Circular RNAs are abundantly expressed and upregulated during human epidermal stem cell differentiation

The expression patterns of endogenous circular RNA (circRNA) molecules during epidermal stem cell (EpSC) differentiation have not previously been explored. Here, we show that circRNAs are abundantly expressed in EpSCs and that their expression change dramatically during differentiation in a coordinated manner. Overall, circRNAs are expressed at higher levels in the differentiated cells, and many upregulated circRNAs are derived from developmental genes, including four different circRNAs from DLG1. The observed changes in circRNA expression were largely independent of host gene expression, and circRNAs independently upregulated upon differentiation are more prone to AGO2 binding and have more predicted miRNA binding sites compared to stably expressed circRNAs. In particular, upregulated circRNAs from the HECTD1 and ZNF91 genes have exceptionally high numbers of AGO2 binding sites and predicted miRNA target sites, and circZNF91 contains 24 target sites for miR-23b-3p, which is known to play important roles in keratinocyte differentiation. We also observed that upregulated circRNAs are less likely to be flanked by homologues inverted Alu repeats compared to stably expressed circRNAs. This coincide with DHX9 being upregulated in the differentiated keratinocytes. Finally, none of the circRNAs upregulated upon differentiation were also upregulated upon DNMT3A or DNMT3B knockdown, making it unlikely that epigenetic mechanisms are governing the observed circRNA expression changes. Together, we provide a map of circRNA expression in EpSCs and their differentiated counterparts and shed light on potential function and regulation of differentially expressed circRNAs.