AUG 21, 2013 10:00 AM PDT

Azole resistance in Aspergillus fumigatus - clinical isolate screening, culture selection, and genetics

  • Professor, Director, Infectious Diseases Program, J. Craig Venter Institute (JCVI)
      Dr. William Nierman is the Director of the Infectious Disease Program at the J. Craig Venter Institute (JCVI). He is also a Professor the George Washington University School of Medicine and has taught Human Genetics at The Johns Hopkins University. He received his BS degree from the US Naval Academy and his PhD degree from the University of California, Berkeley. Dr. Nierman has broad experience in microbial pathogen genomics. His research focus is the genomic and functional analysis of two the levels of pathogen interaction with the human host, that caused by severe acute disease-causing bacterial pathogens, and that caused by fungi that can cause disease only in an immune-system-compromised host. Burkholderia mallei and Burkholderia pseudomallei are severe bacterial pathogens that cause difficult to diagnose but very life threatening diseases, glanders and melioidosis. At the other end of the pathogenicity scale are Aspergillus and Penicillium fungal pathogens which cause invasive or systemic disease in immune compromised or immune suppressed human hosts. Management of the disease in both classes of infections is becoming increasingly compromised by the rapid evolution of drug resistance in the pathogens. Both groups of organisms pose serious public health issues in both developed and in developing countries.


    In the United States, invasive aspergillosis (IA), an invasive fungal infection of the upper respiratory tract of immune compromised patients, is usually caused by Aspergilus fumigatus, while Aspergillus flavus represents only a small fraction of IA cases. In some countries A. flavus is the most frequent IA pathogen. Recently, we described a novel, highly virulent, aggressively invasive, and drug resistant IA pathogen, Aspergillus tanneri. In mouse models of IA and in a non-vertebrate insect model we observed distinct virulence profiles for the three Aspergillus species. Comparative genomics showed that A. tanneri had a larger genome than the other Aspergilli, encoding nearly 1900 more genes than A. fumigatus. A. tanneri genes had numerous orthologs in the other two genomes, however an abundance of genes are unique to A. tanneri. Among the unique genes were multiple gene clusters that encode biosynthetic genes for the synthesis of secondary metabolites, suggesting that A. tanneri produces novel secondary metabolites that may play a role in its high level of pathogenicity. Analysis of genes commonly associated with drug resistance showed that A. tanneri carried CYP51A mutations resulting in or contributing to azole resistance. An important issue featured in the A. tanneri fatal cases and in clinical management of IA is the general limitation in treatment options - only four classes of drugs are available in the context of ever-increasing drug resistance. Drugs used to treat fungal infections target only two differences between human and fungal cells: the presence of ergosterol in fungal cell membranes and of glucans in their cell walls. There remains an urgent need to understand the broad range of genes encoded in the genomes of fungal pathogens that participate in the resistance to the clinically therapeutic antifungals employed in treating infections. To identify novel mechanisms that mediate azole resistance in A. fumigatus, we used whole genome sequencing of in vitro selected azole-resistant strains. To further refine the most significant mechanisms required for resistance, we developed a genetic-sexual system that enables the analysis of complex traits in A. fumigatus and revealed that at least 6 mechanisms for azole resistance exist in this organism. These include mutations in the target protein, CYP51A, and in an additional co-target HMG CoA reductase. The results from this study identify novel drug targets in A. fumigatus and also show that next-generation sequencing coupled with classical genetics experiments is a powerful way to identify genes involved in complex traits.

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