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Advances in Brief |
National Cancer Center Research Institute, Tokyo, Japan [T. A. D., S. H., T. N., J. Y., M. N.]; Istituto Nazionale Tumori, 20133 Milan, Italy [T. A. D., G. M.]; Japanese Red Cross Central Blood Center, Tokyo, Japan [T. J.]; Saitama Cancer Center Research Institute, Saitama, Japan [K. K.]; Kanagawa Cancer Center Research Institute, Yokohama, Japan [M. K., M. O-K.]; Hamamatsu University of Medicine, Hamamatsu, Japan [H. S.]; National Okinawa Hospital, Okinawa, Japan [K. G.]; and Aichi Cancer Center Research Institute, Nagoya, Japan [T. T., T. M.]
| ABSTRACT |
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1200 kb in the KRAS2 region. Allele-specific
oligonucleotide hybridization revealed the same
KRAS2/RsaI polymorphism associated with risk and
prognosis as in Italian lung ADCA patients; the polymorphism was
significantly associated with clinical stage
(P < 0.001) and survival rate (log
rank = 0.0014), confirming the mapping of PAS1 and
pointing to the role of this locus in human lung cancer. | Introduction |
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In an Italian population study, we demonstrated a significant association between genetic polymorphisms located in the human 12p12 chromosomal region, i.e., the region homologous to the Pas1 site in the mouse, and risk and prognosis of lung ADCA. However, population-based studies may be biased because of population admixture and stratification (4) . Thus, we have designed a case-control study in the Japanese population to determine whether a similar association exists between genetic markers and risk and prognosis of lung cancer. In lung ADCA patients, we found a significant association between clinical stage, prognosis, and the same marker polymorphism (KRAS2/RsaI) associated with lung cancer risk and prognosis in the Italian population. Pairwise LD between the biallelic markers used showed that LD can extend up to 200300 kb.
| Materials and Methods |
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Genetic Markers.
A pilot study to identify genetic polymorphisms was conducted on
selected Japanese cases and controls, using PCR primers allowing
amplification of 12-kb DNA fragments containing the genetic markers
of interest. Nucleotide sequences of the amplified fragments were
obtained using an ABI PRISM 377 automatic sequencer (Perkin-Elmer).
Nucleotide sequences were aligned and compared using the Genetics
Computer Group software package to identify putative polymorphisms. PCR
primers were designed to amplify DNA fragments of 100200 bp (Table 1)
, and the SNPs identified were tested by ASO hybridization in the
entire sample set, using the oligonucleotide probes reported in Table 1
.
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ASO Hybridization.
SNPs were detected by ASO hybridization. The PCR mix was denatured in
0.4 M NaOH/25 mM EDTA at room temperature and
spotted onto a nylon membrane. Fifteen-mer ASOs, including the SNP at
the central position, were 5'-end-labeled with
[
-32P]dATP (3000 Ci/mM;
Amersham, Branchburg, NY) and T4 polynucleotide kinase (New England
Biolabs, Beverly, MA). ASO hybridizations were performed in
tetramethylammonium chloride as described previously (6)
.
Statistical Analysis.
Molecular genetic analysis was performed without knowledge of the
clinical data, which were available after completion of the marker
analysis. Fishers exact test was used to evaluate LD in the case of
two alleles at both loci because it detects significance with a low
probability of false positives, and P < 0.001 was considered significant (7
, 8)
. Negative
logarithms of the statistical P values were used to present
the pairwise LD evidence more clearly. Allele and genotype associations
with lung cancer risk were also tested by the Fishers exact test. The
Kaplan-Meier product-limit method (9)
was adopted to
estimate survival functions. The null hypothesis concerning the
differential effects of genotypes on univariate (unadjusted) analysis
or after adjustment for gender and smoking habits (adjusted analysis)
was tested by means of the log-rank test (10)
, and
all P values were related to a two-sided significance test.
