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Cancer Prevention |
Authors' Affiliations: Departments of 1 Epidemiology, 2 Pathology, and 3 Head and Neck Surgery, The University of Texas M. D. Anderson Cancer Center, Houston, Texas
Requests for reprints: Qingyi Wei, Department of Epidemiology, The University of Texas M. D. Anderson Cancer Center, Unit 1365, 1515 Holcombe Boulevard, Houston, TX 77030. Phone: 713-792-3020; Fax: 713-563-0999; E-mail: qwei{at}mdanderson.org.
| Abstract |
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Experimental Design: In a case-control study, DNA samples from age- and sex-matched SCCHN patients (n = 818) and cancer-free controls (n = 821) were genotyped for the presence of three variants of TP53BP1 (T-885G, Glu353Asp, and Gln1136Lys) and three variants of TP53 (Arg72Pro, PIN3, and MspI). Multivariate logistic regression was used to assess the adjusted odds ratios (OR) and 95% confidence intervals (95% CI).
Results: Although none of these six genetic variants alone was associated with SCCHN risk, the combined TP53BP1 genotypes were associated with a significant, dose response–dependent decrease in SCCHN risk among carriers of TP53Pro72Pro, TP53PIN3del/del, and TP53Msp1AA genotypes (trend test: P = 0.024, 0.016, and 0.016, respectively). Furthermore, TP53BP1 variant haplotype GGC carriers who were also TP53 variant homozygotes had a significantly lower risk of SCCHN than did TP53BP1 haplotype TCA carriers (adjusted OR, 0.48; 95% CI, 0.25-0.94 for TP53Pro72Pro; adjusted OR, 0.17; 95% CI, 0.04-0.69 for TP53PIN3del/de; and adjusted OR, 0.16; 95% CI, 0.04-0.65 for TP53Msp1AA). There was statistical evidence of interaction between TP53BP1 and TP53 diplotypes (P = 0.017).
Conclusion: Our data suggest that TP53BP1 variants may have protective effects on SCCHN risk but such effects were confined to TP53 variant allele/haplotype carriers.
The tumor suppressor gene TP53 encodes a key cellular component that helps maintain genomic stability by (a) arresting the cell cycle long enough to allow DNA repair, (b) inducing apoptosis, or (c) both (13, 14). Somatic mutations that inactivate the TP53 gene have been found in at least half of all human tumors (15, 16), suggesting that loss of TP53 function plays an important role in carcinogenesis. These mutations may either be acquired or occur naturally in the form of common genetic variants, such as the nonsynonymous single nucleotide polymorphism (SNP) at codon 72 (Arg72Pro). The Arg72Pro SNP has been extensively studied for its association with cancer risk, although the findings have ranged from conflicting (17) to conclusive (17–27).
To function properly, the TP53 protein must interact with many other proteins. One of the TP53-regulated genes is TP53-binding protein 1 (TP53BP1) that encodes a nuclear protein of 1,972 amino acids that contains numerous phosphatidylinositol-like kinase phosphorylation sites (S/TQ) and two NH2-terminal BRCT motifs (28). TP53BP1 takes part in both DNA repair and cell cycle control and interacts specifically with the DNA-binding core domain of TP53 to enhance TP53-mediated transcriptional activation (29). It also helps mediate the DNA damage checkpoint by cooperating with damage sensors and signal transducers (30, 31).
TP53BP1 is polymorphic. Of over 178 SNPs reported to date, 70 are relatively common (e.g., minor allele frequency >0.05) but only 1 in promoter (i.e., T-885G) and 2 nonsynonymous (i.e., Glu353Asp and Gln1136Lys).4 A recent Chinese study of breast cancer (32) found variant genotypes of these three potentially functional TP53BP1 SNPs to be associated with increased breast cancer risk, particularly among TP53Pro72Pro homozygotes. However, this study did not include two other TP53 SNPs known to be associated with cancer risk: TP53PIN3 (a 16-bp insertion/deletion variant in TP53 intron 3 associated with lung cancer risk; ref. 24) and TP53MspI (a 1798G>A SNP in TP53 intron 6 associated with colon cancer risk; ref. 27). We hypothesized that interactions between variants of TP53BP1 and TP53 may collectively contribute to SCCHN risk. To test this hypothesis, we conducted a case-control study in which we genotyped TP53BP1 variants T-885G, Glu353Asp, and Gln1136Lys SNPs and TP53 variants Arg72Pro, PIN3, and MspI SNPs in SCCHN patients and cancer-free controls.
