Cell replacement therapies: is it time to reprogram?

Hematopoietic stem cell transplantations have become a very successful therapeutic approach to treat otherwise life-threatening blood disorders. It is thought that stem cell transplantation may also become a feasible treatment option for many non-blood-related diseases. So far, however, the limited availability of human leukocyte antigen-matched donors has hindered development of some cell replacement therapies. The Nobel-prize rewarded finding that pluripotency can be induced in somatic cells via expression of a few transcription factors has led to a revolution in stem cell biology. The possibility to change the fate of somatic cells by expressing key transcription factors has been used not only to generate pluripotent stem cells, but also for directly converting somatic cells into fully differentiated cells of another lineage or more committed progenitor cells. These approaches offer the prospect of generating cell types with a specific genotype de novo, which would circumvent the problems associated with allogeneic cell transplantations. This technology has generated a plethora of new disease-specific research efforts, from studying disease pathogenesis to therapeutic interventions. Here we will discuss the opportunities in this booming field of cell biology and summarize how the scientists in the Netherlands have joined efforts in one area to exploit the new technology.

Inhibition of DNA topoisomerase II selectively reduces the threat of tumorigenicity following induced pluripotent stem cell-based myocardial therapy

The advent of induced pluripotent stem cell (iPSC) technology creates new opportunities for transplant-based therapeutic strategies. The potential for clinical translation is currently hindered by the risk of dysregulated cell growth. Pluripotent stem cells reprogrammed by three-factor (Sox2, Klf, and Oct4) and four-factor (Sox2, Klf, Oct4, and c-Myc) strategies result in the capacity for teratogenic growth from residual pluripotent progeny upon in vivo transplantation. However, these pluripotent stem cells also have a stage-specific hypersensitivity to DNA-damaging agents that may allow separation of lineage-specific therapeutic subpopulation of cells. We aimed to demonstrate the selective effect of DNA topoisomerase II inhibitor, etoposide, in eliminating pluripotent cells in the early cardiac progenitor population thus decreasing the effect of teratoma formation. Immunodeficient murine hearts were infarcted and received implantation of a therapeutic dose of cardiac progenitors derived from partially differentiated iPSCs. Etoposide-treated cell implantation reduced mass formation in the intracardiac and extracardiac chest cavity compared with the same dose of iPSC-derived cardiac progenitors in the control untreated group. In vivo bioluminescence imaging confirmed the localization and engraftment of transplanted cells in the myocardium postinjection in both groups. Comparatively, the equivalent cell population without etoposide treatment demonstrated a greater incidence and size of teratoma formation. Hence, pretreatment with genotoxic etoposide significantly lowered the threat of teratogenicity by purging the contaminating pluripotent cells, establishing an adjunctive therapy to further harness the clinical value of iPSC-derived cardiac regeneration.

High-risk human papillomavirus E6 protein promotes reprogramming of Fanconi anemia patient cells through repression of p53 but does not allow for sustained growth of induced pluripotent stem cells

DNA repair plays a crucial role in embryonic and somatic stem cell biology and cell reprogramming. The Fanconi anemia (FA) pathway, which promotes error-free repair of DNA double-strand breaks, is required for somatic cell reprogramming to induced pluripotent stem cells (iPSC). Thus, cells from Fanconi anemia patients, which lack this critical pathway, fail to be reprogrammed to iPSC under standard conditions unless the defective FA gene is complemented. In this study, we utilized the oncogenes of high-risk human papillomavirus 16 (HPV16) to overcome the resistance of FA patient cells to reprogramming. We found that E6, but not E7, recovers FA iPSC colony formation and, furthermore, that p53 inhibition is necessary and sufficient for this activity. The iPSC colonies resulting from each of these approaches stained positive for alkaline phosphatase, NANOG, and Tra-1-60, indicating that they were fully reprogrammed into pluripotent cells. However, FA iPSC were incapable of outgrowth into stable iPSC lines regardless of p53 suppression, whereas their FA-complemented counterparts grew efficiently. Thus, we conclude that the FA pathway is required for the growth of iPSC beyond reprogramming and that p53-independent mechanisms are involved.

IMPORTANCE

A novel approach is described whereby HPV oncogenes are used as tools to uncover DNA repair-related molecular mechanisms affecting somatic cell reprogramming. The findings indicate that p53-dependent mechanisms block FA cells from reprogramming but also uncover a previously unrecognized defect in FA iPSC proliferation independent of p53.

