Human 5p-tailed Mirtrons are Processed by RNaseP

Approximately a thousand microRNAs (miRNAs) are documented from human cells. A third appear to transit non-canonical pathways that typically bypass processing by Drosha, the dedicated nuclear miRNA producing enzyme. The largest class of non-canonical miRNAs are mirtrons which eschew Drosha to mature through spliceosome activity. While mirtrons are found in several configurations, the vast majority of human mirtron species are 5p-tailed. For these mirtrons, a 3p splice site defines the 3p end of their hairpin precursor while a tail of variable length separates the 5p base of the hairpin from the nearest splice site. How this tail is removed is not understood. Here we examine sequence motifs in 5p-tailed mirtrons and interactions with RNA turnover processes to characterize biogenesis processes. Through studying the high confidence 5p-tailed mirtron, hsa-miR-5010, we identify RNaseP as necessary and sufficient for severing the 5p tail of this mirtron. Further, depletion of RNaseP activity globally decreased 5p-tailed mirtron expression implicating this endoribonuclease in biogenesis of the entire class. Moreover, as 5p-tailed mirtron biogenesis appears to be connected to tRNA processing we found a strong correlation between accumulation of tRNA fragments (tRFs) and 5p-tailed mirtron abundance. This suggests that dysregulation of tRNA processing seen in cancers may also impact expression of the ~400 5p-tailed mirtrons encoded in the human genome.