Annals of Oncology Advance Access originally published online on October 25, 2005
Annals of Oncology 2006 17(1):43-49; doi:10.1093/annonc/mdj017
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© 2005 European Society for Medical Oncology
Allelic loss analysis of early-stage flat-type colorectal tumors
Departments of 1 Surgery (I), 2 Gastroenterology and 4 Pathology (II), Juntendo University, Tokyo; 3 Division of Molecular & Cellular Medicine, Department of Molecular Genetics, Course for Molecular & Cellular Medicine, Niigata University, Graduate School of Medical & Dental Science, Niigata City; 5 Pathology and Clinical Laboratory Divisions, National Cancer Center Research Institute and Hospital, Tokyo, Japan
* Correspondence to: Dr H. Fujii, Department of Pathology (II), Juntendo University School of Medicine, 2-1-1 Hongo, Tokyo 113-8421, Japan. Tel: +81-3-5802-1039; Fax: +81-3-5684-1646; E-mail: hfujii{at}med.juntendo.ac.jp
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Abstract |
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Background: Flat-type colorectal tumors are rare, but are known for their unusual flat morphology and aggressive clinical behavior despite their small size. To identify distinct genetic alterations, loss of heterozygosity (LOH) analysis was performed on microdissected tissues.
Materials and methods: DNA was extracted from multiple microdissected foci in 43 cases of early-stage flat-type colorectal tumors and LOH analysis was performed on 2q, 4q, 5q, 12q, 14q, 15q, 17p, 18q, 18p and 22q.
Results: LOH patterns were detected in one of two forms: (i) homogeneous LOH throughout the microdissected foci, which indicated the early acquisition of LOH; and (ii) heterogeneous LOH, which were detected in a part of analyzed foci. Homogeneous and heterogeneous LOH were most frequently detected on 17p (92%) followed by 18q (81%), 18p (81%), 5q (61%), 22q (51%), 14q (44%), 15q (41%), 2q (39%), 12q (36%) and 4q (32%). Homogeneous LOH was detected most frequently on 17p (68%) followed by 18p (53%), 18q (53%), 22q (34%) and 12q (27%). The average fractional allelic loss (FAL) for heterogeneous and homogeneous LOH was 0.57 and the average FAL for homogeneous LOH was 0.37.
Conclusions: Early flat-type colorectal tumors frequently shows the early occurrence of multiple LOH including 17p, 18p, 18q and 22q, which is coupled with additional LOH of other loci either simultaneously or in the early clonal progression phase. The extent and sequences of LOH may be the mechanisms responsible for the aggressive clinical behaviors of these tumors.
Key words: flat-type colorectal tumor, fractional allelic loss, loss of heterozygosity, tumor suppressor gene, 17p, 18p, 18q, 22q
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introduction |
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Flat-type colorectal tumors are a rare but relatively aggressive morphological form of colorectal tumor. Despite their relatively small size, they often show high-grade dysplasia and progress relatively quickly to invasive cancer [1
–4]. Although few genetic studies of such flat tumors exist to date, several distinct genetic events and/or sequences of these genetic alterations have been proposed for this unusual form of tumor. For example, mutation of K-ras is infrequently detected in these flat tumors [5–8]. On the other hand, mutational inactivation of p53 as well as 17p LOH tend to occur frequently from an early stage [9–11]. Previously, we reported on the patterns of clonal neoplastic progression in such tumors. These tumors may show genetic progression and/or genetic divergence in flat mucosal lesions, in which some portions may progress to invasive foci. Genetic evidence for de novo type carcinogenesis has also been described [8, 9, 12]. Recently, we also reported on the frequent LOH of 3p and hypermethylation of RASSF1A in these tumors [13]. Methylation of RASSF1A may start in adjacent normal-appearing mucosa, indicating that different modes of Ras signaling abnormalities may characterize these tumors. To investigate other unidentified allelic changes and to search for other critical genes in such flat colorectal tumors, we initially screened the LOH of all chromosomal arms using