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Advances in Brief |
National Center for Toxicological Research, Jefferson, Arkansas 72079 [C. S., G. Y. M., A. S., B. F. C., F. F. K., C. B. A.]; Arkansas Cancer Research Center, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205 [C. S., G. Y. M., M. Y. F., S. K., L. F. H., C. B. A.]; and Department of Epidemiology, University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030 [P. A. T.]
| ABSTRACT |
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| Introduction |
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| Subjects and Methods |
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Genotyping.
Archived paraffin blocks from surgery were the source of tissue for
genotyping. Only women with normal tissue available for genotyping were
included in the study (n = 240). The majority
(76.4%) of normal tissue specimens were normal lymph nodes; the
remainder were skin or breast tissue. For each subject, 50-µm
sections were cut and placed in sterile tubes for DNA extraction.
Tissue was deparaffinized, and DNA was extracted using a commercial kit
(Qiagen). We detected the Ile105Val polymorphism
by a PCR-RFLP-based method that has been described previously
(8)
. Restriction enzyme BSmAI (New England
Biolabs, Hertfordshire, United Kingdom) was substituted for
Alw26I, resulting in an equivalent digestion.
Statistical Analysis.
Association between GSTP1
Ile105Val genotype and patient
characteristics were assessed by
2 test and
Fischers exact test. Survival and recurrence in relation to genotype
were evaluated using Kaplan-Meier survival function and Cox
proportional hazards models. For overall survival analysis, time from
diagnosis to death or last follow-up was calculated. Hazard ratios
estimated from the Cox model represent relative risk of death among
women heterozygous or homozygous for the variant GSTP1
Val105 allele compared with women
homozygous for the more common GSTP1
Ile105 allele. Hazard ratios were
calculated from the Cox model first by univariate analysis and then
from a multivariate model with adjustment for prognostic factors. In
the adjusted model, stage and node status at diagnosis (categories as
shown in Table 1
), and age at diagnosis (four categories: <40, 4049, 5069, and
70) were included as stratifying variables, and race (Caucasian or
African-American, excluding other or unknown) and estrogen and
progesterone receptor status were included as covariates. Trend was
evaluated using likelihood ratio tests comparing models with and
without a variable representing the number of variant alleles (0, 1,
and 2); reported Ps for trend tests are two-sided. For
analysis of disease-free survival, time from disease-free date to
recurrence, death, or last follow-up was calculated, and adjusted
hazard ratios were estimated from the Cox model, including prognostic
factors as described for analysis of overall survival.
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| Results |
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Characteristics of the study population are shown in Table 1
. The study
population included more women <50 years of age at diagnosis, and more
with stage at diagnosis above I than would be expected in an incident
case group. The overrepresentation of younger age and higher stage was
present among all breast cancer cases in the Arkansas Cancer Research
Center tumor registry. Women with these characteristics seemed to have
been more likely to be referred to this research hospital for treatment
than other incident cases. The distribution of GSTP1
genotypes was 48% GSTP1 Ile/Ile, 42%
GSTP1 Ile/Val, and 10% GSTP1
Val/Val among 192 Caucasian cases and 35% GSTP1
Ile/Ile, 56% GSTP1 Ile/Val, and 8%
GSTP1 Val/Val among 48 African-American cases.
Table 1
shows the distribution of demographic and pathological features
according to genotype. Pathological features did not differ
significantly by genotype.
The Kaplan-Meier function for survival by GSTP1 genotype is
shown in Fig. 1
. GSTP1 genotype was associated with overall survival in
analysis by Cox proportional hazards model, adjusted for age, race,
stage at diagnosis, and estrogen and progesterone receptor status.
Hazard ratios are shown in Table 2
. Compared with women with GSTP1 Ile/Ile genotype,
there was a trend of better survival (P = 0.04) with increasing number of GSTP1 Val
alleles. In the Kaplan-Meier plot, survival in the heterozygous
Ile/Val group was very similar to the homozygous
Ile/Ile women, and the hazard ratio from the univariate Cox
model for the Ile/Val group was 1.0. However, there was
confounding by stage at diagnosis. Nine of the 107 women in the
Ile/Val group but only 3 of 110 in the Ile/Ile
group were stage IV, so that after adjustment for stage, the hazard
ratio changed to 0.8. When the analysis was limited to deaths that
occurred within 3 years or within 5 years after diagnosis, adjusted
hazard ratios for the GSTP1 Ile/Val and
Val/Val genotypes were similar to those in Table 2
. When the
analysis was restricted to women treated by chemotherapy, hazard ratios
were essentially unchanged. Subgroup analyses were conducted by age and
by estrogen receptor status (Fig. 2)
. Although there was little statistical power for assessment of
survival differences by GSTP1 within subgroups, on visual
inspection of the Kaplan-Meier functions, it appears that the
association between the homozygous GSTP1
Val105 variant genotype and better survival
is present in each subgroup.
