Many multipotential gene-marked progenitor or stem cell clones contribute to hematopoiesis in nonhuman primates

Biologic and Clinical Efficacy of LentiGlobin for Sickle Cell Disease

BACKGROUND

Sickle cell disease is characterized by the painful recurrence of vaso-occlusive events. Gene therapy with the use of LentiGlobin for sickle cell disease (bb1111; lovotibeglogene autotemcel) consists of autologous transplantation of hematopoietic stem and progenitor cells transduced with the BB305 lentiviral vector encoding a modified β-globin gene, which produces an antisickling hemoglobin, HbA^T87Q.

METHODS

In this ongoing phase 1-2 study, we optimized the treatment process in the initial 7 patients in Group A and 2 patients in Group B with sickle cell disease. Group C was established for the pivotal evaluation of LentiGlobin for sickle cell disease, and we adopted a more stringent inclusion criterion that required a minimum of four severe vaso-occlusive events in the 24 months before enrollment. In this unprespecified interim analysis, we evaluated the safety and efficacy of LentiGlobin in 35 patients enrolled in Group C. Included in this analysis was the number of severe vaso-occlusive events after LentiGlobin infusion among patients with at least four vaso-occlusive events in the 24 months before enrollment and with at least 6 months of follow-up.

RESULTS

As of February 2021, cell collection had been initiated in 43 patients in Group C; 35 received a LentiGlobin infusion, with a median follow-up of 17.3 months (range, 3.7 to 37.6). Engraftment occurred in all 35 patients. The median total hemoglobin level increased from 8.5 g per deciliter at baseline to 11 g or more per deciliter from 6 months through 36 months after infusion. HbA^T87Q contributed at least 40% of total hemoglobin and was distributed across a mean (±SD) of 85±8% of red cells. Hemolysis markers were reduced. Among the 25 patients who could be evaluated, all had resolution of severe vaso-occlusive events, as compared with a median of 3.5 events per year (range, 2.0 to 13.5) in the 24 months before enrollment. Three patients had a nonserious adverse event related or possibly related to LentiGlobin that resolved within 1 week after onset. No cases of hematologic cancer were observed during up to 37.6 months of follow-up.

CONCLUSIONS

One-time treatment with LentiGlobin resulted in sustained production of HbA^T87Q in most red cells, leading to reduced hemolysis and complete resolution of severe vaso-occlusive events. (Funded by Bluebird Bio; HGB-206 ClinicalTrials.gov number, NCT02140554.).

Preleukemia: the normal side of cancer

PURPOSE OF REVIEW

In the present review, we will define the preleukemic state. We aim at increasing awareness and research in the field of preleukemia that will nurture targeted therapy for the earlier steps of leukemia evolution.

RECENT FINDINGS

Emerging evidence supports the role of hematopoietic stem/progenitor cells carrying recurrent leukemia-related mutations as the cell of origin of both myeloid and lymphoid malignancies. The preleukemic stem cells can maintain at least to some extent their functionality; however, they have increased fitness endowed by the preleukemic mutations that lead to clonal expansion.

SUMMARY

The latent preleukemic period before overt leukemia presents can take years, and the majority of carriers will never develop leukemia in their lifetime. The preleukemic state is not rare, with greater than 1% of individuals having acquired one or more of the recognized preleukemic lesions. The high frequency of such abnormalities in the population may be the cost of growing old; however, another view could be that in order to survive to old age, the hematopoietic system must adapt to create robust hematopoietic stem/progenitor cells with an increased fitness and clonal expansion. Hence, leukemia does not necessarily start as a disease, but rather as a need, with the normally functioning preleukemic hematopoietic stem cells trying to maintain health for years but in time succumbing to their own acquired virtues.

Clonal tracking of rhesus macaque hematopoiesis highlights a distinct lineage origin for natural killer cells

Analysis of hematopoietic stem cell function in nonhuman primates provides insights that are relevant for human biology and therapeutic strategies. In this study, we applied quantitative genetic barcoding to track the clonal output of transplanted autologous rhesus macaque hematopoietic stem and progenitor cells over a time period of up to 9.5 months. We found that unilineage short-term progenitors reconstituted myeloid and lymphoid lineages at 1 month but were supplanted over time by multilineage clones, initially myeloid restricted, then myeloid-B clones, and then stable myeloid-B-T multilineage, long-term repopulating clones. Surprisingly, reconstitution of the natural killer (NK) cell lineage, and particularly the major CD16(+)/CD56(-) peripheral blood NK compartment, showed limited clonal overlap with T, B, or myeloid lineages, and therefore appears to be ontologically distinct. Thus, in addition to providing insights into clonal behavior over time, our analysis suggests an unexpected paradigm for the relationship between NK cells and other hematopoietic lineages in primates.

Identification of pre-leukaemic haematopoietic stem cells in acute leukaemia

In acute myeloid leukemia (AML), the cell of origin, nature and biological consequences of initiating lesions and order of subsequent mutations remain poorly understood, as AML is typically diagnosed without observation of a pre-leukemic phase. Here, highly purified hematopoietic stem cells (HSC), progenitor and mature cell fractions from the blood of AML patients were found to contain recurrent DNMT3a mutations (DNMT3a^mut) at high allele frequency, but without coincident NPM1 mutations (NPM1c) present in AML blasts. DNMT3a^mut-bearing HSC exhibited multilineage repopulation advantage over non-mutated HSC in xenografts, establishing their identity as pre-leukemic-HSC (preL-HSC). preL-HSC were found in remission samples indicating that they survive chemotherapy. Thus DNMT3a^mut arises early in AML evolution, likely in HSC, leading to a clonally expanded pool of preL-HSC from which AML evolves. Our findings provide a paradigm for the detection and treatment of pre-leukemic clones before the acquisition of additional genetic lesions engenders greater therapeutic resistance.

High-throughput genomic mapping of vector integration sites in gene therapy studies

Gene therapy has enormous potential to treat a variety of infectious and genetic diseases. To date hundreds of patients worldwide have received hematopoietic cell products that have been gene-modified with retrovirus vectors carrying therapeutic transgenes, and many patients have been cured or demonstrated disease stabilization as a result (Adair et al., Sci Transl Med 4:133ra57, 2012; Biffi et al., Science 341:1233158, 2013; Aiuti et al., Science 341:1233151, 2013; Fischer et al., Gene 525:170-173, 2013). Unfortunately, for some patients the provirus integration dysregulated the expression of nearby genes leading to clonal outgrowth and, in some cases, cancer. Thus, the unwanted side effect of insertional mutagenesis has become a major concern for retrovirus gene therapy. The careful study of retrovirus integration sites (RIS) and the contribution of individual gene-modified clones to hematopoietic repopulating cells is of crucial importance for all gene therapy studies. Supporting this, the US Food and Drug Administration (FDA) has mandated the careful monitoring of RIS in all clinical trials of gene therapy. An invaluable method was developed: linear amplification mediated-polymerase chain reaction (LAM-PCR) capable of analyzing in vitro and complex in vivo samples, capturing valuable genomic information directly flanking the site of provirus integration. Linking this method and similar methods to high-throughput sequencing has now made possible an unprecedented understanding of the integration profile of various retrovirus vectors, and allows for sensitive monitoring of their safety. It also allows for a detailed comparison of improved safety-enhanced gene therapy vectors. An important readout of safety is the relative contribution of individual gene-modified repopulating clones. One limitation of LAM-PCR is that the ability to capture the relative contribution of individual clones is compromised because of the initial linear PCR common to all current methods. Here, we describe an improved protocol developed for efficient capture, sequencing, and analysis of RIS that preserves gene-modified clonal contribution information. We also discuss methods to assess dominant/overrepresented gene-modified clones in preclinical and clinical models.

