Genomic instability has been proposed to play an important role in cancer by accelerating the accumulation of genetic changes responsible for cancer cell evolution (31
). Previous studies by our group and others demonstrated that chromosomal abnormalities were frequent in ESCC and commonly involve chromosomes 3p, 4p, 4q, 5q, 9p, 9q, 11q, 13q, 15q, 17p and 17q (9
). However, the molecular mechanisms underlying this chromosome instability are unclear. In the present study, we demonstrated that high level of telomere attrition in cancer cells was significantly associated with specific chromosome arm instabilities in ESCC. Our findings here are consistent with previous reports that telomere shortening is associated with chromosome instability in ESCC tumors (26
), in Barrett’s esophagus (24
), and in tissues taken from patients with ulcerative colitis, which is associated with increased risk of colon cancer (30
). Kammori et al
examined tumors from 15 ESCC patients and found that telomere length in normal epithelial cells adjacent to tumor was inversely associated with the frequency of chromosome anaphase/telophase bridges and aneuploidy in tumor cells (26
). Finley et al
examined 11 biopsies from Barrett’s esophagus patients and found that high telomere attrition in epithelial cells significantly correlated with chromosome abnormalities for chromosomes 17 and 11, but not for chromosome 9 (24
). Using a genome-wide scan approach, we discovered that high level telomere attrition in cancer cells was significantly associated with instability on chromosomes 13q and 15q in ESCC. To the best of our knowledge, this is the first study to report that telomeres are involved in the chromosome arm specific instabilities in ESCC.
Surprisingly, we also found that long telomeres in tumor stroma cells (carcinoma-associated fibroblasts and infiltrative lymphocytes) were significantly associated with instability on chromosomes 4q, 13q and 15q. This finding is not entirely consistent with previous reports that short telomeres in epithelial cells/cancer cells are associated with chromosome abnormalities in human cancers (32
). However, these previous studies only examined the telomere length in cancer cells or epithelial cells of preneoplastic lesions. No previous study has examined telomere lengths in CAFs and infiltrative lymphocytes and their relationship to chromosome instability in cancer cells. Telomere lengths measured in tumor stroma cells may not be directly comparable to telomere length in cancer cells or epithelial cells of preneoplastic lesions. Shorter telomeres may either have been inherited as a constitutional trait or may have been acquired somatically because of attrition induction by tissuespecific environmental factors, i.e., excessive proliferation. The reduction in telomere length in epithelial cells of preneoplastic lesions may simply reflect a greater number of times that the preneoplastic epithelium has replicated, compared to the surrounding stroma. In the present study, we found that telomeres in stroma cells are significantly longer than in neighboring cancer cells and that there is no significant telomere length correlation between stroma cells and normal epithelial cells adjacent to tumor, and only a weak telomere length correlation between stroma cells and cancer cells in ESCC. These data suggested that cancer cells and stroma cells in ESCC may have experienced different tissue-specific micro-environment of telomere loss/maintenance. We speculate that telomere maintenance in the tumor stroma may be altered to facilitate the development of tumors with high chromosome instability. Previous studies have demonstrated that tumor stroma play a significant role in the initiation and progression of carcinomas (35
). Reactive tumor stroma differs from normal stroma; it has a reactive phenotype that is associated with an increased number of fibroblasts, enhanced capillary density, and increased type-I-collagen and fibrin deposition. Reactive tumor stroma cells have been shown to provide oncogenic signals that facilitate tumorigenesis (37
). Extensive gene expression changes and neoplastic-specific changes were observed in the CAFs of breast cancer (39
). However, the role of telomeres in the reactive tumor stroma in the initiation/progression of ESCC is completely unknown and is an area that warrants further investigation.
Our data suggest that telomere length is associated with chromosome instability only on specific chromosome arms (4q, 13q and 15q). This observation raises the question as to whether these chromosome arms possess generally shorter telomeres than other chromosome arms in ESCC patients. It has been reported that average telomere length may be less important than short telomeres on specific chromosome arms since telomere dysfunction occurs preferentially on chromosomes with critically short telomeres (41
). In humans, chromosome specific telomere lengths are highly polymorphic between chromosome arms (42
) and the telomere length patterns on chromosome arms appear to be heritable (45
). Several reports have suggested that telomere shortening does not occur at the same rate for all telomeres (47
). This view is supported by several studies which demonstrated that those chromosome arms with the shortest telomeres were more often found in the telomere fusions leading to chromosome instability (41
). A recent study compared telomere length between normal breast epithelium, ductal carcinoma in situ (DCIS), and invasive ductal carcinoma (IDC) from 18 patients and found that the shortening of telomeres on chromosome 17q is greater than the average shortening of all telomeres (50
). Future studies that examine specific chromosome arm telomere lengths may provide new insights into the molecular mechanisms controlling the chromosome instability commonly observed in epithelial cancers.
We also observed that telomere length differed significantly among cell types, such that length in infiltrative lymphocytes > CAFs > cancer cells. Short telomeres were observed in cancer cells in 44 of 47 (94%) tumors examined, suggesting that telomere shortening is a common genetic alteration in ESCC. This observation is consistent with most previous reports of telomere length in various cancer types. In cancer cells, telomere length varies widely and its equilibrium depends on the balance between telomere shortening from cell division and telomere elongation from telomerase activity (51
). Telomerase is a reverse transcriptase that synthesizes and adds the telomeric repeats onto the end of chromosome. Very short telomeres have been reported as common early alterations in many human cancers, including gastric (17
), colon (19
), esophageal (25
), lung (20
), breast (21
), pancreatic (22
), and prostate (23
) cancers. In contrast, some tumor types have longer telomeres in cancer cells than in normal cells; for example, intracranial tumors, basal cell carcinomas of the skin, and renal cell carcinoma (54
). Our results indicate that shortened telomeres are a common genetic alteration in ESCC, which is consistent with previous reports (25
In conclusion, our data provide further evidence that telomere shortening is a common genetic alteration in ESCC, and that chromosome instability is related to both telomere attrition in cancer cells and telomere length in tumor stroma cells. The data provide new clues for understanding the molecular mechanisms of chromosome arm specific instabilities in ESCC and suggest that genetic defects in telomeres may be involved in the development of ESCC.