Identification of WASP mutations in patients with Wiskott-Aldrich syndrome and isolated thrombocytopenia reveals allelic heterogeneity at the WAS locus

Abstract

Mutation in the gene encoding the recently isolated WASP protein has now been identified as the genetic defect responsible for the X-linked Wiskott-Aldrich syndrome (WAS), a primary immunodeficiency disease associated with extensive phenotypic variability. To elucidate the range of WASP mutations responsible for WAS, we used PCR-SSCP analysis to screen for WASP gene mutation in 19 unrelated boys with the diagnosis of classical or attenuated WAS or isolated thrombocytopenia. All 19 patients had WASP mutations, each of which localized to the initial three or terminal three exons of the gene, and the majority of which were unique in each case. However, a missense mutation which results in substitution of the arginine at WAS codon 86 was identified in three boys with severe WAS as well as in one boy presenting with thrombocytopenia alone. While the three mutations found in the isolated thrombocytopenia patients leave the reading frame intact, about one-half of the gene alterations detected in both severe and attenuated WAS patients result in frameshifted transcript and premature translation termination. These findings therefore confirm the association of WAS with WASP mutation and identify WASP mutation as a cause for isolated congenital thrombocytopenia in males. While the WASP gene defects responsible for isolated thrombocytopenia and other mild presentations of WAS do not appear distinct from those resulting in severe WAS, these data indicate that analysis of WASP gene mutation provides a valuable tool for distinguishing the spectrum of WAS patients and the subset of males with isolated thrombocytopenia who represent mild cases of WAS.

Missense mutations of the WASP gene cause intermittent X-linked thrombocytopenia

“…1 Cloning of the WASP gene, mutated in WAS, 2 has allowed the recognition of attenuated forms of the syndrome, with thrombocytopenia and mild eczema or infections, also referred to as X-linked thrombocytopenia (XLT, MIM 313 900). [3][4][5] Most patients with XLT have missense mutations within exons 1 and 2, leading to decreased but detectable protein expression, 6,7 whereas a wide spectrum of mutations, most often leading to the absence of protein, have been detected in classical WAS. 7,8 We have identified 2 families in which affected males have a history of intermittent thrombocytopenia with persistently reduced MPV.…”

Computational Analysis of Binding of the GBD Domain of WASP to Different Binding Partners

“…As discussed above, mutations in the VCA domain (K476E and R477K) are known to cause different forms of WAS. 13,22,23 These mutations occur at the C-terminus of the VCA domain, 18 and we have introduced them in our simulations either simultaneously or one at a time. As a consequence of introducing the mutations simultaneously, the whole complex significantly destabilizes ( Figure 2C).…”

“…13,18−20 Second, we analyze the mutations in the VCA domain responsible for WAS, and elucidate the microscopic background of the destabilizing effect they have on the GBD/VCA complex. These mutations in the VCA domain occur in nature and cause the typical form of WAS (K476E) 13,22 and the XLT form of WAS (R477K). 13,23 Overall, our ambition is to provide a microscopic, dynamic picture behind these two main ways, one mediated by an externally introduced protein and the other one by inherited mutations, through which the interactions involving the GBD domain of WASP participate in different disease processes.…”

Two novel mutations identified in the Wiskott-Aldrich syndrome protein gene cause Wiskott-Aldrich syndrome and thrombocytopenia

“…Until now only 6 different mutations have been reported on exon 12 (Table II): two missense mutations in the last codon of the WASP protein (X503R and X503S) (5,15), two deletions (1519delT and 1543delAGTG) (16,17), a splice defect (4) and a complex mutation (18). All these mutations resulted in aberrant WASP protein or an absence of WASP expression, and have been associated with a severe WAS phenotype.…”