In vivo competitive studies between normal and common gamma chain-defective bone marrow cells: implications for gene therapy

Treatment of osteogenesis imperfecta in adults

BACKGROUND

Osteogenesis imperfecta (OI) is a heterogeneous rare connective tissue disorder commonly caused by mutations in the collagen type I genes. Pharmacological treatment has been most extensively studied in children, and there are only few studies comprising adult OI patients.

OBJECTIVES

i) To review the literature on the current medical management of OI in children and adults, and thereby identify unmet medical needs and ii) to present an overview of possible future treatment options.

RESULTS

Individualization and optimization of OI treatment in adults remain a challenge, because available treatments do not target the underlying collagen defect, and available literature gives weak support for treatment decisions for adult patients.

CONCLUSIONS

Bisphosphonates are still the most widely used pharmacological treatment for adult OI, but the current evidence supporting this is sparse and investigations on indications for choice and duration of treatment are needed.

Gene therapy model of X-linked severe combined immunodeficiency using a modified foamy virus vector

X-linked severe combined immunodeficiency (SCID-X1) is an inherited genetic immunodeficiency associated with mutations in the common cytokine receptor γ chain (γc) gene, and characterized by a complete defect of T and natural killer (NK) cells. Gene therapy for SCID-X1 using conventional retroviral (RV) vectors carrying the γc gene results in the successful reconstitution of T cell immunity. However, the high incidence of vector-mediated T cell leukemia, caused by vector insertion near or within cancer-related genes has been a serious problem. In this study, we established a gene therapy model of mouse SCID-X1 using a modified foamy virus (FV) vector expressing human γc. Analysis of vector integration in a human T cell line demonstrated that the FV vector integration sites were significantly less likely to be located within or near transcriptional start sites than RV vector integration sites. To evaluate the therapeutic efficacy, bone marrow cells from γc-knockout (γc-KO) mice were infected with the FV vector and transplanted into γc-KO mice. Transplantation of the FV-treated cells resulted in the successful reconstitution of functionally active T and B cells. These data suggest that FV vectors can be effective and may be safer than conventional RV vectors for gene therapy for SCID-X1.

Genetic mosaic analysis reveals a major role for frizzled 4 and frizzled 8 in controlling ureteric growth in the developing kidney

The developing mammalian kidney is an attractive system in which to study the control of organ growth. Targeted mutations in the Wnt receptors frizzled (Fz) 4 and Fz8 lead to reduced ureteric bud growth and a reduction in kidney size, a phenotype previously reported for loss of Wnt11. In cell culture, Fz4 and Fz8 can mediate noncanonical signaling stimulated by Wnt11, but only Fz4 mediates Wnt11-stimulated canonical signaling. In genetically mosaic mouse ureteric buds, competition between phenotypically mutant Fz4(-/-) or Fz4(-/-);Fz8(-/-) cells and adjacent phenotypically wild-type Fz4(+/-) or Fz4(+/-);Fz8(-/-) cells results in under-representation of the mutant cells to an extent far greater than would be predicted from the size reduction of homogeneously mutant kidneys. This discrepancy presumably reflects the compensatory action of a network of growth regulatory systems that minimize developmental perturbations. The present work represents the first description of a kidney phenotype referable to one or more Wnt receptors and demonstrates a general strategy for revealing the contribution of an individual growth regulatory pathway when it is part of a larger homeostatic network.

Phage phiC31 integrase-mediated genomic integration of the common cytokine receptor gamma chain in human T-cell lines

BACKGROUND

X-linked severe combined immunodeficiency (SCID-X1, X-SCID) is a life-threatening disease caused by a mutated common cytokine receptor gamma chain (gammac) gene. Although ex vivo gene therapy, i.e., transduction of the gammac gene into autologous CD34(+) cells, has been successful for treating SCID-X1, the retrovirus vector-mediated transfer allowed dysregulated integration, causing leukemias. Here, to explore an alternative gene transfer methodology that may offer less risk of insertional mutagenesis, we employed the phiC31 integrase-based integration system using human T-cell lines, including the gammac-deficient ED40515(-).

METHODS

A phiC31 integrase and a neo(r) gene expression plasmid containing the phiC31 attB sequence were co-delivered by electroporation into Jurkat cells. After G418 selection, integration site analyses were performed using linear amplification mediated-polymerase chain reaction (LAM-PCR). ED40515(-) cells were also transfected with a gammac expression plasmid containing attB, and the integration sites were determined. IL-2 stimulation was used to assess the functionality of the transduced gammac in an ED40515(-)-derived clone.

