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.

The zebrafish is a powerful model system for investigating embryonic vertebrate hematopoiesis, allowing for the critical in vivo analysis of cell lineage determination. In this study, we identify zebrafish myeloerythroid progenitor cells (MPCs) that are likely to represent the functional equivalent of mammalian common myeloid progenitors. Utilizing transgenic pu.1-GFP fish, real-time MPC differentiation was correlated with dynamic changes in cell motility, morphology, and gene expression. Unlike mammalian hematopoiesis, embryonic zebrafish myelopoiesis and erythropoiesis occur in anatomically separate locations. Gene knockdown experiments and transplantation assays demonstrated the reciprocal negative regulation of pu.1 and gata1 and their non-cell-autonomous regulation that determines myeloid versus erythroid MPC fate in the distinct blood-forming regions. Furthermore, forced expression of pu.1 in the bloodless mutant cloche resulted in myelopoietic rescue, providing intriguing evidence that this gene can function in the absence of some stem cell genes, such as scl, in governing myelopoiesis.