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Hematopoietic Stem/Progenitor cells (HSPCs) are endowed with the role of maintaining a diverse pool of blood cells throughout the human life. Despite recent efforts, the nature of the early cell fate decisions remains contentious. Using single-cell RNA-Seq, we show that existing approaches to stratify bone marrow CD34+ cells reveal a hierarchically-structured transcriptional landscape of hematopoietic differentiation. Still, this landscape misses important early fate decisions. We here provide a broader transcriptional profiling of bone marrow lineage negative hematopoietic progenitors that recovers a key missing branchpoint into basophils and expands our understanding of the underlying structure of early adult human haematopoiesis. We also show that this map has strong similarities in topology and gene expression to that found in mouse. Finally, we identify the sialomucin CD164, as a reliable marker for the earliest branches of HSPCs specification and we showed how its use can foster the design of alternative transplantation cell products. Human Hematopoietic stem and progenitor cells (HSPCs) are commonly defined by CD34 expression. Here, the authors map single-cell RNA states both inside and outside the CD34 compartment, uncovering previously unappreciated branchpoints and validating CD164 as an efficient marker for early HSPCs.

Re-expression of the paralogous γ-globin genes ( HBG1/2 ) could be a universal strategy to ameliorate the severe β-globin disorders sickle cell disease (SCD) and β-thalassemia by induction of fetal hemoglobin (HbF, α 2 γ 2 ) 1 . Previously, we and others have shown that core sequences at the BCL11A erythroid enhancer are required for repression of HbF in adult-stage erythroid cells but are dispensable in non-erythroid cells 2 – 6 . CRISPR–Cas9-mediated gene modification has demonstrated variable efficiency, specificity, and persistence in hematopoietic stem cells (HSCs). Here, we demonstrate that Cas9:sgRNA ribonucleoprotein (RNP)-mediated cleavage within a GATA1 binding site at the +58 BCL11A erythroid enhancer results in highly penetrant disruption of this motif, reduction of BCL11A expression, and induction of fetal γ-globin. We optimize conditions for selection-free on-target editing in patient-derived HSCs as a nearly complete reaction lacking detectable genotoxicity or deleterious impact on stem cell function. HSCs preferentially undergo non-homologous compared with microhomology-mediated end joining repair. Erythroid progeny of edited engrafting SCD HSCs express therapeutic levels of HbF and resist sickling, while those from patients with β-thalassemia show restored globin chain balance. Non-homologous end joining repair-based BCL11A enhancer editing approaching complete allelic disruption in HSCs is a practicable therapeutic strategy to produce durable HbF induction. Optimized conditions for ribonucleoprotein delivery of Cas9–sgRNA complexes enables precise and efficient gene editing to restore fetal hemoglobin expression in sickle cell disease patient-derived HSCs

Hematopoietic stem and progenitor cells (HSPC) are endowed with the role of generating and maintaining lifelong the extremely diverse pool of blood cells 1 . Clinically, transplantation of human HSPC from an allogeneic healthy donor or infusion of autologous gene-corrected HSPC can effectively replenish defective blood cell production caused by congenital or acquired disorders 2 – 9 . However, due to methodological and ethical constraints that have limited the study of human HSPC primarily to in vitro assays 10 or xenotransplantation models 11 , 12 , the in vivo activity of HSPC has to date remained relatively unexplored in humans 13 – 16 . Here we report a comprehensive study of the frequencies, dynamics and output of seven HSPC subtypes in humans that was performed by tracking 148,093 individual clones in six patients treated with lentiviral gene therapy using autologous HSPC transplantation and followed for up to 5 years. We discovered that primitive multipotent progenitor and hematopoietic stem cell (HSC) populations have distinct roles during the initial reconstitution after transplant, compared with subsequent steady-state phases. Furthermore, we showed that a fraction of in vitro–activated HSC are resilient and undergo a defined delayed activation period upon transplant. Finally, our data support the concept that early lymphoid-biased progenitors might be capable of long-term survival, such that they can be maintained independently of their continuous production from HSC. Overall, this study provides comprehensive data on HSPC dynamics after autologous transplantation and gene therapy in humans. In vivo tracking of hematopoietic stem and progenitor cells in humans treated with lentiviral gene therapy unveils the resilience of hematopoietic stem cells and the long-term survival of lymphoid-biased precursors.

Background Integration site (IS) analysis is a fundamental analytical platform for evaluating the safety and efficacy of viral vector based preclinical and clinical Gene Therapy (GT). A handful of groups have developed standardized bioinformatics pipelines to process IS sequencing data, to generate reports, and/or to perform comparative studies across different GT trials. Keeping up with the technological advances in the field of IS analysis, different computational pipelines have been published over the past decade. These pipelines focus on identifying IS from single-read sequencing or paired-end sequencing data either using read-based or using sonication fragment-based methods, but there is a lack of a bioinformatics tool that automatically includes unique molecular identifiers (UMI) for IS abundance estimations and allows comparing multiple quantification methods in one integrated pipeline. Results Here we present IS-Seq a bioinformatics pipeline that can process data from paired-end sequencing of both old restriction sites-based IS collection methods and new sonication-based IS retrieval systems while allowing the selection of different abundance estimation methods, including read-based, Fragment-based and UMI-based systems. Conclusions We validated the performance of IS-Seq by testing it against the most popular  analytical workflow available in the literature (INSPIIRED) and using different scenarios. Lastly, by performing extensive simulation studies and a comprehensive wet-lab assessment of our IS-Seq pipeline we could show that in clinically relevant scenarios, UMI quantification provides better accuracy than the currently most widely used sonication fragment counts as a method for IS abundance estimation.

