Circular STAG2 RNA Modulates Bladder Cancer Progression via miR-145-5p/TAGLN2 and Is Considered as a Biomarker for Recurrence

Simple Summary Bladder cancer (BCa) is a major urologic malignancy with a high potential for death-threatening complications. Successful treatment relies on timely diagnostics and prediction of clinical outcomes. Molecular markers can be helpful for earlier diagnosis and for making treatment decisions. The biomarker landscape for BCa is not satisfactory yet. The discovery of novel diagnostic indicators for making treatment decisions is critical. Biological fluids are the most convenient and reliable source of these molecules. Circular RNAs (CircRNAs), special non-coding RNAs with a stable structure that are formed by a unique backsplicing process, recently became popular candidates for biomarkers in cancer biology. CircRNAs are widely distributed within the body including biological fluids. Their biological functions include gene regulation via sponging microRNAs, another type of small regulatory RNA. Our study characterized the circRNA hsa_circ_0139697 (circSTAG(16-25)) derived from exons 16 to 25 of STAG2, a gene known to play an important role in BCa biology that can serve as a biomarker predicting recurrence. We found that circSTAG2(16-25) could promote the proliferation, migration, and invasion of BCa cells by binding miR-145-5p, which causes the upregulation of the potential onco-gene TAGLN2. In addition, circSTAG(16-25) can serve as a biomarker for recurrence prediction.Abstract The current study aimed to elucidate the regulatory mechanisms of the circRNA hsa_circ_0139697 (circSTAG2(16-25)) in BCa and to consider the opportunity of using circSTAG2(16-25) isolated from BCa patient urine as a marker for disease development prediction. The selection of this circRNA was determined by the special role of its parental gene STAG2 in BCa biology. The circRNA hsa_circ_0139697 was chosen from 25 STAG2 circRNAs due to its differential expression in the urine of BCa patients and healthy volunteers. Higher levels of circSTAG2(16-25) were detected in urine samples obtained from patients with recurrent tumors. A higher expression of circSTAG2(16-25) was also detected in more tumorigenic BCa cell lines. The overexpression of circSTAG2(16-25) in BCa cells induced the elevation of proliferation, motility, and invasion. To study the mechanisms of circSTAG2(16-25) activity, we confirmed that circSTAG2(16-25) can bind miR-145-5p in vitro as was predicted by bioinformatic search. miR-145-5p was shown to suppress some genes that promoted BCa progression. One of these genes, TAGLN2, encodes the protein Transgelin 2, which plays a role in BCa cell motility and invasion. Therefore, the possible mechanism of action of circSTAG2(16-25) could be sponging the tumor suppressor miR-145-5p, which results in activation of TAGLN2. In addition, circSTAG2(16-25) might be considered as a potential biomarker for recurrence prediction.