Antiviral potential coupled to genome stability: the multifaceted roles of DDX3X

The human RNA helicase DDX3X is a real multifaceted enzyme. Like all the other DEAD-box proteins of the same family, DDX3X participates into different steps of RNA metabolism. Moreover, DDX3X is one of the actors of cell cycle regulation, innate immunity and apoptosis processes. Our group started to look at DDX3X as an interesting protein since it has primary roles in viral infections and tumor development too. In the context of viral infections, DDX3X possesses dual roles: it acts as an antiviral or proviral factor regulating viral replication at different levels (regulation of genome duplication and/or gene expression and host innate immunity activation). From these observations, it came the idea to use DDX3X as a possible therapeutic target to inhibit a function essential for the viral replication, but dispensable for the human cell. In collaboration with the University of Siena (Prof. Maurizio Botta), we developed some inhibitor molecules able to recognize two different DDX3X pockets: the helicase binding pocket and the unique motif of DDX3X. Both two compounds families showed selectivity and no toxicity in cells; even more interesting, our molecules showed considerable broad-spectrum antiviral effects being able to suppress both the replication of WNV and DENV-2 viruses in infected cells. The development of DDX3X-specific inhibitor molecules in the context of different viral infections is just one of the two projects that I followed during my PhD internship. In parallel, I shifted my attention on the characterization of this enzyme in the context of DNA damage response. Published data have already implicated DDX3X in the resolution of RNA/DNA hybrids and RNA secondary structures, as well as in the degradation of RNAs. Moreover, unpublished data from our lab demonstrated also a possible exoribonuclease activity of DDX3X in addition to the already well-known helicase/ATPase activities. Trying to envision a physiological relevance of this activity, we focused on the possible situations in which RNA/DNA hybrids could be present into the cell. Ribonucleotides (rNMP) are the most abundant type of DNA damage and all the cells have specific enzymes able to remove them avoiding deleterious consequences. This removal activity is essential and it is primarily performed by the ribonucleotide excision repair (RER) pathway. RNaseH2 is believed to be the only enzyme able to incise single rNMPs within a DNA strand activating an error-free RER pathway. Surprisingly, our data demonstrated that DDX3X is able to promote RER initiation, thereby suggesting a possible new role of this enzyme also in genome stability.