Hematopoietic stem cell gene therapy: progress toward therapeutic targets

Adeno-Associated Virus Vectors and Stem Cells: Friends or Foes?

The infusion of healthy stem cells into a patient-termed "stem-cell therapy"-has shown great promise for the treatment of genetic and non-genetic diseases, including mucopolysaccharidosis type 1, Parkinson's disease, multiple sclerosis, numerous immunodeficiency disorders, and aplastic anemia. Stem cells for cell therapy can be collected from the patient (autologous) or collected from another "healthy" individual (allogeneic). The use of allogenic stem cells is accompanied with the potentially fatal risk that the transplanted donor T cells will reject the patient's cells-a process termed "graft-versus-host disease." Therefore, the use of autologous stem cells is preferred, at least from the immunological perspective. However, an obvious drawback is that inherently as "self," they contain the disease mutation. As such, autologous cells for use in cell therapies often require genetic "correction" (i.e., gene addition or editing) prior to cell infusion and therefore the requirement for some form of nucleic acid delivery, which sets the stage for the AAV controversy discussed herein. Despite being the most clinically applied gene delivery context to date, unlike other more concerning integrating and non-integrating vectors such as retroviruses and adenovirus, those based on adeno-associated virus (AAV) have not been employed in the clinic. Furthermore, published data regarding AAV vector transduction of stem cells are inconsistent in regards to vector transduction efficiency, while the pendulum swings far in the other direction with demonstrations of AAV vector-induced toxicity in undifferentiated cells. The variation present in the literature examining the transduction efficiency of AAV vectors in stem cells may be due to numerous factors, including inconsistencies in stem-cell collection, cell culture, vector preparation, and/or transduction conditions. This review summarizes the controversy surrounding AAV vector transduction of stem cells, hopefully setting the stage for future elucidation and eventual therapeutic applications.

TALEN-mediated functional correction of human iPSC-derived macrophages in context of hereditary pulmonary alveolar proteinosis

Hereditary pulmonary alveolar proteinosis (herPAP) constitutes a rare, life threatening lung disease characterized by the inability of alveolar macrophages to clear the alveolar airspaces from surfactant phospholipids. On a molecular level, the disorder is defined by a defect in the CSF2RA gene coding for the GM-CSF receptor alpha-chain (CD116). As therapeutic options are limited, we currently pursue a cell and gene therapy approach aiming for the intrapulmonary transplantation of gene-corrected macrophages derived from herPAP-specific induced pluripotent stem cells (herPAP-iPSC) employing transcriptional activator-like effector nucleases (TALENs). Targeted insertion of a codon-optimized CSF2RA-cDNA driven by the hybrid cytomegalovirus (CMV) early enhancer/chicken beta actin (CAG) promoter into the AAVS1 locus resulted in robust expression of the CSF2RA gene in gene-edited herPAP-iPSCs as well as thereof derived macrophages. These macrophages displayed typical morphology, surface phenotype, phagocytic and secretory activity, as well as functional CSF2RA expression verified by STAT5 phosphorylation and GM-CSF uptake studies. Thus, our study provides a proof-of-concept, that TALEN-mediated integration of the CSF2RA gene into the AAVS1 safe harbor locus in patient-specific iPSCs represents an efficient strategy to generate functionally corrected monocytes/macrophages, which in the future may serve as a source for an autologous cell-based gene therapy for the treatment of herPAP.

Interleukin-10 delivery via mesenchymal stem cells: a novel gene therapy approach to prevent lung ischemia-reperfusion injury