RESULTS

Following co-introduction of the phiC31 integrase expression plasmid and the plasmid carrying attB, the efficiency of integration into the unmodified human genome was assessed. Several integration sites were characterized, including new integration sites in intergenic regions on chromosomes 13 and 18 that may be preferred in hematopoietic cells. An ED40515(-) line bearing the integrated gammac gene exhibited stable expression of the gammac protein, with normal IL-2 signaling, as assessed by STAT5 activation.

CONCLUSIONS

This study supports the possible future use of this phiC31 integrase-mediated genomic integration strategy as an alternative gene therapy approach for treating SCID-X1.

Gene therapy: therapeutic gene causing lymphoma

The development of T-cell leukaemia following the otherwise successful treatment of three patients with X-linked severe combined immune deficiency (X-SCID) in gene-therapy trials using haematopoietic stem cells has led to a re-evaluation of this approach. Using a mouse model for gene therapy of X-SCID, we find that the corrective therapeutic gene IL2RG itself can act as a contributor to the genesis of T-cell lymphomas, with one-third of animals being affected. Gene-therapy trials for X-SCID, which have been based on the assumption that IL2RG is minimally oncogenic, may therefore pose some risk to patients.

Nuclear-localizing O6-benzylguanine-resistant GFP-MGMT fusion protein as a novel in vivo selection marker

OBJECTIVE

We characterized a novel in vivo selectable fusion protein, green fluorescence protein-O6-benzylguanine (BG)-resistant O6-methylguanine-methyltransferase (GFP-MGMT* [*refers to mutant MGMT]) used to delineate optimum selection regimens for transduced hematopoietic stem cells (HSC) ex vivo and in vivo.

MATERIALS AND METHODS

We transduced human or mouse HSC with retrovirus vector encoding GFP-MGMT* where BG-resistant forms of human P140K-hMGMT* and mouse P144K-mMGMT* were studied. We evaluated selection of transduced HSC ex vivo and in vivo using either BG/1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) or BG/temozolomide (TMZ) combinations, evaluating transduction marking by flow cytometry and real-time TaqMan PCR.

RESULTS

GFP-MGMT* transduction confers nuclear-localized GFP fluorescence and BG resistance. Optimum selection ex vivo of GFP-MGMT*-transduced HSC occurred with BG (2.5-10 microM)/BCNU (5-10 microM) or TMZ (100-200 microM), which increases marking while preserving maximum viable transduced cells. Starting at low levels (0.1%) or high levels (>30%) of in vivo bone marrow gene making in mice, in vivo selection with BG/BCNU (20/6 mg/kg) (weeks 4 and 5) or BG/TMZ (20/60 mg/kg) (daily x 5 at week 4) increased bone marrow marking to 8.58% +/- 3.52% or 82.0% +/- 3.4% GFP+ cells, respectively, in the low- or high-level initial marking mice.

CONCLUSIONS

GFP-MGMT* is an informative tool to explore optimization of in vivo selection regimens using BG/BCNU or BG/TMZ to increase gene marking of HSC. Both timing and dosing of selection regimens and the starting level of marking may all be important to the level of selective increase of in vivo marking achieved.

Gene therapy in infants with severe combined immunodeficiency

Severe combined immunodeficiencies (SCID) are rare disorders that represent paediatric medical emergencies, as the outcome for affected patients can easily be fatal unless proper treatment is performed. The only curative treatment for SCID is reconstitution of the patient's immunity. For more than 30 years, allogeneic bone marrow transplantation (BMT) has been extremely successful for SCID. However, BMT often results in only incomplete restoration of B cell function in treated patients, especially when haploidentical donors are used. In addition, BMT can be associated with severe complications such as graft-versus-host disease (GVHD). Alternative forms of therapy for SCID are therefore desirable. Genetic correction of peripheral T lymphocytes and/or haematopoietic stem cells (HSCs) by retrovirally mediated gene transfer has been attempted for patients with SCID due to adenosine deaminase deficiency, the first genetic disease targeted in clinical gene therapy trials with very limited success, overall. After these pioneer trials, recent progress has led to significant improvement of gene transfer techniques and better understanding of HSC biology which has culminated in the recent success of a gene therapy trial for patients affected with X-linked SCID (X-SCID). In this trial, patients with X-SCID received autologous bone marrow stem/progenitor cells which had been retrovirally transduced with a therapeutic gene. Based on the current follow-up, the overall efficacy of this gene therapy procedure is to be considered similar to or even better than that achievable by allogeneic BMT, because patients were not exposed to the risks of GVHD. Although these exciting results have clearly demonstrated that gene therapy is a feasible therapeutic option for X-SCID, they have also raised important questions regarding the long-term outcome of this experimental procedure and the possibility of translating this success into applications for other forms of SCID.

