Homologous recombination between direct repeat sequences yields P-glycoprotein containing amplicons in arsenite resistant Leishmania

Impact of Genetic Diversity and Genome Plasticity of Leishmania spp. in Treatment and the Search for Novel Chemotherapeutic Targets

Leishmaniasis is one of the major public health concerns in Latin America, Africa, Asia, and Europe. The absence of vaccines for human use and the lack of effective vector control programs make chemotherapy the main strategy to control all forms of the disease. However, the high toxicity of available drugs, limited choice of therapeutic agents, and occurrence of drug-resistant parasite strains are the main challenges related to chemotherapy. Currently, only a small number of drugs are available for leishmaniasis treatment, including pentavalent antimonials (Sb^V), amphotericin B and its formulations, miltefosine, paromomycin sulphate, and pentamidine isethionate. In addition to drug toxicity, therapeutic failure of leishmaniasis is a serious concern. The occurrence of drug-resistant parasites is one of the causes of therapeutic failure and is closely related to the diversity of parasites in this genus. Owing to the enormous plasticity of the genome, resistance can occur by altering different metabolic pathways, demonstrating that resistance mechanisms are multifactorial and extremely complex. Genetic variability and genome plasticity cause not only the available drugs to have limitations, but also make the search for new drugs challenging. Here, we examined the biological characteristics of parasites that hinder drug discovery.

DNA Double-Strand Breaks: A Double-Edged Sword for Trypanosomatids

For nearly all eukaryotic cells, stochastic DNA double-strand breaks (DSBs) are one of the most deleterious types of DNA lesions. DSB processing and repair can cause sequence deletions, loss of heterozygosity, and chromosome rearrangements resulting in cell death or carcinogenesis. However, trypanosomatids (single-celled eukaryotes parasites) do not seem to follow this premise strictly. Several studies have shown that trypanosomatids depend on DSBs to perform several events of paramount importance during their life cycle. For Trypanosoma brucei, DSBs formation is associated with host immune evasion via antigenic variation. In Trypanosoma cruzi, DSBs play a crucial role in the genetic exchange, a mechanism that is still little explored but appear to be of fundamental importance for generating variability. In Leishmania spp., DSBs are necessary to generate genomic changes by gene copy number variation (CNVs), events that are essential for these organisms to overcome inhospitable conditions. As DSB repair in trypanosomatids is primarily conducted via homologous recombination (HR), most of the events associated with DSBs are HR-dependent. This review will discuss the latest findings on how trypanosomatids balance the benefits and inexorable challenges caused by DSBs.

Metabolic selection of a homologous recombination-mediated gene loss protects Trypanosoma brucei from ROS production by glycosomal fumarate reductase

The genome of trypanosomatids rearranges by using repeated sequences as platforms for amplification or deletion of genomic segments. These stochastic recombination events have a direct impact on gene dosage and foster the selection of adaptive traits in response to environmental pressure. We provide here such an example by showing that the phosphoenolpyruvate carboxykinase (PEPCK) gene knockout (Δpepck) leads to the selection of a deletion event between two tandemly arranged fumarate reductase (FRDg and FRDm2) genes to produce a chimeric FRDg-m2 gene in the Δpepck∗ cell line. FRDg is expressed in peroxisome-related organelles, named glycosomes, expression of FRDm2 has not been detected to date, and FRDg-m2 is nonfunctional and cytosolic. Re-expression of FRDg significantly impaired growth of the Δpepck∗ cells, but FRD enzyme activity was not required for this negative effect. Instead, glycosomal localization as well as the covalent flavinylation motif of FRD is required to confer growth retardation and intracellular accumulation of reactive oxygen species (ROS). The data suggest that FRDg, similar to Escherichia coli FRD, can generate ROS in a flavin-dependent process by transfer of electrons from NADH to molecular oxygen instead of fumarate when the latter is unavailable, as in the Δpepck background. Hence, growth retardation is interpreted as a consequence of increased production of ROS, and rearrangement of the FRD locus liberates Δpepck∗ cells from this obstacle. Interestingly, intracellular production of ROS has been shown to be required to complete the parasitic cycle in the insect vector, suggesting that FRDg may play a role in this process.

Casein kinase 1.2 over expression restores stress resistance to Leishmania donovani HSP23 null mutants

Leishmania donovani is a trypanosomatidic parasite and causes the lethal kala-azar fever, a neglected tropical disease. The Trypanosomatida are devoid of transcriptional gene regulation and rely on gene copy number variations and translational control for their adaption to changing conditions. To survive at mammalian tissue temperatures, L. donovani relies on the small heat shock protein HSP23, the loss of which renders the parasites stress sensitive and impairs their proliferation. Here, we analysed a spontaneous escape mutant with wild type-like in vitro growth. Further selection of this escape strains resulted in a complete reversion of the phenotype. Whole genome sequencing revealed a correlation between stress tolerance and the massive amplification of a six-gene cluster on chromosome 35, with further analysis showing over expression of the casein kinase 1.2 gene as responsible. In vitro phosphorylation experiments established both HSP23 and the related P23 co-chaperone as substrates and modulators of casein kinase 1.2, providing evidence for another crucial link between chaperones and signal transduction protein kinases in this early branching eukaryote.

Global genome diversity of the Leishmania donovani complex

Protozoan parasites of the Leishmania donovani complex - L. donovani and L. infantum - cause the fatal disease visceral leishmaniasis. We present the first comprehensive genome-wide global study, with 151 cultured field isolates representing most of the geographical distribution. L. donovani isolates separated into five groups that largely coincide with geographical origin but vary greatly in diversity. In contrast, the majority of L. infantum samples fell into one globally-distributed group with little diversity. This picture is complicated by several hybrid lineages. Identified genetic groups vary in heterozygosity and levels of linkage, suggesting different recombination histories. We characterise chromosome-specific patterns of aneuploidy and identified extensive structural variation, including known and suspected drug resistance loci. This study reveals greater genetic diversity than suggested by geographically-focused studies, provides a resource of genomic variation for future work and sets the scene for a new understanding of the evolution and genetics of the Leishmania donovani complex.

