| dc.description.abstract | The pleomorphic adenoma is a benign epithelial tumor originating from the major and minor salivary glands. It is the most common type of salivary gland tumor, accounting for almost half of all neoplasms in these organs. Cytogenetically, pleomorphic adenomas are characterized by recurrent rearrangments involving the chromosome regions 3p21, 8q12 and 12q13-15. The most frequent abnormality is a reciprocal t(3;8)(p21;q12) translocation, or variants thereof in which the 8q segment is translocated to a variety of chromosome regions. The purpose of the present series of investigations was to identify the genes involved in the t(3;8)(p21;q12) using a positional cloning approach. YAC clones corresponding to known genetic markers flanking band 8q12 were used to initiate a walk towards the 8q12 breakpoints. This resulted in the establishment of two non-overlapping YAC contigs covering approximately 75% of band 8q12. The centromeric contig covers approximately 2 Mb of genomic DNA and consists of 34 overlapping YAC clones containing at least seven putative CpG islands, and three ESTs. The telomeric contig consists of 23 YACs and covers about 5 Mb of genomic DNA. FISH mapping of YACs and cosmids from these contigs revealed that the majority of breakpoints clustered within a 300 kb subregion in the centromeric contig. Subsequent studies of new STSs and ESTs from this region led to the discovery of the PLAG1 gene, a novel, developmentally regulated gene coding for a zinc finger protein. The gene, which consists of 5 exons of which the first three are non-coding, spans about 35 kb, with a large intron (approximately 25 kb) between exon 1 and exon 2. The deduced amino acid sequence of the PLAG1 protein reveals seven canonical C2H2 zinc finger domains in the N-terminal region and a serine-rich C-terminus. There are two potential nuclear localization signals in the N-terminal region. 5' RACE analysis of tumors with t(3;8) enabled us to identify CTNNB1 as the chromosome 3 gene fused to PLAG1. CTNNB1 codes for ß-catenin, a protein interface functioning in the WG/WNT signalling pathway and in the specification of cell fate during embryogenesis. The t(3;8) results in promoter swapping between PLAG1 and CTNNB1. Fusions invariably 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) PLAG1 is activated whereas the expression of CTNNB1 is reduced. Activation of PLAG1 was also observed in an adenoma with a t(8;15)(q12;q14) translocation, demonstrating that PLAG1 activation is not restricted to tumors with t(3;8). To our knowledge this is the first example of promoter swapping in solid tumors. The identification and cloning of a novel "benign oncogene" activated by chromosome translocation in pleomorphic adenomas constitute an important step towards an increased understanding of the molecular pathogenesis of benign neoplastic growth. The results potentially allow for the design of novel diagnostic tools and targeted therapy of pleomorphic adenomas. | en |
