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MDM2, an E3 ubiquitin ligase, is an important negative regulator of tumor suppressor p53. In turn the  Mdm2  gene is a transcriptional target of p53, forming a negative feedback loop that is important in cell cycle control. It has recently become apparent that the ubiquitination of p53 by MDM2 can be inhibited when certain ribosomal proteins, including RPL5 and RPL11, bind to MDM2. This inhibition, and the resulting increase in p53 levels has been proposed to be responsible for the red cell aplasia seen in Diamond-Blackfan anemia (DBA) and in 5q- myelodysplastic syndrome (MDS). DBA and 5q- MDS are associated with inherited (DBA) or acquired (5q- MDS) haploinsufficiency of ribosomal proteins. A mutation in  Mdm2  causing a C305F amino acid substitution blocks the binding of ribosomal proteins. Mice harboring this mutation ( Mdm2   C305F  ), retain a normal p53 response to DNA damage, but lack the p53 response to perturbations in ribosome biogenesis. While studying the interaction between RP haploinsufficiency and the  Mdm2   C305F   mutation we noticed that  Mdm2   C305F   homozygous mice had altered hematopoiesis. These mice developed a mild macrocytic anemia with reticulocytosis. In the bone marrow (BM), these mice showed a significant decrease in Ter119 hi  cells compared to wild type (WT) littermates, while no decrease in the number of mature erythroid cells (Ter119 hi CD71 low ) was found in the spleen, which showed compensated bone marrow hematopoiesis. In methylcellulose cultures, BFU-E colonies from the mutant mice were slightly reduced in number and there was a significant reduction in CFU-E colony numbers in mutant mice compared with WT controls ( p  < 0.01). This erythropoietic defect was abrogated by concomitant p53 deficiency ( Trp53   ko/ko  ). Further investigation revealed that in  Mdm2   C305F   animals, there was a decrease in Lin - Sca-1 + c-Kit +  (LSK) cells, accompanied by significant decreases in multipotent progenitor (MPP) cells ( p  < 0.01). Competitive BM repopulation experiments showed that donor BM harboring the  Mdm2   C305F   mutation possessed decreased repopulation capacity compared to WT BM, suggesting a functional stem cell deficit. These results suggest that there is a fine tuned balance in the interaction of ribosomal proteins with the MDM2/p53 axis which is important in normal hematopoiesis.

Mice with a Mutation in the Mdm2 Gene That Interferes with MDM2/Ribosomal Protein Binding Develop a Defect in Erythropoiesis