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Volume: Barrett's Esophagus
Chapter: Markers

Have environmental factors been identified for each genetic step in the dysplasia-carcinoma sequence?

M. Conio, R. Filiberti, S. Blanchi (Genova)

Barrett's esophagus is a premalignant lesion in which the squamous epithelium that normally lines the distal esophagus is replaced by metaplastic columnar epithelium, as a result of chronic gastroesophageal reflux (GERD).

Three types of columnar epithelium have been described. The intestinal type is the most common and it is suggested to be the initiating factor of a sequence of changes till development of adenocarcinoma. This so-called Barrett cancer is now rising faster than any other tumor in the western world [1] and the risk for the development of an adenocarcinoma in patients with Barrett's esophagus is 30/125 times higher when compared to the normal population [2].

It is becoming established that neoplasia in Barrett's esophagus is the result of a multistep process in which a sequence of genetic mutation occurs [2]. The metaplastic epithelium sequentially changes through low-grade dysplasia (LGD), high-grade dysplasia (HGD), carcinoma in situ and finally to invasive cancer. Nowadays, however, the pathophysiology of this disease is unknown. It is impossible to predict which patients are at risk of neoplastic progression, so an early diagnosis is necessary.

At present, subjects with Barrett's esophagus are followed-up with endoscopies and multiple biopsies every 6-24 months, depending on the presence of dysplasia. However, a large disagreement of the pathologists on the grade of dysplasia and the high cost of endoscopic examination impair this surveillance [3]. An alternative to these practices is molecular biology and the research of specific markers predictive of malignant potential of Barrett's esophagus, in association with the finding of environmental factors that may interact with the genetic factors. The tests should be clinically useful, simple, non invasive, cost-effective highly sensitive and specific [4].

Generally, cancer is a genetic disease derived from genomic alterations and characterized by dysregulated cell proliferation and differentiation, with changes in their regulatory molecular factors. Genomic instability, allelic loss of some chromosomes, inactivation of tumor suppressor genes or activation of specific oncogenes are supposed to act in the neoplastic progression of Barrett's esophagus. The abnormalities in DNA can cause morphological changes detectable upon histological examination before genetic mutations lead to invasive spread.

The next pages report the genetic alterations known to be involved in carcinogenesis of Barrett's esophagus (Table I).


Aneuploidy is a modification in the cellular DNA content, resulting from uncontrolled cell division. Recent studies have found aneuploid cell populations in patients with Barrett's esophagus [5,6] . The proportion of these cell seems increases with the progression to different levels of dysplasia or adenocarcinoma. However, data are unclear, since it is

Table I. Environmental factors and molecular alterations involved in the multistep Barrett's esophagus carcinogenesis (from [3]).

difficult to differentiate HGD from early carcinomas and dysplasia can often occur in absence of aneuploidy, and vice versa.

Microsatellite instability

Microsatellites are alterations of the genome's regions consisting in short sequences of repeated DNA-nucleotides [3]. In malignant tissues, there is a rapid cell proliferation resulting in uncorrected DNA replication errors and in a variation of size in microsatellite sequences. Microsatellite instability occurs in 13-35% of Barrett's esophagus adenocarcinomas and can develop as an early event in metaplasia and in diploid tumors cells, before aneuploidy occurs [7].

Figure 1. Contribution of environmental factors and molecular changes in the progression from normal squamous epithelium to Barrett's esophagus and adenocarcinoma.

Gene abnormalities

Proto-oncogene, tumor-suppressor genes, growth factors and their receptors are the most common gene abnormalities in neoplastic progression.

P53 is a tumor suppressor gene located on the short arm of chromosome 17, which appears to play an important role in cellular growth control, influencing the proliferation, suppressing genome instability, signaling apoptosis and moduling differentation. The product of an intact p53 gene restrains cells, with damaged DNA from entering the S-phase of the cell cycle at the G1 checkpoint, leading to cell cycle arrest, DNA repair and apoptosis [8]. Its inactivation causes a rapid clonal expansion of the cell population. p53 mutation may be quite common in Barrett's esophagus adenocarcinoma and arises as an early event, preceding the appearance of aneuploidy. Some studies have shown that p53 mutations are involved in the pathogenesis of Barrett's cancer in a subset of about 50% of the patients and that the mutations are present mostly in dysplasia, but not in metaplasia [9]. Patients with p53-immunoreactive LGD seem to progress more rapidly to HGD and/or carcinoma [4]. Finally, p53 may be a potential molecular marker for determining the risk of development of cancer in patients with Barrett's esophagus, especially if it is used in conjunction with the detection of aneuploidy [10].