The role of common gamma chain in human monocytes in vivo; evaluation from the studies of X-linked severe combined immunodeficiency (X-SCID) carriers and X-SCID patients who underwent cord blood stem cell transplantation

Expression of common gamma chain (gammac) on monocytes was studied in five carriers of X-linked severe combined immunodeficiency (X-SCID) and two X-SCID patients who underwent cord blood stem cell transplantation (CBSCT). Flow cytometric analysis revealed that both gammac-negative and positive monocytes co-existed in X-SCID carriers, whereas no gammac-negative T, B or NK cells were observed in them. Clonal analysis and reverse transcription polymerase chain reaction studies revealed that 13.2-45.0% of monocytes from these carriers expressed the mutant gammac message. X-SCID patients who received CBSCT persistently possessed the majority of gammac-negative monocytes with a good clinical course. These results, together, may indicate that gammac is not essential for monocyte development/function in vivo.

Reconstitution of lymphoid development and function in ZAP-70-deficient mice following gene transfer into bone marrow cells

Mutations in the ZAP-70 protein tyrosine kinase gene result in a severe combined immunodeficiency (SCID) characterized by a selective inability to produce CD8(+) T cells and a signal transduction defect in peripheral CD4(+) cells. Transplantation of genetically modified hematopoietic progenitor cells that express the wild-type ZAP-70 gene may provide significant benefit to some of these infants. The feasibility of stem cell gene correction for human ZAP-70 deficiency was assessed using a ZAP-70 knock-out model. ZAP-70-deficient murine bone marrow progenitor cells were transduced with a retroviral vector expressing the human ZAP-70 gene. Engraftment of these cells in irradiated ZAP-70-deficient animals resulted in the development of mature CD4(+) and CD8(+) T cells. In marked contrast, both populations were absent in ZAP-70(-/-) mice undergoing transplantation with bone marrow progenitor cells transduced with a control vector. Importantly, ZAP-70-reconstituted T cells proliferated in response to T-cell receptor stimulation. Moreover, these ZAP-70-expressing T cells demonstrated a diverse T-cell receptor repertoire as monitored by the relative usage of each T-cell receptor beta chain hypervariable region subfamily. The presence of ZAP-70 in B cells did not affect either lipopolysaccharide- or lipopolysaccharide/interleukin-4-mediated immunoglobulin isotype switching. Altogether, these data indicate that retroviral-mediated gene transfer of the ZAP-70 gene may prove to have a therapeutic benefit for patients with ZAP-70-SCID.

Gene therapy for primary immune deficiencies

Primary immunodeficiency diseases have been important targets of corrective gene transfer approaches since the very early days of gene therapy. The potential for selective survival advantage of gene-corrected cells over populations carrying the mutated, causative gene translates into the possibility of obtaining clinical meaningful results in patients with primary immunodeficiency diseases even if levels of gene transfer are low. This critical prospect has fueled the interest of researchers since the mid-1980s and has recently determined the success of a clinical trial of gene therapy for X-linked severe combined immunodeficiency.

Lack of dominant-negative effects of a truncated gamma(c) on retroviral-mediated gene correction of immunodeficient mice

A recent clinical trial of gene therapy for X-linked severe combined immunodeficiency (XSCID) has shown that retroviral-mediated gene correction of bone marrow stem cells can lead to the development of normal immune function. These exciting results have been preceded by successful immune reconstitution in several XSCID mouse models, all carrying null mutations of the common gamma chain (gamma(c)). One question not formally addressed by these previous studies is that of possible dominant-negative effects of the endogenous mutant gamma(c) protein on the activity of the wild-type transferred gene product. The present work was therefore undertaken to study whether corrective gene transfer was applicable to an XSCID murine model with preserved expression of a truncated gammac molecule (Deltagamma(c+)-XSCID). Gene correction of Deltagamma(c+)-XSCID mice resulted in the reconstitution of lymphoid development, and preferential repopulation of lymphoid organs by gene-corrected cells demonstrated the selective advantage of gamma(c)-expressing cells in vivo. Newly developed B cells showed normalization of lipopolysaccharide-mediated proliferation and interleukin-4 (IL-4)-induced immunoglobulin G1 isotype switching. Splenic T cells and thymocytes of treated animals proliferated normally to mitogens and responded to the addition of IL-2, IL-4, and IL-7, indicating functional reconstitution of gammac-sharing receptors. Repopulated thymi showed a clear increase of CD4-/CD8- and CD8+ fractions, both dramatically reduced in untreated Deltagamma(c+)-XSCID mice. These improvements were associated with the restoration of Bcl-2 expression levels and enhanced cell survival. These data indicate that residual expression of the endogenous truncated gamma(c) did not lead to dominant-negative effects in this murine model and suggest that patient selection may not be strictly necessary for gene therapy of XSCID.