In addition, Coxs multiple regression analysis (11)
was
performed. The regression coefficients have been estimated by maximum
likelihood criteria, and their significance was tested by Walds test
(12)
. The relative risks reported in the text were
estimated as hazard rate ratios.
| Results |
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60% of
controls, which showed a distribution similar to that of ADCA
cases (Table 2)
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1200 kb; Table 3
|
20
kb. As expected, intragenic polymorphisms maintained a high
degree of disequilibrium (15)
. The two M4 polymorphisms,
located within a 1.3-kb DNA fragment that coamplifies with KRAS2 in
some tumors (16)
, also showed a highly significant LD
(-log P = 43). Pairwise LD results were in
agreement with the marker order predicted by the physical map and
showed that in our population sample, a statistically significant LD
can be detected up to 1 cR (200300 kb), as in the case of KRAG-M4
markers (-log P = 14.7; Table 3
Case-Control and Clinical Associations.
Allele frequencies at all loci were in Hardy-Weinberg equilibrium for
both patients and controls (data not shown). Analysis of association of
genetic markers with risk of lung cancer showed no statistically
significant association at any markers, even after adjustment for
gender and smoking history. However, genotype A2/A2 at marker
KRAS2/RsaI, i.e., homozygous for the presence of
the restriction site, was
2-fold more frequent in ADCA patients than
in controls (Table 4)
. In SCC, only three individuals carried the A2/A2 genotype, and no
significant difference from controls was observed.
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The median follow-up time for all survivors was 1490 days (range,
190-7027 days). All markers were tested for possible allelic
association with survival rate in ADCA patients and SCC patients
separately. In ADCA patients, a statistically significant association
was found only for marker KRAS2/RsaI; the A2 allele, which
was present at an excess frequency in ADCA cases as compared with the
general population controls, was associated with poorer survival
(P = 0.0014; Fig. 1
). The probabilities of survival at 1200 days were 0.58 ± 0.05 (n = 119), 0.30 ± 0.07 (n = 47), and 0.20 ± 0.18 (n = 5) for those with the
A1/A1, A1/A2, and A2/A2 genotypes, respectively (P = 0.0030), indicating a significant trend toward poor survival by
copy number of the A2 allele. Coxs regression analysis, including
genotypes, sex, and smoking habits, was carried out by resorting to a
backward procedure. Genotypes (relative risk, 1.8; 95% confidence
interval, 1.142.85; P = 0.01) and sex
(relative risk, 2.17; 95% confidence interval, 1.243.79; P = 0.006) remained statistically significant, whereas smoking
habits failed to reach statistical significance.
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| Discussion |
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2-fold more
frequent in the general population controls than in lung ADCA patients,
with a difference that was of borderline statistical significance
(13)
. The same A2 allele associated with a protective
effect on lung cancer risk was significantly associated with better
survival (13)
. In the Japanese population, there was a
2-fold excess frequency of the homozygous A2 genotype in lung ADCA
patients as compared with controls ("risk allele"), although this
was not statistically significant, due at least in part to the lower
frequency of the KRAS2/RsaI A2 allele in the Japanese
general population as compared with the Italian population. The A2 allele was seen more frequently in the general population than in cancer patients in the Italian population, but the opposite distribution was seen in the Japanese population; the reason for this difference may rest in the different creation of the LD between the KRAS2/RsaI marker and the PAS1 locus in the two populations. The A2 allele, in excess frequency in lung ADCA patients, was significantly associated with poor prognosis in the Japanese cases. Therefore, the same KRAS2/RsaI A2 allele showed a significant and consistent pattern of association with disease prognosis in lung ADCA patients from two unrelated populations. The maintenance of LD between a disease allele (PAS1) and the same marker allele (KRAS2/RsaI) in two unrelated populations provides strong support for the implication of the PAS1 gene in human lung tumorigenesis and suggests a close relationship between the loci of the disease and the marker.
In a study of LD in human populations, Laan and Paabo (17) showed that recently expanded populations, such as the Finns, are well suited to map rare single-disease genes affected by recent mutations, whereas populations that have been of constant size may be much better suited to map genes involved in complex traits caused by older mutations. Slatkin (8) reached the same conclusions, finding that there is a substantial probability of obtaining significant nonrandom associations between closely linked polymorphic loci in a population of constant size at equilibrium, whereas in a rapidly growing population, the probability of detecting significant LD is low, even between completely linked loci. Our results indicate that both the Italian and Japanese populations are well suited for mapping lung cancer predisposition loci, in agreement with the findings of other studies (18) .
