No abstract

DICER1 plays a critical role in microRNA (miRNA) biogenesis. Recurrent somatic 'hotspot' mutations at the four metal-binding sites within the RNase IIIb domain of DICER1 were identified in ovarian sex cord-stromal tumours and have since been described in other paediatric tumours. In this study, we screened the RNase IIIb domain of DICER1 in 290 endometrial tumours and identified six cases with hotspot mutations, including two cases affected by an atypical G1809R mutation directly adjacent to a metal-binding site. Using Illumina and Sanger targeted resequencing, we observed and validated biallelic DICER1 mutations in several cases with hotspot mutations. Through in vitro DICER1 cleavage assays, small RNA deep sequencing and real-time PCR, we demonstrated that mutations adding a positively charged side chain to residue 1809 have similar detrimental effects on 5p miRNA production to mutations at the metal-binding sites. As expected, 5p miRNAs were globally reduced in tumours and cell lines with hotspot mutations. Pathway analysis of gene expression profiles indicated that genes de-repressed due to loss of 5p miRNAs are strongly associated with pathways regulating the cell cycle. Using a Dicer1-null mouse cell line model, we found that expression of DICER1 hotspot mutants promoted cell proliferation, whereas wild-type (WT) DICER1 inhibited cell proliferation. Furthermore, targets of let-7 family miRNAs are enriched among the up-regulated genes, suggesting that loss of let-7 may be impacting downstream pathways. Our results reveal that DICER1 hotspot mutations are implicated in common malignancies and may constitute a unique oncogenic pathway.

Cyclin-dependent kinase 12 (CDK12) promotes transcriptional elongation by phosphorylation of the RNA polymerase II C-terminal domain (CTD). Structure-function studies show that this activity is dependent on a C-terminal kinase extension, as well as the binding of cyclin K (CycK). To better define these interactions we determined the crystal structure of the human CDK12/CycK complex with and without the kinase extension in the presence of AMP-PNP. The structures revealed novel features for a CDK, including a large β4-β5 loop insertion that contributes to the N-lobe interaction with the cyclin. We also observed two different conformations of the C-terminal kinase extension that effectively open and close the ATP pocket. Most notably, bound AMP-PNP was only observed when trapped in the closed state. Truncation of this C-terminal structure also diminished AMP-PNP binding, as well as the catalytic activity of the CDK12/CycK complex. Further kinetic measurements showed that the full length CDK12/CycK complex was significantly more active than the two crystallised constructs suggesting a critical role for additional domains. Overall, these results demonstrate the intrinsic flexibility of the C-terminal extension in CDK12 and highlight its importance for both ATP binding and kinase activity.