Ischemia-reperfusion (IR) injury is an important cause of primary graft failure in lung transplantation. In this study, viral interleukin-10 (vIL-10)-engineered mesenchymal stem cells (MSCs) were tested for their ability to prevent lung IR injury. Bone marrow-derived MSCs were transduced with rvIL-10-retrovirus. After 120 min of warm left lung ischemia, rats received approximately 15 x 10(6) vIL-10-engineered MSCs (MSC-vIL-10), empty vector-engineered MSCs (MSC-vec), or saline intravenously. Mean blood oxygenation (PaO(2)/FiO(2) ratio, mmHg) was measured at 4 hr, 24 hr, 72 hr, and 7 days. As early as 4 hr post-IR injury with MSC-vIL-10 treatment, blood oxygenation was significantly (p < 0.05) improved (319 +/- 94; n = 7) compared with untreated (saline) controls (63 +/- 19; n = 6). At 24 hr post-IR injury, in the MSC-vIL-10-treated group there was a further increase in blood oxygenation (353 +/- 105; n = 10) compared with the MSC-vec group (138 +/- 86; n = 9) and saline group (87 +/- 39; n = 10). By 72 hr, oxygenation reached normal (475 +/- 55; n = 9) in the MSC-vIL-10-treated group but not in the saline-treated and MSC-vec-treated groups. At 4 hr after IR injury, lungs with MSC-vIL10 treatment had a lower (p < 0.05) injury score (0.9 +/- 0.4) compared with lungs of the untreated (saline) group (2.5 +/- 1.4) or MSC-vec-treated group (2 +/- 0.4). Lung microvascular permeability and wet-to-dry weight ratios were markedly lower in the MSC-vIL10 group compared with untreated (saline) controls. ISOL (in situ oligonucleotide ligation for DNA fragmentation detection) and caspase-3 staining demonstrated significantly (p < 0.05) fewer apoptotic cells in MSC-vIL10-treated lungs. Animals that received MSC-vIL10 therapy had fewer (p < 0.05) CD4(+) and CD8(+) T cells in bronchoalveolar lavage fluid compared with untreated control animals. A therapeutic strategy using vIL-10-engineered MSCs to prevent IR injury in lung transplantation seems promising.

Wharton's jelly stromal cells as potential delivery vehicles for cancer therapeutics

There is now compelling evidence that stem cells can be used as gene therapy delivery cells. Stem cells isolated from the Wharton's jelly of the umbilical cord (termed Wharton's jelly stromal cells) can be harvested noninvasively in large numbers and have been shown to traffic to tumors but do not form tumors themselves. WJS cells have low immunogenicity and they have also been engineered to secrete a cytokine, shown to home locally into the tumors in mice and with subsequent tumor attenuation. Naive rat and human Wharton's jelly stromal cells that are not engineered to secrete an exogenous protein also exert a potent anticancer effect in preclinical models.

Down-regulation of CXCR4 expression by SDF-KDEL in CD34(+) hematopoietic stem cells: An anti-human immunodeficiency virus strategy

CXCR4 plays an essential role as the first discovered coreceptor for the entry of T cell tropic isolates of HIV-1. Blocking the surface expression of this receptor may be a potential strategy to prevent HIV-1 infection. A lentiviral vector, pLenti6/V5-S-K, expressing a SDF-KDEL fusion protein was constructed and a replication-incompetent lentiviral stock was produced. The lentiviral stock was transduced into CD34(+) hHSC and the transient expression of the recombinant protein, SDF-1, was assayed using indirect immunofluorescence. The surface expression of CXCR4 in CD34(+) hHSC pretreated with different amounts of recombinant lentiviral vectors was detected by flow cytometric analysis. A marked down-regulation of CXCR4 expression in the cells transduced with recombinant lentiviral vectors pLenti6/V5-S-K was observed by flow cytometry with PE-conjugated anti-human CXCR4 monoclonal antibodies which showed the percentages of the inhibition effects of CXCR4-SDF-1 mediated syncytium formation are presented by concentration. P24 antigen levels of cell culture supernatants were detected on the 4th, 7th, and 10th day, with 10(3) TCID50 HIV-1 infected CD34(+) hHSC to evaluate the inhibitory effect of pLenti6/V5-S-K transduction on HIV-1 infection. The cells transfected with pLenti6/V5-S-K had a significant reduction of HIV-1 DP27 infection compared to controls (P<0.05).

