GENOMIC ORGANIZATION OF MOUSE AND HUMAN BRUTONS AGAMMAGLOBULINEMIA TYROSINE KINASE (BTK) LOCI

Btk is a cytoplasmic protein tyrosine kinase (PTK) that has been direc tly implicated in the pathogenesis of X-linked agammaglobulinaemia (XL A) in humans and X-linked immunodeficiency (Xid) in mice. We have isol ated phage and cosmid clones that allowed us to deduce the genomic str ucture of mouse and human Btk loci. The mouse and human genes are cont ained within genomic regions that span approximately 43.5 kb and 37.5 kb, respectively. Both loci contain 18 coding exons ranging between 55 and 560 bp in size with introns ranging in size from 164 bp to approx imately 9 kb. The 5'-untranslated regions are encoded by single exons located approximately 9 kb upstream of the first coding exon. Exon 18 encodes for the last 23 carboxyl-terminal amino acids and the entire 3 '-untranslated region. The location of intron/exon boundaries in the c atalytic domains of the mouse and human Btk loci differs from that fou nd in other described sub-families of intracellular PTKs, namely that of Src, Fes/Fer, Csk, and Abl/Arg. This observation is consistent with the classification of Btk together with the recently characterized ki nases, Tec and Itk, into a separate sub-family of cytoplasmic PTKs. Pu tative transcription initiation sites in the mouse and human Btk loci have been determined by using the rapid amplification of cDNA ends ass ay. Similar to many other PTK specific genes, the putative Btk promote rs lack obvious TATAA and CAAAT motifs. Putative initiator elements an d potential binding sites for Ets (PEA-3), zeste, and PuF transcriptio n factors are located within the 300 bp which are located upstream of the major transcription start site in both species. These sequences ca n mediate promoter activity when placed upstream of a promotorless chl oramphenicol acetyl transferase reporter gene in an orientation-depend ent manner. The present analysis will significantly facilitate the mut ational analyses of patients with XLA and the further characterization of the function and regulation of the Btk molecule.