ADA-deficient Severe Combined Immunodeficiency Research Articles

Clinical implications of inherited chromosomally integrated HHV-6 in the setting of ADA-deficient severe combined immunodeficiency (SCID)

Clinical implications of inherited chromosomally integrated HHV-6 in the setting of ADA-deficient severe combined immunodeficiency (SCID)

Outcomes following treatment for ADA-deficient severe combined immunodeficiency: a report from the PIDTC

AbstractAdenosine deaminase (ADA) deficiency causes ∼13% of cases of severe combined immune deficiency (SCID). Treatments include enzyme replacement therapy (ERT), hematopoietic cell transplant (HCT), and gene therapy (GT). We evaluated 131 patients with ADA-SCID diagnosed between 1982 and 2017 who were enrolled in the Primary Immune Deficiency Treatment Consortium SCID studies. Baseline clinical, immunologic, genetic characteristics, and treatment outcomes were analyzed. First definitive cellular therapy (FDCT) included 56 receiving HCT without preceding ERT (HCT); 31 HCT preceded by ERT (ERT-HCT); and 33 GT preceded by ERT (ERT-GT). Five-year event-free survival (EFS, alive, no need for further ERT or cellular therapy) was 49.5% (HCT), 73% (ERT-HCT), and 75.3% (ERT-GT; P < .01). Overall survival (OS) at 5 years after FDCT was 72.5% (HCT), 79.6% (ERT-HCT), and 100% (ERT-GT; P = .01). Five-year OS was superior for patients undergoing HCT at <3.5 months of age (91.6% vs 68% if ≥3.5 months, P = .02). Active infection at the time of HCT (regardless of ERT) decreased 5-year EFS (33.1% vs 68.2%, P < .01) and OS (64.7% vs 82.3%, P = .02). Five-year EFS (90.5%) and OS (100%) were best for matched sibling and matched family donors (MSD/MFD). For patients treated after the year 2000 and without active infection at the time of FDCT, no difference in 5-year EFS or OS was found between HCT using a variety of transplant approaches and ERT-GT. This suggests alternative donor HCT may be considered when MSD/MFD HCT and GT are not available, particularly when newborn screening identifies patients with ADA-SCID soon after birth and before the onset of infections. This trial was registered at www.clinicaltrials.gov as #NCT01186913 and #NCT01346150.

Gene delivery using AAV8 in vivo for disease stabilization in a bimodal gene therapy approach for the treatment of ADA-deficient SCID

Adenosine deaminase (ADA) deficiency is an inborn error of metabolism affecting multiple systems and causing severe combined immunodeficiency. We tested intravenous administration of recombinant adeno-associated virus (AAV) 2/8-ADA vector in ADA-deficient neonate and adult mice or as part of a bimodal approach comprised of rAAV treatment at birth followed by infusion of lentiviral vector (LV)-modified lineage-depleted bone marrow cells at 8 weeks. ADA−/− mice treated with rAAV and enzyme replacement therapy (ERT) for 30 days were rescued from the lethal pulmonary insufficiency, surviving out to 180 days without further treatment. rAAV vector copy number (VCN) was highest in liver, lung, and heart and was associated with near-normal ADA activity and thymocyte development. In the bimodal approach, rAAV-mediated ADA expression supported survival during the 4 weeks before infusion of the LV-modified bone marrow cells and during the engraftment period. Conditioning prior to infusion may have resulted in the replacement of rAAV marked cells in marrow and liver, with LV VCN 100- to 1,000-fold higher in hematopoietic tissue compared with rAAV VCN, and was associated with immune cell reconstitution. In conclusion, a bimodal approach may be an alternative for patients without reliable access to ERT before receiving a stem cell transplant or gene therapy.

Recombinant Adenosine Deaminase Ameliorates Inflammation, Vascular Disease, and Fibrosis in Preclinical Models of Systemic Sclerosis.

