Mosaicism of NK cells in a patient with Wiskott-Aldrich syndrome

A single-cell atlas of immunocytes in the spleen of a mouse model of Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is a disorder characterized by rare X-linked genetic immune deficiency with mutations in the Was gene, which is specifically expressed in hematopoietic cells. The spleen plays a major role in hematopoiesis and red blood cell clearance. However, to date, comprehensive analyses of the spleen in wild-type (WT) and WASp-deficient (WAS-KO) mice, especially at the transcriptome level, have not been reported. In this study, single-cell RNA sequencing (scRNA-seq) was adopted to identify various types of immune cells and investigate the mechanisms underlying immune deficiency. We identified 30 clusters and 10 major cell subtypes among 11,269 cells; these cell types included B cells, T cells, dendritic cells (DCs), natural killer (NK) cells, monocytes, macrophages, granulocytes, stem cells and erythrocytes. Moreover, we evaluated gene expression differences among cell subtypes, identified differentially expressed genes (DEGs), and performed enrichment analyses to identify the reasons for the dysfunction in these different cell populations in WAS. Furthermore, some key genes were identified based on a comparison of the DEGs in each cell type involved in specific and nonspecific immune responses, and further analysis showed that these key genes were previously undiscovered pathology-related genes in WAS-KO mice. In summary, we present a landscape of immune cells in the spleen of WAS-KO mice based on detailed data obtained at single-cell resolution. These unprecedented data revealed the transcriptional characteristics of specific and nonspecific immune cells, and the key genes were identified, laying a foundation for future studies of WAS, especially studies into novel and underexplored mechanisms that may improve gene therapies for WAS.

Understanding immunoactinopathies: A decade of research on WAS gene defects

Immunoactinopathies caused by mutations in actin-related proteins are a growing group of inborn errors of immunity (IEI). Immunoactinopathies are caused by a dysregulated actin cytoskeleton and affect hematopoietic cells especially because of their unique capacity to survey the body for invading pathogens and altered self, such as cancer cells. These cell motility and cell-to-cell interaction properties depend on the dynamic nature of the actin cytoskeleton. Wiskott-Aldrich syndrome (WAS) is the archetypical immunoactinopathy and the first described. WAS is caused by loss-of-function and gain-of-function mutations in the actin regulator WASp, uniquely expressed in hematopoietic cells. Mutations in WAS cause a profound disturbance of actin cytoskeleton regulation of hematopoietic cells. Studies during the last 10 years have shed light on the specific effects on different hematopoietic cells, revealing that they are not affected equally by mutations in the WAS gene. Moreover, the mechanistic understanding of how WASp controls nuclear and cytoplasmatic activities may help to find therapeutic alternatives according to the site of the mutation and clinical phenotypes. In this review, we summarize recent findings that have added to the complexity and increased our understanding of WAS-related diseases and immunoactinopathies.

Reversion Mosaicism in Primary Immunodeficiency Diseases

Reversion mosaicism has been reported in an increasing number of genetic disorders including primary immunodeficiency diseases. Several mechanisms can mediate somatic reversion of inherited mutations. Back mutations restore wild-type sequences, whereas second-site mutations result in compensatory changes. In addition, intragenic recombination, chromosomal deletions, and copy-neutral loss of heterozygosity have been demonstrated in mosaic individuals. Revertant cells that have regained wild-type function may be associated with milder disease phenotypes in some immunodeficient patients with reversion mosaicism. Revertant cells can also be responsible for immune dysregulation. Studies identifying a large variety of genetic changes in the same individual further support a frequent occurrence of reversion mosaicism in primary immunodeficiency diseases. This phenomenon also provides unique opportunities to evaluate the biological effects of restored gene expression in different cell lineages. In this paper, we review the recent findings of reversion mosaicism in primary immunodeficiency diseases and discuss its clinical implications.

Diagnostic interpretation of genetic studies in patients with primary immunodeficiency diseases: A working group report of the Primary Immunodeficiency Diseases Committee of the American Academy of Allergy, Asthma & Immunology

Genetic testing has become an integral component of the diagnostic evaluation of patients with suspected primary immunodeficiency diseases. Results of genetic testing can have a profound effect on clinical management decisions. Therefore clinical providers must demonstrate proficiency in interpreting genetic data. Because of the need for increased knowledge regarding this practice, the American Academy of Allergy, Asthma & Immunology Primary Immunodeficiency Diseases Committee established a work group that reviewed and summarized information concerning appropriate methods, tools, and resources for evaluating variants identified by genetic testing. Strengths and limitations of tests frequently ordered by clinicians were examined. Summary statements and tables were then developed to guide the interpretation process. Finally, the need for research and collaboration was emphasized. Greater understanding of these important concepts will improve the diagnosis and management of patients with suspected primary immunodeficiency diseases.

