BodyMap transcriptomes reveal unique circular RNA features across tissue types and developmental stages

Circular RNAs (circRNAs) are a novel class of regulatory RNAs. Here, we present a comprehensive investigation of circRNA expression profiles across 11 tissues and 4 developmental stages in rats, along with cross-species analyses in humans and mice. Although positively correlated, circRNAs exhibit higher tissue specificity than cognate mRNAs. Also, genes with higher expression levels exhibit a larger fraction of spliced circular transcripts than their linear counterparts. Intriguingly, while we observed a monotonic increase of circRNA abundance with age in the rat brain, we further discovered a dynamic, age-dependent pattern of circRNA expression in the testes that is characterized by a dramatic increase with advancing stages of sexual maturity and a decrease with aging. The age-sensitive testicular circRNAs are highly associated with spermatogenesis, independent of cognate mRNA expression. The tissue/age implications of circRNAs suggest that they present unique physiological functions rather than simply occurring as occasional by-products of gene transcription.