An epigenetic mechanism of azole tolerance facilitates acquired antifungal resistance in Aspergillus fumigatus

Antibiotic tolerance paves the way for acquired resistance in bacterial pathogens. However, the mechanisms of tolerance and its evolutionary role in acquired resistance in pathogenic fungi, particularly molds, remains elusive. Here, we identified an Inhibitor of Growth domain protein (IngB) as a novel epigenetic regulator of azole tolerance in Aspergillus fumigatus. The loss of ingB promotes supra-MIC growth on agar surface despite susceptible MICs in standardized assays. Moreover, established {Delta}ingB biofilms are less susceptible to azoles in vitro and in vivo. Subsequent exposure of the tolerant strain to high azole concentrations resulted in rapid acquired resistance, most notably a frameshift mutation in a putative 20S proteasome maturation protein, UmpA, while the susceptible wildtype strain failed to acquire adaptive mutations. The data suggest that IngB-mediated tolerance provides an epistatic background for the emergence of azole resistance. Our work shows drug tolerance facilitates resistance emergence in a critical fungal pathogen. ImportanceWhile antimicrobial drug resistance causes a significant adverse effect on human health, drug tolerance can also lead to insufficient pathogen clearance, resulting in infection relapse. However, the mechanisms of antifungal drug tolerance and its evolutionary role in acquired drug resistance in pathogenic fungi, particularly the molds, remains elusive. We identified IngB as a novel regulator of azole tolerance in Aspergillus fumigatus. Importantly, loss of IngB leads to rapid azole drug resistance under azole-selective pressure. Our work identifies a novel regulator of antifungal tolerance and suggests antifungal drug tolerance can pave the way for resistance emergence in a critical fungal pathogen.