Heterogeneity in hematopoietic stem cell populations: implications for transplantation

PURPOSE OF REVIEW

Transplantation of hematopoietic cells is now a well established clinical procedure, although optimal outcomes are not always obtained. This reflects insufficient knowledge of the different subsets of primitive cells required to achieve a rapid and permanent recovery of mature blood cell production. Here we review recent findings that extend our understanding of these cells and their regulation, and implications for the ex-vivo expansion of these cells.

RECENT FINDINGS

Separate subsets of platelet and neutrophil lineage-restricted human hematopoietic cells with rapid but transient repopulating activities have been identified, thus adding to previous evidence of short-term repopulating cells that generate both of these lineages. New studies also suggest intrinsically determined heterogeneity in differentiation potentialities that are sustained at the stem cell level, and have revealed new ways their self-renewal can be influenced.

SUMMARY

Hematopoietic repopulation posttransplant is highly complex both in terms of the differing numbers and types of cells required for optimal hematopoietic recoveries and the factors that will determine the composition and behavior of a given inoculum. Successful ex-vivo expansion protocols will, thus, need to incorporate conditions that will produce adequate numbers of all cell types required with retention of their full functionality.

Stem cell gene therapy: the risks of insertional mutagenesis and approaches to minimize genotoxicity

Virus-based vectors are widely used in hematopoietic stem cell (HSC) gene therapy, and have the ability to integrate permanently into genomic DNA, thus driving long-term expression of corrective genes in all hematopoietic lineages. To date, HSC gene therapy has been successfully employed in the clinic for improving clinical outcomes in small numbers of patients with X-linked severe combined immunodeficiency (SCID-X1), adenosine deaminase deficiency (ADA-SCID), adrenoleukodystrophy (ALD), thalassemia, chronic granulomatous disease (CGD), and Wiskott-Aldrich syndrome (WAS). However, adverse events were observed during some of these HSC gene therapy clinical trials, linked to insertional activation of proto-oncogenes by integrated proviral vectors leading to clonal expansion and eventual development of leukemia. Numerous studies have been performed to understand the molecular basis of vector-mediated genotoxicity, with the aim of developing safer vectors and lower-risk gene therapy protocols. This review will summarize current information on the mechanisms of insertional mutagenesis in hematopoietic stem and progenitor cells due to integrating gene transfer vectors, discuss the available assays for predicting genotoxicity and mapping vector integration sites, and introduce newly-developed approaches for minimizing genotoxicity as a way to further move HSC gene therapy forward into broader clinical application.

The replication rate of human hematopoietic stem cells in vivo

Hematopoietic stem cells (HSCs) replicate (self-renew) to create 2 daughter cells with capabilities equivalent to their parent, as well as differentiate, and thus can both maintain and restore blood cell production. Cell labeling with division-sensitive markers and competitive transplantation studies have been used to estimate the replication rate of murine HSCs in vivo. However, these methods are not feasible in humans and surrogate assays are required. In this report, we analyze the changing ratio with age of maternal/paternal X-chromosome phenotypes in blood cells from females and infer that human HSCs replicate on average once every 40 weeks (range, 25-50 weeks). We then confirm this estimate with 2 independent approaches, use the estimate to simulate human hematopoiesis, and show that the simulations accurately reproduce marrow transplantation data. Our simulations also provide evidence that the number of human HSCs increases from birth until adolescence and then plateaus, and that the ratio of contributing to quiescent HSCs in humans significantly differs from mouse. In addition, they suggest that human marrow failure, such as the marrow failure that occurs after umbilical cord blood transplantation and with aplastic anemia, results from insufficient numbers of early progenitor cells, and not the absence of HSCs.

Transient in vivo beta-globin production after lentiviral gene transfer to hematopoietic stem cells in the nonhuman primate

Inherited disorders of globin synthesis remain desirable targets for hematopoietic stem cell (HSC)-based therapies. Gene transfer using retroviral vectors offers an alternative to allogeneic HSC transplantation by the permanent integration of potentially therapeutic genes into primary autologous HSCs. Although proof of principle has been demonstrated in humans, this approach has been met by formidable obstacles, and large-animal models have become increasingly important for the preclinical development of gene addition strategies. Here we report lentiviral gene transfer of the human beta-globin gene under the control of the globin promoter and large fragments of the globin locus control region (LCR) in the nonhuman primate. Using an HIV-1, vesicular stomatitis virus glycoprotein G (VSV-G)-pseudotyped vector, modified to overcome a species-specific restriction to HIV-1, gene transfer to colony-forming units (CFU) derived from mobilized peripheral blood (PB) rhesus CD34+ cells was 84.4 +/- 2.33%. Erythroid cells derived from transduced rhesus CD34+ cells expressed human beta-globin at high levels as assessed by flow cytometry with a human beta-globin-specific antibody. Two rhesus macaques (RQ3586 and RQ3583) were transplanted with mobilized PB CD34+ cells transduced with our modified HIV vector at a multiplicity of infection of 80. High gene transfer rates to CFUs were achieved in vitro (RQ3586, 87.5%; RQ3583, 83.3%), with efficient human beta-globin expression among erythroid progeny generated in vitro. Early posttransplantation, gene transfer rates of 5% or higher were detectable and confirmed by genomic Southern blotting, with equivalent-level human beta-globin expression detected by flow cytometry. Long-term gene marking levels among mononuclear cells and granulocytes assessed by quantitative polymerase chain reaction gradually decreased to about 0.001% at 2 years, likely due to additional HIV-1 restrictive elements in the rhesus macaque. No evidence of clonal hematopoiesis has occurred in our animals in up to 2 years. Current efforts are aimed at developing a lentiviral vector capable of efficiently transducing both human and rhesus HSCs to allow preclinical modeling of globin gene transfer.

Long-term vector integration site analysis following retroviral mediated gene transfer to hematopoietic stem cells for the treatment of HIV infection

We previously reported the efficacy of nonmyeloablative allogeneic transplantation in 2 HIV positive recipients, one of whom received retrovirus transduced hematopoietic stem cells to confer resistance to HIV. Here we report an assessment of retroviral integration sites (RISs) recovered out to 3 years post-transplantation. We identified 213 unique RISs from the patient's peripheral blood samples by linear amplification-mediated PCR (LAM-PCR). While vector integration patterns were similar to that previously reported, only 3.76% of RISs were common among early (up to 3 months) and late samples (beyond 1 year). Additionally, common integration sites were enriched among late samples (14.9% vs. 36.8%, respectively). Three RISs were found near or within known oncogenes, but 2 were limited to early timepoints. Interestingly, an integration site near the MDS1 gene was detected in long-term follow-up samples; however, the overall contribution of MDS1 integrated clone remained stably low during follow-up.

Reconstitution of the myeloid and lymphoid compartments after the transplantation of autologous and genetically modified CD34+ bone marrow cells, following gamma irradiation in cynomolgus macaques

Background

Prolonged, altered hematopoietic reconstitution is commonly observed in patients undergoing myeloablative conditioning and bone marrow and/or mobilized peripheral blood-derived stem cell transplantation. We studied the reconstitution of myeloid and lymphoid compartments after the transplantation of autologous CD34⁺ bone marrow cells following gamma irradiation in cynomolgus macaques.

Results

The bone marrow cells were first transduced ex vivo with a lentiviral vector encoding eGFP, with a mean efficiency of 72% ± 4%. The vector used was derived from the simian immunodeficiency lentivirus SIVmac251, VSV-g pseudotyped and encoded eGFP under the control of the phosphoglycerate kinase promoter. After myeloid differentiation, GFP was detected in colony-forming cells (37% ± 10%). A previous study showed that transduction rates did not differ significantly between colony-forming cells and immature cells capable of initiating long-term cultures, indicating that progenitor cells and highly immature hematopoietic cells were transduced with similar efficiency. Blood cells producingeGFP were detected as early as three days after transplantation, and eGFP-producing granulocyte and mononuclear cells persisted for more than one year in the periphery.

