MDM4 is an important negative regulator of the tumor suppressor p53. In normal
unstressed cells the activity of p53 is kept under control by MDM4 and its homologue
MDM2. MDM4 is said to possess oncogenic potential based on the evidence of its overexpression in many cancers. Until recently it was believed MDM4 is constitutively transcribed; however a decrease in full length MDM4 in response to genotoxic stress was observed paving way for exploring the mechanism responsible for this.
It was observed miR-34a a member of the miR34 family which is a direct transcriptional targets of p53 could have a potential role in regulation of MDM4 expression. The 3'untranslated region of MDM4 was also seen to contain several miR-34a binding sites. However reporter assays with select regions of the 3'UTR revealed that the 3'UTR was unresponsive to miR-34a mediated regulation. Reassessment of the MDM4 gene revealed presence of a potential miR-34a regulatory site in the protein coding exon eleven of MDM4. This site was further considered to check for functionality in response to miR-34a modulation. A reporter with the miR-34a site from the coding region was constructed. This reporter was responsive to overexpression or inhibition of endogenous miR-34a in H1299 and MCF7 cells respectively ascertaining the functionality of this site. A SNP leading to an A to C transversion in the seed region of this miR-34a site in the exon 11 was predicted to disrupt responsiveness to miR-34a. We confirmed this by creating point mutants and performing reporter assays.
This study was designed to understand the regulation of MDM4 in absence of DNA damage conditions. Understanding the role of miR-34a in regulation of MDM4 will pave way for designing specific therapeutic strategy for reactivation of p53 via inhibition of MDM4 in cancer that overexpress MDM4 and retain wild type p53.