Cell vehicle targeting strategies

Use of cells as therapeutic carriers has increased in the past few years and has developed as a distinct concept and delivery method. Cell-based vehicles are particularly attractive for delivery of biotherapeutic agents that are difficult to synthesize, have reduced half-lives, limited tissue penetrance or are rapidly inactivated upon direct in vivo introduction. Initial studies using cell-based approaches served to identify some of the key factors for the success of this type of therapeutic delivery. These factors include the efficiency of cell loading with a therapeutic payload, the means of cell loading and the nature of therapeutics that cells can carry. However, one important aspect of cell-based delivery yet to be fully investigated is the process of actual delivery of the cell payload in vivo. In this regard, the potential ability of cell carriers to provide site-specific or targeted delivery of therapeutics deserves special attention. The present review focuses on a variety of targeting approaches that may be utilized to improve cell-based therapeutic delivery strategies. The different aspects of targeting that can be applied to cell vehicles will be discussed, including physical methods for directing cell distribution, intrinsic cell-mediated homing mechanisms and the feasibility of engineering cells with novel targeting mechanisms. Development of cell targeting strategies will further advance cell vehicle applications, broaden the applicability of this delivery approach and potentiate therapeutic outcomes.

Can gene delivery close the door to HIV-1 entry after escape?

BACKGROUND

Research efforts to prevent viral entry by developing small molecule inhibitors against HIV-1 chemokine coreceptors have yielded promising clinical results. However, resistance to some chemokine receptor inhibitors has been recently documented, and therefore, alternative methods of HIV-1 coreceptor disruption are needed.

CONCLUSION

We will describe current HIV-1 vector-delivered genetic disruption mechanisms that target HIV-1 chemokine coreceptors, such as RNA interference, ribozymes, zinc fingers, intrakines, and intrabodies, and frame the use of these gene delivery chemokine receptor disruption mechanisms in the context of current small molecule blocker/antagonists of CCR5 and CXCR4. In addition, we will discuss the importance of evaluating HIV-1 vector-delivered viral entry prevention mechanisms in the rhesus macaque SIV non-human primate model in regard to pathogenesis and therapeutic efficacy.

Potential of mesenchymal stem cells in gene therapy approaches for inherited and acquired diseases

The intriguing biology of stem cells and their vast clinical potential is emerging rapidly for gene therapy. Bone marrow stem cells, including the pluripotent haematopoietic stem cells (HSCs), mesenchymal stem cells (MSCs) and possibly the multipotent adherent progenitor cells (MAPCs), are being considered as potential targets for cell and gene therapy-based approaches against a variety of different diseases. The MSCs from bone marrow are a promising target population as they are capable of differentiating along multiple lineages and, at least in vitro, have significant expansion capability. The apparently high self-renewal potential makes them strong candidates for delivering genes and restoring organ systems function. However, the high proliferative potential of MSCs, now presumed to be self-renewal, may be more apparent than real. Although expanded MSCs have great proliferation and differentiation potential in vitro, there are limitations with the biology of these cells in vivo. So far, expanded MSCs have failed to induce durable therapeutic effects expected from a true self-renewing stem cell population. The loss of in vivo self-renewal may be due to the extensive expansion of MSCs in existing in vitro expansion systems, suggesting that the original stem cell population and/or properties may no longer exist. Rather, the expanded population may indeed be heterogeneous and represents several generations of different types of mesenchymal cell progeny that have retained a limited proliferation potential and responsiveness for terminal differentiation and maturation along mesenchymal and non-mesenchymal lineages. Novel technology that allows MSCs to maintain their stem cell function in vivo is critical for distinguishing the elusive stem cell from its progenitor cell populations. The ultimate dream is to use MSCs in various forms of cellular therapies, as well as genetic tools that can be used to better understand the mechanisms leading to repair and regeneration of damaged or diseased tissues and organs.

Glycogen synthase kinase-3 is an in vivo regulator of hematopoietic stem cell repopulation

The in vivo regulation of hematopoietic stem cell (HSC) function is poorly understood. Here, we show that hematopoietic repopulation can be augmented by administration of a glycogen synthase kinase-3 (GSK-3) inhibitor to recipient mice transplanted with mouse or human HSCs. GSK-3 inhibitor treatment improved neutrophil and megakaryocyte recovery, recipient survival and resulted in enhanced sustained long-term repopulation. The output of primitive Lin(-)c-Kit(+)Sca-1(+) cells and progenitors from HSCs increased upon GSK-3 inhibitor treatment without altering secondary repopulating ability, suggesting that the HSC pool is maintained while overall hematopoietic reconstitution is increased. GSK-3 inhibitors were found to modulate gene targets of Wnt, Hedgehog and Notch pathways in cells comprising the primitive hematopoietic compartment without affecting mature cells. Our study establishes GSK-3 as a specific in vivo modulator of HSC activity, and suggests that administration of GSK-3 inhibitors may provide a clinical means to directly enhance the repopulating capacity of transplanted HSCs.