two to three microsatellite markers/arms in 10 cases (data not shown). Chromosomal arms with more than 40% LOH except for 3p, namely 2q, 4q, 5q, 12q, 14q, 15q, 17p, 18q, 18p and 22q, were further assessed for detailed allelic states in 43 tumor cases. Chromosomal arms with 30–40% LOH in 10 cases included 2p, 6q, 7q, 8q, 9p, 9q, 10q, 11q, 17q, 20q, 21q and 22p. Other chromosomal arms (1p, 1q, 3q, 4p, 5p, 6p, 7p, 8p, 10p, 11p, 12p, 13p, 13q, 14p, 15p, 16p, 16q, 19p, 19q, 20p and 21p) showed less than 30% LOH in 10 cases. These chromosomal arms were excluded from the present study. We microdissected multiple foci from each case and submitted them for LOH analysis. In this way, we evaluated homogeneous LOH throughout the microdissected foci, which are indicative of the early occurrence of LOH as well as heterogeneous LOH, which occur during neoplastic progression/divergence in the mucosal and submucosal foci. 17p LOH was most frequently detected at an early stage as previously reported, along with LOH of 18q, 18p and 22q, indicating the involvement of unidentified critical tumor suppressor genes on these chromosomal arms relatively early in this form of colorectal tumor. The high rate of multiple LOH loci with high fractional allelic loss in these early stage tumors may be the genetic basis of their aggressive clinical behavior.
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cases
Paraffin-embedded archival pathology blocks of flat colorectal tumors were obtained from the Pathology Department of Juntendo University Hospital, and the Clinical Laboratory Division of the Cancer Center Hospital, Tokyo, Japan. These cases were initially tested for their quality of DNA preservation for adequate and reproducible PCR amplification. Forty-three cases were submitted for genetic analysis, consisting of 39 males and four females. The patient age ranged from 44 to 88 years, with a mean age of 65 years. The tumor was located in the colon in 29 cases and in the rectum in 14 cases, and their sizes ranged from 4 to 25 mm with an average width of 11 mm. A lesion was defined as being flat when its dysplastic mucosal thickness was less than twice the thickness of adjacent non-neoplastic mucosa. Since histopathological terminology for intramucosal neoplastic lesions is different between Japanese and Western pathologists, and the term intramucosal carcinoma, commonly used in Japan, is not widely used in the West, we used the term high-grade dysplasia (HGD) for intramucosal highly dysplastic lesions. Tumors with submucosal invasive foci were considered definite for invasive carcinoma. When low-grade flat dysplasia was detected, we classified them as flat adenomas. Eight tumors showed high-grade dysplasia (HGD) without submucosal invasion, and the remaining 35 cases showed focal submucosal invasion, indicating that they were unequivocal invasive cancers. No tumors showed invasion to or deeper than the muscularis propria. Submucosal invasion was graded from sm1, shallow, to sm3, deep and close to the muscularis propria. Portions of low-grade flat adenomas were identified in six cases. Table 1 summarizes the clinicopathological profiles together with the results of the LOH analysis.
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DNA extraction
For each case, appropriate tissue blocks were selected and sections 8 µm thick were cut, deparaffinized, and stained with hematoxylin and eosin. For LOH analysis, tumor portions were microdissected using a 27-gauge needle under an inverted microscope. A laser-assisted microdissection system (Leica laser microdissection systems; Leica Microsystems, Wetzlar, Germany) was also used. At least 95% of the microdissected cells were tumor cells. These highly purified samples aided the subsequent detection of allelic losses without contamination by normal DNA. For each case, two to seven foci were selected. Non-malignant stroma and inflammatory cells were microdissected, collected separately and used as normal control DNA. The microdissected tissue was digested overnight at 50°C in buffer containing 0.5% NP-40, 50 mmol/l Tris–HCl pH 8.0, 1 mM EDTA and 200 µg/ml proteinase K. The lysates subsequently were heated at 95°C for 10 min, and then stored at –20°C until use in PCR.
