RNA Interference: Nuclear Dicer Makes The

RESEARCH HIGHLIGHTS

RNA INTERFERENCE Nuclear Dicer makes the cut Post-transcriptional gene silencing, the endogenous protein localized to repressive histone mark dimethyl- which has a central role in regulat- both the cytoplasm and the nucleus. ated Lys9 of histone H3 (H3K9me2), Dicer ing gene expression in eukaryotes, By contrast, overexpressed GFP- which suggests that Dicer induces the processes is mediated through several types tagged Dicer was only detected in the formation of H3K9me2 through the dsRNAs … of small RNAs, such as siRNAs and nucleus of human DICER1‑knockout processing of dsRNAs into siRNAs microRNAs (miRNAs). Their biogen- cells, which suggests that the nuclear and loading of AGO1. In agreement to promote esis requires the cleavage of precursor levels of this enzyme are tightly con- with this, the authors found that the the formation RNAs by the processing enzyme trolled. Dicer co-immunoprecipitated levels of dsRNAs and small RNAs of hetero Dicer (encoded by DICER1) before with RNA polymerase II (Pol II), were increased in DICER1‑knockout they are loaded onto Argonaute but this interaction was decreased in cells and, moreover, that dsRNAs chromatin (AGO) to guide it to target mRNAs in nuclear extracts that had been treated occurred at chromatin-binding sites the cytoplasm. In mammalian cells, with the dsRNA-specific RNase V1; of Dicer. it was thought that Dicer only had this suggests that the Dicer–Pol II Invading viral dsRNA is cytoplasmic functions; however, the association is dependent on the pres- eliminated through the host presence of a non-canonical nuclear ence of dsRNA. Furthermore, using interferon-response pathway, which localization signal in Dicer suggests chromatin immunoprecipitation ultimately kills the infected cell; that this endonuclease might have a followed by sequencing the authors so, the authors examined whether nuclear role. White et al. now show identified multiple genomic loci the accumulated dsRNA in Dicer- that, in mammalian cells, Dicer local- that were enriched in Dicer, and the depleted cells induces this defence izes to the nucleus and is recruited addition of the transcription inhibitor mechanism. Indeed, genes encoding to loci of endogenously overlapping α-amanitin (which is an amatoxin) key proteins of the interferon- transcripts to promote the formation reduced Pol II and Dicer levels at four response pathway were upregulated of heterochromatin. selected genes. These findings suggest in DICER1‑knockdown cells, To investigate the cellular dis- that Dicer and Pol II co-associate at which displayed increased levels of tribution of Dicer, the authors used transcriptionally active genes. apoptosis, as evidenced by morpho- immunofluorescence and cellular The levels of sense and antisense logical abnormalities and increased fractionation analysis and found that transcripts of these four genes were annexin V labelling. augmented in DICER1‑knockdown Together, the results of this study cells, but this increase was reduced reveal a biological role for nuclear in the presence of the RNase V1, Dicer in mammalian cells. Dicer which suggests that Dicer decreases processes dsRNAs that arise during the formation of dsRNA. The authors convergent transcription to promote investigated the possibility that the formation of heterochromatin nuclear Dicer induces transcriptional possibly through AGO1‑mediated gene silencing in mammalian cells, recruitment of methyltransferases a process that has been described in and the establishment of repressive other eukaryotic cells and involves histone marks. the siRNA–AGO-mediated recruit- Andrea Du Toit ment of methyltransferases to target ORIGINAL RESEARCH PAPER White, E. et al. chromatin to promote the generation Human nuclear Dicer restricts the deleterious of repressive chromatin structures. accumulation of endogenous double-stranded They found that Dicer-positive loci RNA. Nature Struct. Mol. Biol. http://dx.doi. org/10.1038/nsmb.2827 (2014) had increased levels of AGO1 and the NPG

NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 15 | JUNE 2014