Systemic sclerosis (SSc) is characterized by fibrosis, vascular disease, and inflammation. Adenosine signaling plays a central role in fibroblast activation. We undertook this study to evaluate the therapeutic effects of adenosine depletion with PEGylated adenosine deaminase (PEG-ADA) in preclinical models of SSc. The effects of PEG-ADA on inflammation, vascular remodeling, and tissue fibrosis were analyzed in Fra-2 mice and in a B10.D2→BALB/c (H-2d ) model of sclerodermatous chronic graft-versus-host disease (GVHD). The effects of PEG-ADA were confirmed in vitro in a human full-thickness skin model. PEG-ADA effectively inhibited myofibroblast differentiation and reduced pulmonary fibrosis by 34.3% (with decreased collagen expression) (P = 0.0079; n = 6), dermal fibrosis by 51.8% (P = 0.0006; n = 6), and intestinal fibrosis by 17.7% (P = 0.0228; n = 6) in Fra-2 mice. Antifibrotic effects of PEG-ADA were also demonstrated in sclerodermatous chronic GVHD (reduced by 38.4%) (P = 0.0063; n = 8), and in a human full-thickness skin model. PEG-ADA treatment decreased inflammation and corrected the M2/Th2/group 2 innate lymphoid cell 2 bias. Moreover, PEG-ADA inhibited proliferation of pulmonary vascular smooth muscle cells (reduced by 40.5%) (P < 0.0001; n = 6), and prevented thickening of the vessel walls (reduced by 39.6%) (P = 0.0028; n = 6) and occlusions of pulmonary arteries (reduced by 63.9%) (P = 0.0147; n = 6). Treatment with PEG-ADA inhibited apoptosis of microvascular endothelial cells (reduced by 65.4%) (P = 0.0001; n = 6) and blunted the capillary rarefication (reduced by 32.5%) (P = 0.0199; n = 6). RNA sequencing demonstrated that treatment with PEG-ADA normalized multiple pathways related to fibrosis, vasculopathy, and inflammation in Fra-2 mice. Treatment with PEG-ADA ameliorates the 3 cardinal features of SSc in pharmacologically relevant and well-tolerated doses. These findings may have direct translational implications, as PEG-ADA has already been approved by the Food and Drug Administration for the treatment of patients with ADA-deficient severe combined immunodeficiency disease.

Narrative review of severe combined immunodeficiency—a purine metabolism disorder

Severe combined immunodeficiency (SCID) is a primary immune deficiency that is a pediatric emergency. Left untreated, patients will die prior to 2 years of age from overwhelming infection. There are many known causes of SCID, but this review focuses on adenosine deaminase (ADA) deficiency. ADA is a ubiquitous “housekeeping” enzyme, and its deficiency leads to buildup of toxic metabolites, which preferentially affects lymphocytes but also causes non-immune manifestations of disease. While previously diagnosed based on recurrent, severe, or unusual infection, SCID is now being diagnosed during its initial asymptomatic period thanks to recent implementation of T cell receptor excision circle (TREC) newborn screening, thus greatly improving survival of patients as treatment can occur before the onset of infection. To date, TREC screening has shown 100% sensitivity for SCID but also has brought to light other causes of T cell lymphopenia that were previously lesser known. ADA-deficient SCID can be treated through enzyme replacement therapy (ERT), hematopoietic stem cell transplantation (HSCT), or gene therapy, and advances continue to be made every day that improve cost-effectiveness and efficacy of these therapies and make them more widely available to patients.

Gene therapy for blood cell diseases with autologous hematopoietic stem cells

Gene therapy is under development to treat genetic diseases of blood cells, including primary immune deficiencies (PID), lysosomal storage and related metabolic disorders, and hemoglobinopathies. Based on the experience with allogeneic hematopoietic stem cell (HSC) transplantation to successfully cure these disorders, gene therapy using autologous HSC holds the potential for safer treatments, avoiding immune complications. The key technical challenge is to achieve highly efficient gene correction in HSC, without impairing the long-term reconstituting capacity of the stem cells nor causing genotoxicity by the genetic manipulations. Initial trials using gamma-retroviral vectors showed clinical efficacy for ADA-deficient SCID, X-SCID, X-Chronic Granulomatous Disease and Wiskott-Aldrich Syndrome, although insertional oncogenesis led to leukoproliferative complications in the latter three conditions. Subsequent trials using retroviral or lentiviral vectors from which the long-terminal repeat enhancers are deleted (SIN vectors) have led to further demonstration of efficacy for these PID and no clinical genotoxic events. X-linked Adrenoleukodystrophy was the first disorder approached using lentiviral vectors to transduce HSC and results in have been were equivalent to those with allogeneic HSCT, without GVHD. Autologous HSC gene therapy for Metachromatic Leukodystrophy produced outcomes that were better than those previously observed using allogeneic HSCT, with over-expression of the relevant gene product (arylsulfatase A) leading to cross-correction and clinically meaningful neuroprotection and excellent development outcomes. For hemoglobinopathies, early reports from clinical trials for beta-thalassemia and sickle cell disease indicate some degree of efficacy, although it remains uncertain whether current vector designs yield sufficient beta-globin gene transfer and expression to ameliorate severe forms of the diseases. The potential game-changer in this area is correction of the underlying defective gene in HSC, using site-specific endonucleases (ZFN, TALEN, Meganucleases and CRISPR/Cas9) to augment homology-driven gene repair. There have been rapid advances in this approach, although the frequency of engrafting HSC that are gene-corrected and without off-target effects are not yet up to levels that would be therapeutic for most disorders. One disorder at a time, autologous HSC gene therapy is advancing to provide effective and safe therapies for genetic diseases of blood cells.

