Gene therapy for adenosine deaminase deficiency

Severe Combined Immunodeficiency Disorders

Severe combined immunodeficiency disorders represent pediatric emergencies due to absence of adaptive immune responses to infections. The conditions result from either intrinsic defects in T-cell development (ie, severe combined immunodeficiency disease [SCID]) or congenital athymia (eg, complete DiGeorge anomaly). Hematopoietic stem cell transplant provides the only clinically approved cure for SCID, although gene therapy research trials are showing significant promise. For greatest survival, patients should undergo transplant before 3.5 months of age and before the onset of infections. Newborn screening programs have yielded successful early identification and treatment of infants with SCID and congenital athymia in the United States.

Severe combined immunodeficiency: recent developments and guidance on clinical management

Severe combined immunodeficiency (SCID) is a rare but important condition. Affected infants are born with profound abnormalities of immune cell function that lead to severe and recurrent infection that are almost always fatal in the first year of life without treatment. Infants with SCID are often initially seen by general paediatricians in the hospital care setting, and the recognition of the cardinal features of the disease and alertness to specific laboratory parameters are important in making an early diagnosis. There is also increasing interest in newborn screening for SCID, which has the potential to significantly improve outcome through early diagnosis and implementation of prophylactic medications. Definitive treatments such as haematopoietic stem cell transplantation and gene therapy have also made major advances over the last decade and again promise to improve the overall outcome for SCID with reduced long-term toxicities. In this review, we highlight some of the major advances in diagnosis and management of the disease, but we also want to emphasise the important role of the general paediatrician in making an early diagnosis and in ongoing management.

Zinc-finger nuclease-mediated gene correction using single AAV vector transduction and enhancement by Food and Drug Administration-approved drugs

An emerging strategy for the treatment of monogenic diseases uses genetic engineering to precisely correct the mutation(s) at the genome level. Recent advancements in this technology have demonstrated therapeutic levels of gene correction using a zinc-finger nuclease (ZFN)-induced DNA double-strand break in conjunction with an exogenous DNA donor substrate. This strategy requires efficient nucleic acid delivery and among viral vectors, recombinant adeno-associated virus (rAAV) has demonstrated clinical success without pathology. However, a major limitation of rAAV is the small DNA packaging capacity and to date, the use of rAAV for ZFN gene delivery has yet to be reported. Theoretically, an ideal situation is to deliver both ZFNs and the repair substrate in a single vector to avoid inefficient gene targeting and unwanted mutagenesis, both complications of a rAAV co-transduction strategy. Therefore, a rAAV format was generated in which a single polypeptide encodes the ZFN monomers connected by a ribosome skipping 2A peptide and furin cleavage sequence. On the basis of this arrangement, a DNA repair substrate of 750 nucleotides was also included in this vector. Efficient polypeptide processing to discrete ZFNs is demonstrated, as well as the ability of this single vector format to stimulate efficient gene targeting in a human cell line and mouse model derived fibroblasts. Additionally, we increased rAAV-mediated gene correction up to sixfold using a combination of Food and Drug Administration-approved drugs, which act at the level of AAV vector transduction. Collectively, these experiments demonstrate the ability to deliver ZFNs and a repair substrate by a single AAV vector and offer insights for the optimization of rAAV-mediated gene correction using drug therapy.

The case for intrauterine stem cell transplantation

The clinical burden imposed by the collective group of monogenic disorders demands novel therapies that are effective at achieving phenotypic cure early in the disease process before the development of permanent organ damage. This is important for lethal diseases and also for non-perinatally lethal conditions that are characterised by severe disability with little prospect of postnatal cure. Where postnatal treatments are limited to palliative options, intrauterine stem-cell therapies may offer the potential to arrest pathogenesis in the early undamaged fetus. Intrauterine stem-cell transplantation has been attempted for a variety of diseases, but has only been successful in immune deficiency states in the presence of a competitive advantage for donor cells. This disappointing clinical record requires preclinical investigations into strategies that improve donor cell engraftment, including optimising the donor cell source and manipulating the microenvironment to facilitate homing. This chapter aims to outline the current challenges of intrauterine stem-cell therapy.