microsatellite markers
PCR amplification was performed using primers for 2q [D2S102 (2q35-q36), D2S95 (2q32-q33), D2S72 (2q32-q34), D2S127 (2q22-q23), D2S206 (2q37) and D2S2304 (2q22)], 4q [D4S175 (4q28-q31.3), D4S1544 (4q22-q23)], 5q [D5S346 (5q21-q22), D5S2072 (5q11.2-q12), D5S1956 (5q11.2-q12), D5S644 (5q15)], 12q [D12S84 (12q24.1), D12S129 (12q24.2)], 14q [D14S1026 (14q23), D14S63 (14q23)], 17p [CHNRB1 (17p12), D17S578, D17S786 (17p12), D17S796 (17p13), D17S1832 (17p13)], 18p [D18S59 (18p11.3), D18S63 (18p11.3), D18S452 (18p11.3-p11.2)], 18q [D18S474 (18q21), D18S487 (18q21), D18S55 (18q22)] and 22q [D22S430 (22q12), D22S684 (22q12), D22S270 (22q13), D22S274 (22q13)]. Primers for 5q were obtained from Research Genetics (Huntsville, AL, USA). All other primers were designed in our laboratory, and their sequences are available upon request.
analysis for LOH
The PCRs contained 1 µl of DNA lysate, 0.4 µm [
-32P]ATP-radiolabeled microsatellite primers, 0.2 mM dNTP, 10 mM Tris–HCl pH 8.3, 1.5 mM MgCl2, 50 mM KCl and 0.4 U Taq polymerase in a reaction volume of 10 µl. Taq was added to reactions pre-warmed to 94°C (hot-start PCR) and the samples were amplified over 35 cycles. The PCR products were then separated on a 5% denaturing polyacrylamide-urea-formamide gel. Next, the gel was dried, the signals were visualized using an image analyzer (Bas 2500; Bio Imaging System, Fuji Film, Tokyo, Japan), and LOH was determined based on a reduction of over 75% of the relative intensity in one of the two alleles compared with those of the normal controls. When only a single allele was detected for a particular marker, we scored it as non-informative. When two alleles were too closely located to precisely determine retention or LOH for some markers, we scored them as non-informative as well. Microsatellite instability (MSI) was noted when a new allele appeared in the tumor tissues compared with the normal controls. MSI-positive cases were also determined by at least two of the five recommended markers (BAT25, BAT26, D2S123, D5S346, D17S250) showing altered alleles as compared with the normal control DNA. LOH and MSI were confirmed by repeating PCR in triplicates as well as using multiple informative markers.
homogeneous versus heterogeneous LOH and heterogeneity index
When LOH of particular chromosomal loci are detected homogeneously throughout all the microdissected foci (homogeneous LOH), this indicates the early occurrence of LOH during neoplastic progression. When LOH is detected in only a part of the microdissected foci (heterogeneous LOH), it is likely that these LOH are acquired during the neoplastic progression and/or divergence. We therefore calculated the heterogeneity index (HI) of each chromosomal arm as the number of cases with heterogeneous patterns of LOH divided by the total number of cases with homogeneous and heterogeneous LOH. Chromosomal arms with low HI are lost early in the clonal neoplastic process, while chromosomal arms with high HI tend to occur during progression of the tumor in the mucosal/submucosal lesion.
fractional allelic loss
Fractional allelic loss (FAL) was calculated as the number of chromosomal arms on which LOH was detected divided by the number of chromosomal arms on which allelic markers were informative in the patient's normal control DNA.