Dysfunction of immune system in the development of large granular lymphocyte leukemia

OBJECTIVES

Large granular lymphocyte (LGL) leukemia is a rare type of lymphoproliferative disease caused by clonal antigenic stimulation of T cells and natural killer (NK) cells.

METHODS

In this review, we focus on the current knowledge of the immunological dysfunctions associated with LGL leukemia and the associated disorders coexistent with this disease. Novel therapeutic options targeting known molecular mechanisms are also discussed.

RESULTS AND DISCUSSION

The pathogenesis of LGL leukemia involves the accumulation of gene mutations, dysregulated signaling pathways and immunological dysfunction. Mounting evidence indicated that dysregulated survival signaling pathways may be responsible for the immunological dysfunction in LGL leukemia including decreased numbers of neutrophils, dysregulated signal transduction of NK cells, abnormal B-cells, aberrant CD8+ T cells, as well as autoimmune and hematological abnormalities.

CONCLUSION

A better understanding of the immune dysregulation triggered by LGL leukemia will be beneficial to explore the pathogenesis and potential therapeutic targets for this disease.

Clinical Manifestations and Pathophysiological Mechanisms of the Wiskott-Aldrich Syndrome

The Wiskott-Aldrich syndrome (WAS) is a rare X-linked disorder originally described by Dr. Alfred Wiskott in 1937 and Dr. Robert Aldrich in 1954 as a familial disease characterized by infections, bleeding tendency, and eczema. Today, it is well recognized that the syndrome has a wide clinical spectrum ranging from mild, isolated thrombocytopenia to full-blown presentation that can be complicated by life-threatening hemorrhages, immunodeficiency, atopy, autoimmunity, and cancer. The pathophysiology of classic and emerging features is being elucidated by clinical studies, but remains incompletely defined, which hinders the application of targeted therapies. At the same time, progress of hematopoietic stem cell transplantation and gene therapy offer optimistic prospects for treatment options aimed at the replacement of the defective lymphohematopoietic system that have the potential to provide a cure for this rare and polymorphic disease.

Clinical Manifestations and Pathophysiological Mechanisms of the Wiskott-Aldrich Syndrome

The Wiskott-Aldrich syndrome (WAS) is a rare X-linked disorder originally described by Dr. Alfred Wiskott in 1937 and Dr. Robert Aldrich in 1954 as a familial disease characterized by infections, bleeding tendency, and eczema. Today, it is well recognized that the syndrome has a wide clinical spectrum ranging from mild, isolated thrombocytopenia to full-blown presentation that can be complicated by life-threatening hemorrhages, immunodeficiency, atopy, autoimmunity, and cancer. The pathophysiology of classic and emerging features is being elucidated by clinical studies, but remains incompletely defined, which hinders the application of targeted therapies. At the same time, progress of hematopoietic stem cell transplantation and gene therapy offer optimistic prospects for treatment options aimed at the replacement of the defective lymphohematopoietic system that have the potential to provide a cure for this rare and polymorphic disease.

A reversion of an IL2RG mutation in combined immunodeficiency providing competitive advantage to the majority of CD8+ T cells

Mutations in the common gamma chain (γc, CD132, encoded by the IL2RG gene) can lead to B(+)T(-)NK(-) X-linked severe combined immunodeficiency, as a consequence of unresponsiveness to γc-cytokines such as interleukins-2, -7 and -15. Hypomorphic mutations in CD132 may cause combined immunodeficiencies with a variety of clinical presentations. We analyzed peripheral blood mononuclear cells of a 6-year-old boy with normal lymphocyte counts, who suffered from recurrent pneumonia and disseminated mollusca contagiosa. Since proliferative responses of T cells and NK cells to γc -cytokines were severely impaired, we performed IL2RG gene analysis, showing a heterozygous mutation in the presence of a single X-chromosome. Interestingly, an IL2RG reversion to normal predominated in both naïve and antigen-primed CD8(+) T cells and increased over time. Only the revertant CD8(+) T cells showed normal expression of CD132 and the various CD8(+) T cell populations had a different T-cell receptor repertoire. Finally, a fraction of γδ(+) T cells and differentiated CD4(+)CD27(-) effector-memory T cells carried the reversion, whereas NK or B cells were repeatedly negative. In conclusion, in a patient with a novel IL2RG mutation, gene-reverted CD8(+) T cells accumulated over time. Our data indicate that selective outgrowth of particular T-cell subsets may occur following reversion at the level of committed T progenitor cells.

