Expression of alternatively spliced mdm2 transcripts correlates with stabilized wild-type p53 protein in human glioblastoma cells

Abstract

A puzzling finding in various human tumors, including glioblastoma multiforme (GBM), is the stabilization of wild-type (wt) p53 protein. The biological significance of this phenomenon and the mechanism by which it occurs are unexplained. Recent reports have revealed that mdm2 exerts its negative regulation on the p53 signal by directly binding p53 protein and thereby instigating its proteasomal degradation. mdm2 has been shown to exist in alternatively spliced forms in human ovarian and bladder carcinomas, and recently in GBM, with loss or disruption of its p53 binding domain. Here we report that alternatively spliced transcripts of mdm2 are present in 7 of 16 human GBM primary cell cultures and in the established GBM cell lines LN 229 and LN 18. Sequencing demonstrated loss of the amino terminal p53 binding domain in these alternatively spliced mdm2 transcripts, and an out-of-frame splicing in the majority of cases. A significant correlation between the presence of mdm2 splice variants and increased expression of wt p53 protein was observed. Furthermore, in the presence of an mdm2 splice variant, wt p53 stabilization occurred despite coincident MDM2 amplification. Our findings suggest that wt p53 protein stabilization may arise as a consequence of alternative splicing of mdm2. Such a mechanism might account for wt p53 protein accumulation in GBM cells, even in the presence of MDM2 gene amplification.