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Sickle cell disease (SCD) and transfusion-dependent β-thalassemia (TDT) are the most prevalent monogenic disorders worldwide. Trial HGB-205 (NCT02151526) aimed at evaluating gene therapy by autologous CD34+cells transduced ex vivo with lentiviral vector BB305 that encodes the anti-sickling βA-T87Q-globin expressed in the erythroid lineage. HGB-205 is a phase 1/2, open-label, single-arm, non-randomized interventional study of 2-year duration at a single center, followed by observation in long-term follow-up studies LTF-303 (NCT02633943) and LTF-307 (NCT04628585) for TDT and SCD, respectively. Inclusion and exclusion criteria were similar to those for allogeneic transplantation but restricted to patients lacking geno-identical, histocompatible donors. Four patients with TDT and three patients with SCD, ages 13–21 years, were treated after busulfan myeloablation 4.6–7.9 years ago, with a median follow-up of 4.5 years. Key primary endpoints included mortality, engraftment, replication-competent lentivirus and clonal dominance. No adverse events related to the drug product were observed. Clinical remission and remediation of biological hallmarks of the disease have been sustained in two of the three patients with SCD, and frequency of transfusions was reduced in the third. The patients with TDT are all transfusion free with improvement of dyserythropoiesis and iron overload.

A primary challenge in lentiviral gene therapy of β-hemoglobinopathies is to maintain low vector copy numbers to avoid genotoxicity while being reliably therapeutic for all genotypes. We designed a high-titer lentiviral vector, LVβ-shα2, that allows coordinated expression of the therapeutic βA-T87Q-globin gene and of an intron-embedded miR-30-based short hairpin RNA (shRNA) selectively targeting the α2-globin mRNA. Our approach was guided by the knowledge that moderate reduction of α-globin chain synthesis ameliorates disease severity in β-thalassemia. We demonstrate that LVβ-shα2 reduces α2-globin mRNA expression in erythroid cells while keeping α1-globin mRNA levels unchanged and βA-T87Q-globin gene expression identical to the parent vector. Compared with the first βA-T87Q-globin lentiviral vector that has received conditional marketing authorization, BB305, LVβ-shα2 shows 1.7-fold greater potency to improve α/β ratios. It may thus result in greater therapeutic efficacy and reliability for the most severe types of β-thalassemia and provide an improved benefit/risk ratio regardless of the β-thalassemia genotype.

Although gene transfer to hematopoietic stem cells (HSCs) has shown therapeutic efficacy in recent trials for several individuals with inherited disorders, transduction incompleteness of the HSC population remains a hurdle to yield a cure for all patients with reasonably low integrated vector numbers. In previous attempts at HSC selection, massive loss of transduced HSCs, contamination with non-transduced cells, or lack of applicability to large cell populations has rendered the procedures out of reach for human applications. Here, we fused codon-optimized puromycin N-acetyltransferase to herpes simplex virus thymidine kinase. When expressed from a ubiquitous promoter within a complex lentiviral vector comprising the βAT87Q-globin gene, viral titers and therapeutic gene expression were maintained at effective levels. Complete selection and preservation of transduced HSCs were achieved after brief exposure to puromycin in the presence of MDR1 blocking agents, suggesting the procedure’s suitability for human clinical applications while affording the additional safety of conditional suicide.

GATA transcription factors and their FOG cofactors play a key role in tissue-specific development and differentiation, from worms to humans. Mammals have six GATA and two FOG factors. We recently demonstrated that interactions between retinoblastoma protein (pRb) and GATA-1 are crucial for erythroid proliferation and differentiation. We show here that the LXCXE pRb-binding site of FOG-2 is involved in adipogenesis. Unlike GATA-1, which inhibits cell division, FOG-2 promotes proliferation. Mice with a knockin of a Fog2gene bearing a mutated LXCXE pRb-binding site are resistant to obesity and display higher rates of white-to-brown fat conversion. Thus, each component of the GATA/FOG complex (GATA-1 and FOG-2) is involved in pRb/E2F regulation, but these molecules have markedly different roles in the control of tissue homeostasis.

Beta-thalassemia is a group of frequent genetic disorders resulting in the synthesis of little or no β-globin chains. Novel approaches are being developed to correct the resulting α/β-globin chain imbalance, in an effort to move beyond the palliative management of this disease and the complications of its treatment (e.g. life-long red blood cell transfusion, iron chelation, splenectomy), which impose high costs on healthcare systems. Three approaches are envisaged: fetal globin gene reactivation by pharmacological compounds injected into patients throughout their lives, allogeneic hematopoietic stem cell transplantation (HSCT), and gene therapy. HSCT is currently the only treatment shown to provide an effective, definitive cure for β-thalassemia. However, this procedure remains risky and histocompatible donors are identified for only a small fraction of patients. New pharmacological compounds are being tested, but none has yet made it into common clinical practice for the treatment of beta-thalassemia major. Gene therapy is in the experimental phase. It is emerging as a powerful approach without the immunological complications of HSCT, but with other possible drawbacks. Rapid progress is being made in this field, and long-term efficacy and safety studies are underway.

Our understanding of system dynamics of mixed cell populations in whole organisms has benefited from the advent of individual cell marking by nonarrayed DNA barcodes subsequently analyzed by high‐throughput DNA sequencing. However, key limitations include statistical biases compromising quantification and the lack of applicability to deconvolute individual cell fate in vivoafter pooling single cells differentially exposed to different conditions ex vivo. Here, we have derived an arrayed lentiviral library of DNA barcodes and obtained a proof‐of‐concept of its resolving capacity by quantifying hematopoietic regeneration after engraftment of mice with genetically modified autologous cells. This method has helped clarify and bridge the seemingly opposed clonal‐succession and continuous‐recruitment models of hematopoietic stem cell behavior and revealed that myeloid‐lymphoid biases are common occurrences in steady‐state hematopoiesis. Arrayed lentiviral barcoding should prove a versatile and powerful approach to deconvolute cell dynamics in vivowith applications in hematology, embryology, and cancer biology. STEMCells2013;31:2162–2171