Induced pluripotent stem cells from CINCA syndrome patients as a model for dissecting somatic mosaicism and drug discovery

Chronic infantile neurologic cutaneous and articular (CINCA) syndrome is an IL-1–driven autoinflammatory disorder caused mainly by NLRP3 mutations. The pathogenesis of CINCA syndrome patients who carry NLRP3 mutations as somatic mosaicism has not been precisely described because of the difficulty in separating individual cells based on the presence or absence of the mutation. Here we report the generation of NLRP3-mutant and nonmutant-induced pluripotent stem cell (iPSC) lines from 2 CINCA syndrome patients with somatic mosaicism, and describe their differentiation into macrophages (iPS-MPs). We found that mutant cells are predominantly responsible for the pathogenesis in these mosaic patients because only mutant iPS-MPs showed the disease relevant phenotype of abnormal IL-1β secretion. We also confirmed that the existing anti-inflammatory compounds inhibited the abnormal IL-1β secretion, indicating that mutant iPS-MPs are applicable for drug screening for CINCA syndrome and other NLRP3-related inflammatory conditions. Our results illustrate that patient-derived iPSCs are useful for dissecting somatic mosaicism and that NLRP3-mutant iPSCs can provide a valuable platform for drug discovery for multiple NLRP3-related disorders.

WIP remodeling actin behind the scenes: how WIP reshapes immune and other functions

Actin polymerization is a fundamental cellular process regulating immune cell functions and the immune response. The Wiskott-Aldrich syndrome protein (WASp) is an actin nucleation promoting factor, which is exclusively expressed in hematopoietic cells, where it plays a key regulatory role in cytoskeletal dynamics. WASp interacting protein (WIP) was first discovered as the binding partner of WASp, through the use of the yeast two hybrid system. WIP was later identified as a chaperone of WASp, necessary for its stability. Mutations occurring at the WASp homology 1 domain (WH1), which serves as the WIP binding site, were found to cause the Wiskott-Aldrich syndrome (WAS) and X-linked thrombocytopenia (XLT). WAS manifests as an immune deficiency characterized by eczema, thrombocytopenia, recurrent infections, and hematopoietic malignancies, demonstrating the importance of WIP for WASp complex formation and for a proper immune response. WIP deficiency was found to lead to different abnormalities in the activity of various lymphocytes, suggesting differential cell-dependent roles for WIP. Additionally, WIP deficiency causes cellular abnormalities not found in WASp-deficient cells, indicating that WIP fulfills roles beyond stabilizing WASp. Indeed, WIP was shown to interact with various binding partners, including the signaling proteins Nck, CrkL and cortactin. Recent studies have demonstrated that WIP also takes part in non immune cellular processes such as cancer invasion and metastasis, in addition to cell subversion by intracellular pathogens. Understanding of numerous functions of WIP can enhance our current understanding of activation and function of immune and other cell types.

Revertant mosaicism in skin: natural gene therapy

Revertant mosaicism is a naturally occurring phenomenon involving spontaneous correction of a pathogenic mutation in a somatic cell. Recent studies suggest that it is not a rare event and that it could be clinically relevant to phenotypic expression and patient treatment. Indeed, revertant cell therapy represents a potential 'natural gene therapy' because in vivo reversion obviates the need for further genetic correction. Revertant mosaicism has been observed in several inherited conditions, including epidermolysis bullosa, a heterogeneous group of blistering skin disorders. These diseases provide a useful model for studying revertant mosaicism because of the visual and accessible nature of skin. This overview highlights the latest developments in revertant mosaicism and the translational implications germane to heritable skin disorders.