Several studies have demonstrated a good correlation between physical
distance and LD (15
, 19
, 20)
. However, the correlation is
necessarily imperfect due to the type, age, frequency, mutation rate,
and so forth of the polymorphisms. In a survey of LD between
microsatellite loci spread over an anonymous genome region in the
Finnish population, LD was detected between loci separated by up to 1
Mb (19)
. However, the detection limit for LD is
50300
kb when only biallelic markers are used (15
, 20)
. In our
total sample set of 1022 Japanese individuals, the biallelic markers
showed a statistically significant LD with their flanking markers
located in close vicinity, i.e., significant LD was detected
up to a physical distance of 200300 kb (Table 3)
. Thus, in light of
the significant association between KRAS2 polymorphism and lung tumor
prognosis, the PAS1 gene is most probably located within 200300 kb of
this polymorphism.
No LD is detectable between genetic markers and a disease locus resulting from multiple mutations, such as the NF1 locus (15) . Our positive results with respect to the association between KRAS2 polymorphisms and risk/prognosis of human lung cancer suggest that the PAS1 susceptibility allele in humans results from a single or a predominant mutation/polymorphism. This prediction is consistent with an ancestral origin of the PAS1 susceptibility allele, which has been hypothesized based on LD analysis of mouse inbred strains (3) .
The putative PAS1 locus showed a more statistically significant LD with prognosis than with risk of lung ADCA in both the Italian and Japanese populations. Prognosis of lung cancer patients may be affected by several factors, including gender and smoking habits (21) . In our ADCA patients, PAS1 and gender, but not smoking habits, were significantly associated with survival. Based on these data, it is possible that the low statistical level of LD with lung cancer risk is due to a relatively high frequency of the susceptibility allele in the general population. The PAS1 susceptibility allele may determine the risk of lung cancer in association with environmental risk factors (e.g., smoking). Indeed, Sellers et al. (22) suggested the involvement of a major predisposition gene in the risk of human lung cancer, i.e., the susceptibility allele of the predisposition gene causing lung cancer by interaction with tobacco smoking. Also, interaction of the PAS1 susceptibility allele with lung cancer resistance (Par) loci may mask the relevance of the PAS1 locus in association studies, underestimating the role of PAS1 in lung cancer risk (23) . In any case, the putative human PAS1 locus showed a consistent association with lung ADCA prognosis, indicating a role for the PAS1 gene in lung tumor progression.
The use of several genetic markers over a short region in our population-based association study allowed us to estimate the physical distance between biallelic markers to which LD extends in the Japanese population and to confirm of the role and fine mapping of PAS1 in humans. The delineation of a short mapping region (200300 kb) for the PAS1 locus by LD provides the basis for the positional cloning of the PAS1 gene. Availability of the cloned gene will enable analysis of the type of mutation/polymorphism associated with the risk and prognosis of lung ADCA and study of the frequency of the disease allele(s) in different populations. Identification of individuals at genetic risk for lung ADCA as well as epidemiological studies to characterize the interaction of genetic and environmental factors will also be facilitated.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported in part by a Grant-in-Aid for
Comprehensive 10-Year Strategy for Cancer Control from the Ministry of
Health and Welfare of Japan, a grant from the Smoking Research
Foundation of Japan, and grants from Associazione and Fondazione
Italiana Ricerca Cancro (AIRC and FIRC) of Italy. T. A. D. was a
recipient of Foreign Research Fellowship of the Foundation for
Promotion of Cancer Research (Tokyo, Japan). ![]()
2 To whom requests for reprints should be
addressed, at Istituto Nazionale Tumori, Via G. Venezian 1,
20133 Milan, Italy. Fax: 39-02-2390764; E-mail: dragani{at}istitutotumori.mi.it ![]()
3 Present address: Kyoto University School of
Public Health, Kyoto, Japan. ![]()
4 The abbreviations used are: LD, linkage
disequilibrium; ADCA, adenocarcinoma; SCC, squamous cell lung
carcinoma; SNP, single-nucleotide polymorphism; ASO, allele-specific
oligonucleotide; RH, radiation hybrid; YAC, yeast artificial
chromosome. ![]()
Received 1/ 3/00. Accepted 8/ 2/00.
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