Isolation of a novel gene mutated in Wiskott-Aldrich syndrome

Management of Wiskott-Aldrich syndrome

A five-month-old boy presented with lower gastrointestinal bleed, recurrent infections and eczema. Blood picture revealed small platelets, high IgA, and IgM levels. A diagnosis of Wiskott-Aldrich Syndrome was made. The recent concepts in molecular pathology of the disease and treatment are discussed.

Wiskott-Aldrich syndrome protein (WASp) is a binding partner for c-Src family protein-tyrosine kinases

BACKGROUND

Receptor-mediated signal transduction requires the assembly of multimeric complexes of signalling proteins, and a number of conserved protein domains, such as the SH2, SH3 and PH domains, are involved in mediating protein-protein interactions in such complexes. The identification of binding partners for these domains has added considerably to our understanding of signal-transduction pathways, and the purpose of this work was to identify SH3-binding proteins in haematopoietic cells.

RESULTS

We performed affinity-chromatography experiments with a panel of GST-SH3 fusion proteins (composed of glutathione-S-transferase appended to various SH3 domains) to search for SH3-binding proteins in a human megakaryocytic cell line. Protein microsequencing identified one of the SH3-binding proteins as WASp, the protein that is defective in Wiskott-Aldrich syndrome (WAS) and isolated X-linked thrombocytopenia. WASp bound preferentially in vitro to SH3 domains from c-Src family kinases, and analysis of proteins expressed in insect cells using a baculovirus vector demonstrated a specific interaction between WASp and the Fyn protein-tyrosine kinase. Finally, in vivo experiments showed that WASp and Fyn physically associate in human haematopoietic cells.

CONCLUSIONS

Haematopoietic cells from individuals with WAS exhibit defects in cell morphology and signal transduction, including reduced proliferation and tyrosine phosphorylation in response to stimulatory factors. Members of the c Src family of protein-tyrosine kinases, including Fyn, are involved in a range of signalling pathways - such as those regulating cytoskeletal structure - in both haematopoietic and non-haematopoietic cells. Our data suggest that binding of Fyn to WASp may be a critical event in such signalling pathways in haematopoietic cells.

Lessons from human genetic variants in the study of B-cell differentiation

Several human B-cell immunodeficiencies result from mutations in signal transducing molecules. The past year has seen significant advances in our understanding of how these molecules are integrated into B cell signaling pathways. The phenotypes of mice deficient in several of these genes have revealed species-specific differences in the requirements for early B cell development.

Bone marrow transplantation in 26 patients with Wiskott-Aldrich syndrome from a single center

We retrospectively analyzed the outcome of bone marrow transplantation (BMT) performed in 26 patients with Wiskott-Aldrich syndrome (WAS) in one center. Twenty-eight transplantation procedures were performed. Ten unselected patients received unmanipulated marrow from a donor with genetically identical human leukocyte antigen (HLA). Eight patients were cured and survive 1.5 to 16.5 years after BMT. One patient successfully received a T-cell-depleted marrow from a matched unrelated donor. Sixteen patients were selected to receive a related HLA partially incompatible BMT because of the occurrence of life-threatening complications from the WAS (i.e., refractory thrombocytopenia, autoimmunity including vasculitis and sepsis). All but one received T-cell-depleted marrow after a conditioning regimen of busulfan and cyclophosphamide. One patient had two BMTs. Engraftment occurred in 12 of 17 attempts. The addition of monoclonal antibodies to lymphocyte function-associated antigen-1 and CD2 molecules appeared to improve engraftment. Six patients were long-term survivors, whereas others died of viral infections (n = 7), among which Epstein-Barr virus-induced B-lymphocyte proliferative disorder was predominant. Delay in development of full T- and B-cell functions accounted for severe infectious complications. These results confirm the excellent outcome of HLA genetically identical BMT in WAS, whereas BMT from HLA partially incompatible donors should be strictly restricted to patients with severe complications of WAS.

