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Short Communication |
Department of Genetics, Institute for Cancer Research, The Norwegian Radium Hospital, 0310 Oslo, Norway [A. L., I. R. K. B., A. B., T. I. A., R. A. L., A-L. B. D.]; Department of Oncology, Haukeland University Hospital, Bergen, Norway [P. E. L.]; and Institute for Forensic Medicine, The National Hospital, Oslo, Norway [T. O. R., G. I. M.]
| Abstract |
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| Introduction |
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2030% of the tumors and in colorectal cancer in 5060% [reviewed in Ref. 1
]. In addition to gene mutations, several reports have focused on TP53 polymorphisms as risk factors for malignant disease. Two of 14 known polymorphisms located in the TP53 gene alter the amino acid (International Agency for Research on Cancer TP53 Mutation Database).5 The alleles of the polymorphism in codon 72, exon 4, encode an arginine amino acid (CGC; Arg72) with a positive-charged basic side chain and a proline residue (CCC; Pro72) with a nonpolar-aliphatic side chain. Significant association between the codon 72 polymorphism and risk of cancer have been reported, although the results with regard to most cancer diseases, including breast (2, 3, 4) and colorectal carcinomas (5, 6, 7) remain inconclusive.
The Arg/Pro polymorphism is located in a proline-rich region (residues 6492) of the TP53 protein, where the Pro72 amino acid constitutes one of five PXXP motifs resembling a SH3 binding domain. The region is required for the growth suppression and apoptosis mediated by TP53 but not for cell cycle arrest [reviewed in Ref. 1 ]. The two polymorphic variants of wild-type TP53 have been shown to have some different biochemical and biological properties (8) such as different binding to components of the transcriptional machinery and different activation of transcription, but they did not differ in their ability to bind DNA.
The TP73 protein, a homologue of the TP53 protein, is able to activate TP53-responsive promoters and induce apoptosis in TP53-deficient cells. Marin et al. (9) recently showed that some TP53 mutants can bind to and inactivate TP73 and that the binding of such mutants was enhanced when the mutation occurred on the Arg72 allele. They also reported a higher frequency of TP53 mutations on the Arg72 compared with the Pro72 allele in different squamous cell cancers. These findings were supported by Tada et al. (10) , which found an overrepresentation of mutations on the Arg72 allele in tumors from different tissues. Interestingly, they found a preferential selection of the Arg72 allele in cancers with recessive TP53 mutants (mutants that do not inactivate wild-type TP53 in a dominant negative manner). It was suggested that recessive TP53 mutants achieve a selective growth advantage by an Arg72-dependent inactivation of TP73, whereas the dominant negative TP53 mutants inactivate the remaining wild-type TP53 allele in an Arg72-independent manner.
We have investigated whether somatic TP53 mutations exist in combination with a specific constitutional allele variant of the codon 72 polymorphism (Arg72 or Pro72) in a series of breast carcinomas and a series of colorectal carcinomas, which are known to have different TP53 mutation spectrum.
| Materials and Methods |
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Genotyping.
DNA from blood samples was analyzed for the genetic variation in codon 72 in exon 4 of the TP53 gene using Restriction Fragment Length Polymorphism analysis (15)
. Genomic DNA (50 ng) was amplified in 25 µl of PCR reactions (Eppendorf Mastercycler Gradient), containing 12.5 pmol of each primer (F: 5'-TTGCCGTCCCAAGCAATGGATGA-3', R: 5'-TCTGGGAAGGGACAGAAGATGAC-3'), 2.5 µl of 10x buffer (Gene Amp from Applied Biosystems, containing 100 mM Tris-HCl, 500 mM KCl, 15 mM MgCl2, and 0.01% W/v gelatin), 10 mM deoxynucleotide triphosphate, and 0.75 units of AmpliTaq DNA Polymerase (Applied Biosystems). A 199-bp fragment was amplified using a PCR program starting with denaturation for 3 min at 94°C, followed by 35 cycles of 15 s at 94°C, 15 s at 68°C, and 30 s at 72°C. Restriction analysis was performed mixing 8 µl of PCR product, 9 µl of H2O, 2 µl of 1x NEBuffer 2, 1 µl of (10 units/µl) BstUI (New England BioLabs), and incubated for 3 h at 60°C. Electrophoresis in 7.5% acryl amide gel gave an allele pattern of the two alleles of 113 bp +86 bp (Arg) and 199 bp (Pro), respectively (Fig. 1A)
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Cloning and Sequencing.
The cloning of the TP53 gene from tumor DNA was performed using the TOPO TA Cloning kit (Invitrogen). Four different fragments were designed to comprise the polymorphism in exon 4, as well as the respective mutation (Fig. 1B)
. DNA (<50 ng) was amplified in 25 µl of PCR reactions (Eppendorf Mastercycler Gradient), containing 12.5 pmol of each primer (for primer sequences see Table 1
), 2.5 µl of 10x buffer (Gene Amp; Applied Biosystems) giving a concentration of 1.5 mM Mg2+, 10 mM deoxynucleotide triphosphate, and 0.75 units of AmpliTaq DNA Polymerase (Applied Biosystems). The PCR program started with denaturation for 2 min at 94°C, followed by 35 cycles of 15 s at 94°C, 15 s at 63°C, 60 s at 72°C, and finally 10 min at 72°C. The PCR product was analyzed by gel electrophoresis (7.5% acrylamide) for quality check, then cloned into the pCR 2.1-TOPO vector and transformed into Escherichia coli according to standard protocols. The plasmid DNA was purified using QIAprep Spin Miniprep Kit (Qiagen), and the complete insert was sequenced (ABI 3100; Applied Biosystems) in overlapping fragments.