Conclusion

The transplantation of CD34⁺ bone marrow cells had beneficial effects for the ex vivo proliferation and differentiation of hematopoietic progenitors, favoring reconstitution of the T- and B-lymphocyte, thrombocyte and red blood cell compartments.

The MDS1-EVI1 gene complex as a retrovirus integration site: impact on behavior of hematopoietic cells and implications for gene therapy

Gene therapy trials have been performed with virus-based vectors that have the ability to integrate permanently into genomic DNA and thus allow prolonged expression of corrective genes after transduction of hematopoietic stem and progenitor cells. Adverse events observed during the X-linked severe combined immunodeficiency gene therapy trial revealed a significant risk of genotoxicity related to retrovirus vector integration and activation of adjacent proto-oncogenes, with several cases of T-cell leukemia linked to vector activation of the LMO2 gene. In patients with chronic granulomatous disease (CGD), rhesus macaques, and mice receiving hematopoietic stem and progenitor cells transduced with retrovirus vectors, a highly non-random pattern of vector integration has been reported. The most striking finding has been overrepresentation of integrations in one specific genomic locus, a complex containing the MDS1 and the EVI1 genes. Most evidence suggests that this overrepresentation is primarily due to a modification of primitive myeloid cell behavior by overexpression of EVI1 or MDS1-EVI1, as opposed to a specific predilection for integration at this site. Three different proteins can be produced from this complex locus: MDS1, MDS1-EVI1, and EVI1. This review will summarize current knowledge regarding this locus and its gene products, with specific focus on issues with relevance to gene therapy, leukemogenesis, and hematopoiesis. Insights into the mechanisms that result in altered hematopoiesis and leukemogenesis when this locus is dysregulated could improve the safety of gene therapy in the future.

Multilineage hematopoietic reconstitution without clonal selection in ADA-SCID patients treated with stem cell gene therapy

Gene transfer into HSCs is an effective treatment for SCID, although potentially limited by the risk of insertional mutagenesis. We performed a genome-wide analysis of retroviral vector integrations in genetically corrected HSCs and their multilineage progeny before and up to 47 months after transplantation into 5 patients with adenosine deaminase-deficient SCID. Gene-dense regions, promoters, and transcriptionally active genes were preferred retroviral integrations sites (RISs) both in preinfusion transduced CD34(+) cells and in vivo after gene therapy. The occurrence of insertion sites proximal to protooncogenes or genes controlling cell growth and self renewal, including LMO2, was not associated with clonal selection or expansion in vivo. Clonal analysis of long-term repopulating cell progeny in vivo revealed highly polyclonal T cell populations and shared RISs among multiple lineages, demonstrating the engraftment of multipotent HSCs. These data have important implications for the biology of retroviral vectors, the dynamics of genetically modified HSCs, and the safety of gene therapy.

Hematopoietic stem-cell behavior in nonhuman primates

Little is known about the behavior of hematopoietic stem cells (HSCs) in primates because direct observations and competitive-repopulation assays are not feasible. Therefore, we used 2 different and independent experimental strategies, the tracking of transgene expression after retroviral-mediated gene transfer (N = 11 baboons; N = 7 rhesus macaques) and quantitation of the average telomere length of granulocytes (N = 132 baboons; N = 14 macaques), together with stochastic methods, to study HSC kinetics in vivo. The average replication rate for baboon HSCs is once per 36 weeks according to gene-marking analyses and once per 23 weeks according to telomere-shortening analyses. Comparable results were derived from the macaque data. These rates are substantially slower than the average replication rates previously reported for HSCs in mice (once per 2.5 weeks) and cats (once per 8.3 weeks). Because baboons and macaques live for 25 to 45 years, much longer than mice ( approximately 2 years) and cats (12-18 years), we can compute that HSCs undergo a relatively constant number ( approximately 80-200) of lifetime replications. Thus, our data suggest that the self-renewal capacity of mammalian stem cells in vivo is defined and evolutionarily conserved.

Stem cell fate analysis revisited: interpretation of individual clone dynamics in the light of a new paradigm of stem cell organization

Many experimental findings on heterogeneity, flexibility, and plasticity of tissue stem cells are currently challenging stem cell concepts that assume a cell intrinsically predefined, unidirectional differentiation program. In contrast to these classical concepts, nonhierarchical self-organizing systems provide an elegant and comprehensive alternative to explain the experimental data. Here we present the application of such a self-organizing concept to quantitatively describe the hematopoietic stem cell system. Focusing on the analysis of individual-stem-cell fates and clonal dynamics, we particularly discuss implications of the theoretical results on the interpretation of experimental findings. We demonstrate that it is possible to understand hematopoietic stem cell organization without assumptions on unidirectional developmental hierarchies, preprogrammed asymmetric division events or other assumptions implying the existence of a predetermined stem cell entity. The proposed perspective, therefore, changes the general paradigm of thinking about stem cells.

Marking of peripheral T-lymphocytes by retroviral transduction and transplantation of CD34+ cells in a canine X-linked severe combined immunodeficiency model

A retrovirus vector containing an enhanced green fluorescent protein complimentary DNA (EGFP cDNA) was used to mark and dynamically follow vector-expressing cells in the peripheral blood of bone marrow transplanted X-linked severe combined immunodeficient dogs. CD34(+) cells isolated from young normal dogs were transduced, using a 2 day protocol, with an amphotropic retroviral vector that expressed enhanced green fluorescent protein (EGFP) and the canine common gamma chain (gammac) cDNAs. Following transplantation of the transduced cells, normal donor peripheral blood lymphocytes (PBL) appeared by 1 month post-bone marrow transplant (BMT) and rescued three of five treated dogs from their lethal immunodeficiency. PCR and flow cytometric analysis of post-BMT PBL documented the peripheral EGFP expressing cells as CD3(+) T cells, which varied from 0% to 28%. Sorting of EGFP(+) and EGFP(-) peripheral blood T cells from two dogs, followed by vector PCR analysis, showed no evidence of vector shutdown. EGFP expression in B cells or monocytes was not detected. These marking experiments demonstrate that the transduction protocol did not abolish the lymphoid engraftment capability of ex vivo transduced canine CD34(+) cells and supports the potential utility of the MSCV retroviral vector for gene transfer to XSCID affected canine hematopoietic progenitor cells (HPC).

The Genetic Engineering of Hematopoietic Stem Cells: the Rise of Lentiviral Vectors, the Conundrum of the LTR, and the Promise of Lineage-restricted Vectors

Recent studies on the integration patterns of different categories of retroviral vectors, the genotoxicity of long-terminal repeats (LTRs) and other genetic elements, the rise of lentiviral technology and the emergence of regulated vector systems providing tissue-restricted transgene expression and RNA interference, are profoundly changing the landscape of stem cell-based therapies. New developments in vector design and an increasing understanding of the mechanisms underlying insertional oncogenesis are ushering in a new phase in hematopoietic stem cell (HSC) engineering, thus bringing the hitherto exclusive reliance on LTR-driven, gamma-retroviral vectors to an end. Based on their ability to transduce non-dividing cells and their genomic stability, lentiviral vectors offer new prospects for the manipulation of HSCs. Tissue-specific vectors, as exemplified by globin vectors, not only provide therapeutic efficacy, but may also enhance safety, insofar that they restrict transgene expression in stem cells, progenitor cells and blood cells in all but the transcriptionally targeted lineage. This review provides a survey of these advances as well as several remaining challenges, focusing in particular on the importance of achieving adequate levels of protein expression from a limited number of vector copies per cell-ideally one to two.

