Molecular cloning of a functional promoter of the human plakoglobin gene

Objective: Plakoglobin (Pg) is the only cytoplasmic protein component common to both junctional complexes mediating cell-cell adhesion, adherens junctions and desmosomes. In these complexes Pg appears to act as a linker protein anchoring transmembrane proteins of the cadherin superfamily to the actin cytoskeleton and intermediate filament system respectively. Intercellular adhesion is frequently disturbed in skin diseases and in carcinomas, enabling tumour progression and metastasis. Whereas Pg expression is lost in some thyroid tumours and carcinoma cell lines, little information on Pg gene regulation is currently available owing to a lack of promoter studies. Design and methods: We have cloned and sequenced genomic DNA from a human library that resulted in 979 bp upstream of the published Pg cDNA. The transcriptional start was mapped by rapid amplification of cDNA ends. Methylation-specific PCR of bisulfite-modified cell fine DNA was applied to probe the methylation status of a promoter-associated CpG island. Reporter-gene constructs of various promoter fragments were transiently transfected in thyroid carcinoma cell lines and their activities were determined by luciferase measurements. Results and conclusions: A I kb DNA fragment harbouring a functional promoter of the human Pg gene was cloned and characterized. The sequence lacks a canonical TATA box, but contains putative CCAAT boxes as well as various putative binding sites for transcription factors, among them SP I and AP2, proximal to the transcriptional start. Considerable promoter activity was found in thyroid cell lines and deletion analysis indicated that a 300 bp region proximal to the 5'-untranslated region of the mRNA represents the minimal promoter of the human Pg gene. As cells lacking endogenous Pg expression were found to contain methylated CpG dinucleotides in a CpG island located around the transcriptional start site, it is suggested that epigenetic mechanisms such as DNA methylation contribute to dysregulated Pg expression.