Stable long-term gene correction with low-dose radiation conditioning in murine X-linked chronic granulomatous disease

Genetic correction of X-linked chronic granulomatous disease with novel foamy virus vectors

OBJECTIVE

The X-linked form of chronic granulomatous disease (X-CGD) results from mutations in the CYBB gene encoding gp91(phox), the larger subunit of the oxidase flavocytochrome b(558). Affected individuals suffer from recurrent life-threatening infections due to impaired superoxide production by reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase in phagocytes. Novel foamy virus vectors expressing the human codon-optimized gp91(phox) were evaluated for the genetic correction of the disease in the X-CGD cell line and in X-CGD mouse model.

MATERIALS AND METHODS

The vectors were evaluated in vitro, in the human X-CGD PLB-985 cell line and in the X-CGD bone marrow Lin(-) cells. Transplantation of transduced Lin(-) cells was performed in X-CGD mice after busulfan conditioning. Real-time polymerase chain reaction was used for chimerism and vector copy number determination. Restoration of reduced NADPH oxidase production was assessed by nitrobluetetrazolium and dihydrorhodamine assays.

RESULTS

High and stable gp91(phox) expression, as well as reconstitution of reduced NADPH activity, was achieved in the human X-CGD PLB-985 cell line and in primary murine X-CGD hematopoietic stem cells ex vivo. Transplantation of transduced bone marrow hematopoietic stem cells in the murine model of X-CGD, even with low multiplicities of infection (MOI), reconstituted the levels of oxidase-producing neutrophils and provided enzymatic activity that reached 70% of normal.

CONCLUSIONS

Foamy virus vectors expressing the human gp91(phox) transgene constitute potential candidates for the gene therapy of CGD because they combine lack of pathogenicity with efficacy even at low MOI.

Gene therapy of chronic granulomatous disease: the engraftment dilemma

The potential of gene therapy as a curative treatment for monogenetic disorders has been clearly demonstrated in a series of recent Phase I/II clinical trials. Among primary immunodeficiencies, gene transfer into hematopoietic stem (HSC)/progenitor cells has resulted in the long-term correction of immune and metabolic defects in treated patients. In most cases, successes were augmented by a recognized biological selection for successfully treated cells in vivo, perhaps even to some extent at the HSC level. In contrast, similar achievements have not turned into reality for immunodeficiencies in which gene-transduced cells lack selective advantages in vivo. This is the case for chronic granulomatous disease (CGD), a primary immunodeficiency, characterized by deficient antimicrobial activity in phagocytic cells. Several attempts to correct CGD by gene transfer in combination with bone marrow conditioning have resulted in low-level long-term engraftment and transient clinical benefits despite high levels of gene marking and high numbers of reinfused cells. This review summarizes the data from clinical trials for CGD and provides some insights into treatment options that may lead to a successful application of gene therapy for CGD.

Retroviral vector integration in post-transplant hematopoiesis in mice conditioned with either submyeloablative or ablative irradiation

X-linked chronic granulomatous disease (X-CGD) is an inherited immunodeficiency with absent phagocyte NADPH-oxidase activity caused by defects in the gene-encoding gp91(phox). Here, we evaluated strategies for less intensive conditioning for gene therapy of genetic blood disorders without selective advantage for gene correction, such as might be used in a human X-CGD protocol. We compared submyeloablative with ablative irradiation as conditioning in murine X-CGD, examining engraftment, oxidase activity and vector integration in mice transplanted with marrow transduced with a gamma-retroviral vector for gp91(phox) expression. The frequency of oxidase-positive neutrophils in the donor population was unexpectedly higher in many 300 cGy-conditioned mice compared with lethally irradiated recipients, as was the fraction of vector-marked donor secondary CFU-S12. Vector integration sites in marrow, spleen and secondary CFU-S12 DNA from primary recipients were enriched for cancer-associated genes, including Evi1, and integrations in or near cancer-associated genes were more frequent in marrow and secondary CFU-S12 from 300 cGy-conditioned mice compared with fully ablated mice. These findings support the concept that vector integration can confer a selection bias, and suggest that the intensity of the conditioning regimen may further influence the effects of vector integration on clonal selection in post-transplant engraftment and hematopoiesis.

Exploitation of stem cell homing for gene delivery

Stem cells have been the focus of numerous investigations to treat diseases as far ranging as diabetes, chronic heart failure and multiple sclerosis over the past decade. The process of stem-cell-based repair of acute injury involves homing and engrafting of the stem cell of interest to the site of injury followed by either differentiation of the stem cell to indigenous end-organ cells or liberation of paracrine factors that lead to preservation and/or optimization of organ function. Recognition of the ability of stem cells to home to sites of acute injury suggests that, if appropriately defined and harnessed, stem cell homing could serve as a means of local drug delivery through the infusion of genetically engineering stem cells that secrete gene products of interest. The authors have recently demonstrated the use of this approach in preclinical studies of acute myocardial function. In addition, the use of engineered cells that home to appropriate niches have been used to correct genetic deficiency states (i.e., severe combined immunodeficiency, diabetes mellitus) in patients with otherwise chronic debilitating diseases. This review focuses on exploiting stem cell homing for gene transfer and on the state of the art and the challenges that face the field.

