Pioneer and PRDM transcription factors coordinate bivalent epigenetic states to safeguard cell fate.

Pioneer transcription factors (TFs) regulate cell fate by establishing transcriptionally primed and active states. However, cell fate control requires the coordination of both lineage-specific gene activation and repression of alternative-lineage programs, a process that is poorly understood. Here, we demonstrate that the pioneer TF FOXA coordinates with PRDM1 TF to recruit nucleosome remodeling and deacetylation (NuRD) complexes and Polycomb repressive complexes (PRCs), which establish highly occupied, accessible nucleosome conformation with bivalent epigenetic states, thereby preventing precocious and alternative-lineage gene expression during human endoderm differentiation. Similarly, the pioneer TF OCT4 coordinates with PRDM14 to form bivalent enhancers and repress cell differentiation programs in human pluripotent stem cells, suggesting that this may be a common and critical function of pioneer TFs. We propose that pioneer and PRDM TFs coordinate to safeguard cell fate through epigenetic repression mechanisms.

Development of functional resident macrophages in human pluripotent stem cell-derived colonic organoids and human fetal colon.

Most organs have tissue-resident immune cells. Human organoids lack these immune cells, which limits their utility in modeling many normal and disease processes. Here, we describe that pluripotent stem cell-derived human colonic organoids (HCOs) co-develop a diverse population of immune cells, including hemogenic endothelium (HE)-like cells and erythromyeloid progenitors that undergo stereotypical steps in differentiation, resulting in the generation of functional macrophages. HCO macrophages acquired a transcriptional signature resembling human fetal small and large intestine tissue-resident macrophages. HCO macrophages modulate cytokine secretion in response to pro- and anti-inflammatory signals and were able to phagocytose and mount a robust response to pathogenic bacteria. When transplanted into mice, HCO macrophages were maintained within the colonic organoid tissue, established a close association with the colonic epithelium, and were not displaced by the host bone-marrow-derived macrophages. These studies suggest that HE in HCOs gives rise to multipotent hematopoietic progenitors and functional tissue-resident macrophages.

iPSC-derived neural precursor cells engineering GBA1 recovers acid ß-glucosidase deficiency and diminishes α-synuclein and neuropathology.

Mutations in GBA1, encoding the lysosomal acid ß-glucosidase (GCase), cause neuronopathic Gaucher disease (nGD) and promote Parkinson disease (PD). The mutations on GBA1 include deletion and missense mutations that are pathological and lead to GCase deficiency in Gaucher disease. Both nGD and PD lack disease-modifying treatments and are critical unmet medical needs. In this study, we evaluated a cell therapy treatment using mouse iPSC-derived neural precursor cells (NPCs) engineered to overexpress GCase (termed hGBA1-NPCs). The hGBA1-NPCs secreted GCase that was taken up by adjacent mouse Gba -/- neurons and improved GCase activity, reduced GCase substrate accumulation, and improved mitochondrial function. Short-term in vivo effects were evaluated in 9H/PS-NA mice, an nGD mouse model exhibiting neuropathology and α-synuclein aggregation, the typical PD phenotypes. Intravenously administrated hGBA1-NPCs were engrafted throughout the brain and differentiated into neural lineages. GCase activity was increased in various brain regions of treated 9H/PS-NA mice. Compared with vehicle, hGBA1-NPC-transplanted mice showed ∼50% reduction of α-synuclein aggregates in the substantia nigra, significant reduction of neuroinflammation and neurodegeneration in the regions of NPC migration, and increased expression of neurotrophic factors that support neural cell function. Together, these results support the therapeutic benefit of intravenous delivery of iPSC-derived NPCs overexpressing GCase in mitigating nGD and PD phenotypes and establish the feasibility of combined cell and gene therapy for GBA1-associated PD.

A review of protocols for brain organoids and applications for disease modeling.

