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Departments of Surgery [S. A. A.] and Biostatistics [M-J. A.], Medical College of Wisconsin, Milwaukee, Wisconsin 53226, and Departments of Otolaryngology-Head and Neck Surgery [J. T. C., L. W., J. J., D. S.] and Surgery [S. C. Y.], The Johns Hopkins School of Medicine, Johns Hopkins University, Baltimore, Maryland 21287
| ABSTRACT |
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| Introduction |
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Molecular epidemiological studies have begun to link specific environmental carcinogens with mutational events in cancer progression (9 , 10) . The p53 gene has been a common target of these mutational studies because it is the most frequently mutated gene in human cancer (10) . These studies have also shown that the p53 mutational spectrum of smoking-related neoplasms is distinctly different from that present in non-smoking-related cancers. Recently, benzo(a)pyrene diol epoxide, the active form of benzopyrene and a potent carcinogen in cigarette smoke, has been shown to bind preferentially to select regions of the p53 gene (11 , 12) . Formation of these benzopyrene adducts likely reflects the high frequency of certain p53 mutations in smoking-associated tumors and correlates with several known mutational hotspots in the p53 gene (12) . In addition, exposure to other environmental agents such as ethanol may further augment the mutational effect of tobacco smoke (13) . In head and neck squamous cell cancer, a cancer long-associated with both tobacco and alcohol use in epidemiological studies, the frequency of p53 mutations and overexpression is significantly higher in patients who smoked cigarettes and drank alcohol than in patients who only smoked cigarettes (13 , 14) . Although heavy alcohol consumption increases lung cancer risk, the effect of cigarette smoking and ethanol intake on the frequency of p53 gene mutations in lung cancer is not known. In the present study, we studied the effect of alcohol consumption and cigarette smoking on mutation of the p53 tumor suppressor gene in patients with non-small cell lung cancer.
| Patients and Methods |
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Demographic data were collected from patient interviews and a review of the hospital charts and from The Johns Hopkins Hospital Tumor Registry. Pathological stage was determined using the revised International System for Staging Lung Cancer (15) . Tumors from patients with squamous cell carcinoma or adenocarcinoma of the lung and a history of a second primary squamous cell or adenocarcinoma at a different site underwent careful pathological review of both cancers to exclude a pulmonary metastasis from a nonlung primary tumor. Four patients, in whom the possibility of metastases could not be excluded, were not included in the study. Demographic data were compiled by staff members who had no knowledge of the mutational analysis of the p53 gene in the patients tumor. This research protocol was approved by the Joint Committee on Clinical Investigation of The Johns Hopkins School of Medicine, and informed consent was obtained from all of the patients.
The history of alcohol consumption and cigarette smoking was obtained from patient interview and clinic notes or from the tumor registry. Patients were classified as alcohol drinkers if they consumed one or more drinks (one drink being defined as 1 ounce of 86-proof hard liquor, one 3.6-ounce glass of wine containing 12% alcohol, or one 12-ounce can of beer) per day on average during the 20 years before being diagnosed with lung cancer, as occasional drinkers if they consumed alcohol but less than one drink per day on average, and as nondrinkers if they consumed less than one drink per day or abstained from alcohol (13) . Nonsmokers were defined as patients who had smoked <100 cigarettes in their lifetime (4) . All of the smokers had at least a 10-pack-year history of smoking. Lifetime cigarette consumption was quantitated by averaging the number of packs smoked over the number of total smoking years (pack-years). Smoking patients were also stratified by the number of years they had quit smoking in 5-year intervals before the diagnosis of lung cancer or whether they were still smoking cigarettes.
Molecular Analysis.
