Biomedical Engineering Reference
In-Depth Information
variants of OATP1B1 (Phe73Leu, Val82Ala, Ile353Thr, and Gly488Ala) show re-
duced function for either estrone 3-sulfate or estradiol 17
-glucuronide (Table 21.1),
but their allele frequency is generally less than 2% in European Americans and African
Americans.
55
The concentrative and equilibrative nucleoside transporters have a relatively small
number of functionally significant variants. In a study of 14 amino acid variants of
CNT1 only two (Val385del and Ser546Pro) were unable to transport thymidine.
24
Despite normal levels of thymidine uptake by the Val189Ile CNT1 variant, differen-
tial interaction with gemcitabine was noted. Similarly, only one of 10 CNT3 variants
(Gly367Arg) showed loss of thymidine and inosine uptake.
18
,
20
Neither of the two
nonsynonymous ENT1 variants had altered transport, but a deletion variant of ENT2
(
845-846) was unable to transport inosine, guanosine, uridine, hypoxanthine, flu-
darabine or gemcitabine.
43
,
45
Reduced transport of these substrates was also noted
for the Asp5Tyr variant of ENT2.
45
Of the organic anion transporters, nonsynonymous variants have been character-
ized functionally for OAT1 and OAT3. The majority of OAT1 variants are able to
transport
p
-aminohippurate, ochratoxin A, and methotrexate to a degree similar to
that of the reference transporter, although the Arg454Gln variant of OAT1 has com-
plete loss of transport of these three substrates.
22
The transport of the antiretrovirals
adefovir, cidofovir, and tenofovir is impaired significantly with the Arg50His variant
of OAT1.
19
The Arg50His variant is found at 17% frequency in African Americans,
suggesting that it could have a significant effect on the disposition of these antiretrovi-
rals in this ethnic population. The uptake of estrone sulfate and cimetidine is abolished
in the Arg149Ser, Gln239del, and Ile305Phe variants of OAT3.
21
,
70
However, only
the Ile305Phe variant is found at a frequency of
1% (3.5% in Asian Americans), so
the clinical significance of these changes might be minimal.
The majority of polymorphic sites with functional consequences have negative ef-
fects on transport function; however, there are rare examples where variants increase
function. A nonsynonymous variant in OCT1 causes a Ser-to-Phe change at residue
14 (Ser14Phe) which was associated with a 75% increased uptake of MPP
+
and TEA
and a
>
twofold increase in Cl
int
(
V
max
/
K
m
) compared to the reference OCT1.
51
One
general assumption is that if an amino acid position is necessary for optimal protein
function, it should be conserved across species. An alignment with other mammalian
sequences orthologous to OCT1, as well as human OCT2 and OCT3, showed that
Phe14 is evolutionarily conserved except for in human OCT1.
51
Although the mecha-
nism for this increased function is unclear, the variation at a highly conserved residue
is consistent with the functional change observed.
The in vitro functional effects of a variant may not always be clear if a lim-
ited number of substrates are tested. As discussed earlier, a variant may show only
functional effects with specific substrates. One such example is OCTN1. Mam-
malian cell lines were transiently transfected with the reference and Leu503Phe
OCTN1 to see how multiple unrelated substrates affected transport.
46
Enhanced func-
tion with Phe503 OCTN1 was observed with tetrabutylammonium, tetraethylammo-
nium, and tetrapentylammonium; however, reduced function was seen with carnitine,
∼
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