Inhibiting homologous recombination decreases extrachromosomal amplification but has no effect on intrachromosomal amplification in methotrexate-resistant colon cancer cells

Gene amplification, which involves the two major topographical structures double minutes (DMs) and homegeneously stained region (HSR), is a common mechanism of treatment resistance in cancer and is initiated by DNA double‐strand breaks. NHEJ, one of DSB repair pathways, is involved in gene amplification as we demonstrated previously. However, the involvement of homologous recombination, another DSB repair pathway, in gene amplification remains to be explored. To better understand the association between HR and gene amplification, we detected HR activity in DM‐ and HSR‐containing MTX‐resistant HT‐29 colon cancer cells. In DM‐containing MTX‐resistant cells, we found increased homologous recombination activity compared with that in MTX‐sensitive cells. Therefore, we suppressed HR activity by silencing BRCA1, the key player in the HR pathway. The attenuation of HR activity decreased the numbers of DMs and DM‐form amplified gene copies and increased the exclusion of micronuclei and nuclear buds that contained DM‐form amplification; these changes were accompanied by cell cycle acceleration and increased MTX sensitivity. In contrast, BRCA1 silencing did not influence the number of amplified genes and MTX sensitivity in HSR‐containing MTX‐resistant cells. In conclusion, our results suggest that the HR pathway plays different roles in extrachromosomal and intrachromosomal gene amplification and may be a new target to improve chemotherapeutic outcome by decreasing extrachromosomal amplification in cancer.

What's new?

Double‐strand DNA breaks (DSBs) initiate gene amplification, a phenomenon associated with therapeutic resistance in cancer that involves two topographical structures, double minutes (DMs) and homogeneously staining regions (HSRs). Whether DSB repair pathways, particularly homologous recombination (HR), also influence gene amplification is unknown. Here, in methotrexate‐resistant colon cancer cells, HR inhibition effectively reduced gene amplification, specifically the DM‐form, by blocking DM formation and promoting DM exclusion via micronuclei. HR inhibition had no influence on the HSR‐form of gene amplification. Loss of gene amplification by HR inhibition, through partial reversal of methotrexate resistance, may contribute to improved chemotherapeutic outcome.

Predominance of Leishmania major and rare occurrence of Leishmania tropica with haplotype variability at the center of Iran

BACKGROUND

Leishmania major is a causative agent of zoonotic cutaneous leishmaniasis in the center of Iran, Abarkouh district. Molecular characterization and precise incrimination of Leishmania species was carried out to perform controlling measurements and to design treatment programs for zoonotic cutaneous leishmaniasis.

METHODS

All smears isolated from ulcers of suspected patients were examined under a light microscope and graded for amastigotes frequency. Extraction of DNA, PCR, RFLP and sequencing of ITS-rDNA genotype were done to increase the efficacy of Leishmania parasites identification at their species-specific level and to detect any Leishmania infections within.

RESULTS

Humans were found to be infected with L. major with high infection frequency and also Leishmania tropica was identified with low occurrence for the first time as non-native species using molecular analyses. The rates of infections was considerable with microscopic observation (n = 65, 73%) out of 89 smears prepared from suspected patients. Molecular analyses showed that the density of L. major was significantly higher (n = 48, 53.93%) than L. tropica (n = 4, 4.49%) (Mann-Whitney U test: p < 0.05) and two samples (2.25%) remained ambiguous after several sequencing. L. major did not have diversity with two common haplotypes but L. tropica were found to exhibit high diversity with three novel haplotypes.

CONCLUSION

L. major was considered the causative agent of leishmaniasis in the region, but the identification of a non-native L. tropica revealed the importance of further isolation of Leishmania parasites following molecular analyses and confirmation, and also revealed the importance of further isolation of Leishmania parasites from patients of the field areas who do not have easily access to health care centers for specialized treatment strategies.

Integrated genomic and metabolomic profiling of ISC1, an emerging Leishmania donovani population in the Indian subcontinent

Leishmania donovani is the responsible agent for visceral leishmaniasis (VL) in the Indian subcontinent (ISC). The disease is lethal without treatment and causes 0.2 to 0.4 million cases each year. Recently, reports of VL in Nepalese hilly districts have increased as well as VL cases caused by L. donovani from the ISC1 genetic group, a new and emerging genotype. In this study, we perform for the first time an integrated, untargeted genomics and metabolomics approach to characterize ISC1, in comparison with the Core Group (CG), main population that drove the most recent outbreak of VL in the ISC. We show that the ISC1 population is very different from the CG, both at genome and metabolome levels. The genomic differences include SNPs, CNV and small indels in genes coding for known virulence factors, immunogens and surface proteins. Both genomic and metabolic approaches highlighted dissimilarities related to membrane lipids, the nucleotide salvage pathway and the urea cycle in ISC1 versus CG. Many of these pathways and molecules are important for the interaction with the host/extracellular environment. Altogether, our data predict major functional differences in ISC1 versus CG parasites, including virulence. Therefore, particular attention is required to monitor the fate of this emerging ISC1 population in the ISC, especially in a post-VL elimination context.

Genomic analysis of Isometamidium Chloride resistance in Trypanosoma congolense

Isometamidium Chloride (ISM) is one of the principal drugs used to counteract Trypanosoma congolense infection in livestock, both as a prophylactic as well as a curative treatment. However, numerous cases of ISM resistance have been reported in different African regions, representing a significant constraint in the battle against Animal African Trypanosomiasis. In order to identify genetic signatures associated with ISM resistance in T. congolense, the sensitive strain MSOROM7 was selected for induction of ISM resistance in a murine host. Administered ISM concentrations in immune-suppressed mice were gradually increased from 0.001 mg/kg to 1 mg/kg, the maximal dose used in livestock. As a result, three independent MSOROM7 lines acquired full resistance to this concentration after five months of induction, and retained this full resistant phenotype following a six months period without drug pressure. In contrast, parasites did not acquire ISM resistance in immune-competent animals, even after more than two years under ISM pressure, suggesting that the development of full ISM resistance is strongly enhanced when the host immune response is compromised. Genomic analyses comparing the ISM resistant lines with the parental sensitive line identified shifts in read depth at heterozygous loci in genes coding for different transporters and transmembrane products, and several of these shifts were also found within natural ISM resistant isolates. These findings suggested that the transport and accumulation of ISM inside the resistant parasites may be modified, which was confirmed by flow cytometry and ex vivo ISM uptake assays that showed a decrease in the accumulation of ISM in the resistant parasites.

Plasticity of the Leishmania genome leading to gene copy number variations and drug resistance

Leishmania has a plastic genome, and drug pressure can select for gene copy number variation (CNV). CNVs can apply either to whole chromosomes, leading to aneuploidy, or to specific genomic regions. For the latter, the amplification of chromosomal regions occurs at the level of homologous direct or inverted repeated sequences leading to extrachromosomal circular or linear amplified DNAs. This ability of Leishmania to respond to drug pressure by CNVs has led to the development of genomic screens such as Cos-Seq, which has the potential of expediting the discovery of drug targets for novel promising drug candidates.

