The Effect of Alternative Splicing Sites on Mirtron Formation and Arm Selection of Precursor microRNAs.

Mirtrons represent a subclass of microRNAs (miRNAs) that rely on the splicing machinery for their maturation. However, the molecular details of this Drosha-independent processing are still not fully understood; as an example, the Microprocessor complex cannot process the mirtronic pre-miRNA from the transcript even if splice site mutations are present. To investigate the influence of alternative splicing sites on mirtron formation, we generated Enhanced Green Fluorescent Protein (EGFP) reporters containing artificial introns to compare the processing of canonical miRNAs and mirtrons. Although mutations of both splice sites generated a complex pattern of alternative transcripts, mirtron formation was always severely affected as opposed to the normal processing of the canonical hsa-mir-33b miRNA. However, we also detected that while its formation was also hindered, the mirtron-derived hsa-mir-877-3p miRNA was less affected by certain mutations than the hsa-mir-877-5p species. By knocking down Drosha, we showed that this phenomenon is not dependent on Microprocessor activity but rather points toward the potential stability difference between the miRNAs from the different arms. Our results indicate that when the major splice sites are mutated, mirtron formation cannot be rescued by nearby alternative splice sites, and stability differences between 5p and 3p species should also be considered for functional studies of mirtrons.