Wiskott-Aldrich syndrome in a family with Fanconi anemia

Thrombocytopenia may be the presenting finding for both Wiskott-Aldrich syndrome and Fanconi anemia. We examined a sibship of four boys who had features of both of these hematologic disorders. Peripheral blood lymphocytes from three of the boys demonstrated DNA instability when cultured with diepoxybutane, confirming the diagnosis of Fanconi anemia in these patients. However, results of linkage analysis and X chromosome inactivation studies were consistent with the diagnosis of Wiskott-Aldrich syndrome in two of the boys, including one of the boys with Fanconi anemia. These findings could be attributed to the occurrence of two rare genetic disorders in a single family or to an unusual variant of Fanconi anemia. The recent identification of the Wiskott-Aldrich gene permitted us to address this question directly. Epstein-Barr virus-transformed cell lines from the two boys thought to have Wiskott-Aldrich syndrome on the basis of linkage analysis failed to express transcripts for the Wiskott-Aldrich gene. Genomic DNA from these two patients demonstrated a G insertion in the tenth exon of the Wiskott-Aldrich gene, resulting in a frameshift and a premature stop codon. Surprisingly, the patient with Fanconi anemia and a null mutation in the Wiskott-Aldrich gene had typical Fanconi anemia but mild Wiskott-Aldrich syndrome.

Isolated X-linked thrombocytopenia in two unrelated families is associated with point mutations in the Wiskott-Aldrich syndrome protein gene

The Wiskott-Aldrich syndrome (WAS) is characterized by defective platelet and lymphocyte function associated with eczema and increased susceptibility to malignancies. It is caused by mutations of the WAS protein-encoding gene (WASP). X-lined thrombocytopenia, defined by low platelet counts and volume, may be an allelic variant of WAS. In patients with XLT from two unrelated families, WASP gene defects were identified by single-strand conformational polymorphism and by sequencing. Point mutations in exon 2 of the WASP gene were found in the patients from both families in which XLT segregated. Several obligate heterozygote female members of these families display a random pattern of X inactivation in their peripheral blood leukocytes. This study shows that XLT may be caused by mutations of the WASP, thus representing an allelic variant of WAS.

Disease mechanism: unravelling Wiskott-Aldrich syndrome

The gene responsible for Wiskott-Aldrich syndrome, a disease affecting platelets and lymphocytes, has been cloned and its protein product (WASp) found to interact with the GTPase Cdc42. WASp seems to provide a link between Cdc42 and the actin cytoskeleton, perhaps explaining the cellular defects underlying the disease.

Thrombocytopenia in the neonate

Pediatricians caring for newborns will eventually be confronted with the problem of thrombocytopenia in the neonatal period. Familiarity with the differential diagnosis of neonatal thrombocytopenia and understanding the pathogenesis of the more common entities allows physicians to design a selective diagnostic and therapeutic plan to benefit these thrombocytopenic infants.

Studies of the expression of the Wiskott-Aldrich syndrome protein

The Wiskott-Aldrich syndrome (WAS) is an X-linked disorder characterized by thrombocytopenia, eczema, disorders in cell-mediated and humoral immunity, and a proclivity to lymphoproliferative disease. The gene responsible encodes a 53-kD proline-rich protein of unknown function (WASP). We produced a FLAG-WASP fusion protein that was used to immunize mice and produce mAbs against WASP. Using monoclonal anti-WASP in Western immunoblots, we have determined that WASP is present in the cytoplasmic but not nuclear fraction of normal human peripheral blood mononuclear cells, in normal human platelets, in T lymphocytes, non-T lymphocytes, and monocytes. The protein is produced in the B cell immunoblastic cell line DS-1, in normal EBV-transformed B cell lines, and in HEL92.1.7, but is barely detectable in MOLT-4 and not detectable in K562. WASP was present in two of four EBV-transformed cell lines from WAS patients. Splenic tissue immunostaining was performed in two patients, and the results correlated with the results of the Western blots. Sequence analysis of WASP cDNA from two patients who produce WASP show mutations causing amino acid substitutions. These studies establish a foundation for further studies aimed at understanding the function of WASP.

Direct interaction of the Wiskott-Aldrich syndrome protein with the GTPase Cdc42

Wiskott-Aldrich syndrome (WAS) is an X-linked immunodeficiency disorder with the most severe pathology in the T lymphocytes and platelets. The disease arises from mutations in the gene encoding the WAS protein. T lymphocytes of affected males with WAS exhibit a severe disturbance of the actin cytoskeleton, suggesting that the WAS protein could regulate its organization. We show here that WAS protein interacts with a member of the Rho family of GTPases, Cdc42. This interaction, which is guanosine 5'-triphosphate (GTP)-dependent, was detected in cell lysates, in transient transfections and with purified recombinant proteins. A weaker interaction was also detected with Rac1 using WAS protein from cell lysates. It was also found that different mutant WAS proteins from three affected males retained their ability to interact with Cdc42 and that the level of expression of the WAS protein in these mutants was only 2-5% of normal. Taken together these data suggest that the WAS protein might function as a signal transduction adaptor downstream of Cdc42, and in affected males, the cytoskeletal abnormalities may result from a defect in Cdc42 signaling.