A patient with βE/β0‐thalassemia major was converted to transfusion‐independence 4.5 years ago by lentiviral gene transfer in hematopoietic stem cells while showing a myeloid‐biased cell clone. Induced pluripotent stem cells (iPSCs) are a potential alternative source of hematopoietic stem cells. If fetal to adult globin class, switching does not occur in vivo in iPSC‐derived erythroid cells, β‐globin gene transfer would be unnecessary. To investigate both vector integration skewing and the potential use of iPSCs for the treatment of thalassemia, we derived iPSCs from the thalassemia gene therapy patient and compared iPSC‐derived hematopoietic cells to their natural isogenic somatic counterparts. In NSG immunodeficient mice, embryonic to fetal and a partial fetal to adult globin class switching were observed, indicating that the gene transfer is likely necessary for iPSC‐based therapy of the β‐hemoglobinopathies. Lentivector integration occurred in regions of low and high genotoxicity. Surprisingly, common integration sites (CIS) were identified across those iPSCs and cells retrieved from isogenic and nonisogenic gene therapy patients with β‐thalassemia and adrenoleukodystrophy, respectively. This suggests that CIS observed in the absence of overt tumorigenesis result from nonrandom lentiviral integration rather than oncogenic in vivo selection. These findings bring the use of iPSCs closer to practicality and further clarify our interpretation of genome‐wide lentivector integration. StemCells2013;31:1785‐1794

High-level production of β-globin, γ-globin, or therapeutic mutant globins in the RBC lineage by hematopoietic stem cell gene therapy ameliorates or cures the hemoglobinopathies sickle cell disease and beta thalassemia, which are major causes of morbidity and mortality worldwide. Considerable efforts have been made in the last 2 decades in devising suitable gene-transfer vectors and protocols to achieve this goal. Five years ago, the first βE/β0-thalassemia major (transfusion-dependent) patient was treated by globin lentiviral gene therapy without injection of backup cells. This patient has become completely transfusion independent for the past 4 years and has global amelioration of the thalassemic phenotype. Partial clonal dominance for an intragenic site (HMGA2) of chromosomal integration of the vector was observed in this patient without a loss of hematopoietic homeostasis. Other patients are now receiving transplantations while researchers are carefully weighing the benefit/risk ratio and continuing the development of further modified vectors and protocols to improve outcomes further with respect to safety and efficacy.

A challenge for gene therapy of genetic diseases is to maintain corrected cell populations in subjects undergoing transplantation in cases in which the corrected cells do not have intrinsic selective advantage over nontransduced cells. For inherited hematopoietic disorders, limitations include inefficient transduction of stem cell pools, the requirement for toxic myelosuppression, and a lack of optimal methods for cell selection after transduction. Here, we have designed a lentiviral vector that encodes human β-globin and a truncated erythropoietin receptor, both under erythroid-specific transcriptional control. This truncated receptor confers enhanced sensitivity to erythropoietin and a benign course in human carriers. Transplantation of marrow transduced with the vector into syngenic thalassemic mice, which have elevated plasma erythropoietin levels, resulted in long-term correction of the disease even at low ratios of transduced/untransduced cells. Amplification of the red over the white blood cell lineages was self-controlled and averaged ∼ 100-fold instead of ∼ 5-fold for β-globin expression alone. There was no detectable amplification of white blood cells or alteration of hematopoietic homeostasis. Notwithstanding legitimate safety concerns in the context of randomly integrating vectors, this approach may prove especially valuable in combination with targeted integration or in situ homologous recombination/repair and may lower the required level of pretransplantation myelosuppression.

The understanding of the hierarchical organization of the human hematopoietic system is of major biologic and clinical significance. The validity of the conventional model in which hematopoiesis is solely maintained by a pool of multipotent long-term hematopoietic stem cells (LT-HSCs) has been recently challenged by several mouse studies. These new data point to the existence of a heterogeneous stem cell population that consists of distinct subsets of LT-HSCs, which include stem cells biased toward lineage-specific differentiation programs. This review attempts to discuss the balanced versus biased patterns of lineage output of human LT-HSCs gathered in 3 different gene therapy trials on the basis of vector integration site analysis by deep sequencing. The distribution of integration sites observed tends to support the validity of the revised model.

X-linked severe-combined immunodeficiency (SCID-X1) has been treated by therapeutic gene transfer using gammaretroviral vectors, but insertional activation of proto-oncogenes contributed to leukemia in some patients. Here we report a longitudinal study of gene-corrected progenitor cell populations from 8 patients using 454 pyrosequencing to map vector integration sites, and extensive resampling to allow quantification of clonal abundance. The number of transduced cells infused into patients initially predicted the subsequent diversity of circulating cells. A capture-recapture analysis was used to estimate the size of the gene-corrected cell pool, revealing that less than 1/100th of the infused cells had long-term repopulating activity. Integration sites were clustered even at early time points, often near genes involved in growth control, and several patients harbored expanded cell clones with vectors integrated near the cancer-implicated genes CCND2 and HMGA2, but remain healthy. Integration site tracking also documented that chemotherapy for adverse events resulted in successful control. The longitudinal analysis emphasizes that key features of transduced cell populations—including diversity, integration site clustering, and expansion of some clones—were established early after transplantation. The approaches to sequencing and bioinformatics analysis reported here should be widely useful in assessing the outcome of gene therapy trials.

Application of gene therapy to induce antigen-specific immune tolerance could be important for transplantation or treatment of autoimmune diseases. Hematopoietic stem cell-based gene therapy has been hampered by relatively weak gene expression in vivo and loss of transduced cells over time. Selective expansion of transduced hematopoietic stem cells has been accomplished by incorporating the dihydrofolate reductase (DHFR) gene into the gene transfer vector.

