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The human and veterinary disease complex known as African trypanosomiasis continues to inflict significant global morbidity, mortality, and economic hardship. Drug resistance and toxic side effects of old drugs call for novel and unorthodox strategies for new and safe treatment options. We designed methyltriazenyl purine prodrugs to be rapidly and selectively internalized by the parasite, after which they disintegrate into a nontoxic and naturally occurring purine nucleobase, a simple triazene-stabilizing group, and the active toxin: a methyldiazonium cation capable of damaging DNA by alkylation. We identified 2-(3-acetyl-3-methyltriazen-1-yl)-6-hydroxypurine (compound 1) as a new lead compound, which showed submicromolar potency against Trypanosoma brucei , with a selectivity index of >500, and it demonstrated a curative effect in animal models of acute trypanosomiasis. We investigated the mechanism of action of this lead compound and showed that this molecule has significantly higher affinity for parasites over mammalian nucleobase transporters, and it does not show cross-resistance with current first-line drugs. Once selectively accumulated inside the parasite, the prodrug releases a DNA-damaging methyldiazonium cation. We propose that ensuing futile cycles of attempted mismatch repair then lead to G 2 /M phase arrest and eventually cell death, as evidenced by the reduced efficacy of this purine analog against a mismatch repair-deficient ( MSH2 −/− ) trypanosome cell line. The observed absence of genotoxicity, hepatotoxicity, and cytotoxicity against mammalian cells revitalizes the idea of pursuing parasite-selective DNA alkylators as a safe chemotherapeutic option for the treatment of human and animal trypanosomiasis.

Parasitic protozoa of the flagellate order Kinetoplastida represent one of the deepest branches of the eukaryotic tree. Among this group of organisms, the mechanism of RNA interference (RNAi) has been investigated in Trypanosoma brucei and to a lesser degree in Leishmania ( Viannia ) spp. The pathway is triggered by long double-stranded RNA (dsRNA) and in T. brucei requires a set of five core genes, including a single Argonaute (AGO) protein, T. brucei AGO1 ( Tb AGO1). The five genes are conserved in Leishmania ( Viannia ) spp. but are absent in other major kinetoplastid species, such as Trypanosoma cruzi and Leishmania major . In T. brucei small interfering RNAs (siRNAs) are methylated at the 3′ end, whereas Leishmania ( Viannia ) sp. siRNAs are not. Here we report that T. brucei HEN1, an ortholog of the metazoan HEN1 2′- O -methyltransferases, is required for methylation of siRNAs. Loss of Tb HEN1 causes a reduction in the length of siRNAs. The shorter siRNAs in hen1 −/− parasites are single stranded and associated with Tb AGO1, and a subset carry a nontemplated uridine at the 3′ end. These findings support a model wherein Tb HEN1 methylates siRNA 3′ ends after they are loaded into Tb AGO1 and this methylation protects siRNAs from uridylation and 3′ trimming. Moreover, expression of Tb HEN1 in Leishmania ( Viannia ) panamensis did not result in siRNA 3′ end methylation, further emphasizing mechanistic differences in the trypanosome and Leishmania RNAi mechanisms.

Graphical abstract T. cruzi II strains accumulate more 8-oxoguanine in the kDNA after hydrogen peroxide-induced 18 oxidative stress than T. cruzi I strains. NT: untreated; T: treated. Research highlights ▶ Distinct levels of DNA mismatch repair activity are found among T. cruzi strains. ▶ In T. cruzi and T. brucei, MSH2 has a mitochondrial function involved in the response to oxidative stress., Components of the DNA mismatch repair (MMR) pathway are major players in processes known to generate genetic diversity, such as mutagenesis and DNA recombination. Trypanosoma cruzi, the protozoan parasite that causes Chagas disease has a highly heterogeneous population, composed of a pool of strains with distinct characteristics. Studies with a number of molecular markers identified up to six groups in the T. cruzi population, which showed distinct levels of genetic variability. To investigate the molecular basis for such differences, we analyzed the T. cruzi MSH2 gene, which encodes a key component of MMR, and showed the existence of distinct isoforms of this protein. Here we compared cell survival rates after exposure to genotoxic agents and levels of oxidative stress-induced DNA in different parasite strains. Analyses of msh2 mutants in both T. cruzi and T. brucei were also used to investigate the role of Tcmsh2 in the response to various DNA damaging agents. The results suggest that the distinct MSH2 isoforms have differences in their activity. More importantly, they also indicate that, in addition to its role in MMR, TcMSH2 acts in the parasite response to oxidative stress through a novel mitochondrial function that may be conserved in T. brucei.