The root of many evils: indolent large granular lymphocyte leukaemia and associated disorders

Large granular lymphocytes (LGL) leukaemia can arise from either natural killer (NK) cells or cytotoxic T lymphocytes (CTL). The T-cell form of LGL leukaemia has significant overlap with other haematological disorders and autoimmune diseases. Here we provide an overview of LGL biology. We also focus discussion on the indolent LGL leukaemia related disorders and their causal relationships. We then discuss the potential relationships and distinctions between indolent LGL leukaemia and non-malignant clonal lymphocyte expansion that occur in otherwise healthy individuals, especially elder people.

Revertant T lymphocytes in a patient with Wiskott-Aldrich syndrome: analysis of function and distribution in lymphoid organs

BACKGROUND

The Wiskott-Aldrich syndrome (WAS) is a rare genetic disease characterized by thrombocytopenia, immunodeficiency, autoimmunity, and hematologic malignancies. Secondary mutations leading to re-expression of WAS protein (WASP) are relatively frequent in patients with WAS.

OBJECTIVE

The tissue distribution and function of revertant cells were investigated in a novel case of WAS gene secondary mutation.

METHODS

A vast combination of approaches was used to characterize the second-site mutation, to investigate revertant cell function, and to track their distribution over a 18-year clinical follow-up.

RESULTS

The WAS gene secondary mutation was a 4-nucleotide insertion, 4 nucleotides downstream of the original deletion. This somatic mutation allowed the T-cell-restricted expression of a stable, full-length WASP with a 3-amino acid change compared with the wild-type protein. WASP(+) T cells appeared early in the spleen (age 10 years) and were highly enriched in a mesenteric lymph node at a later time (age 23 years). Revertant T cells had a diversified T-cell-receptor repertoire and displayed in vitro and in vivo selective advantage. They proliferated and produced cytokines normally on T-cell-receptor stimulation. Consistently, the revertant WASP correctly localized to the immunologic synapse and to the leading edge of migrating T cells.

CONCLUSION

Despite the high proportion of functional revertant T cells, the patient still has severe infections and autoimmune disorders, suggesting that re-expression of WASP in T cells is not sufficient to normalize immune functions fully in patients with WAS.

Revertant somatic mosaicism in the Wiskott-Aldrich syndrome

Up to 11% of patients affected with Wiskott-Aldrich syndrome (WAS) have presented with somatic mosaicism due to spontaneous in vivo reversion to normal of the original mutation or second-site compensatory mutations that restored production of the WAS gene product. The reasons underlying the high prevalence of this phenomenon in WAS are unclear and may include strong selective advantage of revertant cells over mutated populations, abnormally high general mutation rate and/or increased susceptibility of specific WAS gene sequences to DNA polymerase errors. Additional studies in human samples and in vitro/in vivo models of the disease will likely yield further insights into the mechanisms responsible for the occurrence of revertant mosaicism in WAS and elucidate additional biological characteristics of the WAS gene and protein.

Somatic mosaicism in primary immune deficiencies

PURPOSE OF REVIEW

Spontaneous genetic reversions and second-site mutations resulting in revertant somatic mosaicism are poorly understood phenomena with a seemingly frequent occurrence in primary immunodeficiency diseases. Here we summarize the several cases that have been reported thus far with particular focus on the most recent observations.

RECENT FINDINGS

Revertant cells have been associated with attenuated clinical phenotypes in some, although not all, immunodeficient patients who presented with somatic mosaicism. Interestingly, the latest studies suggest that revertant cells may also be responsible for immune dysregulation. In addition, extensive molecular analysis of revertant cells has revealed that an unexpectedly large variety of genetic changes can be responsible for their emergence.

SUMMARY

The occurrence of revertant somatic mosaicism in patients affected with primary immunodeficiency diseases is likely much more common than originally anticipated. The study of this fascinating phenomenon continues to provide clues as to the possible underlying mechanisms and to inform, albeit indirectly, the process of development of cell and gene therapy for these diseases.

