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From the Academic Unit and Polyposis Registry,*Saint Marks Hospital, Harrow; the Molecular and Population Genetics Laboratory
and the Human Cytogenetics Laboratory,
Cancer Research United Kingdom, London; the Mutation Detection Laboratory,
Cancer Research United Kingdom, Saint Jamess University Hospital, Leeds; and the Department of Clinical Genetics,¶Cancer Research United Kingdom, Guys Hospital, London, United Kingdom
| Abstract |
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FAP results from germline adenomatous polyposis coli (APC) mutations (Figure 1)
. Clinical data show that FAP patients with germline mutations at codon 1309 have exceptionally severe colonic polyposis.4
It is also well established that somatic APC mutations in colonic FAP polyps occur nonrandomly, usually being found in the colorectal mutation cluster region (MCR) (approximately codons 1250 to 1450).5
It is also notable that in FAP patients, the site of the germline mutation determines the type of somatic mutation (second hit) in colorectal polyps:6
germline mutations close to codon 1300 are associated with allelic loss and germline mutations elsewhere are associated with truncating somatic mutations in the MCR. Together with corroborative evidence from studies on sporadic colorectal tumors, these data show that simple inactivation of APC function is not optimally selected. Rather, mutations that truncate the protein close to codon 1300 provide the strongest selective advantage to the cell in which they occur.6
Such mutations result in stable, truncated APC proteins7
in which all of the SAMP repeatsinvolved in axin/conductin binding and ß-catenin degradationare lost and only 1 of the 20-amino acid ß-catenin-binding/degradation repeats is retained.8
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We have screened 49 duodenal polyps from 26 FAP patients for somatic mutations and allelic loss. We have determined whether or not the type of somatic mutation in upper gastrointestinal polyps depends on the site of the germline APC mutation. We have combined our data with those from previous studies to test whether or not there exists a somatic MCR for upper gastrointestinal tumors in FAP. Finally, we have extended our study to a much larger sample set to test the hypothesis that germline mutations within the putative upper gastrointestinal MCR are associated with more severe disease.
| Materials and Methods |
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50% neoplastic cells in each lesion, as far as morphological assessment allowed. All polyps that did not meet these criteria were excluded from the analysis. Paired constitutional DNA was derived from blood or normal colonic tissue. Extraction of DNA from the tumor and normal tissue was performed using the Qiagen (Hilden, Germany) Tissue Extraction Kit and from blood using standard methods. For assessing allelic loss at APC, polymorphic microsatellite markers close to the APC gene (D5S346 and D5S656) were chosen from public databases (for example, http://genome.ucsc.edu). The polymerase chain reaction (PCR) typically contained 20 to 100 ng DNA, 50 mmol/L KCl, 0.5 to 2.5 mmol/L MgCl2, 10 mmol/L Tris-HCl, 0.1% Triton, 2.5 µg of bovine serum albumin, 0.2 mmol/L of each dNTPs, 10 pmol of each oligonucleotide, and 1.25 U Taq DNA polymerase (Promega). The PCR reaction consisted of an initial step of 94°C for 4 minutes, then 40 cycles of 1 minute at 94°C, 1 minute at the appropriate annealing temperature, and 1 minute at 72°C in a PTC-225 Peltier thermal cycler (MJ Research). Microsatellites were analyzed for allelic loss using the Genotyper program (ABI). Allelic loss at each marker locus was considered to be present if the area under one allelic peak in the tumor was less than 0.5x or greater than 2x that of the other allele, after correcting for the relative allelic areas using the constitutional DNA.
Owing to relatively small amounts of DNA available from each polyp, APC mutation screening was restricted to regions F, G, H, I, and J of exon 15 (approximately codons 1147 to 1693). Each of these regions was amplified in the PCR using previously published primers13 (or slight modification thereof, details available from authors). Samples were screened for mutations by capillary based fluorescence-single stranded conformational polymorphism analysis (F-SSCP) using the ABI 3100 system. Briefly, fluorescently labeled oligonucleotides were used to generate labeled PCR products of which 1 to 2 µl were combined with 1 µl of size standard, 10.5 µl of deionized formamide, and 0.5 µl of 0.3 N sodium hydroxide. Samples were denatured and analyzed on 2% Genescan polymer with 10% glycerol at 18°C, 22°C, 26°C, 30°C, and 35°C. Data were analyzed using ABI Genescan and Genotyper software. Direct sequencing in forward and reverse orientations was performed on a new PCR product from those samples that showed a mobility shift or extra banding on F-SSCP, alongside a normal control.
For assessment of germline genotype-phenotype associations in duodenal FAP, data were collected from a series of patients with classical colonic disease, from age 25, who were undergoing routine upper gastrointestinal surveillance at St. Marks Hospital. Patients were examined with a side-viewing endoscope between April 1989 and March 2000. Duodenal severity was retrospectively assessed from the most recent endoscopy and histology reports and scored by Spigelman stage. We examined the relationship between carefully assessed esphago-gastro-duodenoscopy findings, patient demographic characteristics (age at endoscopy, sex), and site of the germline APC mutation. A group of 245 patients with classical colonic FAP, of whom 129 had known germline APC mutations, was analyzed; patients without identified mutations were included in the analysis because they provided useful information, for example as regards the age dependence of disease severity. Statistical computations were performed using STATA (Version 7.0; Stata Corporation, College Station, TX). We performed logistic regression analysis to examine the relationship between duodenal FAP severity (the response variable) and the explanatory variables age, sex, and APC mutation position. Age was computed as an integer, days of life. APC mutations were grouped by functional protein domain (pre-armadillo region, codons 168 to 453, n = 32; armadillo repeat region, codons 454 to 1019, n = 15; ß-catenin-binding region, codons 1020 to 1168, n = 27; post-ß-catenin binding, codons 1169 to 1250, n = 13; colorectal MCR, codons 1250 to 1400, n = 32; postcolorectal MCR, codons 1400 to 1580, n = 10) and then coded as binary dummy variables for computational analysis.
