SRSF3 shapes miR-17-92 cluster to control selective miRNA processing in colorectal cancer

MicroRNA (miRNA) biogenesis is tightly regulated to maintain distinct miRNA expression patterns during development and in adult tissues, dysregulation of miRNAs being a critical determinant of disease. Almost a half of miRNAs in mammalian cells are generated from polycistronic primary transcripts encoding multiple miRNAs but it is enigmatic how the levels of individual miRNAs residing within clusters is controlled. The RNA binding protein SRSF3 (Serine-arginine rich splicing factor 3) has been shown to promote miRNA processing through its binding to CNNC motifs downstream of Drosha cleavage sites. Here we demonstrate that in pluripotent and cancer cells SRSF3 binds to multiple positions along the polycistronic miR-17-92 to specifically enhance the processing of two paralog miRNAs, miR-17 and miR-20a, through its RS domain interactions that alter the pri-miRNA structure. The SRSF3-mediated increase in miR-17/20a levels inhibits the mRNA encoding the cell cycle inhibitor p21 and ultimately promotes the self-renewal properties of both normal and cancer cells. Strikingly, SRSF3-miR-17-92-p21 pathway operates in colorectal cancer cells and is strongly associated with poorly differentiated high grade colorectal tumours, suggesting a direct link between SRSF3-mediated pri-miRNA structure and cancer pathogenesis.

微小RNA（miRNA）的生物发生过程受到严格调控，以维持其在发育进程与成体组织中的特异性表达谱；而miRNA表达失调是疾病发生的关键决定因素。哺乳动物细胞中近半数的miRNA均源自可编码多种miRNA的多顺反子初级转录本，但目前仍尚未明确簇状分布的单个miRNA的表达水平是如何被调控的。RNA结合蛋白SRSF3（富含丝氨酸-精氨酸剪接因子3）已被证实可通过结合Drosha剪切位点下游的CNNC基序，促进miRNA的成熟加工。本研究证实，在多能干细胞及癌细胞中，SRSF3可结合至多顺反子miR-17-92的多个位点，并通过其RS结构域与初级miRNA（pri-miRNA）的相互作用改变其空间构象，从而特异性增强两个旁系同源miRNA——miR-17与miR-20a的成熟加工。SRSF3介导的miR-17/20a表达上调可抑制编码细胞周期抑制剂p21的mRNA，最终促进正常细胞与癌细胞的自我更新特性。值得注意的是，SRSF3-miR-17-92-p21信号通路在结直肠癌细胞中发挥调控功能，且与低分化高级别结直肠肿瘤密切相关，这表明SRSF3介导的pri-miRNA结构改变与癌症发病机制之间存在直接关联。