Candida albicans is normally a harmless commensal fungus in humans, occupying the skin, mouth, gastrointestinal tract and vagina. C. albicans is also, however, a prevalent opportunistic pathogen, capable of causing debilitating mucosal infections as well as life-threatening systemic infections. It is especially virulent amongst people with compromised immune systems, often targeting individuals coinfected with HIV. In one study, 84% of HIV-infected patients had oropharyngeal colonization by Candida species on at least one occasion, and 55% developed clinical thrush. Despite its medical importance, a basic understanding of how this organism persists in healthy hosts while still being able to cause a spectrum of diseases in immunocompromised hosts is lacking.
In this proposal, we will examine phenotypic switching in C. albicans and its role in disease. Phenotypic switching is common in many pathogenic organisms and involves strains switching spontaneously, reversibly, and at high frequency between different phenotypes that are distinguishable by colony morphology. It is thought that phenotypic switching helps the organism to rapidly adapt to changes in host physiology, as well as preventing clearance by the immune system. In the case of C. albicans , it has been shown that isolates from HIV-positive individuals switch at higher frequencies than isolates from uninfected people, and that this increased switching also correlates with a higher proportion of strains being drug resistant.
We will investigate the mechanism of white-opaque switching in C. albicans , a common phenotypic switch observed in this organism. The white-opaque switch has been shown to directly influence biofilm formation in vitro and to result in dramatic changes in virulence in animal models of candidiasis. This proposal involves two specific aims:
Specific Aim 1: A genome-wide overexpression screen to identify novel genes that affect the frequency of white-opaque switching . An overexpression library will be constructed in C. albicans and screened to identify genes involved in regulation of the white-opaque switch.
Specific Aim 2: Determine the role of identified genes in white-opaque switching and virulence of C. albicans strains. The function of identified genes from the screen in regulating white-opaque switching will be investigated. Particular attention will be given to known virulence and drug resistance genes, as these will provide insight as to why phenotypic switching regulates these properties in C. albicans strains.
With the completion of these aims, we will gain insight into the mechanism of phenotypic switching in the most common fungal pathogen to target HIV-infected individuals. Little is known about epigenetic mechanisms of variation in fungi, despite the established role of these processes in promoting biofilm formation and disease. It is envisaged that an understanding of mechanisms of phenotypic switching will help elucidate how colonization and infection by C. albicans strains is achieved . In addition, these studies will lead to new strategies for therapeutic interventions in individuals carrying HIV and other susceptible individuals .
|