Drug resistance analysis by next generation sequencing in Leishmania

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WGS revealed the complexity of resistance which is often polyclonal in Leishmania.

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An impressive variation in chromosome copy numbers exist between Leishmania species.

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The genotype heterogeneity complicates the analysis of resistance in field isolates.

The use of next generation sequencing has the power to expedite the identification of drug resistance determinants and biomarkers and was applied successfully to drug resistance studies in Leishmania. This allowed the identification of modulation in gene expression, gene dosage alterations, changes in chromosome copy numbers and single nucleotide polymorphisms that correlated with resistance in Leishmania strains derived from the laboratory and from the field. An impressive heterogeneity at the population level was also observed, individual clones within populations often differing in both genotypes and phenotypes, hence complicating the elucidation of resistance mechanisms. This review summarizes the most recent highlights that whole genome sequencing brought to our understanding of Leishmania drug resistance and likely new directions.

DNA repair pathways in trypanosomatids: from DNA repair to drug resistance

All living organisms are continuously faced with endogenous or exogenous stress conditions affecting genome stability. DNA repair pathways act as a defense mechanism, which is essential to maintain DNA integrity. There is much to learn about the regulation and functions of these mechanisms, not only in human cells but also equally in divergent organisms. In trypanosomatids, DNA repair pathways protect the genome against mutations but also act as an adaptive mechanism to promote drug resistance. In this review, we scrutinize the molecular mechanisms and DNA repair pathways which are conserved in trypanosomatids. The recent advances made by the genome consortiums reveal the complete genomic sequences of several pathogens. Therefore, using bioinformatics and genomic sequences, we analyze the conservation of DNA repair proteins and their key protein motifs in trypanosomatids. We thus present a comprehensive view of DNA repair processes in trypanosomatids at the crossroads of DNA repair and drug resistance.

Genome-wide stochastic adaptive DNA amplification at direct and inverted DNA repeats in the parasite Leishmania

The human parasite Leishmania uses adaptive gene rearrangements and amplification involving repeated sequences on a genome-wide scale as one strategy to adapt to a changing environment.

Gene amplification of specific loci has been described in all kingdoms of life. In the protozoan parasite Leishmania, the product of amplification is usually part of extrachromosomal circular or linear amplicons that are formed at the level of direct or inverted repeated sequences. A bioinformatics screen revealed that repeated sequences are widely distributed in the Leishmania genome and the repeats are chromosome-specific, conserved among species, and generally present in low copy number. Using sensitive PCR assays, we provide evidence that the Leishmania genome is continuously being rearranged at the level of these repeated sequences, which serve as a functional platform for constitutive and stochastic amplification (and deletion) of genomic segments in the population. This process is adaptive as the copy number of advantageous extrachromosomal circular or linear elements increases upon selective pressure and is reversible when selection is removed. We also provide mechanistic insights on the formation of circular and linear amplicons through RAD51 recombinase-dependent and -independent mechanisms, respectively. The whole genome of Leishmania is thus stochastically rearranged at the level of repeated sequences, and the selection of parasite subpopulations with changes in the copy number of specific loci is used as a strategy to respond to a changing environment.

Variations in the copy number of DNA segments account for a substantial amount of genome diversity of most organisms. DNA amplification, a contributor to copy number variation, can occur in response to various stresses or after altered growth conditions, leading to extensive and often reversible genetic variation. DNA amplification in the parasite Leishmania occurs outside the normal chromosomes and arises by DNA rearrangements involving homologous repeated sequences. We show here that such repeated sequences are widespread in the Leishmania genome and that most of the Leishmania genome is subject to stochastic gene rearrangements mediated by these low-copy repeat sequences. Thus, although cells in the population have a common core genome, many individual cells will differ from the rest of the population by carrying one or more distinct extrachromosomal amplicon. Upon selection with either drugs or culture conditions, a subpopulation can emerge where the amplicon copy number per cell increases, and this clone of cells can then expand to dominate the population. We propose that Leishmania uses adaptive gene amplification at a genome-wide scale as one strategy to adapt to a changing environment.

Proteomic and genomic analyses of antimony resistant Leishmania infantum mutant

BACKGROUND

Antimonials remain the primary antileishmanial drugs in most developing countries. However, drug resistance to these compounds is increasing and our understanding of resistance mechanisms is partial.

METHODS/PRINCIPAL FINDINGS

In the present study, quantitative proteomics using stable isotope labelling of amino acids in cell culture (SILAC) and genome next generation sequencing were used in order to better characterize in vitro generated Leishmania infantum antimony resistant mutant (Sb2000.1). Using the proteomic method, 58 proteins were found to be differentially regulated in Sb2000.1. The ABC transporter MRPA (ABCC3), a known marker of antimony resistance, was observed for the first time in a proteomic screen. Furthermore, transfection of its gene conferred antimony resistance in wild-type cells. Next generation sequencing revealed aneuploidy for 8 chromosomes in Sb2000.1. Moreover, specific amplified regions derived from chromosomes 17 and 23 were observed in Sb2000.1 and a single nucleotide polymorphism (SNP) was detected in a protein kinase (LinJ.33.1810-E629K).

CONCLUSION/SIGNIFICANCE

Our results suggest that differentially expressed proteins, chromosome number variations (CNVs), specific gene amplification and SNPs are important features of antimony resistance in Leishmania.

(Post-) Genomic approaches to tackle drug resistance in Leishmania

Leishmaniasis, like other neglected diseases is characterized by a small arsenal of drugs for its control. To safeguard the efficacy of current drugs and guide the development of new ones it is thus of utmost importance to acquire a deep understanding of the phenomenon of drug resistance and its link with treatment outcome. We discuss here how (post-)genomic approaches may contribute to this purpose. We highlight the need for a clear definition of the phenotypes under consideration: innate and acquired resistance versus treatment failure. We provide a recent update of our knowledge on the Leishmania genome structure and dynamics, and compare the contribution of targeted and untargeted methods for the understanding of drug resistance and show their limits. We also present the main assays allowing the experimental validation of the genes putatively involved in drug resistance. The importance of analysing information downstream of the genome is stressed and further illustrated by recent metabolomics findings. Finally, the attention is called onto the challenges for implementing the acquired knowledge to the benefit of the patients and the population at risk.

