Biomedical Engineering Reference
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comparison between resistant and susceptible isolates also identified point muta-
tions in susceptible isolates as well, which were absent in their resistant counter-
parts and thus implied that these may not contribute to enhanced drug susceptibility
(Morio et al.
2010
). While mutations in Erg11 proteins are routinely reported, many
also remain to be confirmed, which could be established by expressing each variant
in a heterologous system. Some studies point out that a change from heterozygosity
to homozygosity for a mutated
ERG11
gene could also contribute to increased
resistance to drugs (Ge et al.
2010
).
2.2 Overexpression of P45014DM
Resistance to FLC in many clinical isolates is commonly associated with the
overexpression of the
ERG11
gene (Hoot et al.
2011
; Flowers et al.
2012
; Sasse
et al.
2012
). The zinc cluster transcription factor Upc2p regulates the expression of
ERG11
and other genes involved in ergosterol biosynthesis (White and Silver
2005
). It has been observed that an overexpression of
UPC2
increases azole
resistance, whereas its disruption results in hypersusceptibility to azoles. A com-
parison of sequence of
UPC2
between matched pair azole susceptible and resistant
isolates resulted in the identification of a base substitution causing a point mutation
in the encoded protein. This gain of function (GOF) mutation led to an
overexpression of
ERG11
and hyper-resistance to azoles (Heilmann et al.
2010
;
Hoot et al.
2011
; Flowers et al.
2012
). In addition, promoter deletion analysis
indicated that azole drugs induce the expression of
ERG11
through its azole
responsive cis-acting elements (ARE) in the promoter. The presence of ARE
alone was not able to stimulate reporter gene expression in the
upc2
Δ
/upc2
Δ
nulls, thus confirming that azoles manifest their effect on
ERG11
expression
through transcription factor (TF) Upc2p (Oliver et al.
2007
).
2.3 Alterations in Other Enzymes of the Ergosterol
Biosynthetic Pathway
Recently,
ERG3
mutations have been found to occur frequently either alone or in
combination with
ERG11
mutations, leading to a change in the ratios of various cell
sterols and increased resistance to azoles and polyenes. Mutations have been found
in
ERG3,
which are imperative in drug resistance even when drug efflux pumps
were not the causal factor (Martel et al.
2010a
,
b
; Morio et al.
2012
). The cyto-
chrome P450 spectral studies performed in a system reconstituted with purified
ERG5
(
22
-desaturase or CYP61) of
C. glabrata
revealed interactions between
azoles and the heme-protein, implying that
ERG5
could also be a target for azoles
and may contribute to antifungal resistance (Lamb et al.
1999
). Recently, a
Δ
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