CO2 adaptation in the global fungal pathogen Cryptococcus neoformans

The opportunistic pathogen Cryptococcus neoformans causes fatal systemic disease in immunocompromised individuals worldwide. As an environmental fungus, adaptation to host conditions is key for its survival and pathogenesis. A major difference between the host and ambient environment is the concentration of CO2. CO2 makes up ~5% or more of the air in the human body, which is 125 times higher than in ambient air (~0.04%). The growth of many environmental isolates of C. neoformans is inhibited by host levels of CO2, and the ability to tolerate this high level of CO2 is correlated with virulence. As humans acquire cryptococcal infections from the environment, how CO2-sensitive environmental isolates have been able to adapt to the higher CO2 level of the host has remained a mystery. Through experimental evolution, comparative genomics, and targeted mutagenesis, we found that a loss of function mutation in Avc1, an ARID domain-containing protein, greatly enhance cryptococcal CO2 tolerance and their virulence in the animal host in multiple environmental isolates. Intriguingly, loss of function mutations in Avc1 were reported previously in relapse patients with cryptococcal meningitis. Overexpression of AVC1 resulted in loss of CO2 tolerance, and caused polyploidization and cell enlargement in the presence of CO2. Transcriptomic analyses of the environmental isolates and their corresponding avc1 deletion mutants revealed conserved CO2-responsive genes regulated by Avc1. These results implicate a critical role for mutation of Avc1 in cryptococcal adaptation to high CO2.