Spatial profiling of circular RNAs in cancer reveals high expression in muscle and stromal cells.

Circular RNAs (circRNAs) are covalently closed molecules that can play important roles in cancer development and progression. Hundreds of differentially expressed circRNAs between tumors and adjacent normal tissues have been identified in studies using RNA-sequencing or microarrays, emphasizing a strong translational potential. Most previous studies have been performed using RNA from bulk tissues and lack information on the spatial expression patterns of circRNAs. Here, we showed that the majority of differentially expressed circRNAs from bulk tissue analyses of colon tumors relative to adjacent normal tissues were surprisingly not differentially expressed when comparing cancer cells directly with normal epithelial cells. Manipulating the proliferation rates of cells grown in culture revealed that these discrepancies were explained by circRNAs accumulating to high levels in quiescent muscle cells due to their high stability; on the contrary, circRNAs were diluted to low levels in the fast-proliferating cancer cells due to their slow biogenesis rates. Thus, different sub-compartments of colon tumors and adjacent normal tissues exhibited striking changes in circRNA expression patterns. Likewise, the high circRNA content in muscle cells was also a strong confounding factor in bulk analyses of circRNAs in bladder and prostate cancer. Together, these findings emphasize the limitations of using bulk tissues for studying differential circRNA expression in cancer and highlight a particular need for spatial analysis in this field of research.