Update on the safety and efficacy of retroviral gene therapy for immunodeficiency due to adenosine deaminase deficiency

AbstractAdenosine deaminase (ADA) deficiency is a rare, autosomal-recessive systemic metabolic disease characterized by severe combined immunodeficiency (SCID). The treatment of choice for ADA-deficient SCID (ADA-SCID) is hematopoietic stem cell transplant from an HLA-matched sibling donor, although <25% of patients have such a donor available. Enzyme replacement therapy (ERT) partially and temporarily relieves immunodeficiency. We investigated the medium-term outcome of gene therapy (GT) in 18 patients with ADA-SCID for whom an HLA-identical family donor was not available; most were not responding well to ERT. Patients were treated with an autologous CD34+-enriched cell fraction that contained CD34+ cells transduced with a retroviral vector encoding the human ADA complementary DNA sequence (GSK2696273) as part of single-arm, open-label studies or compassionate use programs. Overall survival was 100% over 2.3 to 13.4 years (median, 6.9 years). Gene-modified cells were stably present in multiple lineages throughout follow up. GT resulted in a sustained reduction in the severe infection rate from 1.17 events per person-year to 0.17 events per person-year (n = 17, patient 1 data not available). Immune reconstitution was demonstrated by normalization of T-cell subsets (CD3+, CD4+, and CD8+), evidence of thymopoiesis, and sustained T-cell proliferative capacity. B-cell function was evidenced by immunoglobulin production, decreased intravenous immunoglobulin use, and antibody response after vaccination. All 18 patients reported infections as adverse events; infections of respiratory and gastrointestinal tracts were reported most frequently. No events indicative of leukemic transformation were reported. Trial details were registered at www.clinicaltrials.gov as #NCT00598481.

292. Lentivirus Vector Mediated Gene Correction in Artemis-Deficient Severe Combined Immunodeficiency

Mutations in DCLRE1C/Artemis, a DNA repair gene, cause T-B-NK+ SCID by preventing V(D)J recombination in T and B cell progenitors and also confer heightened sensitivity to irradiation and alkylator chemotherapy. SCID newborn screening identifies Artemis-deficient SCID (ART-SCID) early in life and while allogeneic hematopoietic cell transplantation can cure ART-SCID, preparative regimens for conditioning are poorly tolerated. Without alkylating chemotherapy patients may have graft failure while toxic effects of chemotherapy include increased mortality, short stature and abnormal dental development. Thus, building on experience with X-linked and ADA deficient SCID, we found addition of a normal Artemis gene to hematopoietic stem cells (HSC) an attractive strategy to treat ART-SCID.Since overexpression of Artemis protein causes cellular cytotoxicity, a lentivirus vector with human Artemis cDNA and its endogenous promoter (Apro-hART) was produced and used to transduce fibroblasts from ART-SCID patients and controls. Apro-hART transduced ART-SCID fibroblasts showed correction of radiosensitivity by enumeration of foci of DNA damage and proliferation assays. Radiosensitivity of cells lacking the DNA repair enzyme Ligase-4 was not corrected. View Large Image | Download PowerPoint SlideMobilized peripheral blood CD34+ cells from an ART-SCID patient, incapable of differentiation into T and B cells, were transduced with Apro-hART or GFP lentivirus and cultured on OP9 cells or injected into irradiated newborn NSG mice. OP9 cocultures and blood and spleen cells from NSG mice showed that Apro-hART-corrected, but not GFP-transduced, ART-SCID CD34+ cells differentiated into B cells and T and B cells, respectively, as did GFP-transduced control CD34+ cells. Lymphocyte maturation was proven by lineage specific surface markers and measures of V(D)J diversity and recombination, T cell receptor Vbeta spectratyping and Kappa chain receptor excision circles (KRECs), respectively. View Large Image | Download PowerPoint SlideColony forming assays with transduced cells revealed transduction efficiency of 28.6%, a mean vector copy number of 3/cell and a diverse profile of lentivirus integration sites.This successful gene correction and restoration of Artemis function in fibroblasts and HSCs from ART-SCID patients supports institution of a clinical trial of gene addition therapy for ART-SCID.