Busulfan pharmacokinetics, toxicity, and low-dose conditioning for autologous transplantation of genetically modified hematopoietic stem cells in the rhesus macaque model

OBJECTIVE

Gene transfer to hematopoietic stem cells has recently been demonstrated to benefit a small number of patients in whom a selective advantage is conferred upon genetically modified cells; however, in disorders where no such selective advantage is conferred, conditioning appears necessary to allow adequate engraftment. To decrease the toxicity profile, we sought to develop nonmyeloablative conditioning regimens and in this work, explored the use of intravenous busulfan in a large animal model.

METHODS

Busulfan pharmacokinetics and toxicity were monitored in young rhesus macaques at two dosing levels (4 and 6 mg/kg). These doses were then employed to condition two animals at each dose level prior to autologous transplantation of genetically modified cells using our standard methods.

RESULTS

Busulfan pharmacokinetic (PK) data showed the area under the curve (AUC), drug half-life, and drug clearance were consistent within each dose group and similar to those reported in children. Single doses of busulfan were well tolerated and produced dose-dependent myelosuppression, most notably in the neutrophil and platelet counts. Although marking levels reached over 1% early in one animal, the long-term marking was low but detectable at 0.01 to 0.001%.

CONCLUSIONS

We conclude that low-dose intravenous bolus infusion of busulfan is well tolerated, has dose-dependent effects on peripheral blood counts, and allows long-term engraftment of genetically modified cells, but at levels too low for most clinical disorders.

AMD3100 mobilizes hematopoietic stem cells with long-term repopulating capacity in nonhuman primates

AMD3100, a bicyclam antagonist of the chemokine receptor CXCR4, has been shown to induce rapid mobilization of CD34(+) hematopoietic cells in mice, dogs, and humans, offering an alternative to G-CSF mobilization of peripheral-blood hematopoietic stem cells. In this study, AMD3100-mobilized CD34(+) cells were phenotypically analyzed, marked with Neo(R)-containing retroviral vectors, and subsequently transplanted into myeloablated rhesus macaques. We show engraftment of transduced AMD3100-mobilized CD34(+) cells with Neo(R) gene marked myeloid and lymphoid cells up to 32 months after transplantation, demonstrating the ability of AMD3100 to mobilize true long-term repopulating hematopoietic stem cells. More AMD3100-mobilized CD34(+) cells are in the G(1) phase of the cell cycle and more cells express CXCR4 and VLA-4 compared with G-CSF-mobilized CD34(+) cells. In vivo gene marking levels obtained with AMD3100-mobilized CD34(+) cells were better than those obtained using CD34(+) cells mobilized with G-CSF alone. Overall, these results indicate that AMD3100 mobilizes a population of hematopoietic stem cells with intrinsic characteristics different from those of hematopoietic stem cells mobilized with G-CSF, suggesting fundamental differences in the mechanism of AMD3100-mediated and G-CSF-mediated hematopoietic stem cell mobilization. Thus, AMD3100-mobilized CD34(+) cells represent an alternative source of hematopoietic stem cells for clinical stem cell transplantation and genetic manipulation with integrating retroviral vectors.

Pathogenesis of myelofibrosis with myeloid metaplasia

The primary disease process in myelofibrosis with myeloid metaplasia (MMM) is clonal myeloproliferation with varying degrees of phenotypic differentiation. This is characteristically accompanied by secondary intramedullary collagen fibrosis, osteosclerosis, angiogenesis, and extramedullary hematopoiesis. Modern clonality studies have confirmed the multipotent stem-cell origin of the neoplastic process in MMM. The nature of the specific oncogenic mutation(s) is currently being unraveled with the recent discovery of an association between a somatic point mutation of JAK2 tyrosine kinase (V617F) and bcr/abl-negative myeloproliferative disorders, including MMM. The pathogenetic mechanisms that underlie the secondary bone marrow stromal changes in MMM are also incompletely understood. Mouse models of this latter disease aspect have been constructed by either in vivo overexpression of thrombopoietin (TPOhigh mice) or megakaryocyte lineage restricted underexpression of the transcription factor GATA-1 (GATA-1low mice). Gene knockout experiments using such animal models have suggested the essential role of hematopoietic cell-derived transforming growth factor beta1 in inducing bone marrow fibrosis and stromal cell-derived osteoprotegerin in promoting osteosclerosis. However, experimental myelofibrosis in mice does not recapitulate clonal myeloproliferation that is fundamental to human MMM. Other cytokines that are implicated in mediating myelofibrosis and angiogenesis in MMM include basic fibroblast, platelet-derived, and vascular endothelial growth factors. It is currently assumed that such cytokines are abnormally released from clonal megakaryocytes as a result of a pathologic interaction with neutrophils (eg, emperipolesis). This latter phenomenon, through neutrophil-derived elastase, could also underlie the abnormal peripheral-blood egress of myeloid progenitors in MMM.

Biology of Normal and Acute Myeloid Leukemia Stem Cells

The substantial understanding that has been gained over the past 5 decades of the biology of blood formation is largely due to the development of functional quantitative assays for cells at all stages of differentiation, from multipotential stem cells to mature cells. The majority of studies have involved the mouse because the ease with which repopulation studies can be carried out with this animal model allows the assay of complete lineage development from stem cells. In the past decade, advances in repopulation assays for human stem cells using xenotransplantation have greatly enhanced our understanding of human stem cell biology. Importantly, the xenotransplantation methodology has also been used to identify the cancer stem cell that initiates and sustains leukemic proliferation, providing key evidence for the cancer stem cell hypothesis. This hypothesis argues that cancer cells are functionally heterogeneous and hierarchically organized such that only specific cells are capable of sustaining tumor growth and continuously producing the cells that make up the bulk of the tumor. Recent studies have also brought into focus the importance of the intimate relationship between the stem cell (normal or leukemic) and its microenvironment. Coming into view are the molecular players involved in stem cell homing, migration, and adhesion, as well as the cellular components of the microenvironmental niche. Here we review recent studies that have begun, to elucidate the interplay between normal and leukemic human stem cells and their microenvironment.

Recurrent retroviral vector integration at the Mds1/Evi1 locus in nonhuman primate hematopoietic cells

Recent reports linking insertional activation of LMO2 following gene therapy for X-linked severe combined immunodeficiency (X-SCID) have led to a re-evaluation of risks following gene therapy with retroviral vectors. In our analysis of 702 integration sites in rhesus macaques that underwent transplantation up to 7 years earlier with autologous CD34+ cells transduced with amphotropic murine leukemia virus (MLV)-derived retroviral vectors containing marker genes, we detected insertion into one locus, the Mds1/Evi1 region, a total of 14 times in 9 animals. Mds1/Evi1 integrations were observed stably long term, primarily in myeloid cells. We hypothesize that this over-representation likely results from an impact on the self-renewal and engraftment potential of CD34+ progenitor cells via insertional mutagenesis at this specific locus. There is no evidence of ongoing in vivo clonal expansion of the Mds1/Evi1 populations, and all animals are hematologically normal without evidence for leukemia. Characterization of integration sites in this relevant preclinical model provides critical information for gene therapy risk assessment as well as identification of genes controlling hematopoiesis.

Evidence of similar effects of short-term culture on the initial repopulating activity of mobilized peripheral blood transplants assessed in NOD/SCID-beta2microglobulin(null) mice and in autografted patients

OBJECTIVE

Human mobilized peripheral blood (mPB) is known to contain high numbers of cells with rapid but short-term repopulating activity in NOD/SCID-beta2microglobulin(-/-) mice. Here we assessed the effect of short-term culture on these cells and compared the levels of retained activity with the pace of hematologic recovery in myeloablated patients transplanted with similarly cultured autografts of the same cells.

