DISTAL TYROSINASE UPSTREAM ELEMENT STIMULATES GENE-EXPRESSION IN NEURAL-CREST-DERIVED MELANOCYTES OF TRANSGENIC MICE - POSITION-INDEPENDENTAND MOSAIC EXPRESSION

We have assessed the importance of a melanocyte-specific DNase I hyper sensitive 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 al bino 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 pr o,ide position-independent expression of the transgene. Two exceptions to complete position independence were seen; these lines displayed a mosaic expression pattern in which the transgene was expressed fully i n some melanocyte clones but less so in others, resulting in transvers e stripes of colours ranging from near white to dark grey. Unexpectedl y, pigmentation in the eye of all transgenic lines containing the upst ream region was nonuniform, in that the neural-crest-derived melanocyt es of the choroid and anterior iris contained significantly more pigme nt than those derived from the optic cup (retinal pigment epithelium a nd posterior iris). Transgenes containing a small part or none of the upstream region were expressed poorly and in a position-dependent mann er; of those lines that were visibly pigmented, expression was equal i n the neural crest and optic-cup-derived cells of the eye. Mice with t ransgenes containing DNA sequences encompassing the hypersensitive sit e but lacking most of the matrix attachment region were, on average, p oorer expressors than those containing the entire upstream region; the highest expressing line of this series, however, had a pigmentation l evel similar to the lines with the full upstream region. Thus, full tr anscriptional enhancement activity may lie within the segment containi ng the hypersensitive site, but position-independent expression may re quire the flanking matrix attachment region containing sequences.