Genome-wide identification mA modified circRNAs revealed their key roles in skin ulceration syndrome disease development in Apostichopus japonicus.

Circular RNAs (circRNAs) are novel endogenous non-coding RNAs (ncRNAs) and can be acted as competing endogenous RNAs (ceRNAs) to regulate microRNA (miRNA) and downstream gene expression. Recently, mA modification has been found in circRNA, and mA circRNAs also play important roles in various biological processes and a variety of diseases. Our previous study had been demonstrated that circRNAs were differentially expressed in skin ulceration syndrome (SUS) diseased sea cucumber Apostichopus japonicus. However, whether the function of circRNAs are dependent on mA levels are largely unknown. Here, we firstly investigated the genome-wide map of mA circRNAs in sea cucumbers with different stages of Vibrio splendidus challenge, that's Control group, SUS-diseased group, and SUS-resistant group. MeRIP-seq revealed that mA abundances were enriched in circRNAs in all three groups, especially for SUS-resistant group. Among them, more than 62% of modified circRNAs harbor only a single mA peak and about 55% of mA sites in circRNAs were derived from sense overlapping in each group. After V. splendidus infection, we found that most of mA peaks in circRNAs were upregulated and less were downregulated in both SUS-diseased and SUS-resistant groups when compared with Control. Furthermore, GO analysis indicated that the host genes of circRNAs with dysregulated mA peaks in SUS-diseased and SUS-resistant groups were both mainly enriched in the adhesion pathway. More importantly, we discovered that more than 50% mA circRNAs showed a positive correlation between the circRNAs expression and mA methylation levels both in SUS-diseased and SUS-resistant groups. Therefore, a core circRNA-miRNA-mRNA (ceRNA) network whether influenced by mA modification was constructed based on conjoint analysis. Our results indicated that several selected mA circRNAs bind with miRNAs were mainly targeting to ubiquitylation system and adhesion pathway. What's more, three candidate mA circRNAs and three target genes were validated by MeRIP-qPCR and qPCR, whose mA levels in circRNA and mRNA expressions were consistent with disease occurrence or disease resistance. All of our current findings suggested that mA circRNAs could play important roles during pathogen infection and might be served as a new molecular biomarker in SUS disease diagnose of A. japonicus.