Abstract
Abstract Disclosure: F. Chapus: None. C. Day: None. P. Yasar: None. C. Bregio: None. A. Palia: None. E. Scappini: None. C. Tucker: None. J. Rodriguez: None. Intra-tumoral gene expression heterogeneity of estrogen receptor positive breast cancers is a roadblock toward effective treatment due to subpopulations acquiring resistance and metastatic ability. This expression heterogeneity can be explained by the stochastic nature of transcription. Transcriptional initiation is coordinated through distal elements known as enhancers. These elements aid in the recruitment of cofactors and chromatin remodelers to promoters. During transcriptional activation, genes are transcribed during short periods of nascent RNA synthesis known as transcriptional bursts. Furthermore, estrogen responsive genes can transition between transcriptional bursting and variably long periods of inactivity lasting from minutes to days. Here, we investigated whether estrogen responsive enhancers of variably expressed genes also exhibited transcriptional states which reflect promoter states. To observe live cell transcription dynamics, we integrated MS2 stem loops into several endogenous enhancers using CRISPR. After transcription initiation, these stem loops are transcribed and subsequently bound by the RNA binding protein, MS2-GFP. This enrichment in MS2-GFP at the transcription site is visible as a bright punctate spot. We observed that the estrogen responsive TFF1 enhancer exhibited short periods of transcriptional activity and long periods of inactivity. Moreover, the TFF1 enhancer RNA (eRNA) sporadically formed dynamic, but rare and long-lived condensates. To correlate enhancer and promoter transcription we used single molecule fluorescence in situ hybridization of several eRNA. We observed that eRNA aggregates were correlated with an increase of mRNA content and occasionally localized to different nuclear bodies. Lastly, we enriched for chromatin bound RNA and observed long enhancer RNA transcripts. Taken together, our work highlights the dynamic coupling of enhancer and promoter regulation and the importance of understanding the regulators of enhancer transcriptional states in cancer. Presentation: Sunday, June 18, 2023
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