Targeting oncogenic microRNAs in malignant germ cell tumours
Background. MiR-371~373 and miR-302/367 cluster over-expression occurs in all malignant-GCTs, regardless of age (paediatric/adult), site (gonadal/extragonadal), or subtype [seminoma, yolk sac tumour (YST), embryonal carcinoma (EC)]. Six of eight microRNAs from these clusters contain the seed 'AAGUGC', determining mRNA targeting. Here we sought to identify the significance of these observations by targeting these microRNAs functionally. Methods. We targeted miR-371~373 and/or miR-302/367 clusters in malignant-GCT cell lines, using CRISPR-Cas9, gapmer primary miR-302/367 transcripts inhibition, and peptide-nucleic-acid (PNA) or locked-nucleic-acid (LNA)-DNA inhibition targeting miR-302a-d-3p, and undertook relevant functional assays. Results. MiR-302/367 cluster microRNAs made the largest contribution to AAGUGC seed abundance in malignant-GCT cells, regardless of subtype (seminoma/YST/EC). Following unsuccessful use of CRISPR-Cas9, gapmer, and PNA systems, LNA-DNA-based targeting  resulted in growth inhibition in seminoma and YST cells. This was associated with de-repression of multiple mRNAs targeted by 'AAGUGC' seed-containing microRNAs, with pathway analysis confirming predominant disruption of Rho-GTPase signaling, vesicle organization/transport, and cell-cycle regulation, findings corroborated in clinical samples. Further LNA-DNA inhibitor studies confirmed direct cell-cycle effects, with increase of cells in G0/G1-phase and decrease in S-phase.  Conclusion. Targeting of specific miR-371~373 and miR-302/367 microRNAs in malignant-GCTs demonstrated their functional significance, with growth inhibition mediated through cell-cycle disruption.