Correction of sickle cell disease in transgenic mouse models by gene therapy

Gene delivery by lentivirus vectors

The capacity to efficiently transduce nondividing cells, shuttle large genetic payloads, and maintain stable long-term transgene expression are attributes that have brought lentiviral vectors to the forefront of gene delivery vehicles for research and therapeutic applications in a clinical setting. Our discussion initiates with advances in lentiviral vector development and how these sophisticated lentiviral vectors reflect improvements in safety, regarding the prevention of replication competent lentiviruses (RCLs), vector mobilization, and insertional mutagenesis. Additionally, we describe conventional molecular regulatory systems to manage gene expression levels in a spatial and temporal fashion in the context of a lentiviral vector. State of the art technology for lentiviral vector production by transient transfection and packaging cell lines are explicitly presented with current practices used for concentration, purification, titering, and determining the safety of a vector stock. We summarize lentiviral vector applications that have received a great deal of attention in recent years including the generation of transgenic animals and the stable delivery of RNA interference molecules. Concluding remarks address some of the successes in preclinical animals, and the recent transition of lentiviral vectors to human clinical trials as therapy for a variety of infectious and genetic diseases.

HbS-Savaria: the anti-polymerization effect of a single mutation in human alpha-chains

Recombinant alpha-Savaria globin (alpha(S49R)) was assembled with beta(S) chains by the alloplex intermediate pathway to generate tetrameric rHbS-Sarvaria (alpha (2) (S49R) beta (2) (E6V) ) that exhibited normal O(2) affinity and co-operatively at pH 7.4. Allosteric effectors, 2,3-DPG, L35, and NaCl increased O(2) affinity by 15%. Bohr effects were similar for rHbS-Savaria and HbS (0.38 +/- 0.025 vs. 0.46 +/- 0.03, respectively). The C(SAT) of HbS increased from 16.7 +/- 0.8 to 27.0 +/- 1.0 g/dL. Co-polymerization demonstrated inhibition predominantly by the Cis-dimer. Molecular modeling indicated that the positive charge at alpha-49 generated a strong anion-binding site and reduced flexibility of the CD-region by restricting movement in the E and F helices. The molecular distance between Arg-49 and Asn-78 in the neighboring double strand decreased, and electrostatic repulsion between the inter-double strands increased, resulting in inhibition of polymerization. The Savaria mutation may be useful for the design of super-inhibitory alpha-chains and gene therapy of sickle cell anemia.

Locus control region elements HS1 and HS4 enhance the therapeutic efficacy of globin gene transfer in beta-thalassemic mice

Globin gene transfer in autologous hematopoietic stem cells is a promising therapeutic option for subjects with beta-thalassemia major. In this approach, high level, erythroid-specific globin transgene expression should correct ineffective erythropoiesis and hemolytic anemia following the delivery of only 1 to 2 vector copies per cell. The generation of vectors that provide high-level globin expression and require low vector copy (VC) integration is therefore essential for both safety and efficacy. We show here the major roles played by 2 lesser-known locus control region elements, termed HS1 and HS4. Partial deletions within HS4 markedly reduce in vivo globin expression requiring multiple VC per cell to correct the anemia. Most strikingly, addition of HS1 to HS2-3-4 increases globin expression by 52%, yielding 9 g Hb/VC in beta-thalassemic mice. Thus, while vectors encoding HS2-3-4 provide curative levels of hemoglobin at 1 to 2 copies per cell, adding HS1 is a promising alternative strategy if upcoming clinical trials prove higher levels of expression to be necessary.

Gene therapy for treatment of inherited haematological disorders

Gene therapy, a molecular medicine based on vector-mediated transfer of therapeutic genes, holds promise for a cure of monogenetic inherited diseases. In recent years, tremendous progress has been reported in the treatment of haematological disorders: clinical trials in severe combined immune deficiencies have been successful by using retroviral vectors to express target genes in haematopoietic stem cells, which after transplantation efficiently reconstituted the immune system concomitant with substantial improvement in the clinical status of patients. Conversely, unexpected adverse events were also encountered. In other work, progress towards clinical studies on ex vivo gene transfer for Fanconi anaemia and haemoglobinopathies has been made. Each approach features a unique treatment strategy and also faces various impediments to success. In the case of the X-linked bleeding disorder haemophilia, several Phase I/II clinical trials were conducted, including in vivo administration of viral vectors to skeletal muscle and liver. Adeno-associated viral gene transfer of coagulation Factor IX has been documented in human subjects, reaching therapeutic levels after infusion into a hepatic blood vessel. However, sustained expression of therapeutic levels (as shown in large animal models of haemophilia) has not yet been achieved in humans. In general, long-term follow-up will be important for assessment of the safety of all existing gene therapy strategies.

Sickle-cell haemoglobin polymerization: is it the primary pathogenic event of sickle-cell anaemia?

Sickle cell anaemia is associated with a mutant haemoglobin, HbS, which forms polymers in the red blood cells of patients. The primary role of the HbS polymerization for the pathophysiology has been questioned: observations in patients and model organisms contradict deterministic scenarios of sickling crises triggered by polymerization. However, results with knock-out sickle-cell mice, which were cured by delaying HbS polymerization, reconfirm polymerization's primary role. To reconcile the contradictory observations, this article reviews recent findings on two steps in polymerization: homogeneous nucleation of fibres, and their growth. The fibre growth is faster by far than for any other protein ordered structure. This is due to a negligible free-energy barrier for incorporation into a fibre, determined by an entropy gain, stemming from the release of water molecules structured around HbS. The kinetics of fibre nucleation have shown that the formation of the polymer nucleus is preceded by a metastable droplet of a dense liquid. The properties of the dense liquid are sensitive functions of solution composition, including components in micro- and nanomolar amounts. This mechanism allows low-concentration solution components to strongly affect the nucleation kinetics, accounting for the high variability of the disease. These insights can potentially be utilized for control of HbS polymerization and treatment of the disease.