Hematopoietic stem cell gene therapy with drug resistance genes: an update

Transfer of drug resistance genes into hematopoietic stem cells (HSCs) has promise for the treatment of a variety of inherited, that is, X-linked severe combined immune deficiency, adenosine deaminase deficiency, thalassemia, and acquired disorders, that is, breast cancer, lymphomas, brain tumors, and testicular cancer. Drug resistance genes are transferred into HSCs either for providing myeloprotection against chemotherapy-induced myelosuppression or for selecting HSCs that are concomitantly transduced with another gene for correction of an inherited disorder. In this review, we describe ongoing experimental approaches, observations from clinical trials, and safety concerns related to the drug resistance gene transfer.

Expression of cytochrome P450 genes in CD34+ hematopoietic stem and progenitor cells

Expression of major cytochrome P450 forms (P450) was followed in preparation of purified hematopoietic CD34+ stem and progenitor cells. Levels of transcripts as well as mature proteins were traced by quantitative real-time polymerase chain reaction and by Northern and Western blotting. P450 1B1 and P450 2E1 proteins and respective mRNAs were found in all cases. On the other hand, no expression of P450 3A4, P450 3A7, and P450 2C9 was found. The results showed that expression of various P450 enzymes starts at different stages of cell differentiation. Both P450 forms found are known to be connected with cancer cells and with activation of procarcinogens (P450 1B1, polycyclic aromatic hydrocarbons; P450 2E1, nitrosamines, and solvents). Hence, cells at the early stage of differentiation already may be influenced by interaction with xenobiotics. This fact should also be taken into consideration when hematopoietic cell transplant therapy is applied.

Development of mammalian artificial chromosomes for the treatment of genetic diseases: Sandhoff and Krabbe diseases

Mammalian artificial chromosomes (MACs) are being developed as alternatives to viral vectors for gene therapy applications, as they allow for the introduction of large payloads of genetic information in a non-integrating, autonomously replicating format. One class of MACs, the satellite DNA-based artificial chromosome expression vehicle (ACE), is uniquely suited for gene therapy applications, in that it can be generated denovo in cells, along with being easily purified and readily transferred into a variety of recipient cell lines and primary cells. To facilitate the rapid engineering of ACEs, the ACE System was developed, permitting the efficient and reproducible loading of pre-existing ACEs with DNA sequences and/or target gene(s). As a result, the ACE System and ACEs are unique and versatile platforms for ex vivo gene therapy strategies that circumvent and alleviate existing safety and delivery limitations surrounding conventional gene therapy vectors. This review will focus on the status of MAC technologies and, in particular, the application of the ACE System towards an ex vivo gene therapy treatment of lysosomal storage diseases, specifically Sandhoff (MIM #268800) and Krabbe (MIM #245200) diseases.

Optimization of equine infectious anemia derived vectors for hematopoietic cell lineage gene transfer

Gene transfer into hematopoietic cells may allow correction of a variety of hematopoietic and metabolic disorders. Optimized HIV-1 based lentiviral vectors have been developed for improved gene transfer and transgene expression into hematopoietic cells. However, the use of HIV-1 based vectors for human gene therapy may be limited due to ethical and biosafety issues. We report that vectors based on the non-primate equine infectious anemia virus (EIAV) transduce a variety of human hematopoietic cell lines and primary blood cells. To investigate optimization of gene expression in hematopoietic cells, we compared a variety of post-transcriptional elements and promoters in the context of EIAV vectors. We observed cell specific increase in the number of transgene expressing cells with the different post-transcriptional elements, whereas the use of elongation factor alpha 1 (EFalpha1) promoter resulted in significant increases in both the number of transgene expressing cells and the level of transgene protein in all cell types tested. We then demonstrate increased transduction of hematopoietic cells using a second-generation EIAV vector containing a self-inactivating EIAV LTR and the EIAV central polypurine tract (cppt). These data suggest that optimized EIAV vectors may be a suitable alternative to HIV-1 vectors for use in hematopoietic gene therapy.

