Long antisense non-coding RNAs and the epigenetic regulation of gene expression.

Shortly after the completion of the human genome project in 2003, the Encode project was launched. The project was set out to identify the functional elements in the human genome, and unexpectedly it was found that >80% of the genome is transcribed. The Encode project identified those transcribed regions of the genome to be encoded by non-coding RNAs (ncRNAs). With only 2% of the genome carrying gene-encoding proteins, the conundrum was then, what is the function, if any, of these non-coding regions of the genome? These ncRNAs included both short and long RNAs. The focus of this review will be on antisense long non-coding RNAs (lncRNAs), as these transcripts have been observed to play a role in gene expression of protein-coding genes. Some lncRNAs have been found to regulate protein-coding gene transcription at the epigenetic level, whereby they suppress transcription through the recruitment of protein complexes to target loci in the genome. Conversely, there are lncRNAs that have a positive role in gene expression with less known about mechanism, and some lncRNAs have been shown to be involved in post-transcriptional processes. Additionally, lncRNAs have been observed to regulate their own expression in a positive feedback loop by functioning as a decoy. The biological significance of lncRNAs is only just now becoming evident, with many lncRNAs found to play a significant role in several human diseases.