Bagnall Neil H, Ruffell Angela, Raza Ali, Elliott Timothy P, Lamb Jane, Hunt Peter W, Kotze Andrew C
CSIRO Agriculture and Food, Brisbane, Queensland, Australia.
Invetus, Trevenna Road, Armidale, NSW, Australia.
Int J Parasitol Drugs Drug Resist. 2017 Aug;7(2):236-240. doi: 10.1016/j.ijpddr.2017.05.001. Epub 2017 May 4.
Resistance to the anthelmintic drug monepantel (Zolvix) has emerged in parasitic worms infecting sheep and goats. The mechanism of resistance in these cases is unknown. The drug targets nicotinic acetylcholine receptors belonging to the nematode-specific DEG-3 subfamily. We examined the receptor gene, Hco-mptl-1, in a highly Zolvix-resistant and a -susceptible isolate of the parasitic nematode Haemonchus contortus. cDNA coding for the full length receptor protein (Hco-MPTL-1) was present in all clones prepared from a pool of susceptible larvae (21/21 clones) and approximately 50% of those from the resistant isolate (17/33). On the other hand, the remaining clones from the resistant isolate showed various mutations that resulted in truncated predicted proteins, missing at least one transmembrane domain. The most common mutation (11/33 clones) resulted in the retention of intron 15, a premature stop codon, and a truncated protein. Sequencing of intron 15 genomic DNA showed very few SNPs in susceptible larvae and in 12/18 clones from resistant larvae, alongside the presence of at least 17 SNPs in the remaining resistant clones. The present study shows that the highly resistant isolate has a number of mutations in the drug target gene that would most-likely result in a non-functional receptor, thus rendering the larvae insensitive to the drug. The presence of many wild-type sequences in this highly-resistant population suggests that there was a significant presence of heterozygotes in the survivors of the field drench treatment from which the isolate was derived, and hence that at least some of the mutations may be dominant. Alternatively, their presence may be due to the additional influence of mutations at another locus contributing to the resistance phenotype. The presence of multiple separate mutations in the Hco-mptl-1 gene in this viable field-derived worm isolate may at least partly explain why resistance to Zolvix has arisen rapidly in the field.
感染绵羊和山羊的寄生虫对驱虫药莫能菌素（Zolvix）已产生抗性。这些案例中抗性产生的机制尚不清楚。该药物作用于属于线虫特异性DEG - 3亚家族的烟碱型乙酰胆碱受体。我们检测了寄生线虫捻转血矛线虫高度耐Zolvix和敏感品系中的受体基因Hco - mptl - 1。编码全长受体蛋白（Hco - MPTL - 1）的cDNA存在于从敏感幼虫池制备的所有克隆中（21/21个克隆），以及约50%来自抗性品系的克隆中（17/33）。另一方面，来自抗性品系的其余克隆显示出各种突变，导致预测的蛋白质截短，至少缺失一个跨膜结构域。最常见的突变（11/33个克隆）导致内含子15保留、提前出现终止密码子以及蛋白质截短。内含子15基因组DNA测序显示，敏感幼虫以及来自抗性幼虫的12/18个克隆中SNP很少，而其余抗性克隆中至少存在17个SNP。本研究表明，高度抗性品系的药物靶基因存在许多突变，很可能导致受体无功能，从而使幼虫对药物不敏感。这个高度抗性群体中存在许多野生型序列，表明在该抗性品系所源自的田间灌药处理存活者中存在大量杂合子，因此至少一些突变可能是显性的。或者，它们的存在可能是由于另一位点的突变对抗性表型有额外影响。在这个来自田间且存活的虫株中，Hco - mptl - 1基因存在多个独立突变，这至少可以部分解释为什么在田间对Zolvix的抗性迅速出现。