Telomeric gene deletion and intrachromosomal amplification in antimony-resistant Leishmania

Antimonials are still the mainstay of treatment against leishmaniasis but drug resistance is increasing. We carried out short read next-generation sequencing (NGS) and comparative genomic hybridization (CGH) of three independent Leishmania major antimony-resistant mutants. Copy number variations were consistently detected with both NGS and CGH. A major attribute of antimony resistance was a novel terminal deletion of variable length (67 kb to 204 kb) of the polyploid chromosome 31 in the three mutants. Terminal deletions in two mutants occurred at the level of inverted repeated sequences. The AQP1 gene coding for an aquaglyceroporin was part of the deleted region and its transfection into resistant mutants reverted resistance to SbIII. We also highlighted an intrachromosomal amplification of a subtelomeric locus on chromosome 34 in one mutant. This region encoded for ascorbate-dependent peroxidase (APX) and glucose-6-phosphate dehydrogenase (G6PDH). Overexpression of these genes in revertant backgrounds demonstrated resistance to SbIII and protection from reactive oxygen species (ROS). Generation of a G6PDH null mutant in one revertant exhibited SbIII sensitivity and a decreased protection of ROS. Our genomic analyses and functional validation highlighted novel genomic rearrangements, functionally important resistant loci and the implication of new genes in antimony resistance in Leishmania.

Generation of Leishmania hybrids by whole genomic DNA transformation

Genetic exchange is a powerful tool to study gene function in microorganisms. Here, we tested the feasibility of generating Leishmania hybrids by electroporating genomic DNA of donor cells into recipient Leishmania parasites. The donor DNA was marked with a drug resistance marker facilitating the selection of DNA transfer into the recipient cells. The transferred DNA was integrated exclusively at homologous locus and was as large as 45 kb. The independent generation of L. infantum hybrids with L. major sequences was possible for several chromosomal regions. Interfering with the mismatch repair machinery by inactivating the MSH2 gene enabled an increased efficiency of recombination between divergent sequences, hence favouring the selection of hybrids between species. Hybrids were shown to acquire the phenotype derived from the donor cells, as demonstrated for the transfer of drug resistance genes from L. major into L. infantum. The described method is a first step allowing the generation of in vitro hybrids for testing gene functions in a natural genomic context in the parasite Leishmania.

Genetic polymorphisms and drug susceptibility in four isolates of Leishmania tropica obtained from Canadian soldiers returning from Afghanistan

BACKGROUND

Cutaneous leishmaniasis (CL) is a vector-borne parasitic disease characterized by the presence of one or more lesions on the skin that usually heal spontaneously after a few months. Most cases of CL worldwide occur in Southwest Asia, Africa and South America, and a number of cases have been reported among troops deployed to Afghanistan. No vaccines are available against this disease, and its treatment relies on chemotherapy. The aim of this study was to characterize parasites isolated from Canadian soldiers at the molecular level and to determine their susceptibility profile against a panel of antileishmanials to identify appropriate therapies.

METHODOLOGY/PRINCIPAL FINDINGS

Parasites were isolated from skin lesions and characterized as Leishmania tropica based on their pulsed field gel electrophoresis profiles and pteridine reductase 1 (PTR1) sequences. Unusually high allelic polymorphisms were observed at several genetic loci for the L. tropica isolates that were characterized. The drug susceptibility profile of intracellular amastigote parasites was determined using an established macrophage assay. All isolates were sensitive to miltefosine, amphotericin B, sodium stibogluconate (Pentostam) and paromomycin, but were not susceptible to fluconazole. Variable levels of susceptibility were observed for the antimalarial agent atovaquone/proguanil (Malarone). Three Canadian soldiers from this study were successfully treated with miltefosine.

CONCLUSIONS/SIGNIFICANCE

This study shows high heterogeneity between the two L. tropica allelic versions of a gene but despite this, L. tropica isolated from Afghanistan are susceptible to several of the antileishmanial drugs available.

Intrachromosomal tandem duplication and repeat expansion during attempts to inactivate the subtelomeric essential gene GSH1 in Leishmania

Gamma-glutamylcysteine synthetase encoded by GSH1 is the rate-limiting enzyme in the biosynthesis of glutathione and trypanothione in Leishmania. Attempts to generate GSH1 null mutants by gene disruption failed in Leishmania infantum. Removal of even a single allele invariably led to the generation of an extra copy of GSH1, maintaining two intact wild-type alleles. In the second and even third round of inactivation, the markers integrated at the homologous locus but always preserved two intact copies of GSH1. We probed into the mechanism of GSH1 duplication. GSH1 is subtelomeric on chromosome 18 and Southern blot analysis indicated that a 10-kb fragment flanked by 466-bp direct repeated sequences was duplicated in tandem on the same chromosomal allele each time GSH1 was targeted. Polymerase chain reaction analysis and sequencing confirmed the generation of novel junctions created at the level of the 466-bp repeats consequent to locus duplication. In loss of heterozygosity attempts, the same repeated sequences were utilized for generating extrachromosomal circular amplicons. Our results are consistent with break-induced replication as a mechanism for the generation of this regional polyploidy to compensate for the inactivation of an essential gene. This chromosomal repeat expansion through repeated sequences could be implicated in locus duplication in Leishmania.

Artificial linear episome-based protein expression system for protozoon Leishmania tarentolae

The trypanosomatid protozoon Leishmania tarentolae is a well-established model organism for studying causative agents of several tropical diseases that was more recently developed as a host for recombinant protein production. Although several expression architectures based on foreign RNA polymerases have been established for this organism, all of them rely on integration of the expression cassette into the genome. Here, we exploit a new type of expression architecture based on linear elements. These expression vectors were propagated in Escherichia coli as circular plasmids and converted into linear episomes with telomere-like structures prior to transfection of L. tarentolae. Overexpression of recombinant proteins in transgenic organisms exceeding 10% of total cellular protein, one of the highest overexpression levels obtained in a eukaryotic organism for a cytosolic protein. We show that the linear elements are stably propagated in L. tarentolae cells over long periods of time (> 90 generations) without major changes in structure or expression yields. Overexpressing cultures can be obtained without clonal selection of the transfected cells. To establish the utility of the developed system for protein production in a parallelized format, we expressed 37 cytosolic, peripheral, and membrane proteins as fusions with EGFP in L. tarentolae using linear vectors. We detected the expression of 30 of these targets and describe the preparative purification of two arbitrarily selected proteins.