PATIENTS AND METHODS

In a phase 1 clinical study, mPB cells were collected from 6 advanced cancer patients. CD34(+) cells were then harvested, cultured for 3 days in the presence of early-acting growth factors, and transplanted, and posttransplant recovery of blood cell parameters monitored. Assays for primitive hematopoietic activity using both in vivo (in NOD/SCID-beta2microglobulin(-/-) mice) and in vitro (CFC and LTC-IC) endpoints were also performed on the cells pre- and posttransplant.

RESULTS

All patients showed event-free, timely leukocyte recoveries but slightly delayed platelet recoveries in some cases. During the 3-day period of culture, the CFCs doubled but the LTC-IC activity decreased (twofold), as did the short-term repopulating activity in NOD/SCID-beta2microglobulin(-/-) mice.

CONCLUSION

Patients can be transplanted with 3-day cultured autografts with minimal effects on hematologic recovery. This is associated with a variable but, on average, modest loss of short-term repopulating activity detectable in NOD/SCID-beta2microglobulin(-/-) mice.

Conceptual models to understand tissue stem cell organization

PURPOSE OF REVIEW

Theoretic and, in particular, mathematic models can help biologists to select and design experiments, to highlight general principles, to discriminate similar and to link different phenomena, and to predict novel features. Specifically, they contribute to an understanding of latent mechanisms and crucial parameters of biologic processes. The following review gives an overview of recent developments in the field of hematopoietic tissue stem cell modeling.

RECENT FINDINGS

A number of experimental findings on heterogeneity, flexibility, and plasticity of hematopoietic and other tissue stem cells are challenging the classic stem cell concept of a predefined intrinsic stem cell program. Self-organizing systems provide a more elegant and comprehensive alternative to explain experimental data.

SUMMARY

Within the last few decades, modeling approaches in stem cell biology have evolved and now encompass a broad spectrum of phenomena, ranging from the cellular level to the tissue level. The application of theoretic models is currently suggesting that we abandon the classic assumption of a strict developmental hierarchy and understand stem cell organization as a dynamic, functional process. Such a perspective has implications for a prospective characterization of tissue stem cells (eg, regarding gene expression profiles and genetic regulation patterns).

PIG-A mutations in normal hematopoiesis

Paroxysmal nocturnal hemoglobinuria (PNH) is caused by phosphatidylinositol glycan-class A (PIG-A) mutations in hematopoietic stem cells (HSCs). PIG-A mutations have been found in granulocytes from most healthy individuals, suggesting that these spontaneous PIG-A mutations are important in the pathogenesis of PNH. It remains unclear if these PIG-A mutations have relevance to those found in PNH. We isolated CD34+ progenitors from 4 patients with PNH and 27 controls. The frequency of PIG-A mutant progenitors was determined by assaying for colony-forming cells (CFCs) in methylcellulose containing toxic doses of aerolysin (1 x 10(-9) M). Glycosylphosphatidylinositol (GPI)-anchored proteins serve as receptors for aerolysin; thus, PNH cells are resistant to aerolysin. The frequency of aerolysin resistant CFC was 14.7 +/- 4.0 x 10(-6) in the bone marrow of healthy donors and was 57.0 +/- 6.7 x 10(-6) from mobilized peripheral blood. DNA was extracted from individual day-14 aerolysin-resistant CFCs and the PIG-A gene was sequenced to determine clonality. Aerolysin-resistant CFCs from patients with PNH exhibited clonal PIG-A mutations. In contrast, PIG-A mutations in the CFCs from controls were polyclonal, and did not involve T cells. Our data confirm the finding that PIG-A mutations are relatively common in normal hematopoiesis; however, the finding suggests that these mutations occur in differentiated progenitors rather than HSCs.

Mobilized blood CD34+ cells transduced and selected with a clinically applicable protocol reconstitute lymphopoiesis in SCID-Hu mice

We developed a clinically applicable gene transfer procedure into mobilized peripheral blood (MPB) CD34(+) hematopoietic progenitor cells, based on single viral exposure and selection of engineered cells. CD34(+) cells were transduced with a retroviral vector carrying the truncated form of the nerve growth factor receptor (Delta NGFR) marker gene, and immunoselected for Delta NGFR expression. Optimal time and procedure for viral exposure, length of culture, and transgene expression of MPB CD34(+) cells were determined using in vitro assays. The multipotent capacity of MPB CD34(+)-transduced cells was demonstrated in the SCID-hu bone/liver/thymus mouse model. Transduced Delta NGFR(+) cells retained 50% of long-term culture-colony forming cells (LTC-CFC) compared to unmanipulated CD34(+) cells. In SCID-hu mice, 52% of CD45(+) cells, 27% of CD34(+) cells, 49% of B cells, and more than 50% of T cells were derived from transplanted CD34(+)/Delta NGFR(+) cells. Furthermore, transplantation of purified transduced cells greatly reduced the competition with untransduced progenitors occurring in unselected grafts. These data demonstrate that MPB CD34(+) cells, transduced with a single viral exposure and selected by transgene expression, retain multilineage reconstitution capacity and remarkable transgene expression.

Competitive clonal hematopoiesis in mouse chimeras explained by a stochastic model of stem cell organization

Many current experimental results show the necessity of new conceptual approaches to understand hematopoietic stem cell organization. Recently, we proposed a novel theoretical concept and a corresponding quantitative model based on microenvironment-dependent stem cell plasticity. The objective of our present work is to subject this model to an experimental test for the situation of chimeric hematopoiesis. Investigating clonal competition processes in DBA/2-C57BL/6 mouse chimeras, we observed biphasic chimerism development with initially increasing but long-term declining DBA/2 contribution. These experimental results were used to select the parameters of the mathematical model. To validate the model beyond this specific situation, we fixed the obtained parameter configuration to simulate further experimental settings comprising variations of transplanted DBA/2-C57BL/6 proportions, secondary transplantations, and perturbation of stabilized chimeras by cytokine and cytotoxic treatment. We show that the proposed model is able to consistently describe the situation of chimeric hematopoiesis. Our results strongly support the view that the relative growth advantage of strain-specific stem cells is not a fixed cellular property but is sensitively dependent on the actual state of the entire system. We conclude that hematopoietic stem cell organization should be understood as a flexible, self-organized rather than a fixed, preprogrammed process.

Cloning and Functional Analysis of the Rhesus Macaque ABCG2 Gene

Hematopoietic cells can be highly enriched for repopulating ability based upon the efflux of the fluorescent Hoechst 33342 dye by sorting for SP (side population) cells, a phenotype attributed to expression of ABCG2, a member of the ABC transporter superfamily. Intriguingly, murine studies suggest that forced ABCG2 expression prevents hematopoietic differentiation. We cloned the full-length rhesus ABCG2 and introduced it into a retroviral vector. ABCG2-transduced human peripheral blood progenitor cells (PBPCs) acquired the SP phenotype but showed significantly reduced growth compared with control. Two rhesus macaques received autologous PBPCs split for transduction with the ABCG2 or control vectors. Marking levels were similar between fractions with no discrepancy between bone marrow and peripheral blood marking. Analysis for the SP phenotype among bone marrow and mature blood populations confirmed ABCG2 expression at levels predicted by vector copy number long term, demonstrating no block to differentiation in the large animal. In vitro studies showed selective protection against mitoxantrone among ABCG2-transduced rhesus PBPCs. Our results confirm the existence of rhesus ABCG2, establish its importance in conferring the SP phenotype, suggest no detrimental effect of its overexpression upon differentiation in vivo, and imply a potential role for its overexpression as an in vivo selection strategy for gene therapy applications.

