Internal RNA modifications are found both in coding and noncoding RNAs. In the ribosome, two highly conserved proteins catalyse the methylation of guanosine 1639 and pseudouridine 1248 residue of the 18S rRNA and they are known as WBSCR22 and EMG1. Both proteins are key factors for ribosome biogenesis and previous studies have demonstrated that in this context, they both act as scaffolds and that their catalytic activity is dispensable for 18S rRNA maturation. Moreover, high expression of both methyltransferases has been reported in acute myeloid leukaemia (AML) and uveal melanoma (UVM), where it strongly correlates with poor prognosis. However, the specific role of these enzymes in cancer is still unknown.

I identified both WBSCR22 and EMG1 catalytic activity as essential for the growth of AML and UVM cells with a model in which the endogenous proteins are knocked down through a stable shRNA-inducible system and rescued with a wild-type or a catalytically dead variant. In AML, inducible depletion of WBSCR22 and EMG1 through both shRNA knock-down and CRISPR-Cas9 targeting, causes cell cycle arrest which cannot be rescued by catalytically inactive protein variants and is independent from ribosome maturation. Similarly, inactivation of WBSCR22 in uveal melanoma results in proliferation arrest, decreased migration, and reduced anchorage-independent growth, all while preserving ribosome integrity and function. These data show that, WBSCR22 has an additional function extrinsic to ribosome biogenesis as a regulator of oncogenic protein translation. By using polysome profiling coupled with next generation sequencing I characterized genes that showed no differences in total mRNA expression levels but are less translated when the m7G modification is removed from the 18S rRNA. Among these genes, I identified essential drivers of metastatic uveal melanoma such as cMET. Finally, the therapeutic potential of WBSCR22 catalytic inactivation was explored for the first time as a putative anti-metastatic combination therapy with crizotinib. Preliminary data show that sub lethal concentrations of crizotinib are sufficient to reduce uveal melanoma WBSCR22 catalytic mutant cell metastatic potential.

Together, these data define WBSCR22 as a regulator of migration and endothelial-mesenchymal transition pathways that drive metastasis formation in uveal melanoma and identify this enzyme as a potential therapeutic target.