Gene expression modulation is associated with gene amplification, supernumerary chromosomes and chromosome loss in antimony-resistant Leishmania infantum

Antimonials remain the first line drug against the protozoan parasite Leishmania but their efficacy is threatened by resistance. We carried out a RNA expression profiling analysis comparing an antimony-sensitive and -resistant (Sb2000.1) strain of Leishmania infantum using whole-genome 70-mer oligonucleotide microarrays. Several genes were differentially expressed between the two strains, several of which were found to be physically linked in the genome. MRPA, an ATP-binding cassette (ABC) gene known to be involved in antimony resistance, was overexpressed in the antimony-resistant mutant along with three other tandemly linked genes on chromosome 23. This four gene locus was flanked by 1.4 kb repeated sequences from which an extrachromosomal circular amplicon was generated in the resistant cells. Interestingly, gene expression modulation of entire chromosomes occurred in the antimony-resistant mutant. Southern blots analyses and comparative genomic hybridizations revealed that this was either due to the presence of supernumerary chromosomes or to the loss of one chromosome. Leishmania parasites with haploid chromosomes were viable. Changes in copy number for some of these chromosomes were confirmed in another antimony-resistant strain. Selection of a partial revertant line correlated antimomy resistance levels and the copy number of aneuploid chromosomes, suggesting a putative link between aneuploidy and drug resistance in Leishmania.

Modulation of gene expression in drug resistant Leishmania is associated with gene amplification, gene deletion and chromosome aneuploidy

Gene expression and DNA copy number analyses using full genome oligonucleotide microarrays of Leishmania reveal molecular mechanisms of methotrexate resistance.

Background

Drug resistance can be complex, and several mutations responsible for it can co-exist in a resistant cell. Transcriptional profiling is ideally suited for studying complex resistance genotypes and has the potential to lead to novel discoveries. We generated full genome 70-mer oligonucleotide microarrays for all protein coding genes of the human protozoan parasites Leishmania major and Leishmania infantum. These arrays were used to monitor gene expression in methotrexate resistant parasites.

Results

Leishmania is a eukaryotic organism with minimal control at the level of transcription initiation and few genes were differentially expressed without concomitant changes in DNA copy number. One exception was found in Leishmania major, where the expression of whole chromosomes was down-regulated. The microarrays highlighted several mechanisms by which the copy number of genes involved in resistance was altered; these include gene deletion, formation of extrachromosomal circular or linear amplicons, and the presence of supernumerary chromosomes. In the case of gene deletion or gene amplification, the rearrangements have occurred at the sites of repeated (direct or inverted) sequences. These repeats appear highly conserved in both species to facilitate the amplification of key genes during environmental changes. When direct or inverted repeats are absent in the vicinity of a gene conferring a selective advantage, Leishmania will resort to supernumerary chromosomes to increase the levels of a gene product.

Conclusion

Aneuploidy has been suggested as an important cause of drug resistance in several organisms and additional studies should reveal the potential importance of this phenomenon in drug resistance in Leishmania.

Characterization of Leishmania (Leishmania) amazonensis promastigotes resistant to pentamidine

Pentamidine is a second-line agent used in the treatment of leishmaniasis and its mode of action and mechanism of resistance is not well understood. It was previously demonstrated that transfection of promastigotes and amastigotes with the ABC transporter PRP1 gene confers resistance to pentamidine. To further clarify this point, we generated Leishmania amazonensis mutants resistant to pentamidine. Our results indicated that this ABC transporter is not associated with pentamidine resistance in lines generated by drug pressure through amplification or overexpression mechanisms of PRP1 gene.

Genome position and gene amplification

Genomic analyses of human cells expressing dihydrofolate reductase provide insight into the effects of genome position on the propensity for a drug-resistance gene to amplify in human cells.

Background

Amplifications, regions of focal high-level copy number change, lead to overexpression of oncogenes or drug resistance genes in tumors. Their presence is often associated with poor prognosis; however, the use of amplification as a mechanism for overexpression of a particular gene in tumors varies. To investigate the influence of genome position on propensity to amplify, we integrated a mutant form of the gene encoding dihydrofolate reductase into different positions in the human genome, challenged cells with methotrexate and then studied the genomic alterations arising in drug resistant cells.

Results

We observed site-specific differences in methotrexate sensitivity, amplicon organization and amplification frequency. One site was uniquely associated with a significantly enhanced propensity to amplify and recurrent amplicon boundaries, possibly implicating a rare folate-sensitive fragile site in initiating amplification. Hierarchical clustering of gene expression patterns and subsequent gene enrichment analysis revealed two clusters differing significantly in expression of MYC target genes independent of integration site.

Conclusion

These studies suggest that genome context together with the particular challenges to genome stability experienced during the progression to cancer contribute to the propensity to amplify a specific oncogene or drug resistance gene, whereas the overall functional response to drug (or other) challenge may be independent of the genomic location of an oncogene.

A novel ATP-binding cassette transporter, ABCG6 is involved in chemoresistance of Leishmania

ATP-binding cassette (ABC) transporters constitute the biggest family of membrane proteins involved in drug resistance and other biological activities. Resistance of leishmanial parasites to therapeutic drugs continues to escalate in developing countries and in many instances it is due to overexpressed ABC efflux pumps. Progressively adapted camptothecin (CPT)-resistant parasites show overexpression of a novel ABC transporter, which was classified as ABCG6. Transfection and overexpression of LdABCG6 in wild type parasites, shows its localization primarily in the plasma membrane and flagellar pocket region. Overexpressed LdABCG6 confers substantial CPT resistance to the parasites by rapid drug efflux. Various inhibitors have been tested for their ability to revert the CPT-resistant phenotype to specifically understand the inhibition of LdABCG6 transporter. Transport experiments using everted membrane vesicles were carried out to gain an insight into the kinetics of drug transport. This study provides further knowledge of specific membrane traffic ATPase and its involvement in the chemoresistance of Leishmania.

Organization of H locus conserved repeats in Leishmania (Viannia) braziliensis correlates with lack of gene amplification and drug resistance

Resistance to antimonials is a major problem when treating visceral leishmaniasis in India and has already been described for New World parasites. Clinical response to meglumine antimoniate in patients infected with parasites of the Viannia sub-genus can be widely variable, suggesting the presence of mechanisms of drug resistance. In this work, we have compared L. major and L. braziliensis mutants selected in different drugs. The cross-resistance profiles of some cell lines resembled those of mutants bearing H locus amplicons. However, amplified episomal molecules were exclusively detected in L. major mutants. The analysis of the L. braziliensis H region revealed a strong conservation of gene synteny. The typical intergenic repeats that are believed to mediate the amplification of the H locus in species of the Leishmania sub-genus are partially conserved in the Viannia species. The conservation of these non-coding elements in equivalent positions in both species is indicative of their relevance within this locus. The absence of amplicons in L. braziliensis suggests that this species may not favour extra-chromosomal gene amplification as a source of phenotypic heterogeneity and fitness maintenance in changing environments.