Homologous recombination functions universally in the maintenance of genome stability through the repair of DNA breaks and in ensuring the completion of replication. In some organisms, homologous recombination can perform more specific functions. One example of this is in antigenic variation, a widely conserved mechanism for the evasion of host immunity. Trypanosoma brucei, the causative agent of sleeping sickness in Africa, undergoes antigenic variation by periodic changes in its variant surface glycoprotein (VSG) coat. VSG switches involve the activation of VSG genes, from an enormous silent archive, by recombination into specialized expression sites. These reactions involve homologous recombination, though they are characterized by an unusually high rate of switching and by atypical substrate requirements. Here, we have examined the substrate parameters of T. brucei homologous recombination. We show, first, that the reaction is strictly dependent on substrate length and that it is impeded by base mismatches, features shared by homologous recombination in all organisms characterized. Second, we identify a pathway of homologous recombination that acts preferentially on short substrates and is impeded to a lesser extent by base mismatches and the mismatch repair machinery. Finally, we show that mismatches during T. brucei recombination may be repaired by short-patch mismatch repair.

The introduction ten years ago of RNA interference (RNAi) as a tool for molecular exploration in Trypanosoma brucei has led to a surge in our understanding of the pathogenesis and biology of this human parasite. In particular, a genome-wide RNAi screen has recently been combined with next-generation Illumina sequencing to expose catalogues of genes associated with loss of fitness in distinct developmental stages. At present, this technology is restricted to RNAi-positive protozoan parasites, which excludes T. cruzi, Leishmania major, and Plasmodium falciparum. Therefore, elucidating the mechanism of RNAi and identifying the essential components of the pathway is fundamental for improving RNAi efficiency in T. brucei and for transferring the RNAi tool to RNAi-deficient pathogens. Here we used comparative genomics of RNAi-positive and -negative trypanosomatid protozoans to identify the repertoire of factors in T. brucei. In addition to the previously characterized Argonaute 1 (AGO1) protein and the cytoplasmic and nuclear Dicers, TbDCL1 and TbDCL2, respectively, we identified the RNA Interference Factors 4 and 5 (TbRIF4 and TbRIF5). TbRIF4 is a 3′-5′ exonuclease of the DnaQ superfamily and plays a critical role in the conversion of duplex siRNAs to the single-stranded form, thus generating a TbAGO1-siRNA complex required for target-specific cleavage. TbRIF5 is essential for cytoplasmic RNAi and appears to act as a TbDCL1 cofactor. The availability of the core RNAi machinery in T. brucei provides a platform to gain mechanistic insights in this ancient eukaryote and to identify the minimal set of components required to reconstitute RNAi in RNAi-deficient parasites., Author Summary RNA interference (RNAi), a naturally-occurring pathway whereby the presence of double-stranded RNA in a cell triggers the degradation of homologous mRNA, has been harnessed in many organisms as an invaluable molecular biology tool to interrogate gene function. Although this technology is widely used in the protozoan parasite Trypanosoma brucei, other parasites of considerable public health significance, such as Trypanosoma cruzi, Leishmania major, and Plasmodium falciparum do not perform RNAi. Since RNAi has recently been introduced into budding yeast, this opens up the possibility that RNAi can be reconstituted in these pathogens. The key to this is getting a handle on the essential RNAi factors in T. brucei. By applying comparative genomics we identified five genes that are present in the RNAi-proficient species, but not in RNAi-deficient species: three previously identified RNAi factors, and two novel ones, which are described here. This insight into the core T. brucei RNAi machinery represents a major step towards transferring this pathway to RNAi-deficient parasites.

In most eukaryotic cells, expression or delivery of long double-stranded RNA (dsRNA) signals the presence of foreign and/or potentially dangerous nucleic acids, such as viruses or transcripts derived from retroposons and transposons. As a consequence cells go on red alert and activate specific defense mechanisms to eliminate the invaders. Among such mechanisms is the RNA interference (RNAi) pathway, which is triggered by long dsRNAs, destroys target transcripts in a sequence-specific manner, and is widespread throughout eukaryotic evolution. Here we summarize our current understanding of the RNAi mechanism in Trypanosoma brucei, a protozoan parasite and early divergent eukaryote, and highlight similarities and differences with the RNAi machinery and its function in higher eukaryotes.