Inducing pluripotency for disease modeling, drug development and craniofacial applications

INTRODUCTION

The induced pluripotent stem cells (iPSCs) have characteristics similar to embryonic stem cells, including the capability of self-renewal and large-scale expansion and the ability to differentiate into all types of cells including germ cells, which defines pluripotency. Using iPSC avoids problems of immunological rejection and ethical controversy. The possible future uses of iPSC are diverse and go beyond the differentiation into somatic cells for regeneration of damaged tissues.

AREAS COVERED

A unique feature of iPSC is the potential to generate patient disease-specific tissues. Thus, cells from patients can be differentiated into relevant cells of interest for drug screening, characterization of drug effects and cytotoxic assays. This review presents key aspects related to iPSC, such as their generation, potential for disease modeling, treatment, drug development and future contributions to the craniofacial complex.

EXPERT OPINION

It is undisputable that the evolution in iPSC knowledge will improve the approaches for drug screening and development, help to understand and treat disease origins and mechanisms and provide new strategies to clinical treatment. However, it is necessary to fine-tune protocols to establish iPSCs that are cost-effective and safe for clinical use.

Towards a clinical use of human embryonic stem cell-derived cardiac progenitors: a translational experience

AIM

There is now compelling evidence that cells committed to a cardiac lineage are most effective for improving the function of infarcted hearts. This has been confirmed by our pre-clinical studies entailing transplantation of human embryonic stem cell (hESC)-derived cardiac progenitors in rat and non-human primate models of myocardial infarction. These data have paved the way for a translational programme aimed at a phase I clinical trial.

METHODS AND RESULTS

The main steps of this programme have included (i) the expansion of a clone of pluripotent hESC to generate a master cell bank under good manufacturing practice conditions (GMP); (ii) a growth factor-induced cardiac specification; (iii) the purification of committed cells by immunomagnetic sorting to yield a stage-specific embryonic antigen (SSEA)-1-positive cell population strongly expressing the early cardiac transcription factor Isl-1; (iv) the incorporation of these cells into a fibrin scaffold; (v) a safety assessment focused on the loss of teratoma-forming cells by in vitro (transcriptomics) and in vivo (cell injections in immunodeficient mice) measurements; (vi) an extensive cytogenetic and viral testing; and (vii) the characterization of the final cell product and its release criteria. The data collected throughout this process have led to approval by the French regulatory authorities for a first-in-man clinical trial of transplantation of these SSEA-1(+) progenitors in patients with severely impaired cardiac function.

CONCLUSION

Although several facets of this manufacturing process still need to be improved, these data may yet provide a useful platform for the production of hESC-derived cardiac progenitor cells under safe and cost-effective GMP conditions.

Embryonic Decellularized Cardiac Scaffold Supports Embryonic Stem Cell Differentiation to Produce Beating Cardiac Tissue

Regenerative medicine offers a curative approach to treating heart disease through multiple emerging therapeutic concepts. Decellularized organ scaffolds are being optimized to guide and spatially organize stem cell differentiation in efforts to rebuild functional tissues. Additionally, pluripotent stem cells offer a transformative cell source to differentiate into the full spectrum of cellular building blocks. Adult cardiac tissues have been used as extracellular scaffolds as a proof of principle; however, matching the developmental stages of embryonic scaffold with primitive cardiac progenitors may be used to optimize the differentiation and maturation of bioengineered cardiac tissues. Our novel approach uses embryo-derived decellularized hearts as scaffolds to promote embryonic stem cell differentiation. Further, we determined that agitation with 0.25% sodium dodecyl sulfate (SDS) solution was the most effective protocol to maintain matrix integrity while eliminating endogenous cells. The scaffolds were successfully reseeded with different cellular sources derived from pluripotent stem cells to achieve beating cardiac tissues characterized by endothelial, cardiac, and smooth muscle markers. Therefore, embedding stem cells within a tissue-specific environment matched to the developmental stage of the progenitors may offer a practical solution for stem-cell-derived applications such as disease modeling, pharmaceutical safety testing, and screening of novel therapeutic targets.

Rbm20-deficient cardiogenesis reveals early disruption of RNA processing and sarcomere remodeling establishing a developmental etiology for dilated cardiomyopathy