Wiskott-Aldrich syndrome protein, a novel effector for the GTPase CDC42Hs, is implicated in actin polymerization

The Rho family of GTPases control diverse biological processes, including cell morphology and mitogenesis. We have identified WASP, the protein that is defective in Wiskott-Aldrich syndrome (WAS), as a novel effector for CDC42Hs, but not for the other Rho family members, Rac and Rho. This interaction is dependent on the presence of the G protein-binding domain. Cellular expression of epitope-tagged WASP produces clusters of WASP that are highly enriched in polymerized actin. This clustering is not observed with a C-terminally deleted WASP and is inhibited by coexpression with dominant negative CDC42Hs-N17, but not with dominant negative forms of Rac or Rho. Thus, WASP provides a novel link between CDC42Hs and the actin cytoskeleton, which suggests a molecular mechanism for many of the cellular abnormalities in WAS. The WASP sequence contains two novel domains that are homologous to other proteins involved in action organization.

Tissue-specific deoxyribonuclease I-hypersensitive sites in the vicinity of the immunoglobulin C lambda cluster of man

During B cell development, the onset of DNA rearrangements, expression, and somatic hypermutation of Ig genes are regulated through the complex interaction of cis-acting elements with trans-acting factors. Our aim is to identify DNA elements required during activation of the human Ig lambda light chain genes. Determination of deoxyribonuclease (DNase) I-hypersensitive sites in complex regulated genes can lead to the identification of sequence elements which would have been overlooked by employing transient transfection protocols. We have therefore investigated the chromatin structure of human J-C lambda genes and identified three DNase I-hypersensitive sites (HSS-1, -2, and -3) within an 8-kb region downstream of the J-C lambda 7 gene. HSS-2 and HSS-3 are B cell specific. The DNase I-hypersensitive sites are also present in kappa-producing cell lines which have not rearranged the Ig lambda locus and produce germ-line J-C lambda transcripts. We conclude that in mature B cells, both kappa and lambda loci are in an active structure regardless of the type of light chain they produce. This suggests that the chromatin structure of both loci is opened early in B cell development and that the active structure persists in mature B cells. The observed temporal order (first kappa, then lambda) of activation can be explained by consecutive synthesis of the appropriate regulating factors and the tight regulation of the recombination machinery through the products of L chain gene rearrangements.

Two GTPases, Cdc42 and Rac, bind directly to a protein implicated in the immunodeficiency disorder Wiskott-Aldrich syndrome

BACKGROUND

Members of the Rho family of small GTPases play an essential role in controlling the motile behaviour of animal cells. Specifically, Cdc42 and Rac have been shown to induce the formation of filopodia and lamellipodia, respectively, at the cell periphery of Swiss 3T3 fibroblasts. In addition, both GTPases are required for progression through G1 phase of the cell cycle, possibly by regulating the activity of the Jun N-terminal kinase (JNK) signalling pathway. In order to examine more closely the mechanisms underlying the diverse functions of Rho GTPases in mammalian cells, we searched for downstream targets of these proteins.

RESULTS

A yeast two-hybrid screen for proteins interacting with the human Cdc42 GTPase identified WASP, a protein implicated in the immunodeficiency disorder Wiskott-Aldrich syndrome (WAS). Recombinant WASP, expressed in Escherichia coli, also bound to Cdc42 and weakly to Rac, but not at all to Rho. The Cdc42/Rac-binding domain was identified in a region between amino acids 201-321 of WASP, and binding was dependent on Cdc42 and Rac being in the GTP-bound conformation. Furthermore, WASP did not catalyze GTPase activation or nucleotide exchange activity on Cdc42.

CONCLUSIONS

Positional cloning has implicated WASP in causing WAS, and the protein is defective in patients suffering from the disease. WASP is expressed exclusively in cells of hematopoietic lineage, and lymphocytes from WAS patients have a distorted cell-surface and exhibit reduced proliferative capacity. WASP has recently been found to bind to the Src-homology 3 (SH3) domain of the adapter protein Nck. This observation, and the results presented here, suggest that WAS is the result of defects in signal transduction pathways regulated by Cdc42/Rac and Nck.