Cell-specific and efficient expression in mouse and human B cells by a novel hybrid immunoglobulin promoter in a lentiviral vector

The expression of genes specifically in B cells is of great interest in both experimental immunology as well as in future clinical gene therapy. We have constructed a novel enhanced B cell-specific promoter (Igk-E) consisting of an immunoglobulin kappa (Igk) minimal promoter combined with an intronic enhancer sequence and a 3' enhancer sequence from Ig genes. The Igk-E promoter was cloned into a lentiviral vector and used to control expression of enhanced green fluorescent protein (eGFP). Transduction of murine B-cell lymphoma cell lines and activated primary splenic B cells, with IgK-E-eGFP lentivirus, resulted in expression of eGFP, as analysed by flow cytometry, whereas expression in non-B cells was absent. The specificity of the promoter was further examined by transducing Lin(-) bone marrow with Igk-E-eGFP lentivirus and reconstituting lethally irradiated mice. After 16 weeks flow cytometry of lymphoid tissues revealed eGFP expression by CD19+ cells, but not by CD3+, CD11b+, CD11c+ or Gr-1+ cells. CD19+ cells were comprised of both marginal zone B cells and recirculating follicular B cells. Activated human peripheral mononuclear cells were also transduced with Igk-E-eGFP lentivirus under conditions of selective B-cell activation. The Igk-E promoter was able to drive expression of eGFP only in CD19+ cells, while eGFP was expressed by both spleen focus-forming virus and cytomegalovirus constitutive promoters in CD19+ and CD3+ lymphocytes. These data demonstrate that in these conditions the Igk-E promoter is cell specific and controls efficient expression of a reporter protein in mouse and human B cells in the context of a lentiviral vector.

Modification of globin gene expression by RNA targeting strategies

OBJECTIVE

Sickle cell anemia is a genetic blood disease resulting from production of mutant beta-globin (beta(S)) and has severe clinical consequences. It is known that a higher cellular gamma-globin level, e.g., higher ratio of cellular gamma-globin to beta(S)-globin (gamma/beta(S) ratio), inhibits sickle hemoglobin (HbS) polymerization tendency. Hence, therapeutic treatment of sickle cell anemia has been focused on introducing gamma-globin gene into red blood cells to increase the cellular gamma/beta(S) ratio. Here, we have introduced ribozymes and small interfering RNAs (siRNAs) against beta(S)-globin mRNA into blood cells as a means to increase the gamma/beta(S) ratio.

MATERIALS AND METHODS

Single and multiribozymes against beta(S)-globin mRNA have been tested in vitro and in human erythroleukemia K562beta(S) cells that stably express exogenous beta(S)-globin gene. Primary human hematopoietic progenitor cells were also transfected with multiribozyme and the gamma/(gamma + beta) ratio determined and compared with cells transfected with long hairpin beta-globin cDNA and synthetic siRNA genes.

RESULTS

We have found that the multiribozyme zb21A containing two ribozyme units effectively reduces beta(S)-globin mRNA both in vitro and in K562beta(S) cells. The gamma-globin mRNA to beta(S)-globin mRNA ratio in the multiribozyme transfected cells is about a factor of 2 more than that in the control cells. We have also found that the gamma/(gamma + beta) ratio in the transfected hematopoietic progenitor cells is increased by more than twofold in cells treated with multiribozyme zb21A or siRNA ib5.

CONCLUSION

Our results suggest that introducing multiribozymes or siRNAs into red blood cells is comparable in their effectiveness to increase the ratio of cellular gamma-globin mRNA to beta- or beta(S)-globin mRNA, providing possible strategies to increase the effectiveness of gamma-globin gene transfer as gene therapy for treatment of patients with sickle cell anemia.

Long-term results of related myeloablative stem-cell transplantation to cure sickle cell disease

Allogeneic hematopoietic stem-cell transplantation (HSCT) is the only curative treatment for sickle cell disease (SCD); nevertheless, its use has been limited by the risk of transplantation-related mortality (TRM). Between November 1988 and December 2004, 87 consecutive patients with severe SCD ranging from 2 to 22 years of age received transplants in France. Cerebral vasculopathy was the principal indication for transplantation (55 patients). All the patients received grafts from a sibling donor after a myeloablative conditioning regimen (CR). The only change in the CR during the study period was the introduction of antithymocyte globulin (ATG) in March 1992. The rejection rate was 22.6% before the use of ATG but 3% thereafter. With a median follow-up of 6 years (range, 2.0 to 17.9 years), the overall and event-free survival (EFS) rates were 93.1% and 86.1%, respectively. Graft versus host disease (GVHD) was the main cause of TRM. Importantly, cord blood transplant recipients did not develop GVHD. No new ischemic lesions were detected after engraftment, and cerebral velocities were significantly reduced. The outcome improved significantly with time: the EFS rate among the 44 patients receiving transplants after January 2000 was 95.3%. These results indicate that HLA-identical sibling HSCT after myeloablative conditioning with ATG should be considered as a standard of care for SCD children who are at high risk for stroke.

Sickle cell disease: old discoveries, new concepts, and future promise

The discovery of the molecular basis of sickle cell disease was an important landmark in molecular medicine. The modern tools of molecular and cellular biology have refined our understanding of its pathophysiology and facilitated the development of new therapies. In this review, we discuss some of the important advances in this field and the impediments that limit the impact of these advances.

Erythroid-specific expression of beta-globin by the sleeping beauty transposon for Sickle cell disease

Sickle cell disease (SCD) results predominately from a single monogenic mutation that affects thousands of individuals worldwide. Gene therapy approaches have focused on using viral vectors to transfer wild-type beta- or gamma-globin transgenes into hematopoietic stem cells for long-term expression of the recombinant globins. In this study, we investigated the use of a novel nonviral vector system, the Sleeping Beauty (SB) transposon (Tn) to insert a wild-type beta-globin expression cassette into the human genome for sustained expression of beta-globin. We initially constructed a beta-globin expression vector composed of the hybrid cytomegalovirus (CMV) enhancer chicken beta-actin promoter (CAGGS) and full-length beta-globin cDNA, as well as truncated forms lacking either the 3' or 3' and 5' untranslated regions (UTRs), to optimize expression of beta-globin. Beta-globin with its 5' UTR was efficiently expressed from its cDNA in K-562 cells induced with hemin. However, expression was constitutive and not erythroid-specific. We then constructed cis SB-Tn-beta-globin plasmids using a minimal beta-globin gene driven by hybrid promoter IHK (human ALAS2 intron 8 erythroid-specific enhancer, HS40 core element from human alphaLCR, ankyrin-1 promoter), IHbetap (human ALAS2 intron 8 erythroid-specific enhancer, HS40 core element from human alphaLCR, beta-globin promoter), or HS3betap (HS3 core element from human betaLCR, beta-globin promoter) to establish erythroid-specific expression of beta-globin. Stable genomic insertion of the minimal gene and expression of the beta-globin transgene for >5 months at a level comparable to that of the endogenous gamma-globin gene were achieved using a SB-Tn beta-globin cis construct. Interestingly, erythroid-specific expression of beta-globin driven by IHK was regulated primarily at the translational level, in contrast to post-transcriptional regulation in non-erythroid cells. The SB-Tn system is a promising nonviral vector for efficient genomic insertion conferring stable, persistent erythroid-specific expression of beta-globin.

