The trypanocidal benzoxaborole AN7973 inhibits trypanosome mRNA processing
Authors: Begolo Daniela , Vincent Isabel M , Giordani Federica , Witty Michael J , Rowan Timothy G , Bengaly Zakaria , Gillingwater Kirsten , Freund Yvonne , Barrett Michael P , Clayton Christine 
Published: April 2018

Kinetoplastid parasites - trypanosomes and leishmanias - infect millions of humans and cause economically devastating diseases of livestock, and the few existing drugs have serious deficiencies. Benzoxaborole-based compounds are very promising potential novel anti-trypanosomal therapies, with candidates already in human and animal clinical trials. Their targets in trypanosomes were hitherto unknown. We investigated the mechanism of action of several benzoxaboroles, including AN7973, an early candidate for veterinary trypanosomosis.

In all kinetoplastids, transcription is polycistronic. Individual mRNA 5’-ends are created bytranssplicing of a short leader sequence, with coupled polyadenylation of the preceding mRNA. Treatment ofTrypanosoma bruceiwith AN7973 inhibitedtranssplicing within 1h, as judged by loss of the Y-structure splicing intermediate and reduced levels of mRNA, and accumulation of peri-nuclear granules which are typical for splicing inhibition. Methylation of the spliced leader precursor RNA was not affected, but more prolonged AN7973 treatment caused an increase in S-adenosyl methionine and methylated lysine. Together, these results indicate that mRNA processing is the primary target of AN7973. Polyadenylation is required for kinetoplastidtranssplicing. The EC50for AN7973 inT. bruceiwas increased three-fold by over-expression of theT. bruceicleavage and polyadenylation factor CPSF3, identifying CPSF3 as a potential molecular target. Our results thus chemically validate mRNA processing as a viable drug target in trypanosomes.

Several other benzoxaboroles showed metabolomic and splicing effects that were similar to those of AN7973, identifying splicing inhibition as a common mode of action, and suggesting that it might be linked to subsequent changes in methylated metabolites. Granule formation, splicing inhibition, and resistance after CPSF3 expression did not, however, always correlate, and prolonged selection of trypanosomes in AN7973 resulted in only 1.5-fold resistance. This suggests that the modes of action of oxaboroles that target trypanosome mRNA processing may extend beyond CPSF3 inhibition.

Author summary

Trypanosomes and leishmanias infect millions of humans and cause economically devastating diseases of livestock; the few existing drugs have serious deficiencies. Trypanosomosis of cattle, caused mainly byTrypanosoma congolenseandTrypanosoma vivax, is a serious problem in Africa, because bovids are used not only for meat and milk, but also for traction. Only two drugs are in routine use for chemotherapy and chemoprophylaxis of bovine trypanosomosis. A single injection of the benzoxaborole compound AN7973 was sufficient to cureT. congolenseinfection in cattle and goats, but AN7973 was less effective againstT. vivax. This precluded development of AN7973 as a commercially viable treatment against cattle trypanosomosis, but it could still have potential for diseases caused by other salivarian trypanosomes.

We used a large range of methods to find out how AN7973 kills trypanosomes, and compared it with several other benzoxaboroles. AN7973 and some of the other compounds had effects on parasite metabolism that resembled those previously seen for a benzoxaborole that is being tested for human sleeping sickness. The most rapid effect of AN7973, however, was on processing of trypanosome mRNA. As a consequence, amounts of mRNA decreased and synthesis of proteins stopped. We conclude that AN7973 and some other benzoxaboroles kill trypanosomes by stopping gene expression.