Identification of novel factors regulating Candida albicans stress-induced genome instability

Candida albicans is a commensal harmless organism which asymptomatically colonises the human population. However, this organism can become a life-threatening pathogen causing systemic infections which results fatal especially in immunocompromised patients. During colonisation and infection, C. albicans must adapt rapidly to the different environments of the host. C. albicans genome is highly plastic, and it can tolerate large-scale genomic changes adapting to the stress environments it encounters. This is because genomic instability increases genetic diversity allowing the selection of the genotypes favourable to adapt to the stress environment. This genetic instability increases virulence and resistance to the common antifungal drugs. However, it remains unknown how stress-induced genetic instability is regulated and what are the driving mechanisms to genome plasticity in the stress environment. By this means, the aim of this project is to identify novel regulators of C. albicans stress-induced genome instability.

To this end, I performed different parallel genetic screenings using a homozygous C. albicans gene deletion library, using UV radiation and Methyl methanesulfonate (MMS) as DNA damaging agents. This strategy has led to the identification of ULP2 C. albicans gene as novel potential regulator of C. albicans genome instability. ULP2 gene is involved in SUMO deconjugation, and in this project it has proved to be hypersensitive to different genotoxic stresses including UV, MMS, Hydroxy urea (HU), Hydrogen peroxide (H2O2) and high temperature. In addition, cell lacking ulp2ΔΔ exhibited chromosome mis-segregation defects. The high genome instability of the ulp2ΔΔ strain leads to enhanced Fluconazole resistance via selection of novel fitter genotypes. Whole-genome sequencing of ulp2ΔΔ fluconazole resistance cells exhibited stress-induced segmental aneuploidies of chromosome R and chromosome I. Furthermore, intrachromosomal repetitive elements are the site of formation of complex novel genotypes with adaptive potential. To sum up, these results show that C. albicans ULP2 gene regulates genome plasticity and drug resistance.