Wiskott-Aldrich syndrome: a gene, a multifunctional protein and the beginnings of an explanation

Significance of kinase activity in the dynamic invadosome

Invadosomes are actin rich protrusive structures that facilitate invasive migration in multiple cell types. Comprised of invadopodia and podosomes, these highly dynamic structures adhere to and degrade the extracellular matrix, and are also thought to play a role in mechanosensing. Many extracellular signals have been implicated in invadosome stimulation, activating complex signalling cascades to drive the formation, activity and turnover of invadosomes. While the structural components of invadosomes have been well studied, the regulation of invadosome dynamics is still poorly understood. Protein kinases are essential to this regulation, affecting all stages of invadosome dynamics and allowing tight spatiotemporal control of their activity. Invadosome organisation and function have been linked to pathophysiological states such as cancer invasion and metastasis; therapeutic targeting of invadosome regulatory components is thus warranted. In this review, we discuss the involvement of kinase signalling in every stage of the invadosome life cycle and evaluate its significance.

The 'ins' and 'outs' of podosomes and invadopodia: characteristics, formation and function

Podosomes and invadopodia are actin-based dynamic protrusions of the plasma membrane of metazoan cells that represent sites of attachment to - and degradation of - the extracellular matrix. The key proteins in these structures include the actin regulators cortactin and neural Wiskott-Aldrich syndrome protein (N-WASP), the adaptor proteins Tyr kinase substrate with four SH3 domains (TKS4) and Tyr kinase substrate with five SH3 domains (TKS5), and the metalloprotease membrane type 1 matrix metalloprotease (MT1MMP; also known as MMP14). Many cell types can produce these structures, including invasive cancer cells, vascular smooth muscle and endothelial cells, and immune cells such as macrophages and dendritic cells. Recently, progress has been made in our understanding of the regulatory and functional aspects of podosome and invadopodium biology and their role in human disease.

A novel splice site mutation in the WAS gene causes Wiskott–Aldrich syndrome in two siblings of a Saudi family

We report here on a Saudi family with two affected males with Wiskott-Aldrich syndrome (WAS), which includes mild to moderate bleeding and a low platelet count. A novel splice donor-site mutation (811 + 5 G <-- C) in intron 8 of the WAS gene (Genbank accession number NM_000377) was detected in a hemizygous status in both index cases, heterozygous in their mother and absent in the father. RNA from both index cases was transcribed and amplified with primers complementary to sequences in exons 7 and 10. A reverse transcription-polymerase chain reaction (RT-PCR) product of 688 bp (approximately 82%) was produced in addition to the normal RT-PCR product of 485 bp (approximately 18%). cDNA sequence analysis reveals an inclusion of full intron 8 sequence in the final transcript. The resultant protein is predicted to have 68 missense codons and a pre-mature stop codon at amino acid 260. This novel splice donor-site mutation was not detected in 80 normal controls (56 females and 24 males) from the same ethnic background as the index cases. Since no other mutation was detected in the WAS gene and the patients have classical symptoms of WAS, we concluded that it is highly likely that this novel mutation is responsible for the phenotype observed in these patients.

Wasp, the Drosophila Wiskott-Aldrich syndrome gene homologue, is required for cell fate decisions mediated by Notch signaling

Wiskott-Aldrich syndrome proteins, encoded by the Wiskott-Aldrich syndrome gene family, bridge signal transduction pathways and the microfilament-based cytoskeleton. Mutations in the Drosophila homologue, Wasp (Wsp), reveal an essential requirement for this gene in implementation of cell fate decisions during adult and embryonic sensory organ development. Phenotypic analysis of Wsp mutant animals demonstrates a bias towards neuronal differentiation, at the expense of other cell types, resulting from improper execution of the program of asymmetric cell divisions which underlie sensory organ development. Generation of two similar daughter cells after division of the sensory organ precursor cell constitutes a prominent defect in the Wsp sensory organ lineage. The asymmetric segregation of key elements such as Numb is unaffected during this division, despite the misassignment of cell fates. The requirement for Wsp extends to additional cell fate decisions in lineages of the embryonic central nervous system and mesoderm. The nature of the Wsp mutant phenotypes, coupled with genetic interaction studies, identifies an essential role for Wsp in lineage decisions mediated by the Notch signaling pathway.

Unrelated partially matched peripheral blood stem cell transplantation with highly purified CD34+ cells in a child with Wiskott-Aldrich syndrome

Stem cell transplantation is the only curative approach to the treatment of Wiskott-Aldrich syndrome. However, using grafts from partially matched unrelated donors is associated with increased risk of graft rejection and graft-versus-host disease. In an attempt to prevent these problems, a 6-year-old boy with Wiskott-Aldrich syndrome lacking a suitable family donor, was transplanted with large numbers of unrelated highly purified CD34+ peripheral blood stem cells mismatched at one C locus. Conditioning consisted of busulfan 16 mg/kg body weight, cyclophosphamide 200 mg/kg body weight and antithymocyte globulin 20 mg/kg body weight x 3 days. The boy had a rapid hematopoietic engraftment and showed immunologic reconstitution by day +92. Although he did not receive prophylactic immunosuppression he did not develop any graft-versus-host disease and is well and alive up to now, 25 months after transplantation.