442. Decreased Survival and Engraftment in Adult ADA-Deficient Mice Co-Transplanted With Uncorrected Lineage-Positive Bone Marrow Cells Combined With Gene-Corrected Lineage-Negative Bone Marrow Cells

ADA-deficiency primarily results in a severe combined immunodeficiency (SCID) but because it is a disorder of the purine salvage pathway the accumulation adenosine and deoxy-adenosine also causes skeletal, neurologic, hepatic and pulmonary abnormalities. Autologous transplantation of retroviral vector-mediated gene-corrected hematopoietic cells after non-myeloablative conditioning for the treatment of ADA-deficient SCID has been shown to be safe and efficacious, especially in very young children, with more variable results in older children. Previously, we have shown that ADA-deficient (ADA-/-) mice transplanted with gene-corrected whole marrow or ADA+/+ marrow had significantly higher engraftment of gene-corrected cells in the bone marrow, liver, and lung compared to transplantation with gene-corrected lineage negative (Lin-) bone marrow cells. The improved engraftment with whole marrow might be due the presence of specific cell populations and/or factors that could facilitate engraftment, including the additional ADA expressed from terminally differentiated cells in the niche during the engraftment period. In new experiments (n=3) we compared outcomes in ADA-/- mice transplanted with gene-corrected Lin- cells (5×10^5) alone (n=10) to these end-points in ADA-/- mice transplanted with gene-corrected Lin- cells combined with either gene-corrected lineage positive (Lin+) cells (5×10^6)(n=8), or with uncorrected Lin+ cells (5×10^6)(n= 10). Marrow cells were fractionated by immunomagnetic sorting and transduced with the lentiviral vector, EFS-ADA at 3 × 10^7 TU/ml (Lin- MOI 60, Lin+ MOI 6) for 24 hours and transplanted into adult (16 weeks) ADA-/- recipients conditioned with 500 cGy and maintained on enzyme replacement for one month after transplant. Unexpectedly, survival to day 100 was significantly reduced in ADA-/- mice transplanted with gene-corrected Lin- cells combined with uncorrected Lin+ cells (37%) compared to mice transplanted with either gene-corrected Lin- cells alone (90%) or with gene corrected Lin- cells co-transplanted with gene-corrected Lin+ (88%)(p<0.0001). Overall engraftment and VCN were measured in the thymus, spleen, bone marrow, lung and liver and neither was significantly improved with the addition of gene-corrected Lin+ cells when compared to transplant of gene-corrected Lin- cells alone. Even with so few ADA-/- mice transplanted with Lin- and uncorrected Lin+ cells surviving (n=3), overall engraftment in spleen was significantly lower (8%) when compared to ADA-/- co-transplanted with gene-corrected Lin+ cells (66%)(p<0.001). Likewise, these mice also had fewer splenic T and B cells numbers when compared to mice co-transplanted with gene-corrected Lin+ cells (p<0.05). Taken together with the very poor survival, we hypothesize the uncorrected Lin+ cells may place an extra burden systemically or within the bone marrow niche to decrease engraftment and survival.

30. Phase II Clinical Trial of Gene Therapy for Adenosine Deaminase-Deficient Severe Combined Immune Deficiency (ADA-SCID) Using a γ-Retroviral Vector

We report follow-up of subjects treated in a Phase II study of gene therapy for ADA-SCID. Between 2009 and 2012, ten ADA-deficient SCID patients were treated by γ-retroviral-mediated gene transfer (MND-ADA) to their bone marrow CD34+ cells. The subjects were given non-myeloablative chemotherapy (busulfan @ 90 mg/m2) and were withdrawn from PEG-ADA enzyme replacement therapy (ERT) prior to infusion of autologous gene-modified cells. Subject age at the time of treatment ranged from 3 months to 15 years (median = 11.5 months). Follow-up times range from 2 to 5 years. All but one subject, who was 15-years old at the time of treatment, remain off PEG-ADA ERT with immune reconstitution that reached maximal level between 6 and 12 months after transplant and was maintained thereafter. Vector marking in peripheral blood cells remained consistently detectable (> 0.1 copy/PBMC and ≥ 0.003 copy/granulocyte) at 2 years and later after transplant in subjects who discontinued ERT. These subjects also had PBMC ADA enzymatic activity in the normal range and red blood cell deoxynucleotide levels below 10%. Three subjects have discontinued intravenous immunoglobulin; five subjects have discontinued prophylactic antibiotics. All subjects have polyclonal gene marking with no sign of lymphoproliferative disease. The subjects remain in good health without infections or other complications.