Portions of the patients primary lung cancer were collected and
promptly frozen at -80°C. Sections of the primary tumor were stained
with H&E and examined by light microscopy. Tumors with low neoplastic
cellularity (<70%) were further microdissected to remove
contaminating normal cells, and DNA was isolated. A 1.8-kb fragment of
the p53 gene (exons 5 through 9) was amplified from the
primary tumor from the first 95 patients with a history of cigarette
smoking and in all of the 10 nonsmokers. Exons 5 through 9 of the
p53 gene were sequenced directly using cycle sequencing
(16)
. In addition, the same DNA isolated from all of the
105 tumors was further sequenced (exons 2 through 11) using the
GeneChip p53 assay (Affymetrix Inc., Santa Clara, CA) as
described previously (17)
. Discrepancies between these two
techniques (exons 5 through 9) were resolved by a third assay
(oligonucleotide-specific hybridization) as described previously
(17)
.
Statistical Methods.
Clinical and pathological characteristics between smokers and
nonsmokers were compared using
2 or Fishers
exact test for categorical variables and Wilcoxons nonparametric rank
test for continuous variables. The association between multiple
clinical and pathological variables and mutation of the p53
gene were examined by logistic regression. Statistical analysis was
performed using SAS system software.
| Results |
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Clinical characteristics for smokers (n = 95)
and nonsmokers (n = 10) with non-small cell
lung cancer are shown in Table 1
. Nonsmokers who developed non-small cell lung were significantly older,
more often female, and more likely to develop adenocarcinoma and had
smaller cancers than smokers. Nonsmokers with primary lung cancer were
also more likely to have a prior history of cancer [pancreas, breast,
colon, lymphoma, prostate, melanoma (excludes nonmelanoma skin
cancer)] than patients with a history of cigarette smoking (60
versus 24%; P = 0.02). A trend
toward earlier stage cancers was also observed in the nonsmokers.
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Molecular Analysis.
p53 sequence analysis was performed on tumor samples
from 105 patients with non-small cell lung cancer using direct
sequencing and the p53 GeneChip. At least one mutation of
the p53 gene was identified in 56 (53%) of 105 tumors. A
comparison between these two sequencing techniques has been reported
previously (17)
. One tumor contained two p53
mutations for a total of 57 mutations in 105 non-small cell lung
cancers (Table 3)
. The most common p53 mutations were GC
TA transversions
and GC
AT transitions. Only one p53 mutation was present
among the 10 nonsmokers, which precluded any comparison of mutation
spectra among nonsmokers and smokers. GC
AT transitions were more
common in alcohol drinkers [6 (14%) of 43] than in nondrinkers [2
(4%) of 54]; however, this difference failed to reach statistical
significance (P = 0.12). No other
statistically significant differences were observed among the mutation
spectrum from nondrinkers and the mutation spectrum from patients
consuming at least one drink per day (data not shown). Thirty-four %
of the p53 mutations included frame-shifts, deletions,
splice sites, or termination codons. These mutations would be predicted
to encode truncated p53 proteins that immunohistochemical
analysis usually fails to detect.
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Alcohol use, smoking history, and cell type were then examined in a
logistic regression model to define independent variables associated
with mutation of the p53 gene in non-small cell lung cancer.
A history of alcohol intake (
1 drink per day) was the variable with
the strongest statistical association with p53 mutations
(odds ratio, 3.9; 95% confidence interval, 1.510;
P < 0.005). Cell type was also significantly
associated with p53 mutations with squamous cell cancers
having a greater frequency of mutations than adenocarcinomas (odds
ratio, 2.9; 95% confidence interval, 1.27.2; P
< 0.02). In the multivariate analysis, smoking was associated
with the greatest overall risk (odds ratio, 6.5) of harboring a
p53 mutation in non-small cell lung cancer. However, smoking
was also associated with a large confidence interval, in large part
attributable to the low frequency of nonsmokers in the population and
the low incidence of p53 mutations and squamous cell
histology in this group of patients (95% confidence interval,
0.7358; P = 0.09). Therefore, we further
examined the role of smoking in mutations of the p53 gene in
patients with only adenocarcinoma of the lung (9 of 10 nonsmokers
developed adenocarcinoma). Mutations of the p53 gene were
present in 46% of smokers with adenocarcinoma (21 of 46) but in none
of the 9 nonsmokers with adenocarcinoma of the lung
(P = 0.016).