Fetal gene therapy of alpha-thalassemia in a mouse model

Fetuses with homozygous alpha-thalassemia usually die at the third trimester of pregnancy or soon after birth. Hence, the disease could potentially be a target for fetal gene therapy. We have previously established a mouse model of alpha-thalassemia. These mice mimic the human alpha-thalassemic conditions and can be used as preclinical models for fetal gene therapy. We tested a lentiviral vector containing the HS 2, 3, and 4 of the beta-LCR, a central polypurine tract element, and the beta-globin gene promoter directing either the EGFP or the human alpha-globin gene. We showed that the GFP expression was erythroid-specific and detected in BFU-E colonies and the erythroid progenies of CFU-GEMM. For in utero gene delivery, we did yolk sac vessel injection at midgestation of mouse embryos. The recipient mice were analyzed after birth for human alpha-globin gene expression. In the newborn, human alpha-globin gene expression was detected in the liver, spleen, and peripheral blood. The human alpha-globin gene expression was at the peak at 3-4 months, when it reached 20% in some recipients. However, the expression declined at 7 months. Colony-forming assays in these mice showed low abundance of the transduced human alpha-globin gene in their BFU-E and CFU-GEMM and the lack of its transcript. Thus, lentiviral vectors can be an effective vehicle for delivering the human alpha-globin gene into erythroid cells in utero, but, in the mouse model, delivery at late midgestation could not transduce hematopoietic stem cells adequately to sustain gene expression.

Pair-wise interactions of polymerization inhibitory contact site mutations of hemoglobin-S

The linkage of pair-wise interactions of contact site mutations of HbS has been studied using Le Lamentin [His-20 (alpha)-->Gln], Hoshida [Glu-43 (beta)-->Gln] and alpha(2)beta (2) (T87Q) mutations as the prototype of three distinct classes of contact sites of deoxy HbS fiber. Binary mixture experiments established that beta(A)-chain with the Thr-87 (beta)-->Gln mutation is as potent as the gamma-chain of HbF (alpha(2)gamma(2)) in inhibiting polymerization. On combining the influence of Le Lamentin mutation with that of beta (2) (T87Q) mutations; the net influence is only partial additivity. On the other hand, in binary mixture studies, combined influence of Hoshida mutation with that of beta (2) (T87Q) mutations is synergistic. Besides, a significant level of synergistic complementation is also seen when the Le Lamentin and Hoshida mutations are combined in HbS (symmetrical tetramers). Le Lamentin and Hoshida mutation introduced into the cis-dimer of the asymmetric hybrid tetramer completely neutralizes the Val-6 (beta) dependent polymerization. Accordingly, we propose that combining the perturbation of intra-double strand contact site with that of an inter-double strand contact site exhibit synergy when they are present in two different chains of the alphabeta dimer. A comparison of the present results with that of the earlier studies suggest that when the two contact site perturbations are from the same sub-unit of the alphabeta dimer only partial additivity is observed. The map of interaction linkage of the contact site mutations exposes new strategies in the design of novel anti-sickling Hbs for the gene therapy of sickle cell disease.

Two-step mechanism of homogeneous nucleation of sickle cell hemoglobin polymers

Sickle cell anemia is a debilitating genetic disease that affects hundreds of thousands of babies born each year worldwide. Its primary pathogenic event is the polymerization of a mutant, sickle cell, hemoglobin (HbS); and this is one of a line of diseases (Alzheimer's, Huntington's, prion, etc.) in which nucleation initiates pathophysiology. We show that the homogeneous nucleation of HbS polymers follows a two-step mechanism with metastable dense liquid clusters serving as precursor to the ordered nuclei of the HbS polymer. The evidence comes from data on the rates of fiber nucleation and growth and nucleation delay times, the interaction of fibers with polarized light, and mesoscopic metastable HbS clusters in solution. The presence of a precursor in the HbS nucleation mechanism potentially allows low-concentration solution components to strongly affect the nucleation kinetics. The variations of these concentrations in patients might account for the high variability of the disease in genetically identical patients. In addition, these components can potentially be utilized for control of HbS polymerization and treatment of the disease.

Site-specific transfer of an intact β-globin gene cluster through a new targeting vector

The ideal gene-therapy vector for treating genetic disorders should deliver intact therapeutic genes and their essential regulatory elements into the specific "safe genomic site" and realize long-term, self-regulatory expression. For beta-thalassemia gene therapy, viral vectors have been broadly used, but the accompanying insertional mutation and immunogenicity remain problematic. Hence, we aimed to develop new non-viral vectors that are efficient and safe in treating diseases. As previous studies have demonstrated that physiological expression of beta-globin genes requires both a 5' locus control region and 3' specific elements, we constructed a new human chromosome-derived targeting vector to transfer the intact beta-globin gene cluster into K562 cells. The whole beta-globin gene cluster was precisely integrated into the target site and expressed in a self-regulatory pattern. The results proved that the human chromosome-derived vector was specifically targeted to the human genome and this could provide a novel platform for further gene therapy research.

Glycoprotein Ibalpha promoter drives megakaryocytic lineage-restricted expression after hematopoietic stem cell transduction using a self-inactivating lentiviral vector

Megakaryocytic (MK) lineage is an attractive target for cell/gene therapy approaches, aiming at correcting platelet protein deficiencies. However, MK cells are short-lived cells, and their permanent modification requires modification of hematopoietic stem cells with an integrative vector such as a lentiviral vector. Glycoprotein (Gp) IIb promoter, the most studied among the MK regulatory sequences, is also active in stem cells. To strictly limit transgene expression to the MK lineage after transduction of human CD34(+) hematopoietic cells with a lentiviral vector, we looked for a promoter activated later during MK differentiation. Human cord blood, bone marrow, and peripheral-blood mobilized CD34(+) cells were transduced with a human immunodeficiency virus-derived self-inactivating lentiviral vector encoding the green fluorescent protein (GFP) under the transcriptional control of GpIbalpha, GpIIb, or EF1alpha gene regulatory sequences. Both GpIbalpha and GpIIb promoters restricted GFP expression (analyzed by flow cytometry and immunoelectron microscopy) in MK cells among the maturing progeny of transduced cells. However, only the GpIbalpha promoter was strictly MK-specific, whereas GpIIb promoter was leaky in immature progenitor cells not yet engaged in MK cell lineage differentiation. We thus demonstrate the pertinence of using a 328-base-pair fragment of the human GpIbalpha gene regulatory sequence, in the context of a lentiviral vector, to tightly restrict transgene expression to the MK lineage after transduction of human CD34(+) hematopoietic cells. Disclosure of potential conflicts of interest is found at the end of this article.

