DICER1 syndrome mutations lead to a gain of 3p-miRNA function, HERVH activity and increased metastatic potential

The DICER1 gene is mutated in cancer, including DICER1 syndrome, a rare tumour predisposition syndrome. Cancer-associated hotspot mutations have been reported in both catalytic domains of DICER1 and are predicted to disrupt miRNA biogenesis. To understand how these hotspot mutations contribute to cancer development, we have generated cell lines harbouring single amino acid substitutions within the catalytic RNase IIIa (S1344L) or RNase IIIb (D1709N) domains of the endogenous DICER1 gene. We show that both mutations result in a widespread loss of 5p miRNAs, and, unexpectedly, 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 found that changes in the repertoire of miRNAs loaded into AGO2 result in altered gene expression, impacting critical pathways for cancer development, including metastatic potential. Additionally, our results indicate that inactivating the processing activity of DICER1 does not result in genomic instability. Instead, mutations cause specific upregulation of human endogenous retrovirus H (HERVH) through miRNA-independent mechanisms, suggesting that both canonical and non-canonical DICER1 functions are important to understand DICER1 syndrome.