DNA-based mutation analysis of Bruton's tyrosine kinase gene in patients with X-linked agammaglobulinaemia

The identification of the BTK (Bruton's tyrosine kinase) genedefective in human immunoglobulln deficiency X-linked agammaglobulinaemla(XLA) and characterlsation of BTK exon–intron boundarleshas now allowed the analysis of mutations and polymorphismsat the level of genomic DNA. Using Southern blot analysis andthe polymerase chain reaction single strand conformation polymorphism(PCR–SSCP) assay, amplifying all 19 exons and the putativepromoter region with a single annealling temperature, mutationshave been identified in 19 out of 24 unrelated patients diagnosedas having XLA. Apart from a large deletion involving exon 19,nine missense (F25S, R288W, I370M, M509V, R525P, N526K, R562W,A582V and G594R), two nonsense (E277X and R525X), five frameshiftand two splice site mutations have been found affecting mostcoding exons and all major enzyme domains. No mutations or polymorphismswere detected in the putative promoter region. A single nucleotidedeletion located in the last exon, resulting in a truncationof the eight C-terminal residues of Btk and a typical XLA phenotype,indicates structural and/or functional importance of Btk helixI In the catalytic domain. Although allelic heterogeneity atthe BTK locus may partly explain clinical variability In familleswith XLA, compensatory and redundant mechanisms involved inB-cell development must play a role in the phenotypic diversityof the disease.