¹Department of Dermatology, Veterans Affairs Medical Center, University of California, San Francisco, California, USA

Correspondence: Dr Maria L. Wei, Department of Dermatology, Dermatology Service (190), Veterans Affairs Medical Center, 4150 Clement Street, San Francisco, California 94121, USA. E-mail: weim@derm.ucsf.edu

Received 18 May 2006; Revised 30 July 2006; Accepted 3 August 2006; Published online 19 October 2006.

Abstract

The pale ear (ep) mouse strain is a model for the Hermansky–Pudlak syndrome type 1 (HPS-1), an autosomal recessive disorder causing pigmentary dilution, visual disturbances, bleeding diatheses, pulmonary fibrosis, and granulomatous colitis. The ep mice have a coat color very similar to the black-colored parental strain, C57BL/6. However, the ears and tails of ep mice are significantly hypopigmented compared with the control animals, suggesting that the gene mutation in ep mice reveals a differential regulation of melanocyte function in dorsal back skin melanocytes versus tail or ear skin. In this study, we analyzed the mutant phenotype in detail and determined that in the tail, the defective gene causes delayed onset of interfollicular epidermal melanocyte tyrosinase activity, decreased numbers of melanocytes in the interfollicular epidermis and dermis, and severe immaturity of tail epidermal melanosomes, findings not observed in dorsal back follicular melanocytes. These results highlight differences between follicular and interfollicular melanocyte biology and demonstrate that defects in the ep protein not only affect melanosome biogenesis, but also play a developmental role in determining interfollicular epidermal and dermal melanocyte function. The implications of these findings for the mechanisms governing physiologic variation in human pigmentation and for the pathogenesis of vitiligo are discussed.