Upregulation of a Circular BAX Transcript in Breast Cancer Is Associated with Unfavorable Prognosis

Recently, we have discovered several alternative circRNAs produced by alternative circularization of the primary transcripts of the apoptosis-related BAX gene. Among them, only three circRNAs, namely circ-BAX-6c, circ-BAX-18 and circ-BAX-74, are expressed in all studied triple-negative breast cancer (BrCa) cell lines and in the normal one; moreover, circ-BAX-18 is the only one comprising a novel microexon with canonical splice sites. Therefore, in this study, we examined circ-BAX-18 expression in BrCa tissues and its potential association with tumor features and patients' prognosis. For this purpose, tumor samples from a cohort of 144 female BrCa patients were used, along with paired non-cancerous breast tissue for 14 cases. circ-BAX-18 expression levels were quantified using an optimized, in-house-developed quantitative real-time PCR assay. Extensive biostatistical analysis was performed, including survival analysis. The expression of circ-BAX-18 differed among the 14 pairs of normal and cancerous breast tissues (p = 0.002). On the other hand, circ-BAX-18 expression did not appear to be associated with any clinicopathological characteristics. After splitting at the median value, higher circ-BAX-18 expression was rather associated with poorer disease-free survival (p = 0.009) and overall survival (p = 0.012) of BrCa patients. According to the multivariate Cox regression analysis results, the prognostic value of circ-BAX-18 positivity was rather independent of the other established predictors of prognosis incorporated in the Cox regression models, such as the molecular subtype of the tumors and the prognostic stage of the disease (regarding DFS: HR = 2.53, 95% CI = 1.30-4.92, p = 0.006; regarding OS: HR = 2.32, 95% CI = 1.20-4.49, p = 0.012). Moreover, the stratification of patients based on prognostic staging showed that high circ-BAX-18 expression may distinguish among stage II patients those with worse prognosis. In conclusion, increased circ-BAX-18 expression in BrCa is most likely associated with poor prognostic outcomes.