Transplantation of hepatocytes is a promising alternative to liver transplantation for the treatment of severe liver diseases. However, this approach is hampered by the shortage of donor organs and intrinsic limitations of adult hepatocytes. To investigate whether most of the hurdles faced with adult hepatocytes could be surmounted by the use of human fetal hepatoblasts, we have developed a method to isolate, transduce, and cryopreserve hepatoblasts from human livers at an early stage of development (11–13 weeks of gestation). Cells were characterized in vitro for expression of specific markers, and in vivo for their proliferation and differentiation potential after transplantation into athymic mice. Most of the cells (80–90%) harbored a bipotent phenotype, expressing cytokeratins 8/18, albumin, and CK19. They proliferated spontaneously in culture and were efficiently transduced by a β-galactosidase-expressing retrovirus (90%). After transplantation, cryopreserved cells engrafted into the liver of athymic mice and proliferated, resulting in up to 10% repopulation. Engrafted cells expressed markers of differentiated adult hepatocytes including albumin, α1-antitrypsin, cytochrome P450 3A4, and α-glutathione-S-transferase. When retrovirally transduced before transplantation they expressed the transgene in vivo. In summary, early human fetal hepatoblasts engraft, proliferate, and mature in athymic mouse liver, without conditioning the donor.

Liver endothelial cells (LECs) perform an essential role in important pathophysiologic functions in the liver. Establishment of a human LEC line facilitates advances in LEC research. Here, we present immortalization of human LECs using retroviral gene transfer of simian virus 40 large T antigen (SV40T) and human telomerase reverse transcriptase (hTERT). We also demonstrate excision of SV40T and hTERT with TAT-mediated Cre/loxP recombination and subsequent cell sorting.

Cholangiocytes perform an essential role in important pathophysiologic functions in the liver. Establishment of a human cholangiocyte line facilitates advances in cholangiocyte research and clinical applications for cell therapies. Here, we describe the immortalization of human cholangiocytes using serial transfection of simian virus 40 large T (SV40T) followed by human telomerase reverse transcriptase (hTERT).

Central to the pathophysiology of sickle cell disease are the vaso-occlusive events that lead to tissue damages and life-threatening complications. Lungs are particularly vulnerable to vaso-occlusion because of their specific vasculature. We developed a mouse model of hypoxia/reoxygenation lung injury closely mimicking the lung pathology of patients with sickle cell disease. This model involves the exposure of transgenic sickle cell (SAD) mice to hypoxia (8% oxygen) for 4, 10, and 46 hours followed by 2 hours of reoxygenation. Gene expression profiling of SAD lung tissue pointed to the specific induction of genes involved in the response to ischemic stress and microcirculation remodeling: Hspcb, Hsp86-1, Nfe2l2, Ace, and Fgf7. Hypoxia/reoxygenation also induced a marked increase in bronchoalveolar (BAL) total leukocyte and neutrophil counts, BAL total protein content, and BAL tumor necrosis factor α (TNF-α), interleukin 6 (IL-6), IL-1α, and macrophage inflammatory protein 2 (MIP-2) levels, all indicators of enhanced inflammatory response as compared with control mice. Nitric oxide (NO) was administered to SAD mice. NO (40 ppm) inhalation protected SAD mice from the histopathologic lesions of ischemic/reperfusion lung injury with corresponding normalization and/or modulation of tissue gene expression profiles. Inhaled NO (1) significantly reduced the increase in BAL total protein content, BAL total leukocyte, and neutrophil counts; (2) modulated BAL cytokine network; and (3) did not affect hemoglobin and methemoglobin levels. The present study provides evidences for the beneficial effects of inhaled NO in pulmonary injury induced by hypoxia/reoxygenation in a mouse model of sickle cell disease (SCD) and opens new avenues in drug design based on tissue gene expression profiling.

Maintenance of liver-specific functions has been shown to be stabilized by co-cultivation of hepatocytes with hepatic stellate cells (HSC). Because the limited lifespan of human HSC is a major hurdle to their use, the authors report here the amplification of human HSC populations in vitro by retroviral transfer of human telomerase reverse transcriptase (hTERT).

Transfer of therapeutic genes to human hematopoietic stem cells (HSCs) using complex vectors at clinically relevant efficiencies remains a major challenge. Recently we described a stable retroviral vector that sustains long-term expression of green fluorescent protein (GFP) and a human β-globin gene in the erythroid progeny of transduced murine HSCs. We now report the efficient transduction of primitive human CD34+ fetal liver or cord blood cells with this vector and expression of the β-globin transgene in the erythroid progeny of these human cells for at least 2 months. After growth factor prestimulation and then a 2- to 3-day exposure to the virus, 35% to 55% GFP+ progeny were seen in assays of transduced colony-forming cells, primitive erythroid precursors that generate large numbers of glycophorin A+ cells in 3-week suspension cultures, and 6-week long-term culture-initiating cells. In immunodeficient mice injected with unselected infected cells, 5% to 15% of the human cells regenerated in the marrow (including the erythroid cells) were GFP+ 3 and 6 weeks after transplantation. Importantly, the numbers of GFP+ human lymphoid and either granulopoietic or erythroid cells in individual mice 6 weeks after transplantation were significantly correlated, indicative of the initial transduction of human multipotent cells with in vivo repopulating activity. Expression of the transduced β-globin gene in human cells obtained directly from the mice or after their differentiation into erythroid cells in vitro was demonstrated by reverse transcriptase–polymerase chain reaction using specific primers. These experiments represent a significant step toward the realization of a gene therapy approach for human β-globin gene disorders.