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LOH of chromosomal arms
Of 10 chromosomal arms analyzed, homogeneous and heterogeneous LOH was most frequently detected on 17p (92%) followed by 18q (81%), 18p (81%), 5q (61%), 22q (51%), 14q (44%), 15q (41%), 2q (39%), 12q (36%) and 4q (32%). Homogeneous LOH was most frequently detected on 17p (68%) followed by 18p (53%), 18q (53%), 22q (34%) and 12q (27%). As described previously [8
], heterogeneous patterns of LOH, which are indicative of genetic progression and divergence, were detected within the mucosal foci as well as in the invasive foci. Representative sections of the flat tumors and gels showing homogeneous and heterogeneous LOH are shown in Figures 1 and 2. A summary of the genetic analysis is shown in Table 1 and Figure 3, and the frequencies of LOH for each microsatellite marker loci for 18p, 18q and 22q are shown in Table 2. Relatively high numbers of non-informative cases for some markers/loci as shown in Table 1 are possibly due to low heterozygosity score for a particular marker in the Japanese patient population, or to alleles being too closely located to determine the exact allelic status.
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heterogeneity index
Of chromosomal arms with more than 40% LOH, the lowest HI was detected for 17p (0.26), followed by 22q (0.33), 18p (0.34), 18q (0.34), 15q (0.43), 14q (0.5) and 5q (0.64) (Table 1).
concordant LOH of 17p, 18p, 18q and 22q
Concordant allelic loss including heterogeneous and homogeneous LOH of 17p, 18p and 18q was found in 21 of 29 (72%) of the informative cases, and homogeneous LOH for the three chromosomal arms were detected in 11 of 29 (38%) of the cases. Of these 11 cases, only a single case (#FL4) showed histological components of low-grade adenoma, and all the other cases showed mucosal HGD or mucosa HGD/invasive cancer components. Concomitant LOH of 17p, 18p, 18q and 22q (both heterogeneous and homogeneous) was detected in 10 of 27 (37%) cases, and homogeneous LOH were detected in four of 27 (15%).
fractional allelic loss
When heterogeneous and homogeneous LOH were included, the average FAL was 0.57 and median FAL was 0.57. The average and median FAL for homogeneous LOH was 0.35 and 0.33, respectively.
microsatellite instability
Only two cases (cases #FL36 and #FL59) were classified as MSI-positive as determined by the five recommended markers.
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discussion |
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In the present LOH studies on 43 cases of early-stage flat-type colorectal tumors, several unique features were detected clearly. Since homogeneous LOH detected throughout the microdissected tumor foci occurs early during the neoplastic process, chromosomal loci with frequent homogeneous LOH may play an important role in the initial phase of these tumors. As expected based on previous studies [5
–8], 17p LOH was most frequently detected. 17p LOH showed the lowest HI, suggesting that its occurrence together with alterations of p53 is one of the earliest genetic alterations that occur in the clonal evolution of these tumors.
We found that the frequency of 18p and18q LOH was second to that of 17p and was also one of the earliest events with low HI in these early lesions. 18q is the second most frequent chromosomal arm with allelic loss in conventional colorectal cancer, and that including loci for DCC and SMAD4 has been reported to be up to 75% [14]. Colorectal cancers with 18q LOH have a more aggressive clinical behavior, more advanced stage and metastatic disease state than those that retain 18q [15–18]. Thus, the frequent involvement of 18q LOH in our cases may have conferred relatively aggressive clinical behavior on these tumors.