Novel mutations in the Wiskott-Aldrich syndrome protein gene and their effects on transcriptional, translational, and clinical phenotypes

Wiskott-Aldrich syndrome (WAS) is an X-linked recessive immunodeficiency characterized by thrombocytopenia, eczema, and recurrent infections, and caused by mutations in the WAS protein (WASP) gene. WASP contains several functional domains through which it interacts with proteins involved in intracellular signaling and regulation of the actin cytoskeleton. In this report, 17 WASP gene mutations were identified, 12 of which are novel. DNA of affected males and obligate carriers was PCR amplified and analyzed by SSCA, heteroduplex analysis, and direct sequencing. The effects of the mutations at the mRNA and protein level were ascertained by RT-PCR and Western blot analyses. All missense mutations were located in exons 1-4. Most of the nonsense, frameshift and splice site mutations were found in exons 6-11. Mutations that alter splice sites led to the synthesis of several types of mRNAs, a fraction of which represented the normally spliced product. The presence of normally spliced transcripts was correlated with a milder phenotype. When one such case was studied by Western blotting, reduced amounts of normal-size WASP were present. In other cases as well, a correlation was found between the amount of normal or mutant WASP present and the phenotypes of the affected individuals. No protein was detected in two individuals with severe WAS. Reduced levels of a normal-size WASP with a missense mutation were seen in two individuals with XLT. It is concluded that mutation analysis at the DNA level is not sufficient for predicting clinical course. Studies at the transcript and protein level are needed for a better assessment.

Structure of the Homer EVH1 domain-peptide complex reveals a new twist in polyproline recognition

Homer EVH1 (Ena/VASP Homology 1) domains interact with proline-rich motifs in the cytoplasmic regions of group 1 metabotropic glutamate receptors (mGluRs), inositol-1,4,5-trisphosphate receptors (IP3Rs), and Shank proteins. We have determined the crystal structure of the Homer EVH1 domain complexed with a peptide from mGluR (TPPSPF). In contrast to other EVH1 domains, the bound mGluR ligand assumes an unusual conformation in which the side chains of the Ser-Pro tandem are oriented away from the Homer surface, and the Phe forms a unique contact. This unusual binding mode rationalizes conserved features of both Homer and Homer ligands that are not shared by other EVH1 domains. Site-directed mutagenesis confirms the importance of specific Homer residues for ligand binding. These results establish a molecular basis for understanding the biological properties of Homer-ligand complexes.

X-Linked syndrome of polyendocrinopathy, immune dysfunction, and diarrhea maps to Xp11.23-Xq13.3

We describe genetic analysis of a large pedigree with an X-linked syndrome of polyendocrinopathy, immune dysfunction, and diarrhea (XPID), which frequently results in death during infancy or childhood. Linkage analysis mapped the XPID gene to a 17-cM interval defined by markers DXS8083 and DXS8107 on the X chromosome, at Xp11. 23-Xq13.3. The maximum LOD score was 3.99 (recombination fraction0) at DXS1235. Because this interval also harbors the gene for Wiskott-Aldrich syndrome (WAS), we investigated mutations in the WASP gene, as the molecular basis of XPID. Northern blot analysis detected the same relative amount and the same-sized WASP message in patients with XPID and in a control. Analysis of the WASP coding sequence, an alternate promoter, and an untranslated upstream first exon was carried out, and no mutations were found in patients with XPID. A C-->T transition within the alternate translation start site cosegregated with the XPID phenotype in this family; however, the same transition site was detected in a normal control male. We conclude that XPID maps to Xp11.23-Xq13.3 and that mutations of WASP are not associated with XPID.

Wiskott-Aldrich syndrome protein regulates podosomes in primary human macrophages

Wiskott-Aldrich syndrome protein (WASp) is a hematopoietic-specific, multidomain protein whose mutation is responsible for the immunodeficiency disorder Wiskott-Aldrich syndrome. WASp contains a binding motif for the Rho GTPase CDC42Hs as well as verprolin/cofilin-like actin-regulatory domains, but no specific actin structure regulated by CDC42Hs-WASp has been identified. We found that WASp colocalizes with CDC42Hs and actin in the core of podosomes, a highly dynamic adhesion structure of human blood-derived macrophages. Microinjection of constitutively active V12CDC42Hs or a constitutively active WASp fragment consisting of the verprolin/cofilin-like domains led to the disassemly of podosomes. Conversely, macrophages from patients expressing truncated forms of WASp completely lacked podosomes. These findings indicate that WASp controls podosome assembly and, in cooperation with CDC42Hs, podosome disassembly in primary human macrophages.