B-cell development and functions and therapeutic options in adenosine deaminase–deficient patients

Adenosine deaminase (ADA) deficiency causes severe cellular and humoral immune defects and dysregulation because of metabolic toxicity. Alterations in B-cell development and function have been poorly studied. Enzyme replacement therapy (ERT) and hematopoietic stem cell (HSC) gene therapy (GT) are therapeutic options for patients lacking a suitable bone marrow (BM) transplant donor. We sought to study alterations in B-cell development in ADA-deficient patients and investigate the ability of ERT and HSC-GT to restore normal B-cell differentiation and function. Flow cytometry was used to characterize B-cell development in BM and the periphery. The percentage of gene-corrected B cells was measured by using quantitative PCR. B cells were assessed for their capacity to proliferate and release IgM after stimulation. Despite the severe peripheral B-cell lymphopenia, patients with ADA-deficient severe combined immunodeficiency showed a partial block in central BM development. Treatment with ERT or HSC-GT reverted most BM alterations, but ERT led to immature B-cell expansion. In the periphery transitional B cells accumulated under ERT, and the defect in maturation persisted long-term. HSC-GT led to a progressive improvement in B-cell numbers and development, along with increased levels of gene correction. The strongest selective advantage for ADA-transduced cells occurred at the transition from immature to naive cells. B-cell proliferative responses and differentiation to immunoglobulin secreting IgM after B-cell receptor and Toll-like receptor triggering were severely impaired after ERT and improved significantly after HSC-GT. ADA-deficient patients show specific defects in B-cell development and functions that are differently corrected after ERT and HSC-GT.

Gene therapy for adenosine deaminase–deficient severe combined immune deficiency: clinical comparison of retroviral vectors and treatment plans

AbstractWe conducted a gene therapy trial in 10 patients with adenosine deaminase (ADA)–deficient severe combined immunodeficiency using 2 slightly different retroviral vectors for the transduction of patients' bone marrow CD34+ cells. Four subjects were treated without pretransplantation cytoreduction and remained on ADA enzyme-replacement therapy (ERT) throughout the procedure. Only transient (months), low-level (< 0.01%) gene marking was observed in PBMCs of 2 older subjects (15 and 20 years of age), whereas some gene marking of PBMC has persisted for the past 9 years in 2 younger subjects (4 and 6 years). Six additional subjects were treated using the same gene transfer protocol, but after withdrawal of ERT and administration of low-dose busulfan (65-90 mg/m2). Three of these remain well, off ERT (5, 4, and 3 years postprocedure), with gene marking in PBMC of 1%-10%, and ADA enzyme expression in PBMC near or in the normal range. Two subjects were restarted on ERT because of poor gene marking and immune recovery, and one had a subsequent allogeneic hematopoietic stem cell transplantation. These studies directly demonstrate the importance of providing nonmyeloablative pretransplantation conditioning to achieve therapeutic benefits with gene therapy for ADA-deficient severe combined immunodeficiency.

Biosafety Assessment of Site-directed Transgene Integration in Human Umbilical Cord–lining Cells

Biosafety Assessment of Site-directed Transgene Integration in Human Umbilical Cord–lining Cells

New insights into the pathogenesis of adenosine deaminase-severe combined immunodeficiency and progress in gene therapy

Adenosine deaminase (ADA)- deficient severe combined immunodeficiency (SCID) is a complex metabolic and immunological disorder, characterized by a severe immunodeficiency due to the accumulation of purine metabolites in plasma and cells. This review summarizes recent findings on the pathogenesis of immunological and nonimmunological defects in ADA deficiency and the successful outcome of gene therapy trials for this condition. Recent reports show that ADA-SCID is associated with an increased frequency of autoimmune manifestations and high risk of central nervous system (CNS) complications even after bone marrow transplantation. It remains unclear to what extent infection-related or disease-specific factors correlate with this divergent outcome.Recent trials represented the first demonstration of long-term clinical efficacy of HSC gene therapy for ADA-SCID, underlining that gene therapy has a favorable safety profile and is effective in restoring normal purine metabolism and immune functions. Molecular studies showed that the retroviral integration profile after successful gene therapy did not cause selection or expansion of malignant cell clones in vivo. Gene therapy for ADA-deficient SCID is an effective treatment, providing long-term clinical benefit for affected patients. Future research will be needed to address the occurrence of autoimmune manifestations and nonimmunological defects in order to improve patients' long-term prospects.