We also examined the quantitative effect of alcohol consumption and
cigarette consumption (in pack-years) on the p53 mutation
rate. Smoking intensity was weakly correlated with alcohol intake. The
median number of pack-years smoked (50 pack-years) among the alcohol
drinkers (
1 drink per day) was not significantly greater (Pearson
correlation coefficient, 0.10; P < 0.39)
than the median number of pack-years smoked (40 pack-years) among the
nondrinkers (
1 drink per day). The frequency of p53
mutations was significantly greater in alcohol drinkers than in
nonsmokers and nondrinkers, and this increase was present in both light
(<50 pack-years) and heavy (
50 pack-years) smokers (Table 5)
. There was also a trend toward an increased frequency of
p53 mutations in light smokers (<50 pack-years) who
consumed alcohol when compared with light smokers who consumed less
than one drink per day (82 versus 53%;
P = 0.07).
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1
drink per day) and smoked cigarettes contained p53 mutations
(31 of 41). In contrast, only 42% of tumors from nondrinkers (<1
drink per day) who smoked contained a p53 mutation (20 of
48), and only 14% of the nondrinkers and nonsmokers had a tumor with a
p53 mutation (1 of 7). Only one nonsmoker drank alcohol (wild-type
p53), and eight patients with an unknown history of alcohol
consumption (including two nonsmokers) were excluded from the combined
logistic regression analysis. The odds ratio for tobacco use but no
alcohol use versus no alcohol or tobacco use was 4.3 (95%
confidence interval, 0.838.4; P = 0.19),
alcohol and tobacco use versus no alcohol or tobacco use was
18.6 (95% confidence interval, 2.0174; P =
0.01), and alcohol and tobacco use versus tobacco use but no
alcohol use was 4.3 (95% confidence interval, 1.710.8;
P < 0.01; Fig. 1
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| Discussion |
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Alcohol consumption is an independent risk factor for cancers of the
esophagus, pharynx, larynx, and liver (18)
. A strong
synergism between alcohol and smoking exists for cancers of the head
and neck (18)
. Ethanol is not itself genotoxic, and
neither it nor alcoholic beverages are tumorigenic in animal models
(18)
. However, both tobacco and alcohol use are
independently associated with p53 mutations in head and neck
squamous cell cancers (13)
. Several mechanisms have been
postulated to explain how alcohol may potentiate the effect of
environmental carcinogens. In animal studies, alcohol has been
demonstrated to suppress hepatic clearance of low molecular weight
nitrosoamines present in cigarette smoke by inhibiting several isoforms
of the cytochrome P450 superfamily (18)
. This inhibition
then leads to an increase in the dose-delivery of nitrosoamines to
posthepatic tissues and an increase in the formation of DNA adducts
(18)
. Furthermore, acetaldehyde, the initial breakdown
product of ethanol inhibits the DNA repair enzyme
O6-methylguanine DNA methyltransferase
(19)
. If left unrepaired by
O6-methylguanine DNA
methyltransferase, O6-methylguanine
may pair with thymine during DNA replication. In the present study, a
nonsignificant trend toward an increase in GC
AT transitions was
observed among alcohol drinkers. The present study demonstrates that
alcohol may also play an important role in the pathogenesis of lung
cancer.
Prior studies that examined the effect of cigarette smoking on mutations of the p53 gene in various cancer types have yielded conflicting results (20, 21, 22, 23, 24, 25, 26, 27, 28, 29) . Several studies have used immunohistochemical analysis to evaluate the p53 protein; this method has substantial false-negative and false-positive rates when compared with direct gene sequencing (21, 22, 23) . In the present study, 34% of the 57 mutations would have resulted in a truncated p53 protein, which usually do not stain with labeled anti-p53 antibodies. Several reports have correlated the frequency of p53 mutations with lifetime tobacco exposure and have not included or have included very few nonsmokers to achieve a statistically meaningful result (24, 25, 26) . Other investigators have not demonstrated a statistically significant relationship between cigarette smoking and mutation of the p53 gene but have reported a relatively low frequency of p53 mutations in cigarette smokers (27 , 28) . We identified p53 mutations in 58% of the smokers in the current study using two complementary sequencing methods. Previous work in our laboratory (17) has demonstrated that the use of direct sequencing and the p53 GeneChip assay will detect more mutations than the use of either technique alone.