A proteomics screen implicates HSP83 and a small kinetoplastid calpain-related protein in drug resistance in Leishmania donovani clinical field isolates by modulating drug-induced programmed cell death

The therapeutic mainstay against the protozoan parasite Leishmania is still based on the antiquated pentavalent antimonials (Sb(V)), but resistance is increasing in several parts of the world. Resistance is now partly understood in laboratory isolates, but our understanding of resistance in field isolates is lagging behind. We describe here a comparative analysis of a genetically related pair of Sb(V)-sensitive and -resistant Leishmania donovani strains isolated from kala-azar patients. The resistant isolate exhibited cross-resistance to other unrelated Leishmania drugs including miltefosine and amphotericin B. A comparative proteomics screen has highlighted a number of proteins differentially expressed suggesting that programmed cell death (PCD) is modified in the resistant parasite. Indeed drug-induced PCD progression was altered in the Sb(V)-resistant strain as determined using early and late markers of apoptosis. Two proteins, the heat shock protein HSP83 and the small kinetoplastid calpain-related protein (SKCRP14.1) were shown to be intimately implicated in the drug-induced PCD phenotype. HSP83 increased drug resistance and reduced drug-mediated PCD activation by interfering with the mitochondrial membrane potential, whereas SKCRP14.1 promoted antimonial-induced PCD but protected against miltefosine-induced PCD. This study highlights the important role of PCD in drug susceptibility/resistance in the protozoan parasite Leishmania.

Unresponsiveness to Glucantime treatment in Iranian cutaneous leishmaniasis due to drug-resistant Leishmania tropica parasites

BACKGROUND

Recent circumstantial evidence suggests that an increasing number of Iranian patients with cutaneous leishmaniasis are unresponsive to meglumine antimoniate (Glucantime), the first line of treatment in Iran. This study was designed to determine whether the clinical responses (healing, or non-healing) were correlated with the susceptibility of Leishmania parasites to Glucantime.

METHODS AND FINDINGS

In vitro susceptibility testing was first performed on 185 isolated parasites in the intracellular mouse peritoneal macrophage model. A strong correlation between the clinical outcome and the in vitro effective concentration 50% (EC50) values was observed. Parasites derived from patients with non-healing lesions had EC50 values at least 4-fold higher than parasites derived from lesions of healing patients. A selection of these strains was typed at the molecular level by pulsed-field gels and by sequencing the pteridine reductase 1 (PTR1) gene. These techniques indicated that 28 out of 31 selected strains were Leishmania tropica and that three were Leishmania major. The L. major isolates were part of a distinct pulsed-field group, and the L. tropica isolates could be classified in three related additional pulsed-field groups. For each pulsed-field karyotype, we selected sensitive and resistant parasites in which we transfected the firefly luciferase marker to assess further the in vitro susceptibility of field isolates in the monocyte cell line THP1. These determinations confirmed unequivocally that patients with non-healing lesions were infected with L. tropica parasites resistant to Glucantime. Additional characterization of the resistant isolates showed that resistance is stable and can be reversed by buthionine sulfoximine, an inhibitor of glutathione biosynthesis.

CONCLUSIONS

To the authors' knowledge, this is the first report of proven resistant parasites contributing to treatment failure for cutaneous leishmaniasis and shows that primary Glucantime-resistant L. tropica field isolates are now frequent in Iran.

LD1 amplifications in Leishmania

The genome of Leishmania is quite plastic. Chromosomal rearrangements and DNA amplifications are common events in all the species of the genus. Gene amplification occurs both as a mechanism of drug resistance and in the absence of drug pressure. The best known spontaneous amplification in Leishmania is the so-called LD1 family of amplicons. In the past few years there have been great advances in our knowledge of LD1 elements; here, Manuel Segovia and Ginés Ortiz review all the available data.

Formation of linear inverted repeat amplicons following targeting of an essential gene in Leishmania

Attempts to inactivate an essential gene in the protozoan parasite Leishmania have often led to the generation of extra copies of the wild-type alleles of the gene. In experiments with Leishmania tarentolae set up to disrupt the gene encoding the J-binding protein 1 (JBP1), a protein binding to the unusual base beta-D-glucosyl-hydroxymethyluracil (J) of Leishmania, we obtained JBP1 mutants containing linear DNA elements (amplicons) of approximately 100 kb. These amplicons consist of a long inverted repeat with telomeric repeats at both ends and contain either the two different targeting cassettes used to inactivate JBP1, or one cassette and one JBP1 gene. Each long repeat within the linear amplicons corresponds to sequences covering the JBP1 locus, starting at the telomeres upstream of JBP1 and ending in a approximately 220 bp sequence repeated in an inverted (palindromic) orientation downstream of the JBP1 locus. We propose that these amplicons have arisen by a template switch inside a DNA replication fork involving the inverted DNA repeats and helped by the gene targeting.

The ribosomal RNA gene promoter and adjacent cis-acting DNA sequences govern plasmid DNA partitioning and stable inheritance in the parasitic protozoan Leishmania

Detailed analysis of the Leishmania donovani ribosomal RNA (rRNA) gene promoter region has allowed the identification of cis-acting sequences involved in plasmid DNA partitioning and stable plasmid inheritance. We report that plasmids bearing the 350 bp rRNA promoter along with the 200 bp region immediately 3' to the promoter exhibited a 6.5-fold increase in transformation frequency and were transmitted to daughter cells as single-copy molecules. This is in contrast to what has been observed for plasmid molecules in this organism so far. Moreover, we show that these low-copy-number plasmids displayed a remarkable mitotic stability in the absence of selective pressure. The region in the vicinity of the RNA pol I transcription initiation site, and also in the adjacent 200 nt, displays a complex structural organization and shares sequence similarity to the yeast autonomously replicating consensus sequence and centromere DNA elements. Deletion analyses indicated that these elements were necessary but not sufficient for plasmid DNA partitioning and stable inheritance, and that the rRNA promoter region was required for optimal function. These results suggest an interplay between RNA pol I transcription, DNA replication, DNA partitioning and mitotic stability in trypanosomatids. This is the first example of defined DNA elements for plasmid partitioning and stable inheritance in the protozoan parasite Leishmania.