| Materials and Methods |
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Cancer-free control subjects were recruited from among visitors at the clinics of our institution. Excluded as potential control subjects were any persons genetically related to any enrolled case subject or to any other control subject. Potential control subjects were asked to complete a short questionnaire to (a) determine their willingness to participate in research studies and (b) obtain demographic information for frequency matching to the cases by age (±5 years), sex, and ethnicity.
After giving informed consent, all eligible subjects who agreed to participate were interviewed to collect additional information about risk factors, such as tobacco smoking and alcohol use. Each study subject had 30 mL of blood drawn for later biomarker testing. Because relatively few minority subjects were recruited in this study, only non-Hispanic whites were included in the current analysis. The research protocol was approved by the Institutional Review Board of The University of Texas M. D. Anderson Cancer Center.
Genotyping. Genotyping analyses were done on genomic DNA obtained from the study subjects. Six SNPs were targeted for analysis: TP53BP1 T-885G, Glu353Asp, and Gln1136Lys SNPs and TP53 Arg72Pro, PIN3, and MspI SNPs. The materials, methods, and PCR conditions for genotyping these six genetic variants have been described previously (24, 32). The laboratory personnel doing the genotyping analyses were blinded to the subjects' case-control status. Similar numbers of case and control DNA samples were assayed on each 96-well PCR plate. Approximately 10% of the DNA samples were reanalyzed, and the results of both sets of analyses were 100% concordant.
Statistical analysis. The case and control groups were compared in terms of selected demographic variables, smoking status, and alcohol use. Differences were evaluated using the
2 test. The associations between genotypes or diplotypes of the selected polymorphisms and SCCHN risk were estimated by computing the odds ratios (OR) and 95% confidence intervals (95% CI) from both univariate and multivariate unconditional logistic regression analyses. An analytic software program (PHASE 2.0; ref. 33) was used to infer haplotype frequencies based on observed genotypes. For each individual case or control, "diplotype" was defined as the most probable haplotype pair inferred using the PHASE 2.0 program. Potential gene-gene interactions were evaluated by logistic regression analysis and maximum likelihood testing as follows: the changes in deviance (–2 log likelihood) between the models were compared in terms of main effects with or without the interaction term. All statistical analyses were done using Statistical Analysis System software (v.9.1.3; SAS Institute).
| Results |
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The genotype frequency distributions of the six selected polymorphisms in the controls were consistent with the Hardy-Weinberg equilibrium: TP53BP1 T-885G (P = 0.311), Glu353Asp (P = 0.271), and Gln1136Lys (P = 0.650) and TP53 Arg72Pro (P = 0.411), PIN3 (P = 0.916), and MspI (P = 0.670). In the single-locus analysis (Table 1 ), none of the six variants was associated with SCCHN risk.
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| Discussion |
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That the effects of TP53BP1 on cancer risk might depend on the status of TP53 is biologically plausible. Studies of the association between TP53 polymorphisms and cancer risk have produced inconsistent results (34). It has been variously suggested that the wild-type TP53 Arg72 allele enhances tumor development (via increased inactivation of TP53) in cells expressing the mutated TP53 protein but inhibits tumor development (via increased apoptotic ability) in cells expressing the wild-type TP53 protein (34), that the TP53PIN3 variant causes alternative splicing and RNA instabilization (27), and that the TP53MspI variant influences apoptosis and cell survival (35). Together, these suggest that the joint effect of these three TP53 polymorphisms on cancer risk may be more significant than the individual effect of any one of them alone. Indeed, cells bearing the TP53 haplotype that contains all three variant TP53 alleles (Pro-del-A, the so-called "mutant" MMM haplotype) are associated not only with lung cancer risk but also with a decreased capacity for apoptosis and DNA repair (24). Therefore, the association of this Pro-del-A haplotype with decreased SCCHN risk and the interaction between the TP53BP1 variant diplotypes and the TP53 diplotypes that carry it suggest that the TP53BP1 variant alleles may evolutionarily compensate for the adverse effects of the TP53 variant alleles/haplotype. However, these findings need to be validated in larger studies. Additionally, we found no significant effect of the TP53 Pro-del-A haplotype by itself but a profound effect of the homozygous TP53 Pro-del-A genotype on the effect of the TP53BP1 diplotype on cancer risk.