Gene therapy for chronic granulomatous disease

Patients with chronic granulomatous disease (CGD) cannot generate reactive oxygen metabolites, and suffer from severe recurrent infections and dysregulated inflammation. Haematopoietic stem cell transplantation is the only established option for definitive cure for patients with a suitable donor and is indicated when conventional prophylaxis and therapy with antimicrobial medication fail. Gene therapy has the potential to cure CGD, and several clinical trials have been conducted since 1997. Whereas initial studies resulted in low and short-term engraftment of CGD-corrected cells, recent trials demonstrated clinical benefit when engraftment was enhanced by busulfan conditioning prior to infusion of gene-corrected cells. However, the progress in gene therapy has been hampered by the appearance of insertional mutagenesis causing leukaemia in a trial for patients with X-linked severe combined immunodeficiency and by the emergence of dominant clones in a recent trial for the X-linked form of CGD. These findings stimulated the development of modified vector systems that demonstrate reduced genotoxicity in vitro and in animal models. New gene therapy protocols that allow efficient gene transfer and durable expression but limit the risk for insertional mutagenesis are envisioned to become an important therapeutic option for patients with CGD.

Granulocyte colony-stimulating factor prior to nonmyeloablative irradiation decreases murine host hematopoietic stem cell function and increases engraftment of donor marrow cells

The use of nonmyeloablative conditioning prior to bone marrow transplantation is an important component of transplantation-based therapies for nonmalignant blood diseases. In this study, treatment of recipient mice with granulocyte colony-stimulating factor (G-CSF) prior to low-dose total body irradiation (LD-TBI) enhanced long-term engraftment of freshly isolated congenic marrow 1.5- to 2-fold more than treatment with LD-TBI alone. This combined regimen was also evaluated in a mouse model of X-linked chronic granulomatous disease (X-CGD), where neutrophils have a defective NADPH oxidase due to genetic deletion of the gp91(phox) subunit. Long-term engraftment of male X-CGD bone marrow cells cultured ex vivo for retroviral transduction of gp91(phox) was enhanced by approximately 40% when female X-CGD recipients were pretreated with G-CSF prior to 300 cGy. These data confirm that sequential treatment with G-CSF and LD-TBI prior to transplantation increases long-term engraftment of donor marrow, and they extend this approach to transplantation of murine donor marrow cultured ex vivo for gene transfer. Additional studies showed that the administration of G-CSF prior to LD-TBI did not alter early homing of donor marrow cells. However, the combined regimen significantly decreased the content of long-term repopulating cells in recipient marrow compared with LD-TBI alone, as assessed in competitive assays, which may contribute to the enhanced engraftment of donor marrow cells. Disclosure of potential conflicts of interest is found at the end of this article.

Multiple Reduced-intensity Conditioning Regimens Facilitate Correction of Fabry Mice After Transplantation of Transduced Cells

Hematopoietic cell transplantation can impact lysosomal storage disorders (LSDs) and will be enhanced by gene therapy. Transduced cells in LSDs often secrete the therapeutic hydrolase, which can be used by bystander cells. However, toxicity associated with myeloablative transplant preparative regimens limits many applications of this approach in gene therapy. We hypothesized that reduced-intensity (RI) conditioning regimens would allow stable engraftment of therapeutically transduced cells and allow correction of Fabry disease. We transplanted transduced cells into Fabry mice receiving eight different clinically relevant chemotherapy- and/or radiotherapy-based RI conditioning regimens generating modest and transient lymphoid/myeloid cell depletion. Two comprehensive transplantation Protocols were performed. Firstly, transplantation of 0.38 x 10(6) gene-modified stem/progenitor cells was nominally effective; none of the RI regimens led to stable alpha-galactosidase A (alpha-gal A) correction. Secondly, transduced cells were preselected for functional transgene expression and transplanted at a higher dose (0.72 x 10(6) cells). Each RI regimen yielded engraftment of functional transgene-positive cells through 180 days along with increased plasma alpha-gal A activity. Importantly, the RI regimens mediated broad organ enzyme correction and were not associated with immune responses against alpha-gal A. RI conditioning thus has an important role in gene therapy for LSDs; a variety of regimens can be effective in this context.

A murine model of antimetabolite-based, submyeloablative conditioning for bone marrow transplantation: biologic insights and potential applications

OBJECTIVE

Nonmyeloablative conditioning regimens for marrow transplantation are desirable in many settings. Because repeated doses of the antimetabolite 5-fluorouracil (5-FU) decreases marrow long-term repopulating ability (LTRA) upon transplantation into lethally irradiated hosts, we hypothesized that mice given sequential doses of 5-FU (termed paired dose 5-FU) may permit substantial syngeneic marrow engraftment.

METHODS

C57Bl/6 or X-linked chronic granulomatous disease (X-CGD) mice were administered 5-FU (150 mg/kg) on days -5 and -1. Assessment of host marrow phenotype and repopulating ability occurred on day 0. Transplantation of syngeneic donor marrow occurred on day 0 or day +15.

RESULTS

We confirmed that the number of Sca-1+lin- cells and the LTRA of marrow from paired dose 5-FU-treated animals were diminished. C57Bl/6 hosts conditioned with paired doses of 5-FU followed by transplantation of 20 x 10(6) fresh B6.SJL marrow cells on day 0 displayed 44.9% +/- 7.1% donor chimerism 2 months posttransplant, and 34.4% +/- 8.6% donor chimerism 6 months posttransplant. In contrast, paired dose 5-FU-conditioned hosts transplanted with similar numbers of donor cells on day +15 exhibited only 3.4% +/- 1.2% donor chimerism at 2 months. Paired dose 5-FU-conditioned X-CGD hosts transplanted with MSCV-m91Neo-transduced X-CGD marrow averaged 6.6% +/- 2.3% (range, 4%-10%) NADPH oxidase-reconstituted neutrophils 12-16 months after transplant.

CONCLUSION

These findings support the concept that impairment of host stem cell competitiveness may be an important mechanism for permitting engraftment of donor cells, and suggest that only a brief period of modest host stem cell impairment may be necessary to achieve substantial donor cell engraftment.