Promoter targeted siRNAs induce transcriptional gene silencing (TGS) in Simian Immunodeficiency Virus (SIV)

Abstract

RNA interference is a phenomenon by which double-stranded RNA is processed into small interfering RNAs (siRNAs) that can cause gene silencing in plants, yeast, Drosophila (fruit fly) and human cells. Small interfering double stranded RNAs (siRNAs) can induce gene silencing via two pathways: post-transcriptional gene silencing (PTGS) and transcriptional gene silencing (TGS). PTGS involves siRNA triggering of sequence- specific degradation of mRNA takes place in the cell's cytoplasm. In contrast, siRNAs, which induce TGS have sequence complementary to regions within a gene's promoter and the action happens in the nucleus and is associated with de novo methylation of CpG sites and histone methylation within the promoter region. Suziki et al. utilised siRNAs to target regions within 5'-long-terminal repeat (5'LTR) promoter region of human immunodeficiency virus-1 (HIV-1) and subsequently induce silencing of HIV-1 in HeLa cells. Here I recapitulate the previous findings in HIV-1 by showing that certain promoter-targeted siRNAs can also induce silencing of simian immunodeficiency virus (SIV) replication by inducing CpG methylation and epigenetic changes. A similar silencing effect was observed by using shRNA and miRNA mimics based on the most effective siRNAs. The shRNAs and miRNA mimics after conversion gained a PTGS effect in addition of TGS effect. The dual effect appeared to enhance the silencing efficiency. Here I show that the co-localisation of Ago1 and Ago2 with a TGS inducing siRNA (si2A) in nucleus and at the rim of the nucleus respectively in SIV infected cells but not in non-infected cells. The P-body protein, GW182 complexed with si2A was found in both the nucleus and nuclear envelope and was consistent with the location Ago1/si2A and Ago2/si2A complexes. This indicated a role for this protein in TGS together with Ago proteins. Using SIV 5'-LTR luciferase reporter cells, complexes containing Ago1/Ago2/si2A were showed to work in combination and provide the optimal suppression