Analysis of genetic elements regulating the methionine adenosyltransferase gene in Leishmania infantum

Resistance of Leishmania infantum to allopurinol is associated with chromosome and gene copy number variations including decrease in the S-adenosylmethionine synthetase (METK) gene copy number

Leishmania infantum is one of the causative agents of visceral leishmaniasis (VL), a widespread, life-threatening disease. This parasite is responsible for the majority of human VL cases in Brazil, the Middle East, China, Central Asia and the Mediterranean basin. Its main reservoir are domestic dogs which, similar to human patients, may develop severe visceral disease and die if not treated. The drug allopurinol is used for the long-term maintenance of dogs with canine leishmaniasis. Following our report of allopurinol resistance in treated relapsed dogs, we investigated the mechanisms and markers of resistance to this drug. Whole genome sequencing (WGS) of clinical resistant and susceptible strains, and laboratory induced resistant parasites, was carried out in order to detect genetic changes associated with resistance. Significant gene copy number variation (CNV) was found between resistant and susceptible isolates at several loci, including a locus on chromosome 30 containing the genes LinJ.30.3550 through LinJ.30.3580. A reduction in copy number for LinJ.30.3560, encoding the S-adenosylmethionine synthetase (METK) gene, was found in two resistant clinical isolates and four induced resistant clonal strains. Using quantitative real time PCR, this reduction in METK copy number was also found in three additional resistant clinical isolates. Furthermore, inhibition of S-adenosylmethionine synthetase encoded by the METK gene in allopurinol susceptible strains resulted in increased allopurinol resistance, confirming its role in resistance to allopurinol. In conclusion, this study identified genetic changes associated with L. infantum resistance to allopurinol and the reduction in METK copy number identified may serve as a marker for resistance in dogs, and reduced protein activity correlated with increased allopurinol resistance.

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Allopurinol resistance was previously described in L. infantum isolated from dogs.

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This study aimed at defining the genetic differences between susceptible and resistant strains.

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Gene and chromosome copy numbers differed between susceptible and resistant L. infantum strains.

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Decrease in METK gene copies was associated with increased allopurinol resistance.

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Inhibition of the enzyme encoded by METK increased allopurinol resistance.

Leishmania donovani: proteasome-mediated down-regulation of methionine adenosyltransferase

Methionine adenosyltransferase (MAT) is an important enzyme for metabolic processes, to the extent that its product, S-adenosylmethionine (AdoMet), plays a key role intrans-methylation,trans-sulphuration and polyamine synthesis. Previous studies have shown that a MAT-overexpressing strain ofLeishmania donovanicontrols AdoMet production, keeping the intracellular AdoMet concentration at levels that are compatible with cell survival. This unexpected result, together with the fact that MAT activity and abundance changed with time in culture, suggests that different regulatory mechanisms acting beyond the post-transcriptional level are controlling this protein. In order to gain an insight into these mechanisms, several experiments were carried out to explain the MAT abundance during promastigote cell growth. Determination of MAT turnover in cycloheximide (CHX)-treated cultures resulted in a surprising 5-fold increase in MAT turnover compared to CHX-untreated cultures. This increase agrees with a stabilization of the MAT protein, whose integrity was maintained during culture. The presence of proteasome inhibitors, namely MG-132, MG-115, epoxomycin and lactacystin in the culture medium prevented MAT degradation in both MAT-overexpressing and ‘mock-transfected’ leishmanial strains. The role of the ubiquitin (Ub) pathway in MAT down-regulation was supported using immunoprecipitation experiments. Immunoprecipitated MAT cross-reacted with anti-Ub antibodies, which provides evidence of a proteasome-mediated down-regulation of the leishmanial MAT abundance.

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.

Characterization of the 5' region of the Leishmania infantum LORIEN/MAT2 gene cluster and role of LORIEN flanking regions in post-transcriptional regulation

LORIEN (encoding a protein that contains a SP-RING/Miz zinc-finger motif present in a group of proteins involved in the Small Ubiquitin-related Modifier -SUMO- conjugation pathway) and MAT2 (encoding the methionine adenosyltransferase -MAT-) genes are arranged as two alternating copies in a head-to-tail configuration, with the LORIEN gene as the first copy of the cluster. The 5880bp preceding the first LORIEN gene copy were compared to the same region of L. major, showing a 93% identity between them. Bioinformatic analysis of this region predicted the presence of a 747-bp ORF encoding a hypothetical protein of 248 amino acids. Transcription of this ORF was confirmed by run-on assays and RT-PCR. Expression of the LORIEN gene was tested in both the promastigote and amastigote stages. Transcription arrest evidenced that LORIEN mRNA stability was very similar in both stages of the parasite life cycle. Protein synthesis inhibition by cycloheximide led to an increase in the steady-state levels of LORIEN transcripts only during the promastigote stage, pointing out to the existence of different stage-dependent mechanisms operating on the post-transcriptional regulation of this gene. The role of the LORIEN untranslated regions (5'UTR and 3'UTR) in post-transcriptional regulation was analysed using the luciferase (luc) reporter gene. Results evidenced that the 5'UTR was responsible for a low reporter gene expression, whereas the intergenic region (IR) between LORIEN and MAT2 genes provided high luc levels. However, the 3'UTR seemed to lack regulatory elements. Basing on these results, a model of regulation for the LORIEN gene is proposed.