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Advances in Brief |
Programs in Cancer Biology and Gastrointestinal Oncology, Divisions of Human Biology and Public Health Sciences, Fred Hutchinson Cancer Research Center, Seattle, Washington 98109 [L. J. P., C. A. S., P. C. G., B. J. R.], and Departments of Genetics and Medicine, Gastroenterology Division, University of Washington Medical Center, Seattle, Washington 98195 [B. J. R.]
| ABSTRACT |
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| Introduction |
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The p53 tumor suppressor gene, located on chromosome 17p13, is inactivated in Barretts esophagus by mutation of one allele and loss of the remaining 17p allele (17p LOH3 ; Refs. 4, 5, 6, 7 ). Abnormalities involving p53 (mutation; 17p LOH) are common in esophageal adenocarcinoma and have been detected in premalignant tissue surrounding cancer in esophagectomy specimens (4, 5, 6 , 8) . Although p53 has been studied extensively in patients who have already developed cancer, relatively little is known concerning the biological behavior of somatic p53-mutant clones in patients who have Barretts esophagus without cancer in vivo. In two patients, we recently demonstrated expansion of p53-mutant clones with 8- and 11-cm Barretts esophageal segments 6 and 3 years before the development of cancer (9) . This raises the possibility that p53-mutant clonal expansion is a frequent, early event in neoplastic progression in Barretts esophagus. However, there have been no systematic reports of the distribution of p53 mutations in the Barretts segment of patients without cancer. Therefore, we investigated p53-mutant clonal expansion in 213 mapped, flow-sorted, endoscopic biopsies obtained from 58 patients who had high-grade dysplasia without cancer in Barretts esophagus. Our results indicate that p53-mutant clones undergo expansion involving variable regions of esophageal mucosa, thereby creating an abnormal clonal field that can evolve increased 4N fractions and aneuploid cell populations in the absence of cancer.
| Materials and Methods |
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Flow Cytometric Sorting.
Neoplastic cell populations were purified from biopsies obtained at 2-cm intervals of the Barretts segment by sorting diploid G1, 4N, or aneuploid cell populations using Ki67/DNA content multiparameter flow cytometry (11)
. 4N fractions >6% were classified as abnormal (10)
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DNA Sequencing.
Using genomic DNA extracted from 10003000 sorted cells according to our protocols, exons 5 through 9 of the p53 gene were amplified by PCR as a single product according to published methods (11
, 12)
. PCR products were purified using Microcon-100 filters (Amicon BioSeparations, Beverly, MA) according to the manufacturers instructions. Using the purified PCR products, p53 exons 5 through 9 were amplified as three separate PCR products (exons 56, 7, and 89) and sequenced as described previously (4)
. All mutations were confirmed by at least two independent PCR reactions. Wild-type sequences were confirmed for all patients using constitutive samples.
Statistical Analysis.
Pearson
2 test for comparison of proportions was used to compare the prevalence of p53 mutations in diploid and nondiploid cell populations.
| Results |
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p53 mutations were more prevalent in patients with aneuploidy or increased 4N fractions but occurred in the absence of either flow cytometric abnormality. Aneuploid cell populations were present in 25 of the 58 patients (43%). Twenty-one of 25 patients (84%) with aneuploidy had p53 mutations compared with only 8 of 33 patients (24%) without aneuploidy (P << 0.001). Eight of the 33 patients without aneuploidy had increased 4N fractions, and four of these eight (50%) had p53 mutations. Thus, the prevalence of p53 mutations in patients with either aneuploidy or increased 4N was 25 of 33 (76%) compared with 4 of 25 (16%) in patients with only diploid cell populations (P << 0.001).
