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Hypomorphic mutations in genes associated with severe combined immunodeficiency (SCID) or Omenn syndrome can also cause milder immunodeficiencies. We report 10 new patients with such "atypical" SCID and summarize 63 patients from the literature. The patient groups with T(low)B(low) (n=28), T(low)B(+) (n=16) and ADA (n=29) SCID variants had similar infection profiles but differed in the frequency of immune dysregulation, which was observed predominantly in patients with recombination defects. Most immunological parameters were remarkably similar in the three groups. Of note, 19/68 patients with "atypical" SCID had normal T cell counts, 48/68 had normal IgG and 23/46 had at least one normal specific antibody titer. Elevated IgE was a characteristic feature of ADA deficiency. This overview characterizes "atypical" SCID as a distinct disease with immune dysregulation in addition to infection susceptibility. Lymphopenia, reduced naïve T cells and elevated IgE are suggestive, but not consistent features of the disease., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Introduction: We describe a girl presenting at age 6 years with a history of chronic ulcerating intestinal inflammation since 9 months of age. She exhibited a severe, steroid-dependent clinical course of intestinal inflammation over several years in the absence of serious infections., Results and Discussion: Immunodeficiency was first considered at 6 years of age due to chronic lymphopenia. Immunophenotyping revealed low B and T cell counts with few naïve T cells, a skewed TCR repertoire, and TCR gamma/delta T cell predominance, suggesting a defect of lymphocyte development. Genetic and functional analyses identified a hypomorphic mutation in the DCLRE1C (ARTEMIS) gene compromising V(D)J recombination efficiency, but allowing residual T and B cell development. Hematopoetic stem cell transplantation reconstituted the lymphocyte compartment and cured the inflammatory bowel disease., Conclusion: This report illustrates that a genetic disorder of lymphocyte development can present with chronic inflammatory bowel disease as the dominant phenotype in the absence of severe infection susceptibility.

X-linked severe combined immunodeficiency is a life-threatening disorder caused by mutations in the gene encoding the interleukin-2 receptor gamma chain (IL2RG). Hypomorphic mutations and reversion of mutations in subpopulations of cells can result in variant clinical phenotypes, making diagnosis and treatment difficult. We describe a 5-year-old boy with mild susceptibility to infection who was investigated for a mutation in IL2RG due to persistent natural killer (NK)- and T-cell lymphopenia. A functionally relevant novel T466C point mutation was found in B, NK, and epithelial cells, whereas alpha/beta and gamma/delta T cells showed the normal gene sequence, suggesting reversion of the mutation in a common T-cell precursor. This genetic correction in T cells resulted in a diverse T-cell repertoire and significant immunity despite failure to produce specific antibodies linked to an intrinsic defect of mutant B cells. These observations confirm the potential of revertant T-cell precursors to reconstitute immune function, but questions remain on the longevity of revertant cells implicating the need for careful follow up and early consideration of hematopoietic stem cell transplantation (HSCT).

Griscelli syndrome (GS) was diagnosed in a 2-year-old patient with oculocutaneous albinism and immunodeficiency, but sequencing of RAB27a revealed only a heterozygous mutation. Due to impaired natural killer (NK) and T-cell cytotoxicity implying a high risk of developing hemophagocytic lymphohistiocytosis (HLH), he was prepared for hematopoietic stem cell transplantation (HSCT). Unexpectedly, a severe bleeding episode occurred that led to the demonstration of disturbed platelet aggregation, reduced plateletdense granules, and impaired platelet degranulation. In combination with neutropenia, this suggested the diagnosis of Hermansky-Pudlak syndrome type II (HPSII) and a novel homozygous mutation in AP3B1 was detected. None of the 3 reported HPSII patients had developed HLH, and our patient seroconverted to Epstein-Barr virus (EBV) without clinical symptoms. HSCT was therefore withheld, and granulocyte-colony-stimulating factor (G-CSF) therapy was initiated and prevented further bacterial infections. At 3 years of age, however, the patient developed, without an obvious trigger, fulminant HLH that was resistant to therapy. This patient shows that careful clinical and molecular diagnosis is essential to differentiate the complex disorders of lysosomal trafficking. HPSII belongs to the group of familial hemophagocytic syndromes and may represent an indication for HSCT.

A clinical trial of retroviral-mediated transfer of the adenosine deaminase (ADA) gene into umbilical cord blood CD34(+) cells was started in 1993. ADA-containing peripheral blood mononuclear cells (PBMCs) have persisted in patients from this trial, with T lymphocytes showing the highest prevalence of gene marking. To gain a greater understanding of the nature and number of the transduced cells that were engrafted, we used linear amplification-mediated PCR (LAM-PCR) to identify clonal vector proviral integrants. In one patient, a single vector integrant was predominant in T lymphocytes at a stable level over most of the eight-year time span analyzed and was also detected in some myeloid samples. T-cell clones with the predominant integrant, isolated after eight years, showed multiple patterns of T-cell receptor (TCR) gene rearrangement, indicating that a single pre-thymic stem or progenitor cell served as the source of the majority of the gene-marked cells over an extended period of time. It is important to distinguish the stable pattern of monoclonal gene marking that we observed here from the progressive increase of a T-cell clone with monoclonal gene marking that results from leukemic transformation, as observed in two subjects in a clinical trial of gene therapy for X-linked severe combined immunodeficiency (SCID).

The semirandom location of retroviral integration in the target cell genome introduces a marker in the form of a fusion sequence composed of a genomic and a proviral part that is unique for each transduced cell and its clonal progeny. High-sensitivity detection of these fusion sequences would allow the tracking of clonal contributions of individual, marked hematopoietic progenitor, and stem cells in vivo. Clone detection by Southern blot has helped to analyze models of oligoclonal repopulation but is limited in sensitivity and specificity. Inverse PCR (Nolta et al., Proc. Natl. Acad. Sci. USA 93: 2414-2419) can demonstrate the clonal identity by sequencing but does not permit simultaneous detection of multiple clones. In an efficiently transduced rhesus macaque model (Tisdale et al., Blood 92: 2681-2687; Wu et al., Mol. Ther. 1: 285-293) Kim et al. (Blood 96: 1-8) have identified more than 40 insertion sequences from marrow CFU by inverse PCR. However, no previous study has been able to directly analyze the number of clones active in vivo. Here we demonstrate that the application of a recently developed PCR technology allows the simultaneous visualization of multiple integration sites from small clonal contributions to hematopoietic cells. By combining solid-phase primer extension with ligation-mediated PCR, direct genomic sequencing of retroviral integration sites was obtained in murine bone marrow samples. Further development of this technology will allow analysis of the clonal composition of marked hematopoiesis in small and large animals as well as in human gene transfer.