The allelic deletion of 18p has garnered much less attention than that of 18q, but the LOH of 18p has been reported for lung cancer, brain tumor, breast and gastric cancers [19–21]. In lymph-node positive high-risk breast cancer patients, 18p LOH has an adverse clinical outcome [22]. The critical region of allelic loss has been mapped to 18p11.2, indicating the existence of a potential tumor suppressor gene in this region. Although less attention has been paid than for 18q LOH in colorectal cancer, 18p LOH has been detected in 50% of conventional type colorectal cancers [14], and appears more frequently in advanced stage colorectal cancer. Kochhar et al. [23] reported that 77% of 18p LOH with one marker occurs in Dukes' D colorectal cancer. However, this frequency has no statistically significant correlation with the overall survival or disease-free survival rate of these patients. In the allelotype comparison of primary cancer, the local recurrence and metastases of colorectal cancer, 18p LOH occurred more frequently in metastatic cancer (42%) than do local recurrences (23%) and primary tumors (22%) [24]. In the present study, we frequently detected LOH of 18p and 18q at an early stage, indicating potentially aggressive genetic alterations on both chromosomal arms in small early lesions. Although we used a limited number of markers, concomitant LOH of 18p and 18q observed in most of the cases may be indicative of the complete loss of one chromosome.
22q LOH occurs at frequencies between 20% and 41% in sporadic colorectal cancers, most of which are derived from conventional exophytic tumors [14, 25–31]. 22q LOH occur more frequently in advanced cancer than in intramucosal carcinomas [25, 26]. Also, 22q LOH is correlated with metastatic potential and progression in terms of Dukes' stages [26, 27]. Castells et al. [29, 30] reported on the critical allelic loss to a 0.5 cM interval on 22q13 in sporadic colorectal cancers as well as breast cancers. Although we detected frequent LOH with all four markers on 22q12-q13, a relatively high LOH frequency, homogeneous and heterogeneous LOH in 50% and homogeneous LOH in 36% of the cases, was noted with D22S274, a marker located in the 0.5 cM interval. Thus, in flat colorectal tumors, it is possible that alterations of the same gene responsible for sporadic exophytic adenoma-derived cancers on 22q may also play a critical role, but tend to occur from relatively early stages in its neoplastic evolution.
5q LOH was observed frequently in the present study, but HI was relatively high. Therefore, it may occur later than LOH of 17p, 18p, 18q and 22q during neoplastic progression. The involvement of APC occurs at an equal frequency in flat tumors [7]. Stages of APC alterations and its carcinogenic role may be different in flat colorectal tumors, and this requires future study.
LOH of other chromosomal arms, namely 2q, 4q, 12q, 14q and 15q, were also detected, but occurred less frequently than for other chromosomes as mentioned above. Allelic losses of these arms have also been reported for conventional invasive colorectal cancer [14, 24, 32, 33], and thus they may also contribute to flat colorectal tumorigenesis.
Colorectal carcinomas which show frequent LOH of multiple TSC loci with high fractional allelic loss (FAL >0.2) are more likely to develop into recurrent and metastatic diseases than are low FAL group carcinomas [14, 15]. Similarly, more chromosomal deletions and high FAL occur in metastatic colon cancer than in primary cancer and local recurrence [24, 34]. Although we did not perform complete allelotyping of all of the chromosomal arms, the FAL of 0.57 for heterogeneous/homogeneous LOH and FAL of 0.35 for homogeneous LOH in 10 chromosomal arms indicates that the FAL of these tumors were high. When LOH of 10 chromosomal arms analyzed in the present study was calculated in previous reports, 2.64 of 10 chromosomal arms showed LOH by allelotyping [14]. In Thorstensen et al.'s study [24], an average of 0.9 and 1.8 arms/10 were involved in primary and local recurrence, respectively, while an average of 2.8 arms were lost in the metastases. Thus, it is most likely that many of the early-stage flat colorectal tumors may belong to the group of high-FAL colorectal tumors, which are often associated with advanced cancers.
When we screened for microsatellite instability with the five recommended markers, we only saw two cases showing MSI positivity. Thus, MSI may not be a major pathway for flat tumors.
In conclusion, the majority of early-stage flat-type colorectal tumors show high FAL with multiple LOH events including 17p, 18p, 18q, 3p and 22q, and less frequently 2q, 4q, 12q, 14q and 15q, in early lesions with/without further genetic progression/divergence during mucosal and early submucosal invasion. These distinct genetic patterns coupled with the methylation of certain genes may confer on them unique morphological and aggressive clinical behavior.
