Gene therapy, combined with reduced-intensity conditioning, is a safe and effective treatment for SCID in patients with ADA deficiency. (ClinicalTrials.gov numbers, NCT00598481 and NCT00599781.)

Hematopoietic stem cell (HSC) gene therapy for adenosine deaminase (ADA)-deficient severe combined immunodeficiency (SCID) has shown limited clinical efficacy because of the small proportion of engrafted genetically corrected HSCs. We describe an improved protocol for gene transfer into HSCs associated with nonmyeloablative conditioning. This protocol was used in two patients for whom enzyme replacement therapy was not available, which allowed the effect of gene therapy alone to be evaluated. Sustained engraftment of engineered HSCs with differentiation into multiple lineages resulted in increased lymphocyte counts, improved immune functions (including antigen-specific responses), and lower toxic metabolites. Both patients are currently at home and clinically well, with normal growth and development. These results indicate the safety and efficacy of HSC gene therapy combined with nonmyeloablative conditioning for the treatment of SCID.

Opitz syndrome (OS) is an inherited disorder characterized by midline defects including hypertelorism, hypospadias, lip-palate-laryngotracheal clefts and imperforate anus. We have identified a new gene on Xp22, MiD1 (Midline 1), which is disrupted in an OS patient carrying an X-chromosome inversion and is also mutated in several OS families.MIDI encodes a member of the B-box family of proteins, which contain protein-protein interaction domains, including a RING finger, and are implicated in fundamental processes such as body axis patterning and control of cell proliferation. The association of MIDI with OS suggests an important role for this gene in midline development

Gene transfer into HSCs is an effective treatment for SCID, although potentially limited by the risk of insertional mutagenesis. We performed a genome-wide analysis of retroviral vector integrations in genetically corrected HSCs and their multilineage progeny before and up to 47 months after transplantation into 5 patients with adenosine deaminase-deficient SCID. Gene-dense regions, promoters, and transcriptionally active genes were preferred retroviral integrations sites (RISs) both in preinfusion transduced CD34 + cells and in vivo after gene therapy. The occurrence of insertion sites proximal to protooncogenes or genes controlling cell growth and self renewal, including LMO2, was not associated with clonal selection or expansion in vivo. Clonal analysis of long-term repopulating cell progeny in vivo revealed highly polyclonal T cell populations and shared RISs among multiple lineages, demonstrating the engraftment of multipotent HSCs. These data have important implications for the biology of retroviral vectors, the dynamics of genetically modified HSCs, and the safety of gene therapy.

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IntroductionX-linked lymphoproliferative (XLP) disease is a severe immunodeficiency characterized by variable clinical phenotypes, including fatal infectious mononucleosis, malignant B-cell lymphoma, and progressive dysgammaglobulinemia. 1 The gene responsible for XLP (SH2D1A) encodes for SAP (SLAM [signaling lymphocyteactivation molecule]-associated protein), or SH2D1A, a natural killer (NK) and T-cell-specific signaling adaptor protein. [2][3][4] Mutations in the SH2D1A gene have been detected in patients with hemophagocytic lymphohistiocytosis (HLH) 5 and common variable immunodeficiency syndrome, 6,7 thereby expanding the range of clinical phenotypes associated with this genetic defect. These heterogeneous clinical manifestations might be related to viral infections, which appear to act as secondary factors triggering the severe forms of SAP-deficiency syndrome. In the absence of SAP, dysregulation of T/B-cell interactions and NK-cell functions has been proposed to result in a variable ability to control Epstein-Barr virus (EBV) infection, leading to either fatal infectious mononucleosis, or infectious mononucleosis with progression toward malignant B-cell lymphoma or acquired agammaglobulinemia. 8 However, in some patients with XLP, these manifestations can also occur in the absence of EBV infection. 9 SAP is a protein of 128 residues with a single Src homology 2 (SH2) domain binding to a consensus T.I/V.(p)Y.x.x.V/I motif on the cytoplasmic tail of surface molecules of the SLAM family, including SLAM, 2B4, CD84, Ly-9, and NTB-A. 2,10-13 These molecules form homotypic and heterotypic receptor-ligand pairs during T cell/antigen-presenting cell or NK cell/target cell contacts. SLAM is expressed on activated and memory T cells, in which it can skew cytokine production toward a T helper 1 (Th1) profile 14 and can act as a costimulatory molecule for cytotoxic activity. 15 SAP appears to be a natural inhibitor of the interaction of SLAM-family members with SH2-containing signal molecules such as the SH2-containing phosphatase 2 (SHP-2) phosphatase. 2 In T cells, SAP recruits FynT, which is required for SLAM phosphorylation. Activation of the SLAM/SAP pathway controls the phosphorylation of dok1, dok2, and SH2-containing inositol phosphatase, thereby playing a potential role in the control of interferon-␥ (IFN-␥) production. 16,17 Furthermore, SAP appears to participate in the adhesion of T cells to target cells, as well as in T-cell receptor (TCR)-mediated activation. 18,19 In SAP-deficient murine models, 20,21 infection with lymphocytic-chorio-meningitis virus is associated with higher mortality related to increased T-cell activation and IFN-␥ production, as well as to a reduction in the production of interleukin 4 (IL-4) and the generation of immunoglobulin-secreting cells. However, in this model, T-and NK-cell cytotoxic activities have not been investigated.A number of studies in patients with XLP indicate that 2B4 stimulation on NK cells induces an abnormal dominant abrogation of NK-cell lytic activity. This ...

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