Exon Skipping and Circular RNA Formation in Transcripts of the Human Cytochrome P-450 2C18 Gene in Epidermis and of the Rat Androgen Binding Protein Gene in Testis

Abstract

The cytochrome P-450 2C18 gene was found by reverse transcription-PCR to represent the most abundantly expressed gene of the P-450 2C subfamily in human epidermis. However, in addition to the canonical mRNA of nine exons, transcripts that have skipped exon 4 or 5, exons 4, 5, and 6, or exons 4, 5, 6, and 7 were also identified in this tissue. Remarkably, circular RNA transcripts synthesized by the joining of the donor and acceptor splice sites of the same exon were detected in human epidermis for exons 4 and 5. Moreover, molecules composed of exons 4, 5, and 6 with the donor splice site of exon 6 joined to the acceptor splice site of exon 4 or composed of exons 4, 5, 6, and 7 with the donor splice site of exon 7 joined to the acceptor splice site of exon 4 were also found to be present in this tissue. In rat testis, a similar analysis allowed the detection of a circular RNA molecule composed of exons 6 and 7 of the androgen binding protein (ABP) gene, with the donor splice site of exon 7 joined to the acceptor splice site of exon 6, and of an ABP mRNA which had skipped exons 6 and 7. These results apparently substantiate the hypothesis that alternative pre-mRNA splicing has the potential to generate not only mRNAs that lack one or more exons but also circular RNA molecules that are composed of the exons that are skipped. However, additional 2C18 circular species containing various combinations of exons were also detected in human epidermis, and an exon 6-skipped ABP mRNA molecule was identified in rat testis. This observation is interpreted as indicative that at low frequency, numerous circular RNA formation and exon skipping events may occur, allowing the joining of a variety of different combinations of exons. Moreover, the relative stability of these molecules is apparently the key factor that determines the relative ease of their detection.

Natural Trans-spliced mRNAs Are Generated from the Human Estrogen Receptor-α (hERα) Gene

“…Such RNAs were described for the testis-determining gene Sry in adult mouse testis (32), the human ets-1 gene (33), the human cytochrome p-450 2C18 gene in epidermis, and the rat androgen-binding protein gene in testis (31). Exons skipped during alternative pre-mRNA processing could be indeed present in a circular molecule that has the donor site of the 3Ј exon joined to the acceptor site of the 5Ј exon.…”

“…Exon scrambling is an event that has often been also associated with circular RNA molecules (30,31). Such RNAs were described for the testis-determining gene Sry in adult mouse testis (32), the human ets-1 gene (33), the human cytochrome p-450 2C18 gene in epidermis, and the rat androgen-binding protein gene in testis (31).…”

Circular RNAs: Unexpected outputs of many protein-coding genes

“…The length of the skipped exon appears to be one key determinant, 41 but other factors such as RNA secondary structures and the speed of lariat debranching are also likely to be involved. Interestingly, a correlation between exon skipping and circular RNA biogenesis has been noted at other genes 49,50,52,55 and in a global transcriptome analysis of human endothelial cells. 90 Detailed analyses are now required to determine if re-splicing truly occurs at these loci.…”

“…This was the first hint that the dominant output of a gene could, in some cases, be a circular RNA rather than a linear mRNA. 48 A handful of additional circular RNAs were identified over the ensuing years [49][50][51][52][53][54][55][56] until deep sequencing efforts combined with new computational algorithms all of a sudden revealed thousands of previously missed circular RNAs from eukaryotic cells. 13,14,27,29,31,33,40,57,58 Over 25,000 putative circular RNAs, derived from »15% of actively transcribed genes, were identified in human fibroblasts alone.…”

Inducible RasGEF1B circular RNA is a positive regulator of ICAM-1 in the TLR4/LPS pathway

“…1 Additional studies reported further new examples of circRNAs over the years, [2][3][4][5][6][7][8][9] yet the extent of the prevalence of the circularization phenomenon was not clear, as the available examples remained limited. The advent of high-throughput sequencing revealed that circRNAs are a large class of abundant RNAs with complex developmental-and cellspecific expression patterns.…”