CCAR1 5’UTR As A Natural MiRancer Of MiR-1254 Overrides Tamoxifen Resistance

MicroRNAs (miRNAs) are a class of ～22nt small non-coding RNAs that modulate multiple biological processes including cancer progression by repressing the expression of RNA targets. miRNA:mRNA targeting is mainly mediated by specific base-pairing interactions between the 5’end of miRNA (seed region) and miRNA response elements (MRE) within the coding region (CR) or UTRs (untranslated regions) of mRNA, leading to mRNA destabilization and/or translational inhibition. Typically target RNAs in return titrate or sequester the endogenous miRNA. Conserved seed pairing in unstructured and AU-rich regions, generally located at 3’UTR, have been considered as good indicator of potential miRNA target sites. In comparison, the 5’UTR was typically characterized by GC-richness and a higher degree of predicted secondary structure. Although some miRNAs can bind to the 5’UTR of target mRNAs to stabilize the target mRNA or activate transcription or translation, the interaction between miRNAs and target RNAs with structured miRNA target sites remain largely unexplored. In this study, screening of miRNAs involved in primary breast cancer identified frequent loss of miR-1254 and its host gene CCAR1. Study of the interaction between miR-1254 and the 5’UTR of CCAR1 revealed a fascinating paradigm of reciprocal modulation between this miRNA and its interacting highly structured and GC-rich target sites.We report herein that miR-1254 interacts with CCAR1 5’UTR in association with Ago2, reciprocally reducing the decay of miR-1254/CCAR1 with resultant elevated expression. We demonstrate that the RNA folding pattern of the miRNA-binding sites, either unstructured or structured, directs miR-1254 as either a repressor or enhancer of targets. Interestingly, miR-1254-targeting sites act as both a functional RNA motif sensing miR-1254 binding and an independent RNA functional unit enhancing miR-1254 expression, termed as miRancer. We further demonstrated that CCAR1 5’UTR as a natural miRancer of endogenous miR-1254 re-sensitizes tamoxifen resistant breast cancer cells to tamoxifen through targeting NCOA1, NCOA3, EGFR, ERBB2 and SNAI1, all of which are bona fide miR-1254 target genes and play a central role in tamoxifen resistance and/or EMT. Thus, our studies suggest miRancer-like motif containing RNA fragments or miRancer molecules might be potentially useful for cancer therapeutics.