Authors

Discussion

RNA editing ligase 1 (REL1) plays a crucial role in uridylyl insertion/deletion mRNA editing, a unique and extensive form of post-transcriptional RNA modification in the mitochondria of trypanosomatid parasites. Previous gene knockdown and knockout studies showed that REL1 is essential for the survival of Trypanosoma brucei, the causative agent of sleeping sickness (Schnaufer et al., 2001; Gao & Simpson, 2003). Since REL1 is highly conserved, essentiality is expected for other trypanosomatid species as well. The crystal structure of TbREL1 revealed a well-defined ATP-binding pocket with distinct differences to mammalian DNA and RNA ligases (Deng et al., 2004). This offers exciting potential for the development of specific REL1 inhibitors with broad anti-trypanosomatid properties.

We developed a high-throughput activity assay for REL1 (Zimmermann et al., 2016) and tested over 600,000 compounds in four independent screening campaigns. Promising series of REL1 small molecule inhibitors were identified with an average hit rate of ~1%, and interesting structure-activity relationships emerged for some series. Several compounds inhibited REL1 from different trypanosomatid species, including Leishmania donovani and Trypanosoma cruzi, but are much less potent against a related bacteriophage RNA ligase, suggesting high specificity in vitro. As part of ongoing hit-to-lead development efforts, we are investigating specificity in vivo, optimising the expression of recombinant REL1 orthologues with the help of differential scanning fluorimetry, and using crystallography and surface plasmon resonance as tools to study REL1-inhibitor interactions. In addition, CRISPR-Cas9-based knockout studies in Leishmania mexicana suggest that REL1 is indeed essential in this trypanosomatid.