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.

Chemoprotection effect of multidrug resistance 1 (MDR1) gene transfer to hematopoietic progenitor cells and engrafted in mice with cancer allows intensified chemotherapy

Increasing the proportion of bone marrow cells expression human multidrug resistance (MDR) 1 gene to prevent or circumvent bone morrow toxicity from chemotherapy agent is a high priority of dose intensification protocols. In this study, we have used a BALB/c mouse tumor-bearing model to investigate the chemoprotection effect of MDR1 gene by transfecting retroviral vectors containing and expressing the MDR gene in vivo. Hematopoietic progenitor cells served as a target of MDR1 gene transfer by the mediation of retrovirus vector and engrafted into the BALB/c mice with 60Co-gamma ray exposure in advance. Doxorubicin (5, 10, and 20 mg/kg) suppressed tumor growth of the xenograft significantly in dose-dependence mode if supported by suitable peripheral WBC. WBCs count revealed that the mice that had received gene-transduced cells showed a significant increase in WBCs count compared with their gene-transduced-naive counterparts. The function and expression of MDR1 gene were detected by flow cytometry, RT-PCR and immunohistochemistry (IC) method. MDRl mRNA expression could be detected in BM. Spleens contained measurable amounts of MDRl mRNA. Tail vein blood and tumor tissue detected MDRl DNA but no MDRl mRNA expression. FACS analysis of infected BM cells obtained 6 weeks later showed high levels of P-gp function. Based on these results we conclude that cytostatic drug resistance gene therapy may provide some degree of chemoprotection so can increase the chemotherapy dose to kill tumor cells.

The Kinetics of Nucleation and Growth of Sickle Cell Hemoglobin Fibers

Polymerization of sickle cell hemoglobin (HbS) in deoxy state is one of the basic events in the pathophysiology of sickle cell anemia. For insight into the polymerization process, we monitor the kinetics of nucleation and growth of the HbS polymer fibers. We define a technique for the determination of the rates J and delay times theta of nucleation and the fiber growth rates R of deoxy-HbS fibers, based on photolysis of CO-HbS by laser illumination. We solve numerically time-dependent equations of heat conductance and CO transport, coupled with respective photo-chemical processes, during kinetics experiments under continuous illumination. After calibration with experimentally determined values, we define a regime of illumination ensuring uniform temperature and deoxy-HbS concentration, and fast (within <1 s) egress to steady conditions. With these procedures, data on the nucleation and growth kinetics have relative errors of <5% and are reproducible within 10% in independent experiments. The nucleation rates and delay times have steep, exponential dependencies on temperature. In contrast, the average fiber growth rates only weakly depend on temperature. The individual growth rates vary by up to 40% under identical conditions. These variations are attributed to instability of the coupled kinetics and diffusion towards the growing end of a fiber. The activation energy for incorporation of HbS molecules into a polymer is E(A)=50 kJ mol(-1), a low value indicating the significance of the hydrophobic contacts in the HbS polymer. More importantly, the contrast between the strong theta(T) and weak R(T) dependencies suggests that the homogenous nucleation of HbS polymers occurs within clusters of a precursor phase. This conclusion may have significant consequences for the understanding of the pathophysiology of sickle cell anemia and should be tested in further work.

Recent advances in globin gene transfer for the treatment of beta-thalassemia and sickle cell anemia

PURPOSE OF REVIEW

The beta-thalassemias and sickle cell anemia are severe congenital anemias for which there is presently no curative therapy other than allogeneic hematopoietic stem cell transplantation. This therapeutic option, however, is not available to most patients due to the lack of an HLA-matched bone marrow donor. The transfer of a regulated globin gene in autologous hematopoietic stem cells is therefore a highly attractive alternative treatment. This strategy, simple in principle, raises major challenges in terms of controlling transgene expression, which ideally should be erythroid specific, differentiation and stage restricted, elevated, position independent, and sustained over time.

RECENT FINDINGS

Using lentiviral vectors, May et al. demonstrated that an optimized combination of proximal and distal transcriptional control elements permits lineage-specific and elevated beta-globin expression in vivo, resulting in therapeutic hemoglobin production and correction of anemia in beta-thalassemic mice. Several groups have extended these findings to various models of beta-thalassemia and sickle cell disease. While the addition of the wild-type beta-globin gene is naturally suited for treating beta-thalassemia, several alternatives have been proposed for the treatment of sickle cell disease, using either gamma or mutant beta-globin gene addition, trans-splicing or RNA interference.

SUMMARY

These recent advances bode well for the clinical investigation of stem cell-based gene therapy in the severe hemoglobinopathies.

HIF-dependent induction of adenosine A2B receptor in hypoxia

Adenosine has been widely associated with hypoxia of many origins, including those associated with inflammation and tumorogenesis. A number of recent studies have implicated metabolic control of adenosine generation at sites of tissue hypoxia. Here, we examine adenosine receptor control and amplification of signaling through transcriptional regulation of endothelial and epithelial adenosine receptors. Initial studies confirmed previous findings indicating selective induction of human adenosine A2B receptor (A2BR) by hypoxia. Analysis of the cloned human A2BR promoter identified a functional hypoxia-responsive region, including a functional binding site for hypoxia-inducible factor (HIF) within the A2BR promoter. Further studies examining HIF-1alpha DNA binding and HIF-1alpha gain and loss of function confirmed strong dependence of A2BR induction by HIF-1alpha in vitro and in vivo mouse models. Additional studies in endothelia overexpressing full-length A2BR revealed functional phenotypes of increased barrier function and enhanced angiogenesis. Taken together, these results demonstrate transcriptional coordination of A2BR by HIF-1alpha and amplified adenosine signaling during hypoxia. These findings may provide an important link between hypoxia and metabolic conditions associated with inflammation and angiogenesis.