Our mathematical model of integration site data in clinical gene therapy supported the existence of long-term lymphoid progenitors capable of surviving independently from hematopoietic stem cells. To date, no experimental setting has been available to validate this prediction. We here report evidence of a population of lymphoid progenitors capable of independently maintaining T and NK cell production for 15 years in humans. The gene therapy patients of this study lack vector-positive myeloid/B cells indicating absence of engineered stem cells but retain gene marking in both T and NK. Decades after treatment, we can still detect and analyse transduced naïve T cells whose production is likely maintained by a population of long-term lymphoid progenitors. By tracking insertional clonal markers overtime, we suggest that these progenitors can support both T and NK cell production. Identification of these long-term lymphoid progenitors could be utilised for the development of next generation gene- and cancer-immunotherapies. Gene therapy (GT) using haematopoietic stem cells (HSCs) provides an opportunity to trace cell fates in humans, in vivo. Here the authors present evidence in GT patients for a long term lymphoid progenitor population, surviving and maintaining de novo T and NK cell production for years, independently from HSCs.

Retroviral vectors, including those derived from gammaretroviruses and lentiviruses, have found their way into the clinical arena and demonstrated remarkable efficacy for the treatment of immunodeficiencies, leukodystrophies, and globinopathies. Despite these successes, gene therapy unfortunately also has had to face severe adverse events in the form of leukemias and myelodysplastic syndromes, related to the semi-random vector integration into the host cell genome that caused deregulation of neighboring proto-oncogenes. Although improvements in vector design clearly lowered the risk of this insertional mutagenesis, analysis of potential genotoxicity and the consequences of vector integration remain important parameters for basic and translational research and most importantly for the clinic. Here, we review current assays to analyze biodistribution and genotoxicity in the pre-clinical setting and describe tools to monitor vector integration sites in vector-treated patients as a biosafety readout.

Adverse events stemming from the use of retroviral vectors in humans has prompted the search for methods predicting the fate and biological consequences of gene-modified cells after vector insertion. Methods of integration site analysis, such as linear amplification-mediated PCR (LAM-PCR), rely on use of restriction enzymes and identify only a fraction of all genomic integrants. This report describes a non–restriction enzyme–based LAM-PCR technique that provides comprehensive, unbiased integration site analysis. Retroviral vectors have induced subtle clonal skewing in many gene therapy patients and severe clonal proliferation and leukemia in some of them, emphasizing the need for comprehensive integration site analyses to assess the biosafety and genomic pharmacokinetics of vectors and clonal fate of gene-modified cells in vivo . Integration site analyses such as linear amplification–mediated PCR (LAM-PCR) require a restriction digest generating unevenly small fragments of the genome. Here we show that each restriction motif allows for identification of only a fraction of all genomic integrants, hampering the understanding and prediction of biological consequences after vector insertion. We developed a model to define genomic access to the viral integration site that provides optimal restriction motif combinations and minimizes the percentage of nonaccessible insertion loci. We introduce a new nonrestrictive LAM-PCR approach that has superior capabilities for comprehensive unbiased integration site retrieval in preclinical and clinical samples independent of restriction motifs and amplification inefficiency.

The analysis of genomic distribution of retroviral vectors is a powerful tool to monitor ‘vector‐on‐host’ effects in gene therapy (GT) trials but also provides crucial information about ‘host‐on‐vector’ influences based on the target cell genetic and epigenetic state. We had the unique occasion to compare the insertional profile of the same therapeutic moloney murine leukemia virus (MLV) vector in the context of the adenosine deaminase‐severe combined immunodeficiency (ADA‐SCID) genetic background in two GT trials based on infusions of transduced mature lymphocytes (peripheral blood lymphocytes, PBL) or a single infusion of haematopoietic stem/progenitor cells (HSC). We found that vector insertions are cell‐specific according to the differential expression profile of target cells, favouring, in PBL‐GT, genes involved in immune system and T‐cell functions/pathways as well as T‐cell DNase hypersensitive sites, differently from HSC‐GT. Chromatin conformations and histone modifications influenced integration preferences but we discovered that only H3K27me3 was cell‐specifically disfavoured, thus representing a key epigenetic determinant of cell‐type dependent insertion distribution. Our study shows that MLV vector insertional profile is cell‐specific according to the genetic/chromatin state of the target cell both in vitro and in vivo in patients several years after GT. See accompanying Closeup by Baum DOI: http://dx.doi.org/10.1002/emmm.201000110 .

We prove the existence of Cantor families of small amplitude, analytic, linearly stable quasi-periodic solutions of reversible derivative wave equations.

Background During their lifespan, stem- or progenitor cells have the ability to differentiate into more committed cell lineages. Understanding this process can be key in treating certain diseases. However, up until now only limited information about the cell differentiation process is known. Aim The goal of this paper is to present a statistical framework able to describe the cell differentiation process at the single clone level and to provide a corresponding inferential procedure for parameters estimation and structure reconstruction of the differentiation network. Approach We propose a multidimensional, continuous-time Markov model with density-dependent transition probabilities linear in sub-population sizes and rates. The inferential procedure is based on an iterative calculation of approximated solutions for two systems of ordinary differential equations, describing process moments evolution over time, that are analytically derived from the process’ master equation. Network sparsity is induced by adding a SCAD-based penalization term in the generalized least squares objective function. Results The methods proposed here have been tested by means of a simulation study and then applied to a data set derived from a gene therapy clinical trial, in order to investigate hematopoiesis in humans, in-vivo . The hematopoietic structure estimated contradicts the classical dichotomy theory of cell differentiation and supports a novel myeloid-based model recently proposed in the literature.