Endothelial cells (ECs) play multiple physiological functions and are central to many pathological processes. Various biological studies as well as cell and gene therapy applications would benefit substantially from a procedure that would result in the expansion in culture of large numbers of highly differentiated human ECs. Here, we report the amplification in vitro of human EC populations, which occurred during the first phase of reversible immortalization resulting from the retroviral transfer of an oncogene that was subsequently excised by Cre-loxP-mediated site-specific recombination. Human umbilical vein endothelial cells (HUVECs) and human liver sinusoidal endothelial cells (HLSECs) were transduced with a retroviral vector that expresses the simian virus 40 large T (SV40T) gene flanked by positive and negative selectable markers and a pair of loxP recombination targets. Transduced HUVECs and HLSECs yielded clones with greatly extended life spans, referred to as HNNT-1 and HNNT-2 cells, respectively. HNNT-1 and HNNT-2 cells showed morphological characteristics of ECs and were maintained in culture up to population doubling level (PDL) 80 for HNNT-1 and PDL 65 for HNNT-2 cells. HNNT-1 and HNNT-2 cells were not tumorigenic when transplanted into severe combined immunodeficiency mice and were sensitive to ganciclovir as well as G418. Both cell clones expressed EC markers, which include factor VIII, VEGF receptors (Flt-1 and KDR/Flk-1), and CD34, and endocytosed acetylated low-density lipoproteins. Formation of capillary-like structures in a Matrigel assay was observed with HNNT-1 and HNNT-2 cells until at least PDL 50. Complete elimination of the transferred SV40T gene was achieved in virtually 100% of HNNT-1 and HNNT-2 cells after infection with a recombinant adenovirus expressing the Cre recombinase fused to a nuclear localization signal and subsequent selection with G418. Reverted cells maintained their differentiated EC phenotype. This study extends the utility of the reversible immortalization procedure and provides a means to expand primary human ECs of various sources for basic studies and possible cell and gene therapies.

An ideal alternative to the primary human hepatocytes for hepatocyte transplantation would be to use a clonal cell line that grows economically in culture and exhibits the characteristics of differentiated, nontransformed hepatocytes following transplantation. The purpose of the present studies was to establish a reversibly immortalized human hepatocyte cell line. Human hepatocytes were immortalized with a retroviral vector SSR#69 expressing simian virus 40 large T antigen (SV40Tag) gene flanked by a pair of loxP recombination targets. One of the resulting clones, NKNT-3, showed morphological characteristics of liver parenchymal cells and expressed the genes of differentiated liver functions. NKNT-3 cells offered unlimited availability. After an adenoviral delivery of Cre recombinase and subsequent differential selection, efficient removal of SV40Tag from NKNT-3 cells was performed. Here we represent that elimination of the retrovirally transferred SV40Tag gene can be excised by adenovirus-mediated site-specific recombination.

Current clinical reports have indicated that hepatocyte transplantation (HTX) could be used in patients with liver failure and in children with liver-based metabolic diseases. One of the major limiting factors of HTX is a serious shortage of donor livers for hepatocyte isolation. To address this issue, we immortalized adult human hepatocytes with a retroviral vector SSR#69 expressing the genes of simian virus 40 large T antigen and herpes simplex virus-thymidine kinase simultaneously. One of the resulting clones, NKNT-3, grew steadily in chemically defined serum-free medium without any obvious crisis and showed the gene expression of differentiated liver functions. Under the administration of 5 μM ganciclovir, NKNT-3 cells stopped proliferation and died in in vitro experiments. We have established a tightly regulated immortal human hepatocyte cell line. The cells could allow the need for immediate availability of consistent and functionally uniform cells in sufficient quantity and adequate quality.

Patients with metastatic neuroblastoma are rarely curable with currently available therapy, and the search for new treatment options, which include the use of inhibitors of tumor angiogenesis, is warranted. Here, we have evaluated the efficacy of one of the most promising natural inhibitors of angiogenesis described to date, endostatin, in a human neuroblastoma xenograft model in nude mice. Murine endostatin cDNA was cloned in a bacterial expression vector, expressed as a polyHis-Endostatin fusion protein and purified on Ni2+-NTA beads. The in vitroactivity of soluble endostatin was confirmed on bovine capillary endothelial cells and human umbilical vein endothelial cells. The human neuroblastoma cell line SKNAS was injected subcutaneously in the flank of nude mice and administration of the recombinant angiogenesis inhibitor started when tumors reached the size of 100 μm3. Twenty mg/kg of recombinant precipitated endostatin or PBS was subcutaneously injected daily for 12 days. Serum endostatin levels were measured using a competitive enzyme immunoassay. Tumor growth was only slowed down in endostatin-treated mice when compared to control mice, and no statistically significant difference in serum levels of endostatin was observed between endostatin-treated and control groups. The lack of correlation between serum concentration and tumor response raises concern regarding the mechanism of action of endostatin.

Protein synthesis in reticulocytes depends on the availability of heme. In heme deficiency, inhibition of protein synthesis correlates with the activation of heme-regulated eIF-2a kinase (HRI), which blocks the initiation of protein synthesis by phosphorylating eIF-2a. HRI is a hemoprotein with 2 distinct heme-binding domains. Heme negatively regulates HRI activity by binding directly to HRI. To further study the physiological function of HRI, the wild-type (Wt) HRI and dominant-negative inactive mutants of HRI were expressed by retrovirus-mediated transfer in both non-erythroid NIH 3T3 and mouse erythroleukemic (MEL) cells. Expression of Wt HRI in 3T3 cells resulted in the inhibition of protein synthesis, a loss of proliferation, and eventually cell death. Expression of the inactive HRI mutants had no apparent effect on the growth characteristics or morphology of NIH 3T3 cells. In contrast, expression of 3 dominant-negative inactive mutants of HRI in MEL cells resulted in increased hemoglobin production and increased proliferative capacity of these cells upon dimethyl-sulfoxide induction of erythroid differentiation. These results directly demonstrate the importance of HRI in the regulation of protein synthesis in immature erythroid cells and suggest a role of HRI in the regulation of the numbers of matured erythroid cells.

A worldwide shortage of donor livers is a limiting factor of the clinical application of hepatocyte transplantation (HTX). To resolve this issue, we focused on a reversible immortalization system that allows temporary expansion of primary hepatocyte populations by transfer of an oncogene that can be subsequently excised. As a preliminary test toward this goal, we examined the efficacy of Cre/loxP site-specific recombination in a transformed human liver cell line, HepG2. The present study utilized retroviral transfer of a prototypical immortalizing gene, simian virus 40 large T antigen (SV40Tag), flanked by a pair of loxP recombination targets and adenovirus-mediated Cre/loxP recombination. Here we report that complete elimination of the retroviral transferred oncogene was achieved by site-specific recombination using a replication-deficient recombinant adenovirus vector producing Cre recombinase (Ad-Cre).

