Conserved mechanism of PLAG1 activation in salivary gland tumors with and without chromosome 8q12 abnormalities: Identification of SII as a new fusion partner gene

We have previously shown (K. Kas et al., Nat. Genet., 15: 170-174, 1997) th at the developmentally regulated zinc finger gene pleomorphic adenoma gene 1 (PLAG1) is the target gene in 8q12 in pleomorphic adenomas of the salivar y glands with t(3;8)(p21;q12) translocations, The t(3;8) results in promote r swapping between PLAG1 and the constitutively expressed gene for beta-cat enin (CTNNB1), leading to activation of PLAG1 expression and reduced expres sion of CTNNB1, Here we have studied the expression of PLAG1 by Northern bl ot analysis in 47 primary benign and malignant human tumors with or without cytogenetic abnormalities of 8q12, Overexpression of PLAG1 was found in 23 tumors (49%). Thirteen of 17 pleomorphic adenomas with a normal karyotype and 5 of 10 with 12q13-15 abnormalities overexpressed PLAG1, which demonstr ates that PLAG1 activation is a frequent event in adenomas irrespective of karyotype, In contrast, PLAG1 was overexpressed in only 2 of 11 malignant s alivary gland tumors analyzed, which suggests that, at least in salivary gl and tumors, PLAG1 activation preferentially occurs in benign tumors, PLAG1 overexpression was also found in three of nine mesenchymal tumors, i.e., in two uterine leiomyomas and one leiomyosarcoma. RNase protection, rapid amp lification of 5'-cDNA ends (5'-RACE), and reverse transcription-PCR analyse s of five adenomas with a normal karyotype revealed fusion transcripts in t hree tumors. Nucleotide sequence analysis of these showed that they contain ed fusions between PLAG1 and CTNNB1 (one case) or PLAG1 and a novel fusion partner gene, i.e., the gene encoding the transcription elongation factor S II (two cases), The fusions occurred in the 5' noncoding region of PLAG1, l eading to exchange of regulatory control elements and, as a consequence, ac tivation of PLAG1 gene expression, Because all of the cases had grossly nor mal karyotypes, the rearrangements must result from cryptic rearrangements. The results suggest that in addition to chromosomal translocations and cry ptic rearrangements, PLAG1 may also be activated by mutations or indirect m echanisms. Our findings establish a conserved mechanism of PLAG1 activation in salivary gland tumors with and without 8q12 aberrations, which indicate s that such activation is a frequent event in these tumors.