Genetic confirmation of terbinafine resistance in Trichophyton rubrum mediated by the squalene epoxidase Leu393Phe mutation via targeted gene replacement

Terbinafine (TBF) is a first-line allylamine antifungal used to treat dermatophytosis caused by Trichophyton species, targeting squalene epoxidase (SQLE). The recent emergence of TBF-resistant Trichophyton rubrum (T. rubrum) strains represents a significant global clinical challenge. Although the SQLE Leu393Phe substitution is a commonly reported resistance mechanism in Japan, direct genetic evidence demonstrating its effect when introduced into a T. rubrum recipient strain has been lacking. In this study, we established the causal relationship between the SQLE Leu393Phe substitution and TBF resistance via targeted gene replacement in T. rubrum. A homologous recombination plasmid containing SQLE from the TBF-resistant T. rubrum strain BGUTR13, which harbours the 1177TTA→TTC mutation encoding Leu393Phe, together with the neomycin phosphotransferase II (nptII) selection marker, was constructed and introduced into the TBF-susceptible strain T. rubrum CBS118892 Δku80. The resulting mutant transformants exhibited a >8,000-fold increase in TBF minimum inhibitory concentration compared with the parental strain, whereas control transformants carrying nptII but lacking the SQLE Leu393Phe substitution remained susceptible. Structural modelling indicated that the Leu393Phe substitution causes substantial steric hindrance within the SQLE binding pocket, displacing TBF’s aliphatic chain and disrupting key hydrophobic interactions. Molecular docking simulations predicted reduced TBF binding affinity (ΔG changed from −6.3 to −0.8 kcal/mol) following the mutation. These findings provide the first robust genetic evidence that the single SQLE Leu393Phe substitution is sufficient to confer high-level TBF resistance in T. rubrum. These findings highlight the importance of monitoring SQLE mutations in clinical isolates and provide a foundation for developing strategies to address antifungal resistance in dermatophytes.