Extensive methylation of promoter sequences silences lentiviral transgene expression during stem cell differentiation in vivo

Lentiviral vectors (LV) are widely used to stably transfer genes into target cells investigating or treating gene functions. In addition, gene transfer into early murine embryos may be improved to efficiently generate transgenic mice. We applied lentiviral gene transfer to generate a mouse model transgenic for SET binding protein-1 (Setbp1) and enhanced green fluorescent protein (eGFP). Neither transgenic founders nor their vector-positive offspring transcribed or expressed the transgenes. Bisulfite sequencing of the internal spleen focus-forming virus (SFFV) promoter demonstrated extensive methylation of all analyzed CpGs in the transgenic mice. To analyze the impact of Setbp1 on epigenetic silencing, embryonic stem cells (ESC) were differentiated into cardiomyocytes (CM) in vitro. In contrast to human promoters in LV, virally derived promoter sequences were strongly methylated during differentiation, independent of the transgene. Moreover, the commonly used SFFV promoter (SFFVp) was highly methylated with remarkable strength and frequency during hematopoietic differentiation in vivo in LV but less in γ-retroviral (γ-RV) backbones. In summary, we conclude that LV using an internal SFFVp are not suitable to generate transgenic mice or perform constitutive expression studies in differentiating cells. Choosing the appropriate promoter is also crucial to allow stable transgene expression in clinical gene therapy.

Pseudotype-independent nonspecific uptake of gammaretroviral and lentiviral particles in human cells

The effective entry of retroviruses into target cells depends on the presence of viral envelope (Env) proteins and cognate cellular receptors, such as the murine cationic amino acid transporter-1 (mCAT-1) for the ecotropic murine leukemia virus (MLV-E). Here, we examined whether human cells internalize MLV-E or other retroviral pseudotypes irrespective of the presence of a specific receptor. Using fluorescently tagged Gag to monitor viral internalization, and treating cells with chloroquine or bafilomycin A1, we show that endocytosis is the main pathway for productive transduction with ecotropic particles, but endocytosis of retroviral particles itself does not depend on a suitable receptor or Env. Nonspecific endosomal uptake and lysosomal degradation occurred with all "illegitimate" envelope-receptor combinations tested: MLV particles pseudotyped with the ecotropic envelope or measles virus H and F proteins as well as "ecotropic" or "bald" HIV-1 particles. Kinetic studies in cell lines and primary human T lymphocytes showed the persistence of Gag-GFP signals for more than 10 days after exposure to retroviral vector particles, even in the absence of a suitable receptor. Further studies testing the Gag-mediated transfer of protein or retroviral mRNA revealed that nonspecific endocytosis prevented the release of functional particle-associated proteins and nucleic acids into the cytosol. We conclude that receptor-targeted retroviral particles are unlikely to escape nonspecific cellular uptake unless appropriate protective principles are discovered. Conversely, as lysosomal degradation was found to inactivate mRNA and proteins embedded into retroviral particles, receptor targeting is a useful strategy for both transient and permanent cell modification by retrovirus-like particles.

Stem Cell Transplantation for Hematological Malignancies: Prospects for Personalized Medicine and Co-therapy with Mesenchymal Stem Cells

Bone marrow transplantation is a form of cell therapy that has been in practice for decades for the treatment of hematological disorders and solid tumors. Immunosuppressive therapy has been a mainstay for treatment, but the severity of the adverse effects has made it an undesirable choice. Mesenchymal stem cells (MSCs), which reside in the vascular regions of the bone marrow, have been shown to serve as cellular support for the hematopoietic stem cell (HSC) niche. Furthermore, the immune suppressive properties of MSCs have been explored in the treatment of inflammatory and autoimmune disorders. Thus, co-therapy with MSCs has been shown to facilitate engraftment of hematopoietic cells by suppressive graft versus host disease (GvHD). Although the mechanism by which MSCs suppress GvHD is unclear, the experimental evidence suggests that this partly occurs by modulation of immune response such as the induction of regulatory T cells. This paper discusses the role of MSCs as co-therapy for the future of stem cell transplantation, with the overarching theme of personalized medicine for cell-based health interventions.

[Regulatory framework of innovative therapies : From bench to bedside]

Novel therapies, e.g., cell and gene therapy or tissue engineering, are summarized in the European Union as advanced therapy medicinal products (ATMPs). In terms of composition and product properties, ATMPs are highly complex, and given their multiple potential actions they are subject to continuously developing regulatory requirements. Due to promising basic research findings, there are high expectations by the society toward the therapeutic potential of ATMPs. It is of utmost importance to develop a scientifically sound preclinical and clinical development plan before entering into the first clinical trial. Due to the complex features of ATMPs, this development plan should be discussed early with the regulatory authorities to define the specifics and challenges of each individual product. For planning as well as operational realization of the initial clinical trial involving ATMPs, specific requirements that need to be addressed are discussed in this paper.