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Joseph L. Mailman School of Public Health of Columbia University, Division of Environmental Health Sciences, New York, New York 10032 [R. M. W., F. P. P., R. M. S.]; College of Medicine, Jagiellonian University, Krakow, Poland [W. J.]; Environmental and Occupational Health Sciences Institute, Piscataway, New Jersey 08854 [S. G.]; and National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709 [D. A. B.]
| Abstract |
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G
transition in nucleotide 313 of glutathione S-transferase
P1 (GSTP1), resulting in an ile105val substitution.
CYP1A1 catalyzes the bioactivation of PAH; the CYP1A1 MspI
RFLP has been associated with cancer of the lung. GSTP1 catalyzes the
detoxification of PAH; the val allele has greater catalytic
efficiency toward PAH diol epoxides. The study involves 160 mothers and
their newborns from Poland. Regression models controlled for maternal
smoking and other confounders. No association was seen between maternal
adduct levels and either polymorphism, separately or combined. However,
adduct levels were higher among newborns with the CYP1A1
MspI restriction site (heterozygotes and homozygotes combined)
compared with newborns lacking the restriction site (P = 0.06). Adducts were higher among GSTP1 ile/val and
ile/ile newborns compared with GSTP1 val/val
newborns (P = 0.08). Adduct levels were 4-fold higher
among GSTP1 ile/ile newborns having the CYP1A1
restriction site compared with GSTP1 val/val newborns who
lacked the CYP1A1 restriction site (P =
0.04). This study demonstrates a significant combined effect of
phase I and phase II polymorphisms on DNA damage from PAHs in
fetal tissues. It illustrates the importance of considering
interindividual variation in assessing risks of transplacental exposure
to PAHs. | Introduction |
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This study extends prior evaluations in the current cohort to consider the combined effect of phase I (CYP1A1 MspI RFLP) and phase II (GSTP1) metabolic enzyme polymorphisms on PAH-DNA adduct levels in WBCs of mothers and newborns. The cohort consisted of 70 mother/newborn pairs from Krakow, Poland [an industrial city with elevated ambient air pollution including PAH from coal-burning for industry and residential heating (19) ], and 90 pairs from Limanowa [a small town located 70 km southeast of Krakow with lower ambient pollution levels but 2-fold more frequent use of coal-stoves for indoor home heating (20) ]. We have reported previously on the effects of environmental exposures (cigarette smoke and ambient air pollution) on WBC PAH-DNA adduct levels in mothers and newborns (21 , 22) . Briefly, no difference was seen in adduct levels in mothers and newborns from Krakow compared with Limanowa, possibly because of higher indoor air concentrations of PAHs from coal burning in Limanowa (2) . Ambient pollution monitoring data were available for Krakow subjects only. Among Krakow subjects not employed away from home, a significant association was seen between ambient PM10 levels at the womans residence and adduct levels in both maternal and newborn WBCs. Maternal active and passive cigarette smoking status was significantly associated with maternal, but not newborn, adducts. Newborn adduct levels were significantly inversely associated with birth weight, length, and head circumference (23) .
CYP1A1 codes for an inducible enzyme system involved in PAH
biotransformation to epoxide-containing metabolites, some of which are
mutagenic and carcinogenic (24)
. An MspI RFLP
identified in the 3' noncoding region of CYP1A1 (the
CYP1A1 MspI RFLP) has been associated with cancer of the
lung in some, but not all, studies (reviewed in Ref. 25
). The
CYP1A1 MspI RFLP segregates in linkage disequilibrium with a
polymorphism in exon 7 that results in an ile
val substitution
in the catalytic region (26)
. Although the data are
inconsistent, the exon 7 polymorphism has been associated with
increased CYP1A1 induction or activity in several studies
(27, 28, 29)
. Prior evaluations of the association between
each of these polymorphisms and carcinogen-DNA adducts are limited, and
results are conflicting (30, 31, 32, 33)
. We have reported
previously that PAH-DNA adduct levels were significantly higher in
placental tissue of newborns from the current cohort who had the
CYP1A1 MspI restriction site (heterozygotes and homozygotes
combined) compared with newborns without the restriction site
(34)
.
