RESULTS: We explored
circRNA expression profiles using
RNA-seq data for six adult and fetal normal tissues (colon, heart, kidney, liver, lung, and stomach) and four gland normal tissues (adrenal gland, mammary gland, pancreas, and thyroid gland). A total of 8120, 25,933 and 14,433
circRNAs were detected by at least two supporting junction reads in adult, fetal and gland tissues, respectively. Among them, 3092, 14,241 and 6879
circRNAs were novel when compared to the published results. In each adult tissue type, we found at least 1000
circRNAs, among which 36.97-50.04% were tissue-specific. We reported 33
circRNAs that were ubiquitously expressed in all the adult tissues we examined. To further explore the potential "housekeeping" function of these
circRNAs, we constructed a
circRNA-
miRNA-
mRNA regulatory network containing 17
circRNAs, 22
miRNAs and 90 mRNAs. Furthermore, we found that both the abundance and the relative expression level of
circRNAs were higher in fetal tissue than adult tissue. The number of
circRNAs in gland tissues, especially in mammary gland (9665
circRNA candidates), was higher than that of other adult tissues (1160-3777).
CONCLUSIONS: We systematically investigated
circRNA expression in a variety of human adult and fetal tissues. Our observation of different expression level of
circRNAs in adult and fetal tissues suggested that
circRNAs might play their role in a tissue-specific and development-specific fashion. Analysis of
circRNA-
miRNA-
mRNA network provided potential targets of
circRNAs. High expression level of
circRNAs in mammary gland might be attributed to the rich innervation.