Terbinafine Resistance of Trichophyton Clinical Isolates Caused by Specific Point Mutations in the Squalene Epoxidase Gene

Abstract

Terbinafine is one of the allylamine antifungal agents whose target is squalene epoxidase (SQLE). This agent has been extensively used in the therapy of dermatophyte infections. The incidence of patients with tinea pedis or unguium tolerant to terbinafine treatment prompted us to screen the terbinafine resistance of all Trichophyton clinical isolates from the laboratory of the Centre Hospitalier Universitaire Vaudois collected over a 3-year period and to identify their mechanism of resistance. Among 2,056 tested isolates, 17 (Ϸ1%) showed reduced terbinafine susceptibility, and all of these were found to harbor SQLE gene alleles with different single point mutations, leading to single amino acid substitutions at one of four positions (Leu 393 , Phe 397 , Phe 415 , and His 440 ) of the SQLE protein. Point mutations leading to the corresponding amino acid substitutions were introduced into the endogenous SQLE gene of a terbinafine-sensitive Arthroderma vanbreuseghemii (formerly Trichophyton mentagrophytes) strain. All of the generated A. vanbreuseghemii transformants expressing mutated SQLE proteins exhibited obvious terbinafine-resistant phenotypes compared to the phenotypes of the parent strain and of transformants expressing wild-type SQLE proteins. Nearly identical phenotypes were also observed in A. vanbreuseghemii transformants expressing mutant forms of Trichophyton rubrum SQLE proteins. Considering that the genome size of dermatophytes is about 22 Mb, the frequency of terbinafine-resistant clinical isolates was strikingly high. Increased exposure to antifungal drugs could favor the generation of resistant strains.

Terbinafine‐resistant strain of Trichophyton interdigitale strain isolated from a tinea pedis patient

“…Trichophyton rubrum isolated from human dermatophytosis was reported to exhibit TRF resistance . Resistance of dermatophytes to antifungal drugs frequently results from overexpression of genes encoding ATP‐binding cassette (ABC) transporter proteins, including the pleiotropic drug resistance ( PDR1 ) and multidrug resistance ( MDR2 and MDR4 ) genes .…”

“…Moreover, in recent work, Yamada et al . reported that a TRF‐resistant T. interdigitale strain (TIMM20096; TRF MIC, 3.2 mg/L) harbored missense mutations (Phe397Leu) in the SQLE ‐encoding gene.…”

“…Using sequential passage in the presence of ITZ and TRF, Hryncewicz‐Gwóźdź et al . and Yamada et al . demonstrated in vitro selection of ITZ‐ or TRF‐resistant Trichophyton rubrum .…”

“…demonstrated in vitro selection of ITZ‐ or TRF‐resistant Trichophyton rubrum . These authors warned that T. rubrum can easily develop resistance to ITZ or TRF, a process with important implications for the clinical management of dermatophytoses . Indeed, patients with tinea unguium and tinea pedis have been shown to remain uncured or to become re‐infected, suggesting that antifungal drug‐resistant strains may be emerging in Japan.…”

“…Indeed, patients with tinea unguium and tinea pedis have been shown to remain uncured or to become re‐infected, suggesting that antifungal drug‐resistant strains may be emerging in Japan. However, the manner in which antifungal drug‐resistant T. interdigitale has been isolated in a Japanese patient suggests that strains have naturally acquired resistance genes.…”