MicroRNAs (miRNAs) are ~22 nucleotide (nt) ubiquitous gene regulators that modulate essential cellular processes at the post-transcriptional level. Canonically, miRNAs are produced from primary transcripts that are cleaved by the nuclear Microprocessor complex, with the resulting precursor (pre-) miRNA hairpins exported by Exportin-5 and further processed by cytoplasmic Dicer. Accumulating evidence suggests that various miRNAs are aberrantly expressed in human cancer cells, so it is important to elucidate the mechanisms by which miRNAs are expressed and how their biogenesis is regulated.
Herpesvirus saimiri (HVS) and a unique viral miRNA biogenesis pathway
HVS infection causes fatal T-cell leukemias and lymphomas in New World primates and immortalizes human T cells in vitro, representing an invaluable model for elucidating human T-cell malignancies. In HVS-transformed T cells, viral miRNAs are co-transcribed downstream of viral small nuclear (sn)RNAs. These are the first snRNA-miRNA chimeras identified. Intriguingly, the host Integrator, a 14-subunit complex that is known to process the 3′ ends of snRNAs, recognizes and processes conserved sequence elements flanking the viral pre-miRNAs, therefore replacing Microprocessor cleavage.
Mammalian m7G-capped pre-miRNAs
The search for snRNA-miRNA chimeric precursors in host cells then led to the discovery of a group of Microprocessor-independent 7-methylguanosine (m7G) capped pre-miRNAs, whose 5′ ends coincide with RNA polymerase (Pol) II transcription start sites. Due to the presence of the cap, these pre-miRNAs are exported via the PHAX-Exportin-1 pathway. After Dicer cleavage, only the 3p-miRNA is efficiently loaded into Argonaute to form a functional microRNP (ribonucleoprotein) and guides gene repression. This finding enables the development of special shRNA expression constructs that produce a single 3p-siRNA.
Our future research will address whether such pathways are conserved in other organisms and what the advantage of using alternative pathways over the canonical pathway for generating specific miRNAs is. We will also continue to explore the mechanisms of m7G-capped pre-miRNA 3′-end formation and Integrator-mediated RNA processing.