Recent breakthroughs in human stem cell technologies have enabled the generation of 3D brain organoid platforms for modeling human neurodevelopment and disease. Here, we review advances in brain organoid development, approaches for generating whole-brain or cerebral organoids and region-specific brain organoids, and their applications in disease modeling. We present a comprehensive overview of various brain organoid generation protocols, including culture steps, media, timelines, and technical considerations associated with each protocol, and highlight the advantages and disadvantages of each protocol. We also discuss the current limitations as well as increasing sophistication of brain organoid technology, and future directions for the field. These insights provide a valuable assessment of multiple commonly used brain organoid models and main considerations for investigators who are considering implementing brain organoid technologies in their laboratories.

Aggregation of cryopreserved mid-hindgut endoderm for more reliable and reproducible hPSC-derived small intestinal organoid generation.

A major technical limitation hindering the widespread adoption of human pluripotent stem cell (hPSC)-derived gastrointestinal (GI) organoid technologies is the need for de novo hPSC differentiation and dependence on spontaneous morphogenesis to produce detached spheroids. Here, we report a method for simple, reproducible, and scalable production of small intestinal organoids (HIOs) based on the aggregation of cryopreservable hPSC-derived mid-hindgut endoderm (MHE) monolayers. MHE aggregation eliminates variability in spontaneous spheroid production and generates HIOs that are comparable to those arising spontaneously. With a minor modification to the protocol, MHE can be cryopreserved, thawed, and aggregated, facilitating HIO production without de novo hPSC differentiation. Finally, aggregation can also be used to generate antral stomach organoids and colonic organoids. This improved method removes significant barriers to the implementation and successful use of hPSC-derived GI organoid technologies and provides a framework for improved dissemination and increased scalability of GI organoid production.

Human iPSC-derived hepatocyte system models cholestasis with tight junction protein 2 deficiency.

Background & Aims: The truncating mutations in tight junction protein 2 (TJP2) cause progressive cholestasis, liver failure, and hepatocyte carcinogenesis. Due to the lack of effective model systems, there are no targeted medications for the liver pathology with TJP2 deficiency. We leveraged the technologies of patient-specific induced pluripotent stem cells (iPSC) and CRISPR genome-editing, and we aim to establish a disease model which recapitulates phenotypes of patients with TJP2 deficiency. Methods: We differentiated iPSC to hepatocyte-like cells (iHep) on the Transwell membrane in a polarized monolayer. Immunofluorescent staining of polarity markers was detected by a confocal microscope. The epithelial barrier function and bile acid transport of bile canaliculi were quantified between the two chambers of Transwell. The morphology of bile canaliculi was measured in iHep cultured in the Matrigel sandwich system using a fluorescent probe and live-confocal imaging. Results: The iHep differentiated from iPSC with TJP2 mutations exhibited intracellular inclusions of disrupted apical membrane structures, distorted canalicular networks, altered distribution of apical and basolateral markers/transporters. The directional bile acid transport of bile canaliculi was compromised in the mutant hepatocytes, resembling the disease phenotypes observed in the liver of patients. Conclusions: Our iPSC-derived in vitro hepatocyte system revealed canalicular membrane disruption in TJP2 deficient hepatocytes and demonstrated the ability to model cholestatic disease with TJP2 deficiency to serve as a platform for further pathophysiologic study and drug discovery. Lay summary: We investigated a genetic liver disease, progressive familial intrahepatic cholestasis (PFIC), which causes severe liver disease in newborns and infants due to a lack of gene called TJP2. By using cutting-edge stem cell technology and genome editing methods, we established a novel disease modeling system in cell culture experiments. Our experiments demonstrated that the lack of TJP2 induced abnormal cell polarity and disrupted bile acid transport. These findings will lead to the subsequent investigation to further understand disease mechanisms and develop an effective treatment.

High-Throughput Functional Analysis of CFTR and Other Apically Localized Proteins in iPSC-Derived Human Intestinal Organoids.