Retroviral vectors encoding ADA regulatory locus control region provide enhanced T-cell-specific transgene expression

BackgroundMurine retroviral vectors have been used in several hundred gene therapy clinical trials, but have fallen out of favor for a number of reasons. One issue is that gene expression from viral or internal promoters is highly variable and essentially unregulated. Moreover, with retroviral vectors, gene expression is usually silenced over time. Mammalian genes, in contrast, are characterized by highly regulated, precise levels of expression in both a temporal and a cell-specific manner. To ascertain if recapitulation of endogenous adenosine deaminase (ADA) expression can be achieved in a vector construct we created a new series of Moloney murine leukemia virus (MuLV) based retroviral vector that carry human regulatory elements including combinations of the ADA promoter, the ADA locus control region (LCR), ADA introns and human polyadenylation sequences in a self-inactivating vector backbone.MethodsA MuLV-based retroviral vector with a self-inactivating (SIN) backbone, the phosphoglycerate kinase promoter (PGK) and the enhanced green fluorescent protein (eGFP), as a reporter gene, was generated. Subsequent vectors were constructed from this basic vector by deletion or addition of certain elements. The added elements that were assessed are the human ADA promoter, human ADA locus control region (LCR), introns 7, 8, and 11 from the human ADA gene, and human growth hormone polyadenylation signal. Retroviral vector particles were produced by transient three-plasmid transfection of 293T cells. Retroviral vectors encoding eGFP were titered by transducing 293A cells, and then the proportion of GFP-positive cells was determined using fluorescence-activated cell sorting (FACS). Non T-cell and T-cell lines were transduced at a multiplicity of infection (MOI) of 0.1 and the yield of eGFP transgene expression was evaluated by FACS analysis using mean fluorescent intensity (MFI) detection.ResultsVectors that contained the ADA LCR were preferentially expressed in T-cell lines. Further improvements in T-cell specific gene expression were observed with the incorporation of additional cis-regulatory elements, such as a human polyadenylation signal and intron 7 from the human ADA gene.ConclusionThese studies suggest that the combination of an authentically regulated ADA gene in a murine retroviral vector, together with additional locus-specific regulatory refinements, will yield a vector with a safer profile and greater efficacy in terms of high-level, therapeutic, regulated gene expression for the treatment of ADA-deficient severe combined immunodeficiency.

ADA-deficient SCID is associated with a specific microenvironment and bone phenotype characterized by RANKL/OPG imbalance and osteoblast insufficiency

AbstractAdenosine deaminase (ADA) deficiency is a disorder of the purine metabolism leading to combined immunodeficiency and systemic alterations, including skeletal abnormalities. We report that ADA deficiency in mice causes a specific bone phenotype characterized by alterations of structural properties and impaired mechanical competence. These alterations are the combined result of an imbalanced receptor activator of nuclear factor-κB ligand (RANKL)/osteoprotegerin axis, causing decreased osteoclastogenesis and an intrinsic defect of osteoblast function with subsequent low bone formation. In vitro, osteoblasts lacking ADA displayed an altered transcriptional profile and growth reduction. Furthermore, the bone marrow microenvironment of ADA-deficient mice showed a reduced capacity to support in vitro and in vivo hematopoiesis. Treatment of ADA-deficient neonatal mice with enzyme replacement therapy, bone marrow transplantation, or gene therapy resulted in full recovery of the altered bone parameters. Remarkably, untreated ADA–severe combined immunodeficiency patients showed a similar imbalance in RANKL/osteoprotegerin levels alongside severe growth retardation. Gene therapy with ADA-transduced hematopoietic stem cells increased serum RANKL levels and children's growth. Our results indicate that the ADA metabolism represents a crucial modulatory factor of bone cell activities and remodeling. The trials were registered at www.clinicaltrials.gov as #NCT00598481 and #NCT00599781.