Strategies to expand transduced hematopoietic stem cells in vivo

Data in mice suggest that in vivo selection strategies will expand the numbers of transduced hematopoietic stem cells (HSC) to levels sufficient for clinical therapies, and it is argued that comparable strategies will benefit larger animals and humans. To test this assumption, we performed virtual gene therapy in mouse and cat, species in which the in vivo kinetics of HSC are defined. In the simulated experiments, 10% of HSC and 50% of short-term repopulating cells were transduced with a gene allowing a conditional replication or apoptosis advantage. After transplantation, differentiation proceeded stochastically and contributions of transduced cells were tracked for 2 years. Fifty independent transplantations were simulated per species for each analysis. When transduced HSC had a 2-fold increased chance of replication (self-renewal) extending for 4, 10, or 20 weeks after transplantation, or a 5-fold replication advantage extending for 4 weeks, results in mice were far better than in cat, a larger animal, with slower baseline HSC cell cycle kinetics. Similarly, when transduced HSC had a 2-, 4-, or 10-fold decreased chance of apoptosis, extending for 20 or more weeks after transplantation, the murine studies were poor predictors of feline results. Simulation may allow one to optimize and/or understand the limitations of a gene therapy strategy.

Stem cell clonality and genotoxicity in hematopoietic cells: Gene activation side effects should be avoidable

Two serious adverse events involving activation of the LMO2 oncogene through retrovirus vector insertion in the otherwise extremely successful first gene therapy trial for X-linked severe combined immunodeficieny type 1 (SCID-X1) had initially caused widespread concern in the patient and research communities. Careful consideration 1 year after diagnosis of the second case still finds 12 of the treated patients clearly benefiting from gene therapy (freedom from treatment failure, 80%; survival 100%), a situation that should not portend the end of gene therapy for this disease, and is, in fact encouraging. While current approaches are justified to treat patients with otherwise life-threatening disorders, a broad consensus has developed that systematic basic research is required to further understand the pathophysiology of these serious adverse events and to provide new insights, enabling safer and more effective gene therapy strategies. With the continued success of SCID-X1 gene therapy in the majority of patients treated, it is of even greater importance to understand exactly which vector element or combination of elements predispose to toxicity. An in-depth study of the mechanisms behind the activation of the LMO2 and gammac genes will be highly instructive for the development of safer procedures and vectors. We summarize the central observations, ongoing experimental approaches, new concepts, and developments relevant to understanding, interpreting, and eventually overcoming the real and perceived obstacles posed by insertional mutagenesis due to gene transfer vectors.

Selective pressure for a decreased rate of asymmetrical divisions within stem cell niches may contribute to age-related alterations in stem cell function

Most mammalian continuously renewing tissues are maintained by stem cells located within stem cell niches. Each niche contains a number of stem cells that replicate asymmetrically to give differentiated cells and also undergo periodic deletion and compensatory replacement by symmetrical "self-renewal" divisions of stem cells remaining within the niche. It has been recognized that there is selective pressure for an increased rate of self-renewal divisions and that the increasingly likely consequence is neoplasia. However, to date it has not been appreciated that there is also an independent selective pressure for a decreased rate of asymmetrical divisions. In this article, the origin of this second type of selective pressure is explained and its consequences explored through the use of computer modeling. It is shown that age-related changes in a range of mammalian stem cell compartments can be understood in the context of a decreased rate of asymmetrical stem cell divisions with an increased propensity for self-renewal divisions. It is proposed that a decreased rate of asymmetrical divisions impairs the ability of old stem cell compartments to respond effectively to stress.

Hematopoietic activity of common marmoset CD34 cells isolated by a novel monoclonal antibody MA24

OBJECTIVE

We focused on a small New World monkey, the common marmoset (Callithrix jacchus), to establish a nonhuman primate model of the treatment of hematological disorders. In this study, we developed the first monoclonal antibodies (MAbs) against marmoset CD34 and tested the in vitro and in vivo hemopoietic activity of cell populations isolated using one of these MAbs.

METHODS AND RESULTS

Marmoset cDNA encoding a human CD34 homologue was cloned from bone marrow (BM)-derived RNA using reverse transcription polymerase chain reaction and rapid amplification of cDNA ends. The amino acid sequence of the marmoset CD34 had 81% homology with the human sequence. Five mouse MAbs were raised against marmoset CD34 transfectant. One representative MAb, MA24 (IgM), reacted with approximately 0.5 to 1% of BM mononuclear cells (MNCs), where the colony-forming unit granulocyte/macrophage (CFU-GM) was enriched approximately 11- to 75-fold as compared with the whole BM MNCs. Multilineage differentiation of marmoset CD34+ cells in NOD/SCID mice was confirmed by flow cytometry 1 month after xenotransplantation.

CONCLUSION

These results demonstrated that MA24 is useful for the analysis and enrichment of hematopoietic progenitor cells in the marmoset model for preclinical experiments.

Hematopoietic cells and osteoblasts are derived from a common marrow progenitor after bone marrow transplantation

Bone and bone marrow are closely aligned physiologic compartments, suggesting that these tissues may represent a single functional unit with a common bone marrow progenitor that gives rise to both osteoblasts and hematopoietic cells. Although reports of multilineage engraftment by a single marrow-derived stem cell support this idea, more recent evidence has challenged claims of stem cell transdifferentiation and therefore the existence of a multipotent hematopoietic/osteogenic progenitor cell. Using a repopulation assay in mice, we show here that gene-marked, transplantable marrow cells from the plastic-nonadherent population can generate both functional osteoblasts/osteocytes and hematopoietic cells. Fluorescent in situ hybridization for the X and Y chromosomes and karyotype analysis of cultured osteoblasts confirmed the donor origin of these cells and excluded their generation by a fusion process. Molecular analysis demonstrated a common retroviral integration site in clonogenic hematopoietic cells and osteoprogenitors from each of seven animals studied, establishing a shared clonal origin for these ostensibly independent cell types. Our findings indicate that the bone marrow contains a primitive cell able to generate both the hematopoietic and osteocytic lineages. Its isolation and characterization may suggest novel treatments for genetic bone diseases and bone injuries.

Effect of chronic cytokine therapy on clonal dynamics in nonhuman primates

Hematopoietic cytokines such as filgrastim are used extensively to stimulate granulocyte production or to mobilize hematopoietic progenitors into the circulation; however, their effect on more primitive hematopoietic progenitor and stem cells in vivo is unknown, particularly in large animals or humans. In particular, there is concern that chronic therapy with cytokines could result in stem cell exhaustion or clonal dominance; however, direct assessment of the dynamics of individual stem and progenitor cell clones in vivo has not been previously reported. A number of models can be proposed regarding the mechanisms by which the marrow responds to cytokine stimulation, including recruitment of previously quiescent clones, stimulation of proliferation of already active clones, or prevention of apoptosis of more mature progenitors from all clones. Using retroviral marking and comprehensive insertion site tracking of individual stem and progenitor cell clones in 2 rhesus macaques, we analyzed the effect of chronic administration of granulocyte colony-stimulating factor (G-CSF), or a combination of G-CSF plus stem cell factor (SCF). The overall number of contributing clones remained constant, and the relative output from each clone did not change significantly during or following cytokine treatments. These results suggest that individual transduced stem or progenitor cells can contribute to hematopoiesis for prolonged periods, with no evidence for an effect of G-CSF or G-CSF/SCF on the number, the lifespan, or the relative activity of individual stem or progenitor cell clones. These relevant large animal studies are reassuring regarding clinical applications of cytokines and provide new insights into their mechanisms of action.