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Acknowledgements |
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We thank D. Mrozek for English proof-reading of the manuscript.
Received for publication April 3, 2005. Revision received August 7, 2005. Accepted for publication August 8, 2005.
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References |
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1. Rubio C, Shetye J, Jaramillo E. Histogenesis of small colonic adenocarcinomas. J Surg Oncol 1994; 56: 59–62.[ISI][Medline]
2. Shimoda T, Ikegami M, Fujisaki J et al. Early colorectal carcinoma with special reference to its development de novo. Cancer 1989; 64: 1138–1146.[CrossRef][ISI][Medline]
3. Tada S, Yao T, Iida M et al. A clinicopathologic study of small flat colorectal carcinoma. Cancer 1994; 74: 2430–2435.[CrossRef][ISI][Medline]
4. Teixeira CR, Tanaka S, Haruma K et al. Flat-elevated colorectal neoplasms exhibit a high malignant potential. Oncology 1996; 53: 89–93.[CrossRef][ISI][Medline]
5. Fujimori T, Satonaka K, Yamamura-Idei Y et al. Non-involvement of ras mutations in flat colorectal adenomas and carcinomas. Int J Cancer 1994; 57: 51–55.[ISI][Medline]
6. Minamoto T, Sawaguchi K, Mai M et al. Infrequent K-ras activation in superficial-type (flat) colorectal adenomas and adenocarcinomas. Cancer Res 1994; 54: 2841–2844.
7. Olschwang S, Slezak P, Roze M et al. Somatically acquired genetic alterations in flat colorectal neoplasias. Int J Cancer 1998; 77: 366–369.[CrossRef][ISI][Medline]
8. Fujii H, Ajioka Y, Kazami S et al. Loss of heterozygosity in the clonal evolution of flat colorectal neoplasms. J Pathol 2002; 197: 298–306.[CrossRef][ISI][Medline]
9. Mueller JD, Haegle N, Keller G et al. Loss of heterozygosity and microsatellite instability in de novo versus ex-adenoma carcinomas of the colorectum. Am J Pathol 1998; 153: 1977–1984.
10. Rubio CA, Rodensjo M. Mutation of p53 tumor suppressor gene in flat neoplastic lesions of the colorectal mucosa. Dis Colon Rectum 1996; 39: 143–147.[CrossRef][ISI][Medline]
11. Yukawa M, Fujimori T, Maeda S et al. Comparative clinicopathological and immunohistochemical study of ras and p53 in flat and polypoid type colorectal tumours. Gut 1994; 35: 1258–1261.
12. Yashiro M, Carethers JM, Laghi L et al. Genetic pathways in the evolution of morphologically distinct colorectal neoplasms. Cancer Res 2001; 61: 2676–2683.
13. Sakamoto N, Terai T, Ajioka Y et al. Frequent hypermethylation of RASSF1A in early flat-type colorectal tumors. Oncogene 2004; 23: 8900–8907.[CrossRef][ISI][Medline]
14. Vogelstein B, Fearon ER, Kern SE et al. Allelotype of colorectal carcinomas. Science 1989; 244: 207–211.
15. Kern SE, Fearon ER, Tersmette KW et al. Clinical and pathological associations with allelic loss in colorectal carcinoma [corrected]. JAMA 1989; 261: 3099–3103.[Abstract]
16. Jen J, Kim H, Piantadosi S et al. Allelic loss of chromosome 18q and prognosis in colorectal cancer. N Engl J Med 1994; 331: 213–221.
