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Fungal infections, Treatment of fungal infections - Coggle Diagram

- - - - Resistance to flucytosine
        
        preventing its transport into the cell (mutations in cytosine permease) or its intreacellular conversion to 5FUMP (mutations in cytosine deaminase or uracil phosphoribosyltransferase).
        
        Such mutations are not problematic for the fungi, because they can synthesize nucleotides de novo and do not depend on the salvage pathway.
    - - Resistance to Amphotericin B
        
        Resistence to polyenes is rare, but is observed in some fungi (Aspergillus terreus, Candida lusitaniae, Candida auris)
        
        Alterations in sterol biosynthesis (decreased ergosterol content) can result in reduced susceptibility to amphotericin B
        
        The main problem with amphotericin B is that its selectivity for ergosterol is limited. The drug is also highly toxic to human cells and has severe side effects.
    - - Resistance to Echinocandins
        
        Echinocandins are active against Candida and Aspergillus, but not against Cryptococcus.
        
        Mutations in β-1,3-glucan synthase that abolish drug binding are the main cause of acquired echinocandin resistance.
    - - Resistance to Fluconazoles
        
        Fluconazole is taken up into the cells by an unknown transporter and binds to its target enzyme sterol 14α-demethylase (Erg11, encoded by the ERG11 gene), thereby inhibiting ergosterol biosynthesis in susceptible cells.
        1.) Mutations in the target enzyme result in reduced drug binding
        2.) Overexpression of the ERG11 gene increases the amount of enzyme
        3.) Overexpression of genes encoding multidrug efflux pumps (Cdr1/Cdr2 or Mdr1) reduces the intracellular drug concentration
        
        molecular basis of overexpression of ERG11 and the multidrug efflux pumps
        
        Mrr1 (multidrug resistance regulator 1) positively controls the expression of the MDR1 gene as well as its own expression.
        
        MRR1 deletion in an MDR1-overexpressing isolate resulted in the loss of MDR1 expression and reduced fluconazole resistance.
        
        The sensitive isolate F2 (from the series presented before), which does not detectably express MDR1, contains two different MRR1 allleles. The MDR1-overexpressing resistant isolate F5 has lost allele 2 and is homozygous for a mutated form of allele 1 with a single nucleotide substitution. The resulting Pro-Ser exchange in Mrr1 renders the transcription factor constitutively active.
        
        Such gain-of-function mutations are found in all tested MDR1-overexpressing C. albicans isolates, and almost all of them are homozygous for the mutated MRR1 allele. The mutations are located in different regions of the transcription factor, and it is not clear how they activate it.
        
        Loss of heterozygosity (LOH) for a mutated MRR1 allele can occur by loss of the chromosome on which the wild-type MRR1 is located, and duplication of the homolog with the mutated allele (a) or by a mitotic recombination event that results in partial homozygosity, in the scheme shown above by a single cross over (b). Although local gene conversions affecting only a small region also occur, many LOH events comprise extended chromosomal regions or even multiple chromosomes. This issue will become relevant later in the lecture.
        
        Molecular basis of overexpression of other resistance genes
        
        Besides Mrr1, which regulates MDR1 expression, two other transcription factors are important in the development of fluconazole resistance in C. albicans. Tac1 regulates the expression of CDR1 and CDR2, which encode other multidrug efflux pumps, and Upc2 regulates the expression of ERG11 (encoding the drug target enzyme) and other ergosterol biosynthesis genes. Gain-of-function mutations in these transcription factors result in constitutive overexpression of their target genes and cause increased drug resistance. Again, fluconazole-resistant C. albicans isolates are often homozygous for the mutated alleles.
        
        The C. albicans genome comprises eight pairs of chromosomes. The locations of the genes that are relevant in fluconazole resistance are indicated in this schematic. Homologous chromosomes are distinguished by white and gray lines, and centromeres are indicated by the ovals.
        
        The mating type lous MTL is located on the left arm of Chr5, which also contains ERG11 and TAC1.
        
        A recombination event that results in homozygosity for the mutated TAC1 allele may also render the cell homozygous for the mating type locus, which is located close by.
        This means that especially cells that have acquired a resistance mutation might become able to switch to the mating-competent opaque state!
        
        Indeed, after passaging the heterozygous strain in the presence of fluconazole, we obtained derivatives that exhibited increased drug resistance and had become homozygous for the mutated MRR1 allele (which caused the increased resistance) and either MTLa or MTLα. Simultaneous loss of heterozygosity for different chromosomes can be due to mitotic problems (e.g. nondisjunction), which are also caused by fluconazole treatment.
        
        Summary
        
        Fluconazole treatment not only selects for resistance mutations but also promotes genomic alterations that confer mating competence, which allows cells in an originally clonal population to exchange individually acquired resistance mechanisms and generate highly drug-resistant progeny.