Barrett’s esophagus (BE) is a condition in which the normal squamous lining of the esophagus is replaced by a metaplastic columnar (intestinal type) epithelium. BE develops in the context of chronic gastro-esophageal reflux disease (GERD), with repeated cycles of injury and repair in a genotoxic environment of exposure to acid, bile and chronic inflammation(1
). BE is a pre-malignant condition – it is the only known precursor of esophageal adenocarcinoma (EA), a cancer which is increasing at an exponential rate in the USA. It is estimated that the incidence of GERD within the population is about 10%; Barrett’s esophagus is estimated to develop in 10% of those individuals, and the annual incidence of EA in these patients is estimated to be 0.5–1% per year (3
). Barrett’s esophagus is therefore of considerable clinical significance since the five-year survival rate of esophageal adenocarcinoma is only ~10%, unless detected at an early stage, in which case it is curable. It is therefore recommended that BE patients be managed by endoscopic surveillance; however, at present 95% of patients with esophageal adenocarcinoma do not have a prior diagnosis of Barrett’s esophagus (4
). It is therefore important to define biomarkers which could be readily applied to patients with GERD to identify those who have BE and are at risk for EA, and would therefore benefit from endoscopic surveillance and/or medical or surgical intervention. Although conventional upper GI endoscopy has become widespread in its applications and availability, it is constrained by the requirement for patient sedation, as endoscopes large enough to allow biopsies are not otherwise tolerated (5
). To address this problem, an accurate, sensitive molecular biomarker for the presence of BE would be of great utility.
Widespread genomic instability is believed to facilitate neoplastic progression in BE, as well as many other pre-neoplastic diseases. This process is facilitated the loss and mutation of important cell cycle checkpoint machinery and tumor suppressor loci, such as p16 and p53. In addition, biomarkers of the process of genomic instability itself may be of clinical use. We have documented shortened telomere length and chromosomal instability using fluorescence in-situ hybridization (FISH) in BE (6
). Although we have previously focused on sites of known tumor suppressors, we and others have shown that chromosomal “fragile sites
” and in particular, FRA3B, have an extremely high rate of deletion in BE patients(8
) and in patients that progress to EA(10
). Thus, we hypothesize that fragile sites may serve as a sensitive biomarker of BE and the genotoxic injury that accompanies BE.
Fragile sites are loci that exhibit an increased propensity for sister chromatid exchange, translocation, and breaks under conditions of genotoxic stress (11
). The susceptibility of these loci to damage is believed to be a consequence of their primary and secondary structure, which affects chromatin organization and can stall DNA replication (13
). The resulting DNA gaps, breaks, and other chromosomal aberrations at fragile sites impact genomic stability, and often manifest as deletions and translocations. Currently, there are over one hundred documented fragile sites within the human genome, categorized as “common” (present in all individuals) or “rare” (present in less than 5% of the population) (13
); most are defined cytogenetically and their molecular characterization is not known.
While instability at specific fragile sites has been linked to different cancers (15
) including breast (17
), prostate, and lung (18
), there is still uncertainty as to whether these fragile site alterations causally contribute to cancer development or are merely “silent markers” of genomic stress. Putative tumor suppressors have been suggested to be located within common fragile sites; the fragile histidine triad gene, FHIT
, at FRA3B and the WW-domain containing oxidoreductase, WWOX,
at FRA16D have the best documented evidence for a role in cancer progression (20
), while most other genes known to be at fragile sites, such as parkin
at FRA6E have less clear evidence for roles as tumor suppressors (21
). Alternatively, breakage at fragile sites could contribute to repeated cycles of bridge-breakage-fusion, potentially promoting the amplification of oncogenes (22
) such as Met
within the FRA7G region (23
) or the prolactin-inducible protein (PIP
) gene (24
We propose that as a consequence of chronic acid and nitric oxide exposure and stalling the G1/S transition in Barrett’s epithelium combined with impaired DNA repair pathways (25
), there is an increased instability at fragile sites in this premalignant tissue. In previous studies of BE patients, copy number loss was detected at two fragile sites (FRA3B and FRA13B)(8
). In this report, we include a larger sampling of new early-stage BE specimens and report genomic instability (copy number loss and/or LOH) at many fragile sites in BE, some of which demonstrate increasing alterations with disease progression.