Stem cell bioengineering for regenerative medicine

Stem cells can be used to treat a variety of diseases and several recent studies in animal models demonstrate the potential of bioengineering strategies targeting adult and embryonic stem cells. In order to obtain the desired cells for transplantation, stem cell bioengineering approaches entail the manipulation of environmental signals influencing cell survival, proliferation, self-renewal and differentiation. In that regard, multivariate analytical approaches have been used with success to optimise different stem cell culture processes. The genetic or molecular enhancement of stem cells is also a powerful means to control their proliferation or differentiation or to correct genetic defects in recipients. In the future, systems-level approaches have the potential to revolutionise the field of stem cell bioengineering by improving our understanding of regulatory networks controlling cellular behaviour. This advance in basic biology will be instrumental for the implementation of many stem cell-based regenerative therapies at the clinical level, as treatment accessibility will depend on the development of robust technologies to produce sufficient cell numbers.

Targeting platelet GPIbalpha transgene expression to human megakaryocytes and forming a complete complex with endogenous GPIbbeta and GPIX

Bernard-Soulier Syndrome (BSS) is a severe congenital platelet disorder that results from a deficiency of the platelet membrane glycoprotein (GP) Ib/IX complex that is composed of four subunits (GPIbalpha, GPIbbeta, GPIX, and GPV). Mutations in either GPIbalpha, GPIbbeta, or GPIX can result in BSS with many of the known mutations occurring in GPIbalpha. In this study, we have developed a gene therapy strategy to express hemagglutinin (HA)-tagged GPIbalpha in megakaryocytes and potentially correct a hereditary deficiency. To direct GPIbalpha expression in megakaryocytic lineage cells, we designed a GPIbalpha cassette where human GPIbalpha cDNA was placed under control of the megakaryocytic/platelet-specific alphaIIb promoter and inserted into a lentiviral vector. Human CD34+ peripheral blood cells (PBC) and Dami cells were transduced with alphaIIb-HA-GPIbalpha-WPT virus. Flow cytometry analysis demonstrated that 50.1% of the megakaryocytes derived from CD34+ stem cells and 97.3% of Dami cells were transduced and expressed transgene GPIbalpha protein. Immunoprecipitation with Western blot analysis demonstrated that transgene protein associated with endogenous GPIbbeta and GPIX proteins. To address further the lineage-specific expression of the alphaIIb-HA-GPIbalpha construct, three cell lines, Dami, AtT-20 and HepG2, were transfected with GPIbalpha expression plasmids and analyzed by confocal microscopy. The results demonstrated that among these three cell lines, the tissue-specific alphaIIb promoter was active only in Dami cells. Thus, GPIbalpha can be efficiently and specifically expressed in the megakaryocytic compartment of hematopoietic cells and the transgene product associates with endogenous GPIbbeta and GPIX forming a complete complex. This strategy could potentially be utilized for gene therapy of BSS.

Stem-cell therapy for renal diseases

Significant attention is currently directed to the biological and therapeutic capabilities of stem cells for developing novel treatments for acute and chronic kidney diseases. To date, viable sources of stem cells for renal therapies include adult bone marrow and embryonic tissues, including the metanephric mesenchyme and mesonephros. Native adult kidney stem cells have yet to be identified. Systemically introduced stem cells can engraft in sites of renal disease and injury to show donor phenotypes. Stem cells can differentiate into cells similar to glomeruli, mesangium, and tubules in the kidneys. The impact of stem-cell engraftment and differentiation on renal function presently is unknown. Identification of renal diseases treatable with stem-cell therapies is expected to evolve as stem-cell technologies advance. Methods of modifying stem cells to improve homing, differentiation, and integration into host tissues need further characterization. Ethical and legal controversies about embryonic research and cloning are shaping the regulation and funding of stem-cell research for kidney diseases. Scientific and clinical understanding of stem cells and their potential for renal treatments are in the early stage of development. This field offers great promise, and there are significant opportunities for future investigation in clinical, biological, and ethical aspects of stem-cell therapy for kidney diseases.