The gene Trp53 is among the most frequently mutated and studied genes in human cancer, but the mechanisms by which it suppresses tumour formation remain unclear. We generated mice with an allele encoding changes at Leu25 and Trp26, known to be essential for transcriptional transactivation and Mdm2 binding, to enable analyses of Trp53 structure and function in vivo. The mutant Trp53 was abundant, its level was not affected by DNA damage and it bound DNA constitutively; however, it showed defects in cell-cycle regulation and apoptosis. Both mutant and Trp53-null mouse embryonic fibroblasts (MEFs) were readily transformed by oncogenes, and the corresponding mice were prone to tumours. We conclude that the determining pathway for Trp53 tumour-suppressor function in mice requires the transactivation domain.

Neurons of cranial sensory ganglia are derived from the neural crest and ectodermal placodes, but the mechanisms that control the relative contributions of each are not understood. Crest cells of the second branchial arch generate few facial ganglion neurons and no vestibuloacoustic ganglion neurons, but crest cells in other branchial arches generate many sensory neurons. Here we report that the facial ganglia of Hoxa2 mutant mice contain a large population of crest-derived neurons, suggesting that Hoxa2 normally represses the neurogenic potential of second arch crest cells. This may represent an anterior transformation of second arch neural crest cells toward a fate resembling that of first arch neural crest cells, which normally do not express Hoxa2 or any other Hox gene. We additionally found that overexpressing Hoxa2 in cultures of P19 embryonal carcinoma cells reduced the frequency of spontaneous neuronal differentiation, but only in the presence of cotransfected Pbx and Meis Hox cofactors. Finally, expression of Hoxa2 and the cofactors in chick neural crest cells populating the trigeminal ganglion also reduced the frequency of neurogenesis in the intact embryo. These data suggest an unanticipated role for Hox genes in controlling the neurogenic potential of at least some cranial neural crest cells.

This article described the generation of a mouse in which homologous recombination was used to change codons 25 and 26 of the gene Trp53 to glutamine and serine. The initial sequencing of the full-length cDNA from mouse embryonic fibroblasts expressing this allele was considered to be wild-type at all codons except for 25 and 26. A reanalysis of the sequence showed that this mutant also contains a valine at codon 135, which is referred to as a "provisional wild type codon" in the National Center for Biotechnology Information's LocusLink tool (accession numbers: Trp53 mRNA, NM_011640; p53 protein, NP_035770). Sequence analysis showed that the lambda genomic clone containing an EcoRI fragment encompassing Trp53 (Nature 356, 215-221; 1992) that was used to prepare the targeting construct contained the Val135 codon, whereas Trp53 in laboratory mice (e.g., C57Bl6) encodes alanine at codon 135