Overexpression of MDM2, due to enhanced translation, results in inactivation of wild-type p53 in Burkitt’s lymphoma cells Corinne Capoulade 1 , Brigitte Bressac-de Paillerets 2 , Isabelle Lefre`re 2 , Muriel Ronsin 3 , Jean Feunteun 3 , Thomas Tursz 1 and Joe¨lle Wiels 1 1 Laboratoire de Biologie des Tumeurs Humaines, CNRS URA 1156, 2 Laboratoire de Diagnostic Mole´culaire, 3 Laboratoire de Ge´ne´tique Oncologique, CNRS URA 1967, Institut Gustave Roussy, rue Camille Desmoulins, 94805 Villejuif cedex France Numerous studies have indicated that inactivation of p53 is one of the essential requirements for the unrestrained growth of tumoral cells. When the status of the p53 gene was examined in various types of lymphoid malignancies, mutations in p53 have been predominantly detected in Burkitt’s lymphoma (BL) cells, therefore suggesting that alteration of p53 could specifically contribute to the malignant phenotype of these tumoral cells. In addition to mutations, functional inactivation of p53 can also occur through interaction of the wild-type gene product with various viral or cellular proteins. The cellular MDM2 protein, for example, is able to inhibit p53 tumor suppressor function by concealing its transactivation domain. Mdm2 gene amplification has been described in several types of sarcomas, resulting in overexpression of the MDM2 protein. In this study, we have examined the status of MDM2 and p53 in 20 BL cell lines. Four were found to contain wild-type p53 and to overexpress MDM2 protein. Within these BL cells, both molecules are physically associated since they can be co-precipi- tated and p53 is inactivated as cells neither arrest in G1 nor enter apoptosis following g-radiation. We also report that the high level of the MDM2 protein in BL cells is neither associated with an amplification of the mdm2 gene nor with an elevated level of RNA or an increased protein stability, but is rather due to an enhanced translation ability of the mdm2 RNA. These results indicate that in certain BL cells, overexpression of MDM2 protein regulated at the posttranscriptional level, induces an escape from p53-controlled cell growth. Keywords: MDM2; p53; Burkitt’s lymphoma; enhanced translation Introduction Burkitt’s lymphoma (BL) is a monoclonal B-cell tumor which is the commonest childhood cancer in certain parts of equatorial Africa and Papua New Guinea. A second form of the disease also occurs world wide but at much lower incidence. All BL cells, regardless of their geographical origin, display specific chromosomal translocations leading to the juxtaposition of the c-myc proto-oncogene with either the heavy chain immuno- globulin gene t(8;14) or the light chain immunoglobulin genes t(2;8) or t(8;22). These lesions result in deregulation and activation of c-myc but the amount of the c-myc protein is, in general, not markedly enhanced in BL cells. Histologically, the two forms of BL are also identical but they dier in their association with Epstein – Barr virus (EBV), a human herpes virus. 95% of BL from endemic areas carry the virus, whereas only 10 – 20% of the sporadic BL are EBV(+) (reviewed in Magrath, 1990; Bornkamm et al., 1988). These features have established Burkitt’s lymphoma as a classical human model of multistage oncogenesis. p53, a nuclear, DNA-binding phosphoprotein, is thought to play an important role in the negative regulation of cell growth (reviewed in Haner and Oren, 1995) and its alteration contributes to the deregulated growth of the majority of human cancers. Indeed, the p53 gene is mutated in a wide variety of human cancers with an overall frequency of 50% (Levine et al., 1991; Vogelstein and Kinzler, 1992). Among hematological malignancies, p53 is most often mutated in BL cells. Mutations in the p53 gene have been found in at least 33% of BL biopsies (Gaidano et al., 1991; Bhatia et al., 1992) and as much as 83% of BL cell lines (Wiman et al., 1991; Farrell et al., 1991). Furthermore, analysis of several of these BL p53 mutants showed that they were functionally altered (Farrell et al., 1991; Vousden et al., 1993). It was therefore suggested that, in addition to the deregula- tion of c-myc, p53 inactivation also contributes to the malignant phenotype of BL cells (Gaidano et al., 1991; Imamura et al., 1994). Wild-type p53, which exhibits transcriptional reg- ulatory activities, is involved in the cellular response to DNA damage, resulting in either G1 arrest (allowing repair of damaged DNA prior to DNA replication) (Dulic et al., 1994; el-Deiry et al., 1993) or apoptosis (Yonish-Rouaach et al., 1991; Ramqvist et al., 1993; Miyashita and Reed, 1995). p53 has thus been described as a guardian of the genome (Lane, 1992), limiting the development of genetic abnormalities arising in daughter cells by controlled replication of damaged DNA templates. In addition to mutations of the gene, the functional inactivation of p53 can also occur after interaction of the wild-type gene product with several viral (SV40 T Ag, Adenovirus E1B, Papilloma virus E6) or cellular proteins (MDM2). The mdm2 (murine double minute 2) gene was originally identified by virtue of its amplification in a spontaneously transformed BALB/c 3T3 cell line containing double minutes (Fakharzadeh et al., 1991). Subsequently, the possible involvement of the MDM2 protein in the development of human neoplasia was investigated, leading to the demonstration that, in several types of sarcomas, amplification of its gene, Correspondence: J Wiels Received 21 May 1997; revised 3 November 1997; accepted 4 November 1997 Oncogene (1998) 16, 1603 – 1610  1998 Stockton Press All rights reserved 0950 – 9232/98 $12.00