GST consists of a superfamily of phase II enzymes that catalyze the
conjugation of reduced glutathione with electrophilic compounds,
including many environmental mutagens and carcinogens
(35)
. The currently identified cytosolic GSTs are
categorized into four main classes,
, µ,
, and
, based on
biochemical characteristics (36)
. Human
, µ, and
families contain multiple genes, whereas the
family consists of a
single gene, GSTP1. PAH epoxides are substrates for both
class µ and
GSTs (36, 37, 38)
. GSTP1 is widely expressed
in human epithelial tissue and is the dominant GST present in lung,
brain, esophagus, and erythrocytes (38
, 39) .
is also
the major GST expressed in fetal tissues including liver, lung, kidney,
and placenta (38
, 40, 41, 42)
. A coding sequence polymorphism
in GSTP1, an A
G transition in nucleotide 313, has been
identified. It results in a change in codon 105 from ile to val in the
hydrophobic binding site and impacts catalytic efficiencies
(43)
. The effect of the 105val allele appears
to differ by substrate. Compared with the GSTP1 ile allele,
the GSTP1 105val allele has decreased activity toward
1-chloro-2,4-dinitrobenzene (39
, 44
, 45)
but greater
activity toward PAH diol-epoxides (46, 47, 48)
. Thus, it has
been hypothesized that 105val homozygotes will be more
susceptible to certain mutagens/carcinogens but less susceptible to
PAH-induced DNA damage and carcinogenesis (46
, 47)
. Prior
data on the association between the polymorphism and cancer risk have
been conflicting. The 105val allele was associated with
increased risk of lung, upper aerodigestive tract, bladder, and
testicular cancers in some, but not all, studies (45
, 49, 50, 51, 52, 53, 54, 55)
. Among lung cancer patients, a significant association
was seen between the 105val allele and DNA-adducts measured
in lung tissue of current smokers by a method that detects a complex
mixture of aromatic and/or hydrophobic compounds (49)
. The
present study is the first to evaluate the association between the
polymorphism and PAH-DNA adducts in fetal tissue.
| Materials and Methods |
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A detailed, validated questionnaire administered to the mother within 2 days postpartum included information on smoking (active and passive), residential and employment histories, use of coal stoves for residential heating, and other environmental exposures. Data on the average number of weekly servings of specific PAH-containing foods consumed during pregnancy, such as broiled and smoked meats and fish and smoked cheese, were also collected. In addition, subjects were asked about exposure to sources of PAHs either at home or in the workplace as well as pesticides and other organic chemicals as potential inducers of CYP1A1.
PAH-DNA Adducts.
DNA was extracted from maternal and umbilical cord WBCs by standard
phenol/chloroform extraction and RNase treatment. Quantity obtained
ranged from 16 to 2900 µg DNA. PAH-DNA adducts were measured by a
competitive ELISA with fluorescence endpoint detection, essentially as
described previously (56)
. The detection limit of the
assay is 2 adducts per 108 nucleotides. Samples were
assayed in triplicate at 50 µg of DNA/well and 150 µg of DNA/plate;
the median values were used to determine the percentage of inhibition.
When sufficient DNA was available (63% of samples), the assay was
repeated. Laboratory personnel were blinded to subject status. The
antiserum was elicited against benzo(a)pyrene
diol-epoxide-DNA but recognizes other structurally related PAH
diol-epoxide-DNA adducts, including those formed by
benz[a]anthracene and chrysene (57)
. Thus,
positive reaction with the antiserum may indicate the presence of
multiple PAH-DNA adducts in the sample; values are expressed as the
amount of benzo(a)pyrene diol-epoxide-DNA that would cause a
similar inhibition in the assay. PAH-DNA adduct levels were determined
for 135 maternal and 135 umbilical cord WBC samples, including 112
mother/newborn pairs.
