ORGANIC CATION TRANSPORTERS - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 26

2.7.2. OCT2

Human genetic variants of OCT2 have been investigated comprehensively by rese-
quencing of the coding region in a large ethnically diverse sample. 35 Eight of the
variants identified in this screen result in amino acid substitutions, and one single-
nucleotide insertion (134-135insA) leads to a premature stop codon at amino acid po-
sition 48. Four of the nonsynonymous variants (Met165Ile, Ala270Ser, Arg400Cys,
and Lys432Gln) were polymorphic, with ethnic-specific allele frequencies 1%. The

remaining four nonsynonymous variants (Pro54Ser, Phe161Leu, Met165Val, and
Ala297Gly) as well as the insertion variant were found on only one of 494 chro-
mosomes screened.
Of the four polymorphic OCT2 protein sequence variants (Met165Ile, Ala270Ser,
Arg400Cys, and Lys432Gln), all retained function as measured by uptake of the
prototypical organic cation substrate, MPP + ,in X. laevis oocytes expressing the
variant transporters. However, quantitative differences in MPP + uptake activity and
kinetic differences in interactions with organic cations were observed for the com-
mon variants, with Lys432Gln having twofold increased affinity for MPP + , and the
Met165Ile and Arg400Cys variants having lower maximal transport rates ( V max )
for MPP + than the reference OCT2. 35 In inhibition kinetics studies, Ala270Ser
showed an increased K i value for TBA, while Arg400Cys and Lys432Gln had
lower K i values for TBA inhibition than did the reference OCT2. When the rare
variants of OCT2 were expressed in the same system, all of the amino acid sub-
stitutions were found to have no effect on OCT2 function. 30 However, as ex-
pected, the single nucleotide insertion (frameshift) variant showed a complete loss of
function.
Additional single-nucleotide polymorphisms (SNPs) in the OCT2 gene were iden-
tified by direct sequencing of genomic DNA from 48 unrelated Japanese persons 73
and 116 arrhythmic Japanese patients. 74 In the former study, 27 SNPs and five dele-
tion polymorphisms in the OCT2 gene were identified. 73 Two synonymous variants,
also reported by Leabman et al., 35 were identified at amino acid positions 130 and
150. The remaining SNPs and deletion polymorphisms appeared in introns, 3 un-
translated regions, and 3 flanking regions of the OCT2 gene. In the latter study, 33
genetic variants, including 14 novel ones, were found. 74 Two nonsynonymous vari-
ants were identified (Thr199Ile and Thr201Met) and other SNPs and insertion and
deletion polymorphisms were located in exons, the 3 -untranslated region of exon 11,
introns, and the 3 -flanking region. The functional effects of these polymorphisms
have not been investigated.
In addition to direct sequencing of the OCT2 gene, other lines of evidence exist
for a role of genetic variation in OCT2 in variable drug response. Based on data
from published literature, an estimate of the genetic component contributing to vari-
ation in the renal clearance of metformin, which undergoes transporter-mediated
secretion, was found to be particularly high (

≥

90%). 75 This finding suggests that
variation in the renal clearance of metformin has a strong genetic component and
that genetic variation in OCT2 may explain a large part of this pharmacokinetic
variability.

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