The human ß-globin locus control region (LCR) contains chromatin opening and transcriptional enhancement activities that are important to include in ß-globin gene therapy vectors. We previously used single-copy transgenic mice to map chromatin opening activity to the 5'HS3 LCR element. Here, we test novel hybrid globin genes to identify ß-globin gene sequences that functionally interact with 5'HS3. First, we show that an 850-base pair (bp) 5'HS3 element activates high-level ß-globin gene expression in fetal livers of 17 of 17 transgenic mice, including 3 single-copy animals, but fails to reproducibly activate A?-globin transgenes. To identify the ß-globin gene sequences required for LCR activity by 5'HS3, we linked the 815-bp ß-globin promoter to A?-globin coding sequences (BGT34), together with either the ß-globin intron 2 (BGT35), the ß-globin 3' enhancer (BGT54), or both intron 2 and the 3' enhancer (BGT50). Of these transgenes, only BGT50 reproducibly expresses A?-globin RNA (including 7 of 7 single-copy animals, averaging 71% per copy). Modifications to BGT50 show that LCR activity is detected after replacing the ß-globin promoter with the 700-bp A?-globin promoter, but is abrogated when an AT-rich region is deleted from ß-globin intron 2. We conclude that LCR activity by 5'HS3 on globin promoters requires the simultaneous presence of ß-globin intron 2 sequences and the 260-bp 3' ß-globin enhancer. The BGT50 construct extends the utility of the 5'HS3 element to include erythroid expression of nonadult ß-globin coding sequences in transgenic animals and its ability to express antisickling ?-globin coding sequences at single copy are ideal characteristics for a gene therapy cassette.

Expression of experimental constructs in mammalian cells or transgenic animals is difficult to control because it is markedly influenced by position effects. This has limited both the analysis of cis-DNA regulatory elements for transcription and replication, and the physiological analysis of proteins expressed from transgenes. We report here two new methods based on the concept of recombinase-mediated cassette exchange (RMCE) to perform site-specific chromosomal integration. The first method permits the exchange of a negative selectable marker pre-localized on the chromosome with a transgene via a CRE-mediated double recombination between inverted Lox sites. Integration efficiency is close to 100 % of negatively selected mouse erythroleukemia cells and ranges from 10 to 50 % in embryonic stem cells. The second method allows RMCE with no selection at all except for cells that have taken up plasmid transiently. While less efficient, this technique permits novel experimental approaches.We find that integration of a transgene at a given genomic site leads to reproducible expression. RMCE should be useful to develop artificial genetic loci that impart specific and reproducible regulation of transgenes in higher eukaryotes. This should facilitate the analysis of cis-regulatory DNA elements governing expression and position effects, improve our control over the physiological effects of transgenes, and accelerate the development of animal models for complex human diseases.

Retrovirus‐mediated gene transfer of the human beta‐globin gene into hematopoietic stem cells is an attractive approach to the therapy of human beta‐globin gene disorders. However, expression of the transduced beta‐globin gene linked to its proximal cis‐acting sequences (‐0.8 to +0.3 kb from the cap site) is considerably below the level required for a significant therapeutic effect. The discovery of the beta‐locus control region (beta‐LCR), organized in four major DNase I hypersensitive sites far upstream of the human beta‐like globin gene cluster, provided a potential means to achieve a high level of expression of a linked human beta‐globin gene, but initial attempts to incorporate beta‐LCR derivatives in retroviral vectors resulted in the production of low‐titer viruses with multiple rearrangements of the transmitted proviral structures. We now describe how extensive mutagenesis of the transduced beta‐globin gene, eliminating a 372 bp intronic segment and multiple reverse polyadenylation and splicing signals, increases viral titer significantly and restores stability of proviral transmission upon infection of cell lines and bone marrow‐repopulating cells. These optimized vectors have enabled us to analyze the expression properties of various retrovirally transduced beta‐LCR derivatives in dimethylsulfoxide‐induced murine erythroleukemia cells and to achieve ratios of human beta‐globin/murine beta maj‐globin mRNA, on a per gene basis, as high as 80%.

We have constructed two fusion proteins, DAB389–mIL‐3 and DAB389‐(Gly4Ser)2–mIL‐3, in which the receptor‐binding domain of diphtheria toxin is replaced by mouse interleukin‐3 (IL‐3). Cytotoxic activity of the fusion toxins was observed on three out of six cell lines assayed. This toxicity was mediated through binding to the IL‐3 receptor as it was inhibited in a dose‐dependent manner with murine IL‐3 or anti‐IL‐3 neutralizing antibodies. DAB389–(Gly4Ser)2‐mIL‐3 was up to 5 times more toxic than DAB389–mIL‐3, depending on the cell line (0.8×10−10M

We are focusing on the development of complex retroviral vectors containing human ?-globin gene and ?-LCR for the gene therapy of sickle cell disease and ?-thalassemias. First generation vectors containing mutated splice-sites to insure stability of proviral transfer enabled long-term reconstitution in 1012 transplanted mice for a least 8 months with high expression levels in 2 out of 3 mice analyzed (5 and 20 murine ?). Transfer and expression were also achieved in secondary recipients (range: 3-11 murine ?). Position independent expression was not observed. In an effort to increase the efficiency of gene transfer and obtain complete reconstitution of recipient mice with exclusively transduced cells while enriching for proviral integration into active chromatin regions, we have incorporated a cassette expressing CD24 or the green fluorescent protein (GFP). Stable transfer to murine bone marrow cells allowed efficient FACS-sorting of pure populations of transduced cells. A family of vectors based on these principles and containing segments of ?- or ?-globin genes were also designed for systematic analysis of their anti-sickling properties.

