University of Heidelberg, Heidelberg, Germany.
Clinic and Polyclinic for Cardiology, University Hospital Leipzig, Leipzig, Germany.
Adv Exp Med Biol. 2018;1087:41-52. doi: 10.1007/978-981-13-1426-1_4.
Circular RNAs (circRNAs) are covalently closed single-stranded RNA molecules derived from exons by alternative mRNA splicing. Circularization of single-stranded RNA molecules was already described in 1976 for viroids in plants. Since then several additional types of circular RNAs in many species have been described such as the circular single-stranded RNA genome of the hepatitis delta virus (HDV) or circular RNAs as products or intermediates of tRNA and rRNA maturation in archaea. CircRNAs are generally formed by covalent binding of the 5' site of an upstream exon with the 3' of the same or a downstream exon. Meanwhile, two different models of circRNA biogenesis have been described, the lariat or exon skipping model and the direct backsplicing model. In the lariat model, canonical splicing occurs before backsplicing, whereas in the direct backsplicing model, the circRNA is generated first. In this chapter, we will review the formation of circular RNAs and highlight the derivation of different types of circular RNAs.
环状 RNA（circRNAs）是通过选择性 mRNA 剪接从外显子衍生而来的共价闭合的单链 RNA 分子。1976 年就在植物类病毒中已经描述了单链 RNA 分子的环状化。从那时起，在许多物种中已经描述了几种额外类型的环状 RNA，例如乙型肝炎 delta 病毒（HDV）的环状单链 RNA 基因组，或作为古菌 tRNA 和 rRNA 成熟产物或中间产物的环状 RNA。环状 RNA 通常是通过上游外显子的 5' 位点与同一外显子或下游外显子的 3' 位点的共价结合形成的。同时，已经描述了两种不同的环状 RNA 生物发生模型，套索或外显子跳跃模型和直接反向剪接模型。在套索模型中，反向剪接之前发生规范剪接，而在直接反向剪接模型中，首先产生环状 RNA。在本章中，我们将回顾环状 RNA 的形成，并强调不同类型的环状 RNA 的衍生。
