The neurofibromatosis type 1 (Nf1) tumor suppressor is a modifier of carcinogen-induced pigmentation and papilloma formation in C57BL/6 mice

There is increasing evidence implicating the human NF1 gene in epithelial c arcinogenesis. To test if NF1 can play a part in skin tumor formation, we a nalyzed effects of the skin cancer initiator dimethylbenz-anthracene and/or the tumor promoter 12-O-tetradecanoyl-13-acetylphorbol on mice heterozygou s for null mutations in Nf1 (Nf1+/-). Mice were on the C57BL/6 background, noted for resistance to chemical carcinogens. Nf1+/- mice (18 of 24) develo ped papillomas after treatment with dimethylbenzanthracene and 12-O-tetrade canoyl-13-acetylphorbol; papillomas did not develop in wild-type C57BL/6 mi ce nor Nf1+/- mice treated with 12-O-tetradecanoyl-13-acetylphorbol alone. All papillomas analyzed (six of six) had mutations in codon 61 of H-ras, de monstrating strong cooperation between the Nf1 GTPase activating protein fo r Ras, neurofibromin, and Ras-GTP. After exposure to 12-O-tetradecanoyl-13- acetylphorbol, Nf1+/- keratinocytes showed significant, sustained, increase s in proliferation, implicating Nf1 in phorbol ester responsive pathways. T hus, Nf1 levels regulate the response of keratinocytes to 12-O-tetradecanoy l-13-acetylphorbol. Nf1+/- mice also showed a 2-fold increase in the develo pment of pigmented skin patches stimulated by dimethylbenzanthracene; patch es were characterized by hair follicles in anagen phase, implicating kerati nocytes in the aberrant hyperpigmentation. Our results show that mutation i n the Nf1 gene causes abnormal keratinocyte proliferation that can be revea led by environmental assaults such as carcinogen exposure. The data support a plausible role for NF1 mutation in human epithelial carcinogenesis.