Mismatch Repair of DNA Replication Errors Contributes to Microevolution in the Pathogenic Fungus Cryptococcus neoformans.

Kylie J Boyce,Surbhi Verma,Chaoyang Xue,Alexander Idnurm,Viplendra P S Shakya,Yina Wang,J Andrew Alspaugh

doi:10.1128/mbio.00595-17

Abstract

ABSTRACTThe ability to adapt to a changing environment provides a selective advantage to microorganisms. In the case of many pathogens, a large change in their environment occurs when they move from a natural setting to a setting within a human host and then during the course of disease development to various locations within that host. Two clinical isolates of the human fungal pathogen Cryptococcus neoformans were identified from a collection of environmental and clinical strains that exhibited a mutator phenotype, which is a phenotype which provides the ability to change rapidly due to the accumulation of DNA mutations at high frequency. Whole-genome analysis of these strains revealed mutations in MSH2 of the mismatch repair pathway, and complementation confirmed that these mutations are responsible for the mutator phenotype. Comparison of mutation frequencies in deletion strains of eight mismatch repair pathway genes in C. neoformans showed that the loss of three of them, MSH2, MLH1, and PMS1, results in an increase in mutation rates. Increased mutation rates enable rapid microevolution to occur in these strains, generating phenotypic variations in traits associated with the ability to grow in vivo, in addition to allowing rapid generation of resistance to antifungal agents. Mutation of PMS1 reduced virulence, whereas mutation of MSH2 or MLH1 had no effect on the level of virulence. These findings thus support the hypothesis that this pathogenic fungus can take advantage of a mutator phenotype in order to cause disease but that it can do so only in specific pathways that lead to a mutator trait without a significant tradeoff in fitness.

Highlights

The ability to adapt to a changing environment provides a selective advantage to microorganisms
This collection of strains is currently the best comparator set, in that the virulence of the strains has been tested, showing a correlation between virulence and clinical origin [22]. Mutation rates in these strains were qualitatively assessed by observing the generation of spontaneously resistant 5-fluoroorotic acid (5-FOA) colonies (Materials and Methods). 5-FOA inhibits the growth of wild-type strains as 5-FOA is converted by orotine-5 ́-monophosphate decarboxylase to 5-fluorouracil (5-FU), which is toxic
This study has shown that mutations in MMR components in the human fungal pathogen C. neoformans result in an increase of over 200-fold in the mutation rate compared to the wild-type rate

Summary

Introduction

The ability to adapt to a changing environment provides a selective advantage to microorganisms. Mutation of MSH2, MLH1, and PMS1 (three C. neoformans genes from the mismatch repair [MMR] pathway predicted to be required to repair damage to single bases arising from errors in DNA replication) resulted in increased proliferation in a lung assay of cryptococcosis [13]. This suggests that single-base mutations arising from errors in DNA replication can generate novel traits that facilitate the ability to cause disease and provide an avenue for the pathogen to undergo microevolution during infection. The S. cerevisiae genome encodes six MutS homologues (MSH1 to MSH6) and four MutL homologues (MLH1 to MLH3 and PMS1); only MSH2, MSH3, MSH6, MLH1, and

Methods

Results

Conclusion