Endogenous retroviral sequences regulate tissue-specific expression of a human salivary amylase gene in transgenic mice.

Abstract

The human salivary amylase genes are flanked by an endogenous retroviral-like element, ERVA1, which is not present in the closely related pancreatic amylase genes. To evaluate the contribution of ERVA1 to the tissue specificity of the salivary amylase genes, we analyzed ten constructs in transgenic mice, including three human genomic fragments and seven fusion constructs with human growth hormone gene as the reporter gene. Expression of the AMY1C gene in transgenic mice was detected with a riboprobe specific for human amylase transcripts and expression of human growth hormone was determined by a radioimmunomatrix assay. A functional parotid-specific enhancer was localized to a 677 bp region between $-$1003 to $-$327 of AMY1C by transfer to a heterologous promoter. This parotid enhancer is completely derived from the inserted retrovirus. A positive regulatory element was localized to the region $-$1003 to $-$826 by deletion analysis. This sequence is derived from the 5$\sp\prime$ untranslated region of the retrovirus and is absent in both human pancreatic amylase genes. These results indicate that parotid-specific expression of the human AMY1C gene is dependent upon the adjacent endogenous retroviral-like element. This may be the first example of an endogenous retrovirus which regulates tissue-specific gene expression. Sequence comparison between a 252 bp region of this parotid-specific enhancer and the 5$\sp\prime$ flanking regions of other parotid genes identified three conserved DNA elements. Results from gel mobility-shift assays suggest that these elements can bind parotid nuclear proteins. The interaction of these DNA elements with parotid nuclear proteins may contribute to tissue-specific expression of AMY1C. This work represents the first identification of sequences required for gene expression in the salivary glands. It also provides an example of rapid evolutionary change in gene expression due to transposition of DNA sequences. The presence of several thousand copies of retroviral-like elements in the human genome suggests that this mechanism may be more widespread than previously recognized.