The heat shock protein HSP70 and heat shock cognate protein HSC70 contribute to antimony tolerance in the protozoan parasite leishmania

Antimony-containing drugs are still the drugs of choice in the treatment of infections caused by the parasite Leishmania. Resistance to antimony is now common in some parts of the world, and several mechanisms of resistance have been described. By transfecting cosmid banks and selecting with potassium antimonyl tartrate (SbIII), we have isolated a cosmid associated with resistance. This cosmid contains 2 copies of the heat shock protein 70 (HSP70) and 1 copy of the heat shock cognate protein 70 (HSC70). Several data linked HSP70 to antimony response and resistance. First, several Leishmania species, both as promastigotes and amastigotes, increased the expression of their HSP70 proteins when grown in the presence of 1 or 2 times the Effect Concentration 50% of SbIII. In several mutants selected for resistance to either SbIII or to the related metal arsenite, the HSP70 proteins were found to be overexpressed. This increase was also observed in revertant cells grown for several passages in the absence of SbIII, suggesting that this increased production of HSP70 is stable. Transfection of HSP70 or HSC70 in Leishmania cells does not confer resistance directly, though these transfectants were better able to tolerate a shock with SbIII. Our results are consistent with HSP70 and HSC70 being a first line of defense against SbIII until more specific and efficient resistance mechanisms take over.

A new type of high affinity folic acid transporter in the protozoan parasite Leishmania and deletion of its gene in methotrexate-resistant cells

The protozoan parasite Leishmania is a folate auxotroph and thus depends on the uptake of folate from the environment to meet its folate requirement. We show here that Leishmania contains several putative pteridine transporter genes. Some of these genes are deleted in methotrexate-resistant Leishmania cells where there is no measurable uptake of methotrexate. Transport studies suggest that Leishmania has more than one active folate transporter, and one of these, named FT5, corresponds to a very high affinity folate transporter (K(m) 84 nm). The uptake of both folate and methotrexate was impaired in an FT5 null mutant at low substrate concentrations (50 nm), although transport properties at higher concentrations (1000 nm) were not statistically different between wild-type and the FT5 null mutant. Modulation of the expression of FT5 also changes the susceptibility of Leishmania cells to methotrexate. These results have permitted the characterization of a novel class of folate transporters and suggest that the parasite Leishmania has several gene products possibly transporting folates and related molecules under varying conditions.

Polymorphism in the subtelomeric regions of chromosomes of Kinetoplastida

Leishmania spp. and the related kinetoplastid Trypanosoma brucei are single-celled parasites. In Leishmania, the nuclear genome comprises 36 diploid chromosomes and occasional amplified minichromosomes, while the T. brucei nucleus contains 11 larger diploid chromosomes and a variable number of intermediate-sized and minichromosomes. This paper primarily describes the subtelomeric structure of the larger diploid chromosomes of L. major and T. brucei, although some aspects may also apply to smaller chromosomes. The diploid chromosomes contain most protein-coding genes and vary in size. The telomeric sequence is common to both species, but adjacent subtelomeric repeats vary between species and chromosomes. It is possible that some of the complex repeats described here play a role in stabilizing replication and copy number of the chromosomes. The subtelomeric regions of T. brucei chromosomes differ from those of other protozoan parasites, as they are dedicated to expression sites for variant surface glycoprotein genes, used by the parasite to evade immune destruction by antigenic variation. Variation in these sites creates segmental aneuploidy in many T. brucei chromosomes.

Gene amplification in amphotericin B-resistant Leishmania tarentolae

Two Leishmania tarentolae cells were selected step by step for resistance to the polyene antibiotic amphotericin B, a second-line drug against the parasite Leishmania. One of the mutants was cross-resistant to ketoconazole. DNA amplification was observed in both mutants. The amplicons were extrachromosomal circles and were derived from different chromosomes. In one mutant the circle was unusually stable as it remained within the cell despite numerous passages in the absence of the drug. A circumstantial link between the copy number of amplicons and the resistance levels was established. Gene transfection experiments indicated that the link between the locus amplified and the resistance levels was not straightforward and possibly several mutations act together to lead to amphotericin B resistance.

The Leishmania ATP-binding cassette protein PGPA is an intracellular metal-thiol transporter ATPase

The Leishmania ATP-binding cassette (ABC) transporter PGPA is involved in metal resistance (arsenicals and antimony), although the exact mechanism by which PGPA confers resistance to antimony, the first line drug against Leishmania, is unknown. The results of co-transfection experiments, transport assays, and the use of inhibitors suggest that PGPA recognizes metals conjugated to glutathione or trypanothione, a glutathione-spermidine conjugate present in Leishmania. The HA epitope tag of the influenza hemagglutinin as well as the green fluorescent protein were fused at the COOH terminus of PGPA. Immunofluorescence, confocal, and electron microscopy studies of the fully functional tagged molecules clearly indicated that PGPA is localized in membranes that are close to the flagellar pocket, the site of endocytosis and exocytosis in this parasite. Subcellular fractionation of Leishmania tarentolae PGPAHA transfectants was performed to further characterize this ABC transporter. The basal PGPA ATPase activity was determined to be 115 nmol/mg/min. Transport experiments using radioactive arsenite-glutathione conjugates clearly showed that PGPA recognizes and actively transports thiol-metal conjugates. Overall, the results are consistent with PGPA being an intracellular ABC transporter that confers arsenite and antimonite resistance by sequestration of the metal-thiol conjugates.

The size difference between leishmania major friedlin chromosome one homologues is localized to sub-telomeric repeats at one chromosomal end

Leishmania species are members of the evolutionarily ancient protozoan order Kinetoplastidae and are important human pathogens. The Leishmania genome is relatively small (approximately 34 Mbp) and is distributed among 36 chromosome pairs, ranging in size from 0.3 to 2.5 Mbp. The smallest chromosome of Leishmania major Friedlin, chrl, consists of three homologues which differ in size by approximately 29 kb. Previous sequence and Southern analyses of all three homologues reveal a conserved chromosomal core, consisting of coding and adjacent 'non-informational' sequence. Here we show the size difference between homologues is largely restricted to variation in both the number and content of several sub-telomeric repetitive elements localized on one chromosomal end. These repetitive elements also occur on other chromosomes, but some are more dispersed in the Leishmania genome than others.

Mechanisms of sod2 gene amplification in Schizosaccharomyces pombe

Gene amplification in eukaryotes plays an important role in drug resistance, tumorigenesis, and evolution. The Schizosaccharomyces pombe sod2 gene provides a useful model system to analyze this process. sod2 is near the telomere of chromosome I and encodes a plasma membrane Na(+)(Li(+))/H(+) antiporter. When sod2 is amplified, S. pombe survives otherwise lethal concentrations of LiCl, and >90% of the amplified sod2 genes are found in 180- and 225-kilobase (kb) linear amplicons. The sequence of the novel joint of the 180-kb amplicon indicates that it is formed by recombination between homologous regions near the telomeres of the long arm of chromosome I and the short arm of chromosome II. The 225-kb amplicon, isolated three times more frequently than the 180-kb amplicon, is a palindrome derived from a region near the telomere of chromosome I. The center of symmetry of this palindrome contains an inverted repeat consisting of two identical 134-base pair sequences separated by a 290-base pair spacer. LiCl-resistant mutants arise 200-600 times more frequently in strains deficient for topoisomerases or DNA ligase activity than in wild-type strains, but the mutant cells contain the same amplicons. These data suggest that amplicon formation may begin with DNA lesions such as breaks. In the case of the 225-kb amplicon, the breaks may lead to a hairpin structure, which is then replicated to form a double-stranded linear amplicon, or to a cruciform structure, which is then resolved to yield the same amplicon.