Induced Pluripotent Stem Cells (iPSCs) can be differentiated into epithelial organoids that recapitulate the relevant context for CFTR and enable testing of therapies targeting Cystic Fibrosis (CF)-causing mutant proteins. However, to date, CF-iPSC-derived organoids have only been used to study pharmacological modulation of mutant CFTR channel activity and not the activity of other disease-relevant membrane protein constituents. In the current work, we describe a high-throughput, fluorescence-based assay of CFTR channel activity in iPSC-derived intestinal organoids and describe how this method can be adapted to study other apical membrane proteins. Specifically, we show how this assay can be employed to study CFTR and ENaC channels and an electrogenic acid transporter in the same iPSC-derived intestinal tissue. This phenotypic platform promises to expand CF therapy discovery to include strategies that target multiple determinants of epithelial fluid transport.

Intravenous infusion of iPSC-derived neural precursor cells increases acid ß-glucosidase function in the brain and lessens the neuronopathic phenotype in a mouse model of Gaucher disease.

Gaucher disease (GD) is caused by GBA1 mutations leading to functional deficiency of acid-ß-glucosidase (GCase). No effective treatment is available for neuronopathic GD (nGD). A subclass of neural stem and precursor cells (NPCs) expresses VLA4 (integrin α4ß1, very late antigen-4) that facilitates NPC entry into the brain following intravenous (IV) infusion. Here, the therapeutic potential of IV VLA4+NPCs was assessed for nGD using wild-type mouse green fluorescent protein (GFP)-positive multipotent induced pluripotent stem cell (iPSC)-derived VLA4+NPCs. VLA4+NPCs successfully engrafted in the nGD (4L;C*) mouse brain. GFP-positive cells differentiated into neurons, astrocytes and oligodendrocytes in the brainstem, midbrain and thalamus of the transplanted mice and significantly improved sensorimotor function and prolonged life span compared to vehicle-treated 4L;C* mice. VLA4+NPC transplantation significantly decreased levels of CD68 and glial fibrillary acidic protein, as well as TNFα mRNA levels in the brain, indicating reduced neuroinflammation. Furthermore, decreased Fluoro-Jade C and NeuroSilver staining suggested inhibition of neurodegeneration. VLA4+NPC-engrafted 4L;C* midbrains showed 35% increased GCase activity, reduced substrate [glucosylceramide (GC, -34%) and glucosylsphingosine (GS, -11%)] levels and improved mitochondrial oxygen consumption rates in comparison to vehicle-4L;C* mice. VLA4+NPC engraftment in 4L;C* brain also led to enhanced expression of neurotrophic factors that have roles in neuronal survival and the promotion of neurogenesis. This study provides evidence that iPSC-derived NPC transplantation has efficacy in an nGD mouse model and provides proof of concept for autologous NPC therapy in nGD.

Noncoding deletions reveal a gene that is critical for intestinal function.

Large-scale genome sequencing is poised to provide a substantial increase in the rate of discovery of disease-associated mutations, but the functional interpretation of such mutations remains challenging. Here we show that deletions of a sequence on human chromosome 16 that we term the intestine-critical region (ICR) cause intractable congenital diarrhoea in infants1,2. Reporter assays in transgenic mice show that the ICR contains a regulatory sequence that activates transcription during the development of the gastrointestinal system. Targeted deletion of the ICR in mice caused symptoms that recapitulated the human condition. Transcriptome analysis revealed that an unannotated open reading frame (Percc1) flanks the regulatory sequence, and the expression of this gene was lost in the developing gut of mice that lacked the ICR. Percc1-knockout mice displayed phenotypes similar to those observed upon ICR deletion in mice and patients, whereas an ICR-driven Percc1 transgene was sufficient to rescue the phenotypes found in mice that lacked the ICR. Together, our results identify a gene that is critical for intestinal function and underscore the need for targeted in vivo studies to interpret the growing number of clinical genetic findings that do not affect known protein-coding genes.

Modeling Steatohepatitis in Humans with Pluripotent Stem Cell-Derived Organoids.

