Acetylcholine receptor subunit genes from Ancylostoma caninum: Altered transcription patterns associated with pyrantel resistance

The molecular mechanism of resistance to nicotinic agonist anthelmintics such as pyrantel and levamisole in nematodes of medical and veterinary significance is poorly understood. The identification of pyrantel-resistant isolates of the canine hookworm, Ancylostoma caninum, provides an opportunity to explore, at a molecular level, the mechanism of cholinergic resistance in a species that is a model for the human hookworms. Here we describe the cloning of three A. caninum genes orthologous to components of the pyrantel-sensitive nicotinic acetylcholine receptor in Caenorhabditis elegans (UNC-29, -38, -63). Analysis of mRNA levels by quantitative PCR was also performed on these genes, plus an additional three nicotinic acetylcholine receptor subunit genes thought not to be constituents of the pyrantel-sensitive receptor, for which a partial sequence was obtained. Gene sequences and mRNA levels were compared between two isolates of A. caninum showing either high- or low-level resistance to pyrantel (as shown previously by in vivo efficacy and in vitro comparative studies). While no polymorphisms of likely significance between the two A. caninum isolates were observed, quantitative analysis of transcription revealed significantly lower levels for the three putative pyrantel receptor subunits (AAR-29, -38 and -63) in the highly pyrantel-resistant isolate compared with the isolate with low-level resistance. In contrast, transcription of the three subunits thought not to constitute the pyrantel receptor (AAR-8, -15 and -19) was either not significantly different between the two isolates, or slightly higher in the highly-resistant isolate. This data suggests that reduced transcription of the mRNA coding for nicotinic acetylcholine receptor subunits that form the pyrantel-sensitive receptors may be a component of the pyrantel resistance mechanism in A. caninum.