Biomedical Engineering Reference
In-Depth Information
rhodamine 123.
83
The Ser400Asn variant also has altered affinity to several protease
inhibitors.
82
Limited in vitro functional data exist for polymorphisms of
ABCC1
encoding
MRP1. In a single study the function of ten nonsynonymous variants were exam-
ined using three different MRP1 substrates. There appeared to be minor substrate-
dependent changes for some of the variants; however, only the Ala989Thr was deter-
mined to have a significant functional effect. Specifically, the 989Thr MRP1 showed
a 50% decrease in function when transporting estradiol 17
-glucuronide but had nor-
mal transport function with leukotriene C
4
and glutathione.
84
Amino acid 989 is at
the membrane interface of the seventh extracellular loop and may play an important
role in substrate interactions. The functional effects of a variant of amino acid 433,
located at the interface between the cytoplasm and the plasma membrane, have also
been investigated.
85
The Arg433Ser polymorphism occurs in the fourth cytoplasmic
loop close to the membrane interface of transmembrane domain 8. MRP1 was ex-
pressed transiently in HEK293 and HeLa cells, and membrane vesicles were used to
measure transport of leukotriene C
4
, estradiol 17
-glucuronide, and estrone sulfate.
The Ser433 MRP1 showed a 50% decrease in transport of leukotriene C
4
and es-
trone sulfate as well as a decrease in
V
max
for both substrates relative to the reference
transporter.
85
It is possible that the conversion from a positively charged amino acid
(Arg) to a neutral one (Ser) disrupted substrate affinity.
86
A deficiency in MRP2 function can lead to altered transport of conjugated biliru-
bin in the liver, resulting in the hyperbilirubinemic disease known as Dubin-Johnson
syndrome.
87
,
88
Certain mutations in
ABCC2
are thought to form an inactive protein
product and are regarded as the molecular basis of Dubin-Johnson syndrome.
89
-
91
In some cases, including the Arg768Trp and I1173Phe variants, loss of MRP2 trans-
port function is a result of defects in protein maturation and sorting to the apical
membrane.
90
,
92
In contrast, the Gln1382Arg and Arg1150His polymorphisms have no
effect on localization but disrupt the nucleotide-binding domain and ATP-dependent
transport of MRP2 substrates.
90
,
91
A rare mutation resulting in the Arg412Gly vari-
ant MRP2 has recently been associated with loss of methotrexate function in vitro
and higher plasma methotrexate levels in vivo.
93
The most common nonsynony-
mous variant of MRP2 described to date is a Val417Ile change found at a fre-
quency of 12 to 17% in major ethnic populations (see www.pharmgkb.org). A
comparison of reference and Ile174 MRP2 transport of estradiol 17
-glucuronide,
leukotriene C
4
, and 2,4-dinitrophenol-
S
-glutathione showed no significant changes in
function.
94
The ATP-binding domains for ABC transporters are important for function as
is evident in their high degree of conservation across subfamilies and species.
95
A
priori any modifications made to these regions would be predicted to alter energy-
dependent transport of substrates. Pseudoxanthoma elasticum is a rare heritable
disorder defined by the calcification of elastic fibers, and dozens of mutations in
ABCC6
(encoding MRP6) have been associated with pseudoxanthoma elasticum.
96
Many of the mutations cause nonsynonymous changes in or around the nucleotide-
binding domains (NBDs) of MRP6, which most likely produce an aberrant MRP6
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