Human organoid systems recapitulate in vivo organ architecture yet fail to capture complex pathologies such as inflammation and fibrosis. Here, using 11 different healthy and diseased pluripotent stem cell lines, we developed a reproducible method to derive multi-cellular human liver organoids composed of hepatocyte-, stellate-, and Kupffer-like cells that exhibit transcriptomic resemblance to in vivo-derived tissues. Under free fatty acid treatment, organoids, but not reaggregated cocultured spheroids, recapitulated key features of steatohepatitis, including steatosis, inflammation, and fibrosis phenotypes in a successive manner. Interestingly, an organoid-level biophysical readout with atomic force microscopy demonstrated that organoid stiffening reflects the fibrosis severity. Furthermore, organoids from patients with genetic dysfunction of lysosomal acid lipase phenocopied severe steatohepatitis, rescued by FXR agonism-mediated reactive oxygen species suppression. The presented key methodology and preliminary results offer a new approach for studying a personalized basis for inflammation and fibrosis in humans, thus facilitating the discovery of effective treatments.

Digitalized Human Organoid for Wireless Phenotyping.

Radio frequency identification (RFID) is a cost-effective and durable method to trace and track individual objects in multiple contexts by wirelessly providing digital signals; RFID is thus widely used in many fields. Here, we implement this concept to biological tissues by producing a compact RFID chip-incorporated organoid (RiO). The 0.4 mm RFID chips are reproducibly integrated inside the self-assembling organoids from 10 different induced pluripotent stem cell (iPSC) lines from healthy and diseased donors. We use the digitalized RiO to conduct a phenotypic screen on a pool of RiO, followed by detection of each specific donor in situ. Our proof-of-principle experiments demonstrated that a severely steatotic phenotype could be identified by RFID chip reading and was specific to a genetic disorder of steatohepatitis. Given evolving advancements surrounding RFID technology, the digitalization principle outlined here will expand organoid medicine potential toward drug development, precision medicine, and transplant applications.

Cultivate Primary Nasal Epithelial Cells from Children and Reprogram into Induced Pluripotent Stem Cells.

Nasal epithelial cells (NECs) are the part of the airways that respond to air pollutants and are the first cells infected with respiratory viruses. They are also involved in many airway diseases through their innate immune response and interaction with immune and airway stromal cells. NECs are of particular interest for studies in children due to their accessibility during clinical visits. Human induced pluripotent stem cells (iPSCs) have been generated from multiple cell types and are a powerful tool for modeling human development and disease, as well as for their potential applications in regenerative medicine. This is the first protocol to lay out methods for successful generation of iPSCs from NECs derived from pediatric participants for research purposes. It describes how to obtain nasal epithelial cells from children, how to generate primary NEC cultures from these samples, and how to reprogram primary NECs into well-characterized iPSCs. Nasal mucosa samples are useful in epidemiological studies related to the effects of air pollution in children, and provide an important tool for studying airway disease. Primary nasal cells and iPSCs derived from them can be a tool for providing unlimited material for patient-specific research in diverse areas of airway epithelial biology, including asthma and COPD research.

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

Pluripotent stem cells (PSCs) maintain a low mutation frequency compared with somatic cell types at least in part by preferentially utilizing error-free homologous recombination (HR) for DNA repair. Many endogenous metabolites cause DNA interstrand crosslinks, which are repaired by the Fanconi anemia (FA) pathway using HR. To determine the effect of failed repair of endogenous DNA lesions on PSC biology, we generated iPSCs harboring a conditional FA pathway. Upon FA pathway loss, iPSCs maintained pluripotency but underwent profound G2 arrest and apoptosis, whereas parental fibroblasts grew normally. Mechanistic studies revealed that G2-phase FA-deficient iPSCs possess large γH2AX-RAD51 foci indicative of accrued DNA damage, which correlated with activated DNA-damage signaling through CHK1. CHK1 inhibition specifically rescued the growth of FA-deficient iPSCs for prolonged culture periods, surprisingly without stimulating excessive karyotypic abnormalities. These studies reveal that PSCs possess hyperactive CHK1 signaling that restricts their self-renewal in the absence of error-free DNA repair.

Properties of neurons derived from induced pluripotent stem cells of Gaucher disease type 2 patient fibroblasts: potential role in neuropathology.