The impact of low-dose busulfan on clonal dynamics in nonhuman primates

An understanding of the number and contribution of individual pluripotent hematopoietic stem cells (HSCs) to the formation of blood lineages has important clinical implications for gene therapy and stem cell transplantation. We have been able to efficiently mark rhesus macaque long-term repopulating stem and progenitor cells with retroviral vectors, and track their in vivo contributions to hematopoiesis using the linear amplification mediated-polymerase chain reaction (LAM-PCR) technique of insertion site analysis. We assessed the impact of busulfan on contributions of individual retrovirally marked clones to hematopoiesis. There were 2 macaques that received transplants of retrovirally transduced CD34(+) cells 2 years previously that were then treated with 4 mg/kg busulfan. Despite only transient and mild suppression of peripheral blood counts, the numbers of individual stem/progenitor clones contributing to granulocyte production decreased dramatically, by 80% in the first monkey and by 60% in the second monkey. A similar impact was seen on clones contributing to T cells. The clone numbers recovered gradually back toward baseline by 5 months following busulfan in the first monkey and by 3 months in the second monkey, and have remained stable for more than one year in both animals. Tracking of individual clones with insertion-site-specific primers suggested that clones contributing to hematopoiesis prior to busulfan accounted for the majority of this recovery, but that some previously undetected clones began to contribute during this recovery phase. These results indicate that even low-dose busulfan significantly affects stem and progenitor cell dynamics. The clonal diversity of hematopoiesis was significantly decreased after even a single, clinically well-tolerated dose of busulfan, with slow but almost complete recovery over the next several months, suggesting that true long-term repopulating stem cells were not permanently deleted. However, the prolonged period of suppression of many clones suggests that transplanted HSCs may have a marked competitive advantage if they can engraft and proliferate during this time period, and supports the use of this agent in nonmyeloablative regimens

Retrovirally transduced muscle-derived cells contribute to hematopoiesis at very low levels in the nonhuman primate model

Recent studies have suggested a remarkable potential of adult stem cells from a variety of organs to give rise to cells of disparate organs, but evidence of such potential at a clonal level is lacking in most if not all studies to date. To assess directly the hematopoietic potential of muscle-derived cells in a relevant large animal, we initiated retroviral-tagging studies in the rhesus macaque to allow tracking at the clonal level by integration site analysis. Four rhesus macaques underwent transplantation with transduced muscle-derived cells after lethal irradiation followed by delayed infusion of an autologous hematopoietic graft. The first animal showed no evidence of hematopoietic recovery and, despite infusion of the backup hematopoietic graft, succumbed due to complications of prolonged cytopenias. In the remaining three animals, the overall contribution of retrovirally tagged muscle-derived cells toward hematopoiesis was exceedingly low. Retroviral integration site analysis among clonally derived muscle cells and bone marrow cells in vivo in one animal suggests a common source. These results demonstrate that harvesting disparate organs for cellular therapy is currently highly inefficient at best.

Genetic modification of hematopoietic stem cells: recent advances in the gene therapy of inherited diseases

Hematopoietic stem cells constitute a rare population of precursor cells with remarkable properties for being used as targets in gene therapy protocols. The last years have been particularly productive both in the fields of gene therapy and stem cell biology. Results from ongoing clinical trials have shown the first unquestionable clinical benefits of immunodeficient patients transplanted with genetically modified autologous stem cells. On the other hand, severe side effects in a few patients treated with gene therapy have also been reported, indicating the usefulness of further improving the vectors currently used in gene therapy clinical trials. In the field of stem cell biology, evidence showing the plastic potential of adult hematopoietic stem cells and data indicating the multipotency of adult mesenchymal precursor cells have been presented. Also, the generation of embryonic stem cells by means of nuclear transfer techniques has appeared as a new methodology with direct implications in gene therapy.

Cotransplantation of third-party mesenchymal stromal cells can alleviate single-donor predominance and increase engraftment from double cord transplantation

Although the infusion of umbilical cord blood (UCB) from multiple donors can be a strategy to overcome the cell dose limitation frequently encountered in UCB transplantation, clinical trials have revealed that cells from one donor dominate engraftment. To investigate the origin of and the factors influencing this inequality, we performed mixed transplantation of 2 UCB units with varying degrees of HLA disparities into NOD/SCID mice and determined donor origins by polymerase chain reaction-sequence-specific oligonucleotide probe (PCR-SSOP) or real-time quantitative (RQ)-PCR for human short tandem repeats (STRs). When total mononuclear cells from 2 units were transplanted as a mixture, cells from one donor predominated (ratio, 81:19), despite comparable overall engraftment when infused as single units, and no augmentation in overall engraftment was observed when compared with the single-unit controls. However, lineage depletion or cotransplantation of mesenchymal stromal cells (MSCs) expanded from third-party bone marrow resulted in more balanced coengraftment. Direct comparison of double UCB transplantation in the presence or absence of MSCs showed that the reduced deviation in the donor ratio (1.8:1 vs. 2.8:1) correlated with a higher overall level of engraftment with MSC cotransplantation. These results indicate that third-party MSCs can be used to alleviate donor deviation and to facilitate engraftment of multidonor UCB.

Proliferation kinetics of subpopulations of human marrow cells determined by quantifying in vivo incorporation of [2H2]-glucose into DNA of S-phase cells

This report investigated in vivo turnover kinetics of marrow hematopoietic progenitors and precursors using a recently developed stable isotope-mass spectrometric technique (SIMST). Human subjects were administered a 2-day infusion of 6,6-[2H2]-glucose, a nontoxic stable isotope-labeled form of glucose, which becomes incorporated into DNA of all S-phase cells. The percent [2H2]-glucose incorporated into DNA in the form of [2H2]-deoxyadenosine (%[2H2]-dA enrichment) was determined by gas chromatography-mass spectrometry. The rate constant of replacement of unlabeled by labeled DNA strands (labeling kinetics) was used to calculate population turnover kinetics of CD34+ cells, CD133+ cells, and CD133-CD34+ cells. The observed mean replacement half-life (t1/2) was 2.6 days for CD34+ cells, 2.5 days for CD133-CD34+ cells, and 6.2 days for CD133+ cells. Results from the estimated rate constant of replacement of labeled by unlabeled DNA (delabeling kinetics) also demonstrated slower turnover rates for CD133+ cells than for CD133-CD34+ cells. Although there was a relatively rapid initial decrease in the %[2H2]-dA enrichment, low levels of labeled DNA persisted in CD34+ cells for at least 4 weeks. The results indicate the presence of subpopulations of CD34+ cells with relatively rapid turnover rates and subpopulations with a slower t1/2 of 28 days. Results also demonstrate that in vivo [2H2]-glucose-SIMST is sensitive enough to detect differences in turnover kinetics between erythroid and megakaryocyte lineage cells. These studies are the first to demonstrate the use of in vivo [2H2]-glucose-SIMST to measure in vivo turnover kinetics of subpopulations of CD34+ cells and precursors in healthy human subjects.

AML1/RUNX1 mutations are infrequent, but related to AML-M0, acquired trisomy 21, and leukemic transformation in pediatric hematologic malignancies

AML1/RUNX1, located on chromosome band 21q22, is one of the most important hematopoietic transcription factors. AML1 is frequently affected in leukemia and myelodysplastic syndrome with 21q22 translocations. Recently, AML1 mutations were found in adult hematologic malignancies, especially acute myeloid leukemia (AML)-M0 or leukemia with acquired trisomy 21, and familial platelet disorder with a predisposition toward AML. Through the use of polymerase chain reaction-single-strand conformation polymorphism analysis, we examined the AML1 gene for mutations in 241 patients with pediatric hematologic malignancies, and we detected AML1 mutations in seven patients (2.9%). Deletion was found in one patient, and point mutations in four patients, including three missense mutations, two silent mutations, and one mutation within an intron resulting in an abnormal splice acceptor site. All of the mutations except for one were heterozygous. Mutations within the runt domain were found in six of seven patients. Six of seven patients with AML1 mutations were diagnosed with AML, and one had acute lymphoblastic leukemia. In three of these seven patients, AML evolved from other hematologic disorders. AML1 mutations were found in two of four AML-M0 and two of three patients with acquired trisomy 21. Patients with AML1 mutations tended to be older children. Three of four patients with AML1 mutations who received stem cell transplantation (SCT) are alive, whereas the remaining three patients with mutations without SCT died. These results suggest that AML1 mutations in pediatric hematologic malignancies are infrequent, but are possibly related to AML-M0, acquired trisomy 21, and leukemic transformation. These patients may have a poor clinical outcome.