The pattern of mutations in the p53 gene varies among cancer
types and often provides clues to the pathogenesis of the tumor
(10)
. GC
TA transversions were the most common mutation
observed in the 95 smokers in this study and have also been commonly
reported in other smoking-related cancers (esophagus, head and neck;
Refs. 10
, 13
). These mutations are the hallmark of
mutagenesis involving certain types of polycyclic aromatic
hydrocarbons, such as benzopyrene (10
, 12)
. Mutations at
CpG dinucleotides also occurred commonly in our series (32%) and have
been reported in from 9 to 35% of non-small cell lung cancer patients
(10
, 25)
. Mutations at CpG dinucleotides are generally
presumed to be secondary to the endogenous deamination of methylated
cytosine, especially in colon cancer, in which 55% of mutations occur
at CpG sites (10
, 30)
. However, preferential binding of
several carcinogens, including benzo(a)pyrene diol epoxide,
has recently been demonstrated at methylated CpG sites corresponding to
mutational hotspots in the p53 gene (30)
.
Benzo(a)pyrene diol epoxide-induced DNA damage at these CpG
sites is poorly repaired and likely explains the high incidence of
mutations at these sites in non-small cell lung cancer
(31)
.
Non-small cell lung cancer occurs infrequently in nonsmokers and
has distinct clinical, pathological, and genetic features from cancer
in patients with a history of cigarette smoking. One limitation to this
study is the low number of nonsmokers with non-small cell lung cancer
available for analysis. Adenocarcinoma is more common in nonsmokers,
and, in our series, 90% of the nonsmokers had adenocarcinoma
(10
, 32)
. p53 mutations in non-small cell lung
cancer are also more common in large-cell and squamous cancers than in
adenocarcinoma. However, the frequency of p53 mutations in
smokers with adenocarcinoma was still significantly higher than in the
nonsmokers with adenocarcinoma (46 versus 0%). In the
current series, too few mutations occurred in the nonsmokers to analyze
the p53 mutational spectrum in this group. However, other
series have reported an increased frequency of GC
AT transitions and
deletions in nonsmokers (27)
. An increased incidence of
K-ras mutations and chromosomal loss at the FHIT
gene locus has also been associated with cigarette smoking (27
, 33) .
Loss of p53 tumor suppressor gene function allows the evolution of a clonal population of cells with a selective growth advantage that may result in cancer progression. Aberrant p53 expression has been documented in preneoplastic bronchial lesions and may be an important step in the progression of non-small cell lung cancer (34) . Moreover, p53 mutations may be associated with more aggressive behavior and shorter survival in non-small cell lung cancer (35) . Previous epidemiological data have strongly linked lung cancer with cigarette smoking and to a lesser degree heavy alcohol consumption. The present study did not examine the effect of cigarette smoking or alcohol consumption on the overall risk of developing lung cancer. However, our results demonstrate that alcohol consumption and cigarette smoking together increase the frequency of p53 gene mutations in patients with non-small cell lung cancer more than that observed with cigarette smoking alone. These data suggest that, although not necessarily mutagenic by itself, alcohol may potentiate the effects of other known carcinogens, thereby influencing the development and behavior of lung cancer.
| FOOTNOTES |
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1 Supported in part by the Lung Cancer
Specialized Programs of Research Excellence (CA-58184-03). ![]()
2 To whom requests for reprints should be
addressed, at Department of Otolaryngology-Head and Neck Surgery, The
Johns Hopkins School of Medicine, 720 Rutland Avenue, Baltimore, MD
21205-2196. Phone: (410) 502-5153; Fax: (410) 614-1411; E-mail: dsidrans{at}welchlink.welch.jhu.edu ![]()
Received 12/28/99. Accepted 5/ 2/00.
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