A conserved binding motif defines numerous candidate target proteins for both Cdc42 and Rac GTPases

Rho, Rac, and Cdc42 are small GTPases that regulate the formation of a variety of actin structures and the assembly of associated integrin complexes, but little is known about the target proteins that mediate their effects. Here we have used a motif-based search method to identify putative effector proteins for Rac and Cdc42. A search of the GenBankTM data base for similarity with the minimum Cdc42/Rac interactive binding (CRIB) region of a potential effector protein p65PAK has identified over 25 proteins containing a similar motif from a range of different species. These candidate Cdc42/Rac-binding proteins include family members of the mixed lineage kinases (MLK), a novel tyrosine kinase from Drosophila melanogaster (DPR2), a human protein MSE55, and several novel yeast and Caenorhabditis elegans proteins. Two murine p65PAK isoforms and a candidate protein from C. elegans, F09F7.5, interact strongly with the GTP form of both Cdc42 and Rac, but not Rho in a filter binding assay. Three additional candidate proteins, DPR2, MSE55, and MLK3 showed binding to the GTP form of Cdc42 and weaker binding with Rac, and again no interaction with Rho. These results indicate that proteins containing the CRIB motif bind to Cdc42 and/or Rac in a GTP-dependent manner, and they may, therefore, participate in downstream signaling.

Two sisters with clinical diagnosis of Wiskott-Aldrich syndrome: is the condition in the family autosomal recessive?

We report two sisters in a family representing manifestations of Wiskott-Aldrich syndrome (WAS), an X-linked immunodeficiency disorder. An elder sister had suffered from recurrent infections, small thrombocytopenic petechiae, purpura, and eczema for 7 years. The younger sister had the same manifestations as the elder sister's for a 2-year period, and died of intracranial bleeding at age 2 years. All the laboratory data of the two patients were compatible with WAS, although they were females. Sialophorin analysis with the selective radioactive labeling method of this protein revealed that in the elder sister a 115-KD band that should be specific for sialophorin was reduced in quantity, and instead an additional 135-KD fragment was present as a main band. Polymerase chain reaction (PCR) analysis of the sialophorin gene and single-strand conformation polymorphism (SSCP) analysis of the PCR product demonstrated that there were no detectable size-change nor electrophoretic mobility change in the DNA from both patients. The results indicated that their sialophorin gene structure might be normal. Studies on the mother-daughter transmission of X chromosome using a pERT84-MaeIII polymorphic marker mapped at Xp21 and HPRT gene polymorphism at Xq26 suggested that each sister had inherited a different X chromosome from the mother. Two explanations are plausible for the occurrence of the WAS in our patients: the WAS in the patients is attributable to an autosomal gene mutation which may regulate the sialophorin gene expression through the WAS gene, or, alternatively, the condition in this family is an autosomal recessive disorder separated etiologically from the X-linked WAS.

Wiskott-Aldrich syndrome protein physically associates with Nck through Src homology 3 domains

In the second of a series of experiments designed to identify p47nck-Src homology 3 (SH3)-binding molecules, we report the cloning of SAKAP II (Src A box Nck-associated protein II) from an HL60 cDNA expression library. This molecule has been identified as a cDNA encoding the protein product of WASP, which is mutated in Wiskott-Aldrich syndrome patients. Studies in vivo and in vitro demonstrated a highly specific interaction between the SH3 domains of p47nck and Wiskott-Aldrich syndrome protein. Furthermore, anti-Wiskott-Aldrich syndrome protein antibodies recognized a protein of 66 kDa by Western blot (immunoblot) analysis. In vitro translation studies identified the 66-kDa protein as the protein product of WASP, and subcellular fractionation experiments showed that p66WASP is mainly present in the cytosol fraction, although significant amounts are also present in membrane and nuclear fractions. The main p47nck region implicated in the association with p66WASP was found to be the carboxy-terminal SH3 domain.

Addison disease 10 years after bone marrow transplantation for Wiskott-Aldrich syndrome

We report a 10-year-old boy with familial Wiskott-Aldrich syndrome (WAS) who underwent successful bone marrow transplantation (BMT) at the age of 9 months. With the exception of auto-immune haemolytic anaemia due to warm antibodies lasting 15 months there had not been any complication after BMT. Ten years later the patient presented with diarrhoea, hyperpigmentation of skin and oral mucosa, fatigue and polyuria. Diagnosis of Addison disease was confirmed by typical electrolyte imbalance and absent cortisol response to adrenocorticotrophic hormone. Adrenal antibodies were positive. On therapy with oral gluco- and mineralocorticoids, the symptoms disappeared and electrolytes normalized. To our knowledge auto-immuno endocrinopathy after BMT for WAS has not yet been reported.

CONCLUSION

This is the first report of auto-immune adrenal insufficiency after BMT for WAS. The aetiopathogenesis of this condition remains unknown since auto-immune diseases as toxic side-effects of the ablative treatment before BMT have not yet been reported, and a relapse of WAS and cotransplantation of auto-immune adrenal insufficiency have been ruled out.

