Spontaneous melanocytosis in transgenic mice.

Clone B is a 2-kb fragment of cloned genomic DNA involved in adipocyte differentiation in vitro. Insertion of this DNA fragment into the genome of a variety of cell lines results in committing the recipient cells to undergo adipocyte differentiation. Construction of transgenic mice with Clone B DNA resulted in an unexpected phenotype--spontaneous melanocytosis. The present study describes the distribution and morphology of melanin-containing lesions in these transgenic mice. Spontaneous melanin-containing dermal lesions appeared on the ears, snout, and perianal regions of transgenic mice by the age of 3-4 months. Multifocal dermal masses rapidly developed into raised lesions, which appeared to spread to adjacent skin. Ultrastructural examination of lymph nodes, spleen, and dermal lesions of these mice revealed membrane-bound melanin with effacement f the organelle structure of severely affected cells. Protein gel electrophoresis revealed elevated activity of tyrosinase in the pinnae, skin, perianal mass, and lymph nodes. This line of transgenic mice may provide a useful model for investigation of the etiology and progression of benign and malignant melanin-containing tumors.