A Functional SNP in the MDM2 Promoter Mediates E2F1 Affinity to Modulate Cyclin D1 Expression in Tumor Cell Proliferation

Background: The MDM2 oncogene, a negative regulator of p53, has a functional polymorphism in thepromoter region (SNP309) that is associated with multiple kinds of cancers including non-melanoma skincancer. SNP309 has been shown to associate with accelerated tumor formation by increasing the affinity of thetranscriptional activator Sp1. It remains unknown whether there are other factors involved in the regulation ofMDM2 transcription through a trans-regulatory mechanism.
Methods: In this study, SNP309 was verified to beassociated with overexpression of MDM2 in tumor cells. Bioinformatics predicts that the T to G substitution atSNP309 generates a stronger E2F1 binding site, which was confirmed by ChIP and luciferase assays.
Results: E2F1knockdown downregulates the expression of MDM2, which confirms that E2F1 is a functional upstream regulator.Furthermore, tumor cells with the GG genotype exhibited a higher proliferation rate than TT, correlating withcyclin D1 expression. E2F1 depletion significantly inhibits the proliferation capacity and downregulates cyclin D1expression, especially in GG genotype skin fibroblasts. Notably, E2F1 siRNA effects could be rescued by cyclinD1 overexpression.
Conclusion: Taken together, a novel modulator E2F1 was identified as regulating MDM2expression dependent on SNP309 and further mediates cyclin D1 expression and tumor cell proliferation. E2F1might act as an important factor for SNP309 serving as a rate-limiting event in carcinogenesis.