Regulation of the cell cycle is orchestrated by cyclins and cyclin-dependent kinases. We have demonstrated previously that overexpression of eukaryotic translation initiation factor 4E (eIF-4E) in NIH 3T3 cells growing in 10% fetal calf serum leads to highly elevated levels of cyclin D1 protein without significant increase in cyclin D1 mRNA levels, suggesting that a post-transcriptional mechanism is involved. (Rosenwald, I. B., Lazaris-Karatzas, A., Sonenberg, N., and Schmidt, E. V. (1993) Mol. Cell. Biol. 13, 7358-7363). In the present research, we did not find any significant effect of eIF-4E on polysomal distribution of cyclin D1 mRNA. However, the total amount of cyclin D1 mRNA associated with polysomes was significantly increased by eIF-4E overexpression. Further, we determined that the levels of both cyclin D1 protein and mRNA are increased in serum-deprived cells overexpressing eIF-4E. Nuclear run-on experiments demonstrated that the rate of the cyclin D1 transcription is not down-regulated in serum-deprived cells overexpressing eIF-4E. Thus, elevated levels of eIF-4E may lead to increased transcription of the cyclin D1 gene, and this effect becomes visible when serum deprivation down-regulates the rate of cyclin D1 mRNA synthesis in control cells. However, artificial overexpression of cyclin D1 mRNA in serum-deprived cells in the absence of eIF-4E overexpression did not cause the elevation of cyclin D1 protein, and this overexpressed cyclin D1 mRNA accumulated in the nucleus, suggesting that one post-transcriptional role of eIF-4E is to transport cyclin D1 mRNA from the nucleus to cytoplasmic polysomes.

Somatic gene therapy of hemoglobinopathies depends initially on the demonstration of safe, efficient gene transfer and long-term, high-level expression of the transferred human β-globin gene in animal models. We have used a β-globin gene/β-locus control region retroviral vector containing several modifications to optimize gene transfer and expression in a mouse transplant model. In this report we show that transplantation of β-globin–transduced hematopoietic cells into lethally irradiated mice leads to the continued presence of the gene up to 8 months posttransplantation. The transferred human β-globin gene is detected in 3 of 5 mice surviving long term (>4 months) transplanted with bone marrow cells transduced with high-titer virus. Southern blotting confirms the presence of the unrearranged 5.1-kb human β-globin gene-containing provirus in 2 of these mice. In addition, long-term expression of the transferred gene is seen in 2 mice at levels of 5% and 20% that of endogenous murine β-globin at 6 and 8 months posttransplantation. We further document stem cell transduction by the successful transfer and high-level expression of the human β-globin gene from mice transduced 9 months earlier into irradiated secondary recipient mice. These results demonstrate high-level, long-term somatic human β-globin gene transfer into the hematopoietic stem cells of an animal for the first time, and suggest the potential feasibility of a retroviral gene therapy approach to sickle cell disease and the β thalassemias.

Efficient integration of transgenes at preselected chromosomal locations was achieved in mammalian cells by recombinase-mediated-cassette-exchange (RMCE), a novel procedure that makes use of the CRE recombinase together with Lox sites bearing different spacer regions. We have applied RMCE to the study of the human β-globin gene Locus Control Region by integrating at the same genetic locus in MEL cells, a LacZ gene driven by the human β-globin promoter linked to HS2 and HS3 alone or in combination with HS4. Expression studies at the cell population level and in individual cells before and after induction of differentiation with hemin or DMSO show that the presence of these enhancers is associated with variegated patterns of expression. We were able to show that the LCR fragments tested act by controlling both the probability of expression and the rate of transcription of the linked β-globin promoter. Both of these factors were also dependent on the state of differentiation of the MELc and on the presence of a second transcription unit located in cis. The ability to manipulate by RMCE constructs integrated into chromosomes should help in the creation of complex, rationally designed, artificial genetic loci.

The cellular receptors that mediate binding and internalization of retroviruses have recently been identified. The concentration and accessibility of these receptors are critical determinants in accomplishing successful gene transfer with retrovirus-based vectors. Murine retroviruses containing ecotropic glycoproteins do not infect human cells since human cells do not express the receptor that binds the ecotropic glycoproteins. To enable human cells to become permissive for ecotropic retrovirus-mediated gene transfer, we have developed a recombinant adeno-associated virus type 2 (AAV) vector containing ecotropic retroviral receptor (ecoR) cDNA under the control of the Rous sarcoma virus (RSV) long terminal repeat (LTR) promoter (vRSVp-ecoR). Established human cell lines, such as HeLa and KB, known to be nonpermissive for murine ecotropic retroviruses, became permissive for infection by a retroviral vector containing a bacterial gene for resistance to neomycin (RV-Neo(r)), with a transduction efficiency of up to 47%, following transduction with vRSVp-ecoR, as determined by the development of colonies that were resistant to the drug G418, a neomycin analog. No G418-resistant colonies were present in cultures infected with either vRSVp-ecoR or RV-Neo(r) alone. Southern and Northern blot analyses revealed stable integration and long-term expression, respectively, of the transduced murine ecoR gene in clonal isolates of HeLa and KB cells. Similarly, ecotropic retrovirus-mediated Neo(r) transduction of primary human CD34+ hematopoietic progenitor cells from normal bone marrow was also documented, but only following infection with vRSVp-ecoR. The retroviral transduction efficiency was approximately 7% without prestimulation and approximately 14% with prestimulation of CD34+ cells with cytokines, as determined by hematopoietic clonogenic assays. No G418-resistant progenitor cell colonies were present in cultures infected with either vRSVp-ecoR or RV-Neo(r) alone. These results suggest that sequential transduction of primary human cells with two different viral vectors may overcome limitations encountered with a single vector. Thus, the combined use of AAV- and retrovirus-based vectors may have important clinical implications for ex vivo and in vivo human gene therapy.

