Intronic microRNAs: a crossroad in gene regulation

Most human genes transcribed by RNA Pol II (polymerase II) contain short exons separated by long tracts of non-coding intronic sequences. In addition to their role in generating proteomic diversity through the process of alternative splicing, intronic sequences host many ncRNAs (non-coding RNAs), involved in various gene regulation processes. miRNAs (microRNAs) are short ncRNAs that mediate either mRNA transcript translational repression and/or degradation. Between 50 and 80% of miRNAs are encoded within introns of host mRNA genes. This observation suggests that there is co-regulation between the miRNA biogenesis and pre-mRNA splicing processes. The present review summarizes current advances in this field and discusses possible roles for intronic co-transcriptional cleavage events in the regulation of human gene expression.