Oiki, S.; Abe, M.; Hirasawa, A.; Koizumi, A.; Otani, A.; Shinohara, T.; Miyazaki, Y.

Candida auris (Candidozyma auris) is an emerging multidrug-resistant fungal pathogen that poses a significant global health threat. However, the molecular mechanisms underlying its virulence remain incompletely understood. In this study, we performed in vivo transcriptome analysis using an immunosuppressed mouse gastrointestinal infection model to identify genes associated with host-adaptation and virulence during infection. By comparing fungal transcriptomes obtained from colonization and dissemination sites with those from in vitro cultures, we identified genes that were consistently upregulated during infection. Among these genes, the unfolded protein response regulator HAC1 was selected as a candidate virulence-associated gene for further analysis. RT-PCR and sequencing analyses revealed that HAC1 mRNA in C. auris undergoes an unconventional splicing event of 287 bp that is enhanced under ER stress conditions. The excised region spans the annotated open reading frame boundary, suggesting that the translated region of HAC1 may require re-evaluation. Notably, a proportion of HAC1 transcripts appeared to be spliced even under non-stress conditions, indicating a detectable basal level of UPR activation. Differences in splicing dynamics were also observed among clade strains. Functional analyses demonstrated that deletion of HAC1 increased sensitivity to ER stress and heat stress. The HAC1 deletion mutant also exhibited reduced virulence in both Galleria mellonella and immunosuppressed mouse infection models, as evidenced by delayed host mortality and decreased fungal burdens, respectively. These findings indicate that HAC1 contributes to ER stress adaptation, thermotolerance, and survival in the host environment, and identify HAC1 as a virulence-associated gene in C. auris.