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| Discussion |
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Inherited differences in GSTP1 activity in removal of chemotherapy agents, and specifically of cyclophosphamide, is a plausible explanation for the association with survival observed in the present study, particularly in light of laboratory evidence that the GSTP1 enzyme exhibits specific activity in glutathione conjugation of cyclophosphamide intermediates (3) . When survival analysis in the present study was restricted to subjects who received chemotherapy, hazard ratios were essentially the same as those from the analysis including the total study population. GSTP1 activity in detoxification of base propenals (10) , products of hydroxyl radical reaction with DNA, may implicate GSTP1 in protection from radiation damage; however, it is unclear whether base propenals are of critical importance in the context of overall damage to cells from reactive oxidation (11) . Only 51 subjects in our study received radiation therapy only, and most of these were stage I patients who were alive and free of recurrence at the end of observation. Therefore, it was not possible to analyze survival according to GSTP1 status among the radiotherapy-only group or to evaluate statistically whether the association between GSTP1 genotype and survival differed by type of treatment.
GSTP1 expression in tumor tissue as measured by immunohistochemistry predicts poorer prognosis for cancers of several sites, e.g., ovary (12 , 13) . Studies of GSTP1 enzyme expression and prognosis in women treated for breast cancer (14, 15, 16, 17, 18) , however, have not provided consistent evidence of a relationship. Our finding of a survival difference in women with breast cancer according to host constitutive GSTP1 genotype is biologically plausible, even in the absence of a relationship between breast tumor GSTP1 expression and prognosis. Among individuals with similar levels of GSTP1 expression in tumor, enzyme catalytic activity would be expected to vary according to presence of variant GSTP1 genotype. Furthermore, GSTP1 is expressed in many other tissues, including liver and RBCs, and the GSTP1 genotype may modify the effective tumor dose of chemotherapy by altering systemic drug metabolism.
The association between GSTP1 genotype and survival that we observed remained after adjustment for age, stage at diagnosis, node status, race, and hormone receptor status, indicating that the association was not the result of racial variation, nor was the association attributable to relationships between GSTP1 genotype and one of these prognostic factors. In the present study, information on tumor grade, p53 expression, HER2/neu expression, and multidrug resistance protein expression was unavailable; therefore, we cannot exclude a relationship between one of these pathological features and GSTP1 genotype as a mechanism for the association between GSTP1 and survival. For example, Nedelcheva et. al. (19) reported that loss of heterozygosity at the p53 locus in breast cancer was more frequent among women with a GSTP1 Val105 variant allele. However, the relationship between GSTP1 genotype and one of these unmeasured prognostic factors would have to be quite strong to account for the observed association between GSTP1 and survival. Future research on metabolic polymorphisms and breast cancer prognosis should take into account molecular prognostic factors to consider whether genetic variation of the host acts independently, or through a causal pathway with other prognostic markers as intermediates, in predicting therapeutic response and survival.
In summary, our data indicate that host constitutional metabolic variability may greatly impact the efficacy of treatment for breast cancer. Although the role of genetic variability in metabolic enzymes has been studied extensively in relation to chemical carcinogenesis and cancer risk, there has been little attention to the impact of pharmacogenetics on response to treatment for cancer, particularly for breast cancer. Further research in this field could contribute to more individualized cancer treatment strategies in the future.
| FOOTNOTES |
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1 This research was supported by the Arkansas
Breast Cancer Research Program. C. S. and G. Y. M. were supported by
fellowships from the National Center for Toxicological Research/Oak
Ridge Institute for Science and Engineering. ![]()
2 To whom requests for reprints should be
addressed, at Arkansas Cancer Research Center, University of Arkansas
for Medical Sciences, 4301 West Markham Street, Slot 795, Little Rock,
AR 72205. Phone: (501) 296-1248; Fax: (501) 686-8297; E-mail: sweeneycarol{at}exchange.uams.edu ![]()
3 The abbreviations used are: GST, glutathione
S-transferase; CI, confidence interval. ![]()
Received 7/ 5/00. Accepted 8/22/00.
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