The Identification and Characterization of Anti-Virulence Genes Within Candida albicans Phospholipid Biosynthetic Pathways

Human fungal pathogens, which cause approximately two million life-threatening infections annually, represent a leading cause of morbidity and mortality for immunocompromised individuals. Numerous fungal pathogens with increased virulence, known as hypervirulence, have been identified, and the study of these organisms merits further investigation as this would lead to a greater understanding of host-pathogen interaction and regulation of virulence in fungi. We have identified two strains of hypervirulent Candida albicans, which is the most frequent cause of systemic fungal infections worldwide. These hypervirulent strains have mutations within the two major phospholipid biosynthetic routes in C. albicans, known as the cytidine-diphosphate diacylglycerol (CDP-DAG) and Kennedy pathways. Deletion of the Pem1 and Pem2 methyltransferases results in a buildup of the phospholipid phosphatidylethanolamine (PE) within the CDP-DAG pathway, and this correlates with hypervirulence. Similarly, overexpression of the Kennedy pathway, via constitutive expression of EPT1, results in hypervirulence that correlates with increased PE synthesis. Deletion of EPT1 correlates with reduced PE and reduced virulence. Prior research indicates that decreasing CDP-DAG derived PE, by deletion of the Psd1 and Psd2 enzymes causes a decrease in virulence. Thus, modulation of PE synthesis acts as a rheostat for virulence in C. albicans. In this dissertation, we have determined that loss of PE synthesis by the CDP-DAG route results in avirulence due to an inability to obtain sufficient ethanolamine from the host. To determine how increased PE causes hypervirulence, transcriptomic analysis was performed for both high PE and low PE strains. Interestingly, while there were no transcriptional differences identified for the EPT1 overexpressor, there were increases in cell wall chitin and also hyphal growth, which suggests that an increase in PE modulates virulence via a post-transcriptional mechanism. While the mechanism has not been fully elucidated, hyphae are a major virulence factor for C. albicans that contribute to tissue damage, and it has been demonstrated that chitin modulates immune responses. Taken together, these phenotypes could explain why the EPT1 overexpressor is hypervirulent. To our knowledge, the EPT1 overexpressor and PEM1PEM2 null mutants are the first descriptions of fungal hypervirulence resulting from mutations in phospholipid biosynthetic pathways