Dilated cardiomyopathy (DCM) due to mutations in RBM20, a gene encoding an RNA-binding protein, is associated with high familial penetrance, risk of progressive heart failure and sudden death. Although genetic investigations and physiological models have established the linkage of RBM20 with early-onset DCM, the underlying basis of cellular and molecular dysfunction is undetermined. Modeling human genetics using a high-throughput pluripotent stem cell platform was herein designed to pinpoint the initial transcriptome dysfunction and mechanistic corruption in disease pathogenesis. Tnnt2-pGreenZeo pluripotent stem cells were engineered to knockdown Rbm20 (shRbm20) to determine the cardiac-pathogenic phenotype during cardiac differentiation. Intracellular Ca(2+) transients revealed Rbm20-dependent alteration in Ca(2+) handling, coinciding with known pathological splice variants of Titin and Camk2d genes by Day 24 of cardiogenesis. Ultrastructural analysis demonstrated elongated and thinner sarcomeres in the absence of Rbm20 that is consistent with human cardiac biopsy samples. Furthermore, Rbm20-depleted transcriptional profiling at Day 12 identified Rbm20-dependent dysregulation with 76% of differentially expressed genes linked to known cardiac pathology ranging from primordial Nkx2.5 to mature cardiac Tnnt2 as the initial molecular aberrations. Notably, downstream consequences of Rbm20-depletion at Day 24 of differentiation demonstrated significant dysregulation of extracellular matrix components such as the anomalous overexpression of the Vtn gene. By using the pluripotent stem cell platform to model human cardiac disease according to a stage-specific cardiogenic roadmap, we established a new paradigm of familial DCM pathogenesis as a developmental disorder that is patterned during early cardiogenesis and propagated with cellular mechanisms of pathological cardiac remodeling.

Survivin and YM155: how faithful is the liaison?

Survivin belongs to the family of apoptosis inhibitors (IAPs), which antagonizes the induction of cell death. Dysregulated expression of IAPs is frequently observed in cancers, and the high levels of survivin in tumors compared to normal adult tissues make it an attractive target for pharmacological interventions. The small imidazolium-based compound YM155 has recently been reported to block the expression of survivin via inhibition of the survivin promoter. Recent data, however, question that this is the sole and main effect of this drug, which is already being tested in ongoing clinical studies. Here, we critically review the current data on YM155 and other new experimental agents supposed to antagonize survivin. We summarize how cells from various tumor entities and with differential expression of the tumor suppressor p53 respond to this agent in vitro and as murine xenografts. Additionally, we recapitulate clinical trials conducted with YM155. Our article further considers the potency of YM155 in combination with other anti-cancer agents and epigenetic modulators. We also assess state-of-the-art data on the sometimes very promiscuous molecular mechanisms affected by YM155 in cancer cells.

New and TALENted genome engineering toolbox

Recent advances in the burgeoning field of genome engineering are accelerating the realization of personalized therapeutics for cardiovascular disease. In the postgenomic era, sequence-specific gene-editing tools enable the functional analysis of genetic alterations implicated in disease. In partnership with high-throughput model systems, efficient gene manipulation provides an increasingly powerful toolkit to study phenotypes associated with patient-specific genetic defects. Herein, this review emphasizes the latest developments in genome engineering and how applications within the field are transforming our understanding of personalized medicine with an emphasis on cardiovascular diseases.

Natural cardiogenesis-based template predicts cardiogenic potential of induced pluripotent stem cell lines

BACKGROUND

Cardiac development is a complex process resulting in an integrated, multilineage tissue with developmental corruption in early embryogenesis leading to congenital heart disease. Interrogation of individual genes has provided the backbone for cardiac developmental biology, yet a comprehensive transcriptome derived from natural cardiogenesis is required to gauge innate developmental milestones.

METHODS AND RESULTS

Stage-specific cardiac structures were dissected from 8 distinctive mouse embryonic time points to produce genome-wide expressome analysis across cardiogenesis. With reference to this native cardiogenic expression roadmap, divergent induced pluripotent stem cell-derived cardiac expression profiles were mapped from procardiogenic 3-factor (SOX2, OCT4, KLF4) and less-cardiogenic 4-factor (plus c-MYC) reprogrammed cells. Expression of cardiac-related genes from 3-factor-induced pluripotent stem cell differentiated in vitro at days 5 and 11 and recapitulated expression profiles of natural embryos at days E7.5-E8.5 and E14.5-E18.5, respectively. By contrast, 4-factor-induced pluripotent stem cells demonstrated incomplete cardiogenic gene expression profiles beginning at day 5 of differentiation. Differential gene expression within the pluripotent state revealed 23 distinguishing candidate genes among pluripotent cell lines with divergent cardiogenic potentials. A confirmed panel of 12 genes, differentially expressed between high and low cardiogenic lines, was transformed into a predictive score sufficient to discriminate individual induced pluripotent stem cell lines according to relative cardiogenic potential.

CONCLUSIONS

Transcriptome analysis attuned to natural embryonic cardiogenesis provides a robust platform to probe coordinated cardiac specification and maturation from bioengineered stem cell-based model systems. A panel of developmental-related genes allowed differential prognosis of cardiogenic competency, thus prioritizing cell lines according to natural blueprint to streamline functional applications.