CYP1A1 MspI RFLP.
The CYP1A1 MspI genotype was determined using one of two
methods at either the New York University or National Institute of
Environmental Health Sciences laboratories. At New York University,
prior to the development of a PCR-based assay, high molecular weight
genomic DNA from placental villus (fetal) samples was digested with
MspI, and resultant fragments were electrophoretically
separated and visualized by audioradiograph after hybridization with
radiolabled cDNA probes (20)
. A simpler PCR-based RFLP
procedure was implemented at the National Institute of Environmental
Health Sciences laboratory utilizing DNA from umbilical cord and
maternal blood samples as described previously (20
, 34)
.
As a validation and quality control procedure, 131 samples were
analyzed by both methods, and the concordance was 100%. The
CYP1A1 MspI RFLP was determined for 142 mothers and 158
newborns.
GSTP1.
The GSTP1 (ile105val) genotype was determined by use of the
PCR-RFLP method of Watson et al. (39)
and
Helzlsouer et al. (35)
as described previously.
Briefly, genomic DNA (50 ng) was added to a PCR mix of GSTP1
primers 2306F (5'-GTA GTT TGC CCA AGC TCA AG) and 2721R (5'-AGC CAC CTG
AAG GGT AAG; 15 pmol each) and other PCR reagents as described
previously (35)
. PCR products were digested overnight with
the restriction enzyme Alw26I, which distinguishes between
the restriction sites on the ile allele (ATC) and the
val allele (GTC). For all genotype analysis, laboratory personnel were
blinded to subject status; photographs were interpreted by at least two
independent readers, and
10% of samples were tested a second time
as a quality control measure. The GSTP1 genotype was
determined for 142 mothers and 143 newborns.
Statistical Analyses.
PAH-DNA adduct levels were log-transformed to stabilize the variance
and obtain a more symmetrical distribution. For samples below the
detection limit (34% of maternal and 42% of infant samples), a value
of half the detection limit was assigned prior to transformation. Means
and SDs are presented as untransformed values for ease of
interpretation. Associations between adduct levels and the
CYP1A1 and GSTP1 polymorphisms were evaluated by
multiple linear regression. All models controlled for place of
residence (Krakow versus Limanowa), cigarette smoking
status, average number of servings per week during pregnancy of foods
high in PAH (smoked meat, cheese, and fish), use of coal stoves for
residential heating, and home/occupational exposures to PAH and other
organics (21)
. Maternal age was not included in the models
because it was not associated with PAH-DNA adduct levels in either
maternal or newborn WBCs. Ethnicity was not controlled for because the
Polish population is ethnically homogeneous and subjects were
predominantly Slavic. Associations of borderline significance (0.1 > P > 0.05) are reported but are considered
statistically significant at P
0.05.
| Results |
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Relationships between maternal and newborn WBC PAH-DNA adduct levels
and the combined CYP1A1/GSTP1 genotypes are presented in
Table 4
. There was no significant difference in maternal WBC PAH-DNA adduct
levels between any of the maternal genotype combinations (groups
IVI). However, PAH-DNA adducts were 4-fold higher (P = 0.04) among GSTP1 ile/ile newborns with the
CYP1A1 restriction site (group VI, the highest adduct group)
compared with GSTP1 val/val newborns who lacked the
CYP1A1 restriction site (group I, the lowest adduct group).
Compared with group I, adduct levels were also significantly higher
among GSTPI ile/val newborns with the CYP1A1 MspI
restriction site (group V, P = 0.02) and without the
CYP1A1 MspI restriction site (group II, P =
0.04). Compared with adduct levels in group I, newborn adducts were
also significantly higher in all other genotype groups combined (groups
IIVI; P = 0.03; Table 4
). There was no significant
difference in newborn adduct levels between any other newborn genotype
combinations. Nor was the association between the interaction term
(CYP1A1*GSTP1) and newborn adduct levels statistically
significant.