17. Martinez-Lopez E, Abad A, Font A et al. Allelic loss on chromosome 18q as a prognostic marker in stage II colorectal cancer. Gastroenterology 1998; 114: 1180–1187.[CrossRef][ISI][Medline]
18. Ogunbiyi OA, Goodfellow PJ, Herfarth K et al. Confirmation that chromosome 18q allelic loss in colon cancer is a prognostic indicator. J Clin Oncol 1998; 16: 427–433.[Abstract]
19. Tran Y, Benbatoul K, Gorse K et al. Novel regions of allelic deletion on chromosome 18p in tumors of the lung, brain and breast. Oncogene 1998; 17: 3499–3505.[CrossRef][ISI][Medline]
20. Osborne RJ, Hamshere MG. A genome-wide map showing common regions of loss of heterozygosity/allelic imbalance in breast cancer. Cancer Res 2000; 60: 3706–3712.
21. Yu JC, Sun KL, Liu B, Fu SB. Allelotyping for loss of heterozygosity on chromosome 18 in gastric cancer. World J Gastroenterol 2004; 10: 1964–1966.[Medline]
22. Climent J, Martinez-Climent JA, Blesa D et al. Genomic loss of 18p predicts an adverse clinical outcome in patients with high-risk breast cancer. Clin Cancer Res 2002; 8: 3863–3869.
23. Kochhar R, Halling KC, McDonnell S et al. Allelic imbalance and microsatellite instability in resected Duke's D colorectal cancer. Diagn Mol Pathol 1997; 6: 78–84.[CrossRef][ISI][Medline]
24. Thorstensen L, Qvist H, Nesland JM et al. Allelotype profiles of local recurrences and distant metastases from colorectal-cancer patients. Int J Cancer 1996; 69: 452–456.[CrossRef][ISI][Medline]
25. Miyaki M, Seki M, Okamoto M et al. Genetic changes and histopathological types in colorectal tumors from patients with familial adenomatous polyposis. Cancer Res 1990; 50: 7166–7173.
26. Iino H, Fukayama M, Maeda Y et al. Molecular genetics for clinical management of colorectal carcinoma. 17p, 18q, and 22q loss of heterozygosity and decreased DCC expression are correlated with the metastatic potential. Cancer 1994; 73: 1324–1331.[CrossRef][ISI][Medline]
27. Yana I, Kurahashi H, Nakamori S et al. Frequent loss of heterozygosity at telomeric loci on 22q in sporadic colorectal cancers. Int J Cancer 1995; 60: 174–177.[ISI][Medline]
28. Tomlinson I, Ilyas M, Johnson V et al. A comparison of the genetic pathways involved in the pathogenesis of three types of colorectal cancer. J Pathol 1998; 184: 148–152.[CrossRef][ISI][Medline]
29. Castells A, Ino Y, Louis DN et al. Mapping of a target region of allelic loss to a 0.5-cM interval on chromosome 22q13 in human colorectal cancer. Gastroenterology 1999; 117: 831–837.[CrossRef][ISI][Medline]
30. Castells A, Gusella JF, Ramesh V, Rustgi AK. A region of deletion on chromosome 22q13 is common to human breast and colorectal cancers. Cancer Res 2000; 60: 2836–2839.
31. Zhou CZ, Peng ZH, Zhang F et al. Loss of heterozygosity on long arm of chromosome 22 in sporadic colorectal carcinoma. World J Gastroenterol 2002; 8: 668–673.[ISI][Medline]
32. Thorstensen L, Qvist H, Nesland JM et al. Identification of two potential suppressor gene regions on chromosome arm 14q that are commonly lost in advanced colorectal carcinomas. Scand J Gastroenterol 2001; 36: 1327–1331.[CrossRef][ISI][Medline]
33. Umetani N, Fujimoto A, Takeuchi H et al. Allelic imbalance of APAF-1 locus at 12q23 is related to progression of colorectal carcinoma. Oncogene 2004; 23: 8292–8300.[CrossRef][ISI][Medline]
34. Choi SW, Lee KJ, Bae YA et al. Genetic classification of colorectal cancer based on chromosomal loss and microsatellite instability predicts survival. Clin Cancer Res 2002; 8: 2311–2322.
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