Biology Reference
In-Depth Information
The success of these will have a huge impact in transforming the current
ex vivo
assays into a more robust
in vitro
test that will permit its application
repeatedly to the same parasite isolate. However, despite its limitations, the
current schizont maturation assay has shown utility in discriminating para-
site populations with different degrees of chloroquine resistance (
Hasugian
et al., 2009
;
Suwanarusk et al., 2007
;
Chotivanich et al., 2009
), characteriz-
ing drug susceptibility profiles (
Chotivanich et al., 2009
;
Suwanarusk et al.,
2008
) and the screening of susceptibility to therapeutic agents (
Price et al.,
2010
;
Lek-Uthai et al., 2008
;
Marfurt et al., 2011a
,
2011b
). This work per-
mits definitive drug resistance phenotyping, and thus, the vitally important
search for its genetic determinants.
8.2.2. Genetic basis of
P. vivax
drug resistance
Preliminary studies on drug resistance in
P. vivax
have focused on the ortho-
logues of the transporter genes
pfcrt
and
pfmdr1
, known to be key determi-
nants in
P. falciparum
resistance. The role of
pvcrt-o
in chloroquine-resistant
P. vivax
is not immediately apparent.
Nomura et al. (2001)
compared
pfcrt
homologues from a number of species and although synteny was observed
between
P. falciparum
and
P. vivax
, they found no relationship between muta-
tions in
pvcrt-o
and the clinical. Using an experimental system, in which
pvcrt-o
was transfected into
P. falciparum
,
Sa et al. (2006)
demonstrated a
potential role for
pvcrt-o
in chloroquine transport across membranes, a thre-
onine mutation at position 76 in their
Dictyostelium
system being associ-
ated with impaired accumulation of chloroquine. However, the relationship
between
pvcrt-o
sequence and either clinical or
in vitro
phenotype was not
apparent in studies of more than 330 field isolates from Thailand, Indonesia
and Madagascar (
Barnadas et al., 2008
;
Suwanarusk et al., 2007
). An interest-
ing study of returning travellers with severe vivax malaria contracted from
the Brazilian Amazonia, found twofold increased
pvmdr1
copy number and
21-fold increased copy number of
pvcrt-0
compared to a patient with non-
severe vivax malaria (
Fernandez-Becerra et al., 2009
). No studies have eval-
uated the role of
pvcrt-o
amplification as a marker of CQR in this parasite.
More progress has been made correlating CQR
P. vivax
with single-
nucleotide polymorphisms of
pvmdr1
. Field studies of a large number of
clinical isolates demonstrated that aY976F
pvmdr1
polymorphism was pres-
ent in nearly all patients presenting to a clinic in Papua Indonesia, where
high-grade CQR is known to be present (
Suwanarusk et al., 2007
). The
same study showed that the same mutation was only present in 25% of iso-
lates in Thai isolates from an area where chloroquine retains high efficacy,