Current status of gene delivery and gene therapy in lacrimal gland using viral vectors

Gene delivery is one of the biggest challenges in the field of gene therapy. It involves the efficient transfer of transgenes into somatic cells for therapeutic purposes. A few major drawbacks in gene delivery include inefficient gene transfer and lack of sustained transgene expression. However, the classical method of using viral vectors for gene transfer has circumvented some of these issues. Several kinds of viruses, including retrovirus, adenovirus, adeno-associated virus, and herpes simplex virus, have been manipulated for use in gene transfer and gene therapy applications. The transfer of genetic material into lacrimal epithelial cells and tissues, both in vitro and in vivo, has been critical for the study of tear secretory mechanisms and autoimmunity of the lacrimal gland. These studies will help in the development of therapeutic interventions for autoimmune disorders such as Sjögren's syndrome and dry eye syndromes which are associated with lacrimal dysfunction. These studies are also critical for future endeavors which utilize the lacrimal gland as a reservoir for the production of therapeutic factors which can be released in tears, providing treatment for diseases of the cornea and posterior segment. This review will discuss the developments related to gene delivery and gene therapy in the lacrimal gland using several viral vector systems.

Development of a synthetic promoter for macrophage gene therapy

Macrophages have the potential to deliver therapeutic genes to many target tissues. Macrophage-specific synthetic promoters (SPs) generated by random ligation of myeloid/macrophage cis elements had activity up to 100-fold that of a native macrophage promoter in macrophage cell lines, but were minimally active in nonmyeloid cells. Mouse bone marrow cells (BMCs) transduced ex vivo with lentivectors expressing green fluorescent protein (GFP) driven either by an SP (SP-GFP) or a cytomegalovirus (CMV) promoter (CMV-GFP) were used for syngeneic transplantation of lethally irradiated mice. Blood leukocytes showed stable GFP expression for up to 15 months after transplantation. SP-GFP expression was selective for CD11b+ macrophages, whereas CMV-GFP expression was observed in erythrocytes, as well as in both CD11b+ and CD11b- leukocytes. Furthermore, SP-GFP expression was much stronger than CMV-GFP expression in CD11b+ macrophages. apoE-/- BMCs transduced with the lentiviral vector encoding human apoE were used to transplant apoE-/- mice. Macrophage expression of apoE from 10 to 26 weeks of age significantly reduced atherosclerotic lesions in recipient apoE-/- mice. Thus, the novel SPs, especially when combined with lentivectors, are useful for macrophage-specific delivery of therapeutic genes.

Stable RNA interference: comparison of U6 and H1 promoters in endothelial cells and in mouse brain

BACKGROUND

RNA interference (RNAi) is a post-transcriptional RNA degradation process, which has become a very useful tool in gene function studies and gene therapy applications. Long-term cellular expression of small interfering RNA (siRNA) molecules required for many gene therapy applications can be achieved by lentiviral vectors (LVs). The two most commonly used promoters to drive the short hairpin RNA (shRNA) expression are the human U6 small nuclear promoter (U6) and the human H1 promoter (H1).

METHODS

We investigated whether there is any significant difference between the efficiencies of U6 and H1 in LV-mediated RNAi using green fluorescent protein (GFP) as a target gene by flow cytometry and real-time reverse-transcription polymerase chain reaction (RT-PCR) in endothelial cells. Also, we compared the efficiencies of U6 and H1 in the GFP transgenic mouse brain after stereotactic LV injection.

RESULTS

We show that the U6 promoter is more efficient than H1 in GFP silencing in vitro, leading to 80% GFP knockdown at an average of one integrated vector genome per target cell genome. The silencing is persistent for several months. In addition, the U6 promoter is superior to H1 in vivo and leads to stable GFP knockdown in mouse brain for at least 9 months.

CONCLUSIONS

These results show that LV-mediated RNAi is a powerful gene-silencing method for the long-term inhibition of gene expression in vitro and in vivo.

Investigational agents for sickle cell disease

Developments in the treatment of sickle cell disease (SCD) have not kept pace with advances in understanding the pathophysiology of this haemoglobinopathy. Drugs undergoing preclinical and clinical assessment for the therapy of these globin gene disorders are discussed in this article. Beginning with investigational agents for treatment of SCD as a whole, the discussion proceeds to drugs being developed for specific manifestations or iatrogenic complications. Despite being licensed in the USA, the prototype antisickling agent, hydroxycarbamide, has not attained worldwide clinical use because of concerns about long-term toxicity. The less toxic decitabine, which (as with hydroxycarbamide) increases fetal haemoglobin level, cannot be administered orally; therefore, the search continues for effective and safe antisickling drugs that can be taken orally. The naturally occurring benzaldehyde 5-hydroxymethyl-2-furfural has shown promising antisickling properties in vitro, and when administered to transgenic sickle mice. These effects are surpassed by the new synthetic pyridyl derivatives of benzaldehyde. Studies in humans with SCD are required to assess the clinical efficacy of these benzaldehydes. Niprisan, another antisickling agent with significant clinical efficacy and an attractive safety profile, is undergoing further development. The prospects of antiadhesion therapy in SCD are demonstrated by a recombinant protein containing the Fc fragment of IgG fused to the natural ligand for selectins: the conjugate significantly inhibited blood vessel occlusion in transgenic sickle mice. Whereas the orally administrable iron-chelating agent deferasirox is likely to increasingly take the place of desferioxamine (which can only be given parenterally), effective treatment of priapism in SCD remains a distressing challenge.

