Synthetic small duplex RNAs that are fully complementary to gene promoters can silence transcription in mammalian cells. microRNAs (miRNAs) are endogenous small regulatory RNAs that sequence specifically regulate gene expression. We have developed a computational method to identify potential miRNA target sites within gene promoters. Ten candidate miRNAs predicted to target the human progesterone receptor (PR) gene promoter were tested for their ability to modulate gene expression. Several miRNA mimics inhibited PR gene expression and miR-423-5p, which targets a highly conserved region of the PR promoter, was chosen for detailed analysis. Chromatin immunoprecipitation revealed that the miR-423-5p mimic decreased RNA polymerase II occupancy and increased histone H3 lysine 9 dimethylation (H3K9me2) at the PR promoter, indicative of chromatin-level silencing. Transcriptional silencing was transient, independent of DNA methylation, and associated with recruitment of Argonaute 2 (AGO2) to a non-coding RNA (ncRNA) transcript that overlaps the PR gene promoter. The miR-423-5p mimic also silenced expression of immunoglobulin superfamily member 1 (IGSF1), an additional gene with a predicted target site within its promoter. While additional investigations of endogenous miRNA function will be necessary, these observations suggest that recognition of gene promoters by miRNAs may be a natural and general mechanism for regulating gene transcription.
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