A distal tyrosinase upstream element stimulates gene expression in neural-crest-derived melanocytes of transgenic mice: position-independent and mosaic expression

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Abstract
We have assessed the importance of a melanocyte-specific DNase I hypersensitive site and matrix attachment region situated 15 kb upstream of the mouse tyrosinase gene by analysis in transgenic mice. Transgenes containing all, part, or none of this region linked to the tyrosinase promoter and human tyrosinase cDNA were introduced into genetically albino mice, and pigmentation and transgene message levels were analyzed in the resulting transgenic lines. The effect of the upstream region was to enhance significantly gene expression in melanocytes, and to provide position-independent expression of the transgene. Two exceptions to complete position indepen-dence were seen; these lines displayed a mosaic expression pattern in which the transgene was expressed fully in some melanocyte clones but less so in others, resulting in trans-verse stripes of colours ranging from near white to dark grey. Unexpectedly, pigmentation in the eye of all trans-genic lines containing the upstream region was non-uniform, in that the neural-crest-derived melanocytes of the choroid and anterior iris contained significantly more pigment than those derived from the optic cup (retinal pigment epithelium and posterior iris). Transgenes con-taining a small part or none of the upstream region were expressed poorly and in a position-dependent manner; of those lines that were visibly pigmented, expression was equal in the neural crest and optic-cup-derived cells of the eye. Mice with transgenes containing DNA sequences encompassing the hypersensitive site but lacking most of the matrix attachment region were, on average, poorer expressors than those containing the entire upstream region; the highest expressing line of this series, however, had a pigmentation level similar to the lines with the full upstream region. Thus, full transcriptional enhancement activity may lie within the segment containing the hyper-sensitive site, but position-independent expression may require the flanking matrix attachment region containing sequences.