The Wiskott-Aldrich syndrome (WAS) is an X-linked disorder characterized by thrombocytopenia and small platelet volume, without or with eczema, immunodeficiency, and autoimmune disorders. The WAS gene has been cloned and sequenced, and a polyclonal antibody to the WAS protein (WASP) generated. Mutations are found throughout the entire WAS gene and consist of point mutations, insertions and deletions. Of 36 unrelated WAS families undergoing sequence analysis, ten had unique splice site mutations affecting single nucleotides within an exon or intron. Splicing products were identified by subcloning RT-PCR products and sequencing at least 20 clones. The effect on WASP was determined by Western blot analysis. In three patients with classic WAS phenotype, a single population of RT-PCR product was found, consisting of either a deletion or insertion, as expected from the mutated genomic DNA. In seven patients, four of whom had only platelet abnormalities, multiple splicing products, due to insertions and/or deletions of part of or entire introns/exons, were identified. As an example, one patient with mild WAS and a trace amount of WASP had five different RT-PCR products: of 20 clones sequenced, two had normally spliced cDNA; three clones were found to have exon 4 deleted; three had 23 nucleotides of exon 4 deleted; eight showed insertion of intron 3; and four insertion of introns 3 and 4.

These studies demonstrate the complexity of splice site mutations present in approximately 25% of WAS patients. Alternative splicing may generate, in some patients, WAS gene products that maintain some biologic activity resulting in a mild phenotype resembling X-linked thrombocytopenia.