Prostate cancer (PCa) is a clinically heterogeneous disease marked by variability in patient prognosis. The intratumoral and interpatient heterogeneity formed the basis of molecular stratification of the disease. Overexpression of Serine Peptidase Inhibitor, Kazal type-1 (SPINK1), was found to be the second most recurrent (~10-15%) PCa subtype, after highly recurrent (~50%) androgen-driven TMPRSS2-ERG genetic rearrangement. Unlike amplification of ERBB2 in breast cancer, SPINK1 overexpression has not been associated with gene amplification or rearrangement. Nonetheless, molecular mechanism underlying its upregulation in cancer is poorly understood and remains a matter of conjecture.

Using in silico microRNA prediction tools, we shortlisted three miRNAs, of which miR-338-5p and miR-421 revealed an inverse correlation with SPINK1 in The Cancer Genome Atlas (TCGA) prostate adenocarcinoma RNA-Seq dataset (n=106), which was validated in our PCa specimens (n=20). Here, we showed that miR-338-5p and miR-421 post-transcriptionally regulate SPINK1 by binding to its 3′ UTR. We established that miR-338-5p and miR-421 mediate several cellular responses against SPINK1-positive cancer by inducing S-phase cell-cycle arrest, inhibiting epithelial-to-mesenchymal transition (EMT), cancer stemness, and drug resistance. Moreover, ectopic expression of miR-338-5p and miR-421 abrogates SPINK1-mediated oncogenesis, tumor growth, and distant metastases in murine xenograft model. Further, mechanistically we demonstrate that Polycomb group protein enhancer of zeste homolog 2 (EZH2) mediates epigenetic silencing by establishing histone H3K27me3 repressive marks and DNA methylation, 5’ methyl cytosine (5mC) marks on the promoters of miR-338-5p and miR-421 in SPINK1-positive subtype. Thus, restoring miR-338-5p and miR-421 expression using either epigenetic drugs or synthetic mimics could abrogate SPINK1-mediated oncogenesis by targeting multiple oncogenic pathways and eliciting anticancer pleiotropic effects.

Taken together, the present study for the first time unravels the molecular mechanism underlying SPINK1 overexpression and thereby opens up new avenues for the use of miR-338-5p and miR-421 replacement therapy for the treatment of SPINK1-positive malignancies.

Citation Format: Vipul Bhatia, Anjali Yadav, Ritika Tiwari, Shivansh Nigam, Apul Goel, Bushra Ateeq. Repression of microRNA-338/421 by EZH2 represents a novel mechanism for SPINK1-positive prostate cancers [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr A060.