Expression of MDM2 during mammary tumorigenesis

The MDM2 oncoprotein encodes a 90 kDa nuclear phorphoprotein capable of abr ogating the growth suppressive functions of p53 and pRb tumor suppressor pr oteins by direct interaction, Alternative splicing of MDM2 protein coding s equences has been; documented during tumor progression in human ovarian and bladder carcinomas. The aim of this study was to determine whether alterna tive splicing of MDM2 occurs during breast tumorigenesis in mice and humans and whether protein coding sequences were affected. Specimens representing normal and malignant breast tissues from the murine D2 mammary tumor model system and human breast carcinomas were examined. Three distinct mdm2 mRNA transcripts of 3.3, 1.6 and 1.5 kb were detected in normal and malignant m urine mammary tissues by Northern blot analysis using a full-length mdm2 cD NA probe. Additional Northern blot analysis using a probe derived from exon 12 of murine mdm2 demonstrated that the 1.5 and 1.6 kb transcripts lack se quences encoding the C-terminus of the protein. No evidence of internal del etions of protein coding sequences of mdm2 was detected in any of the norma l mammary tissues or D2 murine mammary tumors examined by reverse transcrip tion PCR (RT-PCR). Three distinct MDM2 transcripts of 6.7, 4.7 and 1.9 kb w ere detected in malignant human breast tissue by Northern blot analysis usi ng a cDNA probe specific for the complete open reading frame of human MDM2. However, a cDNA probe specific for the last exon of human MDM2 hybridized only to the 6.7 and 4.7 kb transcripts, demonstrating that the 1.9 kb trans cript lacked protein coding sequences contained in exon 12. Similarly, no i nternal deletions were detected in a panel of malignant human breast tissue s using RT-PCR and analogous primers within human MDM2. Therefore, breast t umors differ from other solid tumors reported previously in that no interna l deletions of MDM2 protein coding sequences were observed. However, the da ta document the presence of multiple MDM2 mRNA transcripts in both normal a nd malignant breast tissues. A subset of MDM2 transcripts were shown to lac k the last exon which contains sequences coding for the RING and zinc finge rs and domains which are targets for caspase-3 mediated proteolytic degrada tion and are required to target p53 for proteosomal degradation. Int. J. Ca ncer 81:292-298, 1999, (C) 1999 Wiley-Liss, Inc.