The Hb A variant (beta73 Asp-->Leu) disrupts Hb S polymerization by a novel mechanism

Polymerization of a 1:1 mixture of hemoglobin S (Hb S) and the artificial mutant HbAbeta73Leu produces a dramatic morphological change in the polymer domains in 1.0 M phosphate buffer that are a characteristic feature of polymer formation. Instead of feathery domains with quasi 2-fold symmetry that characterize polymerization of Hb S and all previously known mixtures such as Hb A/S and Hb F/S mixtures, these domains are compact structures of quasi-spherical symmetry. Solubility of Hb S/Abeta73Leu mixtures was similar to that of Hb S/F mixtures. Kinetics of polymerization indicated that homogeneous nucleation rates of Hb S/Abeta73Leu mixtures were the same as those of Hb S/F mixtures, while exponential polymer growth (B) of Hb S/Abeta73Leu mixtures were about three times slower than those of Hb S/F mixtures. Differential interference contrast (DIC) image analysis also showed that fibers in the mixture appear to elongate between three and five times more slowly than in equivalent Hb S/F mixtures by direct measurements of exponential growth of mass of polymer in a domain. We propose that these results of Hb S/Abeta73Leu mixtures arise from a non-productive binding of the hybrid species of this mixture to the end of the growing polymer. This "cap" prohibits growth of polymers, but by nature is temporary, so that the net effect is a lowered growth rate of polymers. Such a cap is consistent with known features of the structure of the Hb S polymer. Domains would be more spherulitic because slower growth provides more opportunity for fiber bending to spread domains from their initial 2-fold symmetry. Moreover, since monomer depletion proceeds more slowly in this mixture, more homogeneous nucleation events occur, and the resulting gel has a far more granular character than normally seen in mixtures of non-polymerizing hemoglobins with Hb S. This mixture is likely to be less stiff than polymerized mixtures of other hybrids such as Hb S with HbF, potentially providing a novel approach to therapy.

Successful correction of the human Cooley's anemia beta-thalassemia major phenotype using a lentiviral vector flanked by the chicken hypersensitive site 4 chromatin insulator

beta-Thalassemias are the most common single-gene disorders and are potentially amenable to gene therapy. While retroviral vectors carrying the human beta-globin cassette were notoriously unstable and expressed poorly, considerable progress has now been made using lentiviral vectors (LVs), which stably transmit the beta-globin expression cassette. Mouse studies using LVs have shown correction of the beta-thalassemia-intermedia phenotype and a partial, variable correction of the mouse beta-thalassemia major phenotype, despite the use of beta-globin-hypersensitive sites that are known to result in position-independent effects. Our group used the alpha-globin-hypersensitive site in self-inactivating (SIN) LVs with long-term expression in secondary mice that resisted methylation-associated proviral silencing. However, these vectors also suffered from chromatin position effects. We therefore flanked a SIN-lentiviral vector carrying the human beta-globin expression cassette with a chromatin insulator and studied expression in bone marrow from four patients with transfusion-dependent human thalassemia major. We demonstrated normal levels of human beta-globin expression in erythroid cells produced in in vitro cultures for unilineage erythroid differentiation. There was restoration of effective erythropoiesis and reversal of the abnormally elevated apoptosis that characterizes beta-thalassemia. The gene-corrected human beta-thalassemia progenitor cells were transplanted into immune-deficient mice, where they underwent normal erythroid differentiation, expressed normal levels of human beta-globin, and displayed normal effective erythropoiesis 3-4 months after xenotransplantation. Variability of beta-globin expression in erythroid colonies derived in vitro or from xenograft bone marrow was similar to that seen in normal control subjects. Results show genetic correction of primitive human progenitor cells and normalization of the human thalassemia major phenotype.

Effects of human gamma-globin in murine beta-thalassaemia

Murine models of beta-thalassaemia have been used to test therapeutic globin gene vectors. However, the level of gamma-globin expression necessary to achieve full phenotypic correction in these models is unclear. In order to address this issue, we carried out breeding and transplantation studies in murine models of beta-thalassaemia intermedia (Hbb(th-3)/+) and severe beta-thalassaemia major (Hbb(th-3)/Hbb(th-3)) using transgenic lines expressing various levels of human gamma-globin. Expression of gamma-globin RNA at a modest 7-14% of total alpha-globin RNA resulted in the selective survival of HbF(+) erythrocytes, a fivefold increase in total HbF, and a phenotypic improvement in the beta-thalassaemia intermedia model. Full normalisation of erythrocyte indices in this model required gamma-globin RNA expression at 27% of alpha-globin, resulting in an average 40% (6.8 g/dl) HbF. Studies using the homozygous Hbb(th-3) model of lethal beta-thalassaemia major demonstrated that even this high level of gamma-globin expression, for reasons related to the function of the hybrid globin tetramers, could only prolong, but not fully support, survival. Taken together, these results indicate that only the heterozygous Hbb(th-3) model of beta-thalassaemia intermedia can be reliably used for the pre-clinical assessment of gamma-globin gene therapy vectors, as well as other means of gamma-globin gene induction.

Partial correction of murine beta-thalassemia with a gammaretrovirus vector for human gamma-globin

Several studies have demonstrated that recombinant lentivirus vectors containing extended globin gene expression cassettes and regulatory elements can ameliorate the pathogenic sequela in murine models of beta-thalassemia and sickle cell disease. Similarly promising results have not yet been obtained with recombinant gammaretrovirus vectors. Of these two vector classes, only gammaretroviruses have been tested extensively in clinical trials, with a proven ability to transduce long-term reconstituting hematopoietic stem cells with an exceedingly low incidence of serious side effects. Toward the continuing goal of developing retrovirus vectors for the treatment of the beta-chain hemoglobinopathies, we report here the assessment of a recombinant gammaretrovirus vector for human gamma-globin in murine models of beta-thalassemia. In the beta-thalassemia intermedia Hbbth-3/+ model, we observed a dose-dependent but transient increase in total hemoglobin and red blood cells, with a 2.5 +/- 0.2 g/dL increase in hemoglobin for transduction rates > or = 33%. In the severe beta-thalassemia major Hbbth-3/Hbbth-3 model, we observed a modest but statistically significant increase in survival, from a median of 15 days to 30 days (P = 0.001). These studies provide the first evidence that globin gene transfer vectors based on recombinant gammaretroviruses may provide a viable option for the treatment of the beta-chain hemoglobinopathies.

Retrovirus Vectors: Toward the Plentivirus?

Recombinant retroviral vectors based upon simple gammaretroviruses, complex lentiviruses, or potentially nonpathogenic spumaviruses represent relatively well characterized tools that are widely used for stable gene transfer. Different members of the Retroviridae family have developed distinct and potentially useful features related to their life cycle. These natural differences can be exploited for specialized applications in gene therapy and could conceivably be combined to create future retroviral hybrid vectors, ideally incorporating the following features: an efficient, noncytopathic packaging system with low likelihood of recombination; serum resistance; an ability to pseudotype with cell-specific envelopes; high-fidelity reverse transcription before cell entry; unrestricted cytoplasmic transport and nuclear import; an insulated expression cassette; specific chromosomal targeting; and physiologic or regulated levels of transgene expression. We envisage that, compared to contemporary vectors, a hybrid vector combining these properties would have increased therapeutic efficacy and an enhanced biosafety profile. Many of the above goals will require the inclusion of nonretroviral components into vector particles or transgenes.

