Promoter swapping between the genes for a novel zinc finger protein and beta-catenin in pleiomorphic adenomas with t(3;8)(p21;q12) translocations

Pleiomorphic adenoma of the salivary glands is a benign epithelial tumour occurring primarily in the major and minor salivary glands(1). It is by far the most common type of salivary gland tumour. Microscopically, pleiomorphic adenomas show a marked histological diversity with epithelial, myoepithelial and mesenchymal components in a variety of patterns. In addition to a cytogenetic subgroup with normal karyotypes, pleiomorphic adenomas are characterized by recurrent chromosome rearrangements, particularly reciprocal translocations, with breakpoints at 8q12, 3p21, and 12q13-15, in that order of frequency(2,3). The most common abnormality is a reciprocal t(3;8)(p21;q12). We here demonstrate that the t(3;8)(p21;q12) results in promoter swapping between PLAG1, a novel, developmentally regulated zinc finger gene at 8q12, and the constitutively expressed gene for beta-catenin (CTNNB1), a protein interface functioning in the WG/WNT signalling pathway and specification of cell fate during embryogenesis(4). Fusions occur in the 5'-non-coding regions of both genes, exchanging regulatory control elements while preserving the coding sequences. Due to the t(3;8)(p21;q12), PLAG1 is activated and expression levels of CTNNB1 are reduced. Activation of PLAG1 was also observed in an adenoma with a variant translocation t(8;15)(q12;q14). Our results indicate that PLAG1 activation due to promoter swapping is a crucial event in salivary gland tumourigenesis.