Long non-coding RNA
Long non-coding RNAs (long ncRNAs, lncRNA) are a type of RNA, generally defined as transcripts more than 200 nucleotides that are not translated into protein. This arbitrary limit distinguishes long ncRNAs from small non-coding RNAs, such as microRNAs (miRNAs), small interfering RNAs (siRNAs), Piwi-interacting RNAs (piRNAs), small nucleolar RNAs (snoRNAs), and other short RNAs. Long intervening/intergenic noncoding RNAs (lincRNAs) are sequences of lncRNA which do not overlap protein-coding genes. Long non-coding RNAs include intergenic lincRNAs, intronic ncRNAs, and sense and antisense lncRNAs, each type showing different genomic positions in relation to genes and exons. In 2007 a study found only one-fifth of transcription across the human genome is associated with protein-coding genes, indicating at least four times more long non-coding than coding RNA sequences. Large-scale complementary DNA (cDNA) sequencing projects such as FANTOM reveal the complexity of this transcription. The FANTOM3 project identified ~35,000 non-coding transcripts that bear many signatures of messenger RNAs, including 5' capping, splicing, and poly-adenylation, but have little or no open reading frame (ORF). This number represents a conservative lower estimate, since it omitted many singleton transcripts and non-polyadenylated transcripts (tiling array data shows more than 40% of transcripts are non-polyadenylated). Identifying ncRNAs within these cDNA libraries is challenging since it can be difficult to distinguish protein-coding transcripts from non-coding transcripts. It has been suggested through multiple studies that testis, and neural tissues express the greatest amount of long non-coding RNAs of any tissue type. Using FANTOM5, 27,919 long ncRNAs have been identified in various human sources. Quantitatively, lncRNAs demonstrate ~10-fold lower abundance than mRNAs, which is explained by higher cell-to-cell variation of expression levels of lncRNA genes in the individual cells, when compared to protein-coding genes.
