Increased copy number of the target gene squalene monooxygenase as the main resistance mechanism to terbinafine in Leishmania infantum

We use here two genomic screens in an attempt to understand the mode of action and resistance mechanism of terbinafine, an antifungal contemplated as a potential drug against the parasite Leishmania. One screen consisted in in vitro drug evolution where 5 independent mutants were selected step-by-step for terbinafine resistance. Sequencing of the genome of the 5 mutants revealed no single nucleotide polymorphisms related to the resistance phenotype. However, the ERG1 gene was found amplified as part of a linear amplicon, and transfection of ERG1 fully recapitulated the terbinafine resistance phenotype of the mutants. The second screen, Cos-seq, consisted in selecting a gene overexpression library with terbinafine followed by the sequencing of the enriched cosmids. This screen identified two cosmids derived from loci on chromosomes 13 and 29 encoding the squalene monooxygenase (ERG1) and the C8 sterol isomerase (ERG2), respectively. Transfection of the ERG1-cosmid, but not the ERG2-cosmid, produced resistance to terbinafine. Our screens suggest that ERG1 is the main, if not only, target for terbinafine in Leishmania and amplification of its gene is the main resistance mechanism.