Genetic immunodeficiencies: cutaneous manifestations and recent progress

In recent years, remarkable progress has been made in elucidating the pathophysiology of genetic immunodeficiency disorders. Dermatologic manifestations are prominent in these conditions; because of advances in diagnosis and therapy, patients are living longer, increasing the likelihood that dermatologists will encounter patients with these diseases. The genes of many of these disorders have been cloned, including chronic granulomatous disease, X-linked immunodeficiencies, and myeloperoxidase deficiency. Understanding the regulation and function of these genes will not only affect patients with these rare disorders, but may provide an insight into common dermatologic conditions, such as eczema and cutaneous infection. Diagnosis, dermatologic manifestations, and therapy are discussed.

Finding genes involved in human developmental disorders

Recent advances in the human genome initiative have accelerated positional cloning efforts toward identification of a number of genes responsible for human developmental anomalies, particularly those involving the skeletal system. Genotype/phenotype comparison and functional analysis of these genes will further elucidate pathways of normal and abnormal human development of the skeletal and other organ systems.

Identification of mutations in the Wiskott-Aldrich syndrome gene and characterization of a polymorphic dinucleotide repeat at DXS6940, adjacent to the disease gene

The Wiskott-Aldrich syndrome (WAS) is an X-chromosome-linked recessive disease characterized by eczema, thrombocytopenia, and immunodeficiency. The disease gene has been localized to the proximal short arm of the X chromosome and recently isolated through positional cloning. The function of the encoded protein remains undetermined. In this study we have characterized mutations in 12 unrelated patients to confirm the identity of the disease gene. We have also revised the coding sequence and genomic structure for the WAS gene. To analyze further the transmittance of the disease gene, we have characterized a polymorphic microsatellite at the DXS6940 locus within 30 kb of the gene and demonstrate the inheritance of the affected alleles in families with a history of WAS.

Genetic analyses of tattered, an X-linked dominant, developmental mouse mutation

Tattered (Td) is an X-linked dominant mouse mutation that causes prenatal lethality in affected males. To map the locus, we analyzed 199 normal male and affected female progeny from a backcross of Td and Mus castaneus. Pedigree analysis of these animals suggests a gene order of cen-DXWas70-(Td, DXMit26, Gata1, Tcfe3)-(Cybb, Otc)-tel, where Tcfe3 is a transcription factor homologous to a gene involved in the murine microphthalmia (mi) mutation [Hodgkinson et al. Cell 74, 395-404, 1993]. To evaluate Tcfe3 as a candidate for Td, heterozygous tattered females were crossed to xid males to obtain females in which > 95% of B cells expressed genes solely from the Td X Chromosome (Chr). Fluorescent activated cell sorting (FACS) analysis and Western blotting of isolated splenocytes from Td/xid double heterozygotes rule out Tcfe3 as a likely candidate for the Td mutation.

X-linked thrombocytopenia and Wiskott-Aldrich syndrome are allelic diseases with mutations in the WASP gene

X-linked thrombocytopenia (XLT) is a rare recessive hereditary disorder characterized by isolated thrombocytopenia with small-sized platelets. The XLT locus has been located to chromosome Xp11 by linkage analysis, which is also where the recently cloned Wiskott-Aldrich syndrome (WAS) gene, maps. The relationship between XLT and WAS has long been debated; they might be due to different mutations of the same gene or to mutations in different genes. We now show that mutations in the WAS gene, different from those found in WAS patients, are present in three unrelated male patients with isolated thrombocytopenia and small-sized platelets. Our results demonstrate that XLT and WAS are allelic forms of the same disease, but the causes of the differences need to be further investigated.

Insights into lymphocyte development from X-linked immune deficiencies

The genetic immune deficiencies have drawn the attention of physicians and immunologists for more than 40 years. The selectivity of these deficiencies brings into focus the contribution of the response of each arm of the immune system to specific pathogens. Recently, the genes underlying four X-linked defects in immune development in humans have been identified by either positional cloning or candidate-gene cloning techniques. Remarkably, these genetic defects reveal a microcosm of lymphocyte developmental controls involving cell-cell interactions, combinatorial cell surface receptor specificity and lineage-specific signal transduction.

Immunodeficiency. Trapping the nude mouse gene

Hairless nude mice are immunodeficient because they lack a thymus. The nude gene has now been identified; it encodes a winged-helix transcription factor that is expressed specifically in skin and thymus.