Date of Award

2022

Document Type

Thesis (Ph.D.)

Department or Program

Microbiology and Immunology

First Advisor

Deborah Hogan

Abstract

Collectively, Candida species are the most prevalent cause of both superficial and invasive fungal infections worldwide. Invasive Candida infections have a high mortality rate and predominantly affect individuals with underlying diseases, such as diabetes, HIV, or cancer. Unfortunately, many invasive Candida infections are recalcitrant to antifungal treatment, while intrinsically multidrug-resistant pathogens, like Candida auris, are increasing in prevalence. Although the canonical mechanisms of antifungal resistance in Candida species are well established, i.e., overexpression of efflux pumps and overexpression of or mutations in genes encoding drug targets, factors affecting the natural evolution and regulation of resistance mechanisms remain poorly understood.

One cause of antifungal resistance in Candida species is the acquisition of gain-of-function mutations in the transcription factor Mrr1, resulting in overexpression of the multidrug transporter Mdr1. However, little is known about the functions of other genes regulated by Mrr1 or how Mrr1 activity is modulated in vivo. In this work, we demonstrate in Candida lusitaniae and in C. auris that Mrr1 contributes to resistance against methylglyoxal (MG), a toxic, electrophilic dicarbonyl derived from natural metabolic processes, and that Mrr1-mediated MG resistance is driven in part by expression of the methylglyoxal reductase genes MGD1 and MGD2 in C. lusitaniae and MGD1 in C. auris. Furthermore, we show that a sublethal concentration of MG induces expression of MDR1 and MG reductase genes in C. lusitaniae and C. auris, and consequently increases fluconazole (FLZ) resistance in C. lusitaniae. Finally, we characterize the complete Mrr1- dependent and independent transcriptional response of C. auris to MG and to the known inducer of Mrr1-regulated gene expression, benomyl, and show that both compounds cause the differential expression of a multitude of genes involved in metabolism and stress response, which could contribute to pathogen survival while colonizing and infecting a mammalian host.

Together, the work presented herein provides valuable insight into a potential mechanism for the regulation of Mrr1-dependent transcription in vivo as well as a possible selective pressure for gain-of-function mutations in the MRR1 gene. This is particularly noteworthy because MG is elevated in many of the same human diseases that are considered risk factors for Candida infection, and MG is also produced by activated phagocytes in response to pathogens. Thus, it is conceivable that Candida would encounter biologically significant levels of MG in the context of infection. We propose that MG-mediated induction of Mrr1-dependent transcription in Candida species is one factor that plays a role in antifungal treatment failure.

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