Regulation of the tumor suppressor p53 by Mdm2 and Mdm4
Mdm2 and Mdm4 are critical negative regulators of the p53 tumor suppressor. Mdm4-null mutants are severely anemic and exhibit impaired proliferation of the fetal liver erythroid lineage cells. This phenotype may indicate a cell-intrinsic function of Mdm4 in erythropoiesis. In contrast, red blood cell count was nearly normal in mice engineered to express low levels of Mdm2, suggesting that Mdm2 might be dispensable for red cell production. In the first part of the thesis, we further explore the tissue-specific functions of Mdm2 and Mdm4 in the erythroid lineage by crossing the conditional Mdm4 and Mdm2 alleles to an erythroid-specific-cre (EpoRGFP-Cre ) knock-in allele. Our data show that Mdm2 is required for rescuing erythroid progenitors from p53-mediated apoptosis during primitive erythropoiesis. In contrast, Mdm4 is only required for the high erythropoietic rate during embryonic definitive erythropoiesis. Thus, in this particular cellular context, interestingly, Mdm4 only contributes to p53 regulation at a specific phase of the differientation program.
Moreover, a large body of evidence indicates that aberrant expression of either MDM2 or MDM4 impairs p53 tumor suppression function and consequently favors tumor formation. Overexpression of MDM2 was observed in 10% of 8000 human cancers from various sites, including lung or stomach, and MDM4 was found amplified and/or overexpressed in 10-20% of over 800 diverse tumors including lung, colon, stomach and breast cancers. Remarkably, selective MDM4 amplification occurs in about 65% of human retinoblastomas. In contrast, MDM2 amplifications are relatively rare (about 5%) in retinoblastomas, indicating that depending on the tumor context (cell type, initiating oncogene, …), MDM4, rather than MDM2, overexpression might be selected for as a more efficient mean of suppression of p53 function. As part of a large effort to better understand why different cell types require distinct combinations of mutations to form tumours, we will examine the molecular basis for selective up-regulation of Mdm4 in retinoblastomas. In this context, we have successfully generated 2 conditional transgenic mouse lines expressing either mycMdm2 or mycMdm4 driven by the PCAGGs promoters in the ROSA26 locus. Since a cassette containing a floxed transcriptional stop element is inserted upstream of the transgenes, we can achieve tissue-specific expression and spatio-temporal regulation of the transgenes by using different Cre and CreER. By the use of N-terminal myc-tag fused with the transgenes, we are able to compare the expression levels of the transgenes. Finally, due to C-terminal IRES-GFP element, we can easily identify transgene expressing cells. One of our aims is to use this Mdm4 conditional transgenic mouse line as the first, non-chimeric, mouse model of retinoblastoma that can be used as an appropriate preclinical model to improve treatment of this disease.
Advisor:Jacques Urbain; JC Marine; Eric Bellefroid; Stephane Schurmans; Jody Haigh; Josianne Roscam
School:Université libre de Bruxelles
Source Type:Master's Thesis
Keywords:tumorigenesis mouse model p53 mdm4 mdm2
Date of Publication:12/07/2007