Loss of p19ARF enhances the defects of Mdm2 overexpression in the mammary gland

The protein encoded by the murine double minute 2 (Mdm2) gene inactivates the function of the tumor suppressor p53. The targeted expression of the mdm2 transgene (BLGmdm2) to the mammary epithelium disrupts the cell cycle, causing multiple rounds of DNA synthesis without proper cell division and consequently poor mammary gland development. These phenotypes in the mammary epithelia of the transgenic mice are not dependent on either p53 or the transcription factor E2F1, as mice null for these genes carrying the BLGmdm2 transgene exhibit similar defects to mice carrying the BLGmdm2 transgene alone. p19ARF, an alternative splice product of the INK4a/ARF locus, has been shown to interact directly with MDM2. Therefore, BLGmdm2 transgenic mice null for p19ARF were created to gain insight into the mechanism by which mdm2 overexpression disrupts the cell cycle. The BLGmdm2 phenotype in the absence of p19ARF was worse than BLGmdm2 mice and visible as early as day 15 of pregnancy. By day 5 of lactation the phenotype was very pronounced. Histological analysis of the mammary gland showed a decrease in ductal branching, smaller and fewer lobuloalveolar structures, and a decrease in luminal secretions. Multinucleated and enlarged cells were present due to continued replication in the absence of cytokinesis. Thus, the absence of p19ARF in this in vivo system enhanced the defect caused by mdm2 overexpression.