Hematopoietic stem cell gene transfer in a tumor-prone mouse model uncovers low genotoxicity of lentiviral vector integration

Insertional mutagenesis represents a major hurdle to gene therapy and necessitates sensitive preclinical genotoxicity assays. Cdkn2a-/- mice are susceptible to a broad range of cancer-triggering genetic lesions. We exploited hematopoietic stem cells from these tumor-prone mice to assess the oncogenicity of prototypical retroviral and lentiviral vectors. We transduced hematopoietic stem cells in matched clinically relevant conditions, and compared integration site selection and tumor development in transplanted mice. Retroviral vectors triggered dose-dependent acceleration of tumor onset contingent on long terminal repeat activity. Insertions at oncogenes and cell-cycle genes were enriched in early-onset tumors, indicating cooperation in tumorigenesis. In contrast, tumorigenesis was unaffected by lentiviral vectors and did not enrich for specific integrants, despite the higher integration load and robust expression of lentiviral vectors in all hematopoietic lineages. Our results validate a much-needed platform to assess vector safety and provide direct evidence that prototypical lentiviral vectors have low oncogenic potential, highlighting a major rationale for application to gene therapy.

Evaluation of primitive murine hematopoietic stem and progenitor cell transduction in vitro and in vivo by recombinant adeno-associated virus vector serotypes 1 through 5

Conflicting data exist on hematopoietic cell transduction by AAV serotype 2 (AAV2) vectors, and additional AAV serotype vectors have not been evaluated for their efficacy in hematopoietic stem/progenitor cell transduction. We evaluated the efficacy of conventional, single-stranded AAV serotype vectors 1 through 5 in primitive murine hematopoietic stem/progenitor cells in vitro as well as in vivo. In progenitor cell assays using Sca1+ c-kit+ Lin- hematopoietic cells, 9% of the colonies in cultures infected with AAV1 expressed the transgene. Coinfection of AAV1 with self-complementary AAV vectors carrying the gene for T cell protein tyrosine phosphatase (scAAV-TC-PTP) increased the transduction efficiency to 24%, indicating that viral secondstrand DNA synthesis is a rate-limiting step. This was further corroborated by the use of scAAV vectors, which bypass this requirement. In bone marrow transplantation studies involving lethally irradiated syngeneic mice, Sca1+ c-kit+ Lin- cells coinfected with AAV1 +/- scAAV-TC-PTP vectors led to transgene expression in 2 and 7.5% of peripheral blood (PB) cells, respectively, 6 months posttransplantation. In secondary transplantation experiments, 7% of PB cells and 3% of bone marrow (BM) cells expressed the transgene 6 months posttransplantation. Approximately 21% of BM-derived colonies harbored the proviral DNA sequences in integrated forms. These results document that AAV1 is thus far the most efficient vector in transducing primitive murine hematopoietic stem/progenitor cells. Further studies involving scAAV genomes and hematopoietic cell-specific promoters should further augment the transduction efficiency of AAV1 vectors, which should have implications in the optimal use of these vectors in hematopoietic stem cell gene therapy.

Correction of sickle cell disease by homologous recombination in embryonic stem cells

Previous studies have demonstrated that sickle cell disease (SCD) can be corrected in mouse models by transduction of hematopoietic stem cells with lentiviral vectors containing antisickling globin genes followed by transplantation of these cells into syngeneic recipients. Although self-inactivating (SIN) lentiviral vectors with or without insulator elements should provide a safe and effective treatment in humans, some concerns about insertional mutagenesis persist. An ideal correction would involve replacement of the sickle globin gene (beta(S)) with a normal copy of the gene (beta(A)). We recently derived embryonic stem (ES) cells from a novel knock-in mouse model of SCD and tested a protocol for correcting the sickle mutation by homologous recombination. In this paper, we demonstrate the replacement of the human beta(S)-globin gene with a human beta(A)-globin gene and the derivation of mice from these cells. The animals produce high levels of normal human hemoglobin (HbA) and the pathology associated with SCD is corrected. Hematologic values are restored to normal levels and organ pathology is ameliorated. These experiments provide a foundation for similar studies in human ES cells derived from sickle cell patients. Although efficient methods for production of human ES cells by somatic nuclear transfer must be developed, the data in this paper demonstrate that sickle cell disease can be corrected without the risk of insertional mutagenesis.

Determinants of specific RNA interference-mediated silencing of human beta-globin alleles differing by a single nucleotide polymorphism

A single nucleotide polymorphism (SNP) in the sickle beta-globin gene (beta(S)) leads to sickle cell anemia. Sickling increases sharply with deoxy sickle Hb concentration and decreases with increasing fetal gamma-globin concentration. Measures that decrease sickle Hb concentration should have an antisickling effect. RNA interference (RNAi) uses small interfering (si)RNAs for sequence-specific gene silencing. A beta(S) siRNA with position 10 of the guide strand designed to align with the targeted beta(S) SNP specifically silences beta(S) gene expression without affecting the expression of the gamma-globin or normal beta-globin (beta(A)) genes. Silencing is increased by altering the 5' end of the siRNA antisense (guide) strand to enhance its binding to the RNA-induced silencing complex (RISC). Specific beta(S) silencing was demonstrated by using a luciferase reporter and full-length beta(S) cDNA transfected into HeLa cells and mouse erythroleukemia cells, where it was expressed in the context of the endogenous beta-globin gene promoter and the locus control region enhancers. When this strategy was used to target beta(E), silencing was not limited to the mutant gene but also targeted the normal beta(A) gene. siRNAs, mismatched with their target at position 10, guided mRNA cleavage in all cases except when two bulky purines were aligned. The specific silencing of the beta(S)-globin gene, as compared with beta(E), as well as studies of silencing SNP mutants in other diseases, indicates that siRNAs developed to target a disease-causing SNP will be specific if the mutant residue is a pyrimidine and the normal residue is a purine.

