Specificity of modified Drosophila mariner transposons in the identification of Leishmania genes

A transposon-based tool for transformation and mutagenesis in trypanosomatid protozoa

The ability of transposable elements to mobilize across genomes and affect the expression of genes makes them exceptional tools for genetic manipulation methodologies. Several transposon-based systems have been modified and incorporated into shuttle mutagenesis approaches in a variety of organisms. We have found that the Mos1 element, a DNA transposon from Drosophila mauritiana, is suitable and readily adaptable to a variety of strategies to the study of trypanosomatid parasitic protozoa. Trypanosomatids are the causative agents of a wide range of neglected diseases in underdeveloped regions of the globe. In this chapter we describe the basic elements and the available protocols for the in vitro use of Mos1 derivatives in the protozoan parasite Leishmania.

The genetic toolbox for Leishmania parasites

Leishmania parasites cause a variety of devastating diseases in tropical areas around the world. Due to the lack of vaccines and limited availability of drugs, new therapeutic targets are urgently needed. A variety of genetic tools have been developed to investigate the complex biology of this parasite and its interactions with the host. One of the main techniques is the generation of knock-out parasites via targeted gene replacement, a process that takes advantage of the parasites ability to undergo homologous recombination. Studying the effect of gene deletions in vitro and in infectivity models in vivo allows understanding the function of a target gene and its potential as a therapeutic target. Other genetic manipulations available include episomal and chromosomal complementation and the generation of overproducer strains. However, there are also limitations, such as the lack of RNA interference machinery in most Leishmania species and limited options for inducible expression systems. The genomes of several Leishmania species have now been sequenced and will provide powerful resources in combination with the genetic tools that are available. The increasing knowledge of parasite biology and host parasite interactions derived from these studies will raise the number of potential therapeutic targets, which are sorely needed to combat leishmaniasis.

A transposon toolkit for gene transfer and mutagenesis in protozoan parasites

Protozoan parasites affect millions of people around the world. Treatment and control of these diseases are complicated partly due to the intricate biology of these organisms. The interactions of species of Plasmodium, Leishmania and trypanosomes with their hosts are mediated by an unusual control of gene expression that is not fully understood. The availability of the genome sequence of these protozoa sets the stage for using more comprehensive, genome-wide strategies to study gene function. Transposons are effective tools for the systematic introduction of genetic alterations and different transposition systems have been adapted to study gene function in these human pathogens. A mariner transposon toolkit for use in vivo or in vitro in Leishmania parasites has been developed and can be used in a variety of applications. These modified mariner elements not only permit the inactivation of genes, but also mediate the rescue of translational gene fusions, bringing a major contribution to the investigation of Leishmania gene function. The piggyBac and Tn5 transposons have also been shown to mobilize across Plasmodium spp. genomes circumventing the current limitations in the genetic manipulation of these organisms.

The use of Tn5 transposable elements in a gene trapping strategy for the protozoan Leishmania

The use of transposable elements as a gene-trapping strategy is a powerful tool for gene discovery. Herein we describe the development of a transposable system, based on the bacterial Tn5 transposon, which has been used successfully in Leishmania braziliensis. The transposon carries the neomycin phosphotransferase gene, which is expressed only when inserted in-frame with a Leishmania gene present in the target DNA. Four cosmid clones from a L. braziliensis genomic library were used as targets in transposition reactions and four insertional libraries were constructed and transfected in L. braziliensis. Clones resistant to G418 were selected and analysed by immunofluorescence in order to identify the subcellular localisation of the protein coded by the trapped gene. A definitive subcellular localisation for neomycin phosphotransferase/targeted protein fusion was not obtained in any of the four Leishmania clones investigated. However, the constructed transposable element is highly efficient considering the frequency of insertion in large targets and is therefore a useful tool for functional genetic studies in Leishmania. Our data confirm the utility of the Tn5 transposon system for insertion of sequencing priming sites into target DNA. Furthermore, the high frequency of insertion and even distribution are important in studying genomic regions bearing long and polymorphic repetitive sequences.

Shuttle mutagenesis and targeted disruption of a telomere-located essential gene of Leishmania

Leishmania mutants have contributed greatly to extend our knowledge of this parasite's biology. Here we report the use of the mariner in vitro transposition system as a source of reagents for shuttle mutagenesis and targeted disruption of Leishmania genes. The locus-specific integration was achieved by the disruption of the subtelomeric gene encoding a DNA-directed RNA polymerase III subunit (RPC2). Further inactivation of RPC2 alleles required the complementation of the intact gene, which was transfected in an episomal context. However, attempts to generate a RPC2 chromosomal null mutant resulted in genomic rearrangements that maintained copies of the intact locus in the genome. The maintenance of the RPC2 chromosomal locus in complemented mutants was not mediated by an increase in the number of copies and did not involve chromosomal translocations, which are the typical characteristics of the genomic plasticity of this parasite. Unlike the endogenous locus, the selectable marker used to disrupt RPC2 did not display a tendency to remain in its chromosomal location but was targeted into supernumerary episomal molecules.