Biomedical subjects

Franck Toledo

Publications and source records attributed to Franck Toledo.

6 recordsLinked to original sources

PubMed2006

Mouse mutants reveal that putative protein interaction sites in the p53 proline-rich domain are dispensable for tumor suppression.

The stability and activity of tumor suppressor p53 are tightly regulated and partially depend on the p53 proline-rich domain (PRD). We recently analyzed mice expressing p53 with a deletion of the PRD (p53(DeltaP)). p53(DeltaP), a weak transactivator hypersensitive to Mdm2-mediated degradation, is unable to suppress oncogene-induced tumors. This phenotype could result from the loss of two motifs: Pin1 sites proposed to influence p53 stabilization and PXXP motifs proposed to mediate protein interactions. We investigated the importance of these motifs by generating mice encoding point mutations in the PRD. p53(TTAA) contains mutations suppressing all putative Pin1 sites in the PRD, while p53(AXXA) lacks PXXP motifs but retains one intact Pin1 site. Both mutant proteins accumulated in response to DNA damage, although the accumulation of p53(TTAA) was partially impaired. Importantly, p53(TTAA) and p53(AXXA) are efficient transactivators and potent suppressors of oncogene-induced tumors. Thus, Pin1 sites in the PRD may modulate p53 stability but do not significantly affect function. In addition, PXXP motifs are not essential, but structure dictated by the presence of prolines, PXXXXP motifs that may mediate protein interactions, and/or the length of this region appears to be functionally significant. These results may explain why the sequence of the p53 PRD is so variable in evolution.

Amino Acid Sequence↗

PubMed2006

RMCE-ASAP: a gene targeting method for ES and somatic cells to accelerate phenotype analyses.

In recent years, tremendous insight has been gained on p53 regulation by targeting mutations at the p53 locus using homologous recombination in ES cells to generate mutant mice. Although informative, this approach is inefficient, slow and expensive. To facilitate targeting at the p53 locus, we developed an improved Recombinase-Mediated Cassette Exchange (RMCE) method. Our approach enables efficient targeting in ES cells to facilitate the production of mutant mice. But more importantly, the approach was Adapted for targeting in Somatic cells to Accelerate Phenotyping (RMCE-ASAP). We provide proof-of-concept for this at the p53 locus, by showing efficient targeting in fibroblasts, and rapid phenotypic read-out of a recessive mutation after a single exchange. RMCE-ASAP combines inverted heterologous recombinase target sites, a positive/negative selection marker that preserves the germline capacity of ES cells, and the power of mouse genetics. These general principles should make RMCE-ASAP applicable to any locus.

Animals↗

PubMed2006

A mouse p53 mutant lacking the proline-rich domain rescues Mdm4 deficiency and provides insight into the Mdm2-Mdm4-p53 regulatory network.

The mechanisms by which Mdm2 and Mdm4 (MdmX) regulate p53 remain controversial. We generated a mouse encoding p53 lacking the proline-rich domain (p53DeltaP). p53DeltaP exhibited increased sensitivity to Mdm2-dependent degradation and decreased transactivation capacity, correlating with deficient cell cycle arrest and reduced apoptotic responses. p53DeltaP induced lethality in Mdm2-/- embryos, but not in Mdm4-/- embryos. Mdm4 loss did not alter Mdm2 stability but significantly increased p53DeltaP transactivation to partially restore cycle control. In contrast, decreasing Mdm2 levels increased p53DeltaP levels without altering p53DeltaP transactivation. Thus, Mdm4 regulates p53 activity, while Mdm2 mainly controls p53 stability. Furthermore, Mdm4 loss dramatically improved p53DeltaP-mediated suppression of oncogene-induced tumors, emphasizing the importance of targeting Mdm4 in chemotherapies designed to activate p53.

Animals↗

PubMed2006

Regulating the p53 pathway: in vitro hypotheses, in vivo veritas.

Mutations in TP53, the gene that encodes the tumour suppressor p53, are found in 50% of human cancers, and increased levels of its negative regulators MDM2 and MDM4 (also known as MDMX) downregulate p53 function in many of the rest. Understanding p53 regulation remains a crucial goal to design broadly applicable anticancer strategies based on this pathway. This Review of in vitro studies, human tumour data and recent mouse models shows that p53 post-translational modifications have modulatory roles, and MDM2 and MDM4 have more profound roles for regulating p53. Importantly, MDM4 emerges as an independent target for drug development, as its inactivation is crucial for full p53 activation.

Animals↗

PubMed2005

The C-terminal lysines fine-tune P53 stress responses in a mouse model but are not required for stability control or transactivation.

P53 is an unstable transcription factor that is mutated in a majority of human cancers. With a significant role in initiating cell elimination programs, a network has evolved to fine-tune P53 transcriptional output and prevent errant activation. Modifications of the C terminus have long been viewed as critical binary determinants of P53 stability or activation. However, these conclusions are based on in vitro transfection or biochemical analyses where the stoichiometries between P53 and its regulators are perturbed. Therefore, we tested the importance of the C-terminal regulatory region for P53 control in mice where the seven C-terminal lysines were changed to arginine (Trp-53(7KR)). Surprisingly, the homozygous mutant mice are viable and phenotypically normal. We have functionally characterized the mutant protein in both MEFs and thymocytes, revealing the unexpected result that Trp-53(7KR) exhibits a normal half-life and functions like WT P53 in cell cycle arrest and apoptosis, and in an E1A-ras xenograft tumor suppression assay. However, a significant difference is that P53(7KR) is activated more easily by DNA damage in thymus than WT P53. Importantly, although MEFs encoding WT P53 spontaneously emerge from crisis to become immortal in a 3T3 growth protocol, we do not observe any such escape with the P53(7KR) cells. We propose that the C-terminal modifications believed to be critical for proper P53 regulation are not essential, but contribute to a fine-tuning mechanism of homeostatic control in vivo.

Amino Acid Sequence↗

PubMed2004

Replication initiation in mammalian cells: changing preferences.

Accurate duplication of genetic material is central to cell proliferation. In eukaryotes, S phase is tightly controlled during development. For example, initiation events occur at random sites in early embryos but later appear restricted to preferred DNA regions. Epigenetic changes depending on chromatin organization and/or availability of specific factors likely control origin choice. By using the dynamic molecular combing technology, coupled with specific labeling of neo-synthesized DNA and FISH, we recently demonstrated that the efficiency and spacing of initiation sites are still flexible in mammalian somatic cells, and strongly rely on nucleotide availability. In all conditions, initiation events appear confined to short AT-rich sequences previously identified as matrix attachment regions, which suggests a direct involvement of these features in origin specification. Functional relationships between matrix anchorage and origin selection are discussed.

Animals↗