TP53 is the most frequently mutated tumor suppressor gene in human cancer, with nearly 50% of all tumors exhibiting a loss-offunction mutation. To further elucidate the genetic pathways involving TP53 and cancer, we have exploited the zebrafish, a powerful vertebrate model system that is amenable to wholegenome forward-genetic analysis and synthetic-lethal screens. Zebrafish lines harboring missense mutations in the tp53 DNAbinding domain were identified by using a target-selected mutagenesis strategy. Homozygous mutant fish from two of these lines were viable and exhibited mutations similar to those found in human cancers (tp53 N168K and tp53 M214K ). Although homozygous tp53 N168K mutants were temperature-sensitive and suppressed radiation-induced apoptosis only at 37°C, cells in the tp53 M214K embryos failed to undergo apoptosis in response to ␥ radiation at both 28 and 37°C. Unlike wild-type control embryos, irradiated tp53 M214K embryos also failed to up-regulate p21 and did not arrest at the G 1͞S checkpoint. Beginning at 8.5 months of age, 28% of tp53 M214K mutant fish developed malignant peripheral nerve sheath tumors. In addition to providing a model for studying the molecular pathogenic pathways of malignant peripheral nerve sheath tumors, these mutant zebrafish lines provide a unique platform for modifier screens to identify genetic mutations or small molecules that affect tp53-related pathways, including apoptosis, cell-cycle delay, and tumor suppression.

Melanoma is the most lethal form of skin cancer, and the incidence and mortality rates are rapidly rising. Epidemiologically, high numbers of nevi (moles) are associated with higher risk of melanoma . The majority of melanomas exhibit activating mutations in the serine/threonine kinase BRAF . BRAF mutations may be critical for the initiation of melanoma ; however, the direct role of BRAF in nevi and melanoma has not been tested in an animal model. To directly test the role of activated BRAF in nevus and melanoma development, we have generated transgenic zebrafish expressing the most common BRAF mutant form (V600E) under the control of the melanocyte mitfa promoter. Expression of mutant, but not wild-type, BRAF led to dramatic patches of ectopic melanocytes, which we have termed fish (f)-nevi. Remarkably, in p53-deficient fish, activated BRAF induced formation of melanocyte lesions that rapidly developed into invasive melanomas, which resembled human melanomas and could be serially transplanted. These data provide direct evidence that BRAF activation is sufficient for f-nevus formation, that BRAF activation is among the primary events in melanoma development, and that the p53 and BRAF pathways interact genetically to produce melanoma.