Gene therapy for childhood immunological diseases

Gene therapy using autologous hematopoietic stem cells (HSC) that are corrected with the normal gene may have a beneficial effect on blood cell production or function, without the immunologic complications of allogeneic HSC transplantation. Childhood immunological diseases are highly favorable candidates for responses to gene therapy using HSC. Hemoglobinopathies, lysosomal and metabolic disorders and defects of hematopoietic stem and progenitor cells should also be ameliorated by gene therapy using autologous HSC. At present, gene therapy has been beneficial for patients with XSCID, ADA-deficient SCID and chronic granulomatous disease. The principle that partial marrow conditioning increases engraftment of gene-corrected HSC has been demonstrated. Clinical trials are being developed in Europe and the United States to treat several other genetic blood cell disorders. This progress is tempered by the serious complication observed in XSCID patients developing T lymphoproliferative disease. New methods for gene transfer (lentiviral and foamy viral vectors, semi-viral systems and gene correction) may retain or further increase the efficacy and decrease the risks from gene therapy using HSC. Ultimately, the relative benefits and risks of autologous gene therapy will be weighed against other available options (for example, allogeneic HSCT) to determine the treatment of choice.

In Vivo Transduction by Intravenous Injection of a Lentiviral Vector Expressing Human ADA into Neonatal ADA Gene Knockout Mice: A Novel Form of Enzyme Replacement Therapy for ADA Deficiency

Using a mouse model of adenosine deaminase–deficient severe combined immune deficiency syndrome (ADA-deficient SCID), we have developed a noninvasive method of gene transfer for the sustained systemic expression of human ADA as enzyme replacement therapy. The method of delivery is a human immunodeficiency virus 1–based lentiviral vector given systemically by intravenous injection on day 1 to 2 of life. In this article we characterize the biodistribution of the integrated vector, the expression levels of ADA enzyme activity in various tissues, as well as the efficacy of systemic ADA expression to correct the ADA-deficient phenotype in this mouse model. The long-term expression of enzymatically active ADA achieved by this method, primarily from transduction of liver and lung, restored immunologic function and significantly extended survival. These studies illustrate the potential for sustained in vivo production of enzymatically active ADA, as an alternative to therapy by frequent injection of exogenous ADA protein.

1089. Neonatal BMT of ADA-Deficient SCID Mice Results in Phenotypic Correction Despite Low Levels of Engraftment and Absence of the Selective T Lymphoid Expansion Seen in γc Gene Knock-Out Mice

ADA deficiency in humans results in a SCID phenotype, characterized by extremely low levels of T, B and NK cells, and other organ systems also may be affected (skeletal, hepatic and CNS). In mice, however, knockout of the ADA gene is lethal because ADA is required for gestation. A viable murine model of ADA- deficient SCID was developed in which the ADA-deficient embryos are rescued with an ADA mini-gene expressed only during gestation. Post-natally, these mice die from a non-infectious pulmonary insufficiency by day 19-20, which can be rescued using enzyme replacement therapy (ERT) with PEG-ADA.

Cerebral Lymphoma in an Adenosine Deaminase–Deficient Patient With Severe Combined Immunodeficiency Receiving Polyethylene Glycol–Conjugated Adenosine Deaminase

Polyethylene glycol-conjugated adenosine deaminase (PEG-ADA) provides an alternate therapy to mismatched stem cell transplantation for patients with ADA-deficient severe combined immunodeficiency. Although replacement therapy with PEG-ADA is effective in preventing infections, immune function does not return to normal, and most patients remain lymphopenic. Information is limited regarding the prognosis of patients on long-term ADA-replacement therapy. Here we present a case of a 10-year-old child who was diagnosed with ADA-severe combined immunodeficiency at 4 weeks of age after contracting pneumonia. Treatment with PEG-ADA was begun, the biochemical markers of ADA deficiency normalized, and his clinical progress was very good without significant infections. At 10 years of age, after presenting with headaches and cranial nerve deficits, he was diagnosed with Epstein-Barr virus-positive malignant brain lymphoma. It did not respond to various regimens of aggressive chemotherapy, and the patient expired 5 months later. We speculate that in this patient the immunologic surveillance by T cells may have been defective with respect to elimination of Epstein-Barr virus-infected cells, hence the formation of neoplasm. The possible mechanisms underlying such pathology are reviewed.