| Discussion |
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Cancers of the head and neck, lung, and esophagus appear to be associated with a field effect in which development of malignancy at one site is associated with an increased risk of a second cancer at another site (16, 17, 18, 19) . Although this is less frequently considered for Barretts esophagus, we and others have observed multiple neoplasms in the same patients Barretts segment (2 , 3) .4 The nature of these field effects is an important question in human neoplastic progression. A field exposure to a carcinogen such as tobacco smoke could result in polyclonal abnormalities (20 , 21) . Alternatively, mutant cells may spread to create an abnormal clonal field within which multiple cancers can develop (20 , 22) . The answer to this question will have implications for assessing risk of developing a second cancer, the pathogenesis of the development of second malignancies, and their treatment. Our study of p53 mutations provides evidence for a clonal field effect in the majority of cases and possible multifocal mutagenesis in a smaller number of cases. For example, we observed 12 cases in which a single p53-mutant clone expanded to involve the entire Barretts segment. In these cases, subsequent somatic genetic abnormalities developing in this p53-mutant clonal field could lead to cancer. Yet, we have also observed three cases in which multiple p53-mutant clones were present in the same patient. Such cases could provide evidence for multifocal carcinogenesis in the Barretts segment because each clone could have arisen independently and could progress to cancer. We cannot rule out the possibility of a common progenitor in which the p53 mutations developed as subclones. For example, p16 inactivation could precede the observed p53 abnormalities in these cases (9) . Regardless, the clonal expansion we have observed for p53-mutant cells has implications for therapy of high-grade dysplasia or early cancer in Barretts esophagus. These p53-mutant clonal fields may be responsible for the relatively rapid redevelopment of high-grade dysplasia in some patients, as well as the appearance of cancers, after endoscopic ablative therapy (23) .
Most of the mutations we detected were either confined to a single level or found throughout the Barretts segment (Fig. 1)
. The basis for this distribution is unknown. One possibility is that the mutations arose at different stages in the evolution of the Barretts segment. Thus, a mutation that developed as an early event might expand as the Barretts segment expands, whereas a mutation that developed after expansion of the Barretts segment might be more localized. Alternatively, expansion of p53-mutant clones within the Barretts segment might require a second hit. Thus, clones without the second hit would remain localized, whereas those with the second hit could expand rapidly throughout the Barretts segment. It is unlikely that loss of the remaining p53 allele provides the second hit because 94% of p53-mutant clones in this study had 17p LOH (data not shown).
p53 is inactivated in 50% of human cancers, and it is likely that p53-mutant clonal expansion is common to many human neoplasms. Our observations are consistent with case reports of p53 mutations or 17p LOHs found over extensive areas of other premalignant conditions (20 , 24) . p53-mutant clonal expansion has been well described in the progression of central nervous systems malignancies (22) . Studies of premalignant lung tissue from smokers or ex-smokers have demonstrated extensive expansion of p53-mutant clones as well as multiple independent clones in the same patient in the absence of cancer (20) . One study of smokers who developed multiple head and neck cancers reported polyclonal p53 mutations (21) . However, other genetic abnormalities, including 3p and 9p LOH, arise as early events during neoplastic progression in upper aerodigestive cancers, and it is possible these or other events represent a common progenitor to the later p53 abnormalities (25) . Our results indicate that p53-mutant clones can undergo extensive expansion, creating an abnormal clonal field of mutant cells that frequently involves the entire Barretts segment. This clonal expansion can occur in diploid p53-mutant clones that have not developed flow cytometric abnormalities or cancer. We also observed multiple distinct p53-mutant clones in a smaller number of cases. Patient 333 was interesting because four p53-mutant clones, each with mutations in exon 6, arose in close spatial proximity. Multiple p53-mutant clones have been described previously in patients exposed to field carcinogenesis (21 , 26) , and mutation clusters have been reported for the APC gene (27) . However, this case was an exception in our study, and the majority of cases provide evidence for simple clonal expansion. Our data are consistent with previous studies indicating that diploid p53-mutant clones are progenitors to neoplastic cell populations with increased 4N fractions, aneuploid DNA contents, and other LOH events (7 , 9 , 28) . Thus, clonal expansion of p53-mutant cells is an early event in neoplastic progression in Barretts esophagus that creates an abnormal clonal field in which other genetic abnormalities develop over a period of years before culminating in cancer.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported by NIH Grant RO1 CA61202. ![]()
2 To whom requests for reprints should be addressed, at Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North, P. O. Box 19024, Seattle, Washington 98109-1024. Phone: (206) 667-2875; Fax: (206) 667-6132. ![]()
3 The abbreviation used is: LOH, loss of heterozygosity. ![]()
4 B. J. Reid, unpublished data. ![]()
Received 6/24/99. Accepted 8/17/99.
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