Mice with a Mutation in the Mdm2 Gene That Interferes with MDM2/Ribosomal Protein Binding Develop Bone Marrow Failure

MDM2, an E3 ubiquitin ligase, is an important negative regulator of 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 by the binding of some ribosomal proteins, including RPL5 and RPL11, and that the resulting increase in p53 levels may be important in the red cell aplasia seen in DBA and in 5q- MDS. DBA and 5q- MDS are associated with inherited (DBA) and 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, 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 inadvertently noticed that Mdm2 ^{C305F/ C305F} mice had perturbed hematopoiesis. These mice had a mild macrocytic anemia with reticulocytosis and high erythropoietin levels. In the bone marrow, these mice showed a significant decrease compared to wildtype (WT) littermates in Ter119+ cells, while no decrease in the number of immature erythroid cells (Ter119⁺CD71⁺) was found in the spleen, which showed significantly increased hematopoiesis. In methyl cellulose 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). Further investigation revealed that there was a decrease in Lin^-Sca-1⁺c-Kit⁺(LSK)cells, accompanied by significant decreases in short-term hematopoietic stem cells(ST-HSC) (p < 0.05) and multipotent progenitor(MPP) cells(p < 0.05).

These results suggest that control of the cell cycle through interactions between ribosomal proteins and MDM2 is important in the regulation of normal hematopoiesis as well as in the pathogenesis of 5q- MDS and DBA.