A regulatory element within a coding exon modulates keratin 18 gene expression in transgenic mice.

Multiple tissue-specific, DNase-hypersensitive sites are correlated with known or potential regulatory regions of the human keratin 18 (K18) gene. One of these sites is found within exon 6, close to a potential AP-1 binding site. Footprint analysis confirmed that this site is capable of binding c-Jun and c-Fos in vitro. However, exon 6 can stimulate expression of a reporter gene driven by the K18 proximal promoter independent of AP-1 in F9 cells and additionally modulates AP-1 responsiveness when in combination with an intron enhancer. Analysis in transgenic mice and by transient transfections of mutant forms of the K18 gene showed that exon 6 contributes to the expression of the K18 gene. However, substitution of part of exon 6 with the corresponding part of the keratin 19 gene which lacks an AP-1 site decreased but did not destroy the regulatory activity of the exon. Furthermore, this mutation did not alter either the tissue specificity or the position-independent and copy number-dependent behavior of the K18 gene. In contrast, a frameshift mutation within exon 6 dramatically decreased the expression of the gene. K18 RNA expression from the frameshift mutation was less than 10% of the wild type K18 transgene. This decline in expression was the result of a combination of decreased stability of mutant K18 RNA and the creation of a negative regulatory element that can interact with the first intron regulatory elements and actively suppress K18 expression. These results demonstrate that a protein-coding portion of the K18 gene also has a regulatory function.