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BRIEF REPORT
From the Academic Department of Haematology and
Cytogenetics, and the Section of Cancer Genetics, Institute of Cancer
Research, Sutton, Surrey, United Kingdom.
Interindividual differences in susceptibility to hematologic
malignancies may be mediated in part through polymorphic variability in
the bioactivation and detoxification of carcinogens. The glutathione S-transferases (GSTs) have been implicated as susceptibility genes in
this context for a number of cancers. The aim of this study was to
examine whether polymorphic variation in GSTs confers susceptibility to
chronic lymphocytic leukemia (CLL). GSTM1, GSTT1, and
GSTP1 genotypes were determined in 138 patients and 280 healthy individuals. The frequency of both GSTM1 and
GSTT1 null genotypes and the GSTP1-Ile allele
was higher in cases than in controls. There was evidence of a trend in
increasing risk with the number of putative "high-risk" alleles of
the GST family carried (P = .04). The risk of CLL
associated with possession of all 3 "high-risk" genotypes was
increased 2.8-fold (OR = 2.8, 95% confidence interval: 1.1-6.9). Our
findings suggest that heritable GST status may influence the risk of
developing CLL.
(Blood. 2002;99:4216-4218) B-cell chronic lymphocytic leukemia (CLL) is
the most common form of leukemia, accounting for around 30% of all
cases.1 There is increasing evidence that predisposition
to CLL involves both inherited and environmental
factors.2,3 It is likely that part of the inherited
susceptibility to CLL may be determined by interindividual differences
in the bioactivation of procarcinogens and detoxification of carcinogens.
The glutathione S-transferases (GSTs) are a superfamily of genes
whose products are phase II enzymes, catalyzing the conjugation of
reactive intermediates to soluble glutathione.4 GSTM1 and GSTP1 detoxify carcinogenic polycyclic aromatic hydrocarbons such as
benzo(a)pyrene, whereas GSTT1 is responsible for the detoxification of
smaller reactive hydrocarbons, such as ethylene oxide.4
Differences in the activities of some GSTs are determined by genetic
polymorphisms.4 GSTM1 activity is absent in ~50% of whites as a consequence of the inheritance of 2 null alleles (deletion of the gene). Similarly, GSTT1 activity is deficient in ~20% of whites, resulting from homozygous deletion. The GSTP
subfamily comprises only GSTP1. The 1578A>G substitution in
GSTP1 creates the Ile105Val polymorphism that
leads to expression of an enzyme with reduced activity.4
There is epidemiologic evidence that exposure to aliphatic
hydrocarbons and chlorinated hydrocarbons plays a role in the etiology of CLL.3,5-8 This, coupled with the proposed role of GSTs
in the etiology of a number of common cancers9 provides a
strong rationale for evaluating GSTM1, GSTT1, and
GSTP1 polymorphisms as risk factors for CLL.
Patients
Genotyping
Statistical analysis The relationship between GSTM1, GSTT1, and GSTP1 genotypes and risk of CLL was assessed by means of the odds ratio (OR) with 95% confidence limits calculated by logistic regression. GSTM1 and GSTT1 genotypes were classified as either null (homozygous deletion) or nondeleted. A test for trend (Ptrend) in increasing the risk of CLL by having more than one putative high-risk allele or genotype was evaluated by means of the chi-square test. The relationship between GSTM1, GSTT1, and GSTP1 genotypes and stage and white blood count was assessed by Anova. Departure in the distribution of genotypes from Hardy-Weinberg equilibrium was assessed by means of the chi-square test. A P value of .05 was considered statistically significant. All computations were calculated using the statistical software package STATA, version 6.0 (Stata Corporation, College Station, TX).
The frequency of the GSTM1 and GSTT1 null
alleles in the controls were 50% (135/270) and 23% (66/270),
respectively, which is in agreement with the previous documented
findings in white populations.4 The frequency of these
genotypes in CLL was 56% (77/138) and 30% (41/138), respectively
(Table 1). The distribution of
GSTP1 genotypes within cases and controls was not
significantly different from that expected under Hardy-Weinberg
equilibrium (P = .9 and .2, respectively). The frequencies
of GSTP1 heterozygotes and GSTP1 homozygotes in
controls were 38% (105/273) and 10% (28/273), respectively, also in
agreement with previous estimates.4 The frequencies of
these genotypes were higher in the cases, 46% (63/138) and 12%
(16/138), respectively (Table 1), but these differences did not attain
formal statistical significance. Sex- and age-adjusted ORs were no
different from crude ratios. In order to assess the existence of any
interaction between the 3 GST genotypes we calculated the frequency of
the simultaneous presence of the 3 putative "high-risk" genotypes.
Individuals carrying all 3 low-risk genotypes
Allelic loss in cells used in genetic analyses is a potential source of bias, as genotyping assays do not always distinguish between homo- and heterozygote states. An apparent increase in GSTM1 and GSTT1 homozygotes may be due to loss of heterozygosity of peripheral leukocytes used for DNA extraction. If this is the case, a relationship between white blood count and GST status should be detectable. There was no evidence for an association between GSTM1, GSTT1, or GSTP1 status and white blood count (P values .5, .7, and .4, respectively). The other potential source of bias is if a "case-case" effect is operating such that individuals with more advanced disease have a higher probability of having a "high-risk" allele. There was no evidence for such an effect as there was no relationship between GSTM1, GSTT1, or GSTP1 status and stage (P values .2, .9, and 1.0, respectively). Many studies have reported a relationship between GST variants and risk of a variety of common cancers including hematologic malignancies such as acute lymphoblastic and myeloid leukemia.8 However, only one study has examined specifically the relationship between polymorphic variation in GSTs and CLL.10 While this study failed to show a relationship between GSTM1 status and CLL, it was only based on 13 cases and hence was severely underpowered to detect a relationship on the basis of the probable genotypic risk associated with any common low-risk allele. In our study we found that carrying more than one of the putative high-risk GST genotypes significantly increases the risk of developing CLL, the risk being highest with possession of all 3 high-risk genotypes. It is conceivable that these variants will interact with environmental carcinogens, and certain combinations will better define at-risk groups. Information about exposure to environmental carcinogens was, however, unfortunately not available from either the cases or controls in our study to examine this possibility. While the risk of CLL associated with GST genotypes may be small and further studies are required to validate our observations, the high population prevalence of these high-risk alleles means that heritable GST status may make a significant impact on CLL incidence.
We acknowledge the assistance of Benjamin Hilditch and Andrea Marossy in the ascertainment and collection of patient samples. We would like to thank all the patients who took part in this study and their clinicians.
Submitted May 21, 2001; accepted January 6, 2002.
Supported by grants from the Leukaemia Research Fund (London WCIN 3JJ) and the Kay Kendall Leukaemia Trust (London EC4A 3FF).
The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked "advertisement" in accordance with 18 U.S.C. section 1734.
Reprints: R. S. Houlston, Section of Cancer Genetics, Institute of Cancer Research, Sutton, Surrey, United Kingdom SM2 5NG; e-mail: r.houlston{at}icr.ac.uk.
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© 2002 by The American Society of Hematology.
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