To date, only one genetic variant of human AChE has been described, which is not associated with any abnormality in activity [54]. Based on work in AChE knockout mice, it has been suggested, however, that silent AChE alleles may exist in humans [54a]. This work on AChE in transgenic mice models is providing interesting and important insights on the roles of AChE and BChE in OP toxicity. Targeted deletion of four exons in the AChE gene, which totally eliminated AChE activity in nullizygous mice, was not lethal, but AChE−/− mice displayed a number of physical, behavioral and biochemical abnormalities [55], [56] and [57]. BChE activity was similar in AChE−/−, wild-type and AChE+/− mice; differently from AChE−/− animals, the latter had normal appearance and development, and were fertile [55] and [58]. AChE knockout mice had 50–80% less muscarinic receptors (M1, M2 and M4) in the hippocampus, and decreased muscarinic receptor signaling [59]. These effects are also seen in the development of tolerance to OPs, and may be due to receptor down regulation due to overstimulation by acetylcholine. AChE−/− mice share some characteristics with a wild-type mouse intoxicated with an OP. These include loss of AChE catalytic activity, body tremors, muscle weakness, reduced pain response, pinpoint pupils and increased salivation and lacrimation [60]. However, a 100% inhibition of AChE by an OP, normally results in lethality. Hence, in AChE−/− mice, other enzymes must take over in hydrolyzing, albeit partially, acetylcholine. This role has been shown to be fulfilled by BChE, which in AChE knockout mice can hydrolyze acetylcholine in the central and peripheral nervous systems [58] and [61]. The BChE inhibitor bambuterol was indeed lethal to AChE−/− mice, while it had no effect in wild-type animals [55]. The presence of BChE in mammals may explain why deletion of AChE in lethal in Drosophila, where there is no BChE [62].
Of interest is the response to OPs of AChE−/− mice that lack their primary target. These animals are supersensitive to the acute toxicity of diisopropylfluorophosphate (DFP) and of VX [55] and [60], as well as of chlorpyrifos oxon [57] (Table 3). The signs of toxicity upon VX administration were identical in wild-type and AChE−/− mice, indicating that they were due to cholinergic hyperstimulation. This may be explained by the fact that VX inhibited also BChE; however, inhibition was only partial. Furthermore, atropine protected wild-type mice, but not AChE−/− mice from VX toxicity [60]. Why is then the AChE−/− mouse supersensitive to OP toxicity? Inhibition of BChE appears to be the primary explanation. However, the possibility that additional targets for OPs may be involved has also been raised [57] and [60] (see Section 3.6). Finally, it should be noted that mice carrying only one deficient AChE allele (AChE+/−) are healthy, but display an intermediate supersensitivity to OPs [57] and [60] (Table 3). The existence of genetic polymorphisms of AChE in humans that would cause partial AChE deficiency has not been yet demonstrated, but has been hypothesized to exist [57]. These individuals would be healthy, but would display increased susceptibility to OP toxicity.