Wiskott-Aldrich syndrome protein is required for homeostasis and function of invariant NKT cells

NKT cells comprise a separate T lineage expressing semi-invariant T cell receptors. Canonical invariant NKT (iNKT) cells specifically recognize lipid Ags presented by CD1d, a MHC class I-like molecule. iNKT cells function, in part, as initial responders to bacterial infection and play a role in immune surveillance and tumor rejection. The Wiskott-Aldrich Syndrome protein (WASp) serves as a crucial link between cellular stimuli and cytoskeletal rearrangements. Although we and others have identified a key role for WASp in homeostasis of T-regulatory and marginal zone B cells, little data exist regarding the role for WASp within the iNKT lineage. Analysis of WASp-expressing cell populations in heterozygous female WASp mice revealed a substantial selective advantage for WASp(+) vs WASp(-) iNKT cells. Although adult WASp-deficient (WASp(-/-)) mice had normal thymic and bone marrow iNKT numbers, we observed 2- to 3-fold reduction in the numbers of iNKT cells in the spleen and liver. This peripheral iNKT deficit is manifested, in part, due to defective iNKT homeostasis. WASp(-/-) iNKT cells exhibited reduced levels of integrin surface expression and decreased homing and/or retention within peripheral tissues in a competitive repopulation model. In addition, analysis of young mice showed that WASp is important for both maturation and egress of thymic iNKT cells. WASp(-/-) iNKT cells also exhibited a marked reduction in Ag-induced proliferation and cytokine production. Our findings highlight the crucial role for WASp in iNKT development, homeostasis, and activation, and identify iNKT dysfunction as an additional factor likely to contribute to the clinical features observed in WAS patients.

Clinical and immunologic consequences of a somatic reversion in a patient with X-linked severe combined immunodeficiency

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 α/β and γ/δ 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).

Evolution of highly polymorphic T cell populations in siblings with the Wiskott-Aldrich Syndrome

Population level evolutionary processes can occur within a single organism when the germ line contains a mutation that confers a cost at the level of the cell. Here we describe how multiple compensatory mutations arose through a within-individual evolutionary process in two brothers with the immune deficiency Wiskott-Aldrich Syndrome (WAS). As a result, both brothers have T lymphocyte populations that are highly polymorphic at the locus of the germ line defect, and no single allele achieves fixation. WASP, the gene product affected in this disease, is specific to white blood cells where it is responsible for regulating actin cytoskeleton dynamics in a wide range of cellular responses. The brothers inherited a rare allele predicted to result in truncated WASP lacking the carboxy-terminal VCA domains, the region that directly catalyzes actin filament generation. Although the brothers' T cell populations are highly polymorphic, all share a corrective effect relative to the inherited allele in that they restore the VCA domain. This indicates massive selection against the truncated germ line allele. No single somatic allele becomes fixed in the circulating T cell population of either brother, indicating that a regulated step in maturation of the affected cell lineage is severely compromised by the germ line allele. Based on the finding of multiple somatic mutations, the known maturation pathway for T-lineage cells and the known defects of T cells and precursor thymocytes in mice with truncated WASP, we hypothesize that the presence of truncated WASP (WASPΔVCA) confers an extreme disadvantage in early developing thymocytes, above and beyond the known cost of absence of full-length WASP, and that the disadvantage likely occurs through dominant negative competition of WASPΔVCA with N-WASP, a protein that otherwise partially compensates for WASP absence in developing thymocytes.

Wiskott-Aldrich syndrome

PURPOSE OF REVIEW

Wiskott-Aldrich syndrome is caused by mutations of the Wiskott-Aldrich syndrome protein gene, which codes for a cytoplasmic protein with multiple functions. This review will focus on recent progress in understanding the molecular basis of Wiskott-Aldrich syndrome and its ramifications for the cure of this lethal disease.

RECENT FINDINGS

The discovery of the causative gene has revealed a spectrum of clinical phenotypes demonstrating a strong genotype/phenotype correlation. The discovery of unique functional domains of Wiskott-Aldrich syndrome protein has been instrumental in defining mechanisms that control activation of Wiskott-Aldrich syndrome protein. Long-term follow up of patients undergoing hematopoietic stem cell transplantation has led to important modifications of the procedure. Studies of Wiskott-Aldrich syndrome protein-deficient cell lines and wasp-knockout mice have paved the way for possible gene therapy.

SUMMARY

Wiskott-Aldrich syndrome protein gene mutations result in four clinical phenotypes: classic Wiskott-Aldrich syndrome and X-linked thrombocytopenia, intermittent thrombocytopenia and neutropenia. Wiskott-Aldrich syndrome protein is a signaling molecule and instrumental for cognate and innate immunity, cell motility and protection against autoimmune disease. The success of hematopoietic stem cell transplantation is related to the recipient's age, donor selection, the conditioning regimen and the extent of reconstitution. Since Wiskott-Aldrich syndrome protein is expressed exclusively in hematopoietic stem cells, and because Wiskott-Aldrich syndrome protein exerts a strong selective pressure, gene therapy is expected to cure the disease.