We have constructed two fusion proteins, DAB 389–mIL-3 and DAB 389-(Gly 4Ser) 2–mIL-3, in which the receptor-binding domain of diphtheria toxin is replaced by mouse interleukin-3 (IL-3). Cytotoxic activity of the fusion toxins was observed on three out of six cell lines assayed. This toxicity was mediated through binding to the IL-3 receptor as it was inhibited in a dose-dependent manner with murine IL-3 or anti-IL-3 neutralizing antibodies. DAB 389–(Gly 4Ser) 2-mIL-3 was up to 5 times more toxic than DAB 389–mIL-3, depending on the cell line (0.8×10 −10M

Human gamma-globin and delta-globin chains have been previously identified as strong inhibitors of the polymerization of hemoglobin S, in contrast to the beta-globin chain, which exerts only a moderate antisickling effect. However, gamma-globin and delta-globin are normally expressed at very low levels in adult erythroid cells, in contrast to beta-globin. We report the design of a beta-globin/delta-globin hybrid gene, beta/delta-sickle cell inhibitor 1 (beta/delta-SCI1) and its transduction by retrovirus-mediated gene transfer. The beta/delta-SCI1-encoding gene retains the overall structure of the human beta-globin gene, while incorporating specific amino acid residues from the delta chain previously found responsible for its enhanced antisickling properties. To achieve high expression levels of beta/delta-SCI1 in adult erythrocytes, the hybrid gene was placed under the transcriptional control of the human beta-globin promoter and the DNase I hypersensitive site 2 of the human beta locus control region. High-titer retroviruses were generated, and stable proviral transmission was achieved in infected cells. The mRNA expression levels of the beta/delta-SCI1 gene in infected, dimethyl sulfoxide-induced murine erythroleukemia cells approached 85% of the endogenous murine beta maj-globin mRNA, on a per gene basis, evidence that high gene expression levels were achieved in adult erythroid cells. Further evaluation of this strategy in transgenic animal models of sickle cell disease should assess its efficacy for the gene therapy of human patients.

The human gene coding for porphobilinogen deaminase (PBG-D) is transcribed into two distinct transcription units giving two mRNAs. These units originate from two adjacent promoters distant of 3 kilobase pairs. The upstream promoter is active in all cell types, whereas the downstream promoter is active only in erythroid cells. We have studied the expression of this gene either after introduction of the corresponding human chromosome into murine erythroid cells using somatic hybrids or after transfection into both erythroid and non-erythroid cells. Using somatic hybrids, we showed that activation of the erythroid-specific promoter of the PBG-D gene did not reduce the rate of initiation of the ubiquitous promoter. Transfection experiments in erythroid cells showed that the PBG-D erythroid transcription unit, controlled by the PBG-D erythroid promoter, was correctly transcribed and regulated. Furthermore, we found that the PBG-D erythroid promoter alone was sufficient for correct expression and regulation of a reporter gene during erythroid differentiation. When the human PBG-D gene was transfected into non-erythroid cells, only the ubiquitous promoter was active. Deletion of the ubiquitous promoter did not lead to any activation of the erythroid promoter, suggesting that its inactivity in non-erythroid cells was not due to promoter occlusion but to a strict erythroid specificity.

Rasko: Rarecyte: Consultancy, Equity Ownership; IMAGO Biosciences: Consultancy; Takeda: Speakers Bureau; Abbvie: Speakers Bureau; Gilead: Honoraria; Novartis: Consultancy, Speakers Bureau; Cynata: Consultancy, Honoraria; bluebird bio: Honoraria, Other: Clinical trials ; Spark: Consultancy; Genea: Equity Ownership; Current Cure The Future Foundation: Membership on an entity's Board of Directors or advisory committees; FSHD Global Research Foundation: Membership on an entity's Board of Directors or advisory committees; Gene Technology Technical Advisory, OGTR, Australian Government: Other: Chair; Advisory Committee on Biologics, Therapeutics Goods Administration, Australian Government: Other: Past Chair; International Society for Cellular Therapy: Membership on an entity's Board of Directors or advisory committees; GSK: Honoraria; Pfizer: Honoraria; Celgene: Honoraria. Thompson:Baxalta/Shire: Research Funding; La Jolla Pharmaceutical: Research Funding; Amgen: Research Funding; Biomarin: Research Funding; Celgene: Research Funding; bluebird bio: Consultancy, Research Funding; Novartis: Research Funding. Kwiatkowski:Apopharma: Research Funding; Agios Pharmaceuticals: Consultancy, Research Funding; Novartis: Research Funding; Terumo: Research Funding; bluebird bio: Consultancy, Honoraria, Research Funding. Schiller:bluebird bio: Research Funding; Astellas Pharma: Membership on an entity's Board of Directors or advisory committees, Research Funding. Ho:Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Takeda: Honoraria, Other: Travel to meeting; Celgene: Other: Travel to meeting . Schmidt:GeneWerk GmbH: Employment; German Cancer Research Center: Employment; bluebird bio: Consultancy. Leboulch:bluebird bio: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties. Vichinsky:bluebird bio: Membership on an entity's Board of Directors or advisory committees; Global Blood Therapeutics: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Protagonist: Research Funding. Deary:bluebird bio: Employment, Equity Ownership. Chen:bluebird bio: Consultancy. Asmal:bluebird bio: Employment, Equity Ownership. Walters:Sangamo Therapeutics: Consultancy; ViaCord Processing Lab: Other: Medical Director; AllCells Inc.: Other: Medical Director; bluebird bio: Research Funding.

Background

Background:LentiGlobin Drug Product (DP) contains autologous CD34+ cells transduced with the betibeglogene darolentivec (BB305) lentiviral vector, encoding a human β-globin gene with a point mutation (AT87Q) that confers anti-sickling properties similar to those with γ-globin. Human proof of concept for LentiGlobin treatment in transfusion-dependent β-thalassemia (TDT) and severe sickle cell disease (SCD) has been established in HGB-205 (NCT02151526), the original clinical study of gene therapy for hemoglobinopathies. Here, we provide an update on 5 previously-reported patients (4 TDT, 1 SCD) as well as early data from 2 additional treated patients with SCD.

Autologous hematopoietic stem cell (HSC) gene therapy could be an effective treatment for transfusion-dependent β-thalassemia (TDT) without some of the limitations and risks associated with allogeneic HSC transplantation. LentiGlobin Drug Product (DP) contains autologous HSC (CD34+ cells) transduced ex vivowith the betibeglogene darolentivec (BB305) lentiviral vector encoding a human β-globin gene with the anti-sickling T87Q mutation (HbAT87Q). The Northstar Study(HGB-204; NCT01745120) is an international, multi-center phase 1/2 clinical trial examining the safety and efficacy of LentiGlobin investigational gene therapy in patients with TDT. As of the most recent data cut (June 2, 2017), all patients have 1 to 3 years of follow-up post DP infusion.

