Cyclization process affects alternative splicing
CircRNA is generally accepted as a unique functional by-products derived from splicing process of protein-coding genes. A large proportion of circRNAs we identified till date are exon-derived. Thus, the forming process of such circRNAs could affect alternative splicing of the relevant pre-mRNAs, potentially leading to altered gene expression (Fig.
4.5). A paper published in Circulation Research pointed that RBM20 is crucial for the formation of a subset of circRNAs that originate from the I-band of the Titin gene. In RBM20-knockout mice, exons that originally corresponded to the formation of circular RNA were found more concentrated in linear RNA [
67]. The authors speculate that these exons may be part of the precursor RNA transcribed by Titin gene, they are cleaved from precursor RNA molecule and further forms circRNAs with the aid of RBM20 in a normal situation. When RBM20 is mutated or knocked out, these exon sequences cannot be normally cut down to start the cyclization, and finally accumulate in the linear RNA molecules.
On the contrary, circRNA biogenesis can compete with pre-mRNA splicing, resulting in lower levels of linear mRNAs and changing the composition of processed mRNA by excluding specific exons from the pre-mRNA. Although no natural eukaryotic endogenous circRNA is yet proved to possess the function of translating into proteins, considering the principle we mention above that the more exons are circularized, the less they will be present in the processed mRNA, it is reasonable to speculate that the overexpression or deletion of some gene splicing associated proteins may also modulate formation of mRNA by affecting the amount of circRNAs, further regulate gene expression. This theory will offer a brand-new perspective for the nosogenesis research of some disease.
In consideration of the numerous unsolved problems about circRNAs as yet, more reliable researches are needed to discover the potential existence of other unknown circRNAs or functional RNAs, and complete their functional characterization in eukaryotic cells.