Development of lentiviral gene therapy for Wiskott Aldrich syndrome

BACKGROUND

Wiskott Aldrich syndrome (WAS) is a rare X-linked primary immunodeficiency. This complex disease is characterised by microthrombocytopenia, recurrent infections, eczema and is associated with a high incidence of autoimmunity and of lymphoid malignancies. WAS is attracting growing attention not only because it highlights the rich cellular and systems biology revolving around cytoskeletal regulation but also because it is candidate for a haematopoietic stem cell gene therapy indication.

OBJECTIVES

As several groups are developing this novel approach, this review discusses the state of the art and challenges in clinical development of gene therapy for WAS, with particular regard to biosafety.

METHODS

In spite of the successes of haematopoietic gene therapy for genetic immune deficiencies, there is a need for more efficient transduction protocols and for vectors with a superior safety profile. Preclinical studies have provided reasonable expectations that haematopoietic gene therapy with a self-inactivated HIV-1-derived vector using the native gene promoter for expression of the WAS transgene will be safe and will lead to the restoration of WAS protein in the haematopoietic and immune system at levels sufficient to provide an improvement in the condition of WAS patients.

CONCLUSIONS

Phase I/II clinical studies will soon be initiated in several European centres to assess the safety and efficacy of this lentiviral vector in WAS patients.

The Phenomenon of Spontaneous Genetic Reversions in the Wiskott-Aldrich Syndrome: A Report of the Workshop of the ESID Genetics Working Party at the XIIth Meeting of the European Society for Immunodeficiencies (ESID). Budapest, Hungary October 4–7, 2006

The Wiskott-Aldrich syndrome (WAS) is a primary immunodeficiency disease caused by mutations in the Wiskott-Aldrich Protein (WASP) gene, which typically leads to absent WASP protein expression in WAS leukocytes. However, some patients have been found with small populations of WASP-expressing cells caused by reverse or second-site mutations that allow protein expression. An international consortium was established to further investigate these phenomena. This paper summarizes data collected by this consortium that was presented at a workshop held during the XIIth Meeting of the European Society for Immunodeficiencies (ESID), October, 2006. WASP reversions were noted in approximately 11% of 272 patients tested. Many different cell lineages showed reversions. These data form the foundation for further investigation into this phenomenon, which has implications for therapy of this disease.

Current understanding of the Wiskott-Aldrich syndrome and prospects for gene therapy

Gene therapy, based on the transplantation of genetically corrected autologous hematopoietic stem cells (HSCs), has proven to be an effective therapeutic approach as an alternative to allogenic HSC transplantation for the cure of severe combined immunodeficiencies (SCID). In this article, the recent preclinical studies aiming towards gene therapy trials for the Wiskott-Aldrich syndrome (WAS), a life-threatening immunodeficiency characterized by infections, hemorrhages, autoimmune disorders and lymphomas, will be reviewed. An update of the safety and efficacy data obtained in studies performed in murine disease models and in cells from WAS patients will be presented. Based on these data and on the clinical results of the recent trials for SCID, the most critical issues regarding the implementation of a gene therapy approach for WAS will be discussed.

A second-site mutation in the initiation codon of WAS (WASP) results in expansion of subsets of lymphocytes in an Wiskott-Aldrich syndrome patient

Wiskott-Aldrich syndrome (WAS) is caused by mutations in the gene encoding WAS protein (WASP ). Recently, somatic mosaicism caused by reversions or second-site mutations has been reported in some inherited disorders including WAS. In this article, we describe somatic mosaicism in a 15-year-old WAS patient due to a second-hit mutation in the initiation codon. The patient originally had a single-base deletion (c.11delG; p.G4fsX40) in the WAS (WASP) gene, which resulted in a frameshift and abrogated protein expression. Subsequently, a fraction of T and natural killer (NK) cells expressed a smaller WASP, which binds to its cellular partner WASP-interacting protein (WIP). The T and NK cells were found to have an additional mutation in the initiation codon (c.1A>T; p.M1_P5del). The results strongly suggest that the smaller WASP is translated from the second ATG downstream of the original mutation, and not only T cells but also NK cells carrying the second mutation acquired a growth advantage over WASP negative counterparts. To our knowledge, this is the first report describing somatic mosaicism due to a second-site mutation in the initiation codon of any inherited disorders.