Undoubtedly, the definition of lncRNA merely based on length is arbitrary. Small ncRNAs include many different RNAs, such as microRNAs (miRNAs), small nucleolar RNAs (snoRNAs), piwiRNAs (piRNAs) (Taft et al., 2010). 6, 7, 9 It conventionally divides ncRNAs into lncRNAs that have more than 200 nt in length and the remaining ones that are considered “small” RNAs. The most commonly used definition is based on the threshold of 200 nucleotides (nt) of the RNA length. 6 - 8 There is no definition of lncRNA that is based on biological argumentation and widely accepted in the community.
1 - 5 Among these ncRNAs, long ncRNAs (lncRNAs) represent the most prevalent and functionally diverse class. In contrast to a small proportion of the mammalian genome (e.g., human, mouse) that are transcribed into mRNAs, the vast majority of the genome is transcribed into what was previously regarded as “dark matter”-non-coding RNAs (ncRNAs) that do not encode information about proteins. We believe that the classifications of lncRNAs as indicated above are of fundamental importance for lncRNA studies, helpful for further investigation of specific lncRNAs, for formulation of new hypothesis based on different features of lncRNA and for exploration of the underlying lncRNA functional mechanisms. Finally, we present our view on potential further studies. In combination with the presently available function annotations, we explore potential relationships between different classification categories, and generalize and compare biological features of different lncRNAs within each category.
Here, we summarize classification methods of lncRNAs according to their four major features, namely, genomic location and context, effect exerted on DNA sequences, mechanism of functioning and their targeting mechanism. To make easier interpretation of lncRNA functionality and conduct deep mining on these transcribed sequences, it is convenient to classify lncRNAs into different groups. Long non-coding RNAs (lncRNAs) have been found to perform various functions in a wide variety of important biological processes.
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