DICER1-related tumour predisposition mutations increase 3p-miRNA function and HERVH activity.

The DICER1 gene is mutated in cancer, including in DICER1-related tumour predisposition. Cancer-associated hotspot mutations have been reported in both catalytic domains of DICER and are predicted to disrupt miRNA biogenesis. To understand how these hotspot mutations contribute to cancer development, we generate cell lines harbouring single amino acid substitutions within the catalytic RNase IIIa (S1344L) or RNase IIIb (D1709N) domains of the endogenous DICER1 gene. Here we show that both mutations result in a widespread loss of 5p miRNAs, and an increase in 3p passenger strands loading into AGO2. The shared similarities between both mutants can be attributed to the structural proximity of the S1344 residue to the RNase IIIb catalytic centre. Functionally, we find that changes in the repertoire of miRNAs loaded into AGO2 result in altered gene expression, impacting critical pathways for cancer development. Additionally, our results indicate that inactivating the processing activity of DICER does not result in genomic instability. Instead, mutations cause upregulation of transposable elements, including the human endogenous retrovirus H through miRNA-independent mechanisms. This suggests that both canonical and non-canonical DICER functions are important to understand DICER1-related tumour predisposition.