MULTIDRUG RESISTANCE PROTEINS OF THE ABCC SUBFAMILY - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 289

disposition, and adverse drug effects). 1 One approach is to associate the frequency of
a sequence variant with the frequency of a specific trait in the population. 267 Using this
type of linkage analysis, a sequence variant of ABCC11 has recently been identified
that is associated with wet or dry earwax in humans. 268 Among others, the online tool
PolyPhen (polymorphism phenotyping, http://genetics.bwh.harvard.edu/pph) repre-
sents an approach to assessing the potential effects of single amino acid substitu-
tions. Based on multiple sequence alignments and information from known three-
dimensional protein structures, this algorithm predicts with some probability whether
an amino acid substitution has an impact on protein structure and function. 269 How-
ever, neither association studies nor in silico predictions can substitute for the ex-
perimental analysis of each amino acid variant to proof functional changes of the
respective ABCC/MRP efflux pump. Accordingly, several ABCC1 and ABCC3 vari-
ants have been characterized functionally by expression in mammalian cells (Section
11.7.3).

11.7.1. Hereditary ABCC2 Sequence Variants Causing
Dubin-Johnson Syndrome

An increasing number of sequence variants in the human ABCC2 gene have been
identified in patients with Dubin-Johnson syndrome (Table 11.4) since the initial
demonstration that the absence of a functionally active ABCC2 protein from the
hepatocyte canalicular membrane is the molecular basis of this hereditary disorder. 7 , 8
Dubin-Johnson syndrome was originally described in 1954 and characterized by the
presence of a dark liver and by conjugated hyperbilirubinemia, 9 , 10 because bilirubin
conjugates are effluxed from hepatocytes into blood via the basolateral ABCC3, which
compensates for the deficiency in ABCC2-mediated biliary elimination. 205 The liver
of persons affected by Dubin-Johnson syndrome appears dark blue or black, due to
deposition of a dark pigment in the pericanalicular area of the hepatocytes. 280 The
disorder is inherited in an autosomal recessive mode, 9 , 10 , 281 and its incidence ranges
from 1 : 1300 among Iranian Jews 281 and 1 : 300,000 in the Japanese population. 276
Many Dubin-Johnson syndrome-associated variants (Table 11.5) are single
nucleotide changes, resulting in premature stop codons, 156 , 270 , 274 , 282

amino acid

substitutions, 20 , 272 , 274 , 275 , 277 , 279 or alternative splicing. 20 , 271 , 275 , 276 The NCBI-SNP

database also comprises one entry (rs17222547:C A, dbSNP build 126) that predicts

a premature stop codon at codon 967; however, this variant has not yet been iden-
tified in patients with Dubin-Johnson syndrome. Other Dubin-Johnson syndrome-
associated variants include a 6-nucleotide deletion leading to the loss of two amino
acids from NBD2, 19 large deletions causing frame shifts and subsequent premature
stop codons, 273 , 274 and an in-frame deletion/insertion. 273 Premature stop codons may
lead to rapid degradation of the variant ABCC2 mRNA due to nonsense-mediated
decay , a mechanism that recognizes during translation whether a stop codon pre-
cedes the last splice site. 283 Actually, a truncated ABCC2 protein has not yet been
detected. 7 , 19 , 156 Other Dubin-Johnson syndrome-associated sequence variants result
in the synthesis of immature ABCC2 proteins that are recognized by the cellular

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