Lentivirus-mediated gene transfer of uroporphyrinogen III synthase fully corrects the porphyric phenotype in human cells

Congenital erythropoietic porphyria (CEP) is an inherited disease due to a deficiency in the uroporphyrinogen III synthase, the fourth enzyme of the heme biosynthesis pathway. It is characterized by accumulation of uroporphyrin I in the bone marrow, peripheral blood and other organs. The prognosis of CEP is poor, with death often occurring early in adult life. For severe transfusion-dependent cases, when allogeneic cell transplantation cannot be performed, the autografting of genetically modified primitive/stem cells may be the only alternative. In vitro gene transfer experiments have documented the feasibility of gene therapy via hematopoietic cells to treat this disease. In the present study lentiviral transduction of porphyric cell lines and primary CD34(+) cells with the therapeutic human uroporphyrinogen III synthase (UROS) cDNA resulted in both enzymatic and metabolic correction, as demonstrated by the increase in UROS activity and the suppression of porphyrin accumulation in transduced cells. Very high gene transfer efficiency (up to 90%) was achieved in both cell lines and CD34(+) cells without any selection. Expression of the transgene remained stable over long-term liquid culture. Furthermore, gene expression was maintained during in vitro erythroid differentiation of CD34(+) cells. Therefore the use of lentiviral vectors is promising for the future treatment of CEP patients by gene therapy.

Rapid detection of retroviral vector integration sites in colony-forming human peripheral blood progenitor cells using PCR with arbitrary primers

We have developed a highly sensitive polymerase chain reaction (PCR)-based technique termed two-step PCR, which uses arbitrary primers to identify proviral integration sites in retrovirally marked human colony-forming cells. The two-step PCR was established on cell line clones transduced with the SF1m retroviral vector and independently validated by demonstrating identical integration sites with ligation-mediated PCR, a different technique requiring restriction enzyme digestion and adapter ligation for amplifying unknown DNA flanking the provirus. Two-step PCR was performed on peripheral blood progenitor cell (PBPC) colonies that contained as few as 75 cells, which was estimated by quantitative real-time PCR. We were able to amplify and directly sequence proviral integration sites in 35 % of PBPC colonies (25/72, five donors). Identity to the vector long-terminal repeat was confirmed and flanking DNA was found to match with human database sequences, reaffirming specificity. Two-step PCR is a valuable new tool for rapid analysis of genomic target sites for viral vectors, and will aid significantly in understanding clonal development of hematopoiesis and other cell types. Our protocol has the potential for general applicability as the arbitrary primers described here bind to genomic DNA and are thus independent of the vector backbone used.

Efficient characterization of retro-, lenti-, and foamyvector-transduced cell populations by high-accuracy insertion site sequencing

The identification of unknown genomic flanking DNA sequences can be used for the molecular monitoring of retro-, lenti- and foamyviral integration, transgenes in early embryogenesis, insertional mutagenesis, cell fate, and stem cell plasticity. Most existing methods reflect shortcomings in sensitivity and or specificity, thus limiting genomic sequencing of unknown flanking DNA to clonal preparations. The application of linear amplification-mediated PCR (LAM-PCR), a recently developed direct sequencing technique for flanking DNA, should circumvent current limitations in different research fields. This technique combines preamplification of target DNA with a unique succession of enzymatic reactions on solid-phase. Using LAM-PCR, we show the previously unfeasible in vivo retro-, lenti- and foamyvirus integration site analysis in primate peripheral blood hematopoietic cells and human xenograft hematopoiesis. In light of two severe adverse events that occurred in a clinical SCID-X1 gene therapy trial, in vivo monitoring of the reinfused transduced cell pool by integration site analysis will be an important component of each gene transfer and therapy study aimed at clinical use.

Thirty-four novel mutations detected in factor VIII gene by multiplex CSGE: modeling of 13 novel amino acid substitutions

Detection of causal mutations is required for genetic counseling. Molecular modeling combined with patients' phenotype provides significant insight into structure-function relationship of factor (F)VIII molecule. Our objective was to identify defects in the gene of FVIII by a sensitive and simple scanning technique with high throughput in order to study molecular mechanisms by which novel amino acid substitutions may lead to hemophilia A. A cohort of 81 families with mild, moderate and severe hemophilia A negative in intron 22 inversion was studied. For detection of mutations in the FVIII gene a conformation sensitive gel electrophoresis (CSGE) was modified by multiplexing. Thirteen novel amino acid substitutions were studied by molecular modeling and a correlation with the cross-reactive material (CRM) phenotype was performed. In 74 families, 59 different mutations were detected. Six different mutations were recurrent in 21 unrelated families. Thirty-four novel mutations included 19 point mutations, four small insertions, nine small deletions and two complex mutations. Thirteen novel amino acid substitutions occurred at residues conserved in FVIII orthologs. Five of them were associated with a discrepancy between FVIII activity and antigen; another five with CRM reduced phenotype and one with undetectable FVIII antigen. Multiplexing of the CSGE significantly increased its throughput without substantial loss of sensitivity. Molecular modeling suggested mechanisms by which substitutions at residues 382 and 569, located outside the proposed FIXa-binding region, may influence FVIII/FIXa interaction. His2155 was predicted to participate in FVIII/VFW binding.

Clonality analysis after retroviral-mediated gene transfer to CD34+ cells from the cord blood of ADA-deficient SCID neonates

A clinical trial of retroviral-mediated transfer of the adenosine deaminase (ADA) gene into umbilical cord blood CD34(+) cells was started in 1993. ADA-containing peripheral blood mononuclear cells (PBMCs) have persisted in patients from this trial, with T lymphocytes showing the highest prevalence of gene marking. To gain a greater understanding of the nature and number of the transduced cells that were engrafted, we used linear amplification-mediated PCR (LAM-PCR) to identify clonal vector proviral integrants. In one patient, a single vector integrant was predominant in T lymphocytes at a stable level over most of the eight-year time span analyzed and was also detected in some myeloid samples. T-cell clones with the predominant integrant, isolated after eight years, showed multiple patterns of T-cell receptor (TCR) gene rearrangement, indicating that a single pre-thymic stem or progenitor cell served as the source of the majority of the gene-marked cells over an extended period of time. It is important to distinguish the stable pattern of monoclonal gene marking that we observed here from the progressive increase of a T-cell clone with monoclonal gene marking that results from leukemic transformation, as observed in two subjects in a clinical trial of gene therapy for X-linked severe combined immunodeficiency (SCID).

Transfer of drug resistance genes in hematopoietic progenitors for chemoprotection: is it still an option?

For numerous malignancies a relationship between the intensity of antineoplastic chemotherapy and tumor response has been demonstrated. Myelotoxicity is the main cause of chemotherapy-associated morbidity and of treatment delays. The concept of myeloprotective cytostatic drug resistance gene transfer to normal hematopoietic stem cells (HSC) therefore sparks great enthusiasm. While initial studies using murine retroviral vectors on murine HSC showed that the concept works, a number of clinical studies in the last decade were not informative because of limitations in transduction efficiency and transgene expression.Furthermore, possible side effects such as unforeseen transgene activity and vector integration-based leukemogenesis have been reported. Among others, these developments raised some scepticism against the feasibility of myeloprotective gene transfer. Recently, considerable improvements have been achieved in vector design, HSC manipulation, selection protocols and risk assessment methods which are discussed in detail here. Based on these experimental studies successful clinical trials can now be anticipated.