Abstract 1480 Heat Stress, Thermal Adaptation and the Rise of Fungal Diseases

Climate change is predicted to increase the incidence of known fungal diseases and to facilitate the emergence of new fungal diseases in humans and mammals. Most environmental fungi do not cause disease and are critical symbionts that support plant growth, though some cause devastating crop diseases (wheat, corn, rice) and to a lesser extent, life-threatening diseases in humans. While fungal disease in plants and amphibians is common, humans and mammals are largely protected. In addition to our innate immune defenses, core body temperatures of 37°C serve as a thermal barrier to most fungal infections. Rising global temperatures, however, are predicted to increase the ability of fungi to survive at higher temperatures, which could increase their pathogenic potential. In addition, severe weather that disrupts the soils, such as hurricanes, droughts and flooding, will lead to greater spore dispersal and exposure of humans to potentially hazardous fungi. Cryptococcus species are environmental fungi that cause invasive diseases primarily in populations with weakened immune systems. Inhaled into the lungs, Cryptococcus causes a pulmonary infection that can spread to the brain to cause a fatal swelling and inflammation known as cryptococcal meningitis. To better understand the impact of heat stress on Cryptococcus adaptation in the environment and during host infection, we compared the mutation rate in cells grown at ambient temperature (30°C) and at human body temperature (37°C) using a forward mutation assay measuring drug resistance. When cells were grown under heat stress, the rate of drug resistance increased 8 to 25-fold. Surprisingly, the increased mutation rate into target genes at higher temperature was due to the insertion of large 3-5 kb mobile DNA elements called transposons (Gusa and Williams et al., 2020). Transposable elements (TEs) are prevalent in eukaryotic genomes but their mobility is usually repressed by silencing mechanisms including RNA interference and heterochromatin. When they mobilize, TEs can insert within genes to alter/disrupt gene function or insert between genes to affect gene expression. We have now identified multiple heat stress-stimulated TEs in different species of Cryptococcus and shown that TE mutations are the major source of genetic variation under conditions of heat stress, compared to other sequence variations such as base substitutions or small insertions/deletions. Additionally, we found that transposons mobilize in Cryptococcus during murine infection, suggesting a role in stress adaptation during human infection (Gusa et al, 2023). Studies in our lab are underway to 1) explore the mechanism of heat stress-induced TE movement and 2) determine whether TEs contribute to rapid adaptive changes in Cryptococcus that may increase their pathogenicity, such as acquired thermal tolerance and drug resistance. This research is supported by a K99/R00 MOSAIC fellowship from the NIH/NIAID.