GenPhySE, University of Toulouse, INRAE, ENVT, 31326 Castanet Tolosan, France.
Institute of Genome Biology, Leibniz Institute for Farm Animal Biology (FBN), 18196 Dummerstorf, Germany.
Genes (Basel). 2020 Sep 22;11(9):1111. doi: 10.3390/genes11091111.
Most of the circRNAs reported to date originate from back splicing of a pre-mRNA, and these exonic circRNAs are termed canonical circRNAs. Our objective was to provide an overview of all other (non-canonical) circRNAs that do not originate from the junction of two exons and to characterize their common properties. Those generated through a failure of intron lariat debranching are the best known, even though studies on them are rare. These circRNAs retain the 2'-5' bond derived from the intron lariat, and this feature probably explains the difficulties in obtaining efficient reverse transcription through the circular junction. Here, we provide an unprecedented overview of non-canonical circRNAs (lariat-derived intronic circRNAs, sub-exonic circRNAs, intron circles, tricRNAs), which all derive from non-coding sequences. As there are few data suggesting their involvement in cellular regulatory processes, we believe that it is early to propose a general function for circRNAs, even for lariat-derived circRNAs. We suggest that their small size and probably strong secondary structures could be major obstacles to their reliable detection. Nevertheless, we believe there are still several possible ways to advance our knowledge of this class of non-coding RNA.
大多数已报道的 circRNAs 来源于前体 mRNA 的反向剪接，这些外显子 circRNAs 被称为经典 circRNAs。我们的目标是提供一个关于所有其他（非经典）circRNAs 的概述，这些 circRNAs 并非来自两个外显子的连接，并对它们的共同特性进行描述。那些通过内含子套索分支失败产生的 circRNAs 是最知名的，尽管对它们的研究很少。这些 circRNAs 保留了来自内含子套索的 2'-5' 键，这一特征可能解释了通过环状连接获得有效逆转录的困难。在这里，我们提供了一个前所未有的非经典 circRNAs（套索衍生的内含子 circRNAs、亚外显子 circRNAs、内含子环、tricRNAs）概述，它们都源自非编码序列。由于几乎没有数据表明它们参与细胞调节过程，我们认为，即使是套索衍生的 circRNAs，提出 circRNAs 的一般功能也为时过早。我们认为，它们的小尺寸和可能的强二级结构可能是其可靠检测的主要障碍。尽管如此，我们认为仍有几种可能的方法可以提高我们对这一类非编码 RNA 的认识。