| Results |
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Using the analogy of colorectal polyposis, we then tested the hypothesis that somatic APC mutations in upper gastrointestinal FAP tended to cluster after codon 1400. Inspection of the location of somatic APC mutations reported in duodenal and gastric polyps14
showed clear clustering between codons 1400 and 1580 (Figure 3
, Table 2
); 74% of mutations occurred in this region, which we term the "upper gastrointestinal MCR." There was a highly significant difference between the distributions of mutations in tumors from the upper and lower gastrointestinal tracts (Table 2
, Figure 3
). This difference was also seen when gastric and duodenal tumors were compared separately with the colorectal tumors (Table 2)
. There was no significant difference between the location of mutations in gastric and duodenal tumors (Fishers exact test, P > 0.07).
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| Discussion |
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Analysis and interpretation of these data are inevitably subject to certain caveats. First, we have found an apparently low frequency of somatic APC mutations. Our data are, however, consistent with previous studies.16,17 The low mutation detection rate probably results in part from studying small lesions, which may have contained less neoplastic material than estimated by morphology (for example, sections taken may have been unrepresentative of the neoplastic content of the polyp as a whole, or normal crypts may have been entrapped within adenomas19 ) and from screening only part of the APC gene for mutations. However, F-SSCP is generally recognized to be highly sensitive and we detected all previously identified germline mutations and known common polymorphisms in the regions we analyzed. We cannot, therefore, entirely exclude the possibility that early FAP tumors may start to grow without a second hit.20
Second, many studies have been unable to analyze the whole of the APC gene for genetic changes and/or have focused on specific regions of the gene, with many studies of colorectal tumors failing to analyze mutations 3' of codon 1500. Moreover, different methods of mutation detection have been used for different regions of the gene (for example, protein truncation test for exon 15 and SSCP or denaturing gradient gel electrophoresis for other exons). Perhaps the best standardized comparison of colorectal and upper gastrointestinal tumors is provided by the data of Toyooka and colleagues12 and Miyaki and colleagues21 who screened for mutations in exons 5 to 9 and 13 to 15I in upper gastrointestinal tumors and, as far as can be ascertained, analyzed the same region by the same methods in colorectal tumors. These authors data show 73 somatic mutations in colorectal tumors before codon 1400, compared with 107 mutations after this codon; for upper gastrointestinal tumors, the numbers are zero and 47 mutations, respectively, a highly significant difference from the colorectal lesions (Fishers exact test, P < 7 x 10-10).
Third, we have combined data from duodenal and gastric tumors when analyzing first hit-second hit associations using previously published data. The very similar distributions of somatic mutations in gastric and duodenal tumors (Table 2)
support the use of this approach. Furthermore, we have combined data from gastric fundic polyps and gastric adenomas in our analyses, based on evidence that both these lesions harbor second hits at APC.2
Fourth, our use of codon 1400 as a proximal boundary for the MCR and for association with allelic loss is heuristic and inevitably an approximation, although it does correspond to a functional domain of APC. We have also made the plausible assumption that the distal boundary for somatic mutations APC is at approximately codon 1580, before the APC SAMP repeats, although some families with germline mutations distal to this site have developed severe duodenal disease.9
Fifth, the site of the germline mutation evidently only explains part of the variation in the severity of duodenal FAP; modifying genes, environment, and chance may also have effects.
Despite these reasons for caution, our data from upper gastrointestinal FAP tumors consistently show that the same region of the APC gene (approximately codons 1400 to 1580) forms the somatic MCR, harbors mutations associated with allelic loss, and contains germline mutations associated with severe disease. Although this region seems to be similar in upper gastrointestinal FAP tumors and desmoids,6 the corresponding region in colorectal tumors lies in the 5' direction, around codon 1300.4-6 We suspect that the reason for the different regions in colorectal and upper gastrointestinal tumors may lie in the effects of the resulting truncated proteins on ß-catenin levels within the cell.22,23 Tumorigenesis may benefit most from a level of ß-catenin that is raised above normal, yet is not excessive.22 It is likely that a truncated APC species with an extra ß-catenin-binding/degradation repeat, as would typify mutant APC in upper gastrointestinal polyps and desmoids relative to colorectal tumors, would result in a lower level of ß-catenin in the cell. Thus, the progenitor cells of in upper gastrointestinal polyps and desmoids would require a lower, specific level of ß-catenin for them to produce a tumor. Other explanations for the differing MCRs include effects on stability of the truncated protein,7 although explanations in terms of hypermutability of certain regions of APC24 seem less likely.
Associations between first and second hits seem, therefore, to be a general feature of the APC tumor suppressor gene. It is likely that this phenomenon results because different selective advantages are associated with different APC genotypes. Tumors with the optimal genotypes grow more rapidly and are more likely to come to clinical attention than tumors with suboptimal genotypes. As we have hypothesized for colorectal disease, one of the reasons for the greater severity of upper gastrointestinal disease in patients with mutations after codon 1400 may be that susceptible cells from these individuals can readily acquire a strongly selected genotype by allelic loss, which occurs, by chance, more frequently than specific truncating mutations after codon 1400.22 A particularly interesting aspect of investigating these phenomena and their causes will be to determine why tumors arising from different tissues show different patterns of APC mutation. The answer to this question may help to explain what is perhaps the great conundrum of tumorigenesis, namely why genes with widespread expression are only associated with tumors of specific sites.
| Acknowledgements |
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| Footnotes |
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C. G., H. L., and M. C. contributed equally to this work.
Accepted for publication March 5, 2002.
| References |
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