Gaucher disease (GD) is caused by insufficient activity of acid ß-glucosidase (GCase) resulting from mutations in GBA1. To understand the pathogenesis of the neuronopathic GD, induced pluripotent stem cells (iPSCs) were generated from fibroblasts isolated from three GD type 2 (GD2) and 2 unaffected (normal and GD carrier) individuals. The iPSCs were converted to neural precursor cells (NPCs) which were further differentiated into neurons. Parental GD2 fibroblasts as well as iPSCs, NPCs, and neurons had similar degrees of GCase deficiency. Lipid analyses showed increases of glucosylsphingosine and glucosylceramide in the GD2 cells. In addition, GD2 neurons showed increased α-synuclein protein compared to control neurons. Whole cell patch-clamping of the GD2 and control iPSCs-derived neurons demonstrated excitation characteristics of neurons, but intriguingly, those from GD2 exhibited consistently less negative resting membrane potentials with various degree of reduction in action potential amplitudes, sodium and potassium currents. Culture of control neurons in the presence of the GCase inhibitor (conduritol B epoxide) recapitulated these findings, providing a functional link between decreased GCase activity in GD and abnormal neuronal electrophysiological properties. To our knowledge, this study is first to report abnormal electrophysiological properties in GD2 iPSC-derived neurons that may underlie the neuropathic phenotype in Gaucher disease.

In vitro generation of human pluripotent stem cell derived lung organoids.

Recent breakthroughs in 3-dimensional (3D) organoid cultures for many organ systems have led to new physiologically complex in vitro models to study human development and disease. Here, we report the step-wise differentiation of human pluripotent stem cells (hPSCs) (embryonic and induced) into lung organoids. By manipulating developmental signaling pathways hPSCs generate ventral-anterior foregut spheroids, which are then expanded into human lung organoids (HLOs). HLOs consist of epithelial and mesenchymal compartments of the lung, organized with structural features similar to the native lung. HLOs possess upper airway-like epithelium with basal cells and immature ciliated cells surrounded by smooth muscle and myofibroblasts as well as an alveolar-like domain with appropriate cell types. Using RNA-sequencing, we show that HLOs are remarkably similar to human fetal lung based on global transcriptional profiles, suggesting that HLOs are an excellent model to study human lung development, maturation and disease.

Activated STAT5 confers resistance to intestinal injury by increasing intestinal stem cell proliferation and regeneration.

Intestinal epithelial stem cells (IESCs) control the intestinal homeostatic response to inflammation and regeneration. The underlying mechanisms are unclear. Cytokine-STAT5 signaling regulates intestinal epithelial homeostasis and responses to injury. We link STAT5 signaling to IESC replenishment upon injury by depletion or activation of Stat5 transcription factor. We found that depletion of Stat5 led to deregulation of IESC marker expression and decreased LGR5(+) IESC proliferation. STAT5-deficient mice exhibited worse intestinal histology and impaired crypt regeneration after γ-irradiation. We generated a transgenic mouse model with inducible expression of constitutively active Stat5. In contrast to Stat5 depletion, activation of STAT5 increased IESC proliferation, accelerated crypt regeneration, and conferred resistance to intestinal injury. Furthermore, ectopic activation of STAT5 in mouse or human stem cells promoted LGR5(+) IESC self-renewal. Accordingly, STAT5 promotes IESC proliferation and regeneration to mitigate intestinal inflammation. STAT5 is a functional therapeutic target to improve the IESC regenerative response to gut injury.

Dynamic transcriptional and epigenomic reprogramming from pediatric nasal epithelial cells to induced pluripotent stem cells.

