Mutation screening of the muscarinic m2 and m3 receptor genes in asthmatics, outgrow subjects, and normal controls.

Muscarinic receptors are important in the development of airway hyperresponsiveness. In some patients with asthma and in animal models of hyperreactivity, functional abnormalities in these receptors are suggested to contribute to disease. Here, we have screened for single nucleotide polymorphisms in the coding region of human muscarinic m2 and m3 receptor genes using direct fluorescence sequencing. DNA samples from 102 current asthmatics and 58 who had outgrown asthma ("outgrow" patients) were compared with 70 random non-asthmatic controls. A mutation characterized by a single base substitution (A1050G, Ser350Ser) was identified in the muscarinic m2 receptor gene. This polymorphism was common and was represented in all three groups studied. In contrast, in the m3 receptor coding region examined, we found a very rare nucleotide variant (C261T, Ile87Ile), identified in only one of the 230 samples genotyped. Therefore, neither A1050G in the m2 receptor nor C261T in the m3 receptor is likely to be functionally significant for airway hyperresponsiveness in asthma. Our data suggest that both the m2 and m3 receptor genes are highly conserved, and no significant genetic mutations are related to their possible functional changes in human asthma.