Leboulch: bluebird bio: Patents & Royalties. Payen:bluebird bio: Patents & Royalties. Srivastava:AGTC; Genzyme: Patents & Royalties.

Introduction:β-globin gene transfer into hematopoietic stem cells (HSCs) has the potential to reduce or eliminate the symptoms of severe sickle cell disease (SCD) and reduce or eliminate transfusion requirements in transfusion-dependent β-thalassemia (TDT). LentiGlobin Drug Product (DP) contains autologous CD34+ cells transduced with the BB305 lentiviral vector, which encodes a human β-globin gene containing a single point mutation (AT87Q) designed to confer anti-sickling properties similar to those observed with γ-globin. We previously reported proof of concept for LentiGlobin DP treatment in severe SCD and early data from 4 treated patients with TDT. We now report 18 months of follow-up for the patient with SCD and between 9 and 30 months of follow-up for the 4 patients with TDT.

Thompson: Amgen: Research Funding; bluebird bio: Consultancy, Research Funding; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Research Funding; Mast: Research Funding; ApoPharma: Consultancy, Membership on an entity's Board of Directors or advisory committees; Eli Lily: Research Funding; Baxalta (now part of Shire): Research Funding. Kwiatkowski:Luitpold Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Apopharma: Research Funding; Ionis pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Sideris Pharmaceuticals: Consultancy; Shire Pharmaceuticals: Consultancy. Rasko:GSK: Honoraria; IMAGO BioSciences: Consultancy, Equity Ownership; Genea: Consultancy, Equity Ownership; Rarecyte: Consultancy, Equity Ownership; Australian government and philanthropic foundations: Research Funding; Cure The Future Foundation: Other: Voluntary non-executive Board Member; Royal College of Pathologists of Australasia Foundation: Other: Voluntary non-executive Board Member; Office of the Gene Technology Regulator (OGTR) Australian Government: Membership on an entity's Board of Directors or advisory committees. Schiller:Incyte Corporation: Research Funding. Ho:Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees. von Kalle:bluebird bio: Consultancy; GeneWerk: Equity Ownership. Leboulch:bluebird bio: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Research Funding. Petrusich:bluebird bio: Employment, Equity Ownership. Asmal:bluebird bio: Employment, Equity Ownership. Walters:Kiadis Pharma: Honoraria; Bayer HealthCare: Honoraria; Leerink Partners, LLC: Consultancy; ViaCord Processing Laboratory: Other: Medical Director ; AllCells, Inc./LeukoLab: Other: Medical Director ; bluebirdBio, Inc: Honoraria.

Kanter: Novartis: Consultancy. Walters:Bayer HealthCare: Honoraria; AllCells, Inc./LeukoLab: Other: Medical Director ; ViaCord Processing Laboratory: Other: Medical Director ; Leerink Partners, LLC: Consultancy; Kiadis Pharma: Honoraria; bluebirdBio, Inc: Honoraria. Kwiatkowski:Ionis pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Shire Pharmaceuticals: Consultancy; Sideris Pharmaceuticals: Consultancy; Apopharma: Research Funding; Luitpold Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees. von Kalle:bluebird bio: Consultancy; GeneWerk: Equity Ownership. Kuypers:Children's Hospital Oakland Research Institute: Employment; bluebird bio: Consultancy. Leboulch:bluebird bio: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Research Funding. Joseney-Antoine:bluebird bio: Employment, Equity Ownership. Asmal:bluebird bio: Employment, Equity Ownership. Thompson:bluebird bio: Consultancy, Research Funding; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Research Funding; Amgen: Research Funding; Baxalta (now part of Shire): Research Funding; ApoPharma: Consultancy, Membership on an entity's Board of Directors or advisory committees; Mast: Research Funding; Eli Lily: Research Funding.

DNA mediated gene transfer to hematopoietic cells has been hindered by low gene transfer efficiency. In an attempt to achieve efficient, stable integration of potentially large DNA vectors at a site which confers expression in a defined and a predictable manner, we are exploiting a Cre/lox based recombination strategy. As an initial test in hematopoietic cells, murine BaF3 cells were used to introduce a hygromycin resistant gene flanked by 34 bp lox sites (floxed) using a retrovirus. Subsequently, co-electroporation of a floxed green flourescent protein (GFP) construct and the cre recombinase plasmid should mediate integration through the lox sites replacing hygromycin with GFP. Site directed, stable integration as confirmed by FACS and Southern blot analysis occurred at a high frequency ranging from 1–7% in 5 different BaF3 clones, or some 100 fold higher than the efficiency for a control GFP vector lacking lox sites. To assess if the expression characteristics of an initial integration site can be reutilized to express a transgene in a defined and predictable manner, floxed GFP expressing BaF3 clones were chosen that had clonally distinct levels of GFP expression that spanned over 300 fold. This differential expression was maintained following cre mediated replacement with a floxed cDNA human Thy-1 construct. Analogous findings were also obtained in the human K562 cell line. As an initial effort to extend this approach to primitive primary cells, we introduced lox sites into murine embryonic stem cells (ES) cells either by a retrovirus or by a plasmid. Subsequent cre mediated integration again occurred with high efficiency—up to 22% and clones retained in vitro differentiation capacity and germline competency. Site directed DNA integration should provide a powerful new approach for studying gene function and regulatory mechanisms in hematopoietic cells.

Payen: bluebrid bio: Consultancy. Beuzard:bluebird bio Inc: Consultancy, Equity Ownership. von Kalle:bluebird bio, Inc.: Consultancy. Sandler:bluebird bio, Inc.: Employment, Equity Ownership. Soni:bluebird bio, Inc.: Employment, Equity Ownership. De Montalembert:Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Leboulch:bluebird bio: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties, Research Funding.