Mutagenesis and oncogenesis by chromosomal insertion of gene transfer vectors

Increasing evidence reveals that random insertion of gene transfer vectors into the genome of repopulating hematopoietic cells may alter their fate in vivo. Although most insertional mutations are expected to have few if any consequences for cellular survival, clonal dominance caused by retroviral vector insertions in (or in the vicinity of) proto-oncogenes or other signaling genes has been described for both normal and malignant hematopoiesis. Important insights into these side effects were initially obtained in murine models. Results from ongoing clinical studies have revealed that similar adverse events may also occur in human gene therapy. However, it remains unknown to what extent the outcome of insertional mutagenesis induced by gene vectors is related to (1) the architecture and type of vector used, (2) intrinsic properties of the target cell, and (3) extrinsic and potentially disease-specific factors influencing clonal competition in vivo. This review discusses reports addressing these questions, underlining the need for models that demonstrate and quantify the functional consequences of insertional mutagenesis. Improving vector design appears to be the most straightforward approach to increase safety, provided all relevant cofactors are considered.

HIF-1-dependent repression of equilibrative nucleoside transporter (ENT) in hypoxia

Extracellular adenosine (Ado) has been implicated as central signaling molecule during conditions of limited oxygen availability (hypoxia), regulating physiologic outcomes as diverse as vascular leak, leukocyte activation, and accumulation. Presently, the molecular mechanisms that elevate extracellular Ado during hypoxia are unclear. In the present study, we pursued the hypothesis that diminished uptake of Ado effectively enhances extracellular Ado signaling. Initial studies indicated that the half-life of Ado was increased by as much as fivefold after exposure of endothelia to hypoxia. Examination of expressional levels of the equilibrative nucleoside transporter (ENT)1 and ENT2 revealed a transcriptionally dependent decrease in mRNA, protein, and function in endothelia and epithelia. Examination of the ENT1 promoter identified a hypoxia inducible factor 1 (HIF-1)–dependent repression of ENT1 during hypoxia. Using in vitro and in vivo models of Ado signaling, we revealed that decreased Ado uptake promotes vascular barrier and dampens neutrophil tissue accumulation during hypoxia. Moreover, epithelial Hif1α mutant animals displayed increased epithelial ENT1 expression. Together, these results identify transcriptional repression of ENT as an innate mechanism to elevate extracellular Ado during hypoxia.

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.

Silencing and variegation of gammaretrovirus and lentivirus vectors

Retrovirus vectors integrate into the genome, providing stable gene transfer, but integration contributes in part to transcriptional silencing that compromises long-term expression. In the case of gammaretrovirus vectors based on murine leukemia virus, many integration events are completely silenced in undifferentiated stem cells and in transgenic mice. Gammaretrovirus vectors are also subject to variegation in which sister cells bearing the same provirus differentially express, and cell differentiation can lead to extinction of vector expression. In contrast, lentivirus vectors based on human immunodeficiency virus type 1 appear to express more efficiently, although other reports indicate that lentivirus vectors can be silenced. This review summarizes the key features of gammaretrovirus vector silencing. The evidence for and against gene silencing of lentivirus vectors is described with special emphasis on the potential effects of vector design, provirus copy number, and integration site preferences on silencing. This analysis suggests that the difference between selfinactivating (SIN) lentivirus vectors and their modified SIN gammaretrovirus counterparts may be less dramatic than previously thought. It will therefore be important to further characterize the mechanisms of silencing, in order to create better gammaretrovirus and lentivirus vectors that consistently express at single copy for gene therapy.

A single immunization with a minute dose of a lentiviral vector-based vaccine is highly effective at eliciting protective humoral immunity against West Nile virus

BACKGROUND

Lentiviral vectors, due to their capacity to transduce non-dividing cells, have become precious and worldwide used gene transfer systems. Their ability to efficiently and stably transduce dendritic cells (DCs) has led to their successful use as vaccination vectors for eliciting strong, specific and protective cellular immune responses mostly in anti-tumoral but also in anti-viral applications. However, the ability of lentiviral vectors to elicit an antibody-based protective immunity has, to date, not been evaluated. In the present study, we evaluated the potential of a lentiviral vector-based vaccine to elicit humoral immunity against West Nile virus (WNV). WNV is a mosquito-borne flavivirus that emerged in North America and causes encephalitis in humans, birds and horses. Neutralizing anti-WNV antibodies have been shown to be crucial for protection against WNV encephalitis.

METHODS

The ability of lentiviral vector TRIP/sE(WNV), expressing the secreted soluble form of the envelope E-glycoprotein (sE(WNV)) from the highly virulent IS-98-ST1 strain of WNV, to induce a specific humoral response and protection against WNV infection was assessed in a mouse model of WNV encephalitis.

RESULTS

Remarkably, a single immunization with a minute dose of TRIP/sE(WNV) was efficient at eliciting a long-lasting, protective and sterilizing humoral immunity, only 1 week after priming.

CONCLUSIONS

This study broadens the applicability of lentiviral vectors as efficient non-replicating vaccines against pathogens for which a neutralizing humoral response is one active arm of the protective immunity. The TRIP/sE(WNV) lentiviral vector appears to be a promising tool for veterinary vaccination against zoonotic WNV.

A genetic strategy to treat sickle cell anemia by coregulating globin transgene expression and RNA interference

The application of RNA interference (RNAi) to stem cell-based therapies will require highly specific and lineage-restricted gene silencing. Here we show the feasibility and therapeutic potential of coregulating transgene expression and RNAi in hematopoietic stem cells. We encoded promoterless small-hairpin RNA (shRNA) within the intron of a recombinant gamma-globin gene. Expression of both gamma-globin and the lariat-embedded small interfering RNA (siRNA) was induced upon erythroid differentiation, specifically downregulating the targeted gene in tissue- and differentiation stage-specific fashion. The position of the shRNA within the intron was critical to concurrently achieve high-level transgene expression, effective siRNA generation and minimal interferon induction. Lentiviral transduction of CD34(+) cells from patients with sickle cell anemia led to erythroid-specific expression of the gamma-globin transgene and concomitant reduction of endogenous beta(S) transcripts, thus providing proof of principle for therapeutic strategies that require synergistic gene addition and gene silencing in stem cell progeny.

Lentiviral vectors

We review the use of lentiviral vectors in current human gene therapy applications that involve genetic modification of nondividing tissues with integrated transgenes. Safety issues, including insertional mutagenesis and replication-competent retroviruses, are discussed. Innate cellular defenses against retroviruses and their implications for human gene therapy with different lentiviral vectors are also addressed.