It is known that the p53 codon 72 variants have markedly different apoptotic potential by differentially activating p53-responsive promoters. For example, the Arg72 allele has a higher apoptotic potential than the Pro72 allele because Arg72 has an enhanced association with MDM2 and CRM1 and a greater ability to localize to the mitochondria (36); earlier study also suggested that Arg72 allele enhanced the ability of mutant p53 to bind p73 and neutralized p73-induced apoptosis (37). Therefore, it is likely that TP53 and TP53BP1 variant allele or haplotype may enhance their protein interaction in the absence of TP53 mutations, a hypothesis consistent with a protective effect observed in this study. Larger studies are needed to test this hypothesis.
To date, only two case-control studies have investigated the role of TP53BP1 variants in cancer susceptibility. One was a relatively large German study of 353 breast cancer patients and 960 control subjects that found no overall association between four TP53BP1 SNPs (i.e., D353E, G412S, K1136Q, and 1347_1352delTATCCC) and breast cancer risk (38). The other was the Chinese study of 404 breast cancer cases and 472 cancer-free controls that found no significant main effect of any TP53BP1 genotype (of T-885G, Glu353Asp, and Gln1136Lys) or haplotype (except for GGC) on risk but an increased risk associated with the combined genotypes in TP53 variant Pro72Pro homozygotes only (32). However, we noticed that the allele/haplotype frequency distribution of both TP53BP1 and TP53 variant genotypes in the Chinese study differed dramatically from that in ours and that our study had a much smaller number of TP53 variant homozygotes and diplotypes containing two copies of the TP53 mutant haplotype, thus limiting the statistical power of our study. One possible reason for the difference in frequency distribution between studies is ethnicity. The distribution of TP53BP1 Glu353Asp genotype frequencies in our U.S. non-Hispanic white population (51.6% for CC, 39.3% for CG, and 9.0% for GG) was more comparable with that in the German case-control study (47.6% for CC, 42.5% for CG, and 9.9% for GG in controls; P = 0.233; ref. 36) than to that in the Chinese study (30.9% for CC, 50.9% for CG, and 18.2% for GG; P < 0.0001; ref. 32). The frequency distribution of TP53 Arg72Pro genotypes among the control subjects in our current study was also significantly different from that in the Chinese study (P < 0.0001; ref. 32). Another possible reason for the difference between the Chinese findings and ours is the type of cancer studied. SCCHN and breast cancer likely involve fundamentally different pathways of DNA repair. Indeed, the nucleotide excision repair pathway seems to be more relevant in the etiology of SCCHN (6, 7), whereas DNA double-strand break repair, as suggested by the roles of BRCA1 and BRCA2 (39), seems to be more relevant in the etiology of breast cancer (22).
In summary, our data from a relatively large, although racially homogenous, case-control population suggest that none of the TP53BP1 and TP53 SNPs we studied affects individually the risk of SCCHN. However, the data also suggest that gene-gene interaction between TP53BP1 and TP53 may alter the risk of SCCHN, a notion that warrants further evaluation in larger studies. In any case, our findings warrant functional elucidation of the six SNPs we have studied here to better understand the mechanisms underlying carcinogenesis in SCCHN.
| Acknowledgments |
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| Footnotes |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Note: Current address for K. Chen: Department of Epidemiology, Cancer Institute and Hospital, Tianjin Medical University, Tianjin, China.
4 http://egp.gs.washington.edu/data/tp53bp1/tpbp1.csnps.txt ![]()
Received 2/23/07; revised 4/10/07; accepted 5/ 3/07.
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