BACKGROUND: Induced pluripotent stem cells (iPSCs) hold tremendous potential, both as a biological tool to uncover the pathophysiology of disease by creating relevant human cell models and as a source of cells for cell-based therapeutic applications. Studying the reprogramming process will also provide significant insight into tissue development. OBJECTIVE: We sought to characterize the derivation of iPSC lines from nasal epithelial cells (NECs) isolated from nasal mucosa samples of children, a highly relevant and easily accessible tissue for pediatric populations. METHODS: We performed detailed comparative analysis on the transcriptomes and methylomes of NECs, iPSCs derived from NECs (NEC-iPSCs), and embryonic stem cells (ESCs). RESULTS: NEC-iPSCs express pluripotent cell markers, can differentiate into all 3 germ layers in vivo and in vitro, and have a transcriptome and methylome remarkably similar to those of ESCs. However, residual DNA methylation marks exist, which are differentially methylated between NEC-iPSCs and ESCs. A subset of these methylation markers related to epithelium development and asthma and specific to NEC-iPSCs persisted after several passages in vitro, suggesting the retention of an epigenetic memory of their tissue of origin. Our analysis also identified novel candidate genes with dynamic gene expression and DNA methylation changes during reprogramming, which are indicative of possible roles in airway epithelium development. CONCLUSION: NECs are an excellent tissue source to generate iPSCs in pediatric asthmatic patients, and detailed characterization of the resulting iPSC lines would help us better understand the reprogramming process and retention of epigenetic memory.

Modelling human development and disease in pluripotent stem-cell-derived gastric organoids.

Gastric diseases, including peptic ulcer disease and gastric cancer, affect 10% of the world's population and are largely due to chronic Helicobacter pylori infection. Species differences in embryonic development and architecture of the adult stomach make animal models suboptimal for studying human stomach organogenesis and pathogenesis, and there is no experimental model of normal human gastric mucosa. Here we report the de novo generation of three-dimensional human gastric tissue in vitro through the directed differentiation of human pluripotent stem cells. We show that temporal manipulation of the FGF, WNT, BMP, retinoic acid and EGF signalling pathways and three-dimensional growth are sufficient to generate human gastric organoids (hGOs). Developing hGOs progressed through molecular and morphogenetic stages that were nearly identical to the developing antrum of the mouse stomach. Organoids formed primitive gastric gland- and pit-like domains, proliferative zones containing LGR5-expressing cells, surface and antral mucous cells, and a diversity of gastric endocrine cells. We used hGO cultures to identify novel signalling mechanisms that regulate early endoderm patterning and gastric endocrine cell differentiation upstream of the transcription factor NEUROG3. Using hGOs to model pathogenesis of human disease, we found that H. pylori infection resulted in rapid association of the virulence factor CagA with the c-Met receptor, activation of signalling and induction of epithelial proliferation. Together, these studies describe a new and robust in vitro system for elucidating the mechanisms underlying human stomach development and disease.

An in vivo model of human small intestine using pluripotent stem cells.

Differentiation of human pluripotent stem cells (hPSCs) into organ-specific subtypes offers an exciting avenue for the study of embryonic development and disease processes, for pharmacologic studies and as a potential resource for therapeutic transplant. To date, limited in vivo models exist for human intestine, all of which are dependent upon primary epithelial cultures or digested tissue from surgical biopsies that include mesenchymal cells transplanted on biodegradable scaffolds. Here, we generated human intestinal organoids (HIOs) produced in vitro from human embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs) that can engraft in vivo. These HIOs form mature human intestinal epithelium with intestinal stem cells contributing to the crypt-villus architecture and a laminated human mesenchyme, both supported by mouse vasculature ingrowth. In vivo transplantation resulted in marked expansion and maturation of the epithelium and mesenchyme, as demonstrated by differentiated intestinal cell lineages (enterocytes, goblet cells, Paneth cells, tuft cells and enteroendocrine cells), presence of functional brush-border enzymes (lactase, sucrase-isomaltase and dipeptidyl peptidase 4) and visible subepithelial and smooth muscle layers when compared with HIOs in vitro. Transplanted intestinal tissues demonstrated digestive functions as shown by permeability and peptide uptake studies. Furthermore, transplanted HIO-derived tissue was responsive to systemic signals from the host mouse following ileocecal resection, suggesting a role for circulating factors in the intestinal adaptive response. This model of the human small intestine may pave the way for studies of intestinal physiology, disease and translational studies.

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.

UNLABELLED: 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.