Inhibition of pluripotent stem cell-derived teratoma formation by small molecules

The future of safe cell-based therapy rests on overcoming teratoma/tumor formation, in particular when using human pluripotent stem cells (hPSCs), such as human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs). Because the presence of a few remaining undifferentiated hPSCs can cause undesirable teratomas after transplantation, complete removal of these cells with no/minimal damage to differentiated cells is a prerequisite for clinical application of hPSC-based therapy. Having identified a unique hESC signature of pro- and antiapoptotic gene expression profile, we hypothesized that targeting hPSC-specific antiapoptotic factor(s) (i.e., survivin or Bcl10) represents an efficient strategy to selectively eliminate pluripotent cells with teratoma potential. Here we report the successful identification of small molecules that can effectively inhibit these antiapoptotic factors, leading to selective and efficient removal of pluripotent stem cells through apoptotic cell death. In particular, a single treatment of hESC-derived mixed population with chemical inhibitors of survivin (e.g., quercetin or YM155) induced selective and complete cell death of undifferentiated hPSCs. In contrast, differentiated cell types (e.g., dopamine neurons and smooth-muscle cells) derived from hPSCs survived well and maintained their functionality. We found that quercetin-induced selective cell death is caused by mitochondrial accumulation of p53 and is sufficient to prevent teratoma formation after transplantation of hESC- or hiPSC-derived cells. Taken together, these results provide the "proof of concept" that small-molecule targeting of hPSC-specific antiapoptotic pathway(s) is a viable strategy to prevent tumor formation by selectively eliminating remaining undifferentiated pluripotent cells for safe hPSC-based therapy.

Regenerative medicine primer

The pandemic of chronic diseases, compounded by the scarcity of usable donor organs, mandates radical innovation to address the growing unmet needs of individuals and populations. Beyond life-extending measures that are often the last available option, regenerative strategies offer transformative solutions in treating degenerative conditions. By leveraging newfound knowledge of the intimate processes fundamental to organogenesis and healing, the emerging regenerative armamentarium aims to boost the aptitude of human tissues for self-renewal. Regenerative technologies strive to promote, augment, and reestablish native repair processes, restituting organ structure and function. Multimodal regenerative approaches incorporate transplant of healthy tissues into damaged environments, prompt the body to enact a regenerative response in damaged tissues, and use tissue engineering to manufacture new tissue. Stem cells and their products have a unique aptitude to form specialized tissues and promote repair signaling, providing active ingredients of regenerative regimens. Concomitantly, advances in materials science and biotechnology have unlocked additional prospects for growing tissue grafts and engineering organs. Translation of regenerative principles into practice is feasible and safe in the clinical setting. Regenerative medicine and surgery are, thus, poised to transit from proof-of-principle studies toward clinical validation and, ultimately, standardization, paving the way for next-generation individualized management algorithms.

Molecular imaging of induced pluripotent stem cell immunogenicity with in vivo development in ischemic myocardium

Whether differentiation of induced pluripotent stem cells (iPSCs) in ischemic myocardium enhances their immunogenicity, thereby increasing their chance for rejection, is unclear. Here, we dynamically demonstrated the immunogenicity and rejection of iPSCs in ischemic myocardium using bioluminescent imaging (BLI). Murine iPSCs were transduced with a tri-fusion (TF) reporter gene consisting of firefly luciferase-red fluorescent protein-truncated thymidine kinase (fluc-mrfp-tTK). Ascorbic acid (Vc) were used to induce iPSCs to differentiate into cardiomyocytes (CM). iPSCs and iPS-CMs were intramyocardially injected into immunocompetent or immunosuppressed allogenic murine with myocardial infarction. BLI was performed to track transplanted cells. Immune cell infiltration was evaluated by immunohistochemistry. Syngeneic iPSCs were also injected and evaluated. The results demonstrated that undifferentiated iPSCs survived and proliferated in allogenic immunocompetent recipients early post-transplantation, accompanying with mild immune cell infiltration. With in vivo differentiation, a progressive immune cell infiltration could be detected. While transplantation of allogenic iPSC-CMs were observed an acute rejection from receipts. In immune-suppressed recipients, the proliferation of iPSCs could be maintained and intramyocardial teratomas were formed. Transplantation of syngeneic iPSCs and iPSC-CMs were also observed progressive immune cell infiltration. This study demonstrated that iPSC immunogenicity increases with in vivo differentiation, which will increase their chance for rejection in iPSC-based therapy.