Identification of microRNA Expression Landscapes in Rectal Cancer Undergoing Concurrent Chemoradiotherapy: Investigation Using NanoString nCounter Technology

(1) Background: MicroRNAs (miRNAs) are candidate biomarkers of therapeutic re-sponse; this study sequenced tumor miRNAs before and after neoadjuvant chemoradi-ation (CRT) in cohorts with locally advanced rectal cancer undergoing total mesorectal excision. (2) Materials and Methods: A total of 79 tumor samples, with 36 in the pre-operative (pre-OP) and 43 in the post-operative (post-OP) group, underwent miRNA profiling with the NanoString nCounter Human v3 assay and functional tar-get/pathway analysis using miRDB and TargetScanHuman. (3) Results: NanoString nCounter profiling of 798 miRNAs showed an overall higher expression in post-OP versus pre-OP samples, with 93 miRNAs upregulated in the post-OP group. Tar-get/pathway analyses of the top upregulated miRNAs indicated enrichment across 37 KEGG pathways—including MAPK and Ras signaling and proteoglycans in can-cer—and qRT-PCR validated significant post-OP increases in six miRNAs (miR-143-3p, miR-145-5p, miR-99a-5p, miR-125b-5p, miR-100-5p, and let-7c-5p). (4) Conclusion: We found and validated significant differentially expressed (DE) miRNAs in the post-OP group compared to the pre-OP group in patients with rectal cancer undergoing con-current CRT. These DE miRNAs might serve as the key molecules in CRT-induced suppression of tumor progression and immunomodulation. The further role of DE miRNAs as significant biomarkers needs to be explored in future studies.