Chromatin-associated RNAs as facilitators of functional genomic interactions

Mammalian genomes are extensively transcribed, which produces a large number of both coding and non-coding transcripts. Various RNAs are physically associated with chromatin, through being either retained in cis at their site of transcription or recruited in trans to other genomic regions. Driven by recent technological innovations for detecting chromatin-associated RNAs, diverse roles are being revealed for these RNAs and associated RNA-binding proteins (RBPs) in gene regulation and genome function. Such functions include locus-specific roles in gene activation and silencing, as well as emerging roles in higher-order genome organization, such as involvement in long-range enhancer–promoter interactions, transcription hubs, heterochromatin, nuclear bodies and phase transitions. Enabled by genome-wide profiling approaches, there is growing appreciation for the prevalence and functional importance of various types of chromatin-associated RNAs. As Li and Fu describe in this Review, these RNAs can either be retained in cis at their site of transcription or recruited in cis to other loci, and they have diverse roles in gene regulation, genome organization, nuclear body formation and phase-separation events.