melanogaster ( Brodsky et al., 2000 Ollmann et al., 2000), and C. P53-related genes have been identified in a variety of genetically useful invertebrates, including D. However, owing to the small size of the families, mapping to a single gene on 1q23 and other loci has been difficult. In four families in which a p53 mutation was excluded, linkage mapping implicated chromosome 1q23 and at least one additional locus ( Bachinski et al., 2005), indicating that multiple loci contribute to LFS. Nonetheless, the genes responsible for the remaining 25–45% of LFS families, 60% of LFL families and 94% of LFI families are unknown. In addition, Chk2 −/− mice do not develop tumors ( Takai et al., 2002). Checkpoint kinase 2 (Chk2, also known as Chek2) had been implicated in 5% of LFS families ( Bell et al., 1999) however, subsequent patient analysis has determined that Chk2 is not the cause of LFS ( Evans et al., 2008). Germline mutations in p53 have been identified in 50–70% of LFS families, 40% of LFL families and 6% of LFI families ( Birch et al., 1994 Chompret et al., 2000 Frebourg et al., 1995 Li and Fraumeni, 1969 MacGeoch et al., 1995). Li-Fraumeni-like syndrome (LFL) and incomplete LFS (LFI) are similar to LFS, but with slightly different diagnostic criteria. The criteria for diagnosis of LFS are that the presenting individual has a sarcoma before the age of 45 and has two first-degree relatives who either developed cancer before the age of 45 or who developed a sarcoma at any age. Li-Fraumeni syndrome (LFS) is an autosomal dominant, highly penetrant cancer predisposition that presents with a wide variety of tumor types at an early age, with sarcomas being the hallmarks of the disease ( Kleihues et al., 1997 Varley, 2003). Additionally, we demonstrate that the p53 regulatory pathway, including Mdm2 regulation, is evolutionarily conserved in zebrafish, providing a bona fide biological context in which to systematically uncover novel modifier genes and therapeutic agents for human LFS. Furthermore, as with some LFS mutations, the zebrafish p53 I166T allele is a loss-of-function allele with dominant-negative activity in vivo. This report of LOH indicates that Knudson’s two-hit hypothesis, a hallmark of human autosomal dominant cancer syndromes, can be modeled in zebrafish. As in humans with LFS, tumors arise in heterozygotes and display loss of heterozygosity (LOH). Here, we describe a forward genetic screen in zebrafish embryos that was used to identify LFS candidate genes, which yielded a p53 mutant (p53 I166T) that as an adult develops tumors, predominantly sarcomas, with 100% penetrance. Therefore, modeling LFS in a vertebrate system that is accessible to both large-scale genetic screens and in vivo cell biological studies will facilitate the in vivo dissection of disease mechanisms, help identify candidate genes, and spur the discovery of therapeutic compounds. Although a key role for the tumor suppressor p53 has been implicated in LFS, the genetic and cellular mechanisms underpinning this disease remain unknown. Li-Fraumeni syndrome (LFS) is a highly penetrant, autosomal dominant, human familial cancer predisposition.
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