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2 test. Cross-tabulation and
2 test were performed when studying the polymorphisms association with TP53 mutations. Pearson
2 test or Fishers exact test (when appropriate) was used, and statistical significance level was set to P
0.05. Computations were performed using Excel (Microsoft Excel 97) and SPSS (version 8.0). | Results and Discussion |
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The observed skewed occurrence of somatic TP53 mutations on the Arg72 allele in breast carcinomas suggests that this combination gives breast epithelial cells a growth advantage, which may increase the risk of malignant transformation and development of cancer. The coexistence of the Arg72 with a mutation may modify the TP53 protein structure in a way that interferes either with the proteins ability to achieve sequence-specific binding to DNA or with the interaction and recruitment of the transcription machinery, causing an altered transcription pattern (18) . Another possibility is that the Arg72 may modify the mutant TP53 proteins ability to bind to and interact with other proteins such as, for example, TP73. Interaction between tumor-derived TP53 mutants and TP73 has been observed (19) , and the codon 72 polymorphism has been reported to be a modifier of such an interaction (9) , which may interfere with TP73-induced apoptosis.
The same level of skewed distribution of mutations residing on the Arg72 as seen for all type of mutations was also seen for missense mutations, giving no evidence for a stronger effect of such mutations. However, missense mutations are of many different types, and classifications according to structure or function in different cell types in larger series may give other results. The more severe changes like deletion, insertion, nonsense, and splice mutations may lead to a truncated protein or lack of protein where a codon 72 polymorphism has no modifying impact. Analyzing nonmissense mutations as one group with respect to the codon 72 homozygotes gave no skewed distribution (P = 0.710). The number is, however, small, and even truncated TP53 proteins may have an impact through mechanisms like inactivating other proteins (e.g., TP73) if their interacting domain is intact and the protein is stable. It cannot be excluded that the two polymorphic variants may have different effects also on such mutants.
Genotyping of the 162 colorectal cancer cases revealed allele frequencies of the Arg72 and Pro72 alleles of 0.75 and 0.25, respectively. The TP53 mutation frequency in this cohort was 48.1%. In contrast to the breast cancer cases, no difference in the frequency of mutations between the two different homozygotes was found in the colorectal cancer cases, with 40 TP53 mutations in 97 Arg72 homozygous cases (41.2%) versus 7 TP53 mutations in 16 Pro72 homozygous cases (43.8%). The spectrum of mutations is different between these two tumor types,6 partly attributable to tissue-specific differences in carcinogen exposure and in metabolism (reviewed in Ref. 1 ). Breast cancer is reported to have a high level of insertions, deletions, and nonsense mutations, and GC:AT transitions are the most frequent change, equally distributed between CpG and non-CpG areas, whereas colorectal cancer has a high frequency of CpG transitions leading to mutants with a presumable dominant negative effect (1) . A recent report divided the mutations into two groups according to their predicted dominant negative or recessive characteristics based on the results of a transactivation assay (9) , and the authors proposed that it was only the recessive mutations that preferentially was located on the Arg72 allele (10) . The dominant negative mutants were suggested to be independent of the codon 72 polymorphism. Using the same criteria for classifying the mutations as proposed by Tada et al. (10) on our series, the frequency of dominant negative mutants were higher in the colorectal cancer cases (83.3%, 30 of 36) than in the breast cancer cases (61.1%, 22 of 36; P = 0.064). Although only a minor fraction of our mutants (72 of 175) could be classified according to these categories, these results nevertheless support the hypothesis that a tumorigenic effect of the Arg72 allele only occurs when combined with a somatic mutation of the type seen in breast carcinomas. Additional studies, including functional assays, are warranted to explore the effects of the different combined variants and their role in tumorigenesis in different tissues.
| Acknowledgments |
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| Footnotes |
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1 This study was supported by grants from the Norwegian Cancer Society, The Research Council of Norway, and the Norwegian Womens Public Health Organization. A. L. is a research fellow of the Norwegian Cancer Society. ![]()
2 Present address: Department of Surgery, Akershus University Hospital, 1474 Nordbyhagen, Norway. ![]()
3 Present address: Department of Oncology, Ullevål University Hospital, 0407 Oslo, Norway. ![]()
4 To whom requests for reprints should be addressed, at Department of Genetics, Institute for Cancer Research, The Norwegian Radium Hospital, Montebello, 0310 Oslo, Norway. Phone: 47-22-93-44-19; Fax: 47-22-93-44-40; E-mail: a.l.b.dale{at}labmed.uio.no ![]()
5 Internet address: www.iarc.fr/P53/index.html. ![]()
6 Internet address: www.iarc.fr/P53/index.html. ![]()
Received 11/30/01; revised 7/26/02; accepted 9/16/02.
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