Characterization of a programmed alteration in an 18S ribosomal gene that accompanies the experimental induction of drug resistance in Schistosoma mansoni.

Abstract

Stable resistance to the anthelmintic hycanthone can be produced in the human blood fluke Schistosoma mansoni by exposing immature parasites in mice to the drug. Within a single generation, genomic rearrangements, detected as rRNA-encoding DNA restriction fragment length polymorphisms (RFLPs), accompany the appearance of resistance in this model. One of these RFLPs, an ;"3.6-kilobase BamHI fragment, was shown previously to associate consistently with resistance in independent generations of the JHU strain of S. mansoni. To characterize the genetic changes responsible for this RFLP, the fragment was cloned and sequenced. A comparison of the cloned fragment with a normal 18S rRNA gene demonstrated that the drug resistance-associated RFLP fragment arises through the addition of 732 base pairs into an 18S rRNA gene, 134 base pairs downstream of the junction of the intergenic spacer and the mature 18S rRNA gene. The mutation is nonrandom, targets one, or a few only, of the 100 or so copies of the ribosomal genes, and may represent the incomplete duplication of the gene since the inserted element is identical in sequence to the region contiguous to it. The sequence spanning thejunction of the insertion and the original 18S rRNA gene was used as a specific primer for the BamHI RFLP in PCR experiments. The analysis conclusively demonstrated that the mutation is induced rather than selected by the drug since the junctional sequence was not detectable in the drug-sensitive parent population of schistosomes. In addition, analysis of four, independently derived, resistant lines indicated that the same region of the gene was mutated each time.Together, these data demonstrate that reproducible changes are induced during the acquisition of resistance in schistosomes and suggest that the resistant phenotype is induced rather than selected from preexisting forms.Schistosomiasis is caused by various species of blood flukes of the genus Schistosoma, which as adults live and lay eggs in the vasculature of the intestines and bladder. Chemotherapy is the major method of control for schistosomiasis, and its use is increasing (1). Widespread application of schistosomicidal compounds is expected to lead to increased levels of drug resistance as has occurred with other parasites. Indeed, resistance to thioxanthenones has been detected in natural populations of Schistosoma mansoni (2) and can be derived experimentally (3-5). Nonetheless, the molecular basis of the resistance is poorly understood.In one model developed by Bueding and colleagues (4), resistance to thioxanthenones appears in S. mansoni after drug treatment of mice infected with immature schistosomes. This phenomenon is parasite strain dependent, but it is seen with schistosomes of disparate geographical origin. The resistance can be produced with hycanthone (HC) or the related drug oxamniquine, and the parasites derived show cross-resistance to both compounds. Resistance is induced by exposure of the 27-to 29-day-old parasite to HC, when schistosomes are developmenta...

Schistosomiasis Chemotherapy

“…In addition, stable and transmissible resistance to hycanthone can be produced within a single generation in S. mansoni by exposing immature parasites in mice to the drug. Reproducible genomic rearrangements are induced during the acquisition of resistance and suggest that the resistant phenotype is induced rather than selected from preexisting forms 123. 124…”

Altered Response of Strain of Schistosoma mansoni to Oxamniquine and Praziquantel

“…It is important to study the possible alterations at a genomic level of this strain to PZQ in comparison to another that is sensitive or resistant to OXA/hycanthone (Brindley et al 1991, PicaMattoccia et al 1993, Cioli 2000.…”

Chemotherapie gegen Schistosomiasis

“…Dies lässt darauf schließen, dass sich der resistente Phänotyp durch Induktion und nicht durch Selektion aus schon vorhandenen Formen bildet. [123,124] Es wurde dann versucht, eine Reihe von einfachen N-Alkyl-p-methylanilin-Derivaten herzustellen, die eine ähnliche Effizienz wie Hycanthon haben sollten. Mirasan (Abbildung 6) ist wirksam in Mäusen, aber unwirksam in Affen und Menschen.…”