MULTIDRUG RESISTANCE PROTEINS OF THE ABCC SUBFAMILY - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 294

11.7.3. Naturally Occurring Sequence Variants of ABCC1, ABCC3, ABCC4,
and ABCC5
Many nonsynonymous sequence variants are listed in the NCBI-SNP database and
have been identified by several groups in ABCC1, 21 , 295 , 296 , 306 , 325 − 328 ABCC3, 110 , 202
ABCC4, 21 and ABCC5. 21 , 209 , 295 To date, no human genetic disorders are known that
can be attributed to a deficient transport function of any of these efflux pumps. Knock-
out mice may help to identify human clinical conditions to which a deficient ABCC
transporter might contribute. For instance, Abcc1-deficient mice are hypersensitive
to etoposide, 176 Abcc3-deficient mice show altered morphine pharmacokinetics, 329
and Abcc4-deficient mice are hypersensitive to topotecan. 152
The expression of ABCC/MRP variants in mammalian cells is another valuable
tool to identify functional changes caused by amino acid substitutions. Whereas none
of the currently known ABCC4 21 and ABCC5 21 , 209 , 295 variants have been charac-
terized functionally in vitro, several studies have been performed with ABCC1 and
ABCC3 variants. Table 11.5 lists currently known nonsynonymous variants of hu-
man ABCC1 and ABCC3, their potential effects as predicted by the PolyPhen tool,
and functional consequences of those variants that have been characterized in vitro.
Most of the amino acid substitutions analyzed were predicted to be benign, and
many ABCC1 variants were proven to have transport properties similar to those of
the wild-type ABCC1. 328 Interestingly, in silico prediction and experimental results
were discrepant for several variants. ABCC1-p.C43S and ABCC1-p.R433S, consid-
ered as benign by the PolyPhen algorithm, resulted in impaired plasma membrane
localization and decreased doxorubicin resistance 330 and to decreased transport of
several organic anions and increased resistance to doxorubicin, 326 respectively. On
the contrary, because they are within NBD1 (Gly671 in ABCC1) or NBD2 (Arg1297
in ABCC3), substitution of amino acids that are highly conserved, was expected
to be damaging, but functional consequences were detected for neither the ABCC1
variant p.G617V 331 nor for the ABCC3 variant p.R1297H. 110 Although the ABCC1-
p.G671V variant is apparently functionally active in vitro, it may become deleterious
in combination with other, not yet known factors. This is suggested by two studies
which show that the variant is potentially under negative selection in four different
populations 327 and that it is associated with doxorubicin-induced cardiomyopathy. 332
Taken together, the analysis of phenotypic consequences of ABCC/MRP variants is a
challenging task and it is likely that many variants may have no clinical impact at all.
As for ABCC2, sequence variants have been introduced intentionally into the
ABCC1 and ABCC3 cDNAs. Their functional analysis has provided important insights
into the nature of the substrate recognition sites and the mechanism of transport
(recently reviewed in ref. 333).
11.8. CONCLUSIONS
Multidrug resistance proteins of the ABCC subfamily mediate the unidirectional ef-
flux of many endogenous and xenobiotic organic anions, including drugs, across
the plasma membrane. The ABCC/MRP transporters differ markedly from MDR1