gms | German Medical Science

The p53 tumor suppressor pathway is disabled in human tumors either by p53 mutations or through overexpression of negative regulators, such as HDM2 and HDMX. HDMX is a protein that shares great similarities with the HDM2 oncogene although it is has some distinct features. More than 40 alternatively, as well as, aberrantly spliced transcripts of the HDM2 mRNA with oncogenic properties have been detected in a variety of human cancers. Although some reports describe the presence of few HDMX-splice forms in single cancer cell lines, so far there is no comprehensive study on the expression of HDMX transcript variants. Therefore, the aim of this study was to investigate whether oncogenic HDMX splice variants are generally expressed in human tumors as it has been shown for HDM2. We screened a number of human cancer cell lines, including ovarian cancer, glioblastoma, soft tissue sarcoma cell lines and others. In all of the investigated cell lines, the band corresponding to the full-length HDMX-mRNA could be detected. In addition, one or more shorter PCR products were present. These bands were purified, cloned and subsequently characterized by direct sequencing. We identified HDMX transcripts that were either spliced at genuine exon/intron-boundaries or at cryptic splice sites within exons, including HDMX-S, HDMX-G, and HDMX211 which have been previously described. The HDMX splice variants can be classified into two groups: some isoforms have only retained the p53 binding domain, such as HDMX-S, and in contrast, others consist of an intact C-terminal end including the RING-finger domain, such as HDMX211. Furthermore, we have performed RACE-PCR in order to check the integrity of the mRNA and we have found that both HDMX and HDM2 express a broad pattern of splice variants in tumors, whereas several other genes, including p53 and GAPDH, expressed only the full-length transcript. In conclusion, our results show that HDMX splice variants are frequently expressed in human cancer cell lines of different origin. This suggests that they might contribute to tumorigenesis and/or therapy resistance regardless of the p53 mutational status.