Elevated levels of polyamines and trypanothione resulting from overexpression of the ornithine decarboxylase gene in arsenite-resistant Leishmania

The levels of trypanothione, a glutathione-spermidine conjugate, are increased in the protozoan parasite Leishmania selected for resistance to the heavy metal arsenite. The levels of putrescine and spermidine were increased in resistant mutants. This increase is mediated by overexpression of ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis. Gene overexpression is generally mediated by gene amplification in Leishmania but, here, the mRNA and the enzymatic activity of ODC are increased without gene amplification. This RNA overexpression is stable when cells are grown in the absence of the drug and does not result from gene rearrangements or from an increased rate of RNA synthesis. Transient transfections suggest that mutations in the revertant cells contribute to these elevated levels of RNA. Stable transfection of the ODC gene increases the level of trypanothione, which can contribute to arsenite resistance. In addition to ODC overexpression, the gene for the ABC transporter PGPA is amplified in the mutants. The co-transfection of the ODC and PGPA genes confers resistance in a synergistic fashion in partial revertants, also suggesting that PGPA recognizes metals conjugated to trypanothione.

Role of the locus and of the resistance gene on gene amplification frequency in methotrexate resistant Leishmania tarentolae

The protozoan parasite Leishmania resists the antifolate methotrexate (MTX) by amplifying the R locus dihydrofolate reductase-thymidylate synthase ( dhfr-ts ) gene, the H locus ptr1 pterin reductase gene, and finally by mutation in a common folate/MTX transporter. Amplification of dhfr-ts has never been observed in Leishmania tarentolae MTX resistant mutants while ptr1 amplification is common. We have selected a L.tarentolae ptr1 null mutant for MTX resistance and observed dhfr-ts amplification in this mutant demonstrating that once a preferred resistance mechanism is unavailable, a second one will take over. By introducing the ptr1 gene at the R locus and the dhfr-ts gene at the H locus by gene targeting, we investigated the role of the resistance gene and the locus on the rate of gene amplification. Transfection studies indicated that ptr1 gave higher levels of MTX resistance than dhfr-ts. Consistent with this, when ptr1 was present as part of either the H locus or the R locus it was invariably amplified, while dhfr-ts was only amplified when ptr1 was inactivated. When dhfr-ts was present in a ptr1 null background on both the H locus and the R locus, amplification from the H locus was more frequent suggesting that both the gene and the locus are determining the frequency of gene amplification in Leishmania.

Linear amplicons as precursors of amplified circles in methotrexate-resistant Leishmania tarentolae

Gene amplification is frequently observed in Leishmania cells selected for drug resistance. By gene targeting we have tagged both alleles of the H locus of Leishmania tarentolae with the neomycin and hygromycin phosphotransferase genes ( neo and hyg ). Selection of these recombinant parasites for low level methotrexate resistance led to amplification of the H locus as part of linear amplicons. The availability of tags has permitted us to determine that both alleles can be amplified in the same cell and that chromosomal deletions are frequent. When methotrexate concentration was increased in subsequent selection steps, circles were observed in several mutants. We have introduced a hyg marker into linear amplicons to test whether the circles originated from linear amplicons. After selection with a high methotrexate concentration, circles with the hyg marker were observed, showing that circles can indeed be formed from linear amplicons. The tagging of H locus alleles permits appreciation of the extent of genetic rearrangements leading to amplicon formation in Leishmania cells selected for drug resistance.

Gene amplification in Leishmania tarentolae selected for resistance to sodium stibogluconate

Leishmania tarentolae promastigotes were selected step by step for resistance to sodium stibogluconate (Pentostam). Mutants resistant to antimony-containing drugs and cross-resistant to arsenite were therefore obtained. Amplification of one common locus was observed in several independent sodium stibogluconate-resistant mutants, and the locus amplified was novel. The copy number of the amplified locus was related to the level of resistance to pentavalent antimony. The gene responsible for antimony resistance was isolated by transfection and was shown to correspond to an open reading frame coding for 770 amino acids. The putative gene product did not exhibit significant homology with sequences present in data banks, and the putative role of this protein in antimony resistance is discussed.

Characterisation of Leishmania telomeres reveals unusual telomeric repeats and conserved telomere-associated sequence

Characterisation of the telomeres of Leishmania is important for understanding many aspects of the parasitic life of this primitive protozoan and for the completion of the physical map and sequencing of the genome. After sequencing more than 300 telomere-derived clones from Leishmania braziliensis and Leishmania major, a conserved 100 bp sequence was identified immediately adjacent to the telomere at the chromosome end and was named LCTAS (Leishmania conserved telomere-associated sequence). The LCTAS contains two conserved sequence boxes, and is present in all Leishmania species studied. The organisation of the LCTAS in the telomeric region differs between L. braziliensis and L. major: in L. major the LCTASs are tandemly repeated, while in L. braziliensis the LCTAS is present as a single copy per end. Two additional TASs with 1.6 kb and 274 bp repeat structures, which are apparently different to LCTAS, were isolated and mapped onto a L. braziliensis 250 kb multicopy minichromosome and the L. major chromosome 1, respectively. An unusual feature in L. braziliensis is that the telomeric repeats are often comprised of a novel tandem repeat CCCTAACCCGTGGA. A 'slippage' mechanism for LCTAS formation is proposed in this study as an alternative way for the synthesis and maintenance of telomeres and subtelomere regions.

ABC transporters in Leishmania and their role in drug resistance

ABC transporters have been found in several parasitic protozoa including Leishmania. At least two Leishmania ABC transporters are involved in drug resistance. One is PgpA, which is involved in resistance to arsenic and antimony-containing compounds. Antimonials are the drug of choice against Leishmania infections. Transfection and biochemical studies suggest that PgpA recognizes metals conjugated to thiols. The second ABC transporter is closely related to mammalian P-glycoproteins and confers resistance to anticancer drugs by a mechanism that remains to be elucidated. Additional ABC transporters are likely to be present in Leishmania and these are discussed in relation to the phenomenon of antimony resistance.