Retinoblastoma tumor-suppressor gene (Rb) is a regulatory gene in the G1 phase of the cells cycle. Its mutation results in an uncontrollable cell proliferation and predisposes to numerous human cancers, but the role of this gene in the premalignant stage of Barrett's esophagus has not been demonstrated [11].

Adenomatous polyposis coli (APC) gene has been studied in Barrett's esophagus carcinogenesis since its alterations are highly specific for gastrointestinal tumorigenesis. Data on this topic are uncertain, but APC gene is thought to have a role in a very early step, at metaplasia level and before phenotypic changes of the Barrett's cells [12].

Other oncogenes are involved at different steps of neoplastic development in several types of cancer; for most of them, data on involvement in Barrett carcinogenesis are still uncertain.

c-erbB-2 is an oncogene overexpressed in a several human adenocarcinomas. A c-erbB2 overexpression was observed in 11%-73% of invasive esophageal carcinomas, but in none of dysplastic areas. This suggested that it occurs in a late event of the dysplasiacarcinoma sequence of Barrett's esophagus [13].

Oncogenes of ras family (H, K, N) encode specific proteins (p21) which act as cytoplasmatic mediators. Data on relationship between ras genes and Barrett's esophagus carcinoma are uncertain. Some studies have not found any k-ras gene mutation in specimens with various grades of dysplasia and/or invasive cancer [14], whereas expression of k-ras associated protein was found by other authors in 20-40% of esophageal adenocarcinoma and in 14% of Barrett's esophagus samples [15]. Moreover, H-ras overexpression was found in HGD and carcinoma, but non in LGD or non dysplastic Barrett's esophagus [16].

Other studies suggested a role in the neoplastic progression of Src family kinases, involved in a number of signal transduction pathways. The Src specific activity was found to be 3-4 fold higher in Barrett's esophagus and 6 fold higher in esophageal adenocarcinoma than in control tissues. Its activation seems to occur prior to the development of dysplasia or carcinoma in Barrett's esophagus [17].

Overexpression of Bcl-2, which encodes a protein that blocks apoptosis, was seen early in the dysplasia-carcinoma sequence by Katada et al. [18]. Otherwise, Goldblum and Rice [19] have not seen bcl-2 immunoreactivity in any Barrett's esophagus with or without dysplasia.

The cyclin D1 gene is an important regulator of G1-S phase transition in the cell cycle. Its increase was observed in 46% of dysplastic or non dysplastic samples from patients with Barrett's esophagus, suggesting that the cyclin D1 overexpression might predispose Barrett's esophagus epithelium to malignant transformation in an early phase [20].

Growth factors (TGFa/ and EGF) are proteins that bind to membrane-associated receptors influencing cell proliferation, differentiation and chemotaxis. Barrett's metaplasia is associated with elevated expression of TGFa and EGF which increase the expression of multiple oncogenes such as c-fos, c-jun, c-myc and cyclin D [21].

Adhesion molecules

Cadherins are a family of trans membranous glycoproteins responsible for calciumdependent intercellular adhesion. E-cadherin has been considered a key factor in the intercellular physical adhesion of cancer cells: loss or reduction facilitate metastasis and may play a role in the progression of Barrett's esophagus metaplasia to adenocarcinoma [3].

It is plausible that the development and progression of Barrett's esophagus is a multistep phenomenon involving one or more genetic events and some environmental factors (i.e. life-style related factors and GERD). Several studies have addressed the role of smoking, alcohol, diet and obesity in the pathogenesis of esophageal adenocarcinoma. Alcohol and tobacco are unlikely to play a major role in the natural history of Barrett's esophagus, while an elevated risk was associated with a low intake of fruit and vegetables, dietary fibers, and obesity [22]. In addition, some authors have focused attention on Helicobacter pylori and chemotherapeutic agents, but there is not any evidence of a direct association between these factors and Barrett's esophagus.

It has been hypothesized that an environmental impulse initiates the metaplastic change in Barrett's esophagus, even if it is not clear if this impulse is important also in malignant progression [3].

Genotyping of Barrett's epithelium may provide an important tool in the diagnostic assessment of the pathology. At present, the search to understand the Barrett's esophagus etiopathogenesis is going on. In this way, a better knowledge of the environmental and host/cell factors in disease progression could be achieved for an earlier identification of patients at high risk for the development of cancer.


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Publication date: August 2003 OESO©2015