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| Discussion |
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The current study saw a significant effect of the combined CYP1A1 and GSTP1 polymorphisms on DNA damage in newborn WBCs, whereas there was no significant association between the polymorphisms and DNA damage in maternal WBCs. Possible explanations for this difference include the lack of expression in the fetus of the other class of GST (GST µ) capable of detoxifying PAHs (36 , 38) . GSTP1 is the major GST expressed in all fetal tissues tested, including fetal liver. The µ class genes are expressed rarely and not at all in fetal liver (38) . By contrast, GST µ genes are widely expressed in adult tissues, including the liver. Thus, there is greater redundancy in PAH detoxification capabilities in adults that may compensate for the effects of the CYP1A1/GSTP1 genotype.
Similarly, the lower DNA repair efficiency in the fetus relative to the adult (58, 59, 60) may render the fetus more sensitive to the effects of the polymorphisms. Our prior finding of higher PAH-DNA adduct levels in newborn WBCs compared with paired maternal WBCs (21) lends support to this hypothesis.
Although maternal adducts were not significantly associated with the CYP1A1 or GSTP1 polymorphisms, it is interesting that the genotype associated with the highest level of DNA damage in maternal tissue (the GSTP1 val/val with the CYP1A1 MspI restriction site absent) corresponds to the genotype associated with the lowest level of DNA damage in fetal tissues. The possibility exists that during human evolution, selection could maintain "deleterious" metabolism gene alleles in a population when they are protective during fetal development. Conversely, there may be combinations of maternal/fetal genotypes that result in a high-risk situation for the fetus if the mother is exposed to specific chemicals.
To our knowledge, this is the first study to demonstrate a significant combined effect of phase I and phase II polymorphisms on DNA damage from PAH in fetal tissues. As with any initial finding, the associations seen here require confirmation to rule out the possibility that they are attributable to chance or uncontrolled confounding. If real, it is likely that the genotype acts by modifying the relationship between PAH exposure and net DNA adduct formation. A limitation of the current study is that, because of the small sample size and limited ambient monitoring data, we did not have the power to test for effect modification. Nonetheless, these results suggest that cancer risks from transplacental exposure to PAH may be greater in a subset of infants with the combined phase I and II polymorphisms. They are of concern in light of the association seen previously between PAH-DNA adducts and cancer risk (18 , 61) and illustrate the importance of considering multigene effects on genetic damage from transplacental exposure to these common environmental contaminants. If confirmed, they have implications for risk assessment, which currently does not adequately take into account sensitive subsets of the population.
| Acknowledgments |
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| Footnotes |
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1 This work was supported by USPHS Grants
RO1CA-39174, RO1CA-35809, 1-PO1-ESO5294, RO1CA-53772, CA51196,
RO1-ES06722, RO1-ES08977, and P50-ES09600; Grant DE-FG02-93ER61719 from
the Department of Energy; a grant from the Gladys and Roland Harriman
Foundation; a grant from the March of Dimes Birth Defects Foundation;
and the Fogarty International Center (NIH). ![]()
2 These authors contributed equally to this
work. ![]()
3 To whom requests for reprints should be
addressed, at Joseph L. Mailman School of Public Health of Columbia
University, Division of Environmental Health Sciences, 60 Haven Avenue,
B-1, New York, NY 10032-4206. ![]()
4 The abbreviations used are: PAH, polycyclic
aromatic hydrocarbon; CYP1A1, cytochrome P4501A1; GST, glutathione
S-transferase. ![]()
Received 6/30/99; revised 10/28/99; accepted 11/24/99.
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-oxy-7,8,9,10-tetrahydrobenzo(a)pyrene. Cancer Res., 46: 2220-2224, 1986.
and
isoenzymes of glutathione S-transferase in developing human kidney. Biochim. Biophys. Acta, 990: 321-324, 1989.[Medline]
and
isoenzymes of glutathione S-transferase in human fetal lung: in utero ontogeny compared with differentiation in lung organ culture. Biochim. Biophys. Acta, 1037: 221-226, 1990.[Medline]
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