GENOMIC ORGANIZATION OF THE HUMAN BETA-CATENIN GENE (CTNNB1)

The cytoplasmic beta-catenin protein is implicated in signal transduct ion and associates with both the cell-cell adhesion protein E-cadherin and the tumor suppressor gene product APC. We determined the primary structure of the human beta-catenin gene (CTNNB1) by analysis of cDNA and genomic clones. The size of the complete gene was determined to be 23.2 kb. Restriction mapping and partial sequence analysis revealed 1 6 exons. All splice donor and acceptor sites were conformable to the G T/AG rule. The exon size ranged from 61 to 790 bp. Half of the introns were smaller than 550 bp, with the smallest being 84 bp and the longe st being 6700 bp. The intron-exon boundaries did not coincide either w ith conserved sites in the 12 armadillo repeat sequences of beta-caten in or with intron-exon boundaries in the armadillo gene of Drosophila. A major site for transcription initiation was identified as an A resi due 214 nucleotides upstream of the ATG initiation codon. The resultin g transcript is 3362 nucleotides long. Compared to the previously publ ished mRNA sequence, additional residues were identified, 16 at the 5' end and 766 at the 3' end of the mRNA An alternative splice acceptor site within exon 16 reduced the 3' UTR sequence by 159 bp. Polymerase chain reaction on cDNA from 14 human cell lines demonstrated the gener al occurrence of both splice variants. The 5'-flanking region is highl y GC-rich and lacks a CCAAT box, but contains a TATA box and potential binding sites for several transcription factors, such as NF kappa B, SP1, AP2, and EGR1. Both a 437-bp fragment and a 6 kb fragment, contai ning about 4.7 kb of the 5'-flanking region in addition to the noncodi ng exon 1 and 1 kb of intron 1, showed clear promoter activity when th ese fragments were linked to a secreted alkaline phosphatase reporter gene and transfected into a mouse epithelial cell line. (C) 1996 Acade mic Press, Inc.