Over-expression of mRNA sequences for the Ca++ activated non-selective cation channel, TRPM4, in prostate tumors and tumor cell lines: Identification of a novel prostate tumor selective promoter

Abstract

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Analysis of gene expression profiles in prostate normal and tumor tissues by mining a gene expression data base revealed that mRNA coding for the ion channel TRPM4 is significantly over-expressed in prostate tumor tissue. This was confirmed by expression analysis on an Affymetrix chip array, and by Taqman based analysis using additional prostate and normal tumor tissue samples. TRPM4 is also expressed in a broad range of normal tissues; however, its expression was approximately 4 fold lower than that seen in prostate tumor tissue. Northern analysis demonstrated mRNA’s of approximately 4.2kb and 6.0 kb in most tissues, (consistent with previously published data), but strikingly, prostate tumors and some prostate cell lines also expressed an additional 1.2kb mRNA. The 1.2kb mRNA was the dominant species in the prostate tumor cells and represents the major over-expressed form of the message. This mRNA was cloned and sequenced and found to represent an apparent spliced form of the predicted full length mRNA, covering exons 19 through 24. The exons code for the C-terminal 288 amino acids of the ion channel. The role of this dominant tumor specific mRNA in prostate tumor development is currently under investigation. However, preliminary antisense studies demonstrate reduced growth in LNCaP cells treated with antisense molecules. To understand the basis of elevated mRNA expression in prostate tumors, a 2kb fragment of DNA 5’ to the predicted start sequence was isolated and screened for promoter activity in LNCaP cells. This 2kb element exhibited potent promoter activity in LNCaP cells that was 2-3 fold greater than that of the PSA promoter and 30-40 fold greater than the SV40 promoter. Promoter activity was not significantly above background in non-prostate cells. Dissection of the promoter revealed that activity could be largely found in a 136 bp fragment. Moreover, in contrast to findings with the PSA promoter, activity appeared to be androgen independent. The specificity and strength of this promoter highlight a potential use in prostate tumor gene therapy.